Scout Transom replacement

[MikeV]

I have a 1994 15.3 scout 70hp mariner
cracked transom that needs replacement..

It was constructed with the foam diviny cell i think..

I was wondering what people would suggest for a rebuild original or plywood

[PAR]

This is why I dislike cored transoms. The core isn't self supporting, relying on several elements to insure integrity. If one of these elements fail, the whole piece is compromised.

It's your call, but personally I've done dozens of transoms, without a single failure, all with plywood and epoxy. Even if damaged, the plywood is self supporting, regardless if the skins have detached or holes have been punched through it. Even water soaked, it still functions, though life expectancy is greatly diminished. You pretty much have to use a reciprocating saw to ruin a plywood transom, but foams, honey combs, etc. don't need much damage or over tightened mounting bolts to all but ruin the core.

Use a good marine plywood, tab back into the hull shell well and reattach the skins. It's easier to remove the exterior skin, clean out the mess and bond in a new core. On the other hand this means you'll have to paint the transom and your fairing job will be in plane sight, so it needs to be fairly smooth.

If you remove the inner skin, there's a good bit of additional structure you have to work around, but the net result is the exterior skin remains intact, so things look normal (though a good paint job does the same thing). Of course you have to fair the inside surfaces, which have more nooks and crannies to deal with, but it's less noticeable.

Now some will tell you to try Sea Cast, pour in transom core replacement goo. On your boat this will not work without a good bit of "dam" building. The problem is you hull shell and interior liner don't meet at the bottom and is open. This means when you pour in the Sea Cast, it will run down to the bottom of the transom, flow out onto the bottom of the boat. Yes, you could cut back more skin and build a series of dams to contain the poured in goo, but the dams have to be as strong as the rest of the structure or it'll be the weak link.

In the end, I've found it's faster and cheaper to just use plywood and good epoxy encapsulation techniques. I've got repairs that are over 20 years old, that show no signs of problems. I had one here yesterday. He wanted a transducer installed and the marina said they could do it, but he's learned (keep your customers well trained) to talk to me first. My price was considerably less then the marina's so I squared him away by the end of the day (it helps if you teach your customers to bring beer on Saturdays), which was two days less then what they would have required at nearly twice the price.

Decide how you want to approach the repair (cut the outside or inside), then look up the many previous threads on this repair (it's been covered lots of times on this site). Absorb as much as you can, then take some mean tools out to the butt of your boat and start stripping parts. Everything has to come off, so don't be shy. You probably needed to re-do the bedding (caulk) on most of the stuff anyway. Once everything is off (yep, the engine too) you can start cutting. My favorite tool it the reciprocating saw, but circular saws, jig saws, routers, Roto Zips, etc. have and can be used.

[MikeV]

Thanks so much !!!
I wanted to use the plywood because for my it is simple enough i have seen stories about suprises with the foam core and not ever using it.
plywood it is i thought it would be stronger . i do car body work so have all the tools so i am set ..
I realy liked the detailed answer thanks so much.
what types of epoxy would serve me best
mike v summerville sc

[MikeV]

my concern was that this boat was not able to support the 75hp as stated .
newer designs do not have the same configuration and the hp has been lowered.

Because of this I was not eager to replace the transom with the same.

my question is cutting from the transom would be easir but I was afraid to do this ..

I have enclosed a picture ,, and my question is where do i cut from the stern?

[MikeV]

one picture

[MikeV]

picture 2

[dockdave]

couldn't pull up pic #2. hopefully it is a picture of the interior view of the transom. I to have replaced a few transoms, and rarely (never) CHOOSE to pull it apart from the outside. In your case i would pull motor, bilge pumps, fuel filter, stern eyes. Repair exterior transom. Next cut out transom from the interior, i would cut about 4" outboard of stern eyes down 2" from the deck, across boat to same 4" outboard of stern eyes. Now the FUN part pull skin, get rid of all junk. (Transom probably lost it's shape, no worries) Pattern needed insert. Looks to me to you need 2 layers 3/4 marine ply glassed together. I glass block together, then score aft side of block (grooves every 3", 1/4" deep in both directions). To regain flat plane i use angle iron across the transom in two locations one at the lower motor mount bolts, and the other at the stern eye bolt holes. Next glass block into transom bolting (wax bolts; easier disassembly later) through said locations and lots of wedges at deck flange. In the mean time clean off skin, glass new wood in raised panel (looks like ya need small piece 1/2"). Remove angle iron after full cure, and glass skin in place, fair cut lines, paint, re-install motor, pumps, filter, go fishin

[PAR]

Your boat should be equipped with a "Capacity" plate. Check it to see what she's rated for. I don't think 75 HP isn't unreasonable for that boat.

[MikeV]

Yes the picture that did not load was the inside.
Yes the plate states 75 hp ,
Ok I am all set... marine plywood is availible here locally .
I want to buy a lifting eye for the 75 mariner ,I have a engine lift.

questions
Not sure of the wedges
lots of wedges at deck flange
and
Where to cut across on the top of the inside i will send another picture


btw
I am doing this for my son it is his boat I have a larger scout that works

[marshmat]

West System epoxy is nice stuff, but a bit pricey.

In rebuilding, it might be worth looking at how it failed in the first place. That doesn't look like overpowering damage. (Overpowering damage usually looks like the boat being on its side, basically intact, 20 feet underwater.) It looks more like the fatigue cracking from the heavy motor being bounced around when trailering- by far the harshest conditions a small boat's transom sees. (Consider a "transom saver" bar next time.) The worst of it is at the sharp corner where the gas can shelf meets the transom.

When you go to put it all back together, do try to avoid stress concentrations such as that corner, and don't be afraid to use substantially more fibreglass than the original. (Remember it's the glass cloth that creates strength, the resin is only there to hold it together- lots of glass, wetted out just enough to lay against the previous layer with no bubbles.)

It looks like a big repair but I see no reason why the fix wouldn't work.

[robherc]

for what it's worth, I'd be VERY tempted to try using kevlar (in place of fiberglass) for "glassing" the plywood...kevlar has an amount of stretch to it, where fiberglass doesn't. Therefore, the kevlar is a bit more resilient (i.e. can handle being flexed back-and-forth while bouncing on a trailer better).


Just my 2 cents...I can't disagree much on the rest, except that the "light freak" in me wants to suggest nomex honeycomb in place of plywood...but don't listen to him, he costs me (LOTS) of money more often than he really helps with much ;)

[MikeV]

yes I agree with the added support
i also thought about building the corners up.
what type of fiberglass would you recommend for the corners .
I did buy some kevlar,,

I was also concerned with weight ... but like the benifets of plywood.
he did pull the boat a pretty long distance with no transom saver at first then i insisted on one so maybe a stump or no support on the engine during the trailering at first .


Thanks so much everyone , it is an old subject but ,still a white knuckle trip ..without support or knowin that you are going in the right direction.

[MikeV]

2 questions
what type of fiberglass to wrap the plywood ?
what type for the corners ?
darn shift key ;-)

thanks so much!

[dockdave]

mike,
The wedges are to apply pressure at the bottom of the transom. When you install that transom block without skin you get the chance to apply additional pressure between the deck and transom while your batch (glass) sets up. Cut some small hard wood wedges to drive between deck and transom.
If you were standing on the deck above the transom looking down you should cut that "shelf" off. You should need a saw z all with a long blade to make this cut effortlessly.

Did my pops transom put a 225 hp 30" motor on a 20' 1970's mako. As luck would have it ended up with flush transom no cut out great for inlet. For fun of course.

[dockdave]

wrap the plywood? cover the plywood.
2 layers 1 1/2 oz. mat between layers of plywood (glue and screw block together).
2 layers 1 1/2 oz. mat rolled out on interior of transom
1 layer 1 1/2 mat between skin and block
1 layer 1 1/2 oz. mat between skin and 1/2" ply

Mat for your corners. When the old sfuff is out prep underneath of cracks, slam some glass underneath for strength, prep the cracks with a dremel, drop some bondo, spray some gel. your out

[PAR]

It's important to talk apples and apples folks.

Mat isn't used, nor is desirable when using epoxy.

Bondo absorbs moisture and is prone to "shake out" from vibration, which is fairly common on a transom.

You don't need to wrap the plywood in fabric.

You do need to bond the pieces of plywood together and to tab the plywood into the hull shell (not the liner).

Again many previous threads have covered this topic, but a quick over view:

Grind and cut out all the bad stuff inside the transom. Use a very coarse paper, like 16, 24 or 36 grit and insure all contact areas are clean and well roughed up. Grind back the edges of 'glass pieces to be bonded back together several inches in each direction. This provides a place for the epoxy to bond and make the transition from polyester to epoxy, without a hard point. All areas where the plywood will touch or tabbing will contact should be well "toothed".

Cut and fit the plywood pieces. Drill all holes and cut all notches for a fairly close fit, but it doesn't have to be exact. Remove the plywood, apply two coats of unthickened epoxy all over the plywood. Every side, especially the edges and each hole or cutout, needs epoxy on it.

Bond the two 3/4" plywood pieces (or three 1/2" pieces, which is stronger) together, by applying an epoxy, thickened with a structural filler (50% silica, 50% milled fibers will work) with a 1/16" notched trowel. Some people like to drill a thousand little holes so the two pieces will lie flat and let air escape, but I've found if you start at one side, wedge up the other side with a piece of scrap, then trapping air isn't a problem. Screw the two together with 1 1/4" stainless, flathead screws. Use a tight screw pattern like 3" or 4" center spacing. Apply another coating of unthickened epoxy to the plywood. It should shine on every square inch or it's not completely sealed.

Next wet out the contact areas with unthicknened epoxy, inside the hull shell (again not the liner, unless it's a structural liner, which it's probably not). The wet out areas should include a healthy tabbing area, well back into the hull shell.

Mix up more thickened epoxy and coat the contact surfaces with it. In areas where you expect to fill some gaps, apply more, but generally a reasonably uniform coating min. 1/16" thick. This doesn't include the tabbing areas, these should just be wet from the unthickened epoxy coating.

Install the plywood, you'll see thickened goo oozing out around the edges of the plywood. This is good. If you don't see ooze out, you haven't enough stick'um on the contact areas. From the outside, use a few dozen or so screws to suck the plywood back against the cracked and broken up 'glass transom skin. You could wedge it against this outer skin from inside, but you have enough damage back there that you'll have to paint it anyway, so a few filled screw holes isn't a big deal.

Now the plywood is locked place and the goo oozing out everywhere. Smear the ooze out epoxy into a nice neat corner radius, which we call a fillet, around the inside edges, where it meets the hull. A tongue depressor is the perfect tool for this and I make them up from scrap bits of wood and a belt sander. If you haven't enough ooze out to completely fillet around the inside edges of the plywood, then make some more and do so.

Next apply your tabbing. I use biax, just because its a fair amount stronger then regular woven fabric, don't cost much more, it drapes around things better and places more fibers across the joint then 90/90 weave fabrics do and I have a bunch of it. You can use regular 'glass fabric or cloth tape. Fabrics are sold by weights. There's a many different weights. 3 layers of 8 ounce or 2 layers of 12 ounce will work, which ever is available for you. The tabbing should reach back onto the hull shell several inches. This ties the old to the new. Too short of tabbing will fail. Too long of tabbing will not hurt a thing, error on the too long side.

If the fillets are still wet or green (which is preferred) then just lay the strips of cloth on the transom, crossing over the fillet and back onto the hull shell. The wet epoxy will start to change the color of the cloth. Using a chip brush apply unthickened epoxy to the cloth, until it pretty much seems to disappear. Wipe away excess epoxy with a squeegee, moving it to areas that need more epoxy. When all the tabbing layers are applied, you're pretty much set to finish thing off.

[MikeV]

I finally opened this cracked transom
i see when it was assembled there was no contact between the core you can see the gap in the picture glue was put in but there was still a void.

[MikeV]

hey Par - dock dave or anyone
i was all set to use the plwood then i accuired the original foam insert as it was from the mfg...
now what ?
I cut from the inside will post a picture i have 1/2 in foam left on the skin .. i was think of glassing this the adding the new material the 1/2 does have a crack I repaied the outside skin.

the foam is 1.1 thick original was 1.5 with fiberglass skins .. i was still think of using plywood on this to support the 70 hp mariner


Thanks
MikeV

[MikeV]

added one more picture

Thanks MikeV

[dockdave]

Your off to a good start. Time to get sticky.

[MikeV]

Yes I just am not sure of the glue epoxy oredre should i coat the foam first?
should i glass the inner transom i have some heavy biax plus som kevlar
can i still put a layer of ply over it glassed ?