So my first question is: you want to change the engine because you have noticed that your boat is underpowered while motoring? With a brand new driveline, you'll be taking a loss, regardless of what you want to switch to. If it works really well, why would you tackle a conversion?
But if you're set on creating a project, there are two common conversion ratios used when switching from ICE to electric. The first one is 1kW of electric for every 3hp of ICE. On your boat, that would be about 6.5kW. The problem with this conversion is that it assumes that the ICE was appropriately sized to begin with. The second ratio is more consistent with 1kW for each tonne of displacement. For your 13,000 pound boat, that works out to 6.5kW of electric drive.
The biggest change in going to electric is letting go of the idea of motoring around at hull speed. Even if your motor could do it, your batteries won't last long. Most of us cruise at 1/2 to 3/4 of hull speed to get reasonable range from our battery packs. As a very rough estimate, every time you slow down 1kt, you double your range. So if your boat has a hull speed of 7kts, you'll be able to go 4 times as far at 5kts. Once you accept that you don't need to drive around at hull speed, you can pick a driveline that will support the speeds that you'll actually use.
My 5 tonne Cheoy Lee Bermuda 30 ketch has a hull speed of 6.5kts. The old Yanmar 12hp diesel maxed out around 5.5kts. My new 5.5kW electric drive from Porpulsion Marine pushes the boat to 6kts.
I would suggest getting familiar with your new boat before you make major modifications so that you can determine exactly what changes will best fit the way that you actually use the boat.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, almxwll@... wrote:
>
> Hi,
>
> I just bought a Buchan '37 whose hull is perfect and whose inside is bare,
> and whose engine has run for a total of 15 minutes. Its a Westerbeke Pilot
> 20, reputed to be able to put out 20 HP to the propellor. My calculations
> from displacement and waterline length using Dave Gerr's book, Propellor
> Handbook lead me to believe that the engine is undersized and that I need
> 24 HP or 18 KW.
>
> I looked in on the Nonsuch conversion using the Thoosa 9000 at 9 KW and the
> 35 foot C&C using an electricyacht 180i at I think 8KW and see that their
> owners are pleased. So, my dilemma is why are they happy when they have
> only half the power required in their conversion. Is this a trick?
>
> I'd rather go green, but its a big step to scrap a perfectly fine
> installation of a diesel for a flyer on something that's more expensive.
>
> Cheers,
> Al Maxwell
>
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