Transom Height / Motor Fit question...

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AlexB

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Hey guys,

Below you'll see a couple pics on my 12 foot Gregor U-123 (1977) fitted with a new short shaft 8-HP Tohatsu four stroke.

The Tohatsu manual calls for the anti-ventilation plate to be 0.2-1" below the bottom of the hull, but on my boat it's actually about 1" above.

Everything I've read (and heard directly from Gregor over the phone) indicates that the 12 foot Gregor is built to accept a short shaft outboard.

Do you think this is going to be an issue?

Please let me know.

Cheers,

Alex
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For what it's worth, the transom itself measures about 18". (Measured vertically to a straightedge pressed against the bottom of the hull).

The Tohatsu manual says the Transom Height is 17.1 inches on my short shaft.

Seems to me like the next size up (22.1 inches) would be submerged too deep (especially based on their recommendation that the anti-ventilation plate be 0.2-1" below the hull).




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I inadvertently had this problem on a boat when I made an aluminum brace plate to fit over the transom on an old boat that had an already slightly tall short transom. The anti ventilation plate wound up close to an inch high. The boat ran fine but did cavitate easily on turns. I tried a doel fin hydrofoil stabilizer and it cured it plus made that boat pop up on plane like nobody's business. Try it out first but I think a hydrofoil stabilizer would be worth its weight in gold on your rig.


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Thanks, guys.

I thought for a minute that one of the previous owners had raised the transom and welded on a new aluminum "skin", but on closer inspection it's definitely original (the HIN is stamped into it).

I replaced the transom wood not long ago, and I'm taking it out to the lake tomorrow for its "mayden voyage".

I won't be tearing around the lake at WOT tomorrow (need to follow the break in procedure for the new outboard) but maybe next trip I'll find out if I have any cavitation problems. If I do, I'll definitely look into a hydrofoil.




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As long as the water pickup is getting enough water to keep the motor cool your fine.
 
Thanks again, guys! Headed to the lake in a few!
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Out on the lake now. The prop does "ventilate" a little on sharper turns and going over wakes, but seems to be OK going straight or making wide turns.

I've only run up to half throttle so far (per the break in instructions), but the boat gets up on plane easily at half throttle with my GF and I.


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Well, I've concluded I need to do something to address the prop blow out / ventilation issue. It didn't happen ALL the time, but enough that I'm worried about the added wear and tear on my motor due to over revving when the prop breaks free.

I had the least amount of prop blowout with the motor trimmed down all the way (thrust rod in the hole closest to the transom), but in that position the prop was definitely facing downward... it handled OK at half throttle, but I'm not sure how it'd do at WOT (haven't been able to try it yet... still breaking in the motor).

I guess my options are...

-Try some sort of hydrofoil and see if it prevents prop blowout and allows me to run the motor at the proper trim angle. I'm pretty sure this would involve drilling holes in my brand new outboard - something I'm hesitant to do without KNOWING it'll work.

-Lower the transom... Cut ~1.5" off middle section of the transom and install a new transom cap. I've only got $900 invested in the hull, and I have no plans for selling it in the future.

Thoughts?


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I have the perfect solution for you.

Try the mini-jacker, it is a fixed jack plate for clamp on outboards and can be used to raise or lower the transom. Other cool thing is you can raise or lower the mount to really dial the height in. Problem: solved!

https://www.basspro.com/Mini-Jacker-for-Clamp-on-Outboards/product/8782/?hvarAID=shopping_googleproductextensions
 
Which brand is that? I saw a few no-drill hydrofoils advertised, but they were only "no drill" for larger outboards. (Mine is an 8-HP Tohatsu).




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Read the FAQs first. This product is for long shaft motors in a short transom. Says will not work to lower motor for a short shaft

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First time I drilled holes in my brand spanking evinrude 25 I was sweating bullets! It worked great and did everything it promised. I am a hydrofoil believer. Especially on the smaller hulls when you get some weight in them. Gives stern lift and allows plane at a lower speed. Either way you go will work.


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uncndl1 said:
Read the FAQs first. This product is for long shaft motors in a short transom. Says will not work to lower motor for a short shaft

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I know two guys that have used them to lower their motors and have had no issues. As long as the motor can swing both ways without hitting there will not be a problem. A smaller engine like this should not have any issues.

Also, the 4" set back will likely allow him to keep the motor at the same height, as the water "lifts" past the transom. He could at that point add a short extension arm to the tiller handle.

That being said, I have had "whale tails" on tiller motors and not been happy with them. It really takes from the agility of the tiller. It also makes sharp turns harder because the boat doesn't lean into the turn as easily. Just my opinion on the matter, and another suggestion.
 
Weldorthemagnificent said:
I am a hydrofoil believer. Gives stern lift


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I'd be concerned that stern lift would further increase cavitation.

In any case, I do not believe a hydrofoil will be a fix for a motor that is simply not low enough in the water. Best bet is to set it back some and hope he can keep it at the same height, or maybe drop it 1/2" if needed.
 
And if that don't work, try this!! They are motor extension kits. Your kit should be #301A

https://www.baymfg.com/kits/tohatsu/

Cutting the transom is surely an option, but I like to have all the freeboard that I can have, especially in a small boat.
 
Rotus - I thought about adding an extension kit, but I'm not sure if it would affect my warantee. (This motor is literally brand new.)

Also, adding a 5-inch extension would create a 22 inch shaft length (Tohatsu short shaft is 17"), which would put the anti-ventilation plate a full 4 inches under the bottom of the hull.




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I'm gonna call Gregor in a bit and get their thoughts... (What were they thinking making an 18" transom that doesn't really fit a short OR long shaft?!?)


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AlexB said:
(What were they thinking making an 18" transom that doesn't really fit a short OR long shaft?!?)


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I know right!! That transom is perfect for an 80's Johnson/Evinrude, as their SS motors measure 18.5" if memory serves me right.

Good idea about the warranty. If it were me I went with the extension kit, I would just take the extension kit out if you had to bring it to service for warranty work.

I have set up a good deal of boats in my day. I have learned that some guys are able to raise their engines up 3-4" for 4-6" of setback. I think if you put that minijacker or another kind of fixed 4" setback plate that you could leave the motor at the height that it is at and stop cavitating.
 

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