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Old 07-06-2016, 07:02 PM   #1
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Weight And Engine RPM

Had a friend ask me a question today and did not know the answer, but I told him I would ask on the forum tonight.

He has a boat with a 115hp outboard and weighs 1500lbs, turns up 5800 RPMs. He has now put the engine on a houseboat and the engine only turns up 4500, houseboat weighs 5000lbs. Does weigh affect the RPMs or should the engine turn up the same RPMs no matter what it is pushing.
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Old 07-06-2016, 07:45 PM   #2
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Not only the weight but the hull shape will adversely effect the rpm. He will probably have to pitch way down to get his RPM back up. IMO of course.
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Old 07-06-2016, 07:46 PM   #3
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Whether or not it will turn up is a function of pushing the boat fast enough for the engine to turn up. Simply, you can't expect similar performance on a boat weighing over 3 times as much.

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Old 07-06-2016, 08:49 PM   #4
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Your friend needs to buy a new prop with about 4" less pitch. At least for larger inboard engines, a good rule of thumb is an inch of pitch, changes rpms by 100. Maybe more for high rpm outboards though. Talk to a good dealer for his brand of outboard.


All of which assumes he wants to get up on plane and go fast which may not happen with a 5,000 lb houseboat and 115 hp. If he just wants to go slow at hull speed, then the current prop is probably ok for that. Just keep it down to below 3500 rpm.


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Old 07-07-2016, 12:23 AM   #5
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You asked about weight and that was answered.

So this is unsolicited.

Without knowing the boat, I'm guessing at 4500 that boat is most likely going as fast as it will go.

So, if you want to increase the WOT RPM drop an inch in diameter and pick up about 500 RPM drop an inch of pitch and pick up another 200 RPM. Neither will affect speed.

Even at that you won't want to run much over 4000 so what is the point. Just do what David says.
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Old 07-07-2016, 06:38 AM   #6
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The prop accelerates the water that it "sees"

So a boat running at 20K will have the water accelerated from 20K to perhaps 30K

The same boat at 5K will attempt to accelerate the water to 30K , which will not be done and may overload the engine,.

Frequently the prop will simply cavitate , and you go nowhere.
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Old 07-07-2016, 07:47 AM   #7
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Thanks Will pass all info on;;;
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