Motor height?

Recently put a Cook's CMC PT-130 power trim on my old rig. This set the engine back 6 inches. Its an Evinrude 55hp on a 16' true flat bottom aluminum boat. Cook's instructions say to mount the engine with the anti-vent plate 1-3 inches above the bottom. Originally I was at 3/4" below the bottom. I mounted two holes higher and that put me at 3/4" above the bottom. I gained top end speed probably simply because I can once again trim the motor (factory trim went out about 5 years ago and replacement not available). However I can now trim the engine to a point the prop vents and the motor over-revs, never could do that before, boat would porpoise first. And the hole shot now stinks. Trying to figure if I should lower the motor back to where it was. What is the generally excepted proper height of the anti-vent plate to the bottom on a true flat bottom boat?

For performance you want to continue going up until your speed no longer increases and your RPMs continue to rise. At that point go back down to your last position. What you describe as issues are easily correctable with propeller work. Chances are you are running a standard aluminum propeller. If so, switch into a stainless prop. At that point try going higher and then have additional cup added to the propeller and have the cup brought around the leading edge. These modifications will allow the prop to hook up at the higher transom height and give you the performance you are looking for.
You mention you lost hole shot with your changes. Is the prop blowing out and ventilating? If not there is really no good reason for additional engine height to slow your hole shot. Usually the opposite rings true every time.

No, if anything I am not turning up enough RPM. At WOT I am only turning 5500 of 6000 max for this engine. I am running a 17 pitch SST stainless prop. I picked up a 15 today on loan to try and see if it will get it out of the hole better. May loose a couple MPH on top end but that isn't priority to me. Getting on plane fast and running shallow is what I need in the areas I fish. Leary of going any higher though because I don't have a water pressure gauge and will not be able to tell if pressure drops. Center of prop shaft is 5 1/2 inches below boat bottom now and the water pickups are above that. I suspect the boat no longer porposes because of the increased leverage of the 6 inch setback. Just going to have to learn not to trim it out at WOT to the point it vents and blows out. Engine has a hydrofoil on it. it may be that it has been over prop'ed all along and the hydrofoil was geting it on plane. Raising the motor may have reduced the effectivness of the hydrofoil.

Just my opinion, but IMHO ... hydrofoils slow boats down; raise engines until they are actually too high; (they have no method of stopping the rise when under way) put stress on the motor's anti-cav fin; and might not be needed once you can tilt/trim properly.

Tabs (any tabs) have all of the advantages of foils, with few of the dis-advantages.

Why not pull the hydrofoil off and see what happens?

Around here, ( Texas Gulf Coast) the accepted numbers are....you can raise your engine one inch for every two inches of set-back. Lots of guys (including me ) have done it and have been pleased with the over-all performance.

I am fortunate that, with my present 160W Lowe, I need neither tabs nor a foil. The engine's tilt/trim works fine. R

At 5.5" below you are in no danger whatsoever of losing cooling. For future reference though, trim the engine to where the cavitation plate is parallel to the bottom and take a measurement from the plate to the bottom. Using this I will tell you that you are in no danger of losing cooling if you bring the cav plate up around 3" minimum above the bottom. That is what you bought the plate for, correct?? You will then be able to utilize the 17" prop. Your RPMs for that engine aren't too bad and you will or should be up in the RPM range when you do this. You can then start to modify your 17 SST to stay hooked up. For hole shot you can also drill the hub and vent the prop. This works wonders for hole shot. If you do what is asked you will begin to see the results. May do you well when you are leary of information gathered here to start reading the profiles of those who are trying to help you! Some of us have "been there, done that". The whale tail you have on there at the height you are running is definitely creating parasitic drag and will be totally submerged at your present height.

Skeeterbait said:

Getting on plane fast and running shallow is what I need in the areas I fish.

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4 blade stainless with some cupping in the blades' trailing edge. Keep raising it until water pressure falls off, drop it back down a little and you're good to go. The 4 blade gets out of the hole quicker, stays on plane at a lower RPM and speed, hooks better on higher engine mounting heights and is a little mroe suitable to surface operation than a 3 blade. The cupping will help hold water when the prop blades are slightly out of the water. I found that when switching to a 4 blade (vs 3) that I was able to trim the motor further out before goign into porpoising. Also the stern stays shallower when on plane. But a lot of that is built into the prop design. Might be worth a call to a good prop guy and stress to them that you're looking to get up quick and run as shallow as possible. They'll probably set you up with a SS 4 bld 15 or 16" pitch with a little less agressive blade angle, and some cupping on the tips. With the extra blade, you'll lose a couple hundred RPM at the most if staying with the same pitch on small motors.

Hydrofoils can't slow you down if you're on plane. If the motor is mounted properly, the hydrofoil is not in the water. I pulled mine off of my bass boat and noticed zero speed difference. But it's mroe stable at lower speeds and stays on plane all the way down to about 2600-2700 RPM and 8-10 mph. Good for running shallow lakes with lots of trees where I can't go fast, but can't go so slow that the leg is dragging in the mud. I put my hydrofoil back on just for those reasons.

Is the 55hp a tripple or twin, what year? Reason for asking is the prop hub diameter/prop diameter size. What angle is the motor when trimmed fully down? A picture would help.

I have a cmc pt-130 on my 1448 MV jon with a modified 50 (twin)small hub prop. Found the optimal spot with anti-vent plate 2 1/4 above bottom. Have a lot of cupping on the prop (cupping will increase the prop pitch) so I am able to run motor higher. Only cooling problem I have had is running slow with motor tilted up for very shallow water.

For hole shot you can also drill the hub and vent the prop.

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Just to add what Pappy mentioned, hole diameter and location very important. I have used removable plugs for tuning, drill to large,get a new plug and start small again and work your way back up.

These small jon boats are temperamental with getting them dialed in. Weight displacement plays a big, big part.