Total and Reserve Power

bayview · Feb 25, 2016

Many:
Both engines would use about the same fuel at the same load even though one was running with lower percent load than the other. To reasonable estimate 4 stroke diesels produce 20 Hp per gallon of fuel per hour. Electronic engines may be a bit better.

Mako · Feb 27, 2016

manyboats - those are some good questions about the good ole "50-70 rule." The old 2 strokes (like 6-71s) have to be run hard because those blowers would cool them off at low loads. All the research I've done indicates that mechanical engines (2 and 4 strokes) are happiest at 70% load, continuous. Not sure about electronic engines though. Fuel metering is so precise that they can be run at 30-40% without fouling, but I would still suspect that they are happy at the high numbers.

Perhaps there are some mechanics on the forum who know more.

markpierce · Feb 27, 2016

makobuilders said:
... All the research I've done indicates that mechanical engines (2 and 4 strokes) are happiest at 70% load, continuous. Not sure about electronic engines though. Fuel metering is so precise that they can be run at 30-40% without fouling, but I would still suspect that they are happy at the high numbers.

Perhaps there are some mechanics on the forum who know more.

Are you speaking in relation of fuel consumption or engine revolutions? When speaking in terms of load/work, don't engine makers speak in terms of fuel consumption?

Al · Feb 28, 2016

manyboats said:
Mako,
When head seas cause me to change power/throttle I never increase it. Almost always I lower it 300rpm. Never needed "extra" power. However I frequently slow down a bit but not less than a knot.

Exactly! Hull speed is hull speed. anything more is waste of power/fuel. Eric nails common sense.

Al -Ketchikan

Insequent · Feb 28, 2016

makobuilders said:
..snip....

SPECS - My boat was designed with a displacement of 56.5 cubic meters of seawater at DWL. 56.5 long tons. It's LWL is 47 ft. Tranny is 3.46:1 ratio and prop is calculated at 37"x23" in a 3 blade fixed. We don't need the Kort nozzle in the original design.

...snip....

Mako
Given the design of the boat and expected speeds I don't know why you would not use a nozzle, and indeed I would opt for the Rice Nozzle
Rice Speed Nozzles more efficient than Kort 19a Nozzle

Rice have a website, although the English pages are not that great maybe the Spanish ones are better. The only reason not to go with a nozzle is if you expect to get into low teens speed range as they are less efficient there than an open prop.

Mako · Mar 1, 2016

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?

Nomad Willy · Mar 1, 2016

It's not "propeller slip" ....
It's water sliding over the ends of the propeller. Same thing that led to the development of winglets in aircraft. The nozzel on a boat propeller does the same thing. It reduces the tip loss from fluid spilling over the end of the propeller. That's why long wingspans on airplanes are more efficient and high aspect ratio propeller blades are too. But perhaps you could call it slip ....

The advantage of the shroud or nozzle that I'd like is the elimination of prop walk.

Ski in NC · Mar 1, 2016

It is propeller slip. Nozzle reduces it, and that is a big advantage pulling hard at low vessel speed. When not towing, little is lost spilling over the tips and nozzle adds a good bit of drag just by skin friction. Not a part of Mako's list of decisions to make.

Mako · Mar 1, 2016

So if you compared large prop/slower rpm versus a smaller prop/higher rpm (provided by different gear ratios on same engine) which would result in more pronounced prop walk?

Nomad Willy · Mar 1, 2016

Small dia large pitch = max prop walk. But pitch has the greater effect.

Think about a prop w so much pitch the blades are 90 degrees to the shaft. Zero thrust but the work being done is to throw the water radially and of course the bottom of the prop swing throws the water to one side depending on rotation. That's why prop rotation dictates which way the prop walk moves the stern.

In the first pic w a low aspect ratio prop there will be much higher tip losses. Much more water will be able to get around the end of the blades lowering the vacuum on the back side and lowering the high pressure on the face of the blade.
The second pic is of prop w high aspect ratio blades. Much more efficient than the first pic. The ends of the blades are small so the spillage over the blade tips is also small. Like a sailplane w long skinny wings. High aspect ratio bladed props have limited usage on faster boats. Not even usable on outboards. Airplanes probably couldn't even fly w the low aspect ratio bladed props we use on boats.

drf6 · Mar 1, 2016

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.

Insequent · Mar 1, 2016

makobuilders said:
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.

Mako · Mar 1, 2016

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?

Nomad Willy · Mar 1, 2016

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.

drf6 · Mar 1, 2016

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.

FF · Mar 2, 2016

"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.

bayview · Mar 2, 2016

makobuilders said:
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.

bayview · Mar 2, 2016

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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