97 Nitro 640LX stock mounting height and prop?

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First time boat owner and I picked up a pretty nice 97 640 lx with a Mariner 90 on it. The previous owner had the motor in the bottom mounting hole on the transom and a Laser II 14x20 prop.
When i am by myself and I take off, it has a lot of bow rise, but planes really quick. Runs around 40 mph with full fuel and a light load.
when I have my buddy with me, it won’t plane off unless he sits where the front deck starts, then it planes off and he has to try to get back in the seat. I weigh about 175 and he’s about 300. I’m thinking of putting the motor back to the stock height, but I don’t know what hole they come from factory or what prop size comes on it. Any advice is appreciated
 

byron cooley

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I have an 04' 640LX with tweaked 75hp Optimax. Raise the motor to the middle holes and open prop holes to large plugs.
My stock 75hp turned a 20 laser II to about 5600rpm at 42 43 gps. With mods now turns a 22 laser II to 5700rpm at 49 50 gps with no spray out the side, jumps right up on pad.
 

ronkchar

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This is interesting. My Nitro is a bit older a 91 160TF. It had a 1992 60hp Evinrude that came on it. It struggled a little to get on plane with 2 people. Just a little slow out of the hole with one. Tops out around 5800 rpm and 42mph 1 person and about 37mph 2 and fully loaded. I replaced the outboard with a 2012 Evinrude Etec 75hp. It instantly jumps on plane now even with 2 people fully loaded. However, it tops out at only about 5200rpm and only goes about 38 and 35 respectively. When buying it I thought that it would be faster. I do like the added weight for rough water as my 60hp the boat would be airborne all the time. That being said it almost seems too heavy. When I back up into just a modest chop the wet well instantly fills and the boat starts to take in water. The funny thing is the boat was originally rated for a max 115hp. That seems insane to me.
 

Fishin1.0

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Just my thoughts......Adjust you motor height so that the cavitation plate is in line with the bottom of the hull when trimmed down. I also think your turning too big of a prop..lower the pitch. That 90 should snap that little boat right up on plane even with 2 300 pounders. I had a 2000 Bayliner 1802 with a 2000 90 Merc 2 stroke running a Black Max 19P. It jumped up on plane with no effort and ran 40 mph @ 5700 rpm. I would also check your boat for water logged flotation in the rear.
 
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Just my thoughts......Adjust you motor height so that the cavitation plate is in line with the bottom of the hull when trimmed down. I also think your turning too big of a prop..lower the pitch. That 90 should snap that little boat right up on plane even with 2 300 pounders. I had a 2000 Bayliner 1802 with a 2000 90 Merc 2 stroke running a Black Max 19P. It jumped up on plane with no effort and ran 40 mph @ 5700 rpm. I would also check your boat for water logged flotation in the rear.
some other sites I looked at suggested a 21p stiletto, but I’m not sure what diameter I should be looking at. I’ll try putting it in the 3rd hole on Thursday and see what it does.
 

Fishin1.0

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My thought's....I believe you are running a prop diameter and pitch that is too large for your motor. A 13-13.25 diameter 19 pitch is what your looking for. Check near by prop shops or with someone that has a spare 19P prop for you to test.
 
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My thought's....I believe you are running a prop diameter and pitch that is too large for your motor. A 13-13.25 diameter 19 pitch is what your looking for. Check near by prop shops or with someone that has a spare 19P prop for you to test.
My only real concern is RPM. Right now I’m doing 5500 at WOT. If I raise the motor 2 holes and downsize in prop size, that would put me a few higher 100’s on rpm correct? I think my motor calls for 5000-5500
 
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Well I was wrong. The motor is in the top hole, I took some measurements and the bottom of the pad is 16 inches from ground, the middle of the prop is 9 inches from the ground and the bottom fin is 17 inches from the ground.
 

byron cooley

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Bring it up, 7" below the pad is to low. My 640LX is happy at 4 1/2 -5
 
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I brought it up to the 4th hole and ran her. To high. Couldn’t trim at all without cavitating, so I put it in the 3rd hole. Will try it tomorrow hopefully.
 
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3rd hole (middle) did better and the top speed was 38. I spoke with a mechanic at my local Nitro and he said stock prop is a black max 12.75x23. He recommended a 13.25-13.5x21 prop so I ordered a 13.25x21 Turbo 1 prop.

Here are my current measurements if I did them correctly
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