transom reinforcement, setback and hydraulic jack-plate

[colinJ]

The boat is a 19' 77 witchcraft with a 175hp late 80s Johnson with a land and sea hydraulic jack plate.

It already has a set of bars.. looks like schedule 80 1/2" stainless pipe that triangulate the transom to the floor with a plate at the top behind the top motor mounts to a large floor plate.

I took off the old trim and will need to do some unexpected fiberglass work to seal it up and the way that the jack plate bolts on all the way to the top edge of the motor well there is no good way to work on the area near the engine without removing it, so since I am taking it off I want to make a reinforcement plate for the transom which will also serve as the trim for the motor well area (by wrapping up and around).

First question, I could cover the entire outside of the transom with 5/6 aluminum bolted through say a dozen 1/2 inserts made of stainless schedule 80 epoxied into the transom with corresponding internal plates. It would weight around 65 pounds. I would retain the current stainless bar triangulation.

Would this be sufficient for more setback? The original transom looks to only be 1.5" thick but it doesn't seem to flex I would assume due to the stainless bars.

Next, how much setback is the right amount, the current jack-plate provides about 4 inches of setback. I mainly want to go faster and turn faster.

Should I ditch the current jack plate?

[XstreamVking]

Make an alum pc as a trim /cover across the transom top. (it will add strength) Make a 2nd pc that the same footprint as the motor/jackplate. 1/4'' thick should be good. Prime, paint and install with a good sealant. Keep the weight as low as you can. Set back for your boat is hard to guage w/o seeing it running.

[Gordon02]

I'm confused. Are you talking about using stand-offs thru-bolted to set the outboard away from the transom? I'd be concerned about the stress and strain of outboard weight, torque, steering, and turning forces on a non-fixed, less than solid surface. Even 4" set-back jack plates use 14~15" of 3/8" thick tempered T6 aluminum with large, flat transom surfaces normally in the 5" wide x 15" tall range having 150 square inches of transom surface that is thru bolted in at least 4 places. The outboard energy/forces are applied in straight-line matching side plates through bolted with at least 4 x 1/2" bolts. I did research for an article that I wrote for Bass & Walleye Boats magazine years ago. With a 175 horsepower outboard, the affects of full thrust through a 20" transom outboard and 4" jackplate was 3,560 lbs/feet of torque at the bolted surface. The article included bolt size, grades, pitch, materials, and strengths.

Better safe than sorry - use care. Gordon

[colinJ]

There has been a slight change of plans, and I could really use some advice with getting the setup into the ballpark. I am replacing the deck/stringers transom and would like to move to full transom with the current 20" v6. This is a super simple hull design and I have already done all of the demo with nothing unexpected found. The transom was a little wet but not rotten, the deck was really bad its all out now so there is no going back. For the sake of brevity and because thats really a different discussion lets assume that I can make the transom as strong as it needs to be and then some.

The keel on this boat has no offset and there is no pad, also the transom is nearly perpendicular to the hull. What I am thinking about is either ditching the hydraulic jack-plate and going to something like the 14" setback bobs machine jack plate or hanging the existing hydraulic jack-plate on a shorter setback bracket (8-10").

I think I could probably get away with running with no setback bracket even without the splash well and notch for a test I think the current jack-plate would easily go high enough to get the motor tilted into the trailering position.

Any advise would be appreciated.


Also wasn't talking about using standoffs.

I'm confused. Are you talking about using stand-offs thru-bolted to set the outboard away from the transom? I'd be concerned about the stress and strain of outboard weight, torque, steering, and turning forces on a non-fixed, less than solid surface. Even 4" set-back jack plates use 14~15" of 3/8" thick tempered T6 aluminum with large, flat transom surfaces normally in the 5" wide x 15" tall range having 150 square inches of transom surface that is thru bolted in at least 4 places. The outboard energy/forces are applied in straight-line matching side plates through bolted with at least 4 x 1/2" bolts. I did research for an article that I wrote for Bass & Walleye Boats magazine years ago. With a 175 horsepower outboard, the affects of full thrust through a 20" transom outboard and 4" jackplate was 3,560 lbs/feet of torque at the bolted surface. The article included bolt size, grades, pitch, materials, and strengths.

Better safe than sorry - use care. Gordon

[Gordon02]

Colin, can you post some photos of the transom, keel shape, side profile, and internal structure?

[colinJ]

[colinJ]

The section in the bilge that looks like a repair is not a repair that's where the bilge pump "box" was glassed over the stringer and up to the deck.



Here is one more shot with a little more perspective and second shot about the only one I could find of one of these running in mod VP. The comments said it had about 24" of setback (doesn't look like that much) and that the motor could have been raised a bit more.

[Gordon02]

You've got an opportunity to design, make, and install knee braces. You'll need those to transfer the stresses of the set-back and leverage to the hull. Will you be adding a keel or stringers?

[colinJ]

I am not sure what you mean by keel in this context. There is a keel guard on the front 2/3's of the boat

I will replicate the original single stringer which has already been cut out and probably add a larger knee onto it where it interfaces with the transom. I was also planning on adding 2-4 knee braces. In the original design I think that the splash well added significant rigidity due to its shape and being fairly thick fiberglass. Since I am eliminating that I might add a horizontal brace kind of shelf like between the hull sides a few inches above the top motor mount bolts that that I might even tie the knees too.

I also may retain this tube frame brace that you can see in the below picture with minor modifications.

Really the transom was pretty strong as it was is, the deck/stringer was rotten. I had those out in a few hours and decided that an extra few days spent replacing the transom would not be wasted.

[Gordon02]

I use the term Keel as the center-line - both inside and outside of the boat. Here you see us replacing both the keel and stringers in this smaller 1971 Astroglass fishing boat after making a rigid and carefully measured hull jig. We're replacing the entire keel and stringers structure with full 1" thick White Oak lumber sealed with CPES (epoxy) and then we embed those into a thick layer of epoxy saturated 1708 and thickened epoxy paste (peanut butter), heavily weighted for several days. We then add more epoxy paste to the corners/joints and create rounded fillets and applying 2 layers of 1708 tabs with the same laminating epoxy. rseveral

We use approx 4 gallons of US Composite 3:1 laminating epoxy on these small boats and 70 feet of 50" 1708 biaxial glass. The transom will be tied into the sides, bottom, stringers, keel, and the two large floatation boxes we've built and added to the stern - the transom will have two 30+" vertical supports tieing them into the flooring and splash well - all built from several layers of 1708 and epoxy - very strong.

[colinJ]

Thats more or less what I am going to do though I want to use coosa as I can get some remnants from a friend meaning I would only need to by one sheet to have a full coosa transom, stringers and deck.

I would love to see a picture of the vertical supports. The reason I want to decide on the outboard bracket now is that it will drive the room and shape for supports and knees.

What do you think I should do on the transom thickness? Original was ~1.6" plus the outer glass. It looks like it was 3 layers of 1/2" plywood with a layer of fiberglass between the layers.

This particular boat does not have stringers other than the keel as the deck is glassed into the inside of the strakes which are built up about 1.5" thick with a shelf like structure on the inside.

[XstreamVking]

Transom, 2 layers of 3/4'' coosa with glass between and over . By deck I believe you mean floor? Knees, put them on the 2nd set of strakes from the bottom. Go from deck to hull and cut floor pc to fit around them. Glass to hull transom deck and then floor.

[phillnjack]

if that was mine it would have a transom all the way across as high has possible. do away with the splashwell.

[colinJ]

So I am to the point in my transom replacement where I need to drill the skin and layed up transom core so that I can use the bolt holes as clamps with lumber and all thread.

i plan to use the 14” setback bobs jack plate. Also I am eliminating the splashwell. I made a template on a Cnc mill that starts at the cavetation plate and has holes spaced every few inches.

Where should I drill the transom relative to the cavetation plate and range of adjustment?

I am thinking I should put the lowest possible setting such that the cavetation plate is an inch below the keel then at max lift it will be 5inches above the keel.