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Old 03-01-2014, 02:22 PM   #1
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Seaworthy Dinghy Question

I have an older (1994?) Seaworthy Dinghy which were sold by West Marine way back when.
I have the 10' one which I think measures out at 9' 6".
My transom height is just under 15". It really needs to be about 17" for my O/B motor.
I want to add about 2 1/2" t0 3" to the height of my transom without doing a major transom rebuild.
My idea is to fabricate an aluminum plate about 12" high, 20" wide and 1/4" thick. and bolt it on from the outside. Then bolt some plywood to it on the inside above the existing transom top so I can attach the 90 lb. Tohatsu O/B to it.
Will 1/4" aluminum plate be too thin? Should I use steel and paint it?
has anyone else conquered this problem yet?

Thanks in advance.
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Old 03-01-2014, 02:31 PM   #2
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Originally Posted by Tony B View Post
I have an older (1994?) Seaworthy Dinghy which were sold by West Marine way back when.
I have the 10' one which I think measures out at 9' 6".
My transom height is just under 15". It really needs to be about 17" for my O/B motor.
I want to add about 2 1/2" t0 3" to the height of my transom without doing a major transom rebuild.
My idea is to fabricate an aluminum plate about 12" high, 20" wide and 1/4" thick. and bolt it on from the outside. Then bolt some plywood to it on the inside above the existing transom top so I can attach the 90 lb. Tohatsu O/B to it.
Will 1/4" aluminum plate be too thin? Should I use steel and paint it?
has anyone else conquered this problem yet?

Thanks in advance.
Check out the 4" rise davit heads from Weaver.

Weaver Industries, Inc.: Davits for Inflatable Dinghies
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Old 03-01-2014, 02:57 PM   #3
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Originally Posted by Tony B View Post
I have an older (1994?) Seaworthy Dinghy which were sold by West Marine way back when.
I have the 10' one which I think measures out at 9' 6".
My transom height is just under 15". It really needs to be about 17" for my O/B motor.
I want to add about 2 1/2" t0 3" to the height of my transom without doing a major transom rebuild.
My idea is to fabricate an aluminum plate about 12" high, 20" wide and 1/4" thick. and bolt it on from the outside. Then bolt some plywood to it on the inside above the existing transom top so I can attach the 90 lb. Tohatsu O/B to it.
Will 1/4" aluminum plate be too thin? Should I use steel and paint it?
has anyone else conquered this problem yet?

Thanks in advance.
Standard transom heights are 15" and 20". How did you get to 17"?

I don't think your engineering is sound here. The only thing holding the plywood in place is a piece of 14" aluminum on one side. It's going to bend where the new wood meets the old transom.

Put aluminum plate on both sides and you will have a lot more strength. Bolt through the aluminum and wood sandwich in several places.

A commercial mount to raise the motor will be stronger and look less "homemade". Check the Internet and boating catalogs.
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Old 03-01-2014, 03:14 PM   #4
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I am with Wes. I had a transom riser built for my Avon inflatable so I could use the long shaft Yamaha 9.9 4 cycle (115 lbs) and still have a get home kicker. I never had any problems with it as long as the boat was inflated properly. It had aluminum plate down both sides of the plywood transom and sat very high (the 5" difference) and was through bolted. I had it TIG welded since it would get rolled up inside the boat when deflated and I didn't want any rough edges to wear on the Hypalon. I never did use that engine as a kicker... Should have bought the short shaft :-)
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Old 03-01-2014, 03:57 PM   #5
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Standard transom heights are 15" and 20". How did you get to 17"?

I came up with 17" because that is the the distance from the inside of the O/B mount bracket to the lower vibration plate. The Vibration plate should be from about 1/4" to 1.5" below the transom. In other words, the vibration plate should be in the water when the boat is planning. Tony B
I don't think your engineering is sound here. The only thing holding them plywood in place is a piece of 14" aluminum on one side. It's going to bend where the new wood meets the old transom.
That is what I thought. Tony B

Put aluminum plate on both sides and you will have a lot more strength. Bolt through the aluminum and wood sandwich in several places.
That was my original thought but I can't do this because the transom is already very thick and the motor mount barely fits on now so sandwiching is out. Tony B

A commercial mount to raise the motor will be stronger and look less "homemade". Check the Internet and boating catalogs.
I will see if I could find one. Tony B
My transom does not come straight across. It dips down in the middle section. I'm just trying to avoid a fiberglass project.
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Old 03-01-2014, 04:10 PM   #6
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Tom ...are you talking cavitation/antiventilation plate?

Unless you are talking pretty high performance...being a couple inches too low really doesn't hurt a lot unless you are talking speeds way over 10-12 knots where the increased drag becomes more of an issue.

What HP and speed are you talking about?

My quick stab is I would go steel if down in the 1/4inch range and make sure you get 2 rows of bolts on both sides of the motor and a few inch overlap with the original transom....up the speed slowly and watch what it does (see how solid she stays).
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Old 03-01-2014, 04:16 PM   #7
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The 17"comes from measuring on the flat of your transom. Measure vertically and it will be more like 15".

And it's better to have the cavitation plate lower w a dinghy as speed and low drag is a lesser concern.

Just mount the engine (not motor) and enjoy.

Excellent graphics psneeld.
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Old 03-01-2014, 04:31 PM   #8
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The 17"comes from measuring on the flat of your transom. Measure vertically and it will be more like 15".

And it's better to have the cavitation plate lower w a dinghy as speed and low drag is a lesser concern.

Just mount the engine (not motor) and enjoy.
To be 15" vertical and 17" on the angle (transom)...that means a 28 degree from the vertical rake of the transom...that sounds a bit "excessive".......
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Old 03-01-2014, 05:58 PM   #9
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I think if you dig up the specifications for that dinghy, it either specified a 15" shaft motor or a 20" shaft motor, probably 15".

There are ways to raise the top of the transom and someone with good carpentry skills could do it, probably using a router to make a lap joint. Other ways come to mind but I think you would be better off leaving this to someone with the tools and experience to do a good job if you really believe you need a higher transom.
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Old 03-01-2014, 06:00 PM   #10
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Or use common sense....
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Old 03-01-2014, 10:59 PM   #11
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I did a similar riser on my Livingston. Due to the catamaran, sort of hull, the tunnel left far too much drive in the water even with a short shaft engine.

I raised mine about 3.5", just a bit deeper than ventilation height.

I used two pieces of 1/8" thick alum with a piece of UHMWPE sandwiched between.

I cut the plastic so it is a Tee so the top of the plastic covers the aluminum to protect the engine from sharp edges as I lower the engine into place.

The plates extended below the original transom top untill the inside one just missed riding on the seat.

Levelled it and two 1/4" through bolts have held it to the dingbat now for three years.

Two through bolts through the sandwich and nothing has moved and nothing bends and the boat runs FAR better.

I do have a lighter 8HP 60# Yamaha but I think my riser would hold your engine also.

I would suggest using two pieces of alum., one each side of the transom sandwiching the transom and the engine mounting block. You may prefer to use 3/16 or use the 1/4 if that is what you have.
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Old 03-02-2014, 07:41 AM   #12
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.... you talking cavitation/antiventilation plate? YES. Tony B

Unless you are talking pretty high performance...being a couple inches too low really doesn't hurt a lot unless you are talking speeds way over 10-12 knots where the increased drag becomes more of an issue. YES. Tony B

What HP and speed are you talking about? 7.2 HP top speed unknown. Probably 15 or 20 maybe more. Tony B

My quick stab is I would go steel if down in the 1/4inch range and make sure you get 2 rows of bolts on both sides of the motor and a few inch overlap with the original transom....up the speed slowly and watch what it does (see how solid she stays). That is my thinking. Tony B
Yes, I am talking about the anti-ventilation plate/cavitation plate. The dimension I am talking about is "B" and it is 17".
I have a Tohatsu 9.8 4 stroke. I have the short shaft. The owners manual calls for a transom height of 17.1 inches. I have also measured the dimension "B" with a tape and it is 17+ inches.

This all came about when I put the motor on the dinghy and started using it. This is before I took any measurements. I felt like the cowl was sitting too low in the water. Could be a combination of motor weight (88 lbs) which should be well under the dinghy specs, the gas tank placement and my fat ass.
I tried moving the gas tank up forward and also took a passenger (the admiral) to balance the load. O/B cowl still looked too low. The drag around the cowl was also very evident. So, my problem is not in engine draft but in engine drag. I want to raise engine a few inches to reduce drag on the cowl. The drag is so great that the dink wont plane.
Dinghy specs call for max of 10 HP. I have the 9.8 engine which I believe is actually 7.2 Hp. I also think that the new O/B of 88 Lbs is less weight than a 1994 version of a 10 HP. I could be remembering wrong.
As for skill level, I'm OK on that. I am short on tools but still do-able. I was wondering if a quick fix like a steel plate was a viable choice. I'm sure if the aluminum plate wont work, that a steel plate will. I wanted the quick fix not for ease and speed so much but also to be able to readily un-do.
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Old 03-02-2014, 08:00 AM   #13
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without eyeballing it it's a little disconcerting.....the trouble is leverage versus straight push/pull....if the bottom of the transom mounting plate is pushing against the old transom and the screw tighteners from the front are only catching the addition...the leverage on the 1/4 aluminum plate might be too much....not without backing and more fasteners all the way around.

Maybe not...but again without an eyeball I'm not comfortable with saying one way or another....steel at 1/4 inch I think would be a different story.
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Old 03-02-2014, 08:08 AM   #14
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What HP and speed are you talking about? 7.2 HP top speed unknown. Probably 15 or 20 maybe more. Tony B
I was able to run at 16 mph with a 8 hp engine. I don't think you'll go any faster than that.
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Old 03-02-2014, 08:19 AM   #15
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Thanks J
16 MPH would be fast enough.
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Old 03-02-2014, 08:27 AM   #16
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Rather than use one or a pair of aluminum plates, it would be cheaper and easier to use a pair of Alum or stainless angles on the order of 2" x 2" x 3/16" bolted vertically to the transom, rising upward for mounting the plywood clamp surface. The crack between transom edge and plywood could be sealed with 3M 5200.
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Old 03-02-2014, 09:14 AM   #17
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Rather than use one or a pair of aluminum plates, it would be cheaper and easier to use a pair of Alum or stainless angles on the order of 2" x 2" x 3/16" bolted vertically to the transom, rising upward for mounting the plywood clamp surface. The crack between transom edge and plywood could be sealed with 3M 5200.
It's guys like you that make me feel like if I was around a million years ago, I would have invented the square wheel.
Sometimes the obvious escapes me. This gives me a whole new idea.
Run the bolts through the angle iron and the plywood and then through the transom. This would spread the load and stiffen the whole thing. Then all I would have to do is fabricate another piece of ply to fill in the 'low' area on the transom to give me a thicker, better clamping surface for the O/B.
The only reason I would have to raise it 3" is because of the location of the clamping screws. Before assembly, I could band the edges of the ply and glass with a few layers.
This whole project would only take about a half a day and is completely reversible with the exception of a few bolt holes.
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