Outboard Mounting height

[jonj200369]

Hey everybody, I have a question. My old boats transom was getting pretty bad so I decided to buy a used hull and swap over all my running gear.
When I mounted the outboard (1986 75hp mariner 3cyl) I was not sure exactly how high to mount it on the transom, so I took a guess and went for it.

I took the boat out to the lake and tried to get her going to see how it was. The first thing I noticed was that I was getting major prop wash (some water coming back into the transom). The second was that the bow was riding too high. when I used the power tilt and raised the motor a small amount and the boat began to porpoise quite badly.

I am assuming that the height is the cause of my problems. the cavitation plate is about 5-6 inches below the transoms lowest point, I think it should be higher?
would a hydrofoil help?

Thank you all

Jon

 

[Miroash]

Re: Outboard Mounting height

I have always been told to make the cavitation plate level with the very bottom of the transom +/- a few inches

 

[hwsiii]

Re: Outboard Mounting height

Jon, an you take a piture, beause it sounds like you have a longer shaft motor on a shorter height transom.


H

 

[sho3boater]

Re: Outboard Mounting height

Most boats run well with the cavitation plate even with the bottom when trimmed level. Heavy boat little deeper, light boat higher...in relation to HP you have anyway.

 

[jonj200369]

Re: Outboard Mounting height

Okay I have some pics, But i was mistaken about how low the cav plate is its much higher than i thought. So have a look and let me know what you think

thanks so much

 

[sho3boater]

Re: Outboard Mounting height

That looks like a bow lifting prop, what is it? If so it will cause the bow to lift with any trim. It could run well with the motor higher maybe 2" or so just a guess....if that is a bow lifter. What hull did you run that prop on before?

 

[hwsiii]

Re: Outboard Mounting height

Jon, it is very hard to tell how low the anti ventilation plate is below the boat, but it looks like about 2" to me, and yes that is what is ausing your problem. And no a hydrafoil will not help nor will trim tabs beause the problem is not trying to get the stern up higher in the water olumn but you need the motor to be higher on the transom.
It appears to me that the boat had a gil braket of some kind on the bak that the motor atually attahed to, and that is why the holes were not already there in the right plae for the motor. You will need to know EXATLY how low the anti ventilation plate is below the keel in order to find where the holes need to be plaed in the transom. When you get that number ome bak and lets see what we an do.

H

 

[whofan]

Re: Outboard Mounting height

Hey everybody, I have a question. My old boats transom was getting pretty bad so I decided to buy a used hull and swap over all my running gear.
When I mounted the outboard (1986 75hp mariner 3cyl) I was not sure exactly how high to mount it on the transom, so I took a guess and went for it.

I took the boat out to the lake and tried to get her going to see how it was. The first thing I noticed was that I was getting major prop wash (some water coming back into the transom). The second was that the bow was riding too high. when I used the power tilt and raised the motor a small amount and the boat began to porpoise quite badly.

I am assuming that the height is the cause of my problems. the cavitation plate is about 5-6 inches below the transoms lowest point, I think it should be higher?
would a hydrofoil help?

Thank you all

Jon

I know this is off topic but your transducer for your graph should be moved away from the chine down lower on the transome.

I`m sure if you turn to port at speed you will lose bottom.

 

[jonj200369]

Re: Outboard Mounting height

Hey sho3boater, I don't know too much about the prop other than the fact that is it a speed prop with a 19 degree pitch. I had the motor and prop on a 14.5 ft k&c thermoglass with a shallow v hull and had no problem whatsoever the boat was great.

The new boat has a very similar shaped transom and is also a shallow v design hull. The new hull however has a tri-hull in the front portion as it is an older bow rider.



hwsiii, I will measure it and find out tonight when i get home from work.


Your help is greatly appreciated
Thank you all

 

[jonj200369]

Re: Outboard Mounting height

Thanks whofan for the advice I will move in down to a better spot I noticed it has been acting strangely, maybe this is why
thank you

 

[jonj200369]

Re: Outboard Mounting height

So should the anti ventilation plate be dead even with the bottom of the transom?

 

[hwsiii]

Re: Outboard Mounting height

Atually for best speed it probably be about 2 inhes higher than the keel on that trihull.


H

 

[sho3boater]

Re: Outboard Mounting height

I would take the prop off and search the number to find out what it is, if there is no name on it. If it is a bow lifter, it will work best at a higher height maybe around having the cav plate 2" above the pad or (bottom of hull, and that often works out to 2" higher on a stock transom). This makes less drag on the LU so you go faster, and it drops the bow down to counter the lift the prop is giving (higher motor=less bow lift)...that is why they make a lifting prop so you go faster. You have to lift the bow to unwet the hull. It will grab the water partially surfacing unlike a standard prop that would blow out/cavitate if it were partially out of the water. Just make sure you don't get too high so when you trim up at speed you lose water to the motor, that would be very bad, but should not be a problem at that height. A bow lifting prop will not work well on a heavy hull because it can't lift that bow to unwet the hull and go faster, and they do not work as well fully under water like a standard prop. So a slow heavy boat can not take advantage of one. A race boat can run a fully surfacing prop because they need nearly no bow lift from the prop at all.

It sounds like from your original post the bow is up, and the prop is sucking the stern into the water....just what it should do if you have the trim up some. So if you lift the motor up the bow will come down and you will go faster having less motor in the water. My current boat I stuck a laser 2 prop on it and it did the same thing, the stern was roiling water like crazy until it got on plane and it was making a huge wake. But it was more pitch and could not get any rpm so I did not bother to lift the motor to use it.

 

[jonj200369]

Re: Outboard Mounting height

Ok so I removed the motor and properly sealed up the old holes 3m 4200, wooden doweling, covered in epoxy. I then raised the motor about 3 inches making the anti cavplate 2 inches above the lowest point of the transom. I let everything dry up for a few days then took her out for another test run. I noticed right a way that the boat got out of the hole much faster, seemed much more stable and responsive and had a higher top speed. So it appears I have solved one of the fundamental problems, however the engine is still causing an excessive amount of water to splash into the transom's upper well by creating a crazy rooster tail.. With the boat on plane I looked over the back and attempted to see what was going on, I observed the following; That the water was shooting straight up the back of the transom, the back of the boat is taking up about a foot of water and the anti cavplate is about 1-2 inches underwater. any ideas?

 

[hwsiii]

Re: Outboard Mounting height

I need a picture, but it sure sounds like the motor needs to be higher. Take a couple of pictures of the motor from the back and the side. And if the anti ventilation plate is not on top of the water at cruising speed it needs to be moved up more.


H