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Old 03-01-2016, 04:19 PM   #101
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We are a little late to this discussion but are one of the few who have re-powered with a smaller HP engine.

Our 26,000 lb Pilgrim 40 was powered by a Westerbeke 108 C-6 (108 hp, marinized Kia bus engine) and needed replacement. Since we are a "full displacement" hull and use less than two gallons/hour at seven knots and wanted a Kabota based NA engine we chose the 75hp BetaMarine.

Six months later (390 engine hours and 2800 miles), we couldn't be happier.
The performance graph indicates the result. We almost always run the engine at 2050 rpm (80% of max) which is exactly what we were shooting for during the re-power.

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Old 03-01-2016, 08:28 PM   #102
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Insequent
Nozzles work in situations of propellor slip. That is why they have maximum effectiveness at zero speed, working against a tow or trawl net. At free running speed they are only a detriment. My shipyard installs them on every tug they build, clearly told me I would lose 1/2 knot. So even if the Rice provides higher fuel efficiency I don't think that would be a great advantage against the added drag and slower cruise speed. The prop protection would be wonderful of course, but it is easy enough to add ice horns for that purpose.

In any case, do you have any idea how much a typical 44-48 inch Rice nozzle would cost with the special propellor?
A Kort or Rice nozzle will only be a detriment above something like 12 kn. If you yard loses 1/2 kn then they are not implementing the concept very well. It needs a prop matched to the nozzle.

Fuel savings tell the story: fuel saving is better efficiency! Now, my go-to resource for this stuff is Gerr's Propeller Handbook. It seems that the higher loaded the blade, the more benefit. So tugs, and trawlers returning with a full catch, get maximum benefit. For trawlers the payback could be quite fast given their fuel use.

But, for lower blade loadings, the kind pleasure boats will see, the benefits are much reduced, although still present. So payback could be a very long time. I had not appreciated this aspect before. No idea what a Rice nozzle and matching Kaplan prop would cost, but having now picked up an additional nuance as to when they deliver maximum benefit I have to say Ski is right again: not one of the decisions you need to make.
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Old 03-01-2016, 09:28 PM   #103
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Pilgrim-
I was reading the ISSO 8665 standard online, and if I understand it right then the manufacturer's power curve (below) for your engine is showing the prop-power curve in green.

So at 2050 rpm you're absorbing about 57 hp of the available 62 hp at full load, at the prop. About 90% "power." If this is so then when you increase throttle to full you would not realize any significant increase in speed, maybe a fraction of a knot.

Correct, or am I interpreting this wrong?
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Old 03-01-2016, 09:42 PM   #104
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mako,
Assuming he's propped correctly he's got 550rpm yet to go so lots of power should be availible beyond his cruise.
Pilgrin is admittedly running over 50% of availible power. My Willard is about 50% powered and I run it 700rpm down from rated rpm correctly proped. Fairly similar. I'll bet it took courage to get a smaller engine. An advantage he had/has is the slickest FD hull on this forum.
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Old 03-01-2016, 10:18 PM   #105
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At wide open throttle, maximum speed is 8 knots. This is down 1/2 knot from the max speed produced by the higher hp Westerbeke. Fuel consumption is exactly the same with either engine, 1.7 gallons/hour. The gearbox was chosen to produce the same shaft speed at cruise.
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Old 03-02-2016, 07:16 AM   #106
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"Fuel consumption is exactly the same with either engine, 1.7 gallons/hour."

At a rarely seen 20 hp per gallon 1.7 GPH would only be 35 HP or so , hardly 80% of power available.

You may be using 80% of the RPM available , but less than half the power.

It is right on the prop curve , which frequently is the mfg method of lightly loading the engine for fewer warentee hassles.

A bigger prop using 1800 RPM (40 hp available) might be quieter and more fuel efficient.
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Old 03-02-2016, 09:28 AM   #107
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Pilgrim-
I was reading the ISSO 8665 standard online, and if I understand it right then the manufacturer's power curve (below) for your engine is showing the prop-power curve in green.

So at 2050 rpm you're absorbing about 57 hp of the available 62 hp at full load, at the prop. About 90% "power." If this is so then when you increase throttle to full you would not realize any significant increase in speed, maybe a fraction of a knot.

Correct, or am I interpreting this wrong?
Hard for me to see the colors but the bottom curve is the only one with a proper shape for a prop curve.
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Old 03-02-2016, 09:33 AM   #108
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some seem to get quite worked up over the idea of more power for head seas. I assure you if you face seas on the bow when in a FD hull that is powered to just make hull speed in calm water you will want more power as each wave stops the boat and the next wave arrives just as forward progress resumes.
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