T-Pain Is On A Boat: [Electric Boats] Re: Pure or mostly solar powered boat?

Saturday, April 10, 2010

[Electric Boats] Re: Pure or mostly solar powered boat?

But what size boat needs a 30hp diesel engine?

The diesel motor manufacturer's standard is 3-4hp per ton of displacement, any more is wasted. So a 30hp diesel needs a boat that displaces 7.5-10 tons or 16,500-22,000 pounds, something like a Cal 40. If the boat is any smaller, the engine should be smaller as well. My heavy 30' ketch displaces just over 10,000 pounds or 4.5 tons. therefore any ICE engine larger than 18hp is wasted. A similarly sized Catalina 30 doesn't need any more engine than that, even though many are equipped with 23hp engines. These boats achieve hull speed at 3/4 throttle, so the extra hp doesn't help.

It turns out that most modern sailboats are overpowered, because everyone knows that more power is better. :)

I'm betting that my ketch will be adequately powered with a 5kW (6.6hp) electric drive replacing the Yanmar SVE12 (12hp) that used to drive her, though I feel that she was slightly underpowered before.

So perhaps a good rule of thumb is 1000W for 1 ton (2200 pounds) of displacement. Does this match up with people's conversions in this group that are already sailing? I'll post my actual performance results within the next few months when the conversion is complete.

Fair winds,
Eric
Marina del Rey

--- In electricboats@yahoogroups.com, "gramplarry" <pfister.l@...> wrote:
>
>
> the infamous power curve for ice. Yes there is a continuous rating for most diesel engines it is in the fine print an page 2 and is usually about 5 to 10 percent lower and at a lower rpm just to confuse the issue.
> In reading many motor specifications i have started to believe that not all Horse Power ratings are equal some seem to be for draft horses others are for ponys even though they are reported as the same.
> The curve that may be better for comparison electric to diesel is the propeller power curve reported by yanmar which is dramatically different than the standard max power curves. a closer looki at a yanmar set of curves shows:
>
> from the Yanmar info sheets for 3ym30
>
> max hp 30 or 29.1 at 3600rpm. The difference is fuel temp ratings!!
> continuous rating 27.3 @3400 rpm
> switching to some points on the power curve given as +/- 6%
> 3500 rpm max 27 prop 26 propeller power 25 (18kw)
> 3250 rpm max 27 prop 26 propeller power 21 (16KW)
> 3000 rpm max 26 prop 25 propeller power 16 (12KW)
> 2500 rpm max 25 prop 24 propeller power 9 (7KW)
> 2000 rpm max 20 prop 19 propeller power 5 (4KW)
>
> these are the published numbers anyone trying to compare by picking max hp rating for a deisel is looking at a gap such as this and sliding down a slipery slope.
> No wonder we are scratching our heads trying to find a good replacement number.
>
> Have we been looking at the wrong published curve for our replacement sizing?? Is there annother way that can use the propeller power curve whatever that is?
>
> larry
>