Odd one.....speed 5mph lower but RPM same

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Had my 1989 31 Stamas 20 yrs and with 89 200 Yamahas it has run top speed 39-41 at 5500-5600 rpm. Now 35-36 at same 5500 rpm. Pulled boat cleaned bottom (wasn't bad) but didn't think bottom was issue since before if MPH was down RPM was down at same time. Clean bottom got me maybe 1mph.
So started thinking slipping prop but in past if slipping you really rev up. But tried another set of props (same pitch) and no difference.

Just don't understand why speed down but rpm not......
Any ideas?

[99yam40]

I do not understand what you mean by another set of props.
makes me think more than one prop, guess you have 2 motors
was it used props or brand new ones?

old motors and old props I would think would loose some

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Obviously I have 2 motors on a 31ft boat. Hence yamahas...pleural. Yes just another pair of 13 3/4 17 stainless props in good shape.

[panasonic]

Happen all the sudden or over a long period of time? Sure you not hauling around a bunch of water in the bilge that you can't see? Perhaps a drain plugged up somewhere up foward and can't run back to be pumped out or drained out???

[99yam40]

Happen all the sudden or over a long period of time? Sure you not hauling around a bunch of water in the bilge that you can't see? Perhaps a drain plugged up somewhere up foward and can't run back to be pumped out or drained out???

good point,
does it just take longer to get to that same RPM than it used to??

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Suddenly.... and no water in boat. Weight would lower rpm just like adding passengers. Gets up to rpm just like before but speed just not there...according to GPS. Tried GPS on my map plotter and same mph.

[panasonic]

Odd one. Still have same amount of trim range on engines and trim tabs on hull?? I dont know how that would have changed but can think of nothing else that would happen suddenly???

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Yes trim tabs up and motor trim maximized as I normally do....same rpm on both engines speed just not there.