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 Gtr 37xd Transom height 
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Post Gtr 37xd Transom height
Recently had some mods done to my motor and am trying to gain some answers on proper transom height. 18x60 Uncle J Hull with a 37 efi on it. Heads cam exhaust and bored throttle body. I was getting 24-25 before and am now getting about 30 out of it. I was hoping a more than that. It’s not propped right at the moment so that’s part of it. But my question is..... on the Gtr do you measure the transom height down the side of the transom or straight down plumb. I’ve read that it’s down the side on the GTR. If that’s the case my transom is coming in slightly over 22”and the build sheet says 20* but I haven’t measured that. It looks about right. It’s around 21” measuring plumb to the ground. I’ve attached pictures of it trimmed all the way up and all the way down. The prop saver or whatever you want to call it on the bottom of the shaft is about even with the bottom of the boat when trimmed all the way up. Right now the boat is porpoising pretty bad at around 27 or so. I was wondering if I needed to cut a few inches of the transom to allow the shaft to be flatter when I trim up to plane out. Thus allowing the boat to push forward and not up when the prop is at correct height. Let me know how it looks to you guys.

Also I realize this is a heavy heavy boat. I’m just trying to dial it in as best I can. My fuel tank is in the front and I was sitting on the front trying to keep the front end down but it didn’t help. Gets worse when solo.


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Wed Mar 27, 2019 10:41 am
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Post Re: Gtr 37xd Transom height
You will have to experiment with your setup. I will say a 22" transom plate on a 20* angle is too tall..... Surface drives are not like outboards, we need to lower the motor to get a more parallel push when at optimal trim. The closer we can get to a neutral trim the better. We are always at a negative trim so the taller the transom the worse it is.

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Bayou Surface Drive
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Thu Mar 28, 2019 5:27 am
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Post Re: Gtr 37xd Transom height
Do you measure it along the angle of the transom or plumb to the ground on the 37 xd. I mean it’s 21” one way and 22.5 the other. I did bend down the tab at the bottom of the transom and that seemed to clear my porpoising up. But I also went out with a full tank of fuel. So I’m thinking it could have been the fuel and the tab may be causing me to loose speed. I was still getting 28.5-29 mph at 4700ish rpms. Waiting on my big blade to get here.


Thu Mar 28, 2019 5:38 am
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Post Re: Gtr 37xd Transom height
GT transoms are 21" along the plate Plumb bub is less. So if your transom is 22.5" tall along the plat you are WAY too tall.

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Thu Mar 28, 2019 6:47 am
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Post Re: Gtr 37xd Transom height
Well I’m assuming the full tank of fuel vs just a few gallons in the front is what cured the porpoising. I only the bent the tab at the bottom of the transom this much on both sides. Looks like I need to get this motor off and and trim the transom down a bit. Hopefully pick up a few mph.


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Thu Mar 28, 2019 7:17 am
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Post Re: Gtr 37xd Transom height
bending tabs will force the nose down stopping porpoising but slows you down.

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Thu Mar 28, 2019 7:24 am
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Post Re: Gtr 37xd Transom height
Yes sir that’s what the motor guy told me. Gonna bend them back and see if the fuel was the cause. I didn’t loose much speed if any considering the added weight of the fuel. But I maybe could be gaining some if I didn’t have that drag.


Thu Mar 28, 2019 7:38 am
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Post Re: Gtr 37xd Transom height
I bent the tabs back and cut the transom down. Because of the height of the deck on the back it was hard to get it to the correct height without the bolt ending up in the middle of the deck. I opted to go under the deck and now It seems like I may be a little to low. I’m getting some spray when running on plane from the lower unit contacting the Water. I also figured out my transom angle is 24.5 degrees. Would shimming the angle to 20 degrees help get the lower unit up just a bit? Here is a picture with the trim all the way up. Does it look too low.


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Tue Apr 30, 2019 9:06 pm
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Post Re: Gtr 37xd Transom height
Easy to see if the pic is turned correctly


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Tue Apr 30, 2019 9:20 pm
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Post Gtr 37xd Transom height
If you can trim fully up and down, I don’t think shimming will be your big issue.

On a side note, do the XDs come with the quick trim holes, or is that a thing of the past?


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Wed May 01, 2019 6:21 am
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Post Re: Gtr 37xd Transom height
I can trim it until I blow out of the water when on plane. Yes it’s in the top hole.


Wed May 01, 2019 6:29 am
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Post Re: Gtr 37xd Transom height


Wed May 01, 2019 7:35 am
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Post Re: Gtr 37xd Transom height
Is the belt box dragging on plane? Mine is about that high above the boat and it doesn't hit the water line on plane.

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Wed May 01, 2019 7:40 am
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Post Re: Gtr 37xd Transom height
Just about every uncle j boat I seen has the transom entirely too tall.

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Wed May 01, 2019 7:40 am
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Post Re: Gtr 37xd Transom height
This hull has two talons on the back. Way to much weight for a Mudmotor. Our transoms will measure two ways. Along the transom, and straight up and down. Along the transom will be a bit over 1” more.


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Mon Jul 29, 2019 5:59 pm
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Post Re: Gtr 37xd Transom height
Over an inch? What angle? That seems like a big difference between the 2.

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Mon Jul 29, 2019 6:18 pm
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Post Re: Gtr 37xd Transom height
It’s 21.5/22” along transom

And 20/20.5 straight up and down.


Mon Jul 29, 2019 7:42 pm
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Post Re: Gtr 37xd Transom height
Lawd!

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Tue Jul 30, 2019 1:05 pm
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Post Re: Gtr 37xd Transom height
whats that for?

Its been a while since i've been on here.


Tue Jul 30, 2019 1:32 pm
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