Boat "bobs" while cruising - why?

[Case450]

I have a 16' mirrocraft (it's old) that was a cruiser but i cut the top of the front off and turned it into a tiller. It's a great boat - awesome actually - however now with my 40 hp mariner longshaft on it it tends to bob when i get up to speed.
My transom is rated for 75 hp according to the tag on it and i've rebuilt the transom so it's strong now.
The mariner longshaft might be sitting either too high or too low in the water mounted on the boat.....i'm am unsure of the right height the motor should be mounted. Could this lead to the "bobbing" effect?
My boat is for a 20" shaft outboard and my merc previously mounted was fine. But when i got this mariner it was too low in the water and caused all kinds of backsplash. So i inserted an 1.5" aluminum tube on top of the transom to raise the height of the motor and it stopped the backsplashing.
Any suggestions?

 

[saumon]

Re: Boat "bobs" while cruising - why?

Where is the cavitation plate in relation to the bottom of the transom? You want it even or 1'' above the lowest portion of the hull.

 

[Case450]

Re: Boat "bobs" while cruising - why?

i am unsure what a cavitation plate is but i lined up one of the flat "fins" on the outboard with the bottom of the boat....really by eye to be honest....is this adjustment (or lack thereof in my case) what can cause or relieve the bobbing?

 

[tmcalavy]

Re: Boat "bobs" while cruising - why?

Do you mean porpoising...rising up and down like a dolphin? If so, you need to move some weight forward...shift the balance of your load and adjust the trim of the motor. Tucking the motor right down against the transom pushes the bow/nose down. Setting a notch or three out from the transom pushes the bow up.