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I just took my new (to me) 77 dimarco with a 454 and a vovlo penta 280 outdrive.the first time I opened it up the boat made a right turn on me at 60 mph.so anytime you jump on it above 40 or so it really wants to turn the outdrive and when you back off at that speed or rpm it wants to go left.anyone ever hear about something like this.it has no power steering and steering cable turns easy...maybe too easy.scared me at first.
Years ago I had a 16' runabout with a volvo outdrive and a 283 Chev. It would pull hard at high speed. I found that there was a small skeg right above the prop that was adjustable. I turned it a little and tried it, helped, I eventually got the boat to run pretty straight by just tuning this way.
Can't swear this will work for you but it solved the prob on my old boat.
Get in contact with a company called Teleflex, they have a non power steering system called anti torque steer If I remember correctly. I installed it on a boat I had and it made all the difference in the world. My boat acted the exact same way as yours. My boat originally came with 170 hp and I upped the hp to close to 400 so the original steering system was not designed to handle the increased torque. I think the complete system including the cable was less than $250.
Hey Tomtuma, I have a 280 outdrive on a 18' Donzi and know exactly what you're talking about. Using a 24" Ultra prop and the original cable setup was hairy once the tach got above 5000 RPM. It wandered all over and was a bear to turn into the prop's torque. I switched to a Seastar helm and 9" unbalanced Hynautic cylinder connected to the tiller arm inside and the change was dramatic. All the slop from the cable and helm vanished, and if you let go of the wheel, it doesn't turn. I use the greater volume side of the cylinder to push against the prop torque.
Teleflex Marine, "anti feed back mechanical steering system". 610-495-7011 very helpfull tech support.
thanks for all of your input.I am still looking into what to do.also the boat is asemi v hull,it has strakes in the hull and the boat has a skag in the middle of the hull.I have been told by a few people that skags should only be on flat bottoms.could this have to do with the problem or you think maybe it is just a steering issue.any help with this is totally appreciated t/y guys.I am running about four hundred horse and wanna make thi boat go fast,but I feel it is actually unsafe to go that fast the way it reacts.