obtaining hull speed in flat water is one thing, most inboards for sailboats are oversized to provide extra "push" for wave and wind action. these are not as easily quantified so most boat builders for sailboats "double" the HP quirements. Cost between a 13 hp and 26 hp diesel is only small inpentage to the overall cost.
I had a hunter 31 that had a 13 hp diesel and she could easily make hull speed in calm water, but lost about 1 kt in winds and wave action. ( wave hurts more, sheltered areas without wave only lost about .5 kt.)
--- In electricboats@yahoogroups.com, Bill Spires <spiresac@...> wrote:
>
> Eric,
>
> Great response. The most concise and useful info I have ever seen in the blog.
> The only thing I would add is that it is certain that the 27 hp diesel is way
> overkill for a 26 foot double ender. I have been through the re-power wars for
> small, displacement hulls a few times and the problem is that it is hard to find
> and very expensive to get the right size and weight diesels for small boats.
> The other problem is that single cyl. diesels "Will sound like someone pounding
> on a drum with a hammer" when running in a small boat. On the other hand I
> think adding a small gas or diesel generator to the electric setup makes a lot
> of sense. Really extends your range, reduces the size of the battery bank and
> gives you house power to run the blender. Besides, I have lost electric power
> offshore on enough boats to think a little hand cranked gen set is always a good
> idea. I think there is a safety factor in favor of electric too. I can't tell
> you how many times I have reached the harbor after a rough passage and the
> engine won't crank. All the sludge is stirred up in the fuel tank. A few
> minutes of electric would have got the sails down and us in the slip.
>
> Capt. Bill
>
>
>
> ________________________________
> From: Eric <ewdysar@...>
> To: electricboats@yahoogroups.com
> Sent: Thu, December 30, 2010 5:54:00 PM
> Subject: [Electric Boats] Re: New member: I'm looking to re-power my 26'
> displacement hull launch
>
>
> Hi Gershy,
>
> Welcome to the EB group! There are a few different ways that people here
> usually size the motors for their conversions.
>
>
> The first estiimate is to pick a motor that has 1hp for each 2.5 to 3hp of IC
> engine. In your case, that would be 9hp to replace the 27hp diesel that you've
> got today. This isn't very accurate because if the boat was over or under
> powered to begin with, that will be carried over to the new system.
>
>
> For displacement boats, the next rule of thumb is 1kW of motor for each ton of
> displacement. This usually provides a motor that will do well in most
> conditions, but the motor will be just able to hit hull speed without much
> reserve. That is usually not a problem since hull speed uses much more energy
> than 80% of hull speed and electric boaters will usually throttle back to
> maintain reasonable range.
>
>
> Finally, you can use your typical fuel consumption to calculate your average HP
> demand for your particular boat and usage. If you know your fuel consumption in
> GPH you can use the following formula: GPH times Thermal efficiency (I use .20
> to .25 for deisels operating at slow speed like most marine engines do) divided
> by 0.0226 equals your average HP used. Once you've figured out the HP that you
> need, 1hp roughly equals 750W, i.e. 6hp = 4.5kW.
>
> If the batteries that you mentioned are T-105s or T-125s, then you're talking
> about a bank that has about 23kWh total capacity or 13.5kWh of usable capacity
> to 60% depth of discharge (DoD). Another rule of thumb is that 10 to 11kWh of
> battery is equivalent to about 1 gallon of fuel in an IC engine, already
> adjusted for the increase in electric drive efficiency. I bring this up because
> your 16 6V batteries (about 1000 pounds) will have the usable range of about 1.3
> gallons of fuel through your current engine. If you're OK with that, then
> proceed.
>
>
> In my opinion, as soon as people start discussing adding a deiesel generator to
> extend range, then just going with an IC driveline will be cheaper, simpler and
> more reliable. But that is just my opinion, there are plenty of people here
> that feel differently.
>
> Anyway, welcome and I hope that we will be able to help with your conversion
> plans.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, "gershy@" <gershy@> wrote:
> >
> > Hi all I'm new to the group and I'm hoping to get some advise about going
> >electric with my boat. I have a 26 foot double ender with a 27 HP diesel in it
> >now. the engine is in need of repair and as i only use the boat close to land i
> >was thinking that electric may be the way to go. It looks like around 6 hp is
> >needed to reach hull speed. should i be looking to use a "golf cart" motor? or
> >is there a better choice? I believe i have room for about 16 6V golf cart
> >batteries. If a golf cart motor will work should I try to find a larger motor?
> >like 10 hp or so? Any help that you could give would be appreciated
> >
>
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