15” shaft on a 20” transom

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jbo_c
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15” shaft on a 20” transom

Post by jbo_c »

What’s the best-safest way to do it? Can you use a jack plate that way? 18hp

My build will initially be powered by an old 15” shaft(because I already own one), but will ultimately get a modern 20” shaft. Would prefer to not make a short transom only to go back and add to it.

Jbo

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Re: 15” shaft on a 20” transom

Post by Reid »

jbo_c wrote: Wed Nov 02, 2022 12:59 pm What’s the best-safest way to do it? Can you use a jack plate that way? 18hp

My build will initially be powered by an old 15” shaft(because I already own one), but will ultimately get a modern 20” shaft. Would prefer to not make a short transom only to go back and add to it.

Jbo
You can do a jackplate but you would need it to have a pretty significant set back in order to avoid any clearance issues with tilt/trim of the motor. You also may have an issue with your tiller clearing the transom. I would do some testing and find a way to set your motor on some saw horses to mimic the setback of the jackplate and see if there are any clearance issues.
But I agree, I wouldn't cut the transom for a motor that would be temporary.

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TomW1
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Re: 15” shaft on a 20” transom

Post by TomW1 »

jbo that would be the best way to do it. A 4" set back fixed jack plate would also be the cheap way to go like these https://www.boatbuildercentral.com/prod ... ack-plate/ would be the way to go. Just mount it 5" lower than you would a 20" shaft motor. Oops, was writing this as Reid was posting, he makes a lot of sense the Mini Jack Plate has a max set back of only 6.125" so it will be up to you to decide if that is enough. The only other way is to build out the transom or buy a much more expensive jack plate.

Tom
Last edited by TomW1 on Wed Nov 02, 2022 3:44 pm, edited 1 time in total.
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Re: 15” shaft on a 20” transom

Post by fallguy1000 »

TomW1 wrote: Wed Nov 02, 2022 2:53 pm jbo that would be the best way to do it. A 4" set back fixed jack plate would also be the cheap way to go like these https://www.boatbuildercentral.com/prod ... ack-plate/ would be the way to go. Just mount it 5" lower than you would a 20" shaft motor.

Tom
Actually, this is not ideal Tom. I understand your good intentions here, but I am experiencing some issues with a plate that is higher. What you are doing is installing the plate EVEN LOWER! What will happen with a low plate? Water coming off the hull will hit the plate and go everywhere, worse than mine which is mounted in the regular holes.

Not a good solution. Get a new motor. Sorry, but you will regret a 5" dropped jackplate a lot.
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OrangeQuest
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Re: 15” shaft on a 20” transom

Post by OrangeQuest »

Build a small bracket that will give you the clearance and the depth. Bolt it to the transom in the motor bolt holes. When you upgrade just remove the bracket. Cheapest and easiest way to go. The offset may put your prop in a sweet spot that doesn't require the motor being that low and minimize clearance issues.
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Re: 15” shaft on a 20” transom

Post by cape man »

I'm going to disagree with everyone... :lol:

Build it with a lower transom and add to the transom when you upgrade to a 20" shaft. Easy to do, and cheaper than a bracket.

Or...upgrade to a 20" shaft before you launch.
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jbo_c
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Re: 15” shaft on a 20” transom

Post by jbo_c »

Cape man, I think you have good points, but this is a displacement boat, so I think your concerns may be less(not “no”) concern since speeds will be in the 6knot range vs 25.

I may end up getting a 20” pre-launch, but odds are slim. Just looking for an acceptable, not ideal, option.

Jbo

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Re: 15” shaft on a 20” transom

Post by jbo_c »

The irony, of course, is that the cost of the jackplate could be a significant offset for the cost of a used outboard. Issue is that I never see high thrust outboards on the used market locally.

Jbo

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Re: 15” shaft on a 20” transom

Post by OrangeQuest »

jbo_c wrote: Wed Nov 02, 2022 7:02 pm The irony, of course, is that the cost of the jackplate could be a significant offset for the cost of a used outboard. Issue is that I never see high thrust outboards on the used market locally.

Jbo
That is why I suggested to build a bracket. You should have a few pieces of plywood from cutting windows and angles to laminate a few of them together and make it strong enough to hold up to a 18HP. It can also be custom made for clearance to tilt and steer. If and when you get a new motor just unbolt the bracket. I don't know if it is remote or tiller steer but should be easy to adapt back to a transom mounted motor. Being a houseboat, I don't see it being tiller steered.
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jbo_c
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Re: 15” shaft on a 20” transom

Post by jbo_c »

I like the “build a bracket” idea. Heck, I’m almost always up for build-a-anything ideas. I just have visions of my motor torquing it into pieces because I didn’t make it strong enough. (May still do it, though.)

Yes, remote steer.

Jbo

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