Boat Speed vs. motor RPM

[FiveDog]

If I have a motor with a given prop (17 pitch) and I'm getting 5200 rpms, does that translate directly into a given speed?<br /><br />I have '91 90 HP Merc and my GPS is reading 31 mph. Other boaters in my identical boat are reporting 40+ mph with 90 HP motors and equal props. What's up with that?

 

[Dhadley]

Re: Boat Speed vs. motor RPM

It translates to a theoritical speed. You have to factor in your gear ratio vs theirs. On identical rigs with identical motors one can be faster than the next (remember racing?) because one has more slip percentage than the other (the difference between actual speed vs theoritical).

 

[Dunaruna]

Re: Boat Speed vs. motor RPM

Identical props are not always identical. Cupping, chips, dings etc play a big roll in perfomance. Also the weight you carry (fuel., gear, people) can vary greatly, you would be amazed at how much difference 150Lbs can make.<br /><br />Lastly, your 90HP may not be making 90HP. <br /><br />Perform a thorough tuneup, polish the hull and retest. IMO.<br /><br />Aldo

 

[JB]

Re: Boat Speed vs. motor RPM

What boat, 5dog? How is your loading compared to the other boat? What does it weigh on the water?<br /><br />Many wanna-be speedos are very inaccurate. GPS speaks the (over the bottom) truth.<br /><br />You should be getting more rpms with that engine, too. Some fine tuning of your rig and trim could get you more. If that is optimum you might want a shorter prop.

 

[FiveDog]

Re: Boat Speed vs. motor RPM

It's Boston Whaler 16 SL. My thinking was if the load was greater, the engine rpms would be less, but I guess that's not right.<br /><br />My engine performance is whole different matter discussed in the Merc section.<br /><br />Thanks for the replies.