New transom construction advice needed

PeterZ · Post by **PeterZ** » Thu Jan 05, 2012 6:54 pm

All

Need to replace my 1965 Peshtigo Offshore. A few questions.
First, a little terminology, outboard meaning the transom pieces (horizontal pieces) that you see from the water or outside the boat. Inboard meaning the pieces of the transom (verticle pieces) that face the cockpit of the boat.

1. Any problem using Africam mahogany, that's what I can easily get.

2. I want to use 5/4 mahogany for the horizontal outboard pieces (they are originally 1&1/8" thick, so 5/4 makes sense), 5/4 mahogany for the vertical inboard pieces (they are originally 3/4") and sandwich a piece of 1/2" Okume (sp?), between the mahogany inboard and outboard pieces. I want to strengthen the transom for a heavier newer outboard motor, along with inside bracing still TBD. There was no plywood between the inboard and outboard pieces originally as indicated by some in previous posts.

3. My approach was to have the outboard Mahogany pieces glued up into a panel by a wood shop that can do better then I can (I am sure mine would cup

). Then epoxy a pice of plywood to the mahogany panel, then when that dries, then epoxy/screw the the vertical pieces in place, THEN, cut the entire glued up transom to fit. Which means I have to be careful with plywood and internal vertical pieces that they have proper orientation and screw location with respect to the outboard horizontal pieces and outline of the transom. Or is it better to cut to fit the glued up horizontal outboard piece, then attach the plywood and vertical pieces?

4. I was gonna tell the wood shop to use some waterproof glue to glue up the horizontal pieces. Then I was gonna use epoxy for the plywood and vertical pieces lamination, and screws? Should I be more specific with the glue used for the outside horizontal glue up?

5. I have not really checked yet. But is the edge of the transom square with the face, or is it tapered a bit to match the hull shape over the 2"+ transom thickness?

6. Ok last one. What's the typical shaft length of outboards for this boat. The cut out depth in the old transom from the top of the cut out to the inside edge of the gar board plank is 21"

7. Ok I lied, gotta ask one more. Should I cover the plywood edge with mahogany, at least on top of the transom, and maybe not the whole way around. If its only 1/2 inch Okume. And I cover the outside edge all the way around with say 2" or 3" of Mahogany (1/2" thick to match plywood)' would I want to screw the planking to it, or to just outboard and inboard mahogany pieces.

Thanks for your kind advice and wisdom.
Regards Peter

Torchie · Post by **Torchie** » Thu Jan 05, 2012 9:47 pm

I will address a couple of the questions.

In regards to question 1: Yes you can use African Mahogany, many boat builders do. But be aware of the fact that it looks alot different(in my opion) than Phillipine Mahogany. Tends to be darker and have more grain.

Question 6: You will need a long shaft motor for a transom cut out of this size.
Mine measure about 36" from the bottom of the motor case to the center og the prop hub. You want to make sure the the cavitation fins on the outbaord are below the bottom of the boat.
Hope this helps.
Karl

LancerBoy · Post by **LancerBoy** » Fri Jan 06, 2012 3:58 pm

1. African Mahogany is OK
2. OK to use plywood between layers of solid wood
3. either approach is fine as far as I can tell
4. waterproof glue yes. specific brand, not sure
5. edges are most likely tapered to match the hull planking taper
6. long shaft motor needs a 20" transom, short shaft is 15"
7. not a bad idea to cover the plywood on the top exposed edge of the transom

sounds like fun!

Andreas

PeterZ · Post by **PeterZ** » Fri Jan 13, 2012 10:13 am

Thanks for the replies guys. Much appreciated.

On the shaft length and cut out. On further inspection I see my original cut out is 21" from the top of the cut out to the bottom of the garboards. For some reason there is a portion of the keel missing near the transom. Is it the same width and thickness as the keel amid-ship?

So I need to make the cutout 20" from the top of the transom the the bottom of the keel or garboards? Does the cavitation plate of motor need to be below keel or just the garboards, and by how much: level with exactly, 1" below, 1" above?? Sorry to be anal about this but hate to do it wrong and have to correct later

Phill Blank · Post by **Phill Blank** » Fri Jan 13, 2012 11:09 am

PeterZ, Check the length of the leg of the motor you plan on using on this boat and make the transom to fit that motor.
If the transom is original on your boat duplicate it if you plan on using a long shaft motor. If you are using a short shaft motor make it 15 inches give or take a 1/4" or so.

The main thing is which ever motor you use the cavation plate needs to below the bottom of the hull. I think most of my motors the cavation plate is about an 1" below the bottom of the hull.

The keel does stop short of the transom. Again duplicate what is there if in good enough shape to use as a pattern. If not ask someone for the dimensions from there boat.

Good Luck,

Phill

Torchie · Post by **Torchie** » Fri Jan 13, 2012 11:13 am

Just went out and measured my Off shore Transom.
Measurement from top of transom straight down the center to the bottom edge of the keel is 21 1/8 inches.
Keel is appox. 3/4 inch thick at the end.
Yes the keel does taper down from the mid section back.
If you scroll thru the restoration section look for a post titled "reinstalling the keel" or one titled "removing the keel" It has pictures of the keel removed and will give you an idea on the taper.
With these measurements and a long shaft motor you will be fine.
Hope this helps.
Karl.

Torchie · Post by **Torchie** » Fri Jan 13, 2012 11:25 am

Peter Z.
The post that I was refering to is called "Keel removal Pictorial"
and was posted on May 8th 2011. Shows good shots of keel taper.
Also my keel goes right to the end of the boat and is even with the outside
edge of the transom.
karl.

LancerBoy · Post by **LancerBoy** » Fri Jan 13, 2012 1:58 pm

Starting about 1959 the Thompson of Peshtigo boats had the keel stop short of the transom. This was something to do with cavitation. So on your '65 boats, yes, the keel should stop short of the transom, probably by 20 inches or so.

Shaft length is measured from top of transom down vertically. Not the hypotenuse of the traingle, which will be a greater length. Remember 10th grade geometery????

Andreas

JoeCB · Post by **JoeCB** » Sat Jan 14, 2012 7:47 pm

Yes, what Andreas said ! "Shaft length is measured from top of transom down vertically. Not the hypotenuse of the traingle, which will be a greater length. "

To clairify... TRANSOM HEIGHT is measured vertically, not along the inclined face of the transom

Joe B

PeterZ · Post by **PeterZ** » Sun Jan 22, 2012 7:17 am

Thanks for the replies guys. Yeah, it's all about higher math

. I think I know what to do now.

With regards to the keel, mine looks nothing like What's in Karl's pictirial (thanks Karl, that was a great ref.). Mine stops short of transom by about 30". There seems to have been some kind of strip covering the garboard seam as indicated by some white paint, different from surrounding paint. Also the keel seems to be just a strip about 3/4" thick and 1.5" wide or so. It's not a tall rib like thing you would imagine a keel to be like in Karl's pictorial. Remember it's on a 1965 20' Thompson offshore, .... Anyone else's like this? I will take more pics once I get her flipped over.

GulfCoastThompson · Post by **GulfCoastThompson** » Tue Jan 24, 2012 8:49 pm

I have a source that can provide a single piece of mahoghany for the exterior part of the transom. Other than cost, is there any concern I should have with using a one piece transom over the typical 3 piece outer transom?

LancerBoy · Post by **LancerBoy** » Wed Jan 25, 2012 4:58 am

Your boat is a 1965 and Karl's is circa 1955. There were many changes made over that decade.

Your keel should stop short of the transom by about 30 inches. And yes there should be a piece of metal aluminum trim covering the garboard joint for that last 30 inches.

Keel about 3/4 to 1 inch tall sounds correct.

Andreas

Phill Blank · Post by **Phill Blank** » Wed Jan 25, 2012 2:30 pm

GulfCoastThompson,

The advantage to three piece is not only price, but with three pieces there is less chance of cuping or spliting.
One large piece of wood which is not glued up will tend to cup in time and this can cause internal stress to the wood and cause spliting.

Myself, I would glue up three pieces of wood and alternate the direction of the end grain on each piece and glue up with dowels or biscuits in the joints when glueing.

Good Luck,

Phill

john · Post by **john** » Sun Feb 05, 2012 6:19 pm

Building a stronger transom, does not increase the size outboard the boat can take. Other parts of the boat have lost strength over the years. Even if all parts were strengthened, max hp should not change as the shape of the hull can go just so fast.

By rebuilding the transom with more wood you have increased the total weight the hull must float, add a hever engine and the boat will have less freeboard at the stern, than designed for.