Wide open throttle = Max speed? - SailNet Community

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Old 07-11-2013
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Wide open throttle = Max speed?

Yesterday I did a little experiment. There was no wind, the lake was flat and shiny. I encountered no boat waves or wakes.

My Pearson 26 weighs 5400 lbs dry but is probably closer to 6000 with gear and myself aboard. Outboard is a Nissan 6hp long shaft 4 stroke.

I ran it 2 minutes wide open throttle, then 2 minutes 3/4 throttle, then 2 minutes 1/2 throttle and finally 2 minutes 1/4 throttle.

The average track speed recorded on GPS for each throttle setting in the order of the test was:

WOT-5.8 mph
3/4 - 5.9 mph
1/2 - 5.5 mph
1/4 - 5.1 mph

I was a little surprised to see more speed 3/4 throttle than at WOT. Any ideas why that was?
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

Displacement hulls need less horsepower to move at hull speed, anymore power won't make it go "faster" because the hull will start to drop lower in the water and make a wake that resists it going faster. Some of the Naval engineers on the forum can explain it better.
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

You may have been cavitating?? After reaching hull speed, it's wasteful to try to push a displacement hull faster, just a waste of fuel for little gain. You CAN push above hull speed but it takes a lot more RPMs to gain a few tenths of a knot.
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Re: Wide open throttle = Max speed?

My boat with 16hp inboard diesel moves at hull speed at 1/2-3/4 and does slow down if If I do wot (kinda not really necessary with a diesel)
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Re: Wide open throttle = Max speed?

What I understand about hull speed is 1.34 X the square root of Length of Waterline equal a displacement boat's hull speed in knots.

My boat calculates to 6.242 knots or 7.183 mph. If this is correct, I am under-powered for reaching hull speed with the motor. I don't see any reason to upgrade for more horsepower since we lake sail and stay off the water when thunderstorms are threatening.

Last week our GPS track under sail showed our avg speed over the race course was 4.8 mph with our max speed 7.2 mph. I guess that is about as fast as this pig is going to go??? :-)
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

After reading the above posts, I did an internet search.
Check out this website: Boat Speed Calculator
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Re: Wide open throttle = Max speed?

Your old girl gains waterline when heeled my old cutter was also faster when heeled over.
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

So I think what is more important is the RPM at each benchmark and the manufacturers rated cruising RPM and Max RPM. Are you hitting Max RPM at WOT? If yes, maybe, just maybe your are under prop'd but I would get the advice of a good yard first before making that change.

And your engine is possible governed at a certain RPM so that may throw off the works too. Again, a knowledgeable boat yard would help immensely.

Last edited by kellysails; 07-11-2013 at 11:32 AM.
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

That formula for hull speed is an estimate. The S/L changes with hull shape. Obviosly a
28' DWL with 11' beam will have a slower hull speed than a 28' DWL boat with 9' beam.
But if you are considerting boats with "normal" proportions the 1.34 will get you close.

Also consider that the S/L changes with prismatic coefficient, Cp. The higher your
Cp ( fuller ends) the higher your S/L. You can picture that by imagining the full ends keeping bow and stern waves spread out. Now reduce your Cp (fine ends) and you can imagine how those pointy ends allow the bow and stern waves to creep towards each other reducing your S/L.

I think your one tenth of MPH increase at the lower rpm may be just an error and it's well within the tolerance you can expect with this kind of experiment.

But, if I accept the numbers as accurate I might hypothisize that at full rpm your boat may have squatted immersing the transom and lifting the bow out slightly which could have had a net effect of actually reducing your DWL. Perhaps. Just immersing some of the transom will increase drag. That's why if tyou look at a high performance box rule boat like a TP52 they will have a plumb stem but they always have some overhang aft so at lower than hull speed they do not pick up drag from immersing the transom. At speeds greater than 1.34 the immersed transom can increase Cp and actually help.

Maybe you had a piece of kelp wrapped around your keel when you did the full throttle run.

I did a lot of speed runs years ago when I was testing props. I did them with a stop watch and ran a course next to the Lake Washington floating bridge where they conveniently had marked off a nautical mile. We did each test run three times and took an average. It was a lot of fun and educational.
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Last edited by bobperry; 07-11-2013 at 11:47 AM.
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Old 07-11-2013
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Re: Wide open throttle = Max speed?

Quote:
Originally Posted by bobperry View Post
That formula for hull speed is an estimate. The S/L changes with hull shape. Obviosly a
28' DWL with 11' beam will have a slower hull speed than a 28' DWL boat with 9' beam.
But if you are considerting boats with "normal" proportions the 1.34 will get you close.

Also consider that the S/L changes with prismatic coefficient, Cp. The higher your
Cp ( fuller ends) the higher your S/L. You can picture that by imagining the full ends keeping bow and stern waves spread out. Now reduce your Cp (fine ends) and you can imagine how those pointy ends allow the bow and stern waves to creep towards each other reducing your S/L.

I think your one tenth of MPH increase at the lower rpm may be just an error and it's well within the tolerance you can expect with this kind of experiment.

But, if I accept the numbers as accurate I might hypothisize that at full rpm your boat may have squatted immersing the transom and lifting the bow out slightly which could have had a net effect of actually reducing your DWL. Perhaps. Just immersing some of the transom will increase drag. That's why if tyou look at a high performance box rule boat like a TP52 they will have a plumb stem but they always have some overhang aft so at lower than hull speed they do not pick up drag from immersing the transom. At speeds greater than 1.34 the immersed transom can increase Cp and actually help.

Maybe you had a piece of kelp wrapped around your keel when you did the full throttle run.

I did a lot of speed runs years ago when I was testing props. I did them with a stop watch and ran a course next to the Lake Washington floating bridge where they conveniently had marked off a nautical mile. We did each test run three times and took an average. It was a lot of fun and educational.
I think you may be on to something regarding the squatting stern-except I think it is raising. When I crank the throttle wide open, there is just a little flex of the transom where the motor mounts. It allows the prop to torque forward which I think points the prop thrust downward just a bit. So actually I think it is lifting the stern, causing the bow to dip downward. I'm thinking of beefing up the transom to stop the flexing. What are your thoughts on that?

PS- We don't have kelp here :-)
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