Transom Height / Motor Fit question...

[Weldorthemagnificent]

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.


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[AlexB]

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...


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[rotus623]

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.


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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]

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.


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[rotus623]

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]

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?


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[AlexB]

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.


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[Weldorthemagnificent]

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.


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Sounds like you need a better neighborhood! Lol.


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[rotus623]

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?


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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]

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.


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[rotus623]

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]

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).


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[Weldorthemagnificent]

Stingray classic jr.


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[Tman]

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]

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

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[Weldorthemagnificent]

Looks good!


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[AlexB]

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.


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[AlexB]

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.


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[Weldorthemagnificent]

Thumbs up!


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[onthewatersx3]

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!

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