Horsepower needed?

CobraJet · Dec 10, 2019

This past year I bought a project house on a private lake. This place was really tore up but I'm pretty well finished with house chores and looking to play around a bit now. This lake only allows electric motors. I'd love to put together an electric outboard for next summer!

Lately I've had my eye on a 14' boat that looks like the one in the picture (20') but currently has no engine. I'm thinking I don't want anything fast. It just needs to look cool and go 5-10MPH max.

Any recomendations on how much power I'll need?

Hillhater · Dec 10, 2019

Power will depend directly on the hull design, shape, weight of the complete boat and passenger load, prop size /pitch etc.
But for 5-10 mph, i would be surprised if it needs more than 8-10 kW.
Boat speed /power calculator.. https://www.go-fast.com/boat_speed_predictions.htm

CobraJet · Dec 18, 2019

Hillhater said:
Power will depend directly on the hull design, shape, weight of the complete boat and passenger load, prop size /pitch etc.
But for 5-10 mph, i would be surprised if it needs more than 8-10 kW.
Boat speed /power calculator.. https://www.go-fast.com/boat_speed_predictions.htm

I fully realize that the statement you made about hull shape and weight effect the performance just as you say. Thanks for your thoughts. However, when I plug my parameters of my old boats into the boat speed calculator that you mention, it gives me a WAY Too exagerated optimistic speed. I even tried using many more weights and horsepower ratings, it's over stating the actual stats that I have seen on the boats that I have owned in the past.

Any other links or ideas? I'd like to try just about anything.

Hillhater · Dec 18, 2019

That calculator is for power needed for different hull shapes and probably assumes an ideal prop specification (dia , pitch, rpm, etc)
Unless the prop is correctly matched to the target speed, then all the power i the world wont help.
Even small changes to the prop can make huge changes in speed and power needed.
Prop selection is a completely separate issue.

dodjob · Dec 19, 2019

CobraJet said:
Hillhater said:

Power will depend directly on the hull design, shape, weight of the complete boat and passenger load, prop size /pitch etc.
But for 5-10 mph, i would be surprised if it needs more than 8-10 kW.
Boat speed /power calculator.. https://www.go-fast.com/boat_speed_predictions.htm

Click to expand...

I fully realize that the statement you made about hull shape and weight effect the performance just as you say. Thanks for your thoughts. However, when I plug my parameters of my old boats into the boat speed calculator that you mention, it gives me a WAY Too exagerated optimistic speed. I even tried using many more weights and horsepower ratings, it's over stating the actual stats that I have seen on the boats that I have owned in the past.

Any other links or ideas? I'd like to try just about anything.

Take a look at this calculator. If you don't plan to plan, this will give you a pretty solid estimation. Also beware that most of calculator are talking about propulsive power, the propeller design (take a look at javaprop) will show you the motor power and the unfortunately huge losses on propellers

Hope that helps!
Hadrien

CobraJet · Dec 19, 2019

dodjob said:
.....
Take a look at this calculator. If you don't plan to plan, this will give you a pretty solid estimation. Also beware that most of calculator are talking about propulsive power, the propeller design (take a look at javaprop) will show you the motor power and the unfortunately huge losses on propellers
Hope that helps!
Hadrien

I did do a few scenarios using the calculator above. The ones I tried were all way off in speed estimates! I base this on a couple of boats that I previously owned and a gasoline engine that I had.

For instance:

small rowboat
5 hp engine
entire boat weight including myself = 600lbs.
calculated speed using heavy V-bottom = 21 MPH

small outboard boat
5 hp engine
entire boat weight including myself = 1400lbs.
calculated speed using heavy V-bottom = 13 MPH

Perhaps these results were skewed by using too small of an input as both of these boats were fairly small. There is no way either boat achieved even 1/2 the calculated speed. I tried them both in a real life scenario!

There has got to be a better calculator out there. Anyone have a link?

DogDipstick · Dec 19, 2019

... No, I dont know of any online calculator, but...

I know a boat engineer with a calculator. I'll ask him for you, Ive been thinking about this style of a setup too:.....

He can tell us what you need to know. We are trying to build and power a small boat too at the moment.

What is the length and beam of your boat again (need to know displacement I think.. ) ? Are you open to the idea of Surface-Piercing Edrives?

CobraJet said:
dodjob said:

.....
Take a look at this calculator. If you don't plan to plan, this will give you a pretty solid estimation. Also beware that most of calculator are talking about propulsive power, the propeller design (take a look at javaprop) will show you the motor power and the unfortunately huge losses on propellers
Hope that helps!
Hadrien

Click to expand...

I did do a few scenarios using the calculator above. The ones I tried were all way off in speed estimates! I base this on a couple of boats that I previously owned and a gasoline engine that I had.

For instance:

small rowboat
5 hp engine
entire boat weight including myself = 600lbs.
calculated speed using heavy V-bottom = 21 MPH

small outboard boat
5 hp engine
entire boat weight including myself = 1400lbs.
calculated speed using heavy V-bottom = 13 MPH

Perhaps these results were skewed by using too small of an input as both of these boats were fairly small. There is no way either boat achieved even 1/2 the calculated speed. I tried them both in a real life scenario!

There has got to be a better calculator out there. Anyone have a link?

CobraJet · Dec 19, 2019

DogDipstick said:
... No, I dont know of any online calculator, but...

I know a boat engineer with a calculator. I'll ask him for you, Ive been thinking about this style of a setup too:..... He can tell us what you need to know. We are trying to build and power a small boat too at the moment.

What is the length and beam of your boat again (need to know displacement I think.. ) ? Are you open to the idea of Surface-Piercing Edrives?

Oh that would be really cool, thanks so much!

Width of the hull at the transom is 5',2", length of the hull is 14',2"

It has a shallow V and I roughly estimate the fiberglass hull weight at 1000 lbs.

Here are a couple of pics I took today.

dodjob · Dec 19, 2019

Tried the info you gave in the mentioned calculator, and didnt come at all at the same results

CobraJet said:
dodjob said:

.....
Take a look at this calculator. If you don't plan to plan, this will give you a pretty solid estimation. Also beware that most of calculator are talking about propulsive power, the propeller design (take a look at javaprop) will show you the motor power and the unfortunately huge losses on propellers
Hope that helps!
Hadrien

Click to expand...

I did do a few scenarios using the calculator above. The ones I tried were all way off in speed estimates! I base this on a couple of boats that I previously owned and a gasoline engine that I had.

For instance:

small rowboat
5 hp engine
entire boat weight including myself = 600lbs.
calculated speed using heavy V-bottom = 21 MPH

small outboard boat
5 hp engine
entire boat weight including myself = 1400lbs.
calculated speed using heavy V-bottom = 13 MPH

Perhaps these results were skewed by using too small of an input as both of these boats were fairly small. There is no way either boat achieved even 1/2 the calculated speed. I tried them both in a real life scenario!

There has got to be a better calculator out there. Anyone have a link?

CobraJet · Dec 20, 2019

Wow I like those numbers you're getting! They do seem more in line with what I was kind of expecting. What calculator are you using? The calc I used was the one mentioned and doesn't look anything like what you posted. This is what I get using the posted calc.

dodjob · Dec 20, 2019

Sorry mate, I thought I gave you the link already, epic fail

There you go:

http://www.psychosnail.com/boatspeedcalculator.aspx

It's pretty straight forward, but give very nice ideas for displacement and semi displacement hulls

The guy also made some real life comparison, which is great to have an order of magnitude. And if it helped, give him a beer, seems to be a pretty cool lad

Grüß from Germany!
Hadrien

CobraJet said:
Wow I like those numbers you're getting! They do seem more in line with what I was kind of expecting. What calculator are you using? The calc I used was the one mentioned and doesn't look anything like what you posted. This is what I get using the posted calc.

Horsepower needed?

CobraJet

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