Nitro 640LX struggling to get on plane. Help!!!

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Aug 25, 2023
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At the beginning of the year, I bought a project boat for a steel. It was a 1999 Nitro 640 with a 1995 Mercury 75HP ELPTO 2-Stroke. Obviously this is not the stock motor. I am having issues getting up on plane.

A little history on the boat... I bought this from a guy who only owned it over the winter. He said he bought the boat as a project, but I believe he only wanted the original motor, a 60hp 4-stoke. So he swapped that motor with his crestliner which had a 1995 Merc 75ELPTO. He never took the boat on the water and he was the 2nd owner, making me the 3rd.

In my opinion, this is a upgrade for the nitro. More power and a lighter motor. The Nitro seems to be a little stern heavy. The problem was, it seemed to sit way to low in the water, which gave me reason to believe that the shaft on this motor was 5 inches longer than it needed to be. The motor he put on it also had a 15 pitch prop. I replaced that immediately with a 13.2x21 Solas Amita 3 blade prop as I did not want to over speed the motor.

The first time I ran it, it took about 8 seconds to get on plane with about 5 gal of gas and just me. I also noticed a twisting motion of the boat which got slightly better after getting on plane.Fast forward a bit, I had 2 of my buddies out to check out the new boat. I could not get it on plane unless one of them sat in the front and even then, it took almost half a minute to get on plane. Once on plane she does about 45mph gps @ 5100RPM with just me and 42 mph with a single buddy. My buddy and I's combines weight is about 350lbs.

Back to my original assumption of the motor being to low, I decided to buy a 5.5 inch CMC jackplate. The reasons I thought the motor was to low is as follows; The motor could not be completely trimmed down on the trailer, I could trim up to the max without ever cavitating, and it had a poor hole shot. After installing the jackplate, I set the motor so that the cavitation plate is about 1.5" above the back of the hull. This is just to the point where I can trim all the way down on the trailer. I used a level to ensure my measurements where parallel to the hull.

Took her out again and the hole shot is 1 second worse and she now runs a top speed of 42mph, but I got rid of the twisting motion on the boat. My next plan of action is to lower the motor 0.5 inches until I find a happy medium. My question is, does anybody think it would be beneficial to change the prop to a 4 blade? I was thinking the Spitfire X7 13.2x19p. That might be to aggressive, but I feel that the 17p wouldn't be enough. I was also thinking of adding a hydrofoil.

Let me know your options or if anybody has any experience with these 640s.

As a side note, another thing I have been experiencing is the steering wheel wanting to pull aggressively to the right. This wasn't the case before as the steering cable was coming out of the back of the boat at a sharp 90° angle. I believe this is also a proponent of the wrong engine being installed on the boat. I drilled a hole so the cable wasn't at such an aggressive angle. I also installed a boot over the hole and a cable buddy to lubricate the cable. The steering is nice and loose now, but it wants to steer aggressively to the right. I've also got the trim tab pegged to the starboard (right) side. 'm thinking about getting a non feedback steering system to compensate, bit of there are other ideas, I am open to suggestions.

Thanks and sorry for such a long post. Cheers :)
Most likely the prop is too far below the boat. A good starting number is centerline of the propeller 3 1/2 inches below the bottom of the the lowest part of the transom of the boat. Any lower/deeper an the engine will lift the front of the boat too high and it will not plane without weight on the front of the boat like you described.
I have a 1997 bass boat 70 hp Merc , when I bought it there was a hydrofoil on it. Out of curiosity my buddy and I removed it to see what the difference would be. With out it the bow went straight up could barely get her to plane out. With the hydrofoil she jumped right out of the hole , never thought it would make that much of a difference. Fished out of her for 20 years very reliable
I'll just add, of moving the motor doesn't find a good spot, Ive used a hydrofoil before too, and it worked very well. I've also had a non power steering boat that pulled hard, but moving the trim tab fixed it. If you've moved it as far as you can, and it still torqures bad, probably just the motor being larger than the original. Power steering is awfully nice : )
I had a similar problem with my NX750 opti 115. I had an older 22P laser II with the small square vent holes and it was a dog trying to get up on plane. I recently picked up a newer 22P laser II with the PVS large round vent holes. The difference was night and day, it allowed the prop to spool up much faster thus getting me on plane even with gas and a full live well.
I had a 1995 170DC. 115 on it. Wouldnt plane out very well. Those motors are set "in" to the boat and dont "hang off" the transom, like what a jack plate does for you. I ended up puting a hydrofoil on and helped alot. Dropping 1-2 pitch can help hole shot but lose tpo end.

Good luck.
I had a similar problem with a 929cdx 225hp opti and our solution was removing the plugs on the prop as I recall we tried 3 different size plugs and ended up leaving out one or two plugs which allowed the motor to rev quicker and improve the hole shot.

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