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Transom Fiberlgass Repair

Old 12-19-2017, 09:59 AM
  #101  
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why the scarf joints in the wood?
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Old 12-19-2017, 01:51 PM
  #102  
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Originally Posted by Raybo Marine NY View Post
why the scarf joints in the wood?
Figured I needed the strength? Butt joints would be mush easier otherwise....
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Old 12-19-2017, 02:06 PM
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Butt joints will work...just glue the edges up well as end grain will absorb resin....scarfing is no big deal either...just making the same cut on a 45 degree...a little neater.....peace of mind...but not totally necessary
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Old 12-19-2017, 02:15 PM
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too late now but usually you remove the rub rail and make the cut straight up, then you can just run the core up high
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Old 12-19-2017, 03:06 PM
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Default joins in plywood

Originally Posted by maxie View Post
Butt joints will work...just glue the edges up well as end grain will absorb resin....scarfing is no big deal either...just making the same cut on a 45 degree...a little neater.....peace of mind...but not totally necessary
A butt join is totally next to useless and 45 join is a little better but not much !! even a 10/1 is reasonable with epoxy but a 15 /1 is much better and gives a join that's almost as strong as the original ply !.
Added strength is always an advantage with all and everything you make !! composites is no different ! How the layers are stacked together can make a really big difference to ridged strength !!, the placement of different orientations of fibers not to many guys can comprehend what can be gained stiffness and rigidity or flexural strength so bending can take place to absorb shock loads without breaking or even fracturing at all !!And inn the cases of damage laminates that will last longer and hold together for much much longer!! All these things are achievable by mixing different glasses in different ways !!
So a good and proper scarf join with wood is necessary or you loose rigidity at the join and create and introduce a bending, breaking point weakness and point of potential FAILURE !!
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Old 12-19-2017, 04:16 PM
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In this application it's not going to make a difference whether you make a butt joint or scarf joint.

You aren't relying on the strength of the plywood, the fiberglass skins are providing the strength. Plywood is strong, but one of the main reasons it's used in transoms is because you need a core with fairly good compression strength so you can through bolt the motor in place without crushing the core, plywood does this fairly well. Plus it does it for far less money than other cores of equal compression strength.
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Old 12-19-2017, 04:22 PM
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Originally Posted by ondarvr1 View Post
In this application it's not going to make a difference whether you make a butt joint or scarf joint.

You aren't relying on the strength of the plywood, the fiberglass skins are providing the strength. Plywood is strong, but one of the main reasons it's used in transoms is because you need a core with fairly good compression strength so you can through bolt the motor in place without crushing the core, plywood does this fairly well. Plus it does it for far less money than others cores of equal compression strength.
Gee ondarvar-stop making sense, if I can understand it, you must be good.
Side note, I used multiple layers of 1708 with epoxy on deck replacement with divinycell,it's very strong! Opinions please!
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Old 12-19-2017, 04:38 PM
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Originally Posted by ondarvr1 View Post
In this application it's not going to make a difference whether you make a butt joint or scarf joint.

You aren't relying on the strength of the plywood, the fiberglass skins are providing the strength. Plywood is strong, but one of the main reasons it's used in transoms is because you need a core with fairly good compression strength so you can through bolt the motor in place without crushing the core, plywood does this fairly well. Plus it does it for far less money than other cores of equal compression strength.
Add some exclamation points so we can understand you better.
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Old 12-20-2017, 05:50 AM
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Originally Posted by tunnles View Post
A butt join is totally next to useless and 45 join is a little better but not much !! even a 10/1 is reasonable with epoxy but a 15 /1 is much better and gives a join that's almost as strong as the original ply !.
Added strength is always an advantage with all and everything you make !! composites is no different ! How the layers are stacked together can make a really big difference to ridged strength !!, the placement of different orientations of fibers not to many guys can comprehend what can be gained stiffness and rigidity or flexural strength so bending can take place to absorb shock loads without breaking or even fracturing at all !!And inn the cases of damage laminates that will last longer and hold together for much much longer!! All these things are achievable by mixing different glasses in different ways !!
So a good and proper scarf join with wood is necessary or you loose rigidity at the join and create and introduce a bending, breaking point weakness and point of potential FAILURE !!
In regards to scarfing the wood....if you were building a wooden boat and the wood was carrying the loads, then scarfing is critical.......but when wood or composites are used as a core, not so much....you will see many cored hulls being laid up with nothing more than butt joints and a connecting insert to hold panels in place during the glassing step. I personally like scarfing because it gives me a warm and fuzzy but nothing should come apart if its properly glued in place and glassed afterwards.
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Old 12-20-2017, 10:13 AM
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Originally Posted by ondarvr1 View Post
In this application it's not going to make a difference whether you make a butt joint or scarf joint.

You aren't relying on the strength of the plywood, the fiberglass skins are providing the strength. Plywood is strong, but one of the main reasons it's used in transoms is because you need a core with fairly good compression strength so you can through bolt the motor in place without crushing the core, plywood does this fairly well. Plus it does it for far less money than other cores of equal compression strength.

Respectfully, disagree.

The thinness of the inner transom glass laminate layer makes it clear that the original build design expected and used strength that was in the wood (coring), itself.

When a light weight coring is used (something much lighter than wood), design of the panel must include enough structure (glass) on front and back to carry all of the tension and compression in the panel. In this case, the coring will see large shear loads, small compression loads, and nearly no tension. If the coring is to carry only shear load, something like nidacore, or other rigid foams, or balsa work very well. Very light. Adequate shear strength. Plywood would work, but, would just be heavy. In this case, tightly butting portions of coring up against each other effectively transmits the shear. However, it will not transmit any appreciable compression or tension loads. The tension and compression must be carried by the outer glass layers.


Using butt joints would remove any ability of the wood to carry bending load to the hull sides or bottom. If this transom design had been built to use a light weight coring, the, I'd agree, butt joints would be adequate. However, since the wood is expected to carry load (other than shear) scarfs or laps are needed. Epoxied laps would work and might be easier to build.


Further, I was going to suggest that if the plywood is added in two steps, I'd run my scarfs in different directions. Make the first layer joints vertical and second layer joints horizontal. Or, visa versa.

Last edited by Henry0Hornet; 12-20-2017 at 10:20 AM.
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Old 12-20-2017, 11:00 AM
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Epoxy is typically stronger than the wood, meaning bond lines don't fail, its the wood substrate that's destroyed. This means making the joint stronger on a core doesn't make much of a difference.

Properly built transoms have more of a balanced laminate, the same laminate schedule on both sides, older designs were very sloppy in production and they may just put a small amount of glass on the inside.

Even these older thinner than they should be transoms held up for years until the wood core turned to pulp, and even then most owners had no clue, and there weren't many signs of a problem unless you looked for them.

Cores only need to be strong enough to handle the loads applied to them in the real world, not be the strongest they can possibly be. There are always better and stronger products and methods, they typically cost more, but when the increased strength provides no value it doesn't make sense, other than the warm and fuzzy part of it.

In this case one layer has no joints and one layer does, it's a non issue.
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Old 12-20-2017, 12:14 PM
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Originally Posted by Henry0Hornet View Post
Respectfully, disagree.

The thinness of the inner transom glass laminate layer makes it clear that the original build design expected and used strength that was in the wood (coring), itself.

When a light weight coring is used (something much lighter than wood), design of the panel must include enough structure (glass) on front and back to carry all of the tension and compression in the panel. In this case, the coring will see large shear loads, small compression loads, and nearly no tension. If the coring is to carry only shear load, something like nidacore, or other rigid foams, or balsa work very well. Very light. Adequate shear strength. Plywood would work, but, would just be heavy. In this case, tightly butting portions of coring up against each other effectively transmits the shear. However, it will not transmit any appreciable compression or tension loads. The tension and compression must be carried by the outer glass layers.


Using butt joints would remove any ability of the wood to carry bending load to the hull sides or bottom. If this transom design had been built to use a light weight coring, the, I'd agree, butt joints would be adequate. However, since the wood is expected to carry load (other than shear) scarfs or laps are needed. Epoxied laps would work and might be easier to build.


Further, I was going to suggest that if the plywood is added in two steps, I'd run my scarfs in different directions. Make the first layer joints vertical and second layer joints horizontal. Or, visa versa.
If it were a single piece of core with a joint, I would probably agree, but this transom has 2 layers of ply in the main section where the engine is mounted, basically making it a lap joint if you stagger them.....you see similar joints when builders make stringers out of various types of cores and just stagger the joints every 4 feet, like sheathing on a house.Once the joint is bonded to another layer without a joint, it will be very strong. And if your calcs show no or minimal tensile loads, which skin would that apply to? The 1/4 inch thick outer skin, or the 1/16 inch inner skin? And if the OP goes with 3 layers of 1700, he will exceed the OEM thickness of that inner skin making it that much better. Remember, this is a small boat with moderate horsepower...we're not talking trips or quads of 350hp each.
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Old 12-20-2017, 01:54 PM
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And if your calcs show no or minimal tensile loads, which skin would that apply to? The 1/4 inch thick outer skin, or the 1/16 inch inner skin? And if the OP goes with 3 layers of 1700, he will exceed the OEM thickness of that inner skin making it that much better.




Completely agree with staggering joints.


As mentioned above, OEM clearly expected the plywood to carry substantial load. The thin inner layer would not have done much more than hold the plywood in place. It would have carried very little load. So, if you're going to butt joint the plywood, you gotta' replace a lot of bending strength/stiffness. A lot. I'm not doing any calc's. Just looking at the dynamics. The question is can he be confident that the added glass is adequate to compensate to the loss of strength in the coring? I don't know. Fiberglass is really flexible across the face of a laminate. Stiffness is the problem (floating over the plate - should be knocked out of the park). If it was my boat (its not), I'd try to scarf or lap the joints in the plywood.


just my $0.02....
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Old 12-20-2017, 02:54 PM
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Originally Posted by ondarvr1 View Post
Epoxy is typically stronger than the wood, meaning bond lines don't fail, its the wood substrate that's destroyed. This means making the joint stronger on a core doesn't make much of a difference.

Properly built transoms have more of a balanced laminate, the same laminate schedule on both sides, older designs were very sloppy in production and they may just put a small amount of glass on the inside.

Even these older thinner than they should be transoms held up for years until the wood core turned to pulp, and even then most owners had no clue, and there weren't many signs of a problem unless you looked for them.

Cores only need to be strong enough to handle the loads applied to them in the real world, not be the strongest they can possible be. There are always better and stronger products and methods, they typically cost more, but when the increased strength provides no value it doesn't make sense, other than the warm and fuzzy part of it.

In this case one layer has no joints and one layers does, it's a non issue.
Agree with this 100%
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Old 12-20-2017, 03:02 PM
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I'm with ondarvr1 on this!
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Old 12-20-2017, 03:40 PM
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I've done this for a few decades and been involved with highly engineered and loaded structures, plus I work with the engineers that supply the laminate schedules. You'd be surprised at how little laminate is needed on each side of a core to supply the strength, then they add in the safety factor, then a fudge factor, so it gets thicker. The load is carried in the skins, even the poorly built old school stuff, yes the plywood carried some of the load on the old school stuff because of the sloppy workmanship, but it wasn't really supposed to. Plywood just happens to be a very forgiving core in these applications, plus cheap and easy to work with.

Another aspect is the splash well, once a splash well is incorporated into a transom it can act as one of the main reinforcements, this makes the core even less of an issue.
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Old 12-21-2017, 06:25 PM
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wow you guys have been busy on here I appreciate all the input regarding the butt vs scarf joint for the outer ply. It wont be warm enough to do any of this work soon so I have some time to think about it more.

One other question I do have is I plan to have the bottom of the plywood run above the drain bung parallel to the ground so there will be a triangular area with nothing in it. I have seen this as being referenced as a "mouse hole". Should I be filling this area with something prior to installing the plywood?
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Old 12-22-2017, 06:10 AM
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What kind of drain is that? It doesn't look like a normal garboard drain....from the photo looking down inside the core area, it looks like a PVC drain?? Is it in good shape? is it actually plugable? If its staying, you will need to clean around it well and fill it with a putty mix using high density filler and cabosil...no fairing filler here.
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Old 12-22-2017, 06:15 AM
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Man, this thread is giving me flashbacks...

Any gaps can be filled with thickened epoxy, I used System Three Silica - http://a.co/1dLgcAv I went through two tubs of this stuff. I created a nice thick bed of epoxy/silica goop, and then wet out my plywood and smashed it all in there together.

Lots of good stuff here, but one thing I wanted to mention was replacing the outside skin. If I had to do it again, I probably would not have re-used mine... the amount of work to try and get the old skin to bond fully, and then fair it into the remaining hull around the edges... I would have rather just re-skinned it with a couple layers of 1700.

Now, that may have been a messy project in itself, handling 6-foot wide sheets of fiberglass while everything is tacky would probably be a pain in the ass... but it really was a lot to try and line it up in the first place, then smooth out the 'splice' between the two. Just my experience.
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Old 12-22-2017, 07:16 AM
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Using the old skin is much easier, trying rebuild a surface that size and then get it smooth and looking good is much more difficult and time consuming than doing just the seam where it was cut.

The proper use of clamps will get the old skin lined up fairly good.
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