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Old 11 October 2021, 14:47   #1
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Transom extension and less stress, possible???

I know transom extensions are less than ideal because of the added leverage and thus stress put on the transom joints. Still I made a transom extender. What do you do when you have a fine Tohatsu 18 long shaft, and short shafts are so hard to find? At least if less than 30 years old and in good condition.

I was thinking that maybe there is a way to have a transom extender and not add any stress to the transom, rather the opposite. Has anyone ever tried to make a "pad" low down on the back of the transom that the outboard shaft would rest against under throttle? I have never seen this and perhaps it is a silly idea but if it would be possible to to let the mid/lower part of the shaft touch the back of the transom via some form of shock absorbing pad, that would remove most of of leverage and transom height would not matter much. At least that's my thinking. Is this crazy or worth pursing?

Anyway here is the extender before paint and some other stuff. No new holes were drilled. Only a slight modification to the transom plate was necessary. An arrow is pointing to where the pad would be.
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Old 11 October 2021, 15:18   #2
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It’s not the steady load from the thrust that’s the issue so much as the hammer action of the motor bouncing around. But as this is a sib and the transom moves anyway, I reckon you might get away with it.
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Old 11 October 2021, 15:30   #3
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Looks good to me I wouldnt go for the extra block just run it how it is, looks a pretty strong job to me. Sibs especialy pvc ones are realy a limited life boat anyway & cost way less than the engine to replace. If it dies buy a longshaft boat to suit the engine. In the mean time enjoy what you have

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Old 11 October 2021, 16:59   #4
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If you bolted it on would the bolts be through the existing transom?
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Old 11 October 2021, 17:25   #5
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If you bolted it on would the bolts be through the existing transom?
I assume you mean the pad/block. If I were to put the pad in place I would reuse to lowest transom bolts or just glue something to the transom (now that I'm learning about glue ). This part is not clear to me yet but it would probably be a good idea to use the lower bolts that are already in place.
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Old 11 October 2021, 17:59   #6
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Ahh no sorry I meant if you bolted the outboard on. Like beamishken I'd not worry about the pad.
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Old 11 October 2021, 18:19   #7
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Ahh no sorry I meant if you bolted the outboard on. Like beamishken I'd not worry about the pad.
Ok. I don't plan on bolting the outboard to the transom since this is more of a portable unit. So I haven't taken that into account.
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Old 12 October 2021, 14:08   #8
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I think I understand what you are suggesting. I see three problems:

- you will lose the ability to trim the engine. That might not sound important but with different people, equipment, and even weather conditions it can be handy to move it in or our a hole from your default position

- the engine rotates at this position so making a ďcupĒ for it to sit in would require some thought - I donít think the back of the engine is a nice smooth single radius to make that harder - especially so it sits nicely at the given trim angle - and accommodates the position error each time you fit / remove the engine. Iím sure itís doable - but itís not simply a case of screwing a bit of wood on otherwise the entire thrust of the engine will be delivered through a very small area of wood

- you will almost certainly cause wear to the paintwork on the leg. This will be getting wet with sea water and become a growing problem over time.

Iím not sure if youíve followed all the fun people have with spray from some outboard/transom combos - adding any lump below the waterline may give you new issuesÖ.
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Old 12 October 2021, 14:34   #9
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Great job donít see any problems with it
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Old 13 October 2021, 02:31   #10
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I think I understand what you are suggesting. I see three problems:

- you will lose the ability to trim the engine. That might not sound important but with different people, equipment, and even weather conditions it can be handy to move it in or our a hole from your default position
True. I would not loose the ability to trim out, but this feature would only work at one trim setting. Given that I plan to run at the lowest trim setting this would not be a big issue.

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- the engine rotates at this position so making a ďcupĒ for it to sit in would require some thought - I donít think the back of the engine is a nice smooth single radius to make that harder - especially so it sits nicely at the given trim angle - and accommodates the position error each time you fit / remove the engine. Iím sure itís doable - but itís not simply a case of screwing a bit of wood on otherwise the entire thrust of the engine will be delivered through a very small area of wood
I was thinking a more flat surface and not a cup. Otherwise turning will be impaired.

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- you will almost certainly cause wear to the paintwork on the leg. This will be getting wet with sea water and become a growing problem over time.
A couple of layers of PVC patch on the leg should solve that issue.

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Iím not sure if youíve followed all the fun people have with spray from some outboard/transom combos - adding any lump below the waterline may give you new issuesÖ.
Or it may solve them... This would only be below the waterline before planing.

You certainly understand what I'm suggesting. And you're probably correct, this idea is more trouble than it's worth. But it's fun to toss ideas around.

In this case it'll probably work just fine without this "thrust-pad", especially since the engine is 18 hp and the transom is rated for 40 hp. And the one size up Zodiac takes a long shaft and, I think, the transom is fitted exactly the same way in the same position. My transom is just cut lower. But then anything could be a problem with 20-year-old glue.
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Old 13 October 2021, 12:11   #11
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physics is not on your side!

I may be wrong, but this is how I see it.

The main problem with the raised height is the extra leverage you put on the transom. The bulk of the supported mass being up high and therefore further away from the center of effort of the supporting element, the transom.

Oh no, I'm sounding like a rubbish teacher here!

Doesn't matter how you support the leg, the leverage is still the same.

What it will do is reduce the bending moment or your transom extension, but I don't see that as a major issue, it looks like you have done a good job.

The leverage will come into play in two situations - bumping around on chop, and when accelerating.

How close is the engines weight to the stated maximum?
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Old 13 October 2021, 12:51   #12
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The leverage will come into play in two situations - bumping around on chop, and when accelerating.

How close is the engines weight to the stated maximum?
The engine is pretty light, less than 45 kg. The max rating is 95 kg. So, it's not even close.
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Old 13 October 2021, 15:32   #13
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Great job donít see any problems with it
This .
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Old 13 October 2021, 15:34   #14
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I had a 1985 avon spitfire and the 35HP outboard required a 2.5" spacer added to the transom. This made it level to the height of the outer edges of the original transom. I did a sandwich like you have but a bit wider. Always bolted the engine. None the less I beat that boat and never had a problem. By beating I mean the kind only an 18 year old can give a boat!
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Old 14 October 2021, 02:56   #15
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The engine is pretty light, less than 45 kg. The max rating is 95 kg. So, it's not even close.
Well then I think you'll be absolutely fine. It looks like you've done a good job, and the small amount of extra leverage should more than be made up for by the fact the engine is not even half the quoted max.
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Old 14 October 2021, 14:58   #16
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This .
This what ?
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Old 14 October 2021, 16:45   #17
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This what ?
This:

One-word response placed after a quote on a message board as an affirmation of the author's agreement
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Old 15 October 2021, 04:23   #18
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This:



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Wot he said ^^^^^^^
Iíll translate for you Jeff
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Old 15 October 2021, 07:20   #19
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This:

One-word response placed after a quote on a message board as an affirmation of the author's agreement
Sorry thought you had posted a link that didnít show up
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Old 15 October 2021, 09:43   #20
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Wot he said ^^^^^^^
Iíll translate for you Jeff
Thanks Dave mind I do like the explanation might have to start agreeing with everyone and start using it
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