How to fix the "caterpiller"?

[tmarsh]

New here thanks for the add.
I have a 10ft(3m) Mercury SIB, with only an air floor and inflatable keel. My motor is an 8HP Mercury 4 stroke, weighs about 80lbs (36kg).
When I'm on plane the boat acts like a caterpiller. Or a worm. Does the wave. However you want to describe it!
If I sit on the floor I feel the water rushing and it seems to collect the water until it can't anymore, then it does a hop and pukes it out the back. It hops enough to bounce the gas tank up, so it worries me that the transom seams will go and I'll loose my brand new motor.
What can I do to stiffen it up? I inflate it as much as my pump will allow.
It also has a small board in the very front under the floor to push the keel down, but it's only in the front.
Thanks!

[ShinyShoe]

Have you measured the pressure?

Manual pump or electronic?

[tmarsh]

That gauge on my manual pump, which is the one that came with it, reads 5psi (30kPa)

[Yabadabadoo]

What does the instruction manual for the sib recommend? Does it have a 'sausage keel' or an air floor?

[willk]

Originally Posted by tmarsh

That gauge on my manual pump, which is the one that came with it, reads 5psi (30kPa)

I have a 9' Mercury SIB with an airdeck. I recognise the caterpillar.

It's almost entirely a pressure issue. My unit requires approximately 3.5psi in the main tubes, 6.5psi in the sausage keel and a whopping 11psi in the floor. Your pump isn't man enough.

If inflating in the sun, it pays to top up the floor and keel once afloat.

[tmarsh]

Ahh that is good information. I bought it used and don't have the manual, and this information (if it is on there) has weathered off the plaque on the transom. All I can see is the HIN and weight/engine specifications.

My model is an air floor with a sausage keel.

Do you recommend a certain pump? Electric or manual?

[breezeblock]

Originally Posted by tmarsh

Ahh that is good information. I bought it used and don't have the manual, and this information (if it is on there) has weathered off the plaque on the transom. All I can see is the HIN and weight/engine specifications.

My model is an air floor with a sausage keel.

Do you recommend a certain pump? Electric or manual?

replace it with a sheet of plywood problem solved

[Poly]

Originally Posted by breezeblock

replace it with a sheet of plywood problem solved

No need to be so drastic! Many people have reported the hump issue before and most seem to find a solution. Starting with getting the pressure right in the floor and keel. As Willk says a key point seems to be this needs checking once the boat has been placed on the cold water.