Re: Porpoise help on 16' Starcraft
It is my opinion that the engine needs to be raised on your transom and by quite a bit. The anti-cavitation plate needs to be about 1" above the keel of your boat to function properly. Here is what is happening to your boat I think:<br /><br />When you are fully trimmed in, the anti-cav plate is forcing the bow down. That is as it should be- however, as you increase your speed, you will trim up, bring the anti-cav plate parallel with the keel. That is giving you a straighter line of thrust and better speed. Then you start trimming up a bit more, you raise the bow and reduce the wetted surface of the boat, decreasing drag and picking up even more speed. The issue I think you have is when you get past parallel with the anti-cav plate. Since it is riding so far down, it is still in the water at speed. When you go past parallel, the front edge of the plate is actually digging into the water, pulling the stern down and raising the bow. Eventually the weight of the bow overcomes the downward pull of the plate and it drops. Then the plate pulls the stern down again and raises the bow. This perpetuates the porpoising and until something changes, it's not going to get any better. Raise that engine until the anti-cav plate is NO LESS than 1/2" above the keel; a full inch would be better. <br /><br /> As far as the cavitation, the transducer wouldn't be causing that on take-off; you're not moving too much yet. However, not being able to see the offset, it could cause you an issue at higher speeds. I do wonder one thing though- in the picture you posted, the prop seems small to me- does the front edge of the barrel of the prop fit well into the lower unit? You say the prop is not drilled- considering it is an aluminum prop and is new I'll guess that there are no vent holes. Does the diameter of the prop barrel match the gearcase? If it is too little, you will vent exhaust through that gap and cause the prop to slip. It's a common thing on bigger engines- some guys will run a 4" barrel prop on a large gearcase engine in order to get ventilation and a better holeshot. (Bassboats specifically.) If your prop is not matched well with your gearcase, you may need a prop with an anti-cavitation ring on it. That's a ring that acts as a "gasket" between the prop barrel and the gearcase. I can't tell from your picture- can you get a close-up shot of the barrel/gearcase fit?<br /><br /> I think that boat should run in the low 40's with a 90hp engine- my 16' SeaNymph ran 34 mph with a 50hp and it was set-up pretty close to yours. The 90 should get you up and running! But, there are a couple things you need to fix first. The #1 is raising that engine. I don't believe you are sucking surface air- I think you are drawing exhaust in and aerating the prop. <br /><br /> There's no reason why that boat shouldn't be a beast when set-up right. The engine may be too heavy for the back of that boat but tabs may be able to help that. Get the basic set-up right first and then go from there. <br /><br />UFM82
