how high is outbord supposed to be

[riddo60]

riddo60 here i have a 5.2 metre hains hunter hydrofoil approx 30 years old great stable boat, it has a 75 hp merc on it, at wot it tends to porpose, ive measured it appears the cav plate is above the bottom of the transom, i can lower it one hole will that help it to stop the prob,:-[ any advise will be appereciated cheers riddo

[Horse]

My Oceaneer is a pain in the but without a foil for porpoising. Once fitted it never played up. I even managed to lift the motor another notch

[johncar]

Hi riddo,
Usually more of a problem when the engine is too low, so if porpoising it can often help to raise the engine.
I tend to use a rule of thumb, set a long straight edge on the lowest part of the main planing surface. Trim the engine to it's cruising position then measure the distance between the straight edge and the cav plate. As a starting point approx 1" higher for every foot behind the transom.

So if the front of your leg is about 1' behind the transom set the cav plate 1" above the lowest planing surface.

Then its a water test to see how it goes and depending on the hull design and leg design and prop choice you may have to go lower or higher.
Ultimately the cav plate should be just skimming the surface at speed and trimmed out, but be extremely careful when visually checking this.It can take a little effort to get it right but it will be worth the trouble.

[riddo60]

thanks horse, i have a good foil on the engine, i think ill drop one hole and see what happens thanks riddo

[riddo60]

thanks johncar, i will do what you suggest, and try again hopefully i will fix the prob cheers riddo

[Mister]

What's a - hains hunter hydrofoil?

[riddo60]

the boat i have is a 5.2 metre,hydrolift hains hunter pretty rare boat, ive gone into haines sight looked everywhere, unable to find any info, its a great fishing boat stable big casting deck that i fitted, twin console,lots of room the bloke i bought it off tells me the boat is about thirty years old i will post some photos, i would realy like more info if someone has the same vessel it looks a bit like a whaler cheers riddo

[finga]

Before making any alteration check the engine height whilst it's on the water.
Take the foil off and look down at the cavitation plate whilst the boat is planing.
The Cavitation plate should be a blonde micky hair above the flow of water.
Until the cavitation plate is here all else is a wast of time.
After the plate is in position try the trim positions..
Has the motor got hydraulic trim or a trim pin or both?
Some early Mercs just had hydraulic tilt and not the trim or the hydraulics now leak making the trim function inoperative which effectively makes the trim pin the only trim adjuster.
The Mercs with the hydraulic pump/tank mounted remotely were prone to this problem.
What model Merc is it??

[Andy56]

I'm with finga, cav plate height first point of call. Lining up the cav plate with the bottom of the boat is the START of your line up because it is different for all boats , some want to go higher and some lower. The only thing for sure is you have to experiment. ( mine is 3/4" out of water and 2" above the bottom of the hull)

[Noelm]

Not too sure why you say you will drop the engine down, when it has been stated that it will increase the chance of porpoising!!! as mentioned, take the foil off and see what is going on first. The higher you can run your engine the better (sort of)

[Mister]

Foil? Did you fit this, Why? Or did somebody else make this basic mistake?