Re: Motor Position on Transom
I put a transom in my 16' bayliner, and put the 85hp force back on with a 1/2" spacer to get the cav plate above the bottom at level trim. The issue is you need to have the prop in the water or it blows out when you trim it up and/or turn trimmed up. Now, if you run a bow lifting prop like many bass boat props are, and your boat is not a slug, it will go faster with the motor higher and trimmed more, it lifts the bow more, that type prop lifts the bow more even when surfacing some. You can go higher until it will not lift the bow enough to get your best speed. However a stock type prop will blow out far before that, so the setups are completely different.
I had a 17' open bow vee with a 2" spacer under the motor. I could not trim it much at all with a stock alum prop and it didn't go that fast, rpm was low IIRC 4800. I found a laser prop 1" less pitch that was SS so about the same pitch really, put it on and trimmed way up the boat flew hit 6K rpm and much faster. The boat was setup for that kind of prop. It ran the same speed on the speedo at 4800rpm as the alum prop did.
It depends on the boat too, this bayliner I been running I trim up not quite all the way and it makes no difference after that. The hull will not let it lift the bow more than that; it comes off the strakes and the bow falls, so it is fastest at that point. The trim on the motor does not go as high as other motors I've had, but the exhaust does get a little louder there. Also that is a stock SS MI wheel prop, not a surfacing prop and it will slip on corners so it is high as it can run. I tried a larger bow lifter prop and it would not pull it, but remember my motor was too low for that prop also. I'd like to try a jack on it but requires surgery on the steering I have not gotten to yet. You have to put the height in the range that type of prop needs, then only low enough to lift your bow enough. Also when you do that watch the water pressure you should have a gauge, you can trim up and lift the water intake right out of the water.
The idea with a jack is to give the motor more leverage as well as make height adjustable. So then you don't need as much trim to lift the bow because the motor is further from the transom, as well as the static weight of the motor being further back. Also the boat runs at an attack angle of a few degrees, so you can put the motor higher on a jack just because the water is higher back there due to attack angle.
In the end you need the motor higher and less trimmed to reduce LU drag, but enough in the water and enough lift from prop to get your bow up so you can unwet the bottom and fly (and low enough to cool the motor). That is why you run a jack, the right prop, and right height for your setup as every boat is different and even moving stuff in the boat can change it. In general take everything out of the boat, put only what you need back in and the heaviest stuff you put in the stern and low as you can.
If you get torque steer when trimmed up and high speed, it is because the trim tab (behind the prop) is out of the water. As you go higher with a semi surfacing type prop you will get more torque because the bottom half of the prop is in the water not the top, and it wants to walk sideways. To deal with that you need to put a tab on the skeg where it stays in the water. They make little 'flares' like a wedge that mount on the rear edge of the skeg to push water that way and counter the torque. Some people just put a flat plate above the prop so they still have an anode.
On top of all that your prop needs to be perfect for top speeds, even one little half a dime size dent can take 5mph off the boat.
A hydrofoil should be out of the water when you trim up at planing speeds. It is not safe for it to be in the water at high speeds. This little bayliner will actually holeshot fastest with the trim at WOT height. The bow jumps up and falls, then the prop cavitates a little and it grabs and the bow goes back up repeating the sequence, then it is on plane and gone. The other 17' boat I had I would start with level trim, soon as I hit the throttle down I hit the trim up and in a few seconds the bow dropped and it was on plane and gone. It depends on the weight of the boat and how much power you have.
