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Old 09-02-2019, 18:33   #31
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Re: Info on math of electric boat?

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Originally Posted by roverhi View Post
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With the help of Moyer Marine you can keep an A4 going practically forever. They use significantly more fuel and will take more routine maintenance than a diesel but they are way quieter and smoother than any diesel I've owned. Not unusual to not even be aware the A4 is running in a well done installation.
Agree totally as to smoothness and noise. Case in point, my baby sister has a Catalina 30, as do I. Her's is several years newer than my '78 vinatage. A couple of years ago she came down do with a friend to join my on a local rally (SoCal TaTa). Several times during the week, while in the cockpit, she didn't realize the A4 was already running. Granted, down in the cabin nobody would make that mistake, but in the cockpit, at idle, an A4 is quiet enough to not hear, especially underway. And I don't know about others, but I personally think the stink of diesel is worse than gasoline.

Never really nailed down my fuel consumption, but while I'm certain that it's higher than an equivalent diesel, it's probably not over a gallon an hour at cruise.
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Old 09-02-2019, 18:56   #32
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Re: Info on math of electric boat?

My daughter sails a boat with no engine at all.

She sails it out of the marina, and back in again. When there is little wind, she goes slow. When there is no wind, she waits.

When you have no engine, you learn to sail your boat really well. Rather than it being a problem, it is really a blessing. Coming and going from the dock is usually very slow, quiet, and under control.

I suggest you go with your gut, and put in a small electric motor and a pile of whatever batteries you can afford. Then go sailing. We tend to forget that these are sailboats, first and foremost.

And if you want to save some dollars, put a pair of high thrust trolling motors on the transom. They usually have super long shafts. Add batteries in series for higher voltage.

I used to sail a C&C25 with an outboard on the back. One time the outboard had some problem, so I took it off the boat and up to the outboard engine place who told me 3 weeks. Our season here is short, so I just went sailing without it. I wasn't going far, just cruising my home area. It actually was really nice...quiet, clean, no exhaust, no gasoline aboard.

IMHO, the engine is for harbors and when there is no wind at all. IMHO its just way more fun that way!
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Old 10-02-2019, 07:08   #33
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Re: Info on math of electric boat?

Adding a bit of seamanship to a bit of math:
What is the usual power fitted to your type of boat: Say 30 HP.
30 HP is 22KWatts.
If you do any serious cruising you will require the following minimum power X autonomy:
Ful power x 10 minutes to pull free of grounding
Full power x 3 hours in a grunt, such as heading for shelter against rising sea and wind.
1/4 or 1/2 power x 24 hours for a crossing in light wind.

Convert this in KWatts from solar pannels (acres of these as 1sq meter of Solar Panel generates only 100 W, 8 hours per day at best) and ampxhours ( ie battery size... and weight) and the inescapable conclusion is that no electric arrangement can meet these requirements wether now nor in the foreseable future.
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Old 10-02-2019, 12:00   #34
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Re: Info on math of electric boat?

Hi,
I don't know if this will help, but here's my experience with two Torqeedo 4.0 (4kw) and two 5kWh batteries (48-5000). I have a MaineCat 30 (6,000# displacement); it had been powered by two 8hp Yamahas.
Overall, the 4.0's are about equivalent to (perhaps slightly less than) the 8hp Yamahas in terms of max speed.

[All numbers approximate (+/- 0.5knots), relatively calm sea state]
At max throttle:
knots range (nm) @ 80% battery
1 engine, 1 battery: 5.2 5.2 (double if both engines run sequentially)

2 engines, 2 batteries: 6.5 6.5

Theoretically, halving the speed increases the range by four. E.g.:
1 engine, 1 battery 2.6 20.8
2 engines, 2 batteries 3.25 26
Empirically, the theory seems to hold true, perhaps even better range.

Good luck,
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Old 10-02-2019, 12:12   #35
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Re: Info on math of electric boat?

The calculations on the following link are generally power-boat related, but still can be quite useful when sanity-checking your repower plans: http://www.westlawn.edu/news/Westlaw...d06_June08.pdf
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Old 10-02-2019, 15:28   #36
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Re: Info on math of electric boat?

Thanks
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Old 15-02-2019, 07:22   #37
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Re: Info on math of electric boat?

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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?
Does the boat you are thinking about buying have direct drive(a shaft)? If so, include in your thinking a parallel hybrid. A good example of a parallel hybrid set up is at the link below.

The STEYR HDS - Hybrid Drive System - Steyr Motors
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Old 15-02-2019, 09:38   #38
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Re: Info on math of electric boat?

In my opinion LOL
you only have the power available from the amount of solar you have available and in propulsion terms that is literally nothing!
Yes you have batteries, but if your charging them with Solar and your travelling and not on shore power every night? Then your left with nothing.
Noting on the link they removed 40HP and replaced with 10KW, ok so the 40 HP was to much, but you can get 10KW electric bicycles!
and they attribute 400Kg batteries to Engine weight ( approx 100Kg IMO ) plus fuel
Hmm how far will 300Kg of fuel get you? A one hell of a lot more distance than 400Kg of batteries.
a Tesla works Brilliant, especially in the city where the pollution is moved out of the city or by the use of super Change points, but basically relies on being charged up every night!
as boats consume more fuel than cars, they would need huge battery banks and to be charged from the grid more often - people on here say they struggle just with the living off of batteries! the power for propulsion makes that demand look like nothing.

If you need to save money - rebuild the old Engine or fit a smaller diesel say 50% of the size ( remember the electric conversion was nearer 25%)

As for diesel being safer then Petrol i saw posted on here - yes its a lot safer, what about Batteries vs Diesel vs Petrol?
Batteries are very dangerous a battery fault can take 24 hours to cool, with copious amounts of water ( of course we have water ) luckily they don't go wrong very often

I could do the maths for the above but i cant be bothered as its so obviously a non starter
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Old 15-02-2019, 10:22   #39
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Re: Info on math of electric boat?

The Steyr HDS sounds amazing, except for one little detail I could not find on their website - the price.
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Old 15-02-2019, 10:27   #40
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Re: Info on math of electric boat?

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In my opinion LOL
you only have the power available from the amount of solar you have available and in propulsion terms that is literally nothing!
Yes you have batteries, but if your charging them with Solar and your travelling and not on shore power every night? Then your left with nothing.
Remember, we're talking about sailboats.

If you only need power to get in and out of the marina, and are comfortable with sitting in place waiting for the wind to change, that little bit of power may be all you need.

For a charter where you're on a hard schedule, that'd be a no-go. But for a live-aboard? It might be a reasonable choice.
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Old 15-02-2019, 16:23   #41
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Re: Info on math of electric boat?

Wouldn't it be nice to have a decent diesel generator driving two electric motors to propel the boat?
One could still have solar panels and house batteries I suppose but I am told the big cruise ships use this approach.
I doubt very much that the two Chev 350 V8 motors at 290hp each in my boat is essential as I often motor on one engine at idle speed very comfortably.
I dare suggest few motor cruisers use the full capacity of the boat engines other than in an emergency.

I guess the question is what is the minimum power one can get away with. The size of the auxiliary motor would be a fair indication. I can push/pull my boat around using my inflatable with a 9.9hp Mercury outboard like a tug boat when in a awkward position in a marina for example. Granted it is under ideal conditions but if one could get away with say 20hp thats about 15kw and quite doable.

Im sure a diesel generator and two motors will take up less room, weigh less and is a lot safer than two V8 petrol guzzlers with 3 petrol tanks!
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Old 15-02-2019, 17:45   #42
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Re: Info on math of electric boat?

A diesel generator connected to an electric motor will weigh way more and be less efficient than a direct drive Diesel.
However it enables the generator to be placed anywhere on the ship and enables things like Azipods so therefore the loss of efficiency is acceptable.

However how are large tankers and cargo ships driven? They tend to be very efficient because the less fuel burned, the more money made.
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Old 15-02-2019, 18:16   #43
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Re: Info on math of electric boat?

Everyone is making valid points. Sometimes one needs to consider that the real world is not predictable. Would consider adding a healthy dose of extra horsepower to the equation. Nothing worse than being underpowered when you really needs it.
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Old 15-02-2019, 19:38   #44
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Re: Info on math of electric boat?

Quote:
Originally Posted by Jdege View Post
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?
Jdege, I looked at buying an old Toyota Prius or a Chevy Volt and using the electric motor controls and battery. More engine etc than you need but its all there charging motor, battery, Electric motor and controls. Need a certain amount of space in the boat. New systems and batteries I've seen estimates of $12k or more dependent on size of boat. A new diesel engine for that size boat would run $10K or more. Let me know what you do. DavesIM
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