Engine height question on 16ft Jon Boat

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So i purchased and installed a CMC PT-35 on my 1972 Polarcraft jon boat. The motor is a 1981 Evinrude 35HP model # E35RC1G and I am struggling a bit with the engine height. The manual on the PT-35 says to mount motor with the cavitation plate 1" to 3" above the bottom of the hull. I mounted the PT-35 as high as i could on the transom and at that position the cav plate is about even with the hull. I test ran it at this position and it goes about the same speed as it did before i added the trim kit, but the odd thing was this motor has like a second cav plate located a few inches higher than the true cav plate. At this position the 2nd cav plate was getting water over the top of it and spraying quite a bit, so i tried raising the motor. This motor only has clamps and 1 bolt hole on each side, so i used a shim to raise it about 1 3/8" which is about max that i could do while still using the clamps. I ran it last night and at this position i lost a little speed and RPM, it felt like it wasn't getting the nose up enough. I also noticed i could see the true cav plate and looked like water was flowing good into it, however the 2nd cav plate was having water hit the bottom surface of it and it was sort of spraying it down and outward.

I think the sweet spot may be between these two positions, i'm thinking i want the water just kissing the upper cav plate? Thoughts?
 

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You want to mount the motor as high as possible without it blowing out. The lower anti-ventilation plate is the one you go by. It should be level with the bottom of the boat or at the most 1" above. I doubt you will get it much higher without it blowing out. When the motor blows out you will notice and increase in RPM and the boat will slow down. If it slowed after raising it then you may have been on the verge of it blowing out or you may just need to adjust the trim a little, you may need to trim the motor down slightly when you raise it.

Use a straight edge on the bottom of the boat to determine how high the motor is mounted, your eye will deceive you.
 
surfman said:
You want to mount the motor as high as possible without it blowing out. The lower anti-ventilation plate is the one you go by. It should be level with the bottom of the boat or at the most 1" above. I doubt you will get it much higher without it blowing out. When the motor blows out you will notice and increase in RPM and the boat will slow down. If it slowed after raising it then you may have been on the verge of it blowing out or you may just need to adjust the trim a little, you may need to trim the motor down slightly when you raise it.

Use a straight edge on the bottom of the boat to determine how high the motor is mounted, your eye will deceive you.

With the jackplate he added he needs the motor to be higher due to the rearward offset of the tilt/jack unit.

Im currently going through this process with a similar setup and with the motor mounted directly to the transom Im probably going to wind up with the anti cav plate 1.5-2" above the bottom of the hull.


And yes, Im also trying to get rid of the flying V rooster tail of shame. :oops:

What kind of speed are you getting?


ETA: I just looked at your pics again, you may have the jackplate mounted too low.
 
I'm trying to tinker on this exact issue. My boat has a Jack plate AND the pt130 trim so the setback is near 10 inches! I worry the rooster tail that occurs at some speeds is because of the two large adjustment nuts on the bottom of the jack plate.
b38dc1eff4e4754c51a85abefb3734a6.jpg
ee8c387d07afedf340c4f24b96a14a0b.jpg


Sent from my SM-G973U using Tapatalk

 
thedude said:
I'm trying to tinker on this exact issue. My boat has a Jack plate AND the pt130 trim so the setback is near 10 inches! I worry the rooster tail that occurs at some speeds is because of the two large adjustment nuts on the bottom of the jack plate.
b38dc1eff4e4754c51a85abefb3734a6.jpg
ee8c387d07afedf340c4f24b96a14a0b.jpg


Sent from my SM-G973U using Tapatalk

I think you may be right.
 
RaisedByWolves said:
surfman said:
You want to mount the motor as high as possible without it blowing out. The lower anti-ventilation plate is the one you go by. It should be level with the bottom of the boat or at the most 1" above. I doubt you will get it much higher without it blowing out. When the motor blows out you will notice and increase in RPM and the boat will slow down. If it slowed after raising it then you may have been on the verge of it blowing out or you may just need to adjust the trim a little, you may need to trim the motor down slightly when you raise it.

Use a straight edge on the bottom of the boat to determine how high the motor is mounted, your eye will deceive you.

With the jackplate he added he needs the motor to be higher due to the rearward offset of the tilt/jack unit.

Im currently going through this process with a similar setup and with the motor mounted directly to the transom Im probably going to wind up with the anti cav plate 1.5-2" above the bottom of the hull.


And yes, Im also trying to get rid of the flying V rooster tail of shame. :oops:

What kind of speed are you getting?


ETA: I just looked at your pics again, you may have the jackplate mounted too low.

I have tried it 1.5" above the bottom of the hull and I loose speed. I lowered it to about 0.75" above the hull and i'm getting about the same speed i got with out the trim kit, which is about 25 to 26mph with a 10.5x11 aluminum solas prop. I have a lot of weight in the front of my boat due to a big plywood casting deck, trollin gmotor, tackle, ect. I'm sure if i distributed the weight back a little more it would help. I turn about 5400 rpm. I am thinking about a SS prop in a 12 or 13 pitch. I have a 13 pitch aluminum prop that i could only turn about 4700 rpm. I have read that some guys will go up a pitch or two with a stainless.
 
For not much money you could try the motor higher but add an aftermarket hydrofoil to prevent that blowout. Does it plane quickly?

And a stainless prop would be nice but may not be the answer.

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thedude said:
I'm trying to tinker on this exact issue. My boat has a Jack plate AND the pt130 trim so the setback is near 10 inches! I worry the rooster tail that occurs at some speeds is because of the two large adjustment nuts on the bottom of the jack plate.
b38dc1eff4e4754c51a85abefb3734a6.jpg
ee8c387d07afedf340c4f24b96a14a0b.jpg


Sent from my SM-G973U using Tapatalk


I would change those out with stainless button head cap screws regardless.
 
Taxin_That_Bass said:
RaisedByWolves said:
surfman said:
You want to mount the motor as high as possible without it blowing out. The lower anti-ventilation plate is the one you go by. It should be level with the bottom of the boat or at the most 1" above. I doubt you will get it much higher without it blowing out. When the motor blows out you will notice and increase in RPM and the boat will slow down. If it slowed after raising it then you may have been on the verge of it blowing out or you may just need to adjust the trim a little, you may need to trim the motor down slightly when you raise it.

Use a straight edge on the bottom of the boat to determine how high the motor is mounted, your eye will deceive you.

With the jackplate he added he needs the motor to be higher due to the rearward offset of the tilt/jack unit.

Im currently going through this process with a similar setup and with the motor mounted directly to the transom Im probably going to wind up with the anti cav plate 1.5-2" above the bottom of the hull.


And yes, Im also trying to get rid of the flying V rooster tail of shame. :oops:

What kind of speed are you getting?


ETA: I just looked at your pics again, you may have the jackplate mounted too low.

I have tried it 1.5" above the bottom of the hull and I loose speed. I lowered it to about 0.75" above the hull and i'm getting about the same speed i got with out the trim kit, which is about 25 to 26mph with a 10.5x11 aluminum solas prop. I have a lot of weight in the front of my boat due to a big plywood casting deck, trollin gmotor, tackle, ect. I'm sure if i distributed the weight back a little more it would help. I turn about 5400 rpm. I am thinking about a SS prop in a 12 or 13 pitch. I have a 13 pitch aluminum prop that i could only turn about 4700 rpm. I have read that some guys will go up a pitch or two with a stainless.
Didn’t realize you were running a 11p.

Put the 13 on and raise it back up. That will let the prop unload at speed and bring your RPM/speed up.

Putting the same pitch stainless on will “Be” like going up in pitch.

If you can’t swing the 13p aluminum prop you certainly won’t be able to swing that pitch in stainless unless you raise the motor way up. 2” maybe, but your hole shot will suffer with an SS even more than with the 13p Alu.
 
Interesting comments.

When i had the motor raised up as high as i could (cav plate about 1.5" above the bottom of the hull) i could only turn about 5200 rpm and i lost a little speed (24.5 mph), hole shot seemed not as good but i wasn't blowing out from what i could tell.

I lowered it to about 3/4 inch above the bottom of the hull and I gain rpm to 5400 and a little speed (25.5). When i had the motor mounted without the trim plate and about an inch below the bottom of the hull i would get about 5400 rpm and 25.5 mph. so far the set back and raising the motor hasn't helped my speed situation.

pertaining to stainless props, i have read threads on here where guys went to a stainless and had to go up a pitch or two and go significant performance gains. I will see if i can dig up the thread
 
RaisedByWolves said:
thedude said:
I'm trying to tinker on this exact issue. My boat has a Jack plate AND the pt130 trim so the setback is near 10 inches! I worry the rooster tail that occurs at some speeds is because of the two large adjustment nuts on the bottom of the jack plate.
b38dc1eff4e4754c51a85abefb3734a6.jpg
ee8c387d07afedf340c4f24b96a14a0b.jpg


Sent from my SM-G973U using Tapatalk


I would change those out with stainless button head cap screws regardless.
I'm thinking about swapping the jack plate and the trim unit. I bought it like this and never really loved the setup. But 30 mph is hard to beat.
258ee0fc14b432b89967e356e2487033.jpg


Sent from my SM-G973U using Tapatalk

 
thedude said:
RaisedByWolves said:
thedude said:
I'm trying to tinker on this exact issue. My boat has a Jack plate AND the pt130 trim so the setback is near 10 inches! I worry the rooster tail that occurs at some speeds is because of the two large adjustment nuts on the bottom of the jack plate.
b38dc1eff4e4754c51a85abefb3734a6.jpg
ee8c387d07afedf340c4f24b96a14a0b.jpg


Sent from my SM-G973U using Tapatalk


I would change those out with stainless button head cap screws regardless.
I'm thinking about swapping the jack plate and the trim unit. I bought it like this and never really loved the setup. But 30 mph is hard to beat.
258ee0fc14b432b89967e356e2487033.jpg


Sent from my SM-G973U using Tapatalk

Let me know if you want tp part with it. :wink:
 

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