# proper engine mounting hieght



## heavy-chevy (Jul 19, 2008)

i have a 20" shaft and i have read on another forum that the cavitation plate should sit level with the bottom on a john boat, so i built my transom according to this. last time i ran the boat i looked back for the first time at the engine and noticed that the bottom fin runs under the waterline and about an inch or two above it is dragging. now i have read that on a normal boat it should be 1-1.5 inches above the bottom of the boat. i know on any other boat that plate runs above the water line, but i have no previous experience with john boats. so my question is, is a john boat supposed to run this way or do i need to move the engine up more. it runs fine this way, but i was thinking maybe i could pick up a little more speed/fuel mileage.


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## heavy-chevy (Jul 20, 2008)

no one knows? come on someone must know the way it goes with all the john boat owners on here...


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## bassboy1 (Jul 20, 2008)

That motor has low water pickup right? On many of the smaller OMC motors of the '80s, the water pickup was a split one, both on the top and bottom of the lower cav. plate. On these motors, you need to even up the top cav. plate to the hull. However, if you have a low water pickup (which from your pictures, I think it does) you can raise the motor to the point that the bottom cav. plate is even. If your transom jack added any setback, you might can go higher (biggest thing is to keep the water intake submerged), just see if you feel any prop blowout, especially on turns.


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## heavy-chevy (Jul 20, 2008)

the pickup is low enough. it already loses bite if i corner hard or if i accelerate hard through real choppy water, but its not a big deal to me, i just back it down and roll into the throttle again and its fine. but I'm thinking it may get allot worse since its a commercial/high thrust engine so I'm guessing the prop (10x13 cupped) may not be meant to run this way? what do you guys think, would i have problems with excessive prop slip or is my setup good to be run that way?


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## bassboy1 (Jul 20, 2008)

I highly doubt you will have a noticeable difference with 1.5 inches on the rig you are running, and you would lose bite quite a bit more often, so it probably wouldn't be a worthwhile tradeoff. That 1.5 inches would play a major role in speed on a 80+ mph glass rig, but at the speeds you are running, 1.5 inches doesn't play that major of a role. 

Now, as far as your prop goes, that may be where you gain some speed. Measure your RPMs in some way (Tiny Tach - 40 bucks) and you can determine if you are underpropped. If you indeed have a high thrust lower unit, a 13 is probably a bit light, especially seeing that you have it on such a light boat.


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## heavy-chevy (Jul 20, 2008)

yea you make a good point, i should just leave it alone. my transom is already built up 5", any higher will just put even more stress on it. I actually just installed a tach and speedo for that reason but the last time i ran it i had the tach on the wrong setting so my reading was way off all day, i will have to see what im running next time. also would you know what pulse ignition my 87 would have, because i had it set on 5 pulse, and i put it to i think 4 pulse and it seemed rite when i rev it in neutral.


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## heavy-chevy (Jul 22, 2008)

alright well i measured and it actualy sits an inch bleow the bottom, i guess my transom is a little short for even a 15" shaft engine. and im definitely under proped, its spins 5800+ at wot. maybe il will move it up an inch, it just seems wrong to run it like this.


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## minicuda (Jul 22, 2008)

i would also like clarification on this topic. i just purchased a g3 1648lw with a 25 horse nissan the dealership said i should build the transome up so the cavitation plate sits 2 inches below the bottom of the boat


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## heavy-chevy (Jul 22, 2008)

damn how low does it sit if you need to build it up to 2" below. and to clarify i was getting 5800 on a leaky fuel line. it should spin 6000 when i fix it, it surged to 5900 a few times. anyone know what rpm i should be running wot on this engine, i cant find this info anywhere.


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## minicuda (Jul 22, 2008)

heavy-chevy said:


> damn how low does it sit if you need to build it up to 2" below. and to clarify i was getting 5800 on a leaky fuel line. it should spin 6000 when i fix it, it surged to 5900 a few times. anyone know what rpm i should be running wot on this engine, i cant find this info anywhere.


i had to build it up 2.5 to get it to sit 2 below the boat


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## heavy-chevy (Jul 22, 2008)

and its supposed to run 2 below? so some boats are actually meant to run on the second plate up huh.... maybe i will just leave IY alone, or should I? now im confused i get a different standard on how it should be with everything i read....


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## minicuda (Jul 23, 2008)

heavy-chevy said:


> and its supposed to run 2 below? so some boats are actually meant to run on the second plate up huh.... maybe i will just leave IY alone, or should I? now im confused i get a different standard on how it should be with everything i read....



i have no idea but i need to know. thats just what the sales guy at the boat stealership told me


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## bassboy1 (Jul 23, 2008)

heavy-chevy said:


> it should spin 6000 when i fix it,


Wow. The WOT RPM of that engine is 5000. I think every inch of pitch will change your RPMs by about 200. Now, I don't know if they even make a 10 x 18 prop, as you have such a special demand. First, you have an excess of motor on such a small boat, which isn't providing much drag, so you would need a larger pitch anyway. But, your high thrust lower end changes the gearing a bit. It slows it down, so a small pitch prop can push a real heavy pontoon or work boat at low speeds. So, two identical boats with the same engine, only one having a normal lower unit, and one having the high thrust unit, will need different props. The high thrust one will need extra pitch to make up for the fact that it turns slower. Check around and see, cause I am not sure that a 10 x 18 exists (haven't looked into that yet). You should, however, see a significant speed increase. 

As far as raising it more, every boat is going to be different. All hulls are going to act differently, so some might need it deeper to avoid constant blowout, but others that have hulls with pads, can raise it a few inches above, provided there is setback. Since you said that you were having blowout every now and then, I wouldn't change it. And again, your speed is not going to be as affected by a deeper/shallower lower unit, as would a bass rocket. Change your prop, and see what it does to ya.


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## heavy-chevy (Jul 23, 2008)

yea i don't think they pitch em that deep... i will probably have to move up to a larger diameter prop. omc sells a 10x17, that would be pretty close. but then there 11 inch props only go to like 7 or 8 pitch i think. is there anyway to change the gearing without going to the smaller lower unit? i still need to run it some more to get a true wot rpm before i decide on a prop, i still haven't been able to get wot with one person in it. this is like my last project all over again, to much engine not enough gear, i just gave up on that one, too much money, i let her scream 8-9k all day lol...

also the tach is on 4 pulse, is this right?


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## bassboy1 (Jul 23, 2008)

I think you could change it by replacing the pinion gear, the two drive gears, and maybe some bushings and thrust washers. But, I estimate that those will come to about 300, and I am not guaranteeing that all is the same with the rest of the parts.

I just checked Michigan wheel, and in the aluminum series, they don't carry anything with that much pitch. The 10 x 17 may be your best bet, with some changes elsewhere, although I don't know of what other changes you would use, aside from never opening it all the way (although I feel much safer running it at 5200 than the rod bending 6000 you mentioned)


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## heavy-chevy (Jul 23, 2008)

yea, its probably been run at that rpm for at least 3-4 hours of run time. last weekend i put about 150 miles on her running wot most of the time, but with 3 people so slightly less rpm. also i wanted to put a 30hp carb on it but then i would be right back where i started with the 10x17 and no bigger props for that lower unit. they sell an 11x9, maybe i could get that and have a shop pitch it a little more, but i don't know how big a difference 1" larger diameter makes... also does anyone know exactly what gear ratio my lower unit is? thanks for all the help so far.


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