electric boat, proper rpm and propeller

[Rick Willoughby]

Displacement?

Is the transom submerged at 10 knots?

What area have you got to mount solar panels?

Rick W

[8430017]

displacement=2.65ton
yes
area=13.9875 m^2
thank you for helping

[Rick Willoughby]

I have done the attached sketch of the boat you describe. You will have to advise if I am close to the mark.

The boat drawn will require 17kW to do 10knots if the propulsion can achieve an efficiency of 75%.

Good solar panels can produce up to 160W/sq.m in strong sunlight directly overhead. So the required area of panels is:
Area = 17000/160 = 106sq.m

With your available 13sq.m you could produce 2080W. This will achieve a speed of 3.5kts for the boat I have drawn.

The transom drag is significant. If I have shown it deeper than it is in your boat then this should be modified to get a more accurate calculation of performance. A photograph of the boat would help.

Rick W

[Defender130]

Hi,
I have been reading with great interest this topic on propeller design and electric outboard as I am looking at converting my aluminium dingy to electric propolusion.
I live in Perth, Western Australia so availabilty to your full range of products may be limited.
I currently have a 12ft aluminium dinghy with a 9hp Mariner(mercury) engine.
Can anyone suggest a suitable electric outboard?
Kind Regards.
George.

[Rick Willoughby]

Originally Posted by Defender130

Hi,
I have been reading with great interest this topic on propeller design and electric outboard as I am looking at converting my aluminium dingy to electric propolusion.
I live in Perth, Western Australia so availabilty to your full range of products may be limited.
I currently have a 12ft aluminium dinghy with a 9hp Mariner(mercury) engine.
Can anyone suggest a suitable electric outboard?
Kind Regards.
George.

George
You really need a good reason to spend the money to replace a petrol outboard on a small dinghy intended for planing.

The only commercially available electric outboards are rated around 4HP. Something like these:
http://www.electricmotorsport.com/st...ard_motors.php

There are some others but most are more expensive. The ones shown are USD prices as well.

So you can get an outboard of considerably less power than the existing Mercury. The next step is the batteries. Lead are heavy. For a 4HP outboard you would ideally need about 240kg (three adults of weight) of batteries and they will set you back AUD3000. By the time you do this the boat will be way to heavy to plane. It will be like a barge.

For AUD3000 you could get lithium batteries that would weigh much less and enable full power for about 50 minutes. They might get you on the plane one-up but it would be borderline. You might need a special prop.

I expect by the time you have spent AUD6000 your little tiny will be overcapitalised. Its performance will also be considerably reduced. It will be a little quieter though.

If you are doing it simply to reduce noise with the intention of just mooching along then a Minn Kota or Torqueedo might be worth consideration:
http://www.minnkotamotors.com/
http://www.torqeedo.com/en/hn/home.html

Also be careful with things like static thrust on electric outboards compared with combustion engines. It has no meaning to moving a boat through the water. It only has meaning if you want to tie the boat to a pier and measure the force you can produce. I have no idea of the value in doing this exercise. None the less it seems to be something that helps sell electric outboards. It seems the makers get close to fisticuffs over this nonsense figure accusing each other of exaggerated claims.

You will find most of the developments with electric powered boats are in hull design where the designer starts with efficiency in mind at the outset. You can get impressive results from electric motors matched to an efficient hull and prop.

Rick W

[Defender130]

Thanks for your valuable reply Rick.
Kind Regards.
George.