Weed shedding props

Gentlemen

New to this forum, but not new to boats, in fact I have had more boats in the last thirty years than I care to admit to, I'm sure there is a cure for this illness but I'm too frightened to speak with my GP. Background, retired Airline pilot and Precision engineer, interest in water craft started in the 60's with small hovercraft and slowed down to displacement mode in the early 80's. I have been developing Stirling cycle engines over a similar period with little success until recently when I completed a 100watt engine which I fitted in a motorised 5mtr catamaran. The performance is just about tolerable when everything is going well but this time of year with the weed well developed, any trace on the propellor kills what little performace there is available.

Bright idea! I have two 125 Ahr batteries, in my Dutch barge, which sit there slowly discharging as the Barge rarely goes out, will buy a dc motor and a PWM controller and replace the SE in the catamaran. Wonderfull, power at the turn of a knob, astern at the flick of a switch, almost perfect but for the bl***y weed. Fortunately I came across a excellent thread on another forum with a wealth of knowledge on 'efficient electric boats', you will all be aware of this but for me it made hours of enjoyable reading. I noticed in this thread a folding propellor design which looked as though it would shed weed, so I spent last weekend making a version of this idea and hope to try it later this week when balancing is complete.

The only other aspect of my electric motor conversion, which is not yet to my likeing, is the noise from the PWM controller. It is an inexpensive, Thailand produced box of tricks, with the facility to vary the frequency (still trying to get my brain cell around this aspect of the controller) and some reducton in 'hum' can be achieved with the frequency set at its lowest setting. Is this due to the purchase price, typical of PWM controllers or am I doing something wrong?

Your advice would be most welcome.

Geoff Vaizey

p.s. I tried to attache some pictures of the folding prop but 'file size too big'! will try again soon.

Pictures of prop and cat.

Looks nice, Geoff, kind of familiar looking.

I guess the other forum may have been "Boat Design", with the thread about my first electric boat, with this prop that I designed and made for it:



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Hello Jeremy

I was hoping you would reply as you are indeed the source of my inspiration.

Regarding PWM controller noise, can you shed any light on the matter, electricary is not my strong suite?

I get the impression from your enormous contribution to these forums that I would be well advised to persue 3phase 'outrunner motors' for my next project. I have some experience with them as I used a modified version as a starter generator in an experimental vee twin thermo lag engine to explore the effects of pressurisation on their performance. The biggest nuisance was the need to use a servo tester to control the motor, do you know of a more elegant solution to give ahead and astern plus speed control?

Geoff

Throw an inductor in series with your motor, and a big electrolytic cap after the inductor. The noise will go away, and the penalty will be a bit of power lost in inductor heating, and a tiny delay in throttle changes before the motor reacts (which likely won't be detectable).

Welcome to ES. Jeremy rocks, and I like your craftsmanship on the prop.

I know the velocity is very low in the center of the prop, but doesn't having the flats near the pivot getting dragged through the water cost a substantial penalty in efficiency?

Outrunners are certainly nicer to use for this application, in my view. I've built a few outrunner based boat drives now, the latest being a conversion of a drive I made for a cordless tool powered boat race. Here's the original drive with the cordless drill driving it for the race:



After the race I decided to convert this drive leg to become a general purpose electric outboard, using an outrunner and belt reduction drive, with the controller fitted inside the case. It has about 3/4 hp of power, enough to drive a small boat at a fair clip with the efficiency of the prop. Here's the almost completed outrunner drive leg:



And here's a shot with the cover off, showing the belt drive and controller:

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Luke, there isn't much in the way of flats in the centre of my prop, as the root of the blade is thickened and shaped to a very steep angle of attack foil, albeit a rather inefficient one that's rather thick, to ensure the blade is stiff enough. This doesn't show too clearly in those photos, though. Here's a slightly better picture:

Jeremy

I've been following the CCC, from a distance, very enjoyable, I also noticed your drive leg with both drill and out-runner, some where on the net, very tidy. I'm currently using a long carbon tube propshaft, selected for the Stirling Engine installation to minimise losses, but your use of Hooks joints has set me thinking. I find it interesting that both electric and SE installations require similar disciplines, the SE has little power but no range issues where as the electric motor has adequate power but limited range (without PV panels or similar).

Luke?

Thanks for your thoughts on silencing my controller, will give it a try. Regarding the hub on 'weed shedding' folding props, there is no doubt that the hub configuration is less than ideal but I am hoping any loss of efficiency will be considerable less than when fouled by weed, for the rest of the year when weed is not a problem, I may well return to a fixed prop.

I thank you both for your advise.

Geoff V

Having realised that there is little point in having a weed shedding prop when the propshaft, skeg and rudder of my original installation were collecting weed almost as fast as the prop, I decided to follow in Jeremy H's foot prints and start with a 'clean sheet of paper'. It seemed sensible to dispense with the shaft, skeg and rudder altogether and build a unit whereby the various features have at least one function, and also, if possible, arrange it so that it can be easily transfered to other hulls, the results of my endevours are shown below.

The motor is a HK special, Turnigy I think, the large 80mm diam model, can't be much more specific as there was no documentaion whatsoever. It appears to be the same model as that which 'Thud' has rewound in an associated thread, I may try to follow in his foot steps when I,ve summoned up enough courage, but for now it is as supplied. The belt is a 5mm pitch HTD driving through a double Hooks joint, with a ratio of 3:1 and a motor wound for Kv130 (delta), may be a little fast, the first trials (Sunday weather permitting) will determine if I need to rewire it in star configuration. The whole assembly is mounted on an old Johnson outboard transom clamp so I can transfer it to other hulls quickly and which also permits steering (hence the enlarged fairing) and the ability to 'kick up' if I am stupid enough to drift into shallow water. The controller incorporates a Nasa Marine BM1 battery monitor feeding an RC ESC and reverse is achieved by switching two phases using a pair of relays. Bench trials show a power consumption of 23watts for the motor alone at 2000rpm and a further 14watts with the transmission connected and running at the same speed.

My thanks to Jeremy H and Rick W for their ideas.

Geoff V

Very neat, Geoff. Hope it works as well as mine - I'm really happy with the way the double universal joint drive works, much easier to DIY than a bevel gearbox, and quieter and cheaper too.

Geoff, Jeremy,
I assume those UJ's have roller races on all the link pins ( like the larger versions) or are they simple bush bearings. ?
I am imagining high maintenance requirements if they are bushes.

G'day.

The UJ's I'm using are of the plain bearing variety, they are available with needle roller bearings but as the leg is filled with ATF I suspect the life expectancy will be adequate, if one tried to wear out such an arrangement I think it would take a very long time at 300 watts and 550 rpm, I just hope I'm not being too optimistic and your concerns are unfounded.

For general information, I tested the Leg yesterday for several hours during which time I logged a lot of data which I then compared with previous trials of the original layout of shaft, skeg and rudder. My best estimates are an improvement in overall efficieny of 10-12% at cruising speed (4Kts), which now gives me about 8 hours endurance from the two 125Ah AGM batteries, increasing to 20 hours at 3Kts. The folding prop readily sheds weed and the inclined leg resists collecting weed well, also the prop now works acceptably well in reverse as I can now spin it a little faster than the original installation. All in all a G'day out.

Thanks for your comments.

Geoff V

Very nice catamaran you have Geoff. And as mentioned previously the finishing on your prop looks vey good. I'm on the planning stage for something like this myself, but with a couple of little ones may spare time is easily full. So I often end up reading on forums when I have to, no cleanup in the workshop when I'm finished then, and I've read many of Jeremys contributions on boatdesign.net as well.

I've read a couple of books on sterling engines and I think a 100w engine is quite an achievement. Please post a couple of pictures of your setup. The alibi could be for converting it to a sterling-electric propulsion system for it to be covered by this forum:-D

Definitely a lot of inspiration in these threads.

Regards

Permagne

Thank you for you kind comments, I'm sure the powers that be will not be too upset by my including information about my Stirling Cycle engine, after all they have the potential to be Solar heated. I am currently fitting some upgrades to the engine which dispite its current low output seems quieter than the outrunner motor that I'm using and maybe I can reach the point where the output is enough to propel my catamaran. Regardless of my success or failure these projects keep me off the streets and sometimes away from my Local!

The link below is one of my videos on youtube, I hope it is of interest.

http://youtu.be/TUFUR8lav6o

Geoff V