Info on math of electric boat?

[Jdege]

I'm looking at a very nice old boat at a very good price today would need a new engine.

So I'm musing about electric ad a alternative. Admittedly, an electric motor would have limitations, but would they be limitations I could work around? Or limitations that would be show stoppers?

It comes down to the math.

How much power does it take to push a boat at a given speed? How much motor to generate that power? How many batteries to power that motor for how long? How much charging capacity to charge them?

I don't need precise numbers to judge feasibility, but I do need rough estimates.

Does anyone know of a source that explains how to make such estimates?

[RonRicco]

Do a search from the menu page. There are a quite a few threads with a lot of data and “varying” opinions.

[cj88]

I recently saw this article about an electric conversion on an old yacht. They have a website and YouTube channel. Perhaps you could contact them and ask a few questions.

https://www.sail-worldcruising.com/n...lectric-vessel

[Chardonnay]

Annapolis Hybrid Marine, Electric Propulsion for Sail & Power from AHM will run the numbers on your boat and give you an estimate.

[SteveSails]

What kind of boat? Sail? Power? How big and how heavy? What do you want to do with it? Daysail? Just motor out the harbor? Or run for hours on end? What speed do you expect to make? Recharge on shore power or wind and solar? What size internal combustion engine does it have now?

As for math 1HP = 746Watts or 0.746KW, Watts = Volts x Amps. Vendors of electric propulsion units will do the math for you.

You will be told that you can go around the world or you won't be able to do much more than leave the dock. Both can be true.

[Dave_S]

Someone might correct me but from my understanding..

If you have 0.5kw of solar and you use 4kw to drive your electric motor and your electric gathering / storing / distribution may be 80% efficient which provides you 0.4kws for 5 hours effective sun per day, you need 2 days to produce 1 hr on the motor providing they are sunny days and the sails don't shadow the solar panels.

You will need to be able to store that power with enough batteries, probably 8 x 6v lead acid. EDIT: That's not enough for lead acid you may need 12 off for 1 hr at 4kw.

Then you need to add to this the house bank power requirements.


For some this is fine they run the motor for a few minutes getting on and off the anchor or in and out of the marina for those it seems a great alternative.

Some run large generators but that doesn't make much sense if that is a common occurrence.

[Jdege]

Those are exactly the sort of numbers is need to run.

Some are pretty generic. But some are not.

Any rules of thumb on how much it takes to push a hull of a certain shape tho a given speed?

[Adelie]

A 1hp gas motor provides approximately 25lb thrust.

The trolling motor folks indicate. current draw of 1amp/lb at 12v.

By that measurement 250w electric is about equivalent to 1hp gas motor. This is apples to oranges comparison so I’m going to ignore the apparent conflict for the moment.

As a general approximation you can use the following chart to figure out what thrust you need for a given size boat at a given displacement to get a specific speed.



What’s your waterline length and lightship displacement?

Or more generally what boat are you considering?

[Jdege]

I'm thinking about a great many boats.

But the one that needs a new motor is an Allied Chance 30/30.

Even if the motor worked, it's a gas engine that iI'd want to replace.

[Adelie]

What’s wrong with gas motors?

[Adelie]

OK so 25’ LWL which gives a 5kt speed at 1.0 speed/length ratio.
11,200lb lightship displacement. Let’s use 14,000, that’s 6.25 long tons.

From the chart 1.0 speed to length ratio requires 15lb thrust per long ton.
6.25lt X 15lb/lt = 93lb.

Let’s be conservative and say 20lb/1hp so you will need about a 5hp motor to hit 5kt in flat water.

Checking things with my boat but haven’t got all the data together so I don’t know how good the above chart is.

Drop to 3kt and you need about 30lb thrust or 1-1.5hp.

I found about recently that you can get an electric go kart motor and controller a lot cheaper than a marine version.

[Jdege]

What's wrong with gas?

This one doesn't work. Owner says it needs new points or an electronic ignition system installed.

More generally, I'd not expect a fifty year old gas engine to work for very long.

Diesels last longer. And diesel is safer on a boat than gasoline.

But mostly, the gas engine on this boat doesn't run.

[44'cruisingcat]

New points would likely cost less than $50.

[Adelie]

Originally Posted by Jdege

What's wrong with gas?

This one doesn't work. Owner says it needs new points or an electronic ignition system installed.

More generally, I'd not expect a fifty year old gas engine to work for very long.

Diesels last longer. And diesel is safer on a boat than gasoline.

But mostly, the gas engine on this boat doesn't run.

Diesels in general last longer as you say. On the other hand the gas engine is probably an Atomic4 for which there is a HUGE amount of support. Even 40yr after they stopped making them about half are still running.

Gas engines are much happier running under partial load for long periods whereas diesels don’t like that at all.

Diesel is marginally safer. That doesn’t make gas dangerous, just less safe.

Of larger import to me would be fuel economy and then only if I was going offshore. Coastal or local sailing I wouldn’t bother converting.

If it is an Atomic4 then Moyer Marine is going to have what you need to repair. If it’s points, those are $23 for a late model plus tax and shipping.
https://moyermarine.com/

Whatever. If you still want to convert, putting a little money to repair the existing will give you breathing room to figure out what you want to replace it with. May also be worth a little as it goes which will help defray the costs of the replacement.

Find out what kind of gas motor it has.

[jackheape]

I would stick with the gas engine if it is only a matter of some minor repairs. An electric conversion would cost you quite a bit. I was recently considering replacing my diesel (I was having to pull it to repair an oil leak and was just curious since I had to pull it out what my options for replacement would be) and looked at a new diesel, gas, or electric. Gas won hands down, both for cost and utility. Diesels IMO are way overrated for use on small sailboats.