Wot

[dirtdiver]

I have a new to me 20ft 89 thonpson cuddy cabin witha 180hp 3.7 i/0. Its my first power boat and ive been doing research and still have questions. i read some where that at wide open throttle I should be at 3400 rpm. but at maybe half throtle im alread at 3200 rpm which was about 26mph (according to my gps). I havent pushed it all the way open just because its an older boat and i thouth that maybe that where i had it was full throtle. Should I try a different prop or take it out again and push the thro all the way open and see what happens? Whats redline for a motor like that? Is there anything Ielse I should do?

 

[maxum_man]

Re: Wot

need a Clymer Manual!! I have one and it gives all the specs on that kinda stuff at the end of each chapter. I'm sure they make a manual for you (wheter its a Clymer or not).

Bryan

 

[Bifflefan]

Re: Wot

I belive the 3.7 red line is 4200 or 4500 rpm. something like that. I would have to look on mine for sure. It says it on the valve cover. Along with some other info.

As for the speed? what pitch prop is on it? 19 to 21 should be a good start.

 

[steelespike]

Re: Wot

Look on the motor there may be a label with the max rpm rating.
I'm betting the max rpm is at about 4200.And it will approach close to 40.in fine tune.
If the motor is healthy don't hesitate to take it wide open to get a good rpm and speed.Look on the barrel of the prop or under the prop nut for the prop size.
Also look on the outdrive for the gear ratio.With as much info as possible we can tell if it seems normal or if there is a prop change or other fix in order.
It seems good at 3200/26.Some boats don't even plane at 3200.

 

[dirtdiver]

Re: Wot

It says 4200 to 4800 is max rpm. So is that where it should be at wide open or would it keep reving higher?

 

[QC]

Re: Wot

dirtdiver,

That range at WOT is where she should run with the engine at max output, "guts and feathers", "pedal to the metal", "hammered" or . . . wide open throttle (WOT). If she exceeds or will not reach that RPM when you are trimmed properly with an average load and the engine running right, then it indicates that your propeller is probably not matched as well as it should be. If the WOT RPM is exceeded then generally you go up in pitch, if you cannot reach that range (RPM too low), then you go down in pitch. A rule of thumb is 1" pitch equals 200 RPM change. So let's say that she only turns 3900 RPM with a 19 inch prop, then the simple answer is go down to a 17 inch pitch and she should run 4300 RPM. If she runs 4900 RPM at WOT, then go up to a 21 inch pitch and she should now run 4500 RPM. Things are never that simple, but that's the basics.