What is my approximate top speed?

Dave87

Petty Officer 3rd Class
Joined
Jul 29, 2017
Messages
75
Hi I have a 18ft open bow cadorette with a 4.3l mercruser. My prop is a 1918p21. I was wondering what the top speed should be for this boat. I can't find any info anywhere online. Any idea would be nice thanks
 

GA_Boater

Honorary Moderator Emeritus
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May 24, 2011
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49,038
You may not find that online.

It would be good to say what top speed you are getting and what your WOT RPM is.
 

Rick Stephens

Admiral
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Aug 13, 2013
Messages
6,118
FYI, you can get a phone app for almost any smart phone that will give you GPS speed. With that, and a tachometer, you can answer the basic starting point question for any speed, prop question, which is: what is Speed and RPMs at WOT.
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,605
You can theoretically do the math and come close to a proposed top speed, but even with that, you have to know how fast your engine can spin any prop. So a tachometer is a must to determine if you are getting everything you can from any marine engine and prop combination.

A prop pitch is basically stating that from ever revolution, you will move forward X inches with the prop pitch. In your case you stated you have a 21 pitch prop. So for ever revolution, your hull should move forward 21 inches. However, there is always slip with any prop. And the theoretically is never going to match the actual.

So knowing your engine's WOT (wide open throttle) RPMs will tell you how fast your engine is turning your prop. Then you have to also know the gear reduction ratio in the drive to the prop, and then do the math. But if your engine isn't riving up to what it should be turning, you need to reduce the prop pitch to get you to that WOT RPM. JMHO
 

Rick Stephens

Admiral
Joined
Aug 13, 2013
Messages
6,118
Here's what I get from my 17 footer with a 4 blade prop I got for pulling skiers. Kind of give you an idea of what a prop slip calculator will say.


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