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Old 02-09-2019, 09:35 AM   #41
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To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".
.
Thanks for this! Since I don't have a trawler, I would like to ask a question about this formula.

Say a trawler has a 36 foot LWL. The square root would be 6 feet. Times .9 to 1.15 would still be in the less than 6 to just over 6 range. Or about 6.

Would that speed be miles per hour? Knts per hour or some other measurement. I assume the variable of .9 to 1.15 would be hull design of semi-D hull versus full D hull. Or maybe weight or ??

Am I tracking at least some what correctly???


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Old 02-09-2019, 09:58 AM   #42
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"Run it a hull speed for greatest fuel efficiency ."

Hull speed is NEVER the greatest speed for fuel efficiency.

It is the point where the bow wave is so large the boat can not go faster , or climb up on top.

To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".

Hull speed 1.34 is mostly of use for displacement wind sailors with free propulsion to understand when the boat is being pressed enough to consider sail addition as unproductive and perhaps dangerous.
This assumes your prop is sized correctly, right?

I have another way I look at it.
Max torque for my SP 135 is between 1600 and 1700 rpms.
Thus, that's why I run away that rpm setting, as the engine should be most efficient at max torque.
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Old 02-09-2019, 10:13 AM   #43
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That's a ton of HP for that boat!
A few months ago I was looking at a a 64 GA that had the original Cat engines (current owner didn't know model) removed and JD 6068s put in. Unfortunately with several GAs the first owner specified engines that in retrospect proved wrong.

BTW, at genset speeds - 1800 RPM - the JD 6068 is designed to run 24/7. Tens of thousands doing it. I was recently on two different vessels that had NL gensets with the 6068s. One with about 8,000 hours and the other new. Great reports from the engineers.
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Old 02-09-2019, 11:28 AM   #44
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Originally Posted by Action View Post
Thanks for this! Since I don't have a trawler, I would like to ask a question about this formula.

Say a trawler has a 36 foot LWL. The square root would be 6 feet. Times .9 to 1.15 would still be in the less than 6 to just over 6 range. Or about 6.

Would that speed be miles per hour? Knts per hour or some other measurement. I assume the variable of .9 to 1.15 would be hull design of semi-D hull versus full D hull. Or maybe weight or ??

Am I tracking at least some what correctly???


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Knots are the correct unit of measure using the hull speed formula. The .9 to 1.15 of hull speed as a "sweet" spot applies to FD boats since SD or planing hulls have other options on defining what is a sweet spot and what is not. My son's planing Grady White has greatest efficiency defined as max distance traveled per mile of fuel consumed at around 28 knots, which would be 4x its hull speed.
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Old 02-10-2019, 07:07 AM   #45
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The formula is in Knots and varies as some boats are fat and heavy some are leaner and lighter.

5K = about 6 statute MPH .Inland charts are usually in miles , so folks prefer talking in statute.

On a boat with a tach and even a hand held GPS , simply create a graph of RPM to boat speed .

You will find every RPM increase gives a modest speed increase , until a certain point where much larger RPM increases are needed for more speed .

Looking over the side there will be a bow wave and a stern wave with a hollow amidships.

As you increase RPM the boat will begin to climb the bow wave , and the deck will angle up.

The boat may begin to leave the stern wave and the transom will sink below the water line.

That's when your mostly just warming the ocean and making waves, fun perhaps but expensive .

A Knot slower may cost 1/2 the fuel per hour.
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Old 02-10-2019, 09:48 AM   #46
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True or false, years ago, you can determine the hull speed by watching your stern wake. When the wake comes together, this is supposed to be the hull speed. I dont know if it is true. Gotta be close.
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