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Tunnel hull outboard height?

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Need some advice on my 15' Alumacraft Tunnel Hull; from the top of the tunnel to the top of transom is 15" - I bought a yamaha 20 short shaft, but unfortunately the outboard seems to be more like 18" - as you can see the cavitation plate sits about 3" lower than top of tunnel.

 

I am considering a welder to raise transom, if I do this can someone just 100% confirm for me that the plate directly above the prop should be at the same height as the top of the tunnel?

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Don't know answer to your question, but I would think for just a little more $ you could get a manual jack plate and be able to raise/lower the motor height to the exact position that you were as high as possible and still had plenty of water pressure.

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I've seen people just slap a piece or two of wood on top to raise it. One buddy of mines grandfather did that for a 25 merc on a extra wide 14 ft G3 and it has held for about 2 decades. Not tunnel hull but it raised the motor just fine 

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1 hour ago, bagofdonuts said:

Don't know answer to your question, but I would think for just a little more $ you could get a manual jack plate and be able to raise/lower the motor height to the exact position that you were as high as possible and still had plenty of water pressure.

Trying to avoid a jack plate as my tiller handle would be set back too far.

1 hour ago, TnRiver46 said:

I've seen people just slap a piece or two of wood on top to raise it. One buddy of mines grandfather did that for a 25 merc on a extra wide 14 ft G3 and it has held for about 2 decades. Not tunnel hull but it raised the motor just fine 

I'm considering spacing it up 2" - then my clamp mounts would still hit the very top edge of transom, but not sure how stable this will be with a 20hp - if anyone else can comment would really like to hear. It would still be about 1" too low, but guess maybe I could live with that if it saves me welding the transom! Thanks for the idea.

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I hope you realize the motor does not have to be sitting down on the transom.  It can be several inches above it, as long as the top mounting bolt is not right at the top.  If the top mounting bolts has to be close to the top, I would recommend using a piece of 1/2x 2" flat bar, either aluminum of stainless to go across the inside of the transom. 

You could also get a couple of pieces of 2"x 3"x 3/16 or 1/4" aluminum channel, a couple of feet long and use them like a jack plate.  Bolt one side to the boat, and the motor to the other side.  That keeps you from having to drill new holes in the transom.  You can also do the same thing with 2x3 or 2" x 4" angle and bolt them together.  That used to be a common method of making jackplates. 

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52 minutes ago, Way2slow said:

I hope you realize the motor does not have to be sitting down on the transom.  It can be several inches above it, as long as the top mounting bolt is not right at the top.  If the top mounting bolts has to be close to the top, I would recommend using a piece of 1/2x 2" flat bar, either aluminum of stainless to go across the inside of the transom. 

You could also get a couple of pieces of 2"x 3"x 3/16 or 1/4" aluminum channel, a couple of feet long and use them like a jack plate.  Bolt one side to the boat, and the motor to the other side.  That keeps you from having to drill new holes in the transom.  You can also do the same thing with 2x3 or 2" x 4" angle and bolt them together.  That used to be a common method of making jackplates. 

I can raise the outboard 2" but the clamps would be bordering the very top edge; if your saying you dont want it at the very top then that means maybe 1" raise is all you could get away with. If I could pull off the 2" I would probably do that and call it a day, but it just seems like that would be right on the edge. What do you shim the empty top/vertical space with when you do that?

 

I understand the jack plate idea, was just trying to avoid as it will set my tiller handle back.

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OK, I guess I missed the part where you said it was a tiller handle, so I guess it also has the two screw bolts in the front than clamps it onto the transom for easy removal and not the mount bolted onto the transom that I was thinking you might have.

I know of a couple of methods I have used for raising tiller handle motors that just clamp onto the transom, "BUT" I'm not going to cough up that info because of the risk involved if not done properly, and not knowing your back ground. 

 

So, I'm just gonna have to say, be careful and make sure you have a full understanding of how you try to do it. 

 

I will say, the clamps being near the top, if it's bolted at the bottom, is not a problem.  It you try running them near the top edge without bolting the bottom, you run a very significant risk of snatching the motor off the boat in a turn.   One only has to move a little bit and come over the edge and when it does, it will usually take the other one with it.

 

One other comment, if you don't leave the motor on the boat, and take it one and of, there is a way you can bolt the bottom without having to reseal the bolts every time.  It's a little trick I've used for over 50 years.  3/8" or 7/16" bolts are more than strong enough for a 20hp motor.  So, make you a couple of sleeves out of 1/2" ID copper tubing a little longer than the length needed to do through the transom.  Drill hole just large enough for the tubing and put plenty of 5200 around them and in the holes, stick the sleeves through, then flare out the ends and flatten them down tight against the aluminum. You can do that with a couple of grade 8, 7/16" SAE, bolts with washer and a nut,  Just stick them trough the tubing after both ends and got some flare on them and they will force it down nice and tight.  Just be sure to clean the tubing out before the 5200 dries. 

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11 hours ago, rustybass said:

if I do this can someone just 100% confirm for me that the plate directly above the prop should be at the same height as the top of the tunnel?

 

My 40 hp Tohatsu is sitting 1 1/8 - 11/4" above the tunnel.

 

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3 hours ago, Way2slow said:

I will say, the clamps being near the top, if it's bolted at the bottom, is not a problem.  It you try running them near the top edge without bolting the bottom, you run a very significant risk of snatching the motor off the boat in a turn.   One only has to move a little bit and come over the edge and when it does, it will usually take the other one with it.

 

One other comment, if you don't leave the motor on the boat, and take it one and of, there is a way you can bolt the bottom without having to reseal the bolts every time.  It's a little trick I've used for over 50 years.  3/8" or 7/16" bolts are more than strong enough for a 20hp motor.  So, make you a couple of sleeves out of 1/2" ID copper tubing a little longer than the length needed to do through the transom.  Drill hole just large enough for the tubing and put plenty of 5200 around them and in the holes, stick the sleeves through, then flare out the ends and flatten them down tight against the aluminum. You can do that with a couple of grade 8, 7/16" SAE, bolts with washer and a nut,  Just stick them trough the tubing after both ends and got some flare on them and they will force it down nice and tight.  Just be sure to clean the tubing out before the 5200 dries. 

 

This is good to know, definitely sounds like may have to drill the transom for bolts then - and will use your sleeve trick if I go that route for sure. Do you have any opinions of buying square aluminum tubing and having a welder weld that on to raise the transom? Only concern is the clamp bolts would be on that new piece of tube

14 minutes ago, Catt said:

 

My 40 hp Tohatsu is sitting 1 1/8 - 11/4" above the tunnel.

 

At speed the water flows right at that small top plate you can see in my pic about 1/2-3/4" above the tunnel - I think its the seam where the lower unit can be removed; this is probably 4" above the cavitation plate, so just not sure how high I can get away with raising it? Some guys told me these smaller outboards cavitation plates are meant to be underwater some, but not sure if that is correct.

 

Did you just adjust the height on yours until it ran the best? Is your cavitation plate right at the surface of the water that comes out of the tunnel?

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I definitely would not try welding tubing on top and expect that to hold my motor.  Doing that, you are expecting a thin piece of aluminum across the top of the transom, that's all you have to weld to, to hold all the torque.  Remember, the aluminum hull is not what's holding the motor, it's just protecting the, thick, heavy piece of plywood or composite material inside that aluminum shell that's actually providing all the strength. 

Remember one other thing.  If this is a new boat, it has a warranty, and you start welding on it back in the transom area, you can probably kiss that warranty good bye.   I have seen more than one aluminum boat develop cracks in the welded seams at the transom.  If yours does, they will probably laugh when you ask about it being under warranty.

A common method of raising a 15" transom to run a 20" motor is to take two pieces of 3/4 inch pressure treated plywood about 5" high, sandwiched together and use something like 1/4" x 2" steel and make two U shaped clamps the bend over the plywood and bolts onto the transom on the inside and outside with bolts that go through the transom.  The thickness might have to be adjusted some to match the thickness of your transom. 

Again, this kind of modification would probably void any warranty. 

If your transom does not have a center support, you might want to look at adding one if you raise the motor very much above it.

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3 hours ago, Way2slow said:

I definitely would not try welding tubing on top and expect that to hold my motor.  Doing that, you are expecting a thin piece of aluminum across the top of the transom, that's all you have to weld to, to hold all the torque.  Remember, the aluminum hull is not what's holding the motor, it's just protecting the, thick, heavy piece of plywood or composite material inside that aluminum shell that's actually providing all the strength. 

Remember one other thing.  If this is a new boat, it has a warranty, and you start welding on it back in the transom area, you can probably kiss that warranty good bye.   I have seen more than one aluminum boat develop cracks in the welded seams at the transom.  If yours does, they will probably laugh when you ask about it being under warranty.

A common method of raising a 15" transom to run a 20" motor is to take two pieces of 3/4 inch pressure treated plywood about 5" high, sandwiched together and use something like 1/4" x 2" steel and make two U shaped clamps the bend over the plywood and bolts onto the transom on the inside and outside with bolts that go through the transom.  The thickness might have to be adjusted some to match the thickness of your transom. 

Again, this kind of modification would probably void any warranty. 

If your transom does not have a center support, you might want to look at adding one if you raise the motor very much above it.

This is starting to get really really frustrating, I did all my homework before buying all this - the outboard was advertised by the Yamaha dealer as 15" - but apparently that is just a generic designation they use to say its for 15" transom? The outboard is almost 18" from cavitaiton plate to where it mounts on top of transom. Only other option I see is to run this outboard for a year to get some use for my money and then upgrade to a larger one at that point and make sure its a true 15" outboard - if they exist?!

 

Here was my transom idea - weld 2"x3" .125 rectangular tube across top of transom 3ft, then cut thick aluminum plates maybe 12"x12" or larger  - one plate welded to each side of transom so it helps tie the new top tube to the lower transom area. Maybe I will fill the 2x3 tubing with a cut down treated 2x4 inside of it? At this point the outboard would mount with clamps over the top new rectangular tube area - but it would be clamped onto my 12x12 plate so stress would be distributed down the transom as long as those welds hold. So taking into account these plates on the front and back do you think it may work or will this still be way weaker than my stock transom? My current transom has support brackets welded onto it on the inside and a plate welded in lower area, its pretty beefy looking.

 

 

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What does Alumacraft recommend?

The centerline of the prop shaft is 4" below the cavitation plate. 

Tom

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All the welding you do is probably only going to make it weaker, not stronger.  Like I said, you are only welding to a piece of sheet aluminum that's approximately .080" thick.  When welding, you are burning the main support structure underneath the aluminum and making where your weld seams are more brittle and subject to cracking. 

What ever method you choose, I would not include welding.  It needs to be bolted through the transom and extended 8 to 10" down it.  You can put 1/4" plate on both side with a filler between them and bolt those but you are going to have to be careful how thick you make what ever you use.   I would use a bolt pattern the same as a small engine mount so a second set of holes wouldn't have to be drilled through the transom if an engine was going to be mounted to it in the future if you go that route.    Those mounting screws on the motor don't give you a whole lot of space, even when fully open. 

 

There really is not practical way to extend it without making something that looks as bad as an elephants a** sewed up with a grape vine, short of using a jack plate and those are not normally made for tiller motors with the mounting screws. 

If at all possible to live with the motor set back about three inches, I would just get me some angle aluminum and make me a jack plate.

 

They typically measure from the hooks to the anticav plate for motor shaft length, but that can vary an inch of so each way.

 

If like what Catt has, the weld away.  My experience with most jons is they use plywood for adding the strength to the transom and just an aluminum shell over it..

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@rustybass My 40 hp Tohatsu does not clamp to the transom, it has 4 bolts that bolts through the transom.

 

Your description for raising the transom is pretty close to what aluminum fabricators do here. The difference is after the two plates are mounted on both side a top cap is welded to both side plates. You can for additional piece of mind slide the square tube in & weld the ends.

 

@Way2slow My Alweld has an all aluminum stump guard transom 😉

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5 minutes ago, Catt said:

@rustybass My 40 hp Tohatsu does not clamp to the transom, it has 4 bolts that bolts through the transom.

 

Your description for raising the transom is pretty close to what aluminum fabricators do here. The difference is after the two plates are mounted on both side a top cap is welded to both side plates. You can for additional piece of mind slide the square tube in & weld the ends.

Okay, so they weld a plate on either side and then cap it - that basically looks like how Alumacraft did my transom, though I suppose their is wood inside of that? When fabricators do what your saying do they add wood in that new hollow area then or just leave hollow?

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3 minutes ago, rustybass said:

Okay, so they weld a plate on either side and then cap it - that basically looks like how Alumacraft did my transom, though I suppose their is wood inside of that? When fabricators do what your saying do they add wood in that new hollow area then or just leave hollow?

 

The inside/outside plates are glued & through bolted in place. The only "hollow" area is between the top of the transom & the bottom of the top cap.

 

The whole "modification" is built out of 3/16 or 1/4" aluminum plate.

 

@Way2slow Is correct in saying if your transom is not all aluminum don't weld to it.

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24 minutes ago, Way2slow said:

All the welding you do is probably only going to make it weaker, not stronger.  Like I said, you are only welding to a piece of sheet aluminum that's approximately .080" thick.  When welding, you are burning the main support structure underneath the aluminum and making where your weld seams are more brittle and subject to cracking. 

What ever method you choose, I would not include welding.  It needs to be bolted through the transom and extended 8 to 10" down it.  You can put 1/4" plate on both side with a filler between them and bolt those but you are going to have to be careful how thick you make what ever you use.   I would use a bolt pattern the same as a small engine mount so a second set of holes wouldn't have to be drilled through the transom if an engine was going to be mounted to it in the future if you go that route.    Those mounting screws on the motor don't give you a whole lot of space, even when fully open. 

 

Sounds like your kind of suggesting something similar to this T&R transom elevator? I could have a welder build a more custom version of this and bolt it on; I think this one pictured raises like 5"

 

s-l500.jpg

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All you need for a 20 hp clamp on OB is T-H Marine Mini Jacker and specs from Alumacraft for mounting height.

Tom

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Yep, that's exactly what I was talking about.  Unless you have free access to a fabricator with a TIG welder, or you can't use the full five inch lift, it would probably be a lot cheaper to buy it.  

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5 hours ago, WRB said:

All you need for a 20 hp clamp on OB is T-H Marine Mini Jacker and specs from Alumacraft for mounting height.

Tom

Thanks Tom, main issue I'm running into is that my tiller handle cannot go back any further, its already back pretty far - the TH Mini Jacker is like a 4" setback I believe.

5 hours ago, Way2slow said:

Yep, that's exactly what I was talking about.  Unless you have free access to a fabricator with a TIG welder, or you can't use the full five inch lift, it would probably be a lot cheaper to buy it.  

Okay, this helps out a lot - I didn't even consider bolt on at first as though welding better, but I see what your getting at how the welding could maybe weaken areas - so definitely will consider the bolt on ideas, not many other options. The plate I posted is like 5", so yes would have to have one made a bit smaller, found a mobile welder pretty cheap here - so  could probably build the whole thing for as much money as that plate, we'll see, thanks for the advice.

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Have you looked into a longer tiller handle for the Yamaha?

I would be concerned about raising your engine using the elevator shown without adding  angle supports because of the leverage unless the clamp on engine bracket extends well below onto the transum to take most of the load. If you have a bolt on engine mount at least 2 bolts go through the original transum.

What is the Alumacraft recommended height of the centerline of the prop shaft from the bottom of the boat or the top of the tunnel surface?? You need to know!

Tom

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