Raising transom for long shaft

Hard H2O

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I have a 16' tin boat that currently has a short shaft motor that needs to be replaced. The boat is rated for a 50 horse. It has remote steering and tilt and trim. The motor is shot.

In searching for a replacement I haven't come up with any good options in a short shaft.

Has anyone added 5" on the top of a transom to be able to mount a long shaft?
 

perchin

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I would agree to just find the correct motor.... reason being is with the larger horse motors you'll have problems with it pushing the front end constantly deep....more so the faster you go.... with smaller motors they just don't have the hp to make much difference. clear as mud? :lol: If ya want I'll draw you a diagram to show ya what I mean.
 

Ranchero50

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Perch, as long as the prop is below the boat I can't see how a taller motor is going to push the nose down :?:

We do it all the time for jet installs. Depending on your transom thickness you just add a 2x6 on top of the existing transom and bend a 1/8" peice of aluminum around it in a 'U' shape (upside down of course) Drop the legs 2/3 the way down the transom and through bolt with stainless 3/8 bolts, washers and locking nuts.

Depending on how strong your existing transom is you may have to add additional angle supports.

I'd try to trade around for a short shaft or get a rebuilt powerhead for your existing engine.

Jamie
 

clarkbre

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Ranchero50 said:
Perch, as long as the prop is below the boat I can't see how a taller motor is going to push the nose down :?:

Agreed, please draw up a diagram.

The idea of a transom riser is to essentially make a long shaft motor a short shaft motor. The theory is you want the cavitation plate of the motor within about an inch of the height of the bottom of your boat.

I have done this and it works extremely well. I have a long shaft 15hp Suzuki and a short transom Lund boat. I used aluminum square tubing and plates to raise my motor 6” total. Now, the cavitation plate is even with the bottom of the boat and the performance is perfect.

I’m going to have to disagree with Perchin on this one. Using a transom riser won’t make the front of the boat plow through the water. However, it will improve performance in hole shot and overall speed/smoothness.

If you use a long shaft motor on a short transom boat without a riser, the motor creates leverage at the back of the boat. The prop is about 5” lower than it should be so under acceleration there is more leverage to lift the front of the boat. And, while under full speed, you have an extra 5” of motor in the water creating drag. This not only is bad for performance but makes the engine work much harder to push the same weight boat.
 

Zum

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Pretty sure theres all kinds or atleast a few of the mods done right here.
Thought it was pretty normal?
My thinking is as long as you build it strong enough and your boat can handle the HP anyways ...why not?
Pretty sure if you do a search on here, something will come up.
 

perchin

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lol... you guys are funny. its just a matter of leverage. If you raise the motor there is more leverage pushing the front end DOWN, not because of the location of the prop, rather the attachment point. As far as doing this with a 15 hp.... I believe I already previously stated that the smaller hp motors don't effect it much. I also agree though that if you support your homemade jack plate with knee-brackets it helps a lot.

I guess I'll draw up a diagram tonight.... our last boat had this exact problem, so I'm not just guessing.... though maybe we had a odd boat. At about 3/4 throttle to WOT it would start to dig. Put on the same hp motor but the correct size and VIOLA.
 

Hard H2O

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I looked at a post of a great transom raising mod. There aren't any short shaft motors being made in the HP I want. I have been looking.

The short shaft is 15". A long shaft is 20". If you raise the transom 5" and hang a long shaft it shouldn't change the handling dynamics. Except for maybe a bit of extra weight if the prop and cavitation plate are in the same position relative to the bottom of the hull i should be good to go.

Here is what I am thinking of doing. https://www.tinboats.net/forum/viewtopic.php?f=3&t=10823

I will get a pic of my transom tonight and post it.
 

Hard H2O

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Isn't tilt and trim for adjusting the nose of the boat when under power. You start out and the nose goes up. Adjust the trim to bring it down?
 

perchin

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I think you guys are misunderstanding my point..... I didn't say it can't be done, I said it changes the performance of it on the bigger hp motors. I was by no means saying anything about the tilt, cav. plate, or prop. Adjusting the tilt will yes adjust the bow's ride height.

Think of it this way..... if you have a large L laying on the ground and its too heavy to outright lift it. The higher your hands are on the L while trying to push it, the more the back is going to pick up and the front is going to dig into the ground, the lower you push it will start to slide.
Just simply a leverage senario...... Have you ever used a cheater bar on a stubborn bolt???

Hard H2O said:
I looked at a post of a great transom raising mod.
That mod you posted was not putting on a 50hp motor...... :?

I wouldn't mind someone explaining how it doesn't change a thing...... not saying the guys at I-boats are smarter, but they are the ones I learned it from, and also the ones who said to get rid of the riser plate we had on a boat and to just go with the correct short shaft motor.... problem was solved, so I know that the attachment point height made a difference.
 

Zum

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While I agree with your L theroy,I don't agree in this way.
The propultion(power) is from the prop,which is in the water and always will be in the water if you are motoring.
It's not being pushed from the top.
 

perchin

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Zum said:
While I agree with your L theroy,I don't agree in this way.
The propultion(power) is from the prop,which is in the water and always will be in the water if you are motoring.
It's not being pushed from the top.

absolutly is at the top...... that's where its attached to the boat..... If I push the L at the top with my hands and run, My propultion(power) is from my legs (feet)... the darned ol L is still being pushed at the top. :wink: just saying
 

clarkbre

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perchin said:
Zum said:
While I agree with your L theroy,I don't agree in this way.
The propultion(power) is from the prop,which is in the water and always will be in the water if you are motoring.
It's not being pushed from the top.

absolutly is at the top...... that's where its attached to the boat..... If I push the L at the top with my hands and run, My propultion(power) is from my legs (feet)... the darned ol L is still being pushed at the top. :wink: just saying

It may be attached there but the boats isn't pushed from that point. The point of power is at the prop in the water. Using your theory, a lifted truck or a lowered car are pushed at a different point even though the tires are sitting on the ground. If the engines are higher or lower would make a difference on the pavement?

You're backwards on your theory. where the transom and keel meet is like a fulcrum. When the prop is close to that point, the boat will run smooth and get on plain fairly easy. Givin the same boat, motor, prop, weight, everything set up the same except that the prop is about 6" lower, the prop will provide more torque and leverage to push the bow of the boat out of the water more. Once it gets to speed, it will start to plain out; however, it won't ever handle that great because the prop is sitting too far from the bottom of the boat.


Whatever boat that it was doing the opposite on I would question if you adjusted the trim or pin on that motor. I know when I changed mine, I had to trim the motor out one notch to keep the bow from plowing. The motor was in one notch to compensate for the extra torque being provided at the prop sitting too low.
 

perchin

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Your first paragraph doesn't really make sense nor apply.

No matter what is providing propulsion, it is still transferring it through the point of contact.

Questioning if I tried to adjust its trim and tilt before getting another motor............ are you serious :?
 

clarkbre

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perchin said:
Questioning if I tried to adjust its trim and tilt before getting another motor............ are you serious :?


Stating that it's pushing from the top...................are you serious?

You're saying that all the push comes from the top?

Doesn't the propeller push from the bottom, thus equalling pull from the top? When the propeller pushes forward, the natural movement of the top of the motor is to go back. Similar to a car, motorcycle, or snowmobile popping a wheelie. The propulsion comes from down low and the vehicle pivots up on it's fulcrum.

There is no push from the top. Even if the mtor is mounted on a point 3 feet higher than the transom, it still pushes from the same propeller position.

So, with that set up, with the motor in reverse, slam on the throttle, and the bow will come up instead of dipping down? And, the motor, wouldn't try to lift itself out of the water?

I'm thinking this is simple phisics.
 

perchin

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clarkbre said:
Stating that it's pushing from the top...................are you serious?

You're saying that all the push comes from the top?
That was me wrongly stating what I meant... no the upper portion of the transom...sorry

clarkbre said:
Doesn't the propeller push from the bottom, thus equalling pull from the top? When the propeller pushes forward, the natural movement of the top of the motor is to go back. Similar to a car, motorcycle, or snowmobile popping a wheelie. The propulsion comes from down low and the vehicle pivots up on it's fulcrum.
almost..... but you left out the other half of the equation..... thus it pushes just below the pivot point...like the point of contact.

clarkbre said:
There is no push from the top. Even if the mtor is mounted on a point 3 feet higher than the transom, it still pushes from the same propeller position.
correct........ its push begins directly below the pivot point..... sorry I was not clear.

clarkbre said:
So, with that set up, with the motor in reverse, slam on the throttle, and the bow will come up instead of dipping down? And, the motor, wouldn't try to lift itself out of the water?
Nope.... bow will dip down. Yes motor would try to lift.... your missing the point still.

clarkbre said:
I'm thinking this is simple phisics
Looking at the spelling.... I think not.. :shock:

Look at my drawing below..... I think your just not considering the end result.... If there was no push on the back of the boat, it wouldn't go forwards... :lol: The higher the push is on the transom the more leverage there is...cut and dry, not matter what.
motor.JPG
 

Zum

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I don't agree with you on this one perchin...sorry.
When your transom flexes(lets say it's weak,from whatever reason),when you crack on,your stern will flex outwards.
The push from the prop,makes an opposite reaction at your pivot point.
If anything the bow would come up,like mentioned before pop o wheeely:)
 

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