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Old 15-01-2013, 16:47   #1
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Slow vs fast rpm diesels...

Interesting article by Nigel Calder in this month’s Sail mag. He compares two 110 hp diesel engines.
One is in a 78 ft, 150 ton, vessel that is 100+ years old. Engine is110 HP at 325 rpm. And weighs ten tons!
The other is his recent boat with a 110 HP Yanmar rated at 3200 rpm. And it weighs one fortieth the weight of the old engine.
At 8 knots both vessels burn the same amount of fuel, yet the old tug weighs eight times as much!
His main point seems to be that slow turning, large propeller powerplants are more efficient than the new style engines. The cost of light weight is pretty large….. just an interesting note….
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Old 15-01-2013, 16:57   #2
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Re: Slow vs fast rpm diesels...

Unless his new boat has the same waterline as the tug, the numbers aren't comparable.
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Old 19-01-2013, 13:09   #3
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Re: Slow vs fast rpm diesels...

Large marine engines are always low rpm because of higher effiency.
To give you some numbers
Your average car diesel or 3000 rpm yanmar gets about 40-45% effiency
A medium speed 4 stroke diesel engine (500-800 RPM) gets up to 48-49% efficiency
The large low speed 2 stroke diesel running about 30-120RPM gets up to 51% on its own and up to 57% using exhaust gas to generate steam and drive a steam turbine..

As for prop speed, best RPM for high efficiency depends on cruising speed and prop diameter. Best efficiency is usual obtained at around 100 RPM .
I know of some commercial fishing vessels that have recently been converted to a smaller engine and a slower running bigger prop this gives them a slightly lower max speed but increased pulling power a low speeds and higher efficiency.

Furthermore a low speed engine lasts longer between overhauls.

However on a sailboat,
Do you really want extra weight?
Do you really want a big prop? (a lot more drag when sailing)
On a small boat wont the extra weight out balance the 5% higher efficiency from the engine and propeller.

My trintella 1 still has the original SABB 10hp 200kg engine and I'm happy with it.
But most people wouldn't appreciate it. I just love the sound.
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Old 19-01-2013, 13:11   #4
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Re: Slow vs fast rpm diesels...

I can't give exact number on props but I would expect max 10 % increased efficiency.
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Old 19-01-2013, 13:36   #5
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Re: Slow vs fast rpm diesels...

My trintella 1 still has the original SABB 10hp 200kg engine and I'm happy with it.
But most people wouldn't appreciate it. I just love the sound.

Yeah, those old Saabs are cool sounding for sure....does yours have the variable pitch?
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Old 19-01-2013, 13:56   #6
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Re: Slow vs fast rpm diesels...

Quote:
Originally Posted by Cheechako View Post
My trintella 1 still has the original SABB 10hp 200kg engine and I'm happy with it.
But most people wouldn't appreciate it. I just love the sound.

Yeah, those old Saabs are cool sounding for sure....does yours have the variable pitch?
Yes I have, I had to overhaul the trust bearings and replace a bend blade when I bought it. But now it runs fine and gets me up to 6.2kn that is almost my 6.5-7kn hull speed.

I does have the early motorboat version of the variable pitch. If I ever find a sailboat propeller for a reasonable price I will change them.
The sailboats propellers are identical in shape and dimension but have a zero drag vane stand.
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