# Raising transom for long shaft



## Hard H2O

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?


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## crazymanme2

You want to keep transom the way it is and find a short shaft motor.


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## perchin

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.


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## lckstckn2smknbrls

You can build a jack plate / motor riser.


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## Ranchero50

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


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## clarkbre

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.


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## Zum

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.


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## perchin

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.


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## crazymanme2

Your also going to have a higher center of gravity which will make the boat more tippy. [-X


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## Hard H2O

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.


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## Hard H2O

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?


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## crazymanme2

Correct.Tilt motor out to bring bow up.Tilt motor down to bring bow down. #-o


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## perchin

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.


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## Zum

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.


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## perchin

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


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## clarkbre

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
Click to expand...


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.


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## perchin

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 :?


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## clarkbre

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.


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## perchin

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.


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## Zum

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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## clarkbre

I’m sorry my spelling is so terrible. I didn’t ever claim I was an English Major.

Here’s my sketch demonstrating the lift of the bow in comparison to the pivot point and the thrust of the prop. No matter what, when you rev the throttle in the forward position, the bow is naturally going to come up.

There’s no way that when you throttle up in forward that the bow pushes down.


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## Ranchero50

Perchins transom being weakly built allowed a pivot point causing the bow to plow... I can also see a weak hull flexing from the extra loads. A stiff hull is still pushing the boat by the prop. 

I read some stuff posted on I boats #-o 

Jamie


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## perchin

Then someone wouldnt' mind posting a video of them starting their motor, pulling up on the motor, putting their hand between the transom and the mounting plate and putting the gas to her.... :lol: Seeing how it won't do anything because it does not push there. :?


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## clarkbre

perchin said:


> Then someone wouldnt' mind posting a video of them starting their motor, pulling up on the motor, putting their hand between the transom and the mounting plate and putting the gas to her.... :lol: Seeing how it won't do anything because it does not push there. :?



Funny you should say that...

Seems if some guy did that...
1. He wouldn't be real bright...
2. The pivot point of the motor would be at the top of the transom. In order to really clamp your hand between the transom and motor, you'd have to close that pinch point. To close the pinch point on your hand, the propeller would have to push the motor forward from the bottom with the propeller. That would sure hurt like hell but you'd start to understand thrust and lift. And, that's if the prop is set at the right height on the transom.

If that prop was sitting 6" lower on the transom, it would have more leverage to clamp onto your hand. 

Oh, wait, you said the motor pushes from the top though...stupid me.

It's just like using two wrenches with different length handles. The short handle wrench will tighten to so many foot pounds. Use a wrench with a handle twice as long and it will deliver twice the torque on the bolt.

And again, the outboard and boat are working as a single unit at that point. It wouldn't matter if the pivot point on the motor/transom was 3 feet in the air. The motor pushes the boat from the prop, not the transom.


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## perchin

:LOL2: would you make up your mind .........a couple of posts ago, you posed a drawing disagreeing with whatever you just said..... for instance the pivot point. 

And for like the fifth time your fixation with the prop.... this entire time I've been talking about the point of contact... I believe we are talking about the same Dang thing.....I'm only talking about where the motor meets the back of the transom (point of contact).....

Thanks for the obvious tip with the wrenches....as if I didn't already post it earlier about using a cheater bar..... again were saying the same thing, just talking about different areas.


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## clarkbre

perchin said:


> ... I believe we are talking about the same Dang thing.....



So tell me again how the bow would get pushed down into the water as opposed to being lifted out of the water if the prop was sitting too low?


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## perchin

again i never said that . i said if you build up the transom, that raises the point of contact.... and applies more leverage. Just like a longer wrench....


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## clarkbre

perchin said:


> .... 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.....



Yep, you never said that bow would get pushed down? 

The point of contact does not matter. The reason a bow goes up when you gun the throttle is because the propeller pushes from the bottom. In the cases of an outboard, i/o, or an inboard, the prop pushes from the bottom creating the bow to rise when accelerating.


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## perchin

I meant I never said anything about the depth of the prop like you said I did.

I'm really not trying to make ya mad.... I enjoy a good debate, and also like learning new things, this constantly tends to give off the wrong impression. And also tends to tick a few off.... no harm was intentional...... I just wish someone could explain it so I can understand how my thought is incorrect...... as of now I guess I can see it both ways.... kinda :?


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## Hard H2O

Here are some pictures of my transom


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## ditchen

I have just been through this on my modified v jon. Had a 76 40hp johnny 15" mount to the 15" transom. But I had to have the tilt and trim and have always been a believer in the jack plates. So I obtained a cmc tilt/trim with a set back of 6". No changes of anything else. Big difference. With the motor set back and raise up while on plane back of hull ran bout 4" deep in the water.

Well I moved onto a 50hp, 20" shaft. Mounted it up with no changes in current setup. With the cavitation plate running 3" below the hull, the arse end was pushing so hard down while up on plane bow was high and back of hull ran about 12" (getting water spry from the anchor so I was pretty accurate on the running draft). It rode and handled like a brick.

After talking to CMC I purchase the vertical extension brackets designed for the jet drives and for short to long shaft conversions. No problems and quite happy with the results,,, well other than the one of the kind look.


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## althedj

My thoughts are aluminum boats (open style not the carpeted bass rigs) are generally 12 to 15 feet, max horse power on my Lund is a 10HP. I'm using a 5HP. There's not enough power to do much to begin with, driving the bow down, or making the boat unstable seems to be a bit of an over statement to me on a tiny little aluminum boat. Raising the transom 4" to bring the prop out of deep water shouldn't do anything. Just my thoughts. Raise that sucker up


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## stevenmurray1987

I know this is an old thread, but I wanted to add my experiences concerning raising the point of contact on the transom will contribute to the bow digging.

In my experience it DOES - but not drastically. It would be an issue if I did not have power trim.


Here is my new setup


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## Bateman

Man, this thread made my head hurt..

Hopefully heading to FL tonight to pick up a long shaft 50 for my short transom jon boat. I'll be using a jack plate. Been on several similar setups and never once witnessed a bow dig. Perhaps the jack plate's fulcrum being lower on the transom when compared to the taller jack plate makes the difference. But then, we'd be saying a factory tall transom boat would have the same negative side effects.. Hmm.


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## stevenmurray1987

Bateman said:


> Man, this thread made my head hurt..
> But then, we'd be saying a factory tall transom boat would have the same negative side effects.. Hmm.



Yeah I agree with that logic and also about the headache lol.

I still don't really know the answer, honestly my setup has too many new variables to blame the digging on the transom raise. 

-I also moved my console (my 200# body) 24" forward
-I removed weight
-Not sure of the angle my last outboard sat versus what I was trimmed at when experiencing the issue.


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## vfourmax

I understand the concept of how raising the engine mounting position to a higher point on the rear of the boat at the transom would have an effect of changing or making a difference in how those forces would relate to the hulls position as a result of the difference in height of those forces being applied.

From a simple explanation it is just simple physics but there are many ways to "offset" any negative effects of such a minor change.

As I think in this instance we are talking about a 15" to a 20" transom height difference and I think as a general rule that most outboards designed for a 15" transom are of 50 hp or less with the majority actually being of around 30 horsepower or less.

Granted if we were talking more of a using a 25" motor on a 20" transom then actual weight and the hp being used may be of a much larger effect as we could be dealing with multiple hundreds of lbs of weight and the effects of up to 300 horsepower.

Here I think we are looking at much less as far as the change of physics versus the size of the hull and height of the transom as long as the hulls maximum coast guard horsepower ratings are followed.

First off I would think those differences could be easily negated by utilizing a jackplate to not only lift the motor to provide the correct lower unit depth but provide a setback of 4" or so which should by moving the weight of the engine back further off the back transom should easily offset the change of the slight height increase difference of using a 15" to a 20" outboard with all other variables being the exact same.

The manual jackplate would also offer the benefit of having the ability to make smaller more precise height adjustments to truly dial in the engine to hull mating for the best results and adjustment changes can be made easily with a wrench rather than needing to remove the engine from the transom to change the engines height with every change.

Also the set back would conceivably put the prop back in cleaner water plus the setback would lengthen the hinge effect on the hull again helping offset any changes raising the engine may cause. Again here the largest effects would probably only really be noticed to any degree on the very largest of the engines in the class of mainly available 15" shaft motors.

Actually just doing a quick search of currently available new stock off the shelf jon boat models from LOWE, TRACKER and ALWELD the highest horsepower rated hull I found utilizing a 15" transom was only rated for 35 horsepower. Any higher rated hp models were utilizing a 20" transom.

I do know from personally researching newer 4 stroke motors in the 20-30 horsepower range that I have ran across several models that power trim was only available on the 20" model and not the 15" model.
I personally would not have a problem using a jackplate to fit one of the long shaft motors to a 15" transom hull rated for the horsepower if the motor I wanted to buy was only available in a 20"shaft.

Between the jackplate and the power trim on a motor of this weight and hp the adjustability would more than make up for the minimal 5" increase in mounting height and any performance or handling differences as a result of that slight height increase.

Without power trim using a set trim level every change or distribution of weight in the boat will compromise or enhance the boats performance. I would worry more about that than raising a motor of 50 hp or less up 5".


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