Speed V RPM Drop off

[Trojan]

Maybe someone can explane what is going on. I start at 0 MPH throttle up to WOT 22 MPH at 2650 RPM. I back off to 2200 RPM 16 MPH, Then the boat starts to slow to 13 MPH at 2200 RPM dropping off plane.Increasing back up to 2650 RPM. the boat will not return to plane. Returning the boat to 0 and starting over the boat repeats. What is going on. Why won't it return to plane? Bill

 

[SKYCHENEY]

Do you have a clutch slipping? I'm no transmission expert, but if the RPMS are the same and the speed is less, I would look at the transmissions.

 

[Nonchalant1]

Is this something new that has just started happening? Does it happen consistently in all waves and current conditions? Do the gear oil pressures read the same the first time as they do the following times, or is there a difference after it heats up? If all the readings are the same the 2nd time you go to 2650 but it won't plane, that's very strange, particularly if it'll plane if you start from zero again.

How's this for a wierd scenario: Your trim tabs hydraulics lose pressure slowly and can't hold the position you have them set for. When you are at zero, they can go to the correct position because there's no force against them. Then when you get underway, one or both slowly move back up and you drop off plane and can't get it back up while underway because the trim tabs are now set too level so you're nose up, even at 2650. If you go back to idle, the trim tabs can reset slowly to the correct position so it's now ready to repeat.

If you have stabilizers and one trim tabs goes up, the stabilizers will fight to offset that producing drag and worsen the effect.

Doug Shuman

 

[Trojan]

I thought it was the tabs also but it is not. I don't know what is going on. I'm going to change the props tomarrow. I think ( guessing )there might be something wrong with a prop. Causing air bubbles to develop around it. Loosing thrust. Bill

 

[Passages]

I'd be surprised if the prop has anything to do with this unless it is damaged. Cavitation in our size boats is usually only seen in really nasty seas.

I'd put my bet on something in the tranny. Confirm oil level, pressure then have someone in the bilge while underway try to listen for odd noises such as clutch slippage.

 

[DLCameron2]

What is the water depth where you are doing your trials Bill? These hulls planing characteristics are affected by water depth, i.e. shallow water = hard to plane.

DC

 

[Genesis]

Checking gear slippage is easy - get out your photo tach, check the engine speed, then check the shaft speed. If its slipping it will be instantly obvious.