how much to raise the motor

[R Willis]

14' G3 flat bottom with 25hp Johnson
I have been looking at one of the mounting plates that bolt to the transom and extends the engine back about 4 inches. I was told today that when you move the motor back off of the transom it allows you to raise the engine up higher.
With my engine on the transom (as it is now) the cavitation plate is 1" below the hull. I know I need to raise it. My question is if I move the motor back 4" with the extension how much more (if any) should I raise the motor?

RW

 

[kfa4303]

Re: how much to raise the motor

Hi RWillis. Moving the motor back doesn't necessarily "let" or require that you raise the motor per se. The general rule of thumb is that the cavitation plate should be at the same level, or slightly higher than the keel of the boat whether it's on the transom or on a bracket. If it's only about 1" below the keel now, it may not even notice the change if you did raise the motor, but it might help some. Moving the moor back effectively increases the length of the boat, so if you had a 1' bracket on a 14' boat, it would be as if you had a 15' boat. You may be able to get the 1" in lift you need by simple raising and re-clamping the motor on the transom. The motor doesn't actually have to rest on the transom as long as the clamps are sufficiently tightened. Hope that helps. Good luck.

 

[5150abf]

Re: how much to raise the motor

I have a 16' v hull with a 6'' set back and my engine is @ 2 1/4'' above the top of the transom and my cav plate is @ 1''-1 1/2'' above the bottom of the boat and on my boat with my set up it runs really well but every boat is different so you will need to try several different hights on yours to find what it likes.

Just start as high as it will go and work your way down till it doesn't cavitate, you don't say if you have tilt trim but probably not on a 25 but that helps alsso.

 

[ondarvr]

Re: how much to raise the motor

Not to be picky, but it's not a cavitation plate, it's anti ventilation plate, they are two very different things.

Without a set back 1" lower than the hull is not normally a correct height, it should be even with, or higher than the bottom of the transom. It may be 1" or more inches higher depending on the exact setup. Sometimes flat bottom boats need the motor slightly lower though. You can raise the motor without changing any thing else and see if it helps, but you may need a GPS and a tachometer to notice the changes and then set the height correctly. If the prop ventilates under hard acceleration or when turning it may be too high, but it's sort of up to you and what you are actually looking for, top speed, better handling, etc. By the way, what is it you are looking for by bolting one on.

Proper setup (height) will depend on your testing, there are some general guidelines, but it's a trial and error process for each boat and motor combination.

 

[Frank Acampora]

Re: how much to raise the motor



See if you can picture this in your mind: As the boat is planing it is not parallel and completely on top of the water. It is tilted slightly nose high and moving through the water. Displaced water forms the wake.

So, when it moves through the water it does displace some and after the transom passes this given point, the water tends to replace itself. THUS: water coming off the back of the transom will "rise" some. Exactly how much this rise will be depends upon the type hull, speed,View attachment 126739View attachment 126740 and how flat it planes. The net effect is that if you set back the engine from the transom, it will effectively ride lower in the water. THUS: You raise it up to reduce drag. There are other factors such as leverage involved but for now, you are primarily concerned with getting the prop at the correct depth in the water. The amount you may raise it varies from hull to hull so no one except a person with your exact set-up can give you an accurate guess.

You may start with maximum raising allowed with the set-back plate or start with the anti-ventilation (cavitation) plate even with the transom bottom and work from there. Raise or lower it one hole at a time. Once you find the approximately correct setting you may want to move the engine in increments and when correct position is found, re-drill the set-back plate to accomodate engine transom clamp holes. Ultimately, for maximum speed you want the engine to not ventilate at straight runs and only ventilate slightly during turns. I suspect that with your set-up, you will not go too high but will end up with the plate about 1 to 1 1/2 inches above the bottom of the transom.



This hull is a flat bottom and although it does not have a set back plate, the engine is at maximum height available with the transom clamps. It is one inch above the bottom of the transom. Note how flat the boat rides and how little the wake is. Even though this boat rides high on the water, this engine can probably be raised another inch but I am too lazy to do it. In the first photo, the plate appears to be even with the bottom but that is an illusion due to the angle of the photo and the engine being trimmed all the way out.

 

[Bamaman1]

Re: how much to raise the motor

Ahhh, we're not talking about a bassboat with a 250 hp motor. They all need jack plates for maximum performance.

See if you can get your motor mounted where the "anti cavitation plate" is even with the bottom of the boat. Then, go fishin'.

 

[Outsider]

Re: how much to raise the motor

Set back or not, start raising the motor a little at a time until the cavvent plate is just above water level when at cruise speed. When doing so, make sure the level of the bow is where you want it by adjusting the trim rod in or out. Once you zero in, you can change the position of the rod by a hole either way to compensate for different weight loading without affecting much else ...

 

[Texasmark]

Re: how much to raise the motor

The problem with the usual philosophy on anti-vent plate and water behind the transom level is that aluminum flat bottom boats have ribs and a piece of 1" angle aluminum which covers the joining of the two sections of the hull down the centerline of the boat. This seam cover becomes a keel and also a source of turbulence, especially troublesome when the engine is jacked up as we are discussing and in turns, especially sharp ones.

Trial and error is what you have to do to find your sweet spot and a SS CUPPED prop can surely help you to run higher, cleaner, more bite in the turns and more mph.....more fun. I not only BTDT, but I AM THERE. See my Avatar....that is a tin lizzie upon which that little black beast is mounted producing that wake.

Mark

 

[Frank Acampora]

Re: how much to raise the motor

Come on, Mark: Show us more of the hull, not just that little taste in the avatar.

 

[Texasmark]

Re: how much to raise the motor

View attachment 126800Come on, Mark: Show us more of the hull, not just that little taste in the avatar.

Ok Frank. Back in July, 25th or thereabouts i posted pics of it and the Ballistic SS prop I ported....I knew SS was tough but didn't expect what I ran into in doing that.

I'll see if I can figure out how to get them posted again,

Mark

 

[Texasmark]

Re: how much to raise the motor

Ok Frank. Back in July, 25th or thereabouts i posted pics of it and the Ballistic SS prop I ported....I knew SS was tough but didn't expect what I ran into in doing that.

I'll see if I can figure out how to get them posted again,

Mark

See if this works.

 

[Texasmark]

Re: how much to raise the motor

More: #1 isn't accurate as the engine is tilted up. Anti vent plate is about 1" above hull when vertical.

#2 shows jack up off transom.

#3 self explanatory.

 

[steelespike]

Re: how much to raise the motor

The problem with the usual philosophy on anti-vent plate and water behind the transom level is that aluminum flat bottom boats have ribs and a piece of 1" angle aluminum which covers the joining of the two sections of the hull down the centerline of the boat. This seam cover becomes a keel and also a source of turbulence, especially troublesome when the engine is jacked up as we are discussing and in turns, especially sharp ones.

Trial and error is what you have to do to find your sweet spot and a SS CUPPED prop can surely help you to run higher, cleaner, more bite in the turns and more mph.....more fun. I not only BTDT, but I AM THERE. See my Avatar....that is a tin lizzie upon which that little black beast is mounted producing that wake.

Mark

years ago when boats were wood they used to suggest removing about 6" of the keel near the transom when mounting a motor.
I suppose you could carefully do the same to the seam cover if deemed necessary.
The main purpose of the jack plate is fine tuning the motor height and to get the motor in less disturbed water
In figuring setback; motors with hand clamps may need more clearance in the event the motor needs to be low.
I allways suggest raising the motor to a point just before venting becomes a problem.In the extreme I have seen speeds increase about 2 mph. I would say that is probably the exception but nothing ventured nothing gained.

 

[Texasmark]

Re: how much to raise the motor

years ago when boats were wood they used to suggest removing about 6" of the keel near the transom when mounting a motor.
I suppose you could carefully do the same to the seam cover if deemed necessary.
The main purpose of the jack plate is fine tuning the motor height and to get the motor in less disturbed water
In figuring setback; motors with hand clamps may need more clearance in the event the motor needs to be low.
I allways suggest raising the motor to a point just before venting becomes a problem.In the extreme I have seen speeds increase about 2 mph. I would say that is probably the exception but nothing ventured nothing gained.

I remember the cut away years ago in wooden boats too. I think Alumacraft for one had it smashed flat at the rear. I looked at cutting mine away as you mentioned but I would leave the welded seam exposed and I'd rather not do that. Mine isn't all that bad, the prop obviously helps. Problems show up when I'm horsing around in a tight turn, but I understand that.

Mark