# Transom Height / Motor Fit question...



## AlexB (Mar 18, 2017)

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










Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 18, 2017)

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).




Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Mar 18, 2017)

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. 


Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Mar 18, 2017)

I've also used stingray hydrofoils and they are the cats pj's as well. 


Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 18, 2017)

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.




Sent from my iPhone using Tapatalk


----------



## lckstckn2smknbrls (Mar 19, 2017)

As long as the water pickup is getting enough water to keep the motor cool your fine.


----------



## AlexB (Mar 19, 2017)

Thanks again, guys! Headed to the lake in a few!






Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 19, 2017)

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.


Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 21, 2017)

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?


Sent from my iPhone using Tapatalk


----------



## lckstckn2smknbrls (Mar 21, 2017)

One of the hydrofoils brands does have a no drill mounting system.


----------



## rotus623 (Mar 21, 2017)

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


----------



## AlexB (Mar 21, 2017)

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). 




Sent from my iPhone using Tapatalk


----------



## uncndl1 (Mar 21, 2017)

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

Sent from my SM-G920V using Tapatalk


----------



## Weldorthemagnificent (Mar 21, 2017)

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. 


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

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
> 
> Sent from my SM-G920V using Tapatalk



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.


----------



## rotus623 (Mar 21, 2017)

Weldorthemagnificent said:


> I am a hydrofoil believer. Gives stern lift
> 
> 
> Sent from my iPhone using Tapatalk



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.


----------



## rotus623 (Mar 21, 2017)

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.


----------



## AlexB (Mar 21, 2017)

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.




Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 21, 2017)

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?!?)


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

AlexB said:


> (What were they thinking making an 18" transom that doesn't really fit a short OR long shaft?!?)
> 
> 
> Sent from my iPhone using Tapatalk



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.


----------



## Weldorthemagnificent (Mar 21, 2017)

That hydrofoil will allow the motor to be run at a higher trim than it would run at without. Setting the prop back. It works. My last tinny had an 18 1/2" transom and it cavitated just like this one. The wing cured all woes. On my other boats I have actually raised the motor so the anti ventilation plate was slightly above the bottom of the boat. 


Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 21, 2017)

Just got off the phone with Gregor. Apparently I have one of about a dozen Gregor boats that were made specifically for a certain long(ish) shaft Mercury that was available back in the mid-late 1970s. Lucky me...

According to Gregory at Gregor, this boat will not work for a short shaft without modifying the transom, so I think that's what I'm going to have to do...


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

Weldorthemagnificent said:


> That hydrofoil will allow the motor to be run at a higher trim than it would run at without. Setting the prop back. It works. My last tinny had an 18 1/2" transom and it cavitated just like this one. The wing cured all woes. On my other boats I have actually raised the motor so the anti ventilation plate was slightly above the bottom of the boat.
> 
> 
> Sent from my iPhone using Tapatalk




Cool man good to know!! And I'm with you, most boats that I have had have been run with the anti-vent plate ABOVE the keel by 1" or so.

Sounds like you have a few things to try before cutting into that hull, OP.


----------



## AlexB (Mar 21, 2017)

I guess it might be worth trying a hydrofoil, but I'm hesitant to throw more money at gadgets that might not fix my problem.

Now that I know my boat has a "unique" transom height, the idea of chopping it back down to standard height doesn't sound too bad. It should be as simple as cutting, sealing the cut edge with penetrating epoxy, and installing a new "cap" made of aluminum channel (or two opposing aluminum angles bonded together w/ JB weld). 

Doesn't need to be pretty, just needs to work.


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

No need for a hack job, that's a good looking boat.

Honestly, an 18" transom isn't that rare. Lots of older motors (same era as the boat) had short shaft motors from 15" to 18". The issue is that you either told the boat dealer that you needed a short shaft motor, or you told the boat dealer that you had an 18" transom and they told you that a short shaft Tohatsu motor would cut it. These motors can afford to be clamped up and bolted in a little higher than you have yours mounted. So the 1" offset that you need, maybe 2" higher mounting height would leave you with the anti-vent plate 2" below the keel. I think that would have worked great!


----------



## AlexB (Mar 21, 2017)

rotus623 said:


> No need for a hack job, that's a good looking boat.
> 
> Honestly, an 18" transom isn't that rare. Lots of older motors (same era as the boat) had short shaft motors from 15" to 18". The issue is that you either told the boat dealer that you needed a short shaft motor, or you told the boat dealer that you had an 18" transom and they told you that a short shaft Tohatsu motor would cut it. These motors can afford to be clamped up and bolted in a little higher than you have yours mounted. So the 1" offset that you need, maybe 2" higher mounting height would leave you with the anti-vent plate 2" below the keel. I think that would have worked great!



I'm not sure I'm following you. Are you pretty much saying I should have bought a long shaft motor and mounted it high? 


Sent from my iPhone using Tapatalk


----------



## AlexB (Mar 21, 2017)

For what it's worth, bolting the motor to the transom is not an option. If I leave it outside overnight it WILL be gone in the morning. 

I need to be able to unclamp it and bring it into the garage for safe storage.


Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Mar 21, 2017)

AlexB said:


> For what it's worth, bolting the motor to the transom is not an option. If I leave it outside overnight it WILL be gone in the morning.
> 
> I need to be able to unclamp it and bring it into the garage for safe storage.
> 
> ...


Sounds like you need a better neighborhood! Lol. 


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

AlexB said:


> rotus623 said:
> 
> 
> > No need for a hack job, that's a good looking boat.
> ...



Well "should" is a strong word. But I would have done that if it were me. Maybe even have gotten the mini jacker and put the longshaft engine on there. I'm just saying that "short shaft" and "long shaft" were used pretty loosely back when your boat was in production, so it's not necessarily a problem unique to your boat. Just sucks that you ponied up for a brand new motor and have to dick around with it to get it right.


----------



## AlexB (Mar 21, 2017)

Yeah, it does suck. Hell, I even called Gregor before I bought the motor and "confirmed" that a short shaft was what I needed for my boat. They didn't ask me first for a measurement or HIN at the time, so I guess they weren't thinking of the dozen or so "unique" hulls they made in 1977.

Oh well. I'll make it work. By the time I get this thing fixed up I'll be much more familiar with working on aluminum boats... I'm sure it won't be the last one I ever own.


Sent from my iPhone using Tapatalk


----------



## rotus623 (Mar 21, 2017)

I know that's right! I'm sure you will figure it out but keep us posted. Lots of good ideas to try before you start cutting.


----------



## AlexB (Mar 21, 2017)

Options I'm seeing:

- Hydrofoil (I'm still not seeing any no-drill options for an 8-HP outboard - please point me in the right direction if you know of one). Cost: ~$100

- Jack plate (Is there any way to make this work for a clamp on/tiller outboard? I'm hesitant to drill a bunch of hole through my new transom wood without KNOWING it will solve my problem.) Cost: ~$100 plus ~$20 worth of SS hardware.

- Cut ~2" off the transom, epoxy the exposed end grain, and create a new transom cap from some aluminum channel (or pay a metal shop to fab one). Cost: Maybe $50 if I do it all myself, and more like $200 if I have someone fab the transom cap for me.

I guess it wouldn't hurt to try a hydrofoil before I cut the transom.

Which hydrofoil do you think is best to eliminate prop ventilation and allow me to run my motor at the proper trim? (not trimmed down excessively like it has to be now).


Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Mar 21, 2017)

Stingray classic jr. 


Sent from my iPhone using Tapatalk


----------



## Tman (Mar 22, 2017)

does anyone know about the Attwood Hydrostabilizar Jr? the fins aren't flat like traditional stabilizers. they have a slight upward angle, says the end comes out of the water when on plane to reduce drag.

https://www.amazon.com/gp/product/B001O0D6RY/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=A343JNTQYJ9AIC


----------



## AlexB (Apr 3, 2017)

Went for it... I'm happy with how it turned out. I'll post a report after I give it a whirl.








Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Apr 3, 2017)

Looks good!


Sent from my iPhone using Tapatalk


----------



## AlexB (Apr 4, 2017)

Thanks! I put the motor on there for a minute yesterday, and it looked MUCH better. 

Hopefully I'll get to try it out this weekend.


Sent from my iPhone using Tapatalk


----------



## AlexB (Apr 10, 2017)

I took the boat out yesterday to try out my transom modification. Wow... night and day... I can now run the motor at proper trim, and I couldn't even get the prop to ventilate if I tried. It felt rock solid and performed just how I hoped it would. 


Sent from my iPhone using Tapatalk


----------



## Weldorthemagnificent (Apr 10, 2017)

Thumbs up!


Sent from my iPhone using Tapatalk


----------



## onthewatersx3 (May 8, 2017)

The mini-jacker looks like one single cast piece of metal, a simple bracket with no moving parts. If true I don't believe the earlier blogger saying you could adjust and "dial in" the exact height would be accurate using the mini-jacker. There are more expensive jack plates which do allow about 4 inches of height adjustment BUT they'll run you a lot more money and the one I bought weighs about 50 lbs. which is not a great idea on the tail of a short boat - you could be very transom-low / bow-high at rest. There are Pro's and Con's. Maybe you can position the stationary mini jacker at just the right height to lower the cav plate but still allow the motor to rotate freely over the transom? You can always send it back if it doesn't line up lol. Good Luck! 

Sent from my SM-N900V using Tapatalk


----------



## AlexB (May 9, 2017)

onthewatersx3 said:


> The mini-jacker looks like one single cast piece of metal, a simple bracket with no moving parts. If true I don't believe the earlier blogger saying you could adjust and "dial in" the exact height would be accurate using the mini-jacker. There are more expensive jack plates which do allow about 4 inches of height adjustment BUT they'll run you a lot more money and the one I bought weighs about 50 lbs. which is not a great idea on the tail of a short boat - you could be very transom-low / bow-high at rest. There are Pro's and Con's. Maybe you can position the stationary mini jacker at just the right height to lower the cav plate but still allow the motor to rotate freely over the transom? You can always send it back if it doesn't line up lol. Good Luck!
> 
> Sent from my SM-N900V using Tapatalk



I already modified my transom, and I'm happy with the results (see pic above). The prop doesn't ventilate anymore, and the boat performs just as it should. 

I didn't like the idea of permanently installing a fixed jack plate without KNOWING it would fix my problem. 


Sent from my iPhone using Tapatalk


----------



## onthewatersx3 (May 9, 2017)

Sorry for my Beginner's Mistake, I posted before getting to the end of the thread to see you'd already solved it. Glad it worked out for you! 

Sent from my SM-N900V using Tapatalk


----------



## AlexB (May 9, 2017)

All good! I do the same thing sometimes...


Sent from my iPhone using Tapatalk


----------



## skipper123 (May 10, 2017)

WOW you did a great job cutting that transom looks like a factory job to me. I have rigged out a lot of boats over the years and have found a hydrofoil sized for the outboard always helps getting on plane stays on plane at lower speeds and helps with prop blow out on sharp turns. As a rule you can trim motor up one pin hole just by adding one which some times picks up the speed a little it all depends on the hull design. I have never experienced any negative effects of running a hydrofoil if its sized properly for the outboard as some have said. I drill them holes in a skinny min on my new outboards and enjoy the benefits. I usually run a new outboard for a while so I know exactly how it performs without it and then install so I can see the difference. I am doing some research now as to which one I'm installing on the new 30HP Tohatsu. I want to get the RPM gauge installed first so I can see the difference with and without.


----------



## Stumpalump (May 10, 2017)

skipper123 said:


> WOW you did a great job cutting that transom looks like a factory job to me. I have rigged out a lot of boats over the years and have found a hydrofoil sized for the outboard always helps getting on plane stays on plane at lower speeds and helps with prop blow out on sharp turns. As a rule you can trim motor up one pin hole just by adding one which some times picks up the speed a little it all depends on the hull design. I have never experienced any negative effects of running a hydrofoil if its sized properly for the outboard as some have said. I drill them holes in a skinny min on my new outboards and enjoy the benefits. I usually run a new outboard for a while so I know exactly how it performs without it and then install so I can see the difference. I am doing some research now as to which one I'm installing on the new 30HP Tohatsu. I want to get the RPM gauge installed first so I can see the difference with and without.



The flat aluminum ones are the thinnest and fastest. Permatrim makes a really nice one and so does Bob's Machine shop called a True Tracker. The thick plastic ones add too much drag and are often so big they handle weird in turns. The very best is built exactly like the factory cavitation plate. It's a flat aluminum plate that you cut to size. Here is one I made in this thread that worked really nice. Nice easy DIY project that works better than the plastic fins. https://www.tinboats.net/forum/viewtopic.php?f=3&t=39681&start=30


----------



## AlexB (May 12, 2017)

skipper123 said:


> WOW you did a great job cutting that transom looks like a factory job to me. I have rigged out a lot of boats over the years and have found a hydrofoil sized for the outboard always helps getting on plane stays on plane at lower speeds and helps with prop blow out on sharp turns. As a rule you can trim motor up one pin hole just by adding one which some times picks up the speed a little it all depends on the hull design. I have never experienced any negative effects of running a hydrofoil if its sized properly for the outboard as some have said. I drill them holes in a skinny min on my new outboards and enjoy the benefits. I usually run a new outboard for a while so I know exactly how it performs without it and then install so I can see the difference. I am doing some research now as to which one I'm installing on the new 30HP Tohatsu. I want to get the RPM gauge installed first so I can see the difference with and without.



Thanks! I'm definitely happy with the way it came out. 

I really have no complaints with the way the boat handles at this point, so I probably won't spent the $ to add a hydrofoil. If the day comes when I have some free time and a hankering to tinker with my boat, I might make a DIY hydrofoil using some 1/8" aluminum sheet I have stashed in the garage. 


Sent from my iPhone using Tapatalk


----------

