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  #1  
Old 09-07-2012
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Optimal propulsion system

I have what may be a slightly unusual problem. My wife loves to sail but hates it when I have to put on the engine. I mean really hates it. She would rather sail at 2 to 3 knots than motor at 5 knots.

For coastal sailing however motoring seems to be a pretty common necessity.
The alternative is to get into a strange harbor after dark and miss dinner and a slip assignment.

I'm thinking that an optimal system might be an electric drive system with battery capacity for about 5 hours. This battery bank would be mated to a gen-set that had the horsepower to drive the boat at max throttle.

The system would be reasonably simple.
Genset with charger connected to battery bank.
Batteries connected to electric motor.

During the day if the wind was light I could motor sail for a few hours. The motor would be so quiet no one would know.
In the afternoon while we were off the boat I would run the genset to charge the batteries (The genset would be insulated so its running would not be noticed by any other boats)
If I really had to run the genset to run the motor it would be big enough to do the job and I would just have to deal with the noise.
  • I know this is not a cheap solution. Probably double what a re power would cost.
  • It is also pretty complicated with lots of controllers and electronic controls.
  • It will be somewhat inefficient converting gen-set horsepower to electric instead of shaft power. I'm hoping that the built in inefficiency will be off-set somewhat by the fact that the the gen-set can be set to the most efficient rpm as opposed to a shaft driven motor where the rpm is used control the boat speed.

Weather I would be willing to pay for and actually build such a systems is another question but assuming the above usage what do you think?
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Old 09-07-2012
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Re: Optimal propulsion system

There are a couple boats with this already available. The hunter e33 has this as an option and a cruiser did a conversion on his older boat.

Here is a link to their blog:
biankablog.blogspot.com/2008/05/going-electric.html
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Old 09-08-2012
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Re: Optimal propulsion system

Quote:
Originally Posted by todd740 View Post
There are a couple boats with this already available. The hunter e33 has this as an option and a cruiser did a conversion on his older boat.

Here is a link to their blog:
biankablog.blogspot.com/2008/05/going-electric.html
Neat link. It reinforces how complicated a diesel engine install it. I remember reading some thread and someone wrote off electric propulsion by saying it "wasn't suited for the marine environment"...as if a combustion engine with it's requisite, lubrication, cooling, starting, fuel , systems with their requisite secondary filters, cooling, charging, system are? I'd say moving parts arn't well suited for the marine environment and electric definitely has the advantage there. The only problem is the low amount of energy stored in batteries vs diesel fuel.

Quote:
It will be somewhat inefficient converting gen-set horsepower to electric instead of shaft power. I'm hoping that the built in inefficiency will be off-set somewhat by the fact that the the gen-set can be set to the most efficient rpm as opposed to a shaft driven motor where the rpm is used control the boat speed.
This was the line of thinking when the chevy volt was in development. Then at some point they snuck in a direct drive system because it just wasn't efficient enough. However the boat is clearly a different situation - many have generators anyway for example. So I'm interested to see how this thread goes.
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Old 09-08-2012
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Re: Optimal propulsion system

What you're proposing is gaining traction these days.. there are a few boats offering this kind of package, and you're right, it's not going to be inexpensive. Overall I think the electric drive will be best suited to the in-harbour/out of harbour needs of the daysailer, once you get into generators to power the electric power most of the advantages are lost. Getting enough solar to power up such a system will have its own challenges as well.

Good luck, btw, coming up with a powerful enough generator that no one else will notice running....

Emerging battery technology may well make this more feasible in the near future.. It's a neat idea...
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Re: Optimal propulsion system

Quote:
Originally Posted by Faster View Post
once you get into generators to power the electric power most of the advantages are lost. .
The way I see it all of the advantages are lost except one.

It's more expensive to install and run and more complicated and may be just as noisy.
If you typically sail 50% and motor 50% and catagorize a light electric assist in the sailing side then maybe you can sail 90% and motor 10%.

Lots of if's.
My wife really hates motoring under diesel.
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Re: Optimal propulsion system

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Originally Posted by Faster View Post

Good luck, btw, coming up with a powerful enough generator that no one else will notice running....

.
I saw a generator at the boat show that seemed very quiet.
Your right this is a key component.
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Old 09-08-2012
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Re: Optimal propulsion system

I hope for your sake an expert chimes in but I've been poking around looking at this this morning.

First a number to use as a reference is that a hp is 0.75kW. This means that if you want to run continuously with the equivalent of a 15hp diesel you need roughly 11kw available. Punching that into a search for generators seems to reveal some bad news - 11kw generators are huge. Panda for example is 265 lbs for just 6kW (The 6kW is electrical output so the conversion losses are already taken into account. I bet it's a 6-10hp engine). And although there are a lot of complexities I think that's a fair starting point.

However it does seem like you have quite a lot of places to make compromises. First, assuming you're making a long passage and know it, you can charge the batteries ahead of time. Then you could run the generator and draw off the batteries at the same time and get good speed for a long time.

Second, even if you're limited to say 1/2 throttle average for a long passage that should still net 3/4 or 2/3 speed given the typical power curves. And you'll be able to throttle up for quick bursts anytime you want. Actually, you can probably specifiy a fairly oversized electric motor without a whole lot of extra cost/weight and have some pretty high speed burst potential. That's a neat option to have if you get into trouble.

If you're willing to comprise your passage ability by installing a smaller generator it might work well, if not it looks like it's heavy and expensive.
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Old 09-08-2012
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Re: Optimal propulsion system

Earplugs and Valium would be cheaper!
I'm interested in your projest.
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Re: Optimal propulsion system

here is a place to start
Vetus Electric Drive EP2200 - French Marine Motors Ltd
or D&D Motor Systems - Electric Boat Motors
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Old 09-08-2012
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Re: Optimal propulsion system

davidpm:
What kind of motor setup do y0u have now?
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