Boat lacking in performance

[mike rousseau]

Heres the info...

 

It's a 17 foot Lund mr pike

90hp yammy 2 stroke

 

On plane it seems to be nose diving...

 

If he trims up to try and lift the nose he ends up blowing out...

 

On plane it seems like his cavitation plate is 1-2 inches under the surface...

 

Top speed is around 25mph

 

Rpms are 5200-5300 at WOT

 

 

What do you think is wrong?

 

I think his motor is mounted too low... I thought the cavitation plate was supposed to be out of the water on plane... Or just skimming the surface...

[DRIFTER_016]

Heres the info...

 

It's a 17 foot Lund mr pike

90hp yammy 2 stroke

 

On plane it seems to be nose diving...

 

If he trims up to try and lift the nose he ends up blowing out...

 

On plane it seems like his cavitation plate is 1-2 inches under the surface...

 

Top speed is around 25mph

 

Rpms are 5200-5300 at WOT

 

 

What do you think is wrong?

 

I think his motor is mounted too low... I thought the cavitation plate was supposed to be out of the water on plane... Or just skimming the surface...

 

Look at his boat from the back while it is on the trailer.

With the motor down the cavitation plate should be even to about 1" below the bottom of the hull.

If it's blowing out while trimming the motor up it sounds like the motor is either mounted too high and you can't get enough trim to lift the bow or it's mounted too low and you have to trim up too high to try and lift the nose.

 

I's also possible the hull has a hook in it just forward of the transom or there are improperly adjusted trim tabs on the boat if it has them. Both these scenarios will act to keep the nose of the boat planted.

 

That boat should also be running in the mid 30's at WOT with that motor and mid 20's at cruise so there is definately an issue there somewhere.

[mike rousseau]

As far as I know the hull is ok

 

And the motor is sitting on the transom... Cannot be lowered...

 

That's why I figure it's too low...

[beagle dad]

had the same issue..the motor needs to be lower yes 1" below boat bottom

transfer as much weight to rear of boat as possible...batteries etc

lastly i added fins to cavitation plate making it larger and helps collect water for prop

 

good luck

[beagle dad]

forgot as well torque from my 90 causes the hull to flex in front of the transom....works like a suction cup not letting nose of boat rise.... went to 70 hp fixed mine...flex part came right from lund.factoery rep

[ecmilley]

before all of that make sure the prop hub isn't spun, had this issue when demand is put on the prop it'll blow out.

sounds like the motor requires lowering though, has this been a non-stop issue or did it just start?

[Rizzo]

I don't know what you mean by "blowing out" but I spun the hub in my prop a few years back. Once I got to a certain speed the motor would just scream and I wouldn't go any faster.

[davey buoy]

I don't know what you mean by "blowing out" but I spun the hub in my prop a few years back. Once I got to a certain speed the motor would just scream and I wouldn't go any faster.

 

I'm not sure either but if the hub spun,wouldn't his rpm's hit the roof?. his are where they should be at wot.

[Rizzo]

I'm not sure either but if the hub spun,wouldn't his rpm's hit the roof?. his are where they should be at wot.

 

you're right, missed that part. Don't think you'd get over 20 mph either with a spun hub

[mike rousseau]

The way he described the "blow out" I thought he meant tge prop would catch air and the motor would rev really hight...

 

But now that you mention it the hub sounds about right...

 

He will give it gas... Plane out... Then when he trims up to raise the nose for the proper ride... He says the RPMs go through the roof and he's not getting propulsion....