Getting Into Electric Power Catamarans

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bobkart   1 W

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Getting Into Electric Power Catamarans

Post by bobkart » Feb 04 2021 6:24pm

Greetings from the Pacific Northwest . . . I'm new to the forum.

Last year I got a tiny bit into electric boating, with a 10-foot inflatable and a 55-pound thrust trolling motor. This year I want to go bigger.

After digging into what makes an efficient hull shape, I settled on the catamaran style, with long/thin hulls. I believe this style of boat is referred to as 'semi displacement'. I see these used in human-powered watercraft, like the Open Waterbike Project:

https://openwaterbike.com/

I considered hydrofoils briefly, and they do look to be more efficient, but the extra complexity puts it out of immediate reach for me. This is still a 'crawl/walk/run' type of thing, and I'm not quite to 'walk' yet.

My target boat size is 3-5 meters (10-16 feet), with capacity of 2-4 persons. I feel like 20hp will be enough to move at what I'm considering my target speed, which is 20mph. This is for the 16-foot version, smaller versions shouldn't need that much power to go that fast, but of course won't carry as many passengers and/or batteries.

I'm in line to purchase a 16-foot power cat based on the RS Sailing boat of that size:

https://www.rssailing.com/project/rs-cat-16/

But it won't get here for another month or two. In the meantime, as a way to have some fun now and use as a testbed for ideas that may carry over to the larger boat, I've ordered the Build Your Own Boat version of the Hydrobike:

https://hydrobikesontario.com/product/b ... -own-boat/

We live near a small river that's perfect for putting around; here's a video from one of our trips up the river last year:

https://www.youtube.com/watch?v=xc72AsZgcRc

There's also Lake Washington and the Puget Sound. Last year we hit a dozen different locations, for over 30 hours on the water, at a total electricity cost of under $2.

What's the point? I may need to draw from the body of knowledge represented in this forum. I'm hoping some people here are willing to help me through sticking points I may encounter in my quest. Thanks in advance to all who help.

I'm no stranger to electricity and batteries. I have 10kWh of 200Ah LiFePO4 cells that I just finished top balancing. I used a similar setup last year on a demo of an ePropulsion Navy 6.0 outboard, but it wouldn't quite plane the inflatable boat. I feel like this motor would push the larger cat to close to 15mph.

But back to the personal cat. I have that trolling motor, which I will try first. I'll add a deck, seat, and motor mount first of course. I know that trolling motors don't have the propeller pitch to go much more than 5mph. I got around 4mph in the inflatable with this motor. So this will be a good second data point. I see some questions about how to get higher-pitch props for these motors, but very few good answers (Kipawa propellers being one). So to go faster I'll most likely need a legitimate outboard, with prop pitches suited to the speeds I'm after.

I also have a 2200W Hangkai outboard, and on the inflatable boat, I got 6mph out of that. No doubt I was pushing pretty hard on the hull speed limit for that boat. By looking at the prop on that, and guesstimating the pitch, and combining that with the RPM claimed by the manufacturer, I think this motor could push the little cat to over 10mph. I'll know this soon enough, as the little cat is scheduled to arrive next week.

I'll most likely update this thread as I get results from my various experiments.

Any discussion relating to this project is welcome!

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 05 2021 7:02am

The catamaran is one of the most efficient hull forms possible. Properly designed, it doesn't have a hull speed like monohull, where you need huge amounts of power to step up on a "plane". One of my favorite boats from the past was a 30' cuddy cabin catamaran. It used twin 225 hp outboards, top speed about 45 knots, but a very comfortable cruise at 23-24. It would cut right through huge wakes that would leave behind broken teeth on a monohull.

The cat is weight sensitive. If you can, discuss with the designer your plans, and establish a weight budget. The cat above with full fuel was pretty much limited to two passengers to get good performance. But I loaded it up with accessories like a generator and air conditioner.

Good luck and have fun!

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Re: Getting Into Electric Power Catamarans

Post by Stealth_Chopper » Feb 05 2021 7:56am

My friendly suggestion
Buy an electric outboard, mount it on a comfortable boat ( with thwarts ).
Enjoy yourself, recharge, repeat.
MinKotaOutboard.jpg
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Re: Getting Into Electric Power Catamarans

Post by SlowCo » Feb 05 2021 8:16am

Nice idea.
I think it would be a very nice project to build my own trolling motor using a multi kW brushless electric motor. For me a 3kW motor like this: https://nl.aliexpress.com/item/40010535 ... 2e0erO0ltj with a fiberglass dome attached/glued at the front and a custom machined aluminium rear section with a stainless steel axle in seals to attach a prop to would be great.
In your case two QS 205 90H motors (one at the rear of each catamaran hull) would be better to achieve your power goal. Putting the motor(s) under water would give enough cooling to increase their continuous and peak rating seriously.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 05 2021 10:20am

BlueSeas wrote:
Feb 05 2021 7:02am
The catamaran is one of the most efficient hull forms possible. Properly designed, it doesn't have a hull speed like monohull, where you need huge amounts of power to step up on a "plane". One of my favorite boats from the past was a 30' cuddy cabin catamaran. It used twin 225 hp outboards, top speed about 45 knots, but a very comfortable cruise at 23-24. It would cut right through huge wakes that would leave behind broken teeth on a monohull.

The cat is weight sensitive. If you can, discuss with the designer your plans, and establish a weight budget. The cat above with full fuel was pretty much limited to two passengers to get good performance. But I loaded it up with accessories like a generator and air conditioner.

Good luck and have fun!
Thanks for joining the discussion, and confirming my impressions of catamarans. That 30-footer sounds like a lot of fun!

I'm definitely considering weight. There's a Catch-22 with power and runtime in that the more of either you shoot for, that requires more battery weight, which then impacts your performance. My goal is to have at least one hour of runtime at full power (plus say 10% for reserve and to protect the batteries). Assuming that cruise speed is closer to half power, this could yield two hours of cruising. Counting some idle/off time, this could be more like three hours on the water. My old inflatable boat had a range of about 20 miles, but of course brining a second battery would double that without too much performance penalty. Naturally I'm looking to exceed that range with the new setup(s).

I have a few ideas for extending runtime with less weight, some greener than others. First up is solar panels, I see those light/flexible models as working well, just need a light structure to hold them. On the larger cat I could get somewhere between 1-2kWp, offsetting consumption by a fraction. On the smaller cat, the most I can see fitting is 1kWp, but with a small-enough motor that could greatly extend runtime (the trolling motor I have pulls around 600 watts).

I briefly looked into fuel cells (hydrogen, methanol, LPG), but costs are prohibitive.

The last option for extending runtime is a propane-powered generator (a'la series hybrid). The standard 'backup generator' models unfortunately are heavy for their power output, amounting to weight of around two hours of runtime (~30 pounds per kilowatt). So bringing a second hour-of-runtime-worth of battery would be lighter. Only by cutting the battery size to the bare minimum (LTO cells for example) and sizing the generator to nearly match your average anticipated power draw would that approach come out ahead weight-wise. One nice thing about such an approach is that you can basically run as long as you have propane, although perhaps at reduced power.

An alternative to typical portable backup generators is DC generators. I see a couple of good options there, and the power-to-weight ratios look much more appealing. But this is thinking pretty far ahead for now; I really need to get the baseline functionality in place and better characterize that before heading off into range-extension territory.

There are also a few brands of propane outboards out there. Most are of lower power outputs (5hp). But at that point we're no longer talking about electric boating, although range and weight concerns pretty much go away. This would only be a last resort, basically on option for supporting a use case that simply can't be accommodated with batteries.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 05 2021 10:46am

Stealth_Chopper wrote:
Feb 05 2021 7:56am
My friendly suggestion
Buy an electric outboard, mount it on a comfortable boat ( with thwarts ).
Enjoy yourself, recharge, repeat.
MinKotaOutboard.jpg
The outboard approach does seem easiest. What I'm seeing is that trolling motors typically won't push you much more than 5mph, despite having the motor power to go faster. The propeller parameters for those usually favor thrust over speed, like being stuck in first gear in a manual car transmission. Even that MinnKota E-Drive you pictured, I see this as part of the description:

"E-Drive features 2 HP of muscle to move a full-size pontoon up to 5 mph on a single charge."

So plenty of thrust for pushing a heavy boat slowly. And, if they made higher-pitch propellers for it, I could possibly get 10mph out of the 10-foot cat. But so far I've not found such a propeller, so 5-6mph is more like what I expect I'd see.

While on the subject of propellers, I see two approaches to electric outboards in that regard. One involves using the same style of prop that you see on gas outboards (with the thru-hub exhaust opening at the rear). The other involves propellers designed by the motor maker, and those have considerably better efficiency for two reasons: the large opening at the rear, which is a big source of drag, is gone, and they can break out of the standard (and inefficient) approach of high RPM and smallish diameter, as lower-RPM propellers can be much more efficient due to their slower rotating speed through the water and thus less drag. A typical gas outboard propeller might be 60% efficient (at a given combination of power and speed) while larger diameter and slower-turning propellers can exceed 80% efficiency.

Brands like Elco use the standard propeller approach, which has the upside of being able to more easily change pitch. But they're much less efficient due to the higher RPM and smaller diameter, and the big drag component represented by the now-empty big hole at the rear (even flared out to help exhaust leave!). I'd at least cut that flare off as a first step.

Then brands like ePropulsion and Torqeedo use proprietary propellers, which win on efficiency hands down. But you usually don't get a wide range of pitches to work with. In the case of the ePropulsion Navy 6.0, you get a low- and a high-pitch propeller, the first for heavier boats and topping out around 10mph, then the second can hit 15mph with a light enough boat. The latter will likely work well on my larger cat, but if I were to put that on the smaller cat, I'd most likely be leaving performance on the table. I.e. with more propeller pitch, I suspect 20mph could be achieved. But I've been in discussion with them and a third propeller is not an option. And I'm in no position to fabricate one myself, although there *may* be services out there that have that capability. But it most like would be way too costly to justify that approach.

So that just leaves the inboard-motor approach, if trying to get both efficient propellers and wide choice of propeller parameters (mainly pitch). More on that later.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 05 2021 1:54pm

SlowCo wrote:
Feb 05 2021 8:16am
Nice idea.
I think it would be a very nice project to build my own trolling motor using a multi kW brushless electric motor. For me a 3kW motor like this: https://nl.aliexpress.com/item/40010535 ... 2e0erO0ltj with a fiberglass dome attached/glued at the front and a custom machined aluminium rear section with a stainless steel axle in seals to attach a prop to would be great.
In your case two QS 205 90H motors (one at the rear of each catamaran hull) would be better to achieve your power goal. Putting the motor(s) under water would give enough cooling to increase their continuous and peak rating seriously.
Thanks for joining the discussion. Yes, the thought of putting something together motor/drive-wise has crossed my mind. Except I'm leaning more towards an inboard motor approach. With a raised deck like these have, there's no issue of sealing the hole for the propeller shaft in the hull.

Upsides are much more power/propeller choice flexibility, and most likely lower materials cost. Probably lower overall weight per power too. And less drivetrain losses. Downsides are less steering control (rudder) and of course the actual work to get it built. And less flexibility in changing out the motor for another like is more easily done with outboards.

One problem I've yet to solve with this approach is the thrust bearing. Many electric inboard motors have those built it, so it's easy to tack on the rest: flange coupling, prop shaft, prop strut, prop and attaching hardware. Propeller thrust is transferred to the boat through the motor mounts.

But those electric inboard motors are usually considerably more expensive than a small/medium PMAC motor and controller (air cooled as I'd rather not mess with liquid cooling):

https://www.electricmotorsport.com/pmac ... ystem.html

Under $4K gets you 14.5kW continuous output, with controller. But from what I can tell, there's no thrust bearing built in. So one needs to be added in the drivetrain. The forward (or reverse) thrust from the prop needs to all get transferred to the boat through the thrust bearing and its mounting to the boat. Otherwise the bearings in the motor will have to absorb that, and they're not designed for that much load in that direction (axial).

So my first big question to the forum is: what's out there in the way of standalone thrust bearings? The ideal arrangement I can think of is to have flange adapters on each end of a short shaft that passes through the bearing, then the whole thrust bearing assembly can be mounted to the boat (a'la pillow block bearings). Then the motor would be attached to one end's flange and the prop shaft to the other. I'd most likely use those bearing-saver style of flange couplings, which allow for slight misalignment:

Image

This approach might also work well on the small cat, using a 2-8kW motor.

Anybody out there knowledgable about thrust bearings?

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 05 2021 2:48pm

If you don't know about this forum, check it out:

https://www.cruisersforum.com/

Probably some good threads there on marine electric propulsion. But definitely a more likely place to find/get answers to questions on thrust bearings.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 05 2021 2:54pm

BlueSeas wrote:
Feb 05 2021 2:48pm
If you don't know about this forum, check it out:

https://www.cruisersforum.com/

Probably some good threads there on marine electric propulsion. But definitely a more likely place to find/get answers to questions on thrust bearings.
Thanks BlueSeas, I'll check that out!

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 05 2021 10:17pm

Looks like they call what I'm looking for a thrustblock, at least on ships:

https://www.poseidon-bv.nl/orion-thrustblock/

Image

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 06 2021 8:14am

Never saw one exactly like that on the 20-60 ft boats I worked on. What I did see were systems designed to decouple the shaft alignment from the engine alignment to eliminate vibration. But it wasn't very common. Normally the transmission takes the propulsion force directly from the shaft.

Borrowed this picture from Aquadrive. Something like this is probably what you are looking for.
5ACBA944-FD7D-4EE2-93D0-5C5A929DB063.png
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I poked around at the commercial electric drive packages and found none using "direct drive". Here was a system that looked like it could be copied:
82FA759C-55D9-4FAC-8FA4-4D5E50D2BC99.png
82FA759C-55D9-4FAC-8FA4-4D5E50D2BC99.png (440.16 KiB) Viewed 1473 times

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Re: Getting Into Electric Power Catamarans

Post by John in CR » Feb 06 2021 12:55pm

Before deciding anything about the electric powerplant you've got to get your hull design down. For 20mph, which is 17.4 knots, you'll need a planing hull design, which is more difficult to achieve with a catamaran. Planing hull cats are more typically for achieving greater stability while fishing and cutting through moderate size waves more smoothly being up on 2 fine hulls. 17.4 knots isn't terribly difficult to achieve with a displacement cat, but will require a 25-30ft waterline length depending on the length-to-width ratio of the hulls.

A planing monohull is really the way to go if you need 20knots, and depending on where you plan to use it could be quite easy to design for protected water use.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 06 2021 2:21pm

BlueSeas wrote:
Feb 06 2021 8:14am
Never saw one exactly like that on the 20-60 ft boats I worked on. What I did see were systems designed to decouple the shaft alignment from the engine alignment to eliminate vibration. But it wasn't very common. Normally the transmission takes the propulsion force directly from the shaft.

Borrowed this picture from Aquadrive. Something like this is probably what you are looking for.
<snip>
I poked around at the commercial electric drive packages and found none using "direct drive". Here was a system that looked like it could be copied:
<snip>
That's helpful, BlueSeas.

I've considered that decoupling, and it's still on the table. I'm trying to minimize drivetrain losses, and each of those U-Joint points is such a loss. Also, it seems like there still needs to be *something* in the way of a thrust washer between the prop and the first U-Joint, otherwise the thrust gets pushed all the way up the line (possibly causing problems for those U-Joints), and the same problem occurs when it gets to the motor, which can't handle it (in the approach I'm contemplating).

But yeah, in a typical ICE installation, the reduction gearbox or V-Drive has the thrust washer. And many electric inboard motors have them as integrated components. I just don't believe I'd get that with a small-to-medium PMAC motor brand (like Motenergy). My reasons for considering that direction is that the cost is quite a bit less, and there's more selection in terms of motor parameters (voltage, RPM, power, ...).

For an example of an electric inboard motor with integrated thrust washer (and thus suitable for direct drive applications), look here:

https://www.bellmarine.tech/wp-content/ ... 5_LQ-1.pdf

The good stuff starts on page 6:

"Motor with integrated thrust bearing"

and a bit above that on the same page:

"Separate thrust bearing is recommended above 20 kW"

So this confirms that there is such a thing as a standalone thrust bearing (of the size I'd probably need) . . . I'll probably reach out to Bellmarine at some point for examples.

I do like that last style you posted; similar to a V-Drive, it lets you start the prop shaft lower and more forward, to get closer to horizontal. That belt-drive arrangement will be a not-insignificant source of drivetrain loss though, as well as an additional point of failure. I first ran into that approach on the Plugboats.com site, which brought me to here:

https://www.lynchmotors.co.uk/inboard.html

Those axial flux 'pancake' motors would seem to have the advantage of more torque per weight, so less RPM should be needed to achieve the power level desired, which better fits my requirement for lowish propeller RPM. But they're DC as opposed to AC, and my understanding is that the latter are more efficient. Still, this option is not off the table.

Thanks for those good suggeestions.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 06 2021 4:43pm

John in CR wrote:
Feb 06 2021 12:55pm
Before deciding anything about the electric powerplant you've got to get your hull design down. For 20mph, which is 17.4 knots, you'll need a planing hull design, which is more difficult to achieve with a catamaran. Planing hull cats are more typically for achieving greater stability while fishing and cutting through moderate size waves more smoothly being up on 2 fine hulls. 17.4 knots isn't terribly difficult to achieve with a displacement cat, but will require a 25-30ft waterline length depending on the length-to-width ratio of the hulls.

A planing monohull is really the way to go if you need 20knots, and depending on where you plan to use it could be quite easy to design for protected water use.
I appreciate your input, John. I see this position expressed a lot. In researching the subject online, I see varying schools of thought, ranging on one end from 'Hull Speed Limit is Real' to 'Hull Speed Limit is a Myth'. Here are a couple of webpages that do a good job of debunking the Hull Speed Limit:

https://www.dmsonline.us/the-truth-of-h ... eed-limit/

https://www.storerboatplans.com/boat-de ... and-modes/

There are a few key pieces to at least my understanding of this subject. First, the two components of drag on a hull are wave making and surface ('skin') drag. It's true that when vessel speed matches the computed hull speed, the bow and stern waves will combine, leading to increased drag from wave making (wavelength of generated waves match the boats LWL). But say speed were to "somehow" increase somewhat . . . I don't think anyone can say that no amount of additional propulsive power can do that. What would happen to those two waves? They'd start to get back out of phase, and start slightly interfering with each other, reducing their combined contribution to hull drag (at least where they start to interact at the rear).

A second insight into this subject involves the observation that the waves generated by fine hulls are significantly smaller than those of more blunt hulls. Going back to the two components of hull drag (wave making and surface), if the former shrinks significantly compared to the latter, you can arrive at a situation where surface drag dominates the total. This appears to be the case with these narrow-hulled cats. Hull Speed for a 15.5-foot LWL is 6mph, yet you can find numerous videos of these RS16 cats sailing at speeds easily over 20mph. The maker of the converted-to-power RS16 I'm in line for has test numbers showing speeds of 20mph from 15hp motors. They're rated for 20hp, so something like 23mph (20 knots) seems do-able at that power level. And that rating is really just for the outboard motor mount; an inboard configuration wouldn't have such a limit, although going nuts with power is going to lead to reduced runtime based on capacity limits for batteries (unless you only rarely draw on that extra power), and all that extra weight is going to slow things down somewhat.

The position of the RS16 (power version) team is that speed is primarily a linear function of the square root of the power, which jives with the primary source of hull drag being from surface friction. Of course it's somewhat negatively impacted by total weight, as that drives up wetted area. Now the speed-for-power curve may not be a perfect sideways half-parabola shape, because wave-making drag no doubt factors in to *some* extent (especially around computed hull speed), but beyond that first multiple of hull speed, skin friction dominates the total.

Are there monohull boats that can compete with a power cat on a power-to-speed basis? Sure, but they're typically small and light, with a fairly flat bottom. Think Jon Boat. Not something you want to be in when the waves start picking up, and certainly they lack any degree of unsinkability. Sure, you can add foam but then usable area goes down fast. And swamping is a real possibility. Picking a brand of boat that better checks those boxes (Boston Whaler for example), their 16-foot model has a minimum power rating of 75hp. Granted it will go in the 30mph range (at full power), but enough batteries to run a 75hp motor (at full power) for an hour will weigh around 1,200 pounds, cutting your remaining load-carrying capacity to pretty much zero (excluding even the pilot!). Granted you won't typically run at full power, but I decided on that requirement with that in mind, wanting 2-3 hours of 'cruise time' from that one hour of full power. Dropping back to their 13-foot model, 25hp is the minimum power rating, and you'll get mid-20mph out of that at full power (not counting battery weight), but adding 400 pounds of batteries for an hour of full-power running, and ~150 pounds for the motor/rigging, takes 550 pounds off of your 990-pound capacity. I.e. your four-passenger boat is now just a two-passenger boat. Yes I can cut the battery weight by not requiring a full hour at full power. And I've explored various combination of power/runtime extensively. The result of those explorations led me to the current power cat solution.

Cost is the other motivation moving me to lower-power-requirement solutions. Even that 13-foot Boston Whaler example above, as crippled as it would be compared to the power cat solution, would come in at more like twice the cost of what I'm working on. Those larger motors aren't cheap and neither are LiFePO4 batteries.

Keep in mind that the above is all just "my understanding" . . . I could easily be wrong on parts of it. I welcome alternate points of view. It's pretty much why I joined this forum.

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 07 2021 9:28am

I think the power cat design you are looking for is exemplified here:
D848C484-B91F-4230-B675-A7148D1E1022.jpeg
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A very unusual boat. Now retired, it made the 140 mile round trip day excursion from Key West to the Dry Tortugas park. I'm just not sure how far you can "shrink" the concept.

However...let's say you can do it with 15hp. That's still around 12 kW. A 100 Ah cell is .32 kW, so that's 37 cells to get 1 hour. All of this very back of the envelope....but for 2 hours you need 74 cells. I'm just not sure where you put all that on the RS16. I think you might end up with the 13' Whaler issue.

Planing or not, speed costs energy. You will get a lot more utility if you forgo the speed. How about a pontoon boat? Not exactly a blue water design, but nothing in this size is. On that you could have 6 friends and a keg of beer to pass the time.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 07 2021 4:04pm

Thanks for that great example. The boat I'm headed for looks a lot like that one (scaled down):

Image

They sell an electric outboard motor powered version with a 6.5kW motor, and the batteries are in Pelican-style cases under the seats.

Here are my 10kWh-worth of LiFePO4 cells:

Image

Dimensions in this arrangement are 15.5" x 15.5" x 9". They should fit nicely in an SKB case I've been eying, with outside dimensions of 19.5" x 19.7" x 10.9". Depending on where on the deck I determine that they work best for boat speed/attitude (via experimentation), that case could make a good base for a passenger seat. Weight including case is 160 pounds.

Other than some early experimentation with lower-power options, I most likely will start with the ePropulsion Navy 6.0 motor (6kW draw):

Image

They have an improved model this year that moves the motor into the lower unit (direct drive). That's 80 pounds, plus batteries comes to 240 pounds. Then add us two passengers (call that 360 pounds) for a total of 600 pounds. That leaves 390 pounds from the 990-pound starting point, which is coincidently the same number as the 13-foot Whaler. The difference is that this ~8hp of power won't plane the Whaler, but should push this boat to around 13-15mph (still to be confirmed). And that 390 pounds of leftover capacity is easily enough for two more passengers, although adding them would probably cut another couple of miles per hour off the top speed.

That much battery with that motor is enough for 1.5 hours of full-power running (with ~10% left for reserve). I know what you're saying: that's only half the 15hp you need to hit 20mph. True enough. The larger motor option I have in mind is the Stealth E18kW:

Image

20hp at the prop shaft (18kW draw), and doubling that 10kWh battery pack (a second rear passenger seat?) will just meet that one-hour runtime requirement (still with reserve), while easily meeting the 20mph top speed requirement.

But 160 more pounds gets taken off the remaining capacity, knocking it down from 390 to 230. So I've lost close to one passenger. I agree that it's very close. And keep in mind that it was always understood that some give-and-take would need to happen with these requirements.

One nice feature of the Stealth outboard is that you can run it at lower voltages (all the way down to 24V), and you'll just get proportionately less power. So say the full 96V nominal is cut by one quarter to 72V. That will come to 15hp at the prop shaft, and save 75-80 pounds, bumping the leftover capacity back up to 305-310 pounds. That might be enough for two medium-small passengers, and might still get 'close enough' to 20mph. And regarding priority of requirements, the four-passenger requirement is one of the lower-priority ones (notice I said 2-4 persons).

Also, these coaching launches have a 'rescue capacity' of more like 8 persons. I don't know how well that translates into actually cruising with a hundred or so pounds over the capacity limit, but I suspect I'll find out at some point.
Last edited by bobkart on Feb 07 2021 5:10pm, edited 2 times in total.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 07 2021 4:56pm

I forgot a piece of the above calculations: the larger motor adds 20 pounds to the 80 pounds of the smaller motor. So subtract 20 pounds from those 230-pounds-and-later numbers. Not a bad total weight for that larger motor though, considering it nearly doubles the power output of the smaller one.

One thing better about the Stealth outboard approach is that it lets you use off-the-shelf propellers, so I can better match the prop pitch to the boat. Of course that larger lower unit and thru-hub exhaust is a big downside (appendage drag). I'm thinking of ways to add some kind of bullet-shaped trailing part to the prop. At a minimum, removing that trailing-edge flair on the prop hub should be an improvement . . . reversing it (flaring inward instead of outward) would be even better, but harder to see how to do.

All this prop flexibility versus appendage drag and thru-hub exhaust back-and-forth goes away with the inboard approach. But much more thought needs to go into that approach, so I'll most likely 'get off the ground' with an outboard approach. At a minimum that will help me better determine what kind of inboard power level I want to pursue.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 07 2021 5:33pm

BlueSeas wrote:
Feb 07 2021 9:28am
Planing or not, speed costs energy. You will get a lot more utility if you forgo the speed. How about a pontoon boat? Not exactly a blue water design, but nothing in this size is. On that you could have 6 friends and a keg of beer to pass the time.
I'm fine with giving up on some speed . . . range is the real requirement, but in a reasonable amount of time (none of this 4-5mph stuff). The happy balance between speed and range is what this ends up being about.

Because of the non-planing speed curve of these hulls, it's easy enough to back off from full power to (say) half power and retain 70% of the speed. That's a clear win for range, so as long as the total trip duration is tolerable (3-4 hours), slowing down works as a means of extending range. With the Navy 6.0 solution above, full-power range is not much more than 20 miles, but at half power, 30 miles could be achievable. (Battery-life impact also wins.)

Regarding pontoon boats, I definitely considered that. Here they make build-it-yourself pontoon boat kits:

https://www.tinypontoonboats.com/framekits/

That approach was a step along the away to the power cat approach, which I think does everything I need better (I don't really need more than 4-person capacity). And once you get to a large enough one of those pontoon boat kits, costs aren't that much less. Speeds (per power) are definitely less and so is rough-water-handling capability.
Last edited by bobkart on Feb 07 2021 6:58pm, edited 1 time in total.

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 07 2021 6:33pm

Yeah...but what about the keg?

Seriously, no boat I know of does better MPG at a faster speed. Some planing boats have a "plowing" speed, where after finally after stepping up, see a mileage increase. But this is artificial and should be hardly observable on that cat hull design. Best range is always slower.

I know that cat your looking at is different than a Hobie 16, but I raced those in my younger days. While it was safe in conditions I wouldn't take a runabout out into, it was wet. I think you mentioned you are in the PNW? The water is never warm there!

I poked around a bit, and pontoons aren't as efficient as you might hope. So I understand the dilemma.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 07 2021 7:26pm

In earlier versions of that RS16 power cat, I noticed (from pictures) a small gap (maybe 1") between the bottom of the front of the helm area and the deck. I could definitely see spray making its way through that gap onto our shoes, and I pointed it out to the RS people. In later versions I see they have addressed the gap. There's a windshield that gets close to the top of the passengers' head (about up to eye level), so I feel like that will go a long towards mitigating incoming spray from the bows. And I feel like ducking from time to time is an option. I may also consider lowering the seats a bit.

On seaworthiness, the model from which this power version is derived has a C rating (Force 6 wind, 2m waves). When they rework it they lower that rating to D (Force 4 wind, 0.3m waves). I feel like the reality of it is somewhere between those two (it's a pretty big difference). Obviously I'll adjust my thinking once I get some hours in, but my current impression that it can handle 1m waves. I'm less clear on wind, but Force 5 seems reasonable. Others out there are likely much better at making these judgements than I am.

We *once* get into stuff 'too big' in the 10-foot inflatable (near Naval Station Everett), and quickly adjusted our course to calmer waters. I think it was around 1.5m swells. I know the long-wavelength stuff isn't as challenging as the chop. You can see that in this video, at the end of the run along the coast of Jetty Island, before going for a nice tour of the Everett Marina, which I find particularly entertaining due to the numerous walkways we were able to pass under:

https://www.youtube.com/watch?v=ZpzbtWfmwBU

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Re: Getting Into Electric Power Catamarans

Post by John in CR » Feb 07 2021 8:15pm

bobkart wrote:
Feb 06 2021 4:43pm
John in CR wrote:
Feb 06 2021 12:55pm
Before deciding anything about the electric powerplant you've got to get your hull design down. For 20mph, which is 17.4 knots, you'll need a planing hull design, which is more difficult to achieve with a catamaran. Planing hull cats are more typically for achieving greater stability while fishing and cutting through moderate size waves more smoothly being up on 2 fine hulls. 17.4 knots isn't terribly difficult to achieve with a displacement cat, but will require a 25-30ft waterline length depending on the length-to-width ratio of the hulls.

A planing monohull is really the way to go if you need 20knots, and depending on where you plan to use it could be quite easy to design for protected water use.
I appreciate your input, John. I see this position expressed a lot. In researching the subject online, I see varying schools of thought, ranging on one end from 'Hull Speed Limit is Real' to 'Hull Speed Limit is a Myth'. Here are a couple of webpages that do a good job of debunking the Hull Speed Limit:
Hull speed is definitely not a myth, but calling it a hard limit is incorrect, it just requires significant increases in power to exceed it. For a monohull displacement hull the hull speed in knots is 1.35 X the sqrt of the waterline length in ft, but a fine hulled cat can exceed 3x the sqrt of waterline, which is why that big transport cat above can be so fast, and with such fine hulls at and above the waterline I'd bet it has larger submerged torpedo shapes at the bottoms of those hulls for more load capacity but make it smooth through the waves, something beyond what you'd want to do on a 16ft cat.

I'm still unclear on your intended use other than your mention of 16ft, so I'd assume 2 sheets of plywood long. I see talk above about motors, power and range, and again I think that's a mistake to even think about until you've settled on a hull design. As far as that goes it really depends how far down the rabbit hole you want to go. For me exploring rabbit holes is part of the fun, and the rest is made easy for me, because I have a nice stockpile of batteries and high efficiency hubmotors, making a dual inboard solution my preference for a cat, which I've researched pretty extensively for my plan to build a 30ft cat based primarily on a sailing hull design but capable of offshore charter fishing. With a 1800kg displacement the cat designer I worked with predicts just a 1500W power requirement for 7knots, the high end of trolling speeds for bill fishing.

For a peek into the rabbit hole, check out the simple to build 200lb 16ft cat dubbed the Tesla Catamaran by the youtube scientist Tech Ingredients at https://www.youtube.com/watch?v=6BMskpsLiYA. Looking at his video of the boat in use I think he made 2 mistakes in the hull design. One the flat bottom gives the front an entry to the water that isn't fine enough (his bow wave tells me this), and 2 while the extra plywood at the rear on the bottom keeps it super smooth through the waves, the flat angle of attack makes the bow dig and unlikely to plane.


For your needs I would make several changes, and use his method to set the hull width dimensions and ease of construction:
1. I'd scale it up to 20ft length by scarfing in an extra half sheet length of plywood, scaling up the width at the same time, and move the initial spreader board slightly forward due to number 2 to maintain very close to the same foil dimentions.
2. I'd cut a slight curve at the bottom front of the side panels to give the bow a finer entry.
3. I'd add a triangular bottom to the hulls, but not all the way to the edges, with the idea being to give them a nice hard chine to help it potentially plane, or at least assist in the lift from number 4. I'd put a length of aramid tape (kevlar) along the peak of the triangle and at the bow to really toughen up the bow and bottom for beaching and prevent damage from debris at or just under the surface.
4. This would be the full rabbit hole part...I'd build the hulls with appropriate reinforcement at stations near the front and rear for 2 smallish short foils at the bottom of each hull to give it several hundred pounds of lift at 10 knots or so for greater efficiency and help it get on plane.

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Re: Getting Into Electric Power Catamarans

Post by BlueSeas » Feb 07 2021 8:51pm

John in CR wrote:
Feb 07 2021 8:15pm
[
Hull speed is definitely not a myth, but calling it a hard limit is incorrect, it just requires significant increases in power to exceed it.
I'm not a naval architect...but I owned a power cat built on this hull (Twin Vee bought the mold from Ocean Cat, who made this as a cuddy cabin).

Take a look at the stats on this chart:
9BFA92BB-8844-4DD5-9F86-DB740E304BAC.jpeg
9BFA92BB-8844-4DD5-9F86-DB740E304BAC.jpeg (241.05 KiB) Viewed 1375 times
It really doesn't have the "normal" no go zone before it "planes". At the helm, it feels simply like "more power" more speed. The bow never rises or mushes. Interestingly, it does about 18 knots on one engine, with the second one tilted up and out of the water. Mine had 225hp 2 strokes, so less HP than the example above.

I bought mine from the naval architect who developed the hull. He said it doesn't really plane. The waterline once above about 5-6 knots, remains constant. Some of that is the tunnel. It's really shallow, and at those speeds, fills at the transom. So maybe it does plane on the tunnel? Who knows.

It was the perfect boat, except it "sneezes", that is a slight mist pops up forward and between the hulls. Had to constantly clean the windshield to see clearly.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 07 2021 10:48pm

John in CR wrote:
Feb 07 2021 8:15pm
For a monohull displacement hull the hull speed in knots is 1.35 X the sqrt of the waterline length in ft, but a fine hulled cat can exceed 3x the sqrt of waterline, ...
Yes, I had seen that 3.x factor brought up in other articles I had read on the subject, but wasn't able to find it for inclusion in my response to you above.
John in CR wrote:
Feb 07 2021 8:15pm
I'm still unclear on your intended use other than your mention of 16ft, so I'd assume 2 sheets of plywood long. I see talk above about motors, power and range, and again I think that's a mistake to even think about until you've settled on a hull design. As far as that goes it really depends how far down the rabbit hole you want to go. For me exploring rabbit holes is part of the fun, ...
Intended use is just cruising the local waterways (rivers, lakes, Puget Sound), exploring and sightseeing mostly. I seem to have led you to believe I'm wanting to build the hulls myself. I'll go as far as purchasing *just* hulls and build from there up (as I'm doing with the 10-foot model mentioned in my first post). For the 16-foot cat, as mentioned in the first post, there is a model out there (RS16) that I've identified as meeting my needs. Rowing Solutions makes a power version of it, and I'm in line to buy one in a month or two, when the shipment arrives in the USA from the UK. And yes, the 'rabbit hole' exploration is a big part of the fun, and I've been on that journey for the better part of a year now.
John in CR wrote:
Feb 07 2021 8:15pm
For a peek into the rabbit hole, check out the simple to build 200lb 16ft cat dubbed the Tesla Catamaran by the youtube scientist Tech Ingredients at https://www.youtube.com/watch?v=6BMskpsLiYA.
Great video; I just finished watching all of it. Thanks for passing that on. He showed a couple of ideas that I had already come up with for the 10-foot cat, related to modularity and ease of assembly and disassembly. And separating the trolling motor heads from the shaft and motor was definitely already on my list of things to try. I had seen mention of that RC airplane propeller idea as a way to get around trolling motor pitch limits, but his pointing out a specific model was very helpful.

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Re: Getting Into Electric Power Catamarans

Post by bobkart » Feb 09 2021 10:17am

Something like this might work well for the 10-foot cat (which should arrive today):

https://ecoboats.com.au/products/electr ... drive-2-0/

I see what looks like a thrust washer in the forward part (right of picture) of the sample installation. But I think it only handles forward thrust:

Image

I also see an interesting use of what looks like a hatchback strut as part of the motor mount (could just be a damper).

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Re: Getting Into Electric Power Catamarans

Post by Voltron » Feb 09 2021 1:38pm

It looks like the strut is being used as a hydraulic adjustable belt tensioner.

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