Over revving and slow planing

[CobiaXL]

My wot is suppose to be 4400-4800 rpm.Rpm's don't go over 4500 while planning,which usually takes 10-15 seconds with 5 adults onboard but soon as I'm on plane the rpm's will build to 5300 with a speed of 31mph.I got a 13.25 x 21 pitch on it now.I can't stay on plane unless engine is turning 4000 rpm's.I was thinking of a 4 blade but unsure what pitch to go with.My marina doesn't have any 13 spline props to even try,so If I buy a prop online,im sure its mine to keep and really would hate to waste the cash..Any suggestion would be greatly appreciated.

By the way were working on a 18' cobia monte carlo 189xl.Power plant is 140hp with the OMC stringer 400.Unsure of weight but would guess around 3000lbs.Wet foam was removed before new deck was installed.Trim tabs are in the near future but for now the budget doesn't allow for them just yet.

 

[SparkieBoat]

Re: Over revving and slow planing

you need trim tabs for sure.

 

[CobiaXL]

Re: Over revving and slow planing

you need trim tabs for sure.

Trim tabs will help with over rev issues?

 

[CobiaXL]

Re: Over revving and slow planing

Smart tabs seem to be in my price range and I like the reviews..I guess we'll try them first then play with prop.

 

[craze1cars]

Re: Over revving and slow planing

Trim tabs will help with over rev issues?

No.

"Over-revving AND slow planing" are generally contradictory things. If you're over-revving, it should be coming out of the hole like a rocket. If it's slow planing, it should be under-revving...

If you say it's doing both, that leads me to believe you have some sort of other problem. Prop ventillating at speed, slipping prop hub or engine coupler, inaccurate tachometer, improper use of power trim, badly chewed up/damaged prop...just drawing straws here.

What is your outdrive gear ratio? With that you can run a prop slip calculation, which might help determine if there is something else going on here...

 

[SparkieBoat]

Re: Over revving and slow planing

actually it makes perfect sense, the reason he is over reving once up on a plane is because his boat is dragging and keeping RPMs low while getting up in a plane, I assume his boat takes a while getting up on a plane, the trim tabs will help the boat get up on a plane much quicker.

 

[steelespike]

Re: Over revving and slow planing

An 18 footer with 140 hp and a 21" prop is highly unlikely to over rev.Also hole shot is likely to be slowish. with 5 aboard.I'm leaning toward a tach problem because it seems like a slipping coupler or hub would let go under the strain of a slow developing hole shot.

 

[craze1cars]

Re: Over revving and slow planing

the trim tabs will help the boat get up on a plane much quicker.

Agreed. But they won't fix the other problem he has yet to identify...

 

[CobiaXL]

Re: Over revving and slow planing

My tach was just replaced in the spring,Also checked with a different tach to verify its correct.Im waiting for my smart tabs to arrive thursday.My boat is very stern heavy so Ill wait till sea trial of trim tabs before I get into the prop.

 

[Outsider]

Re: Over revving and slow planing

Fix your RPM problem then chase after the rest. You may find you'll never jump on plane with the load you're carrying ...