eric
iI believe It is not realy about how big a boat the engine will drive using some rather abratrary formula but rather how fast you want the boat to move at crusing speed. Everyone on this forum has pretty much said that in a thousand different ways. The electric has a much simpler power and torque curves than an ice and those ice curves are confusing the issue.
Lets discuss sailboat diesel selection and that seldom discussed engine warenty which is a major driver in motor selection for deisel but never talked about. Many manufacturers of diesels want the propeller sized for the engine to reach within 400 rpm of maximum rpm or they will void the warrenty!! This changes the ground rules in selection. At first it does not seem to be much but at a minimum that means that you must reach hull speed at the continuous rpm rating or the warrenty is void on a new engine. The customer for a new $200,000 sail boat is a busy person who wants to maximize sailing time and demands that the boat get closer to hull speed at a "crusing rpm", He could run the engine faster but that is too noisy and vibrates and spills his cocktail. You could change the hull speed to a different rpm by changing the pitch but that would mean the engine will not reach full rpm and void the warrenty. so at present the power in new sail boats are creeping higher so we can motor faster and longer.
( i have had friends and customers who have to motor for 1 to 2 hours before thay could set sail. The original motors in sailboats were sized and intended just get out of the harbor under fine sailing conditions usually less than 1/2 mile to sail setting)
If you do ignore the warenty and use a larger prop with the ice then the side effect is that when you really need a bit more power in an emergency you are already at the top of the power curve, you get to clean the soot off the transom more often, and spend more time with your engine doing maintence.
Makes you want to switch to electric!
every day goes someone wants a prop change to increase the crusing speed. Most have never run up to full rpm and often do not even know what is the max rpm for the engine.
If you want to compare a deisel to an electric you might be better off selecting your location on the deisel Propeller power curve first rather than the maximum ratings. E.G.
If i am reading my power curves right your 5kw would about the same as a 3YM20 running at about 2600 rpm so in a way you could say you are installing the equivalant to a 21 hp deisel engine. be curious to see what results you have.
because of the non liner nature of the propeller curve for a deisel engine there not be a simple 5kw replaces a 20 hp deisel I consider that your crusing speed may be a more valid comparison criteria.
I think that your comment of kw/ton makes more sense than deisel hp per ton because the electric kw is more closely ralated than the deisel HP which needs to be curved for rpm to be even close.
larry
--- In electricboats@
>
> But what size boat needs a 30hp diesel engine?
>
> The diesel motor manufacturer'
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> 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.
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> 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@
> >
> >
> > 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
> >
>
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