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jag
05-11-2001, 08:30 AM
I'm repairing the transom on my mirage. I was wondering about plywood. How important is it to use marine grade vs treated. The price of marine grade aint cheap $118 for a 4'x8'x1/2 sheet. What do they use in the new ones. Thanx

Liqui-Fly
05-11-2001, 08:42 AM
Marine grade plywood is really only necessary for wooden boats. A good 5-ply construction grade plywood works fine. Laminating with glass and epoxy will give you a strong water tight transom. Pay special attention to voids and edges. LF

Dewey
05-11-2001, 11:52 AM
Use the marine grade. See if you can get a scrap from a boat yard or marine upholstery shop so you don't have to eat the entire sheet. Marine grade is a 7 layer lamination and uses different glue. The marine resorsinol (sp?) glue will not delaminate seperate or blister if it gets wet/damp. It is also stronger. The standard plywood is okay under ideal circumstances, but if you have any penetrations at the transom ( most boats do) and the marine grade gets damp once or twice, you will have less problems than if the standard construction grade gets wet. Whe construction grade gets wet it gets ugly pretty quick.

You have a top of the line boat, why sacrifice its quality by putting sub-standard materials in it just to save $100 bucks or less. It is also a peace of mind thing. It bugs the heck out of me if I know something in my boat is done half ass.

Liqui-Fly
05-11-2001, 12:49 PM
If you don't do anything halffast how would your transom plywood ever get wet? If you think boat yards use marine grade plywood for seats you are definitely mislead. The seven ply would be stronger but not by much and if your transom does get wet you run into more problems with the wood delaminating from the boats skin, especially with PE resins, than you do with rotten wood. Like I said wooden boats require marine grade plywood. It is assumed in their construction and operation that they will absorb a certain amount of water. David

Scott
05-11-2001, 01:01 PM
Whe we finally flip the Baja I'll be replacing the transom, I thought I read somewhere on the board that composition board was the suggested replacment?

Scott
05-11-2001, 01:15 PM
This caught my eye 'caus when I stuck my finger in one of the mount holes in the transom the plywood felt like a soft sponge and this hull aint seen the water in a while! I just scanned the replys and I didn't see the composition board blurb, but I'm sure it there somewhere.

Liqui-Fly
05-11-2001, 01:27 PM
Definitely not compostion board!! A plywood fibreglass composite.....yes. Scott the best advice I can give you is to pick up West System Epoxy's fiberglass boat repair manual. They have excellent technical support also. This board is great but a lot of times you get opinions and not facts. http://www.westsystem.com David

05-11-2001, 03:09 PM
We use 17.5 lb density Klegecell pvc foam sheet material with a 1.4" okoume plywood laminated to the outsides and center in our boats. It will never rot period, and it is very strong.
Eagle One

Scott
05-11-2001, 03:12 PM
I went to Rot Doctor.com and looked at their transom repair suggestions also. I had planed to cut out a section from the inside, remove the wood, epoxy in a new section then re-glass over the entire area. Knees will be added as support and also provide an area for the 18 gal. fuel cell. Rot doctor outlines cutting away the top of the transom and digging out the bad wood. I'm not inclined to cut away the top of the transom unless my original idea proves to be ill advised. Looks like I have a lot to learn, and maybe more work than I originally thought. Maybe sounds corny but I never plan to get rid of this boat when done. Would like to give ot to my son someday, so I need to do it right the first time! Thanks again!

Dewey
05-11-2001, 04:22 PM
David,
First off let me start by saying I respect eveybody's opinion. At least for the time being. I welcome debate because I think that is how we learn. If I post something you don't agree with, please let me know. I will do the same and will do it respectfully.

Construction grade plywood is for houses not boats. Do some boatyards use it? yes, but not the ones that do it right. Marine grade is just that. It is designed to outperform in a marine environment. period.

As far as doin it right the first time and it never getting wet? C-mon, you said it yourself later in your post, the wood in a boat is eventually going to get wet, no matter how good a job you do. If there is a penetration for engine mounting, cables, controls, what ever, there is the potential for a leak. Even 5200 shrinks and peels eventually, even West Systems Epoxy gets old and cracks. The question is how long will it last. If it is done right the first time, using the proper materials, it is going to last alot longer. In my opinion,if I am going to go to the effort, I am not going to scrimp on quality just to save $100. Why do the same job in 5-10 years if you can make it last 15-20?

Cheers,

Bill

Techno
05-11-2001, 11:23 PM
WRONG.WRONG.WRONG. Marine plywood has no voids in it's construction. There is differences in even the construction plywood from A to CX which is crap. Look at the edge of some plyood you'll see gaps-voids in the edge. you'll also see bark filler. This plywood is mostly graded for it's surface quality Cut it open and what you thought was solid wood has gaps running through it. Check the same with marine ply or baltic birch(which is almost non-existant now).
For a transom, the part of the boat that is supporting 300+lbs (and offset at that)and 200+HP I would make sure it was strong.
$100 for a sheet of plywood isn't that bad. consider that you can cut enough pieces out of it to laminate the entire thickness of the transom. How thick were you going to make it?

Having replaced a rotted transom(waterlogged) I can say that reparing one is a waste of time and removeing the punky wood is kind of easy. Whats hard is if you don't give yourself the room to do it.
I left the outer skin of glass in place and removed the splash well since I was modifing it anyway. I worked verticly rather than through the back of the boat. I chopped the transom down the center then removed the two pieces up through the hole I had made. when I installed the laminated marine ply I slid it down the same way. C-clamps with boards to spread the load rebonded the skin to the transom. Screws can be used too then removed and filled.

Your material cost are going to be plywood, epoxy, glass and BEER. Don't underestimate the beer costs or the work slows down.

05-12-2001, 06:05 AM
Dang Randy, you stirred the pot on this topic! Gotta love it!

Instigator
05-12-2001, 08:08 AM
I have built many small O/B power'd plywood hydroplanes, and redone many rotted boards in pleasure/performance boats.
First off, on a performance O/B the motor baord is the backbone of the boat. If your running a worn out 85 hp Chrysler, dig out the rot splice it back togehter and go!
If you are running the boat hard, replace the board and ad strenthening.
I have used a lot of exterior grade (A/C if you can find it) and marine grade. The differences are glue (grade) and no voids in the marine. You can spec. the number of plys in marine as well.
I found a deal one time on 1/4" marine for a hydro I was building. It ended up being 3 ply Luan (under layment)!! You could look at that boat and put a hole in it! Typically 1/4" marine is 5 ply Occume (African Mahogany).
The more the plys, the stronger it is.
For m/b replacement I have used (succesfully) 2 pcs of 3/4" exterior of the best grade you can find, glued together. You can stack em to get whatever total thickness you want.
The variable is, as someone already said, the voids.
If you go exterior (consrustion grade) it may have voids (the football shaped plugs) all over the inner plys that you cant see.
What kills, m/b's are water leaks. Seal all your screws, speedo hoses etc very well when you are finished.
Good luck.
Instigator

tombrown
05-12-2001, 04:23 PM
I'm surprised no one mentioned the Seacast system. The guys I know who have used it (three) say good things about it. Has it fallen from favor here?

<http://www.seawolfindustries.com/seacast.htm>


Regards,

Tom Brown

tombrown
05-12-2001, 04:29 PM
Sorry about that. The URL I posted got lost. Let me try it again.

http://www.seawolfindustries.com/seacast.htm


Tom Brown

Techno
05-12-2001, 08:23 PM
I thought it was an interdesting idea but didn't really like it.
$250-500 for a transom 2'x3-6'x 2.5"
Seems heavy for not having any inherent strength.
Vague claims to strength, no actual figures of shear compression or tension. And no figure for fatigue resistance.
An idiot letter that compares a $1,500 professional repair then does the job.
A few other minor things like mixing several 6lb. amounts of glass powder. no actual strength of this filler.

Ever notice that aluminum plate isn't used in a transom?

If you want any wood part in your boat not to rot then don't drill holes through it and expect it to be sealed. either put several coats of epoxy on the 'opened' wood or fill the hole with filled epoxy and re-drill it.

As a thought rule think of wood as coming from trees. Trees have to pump water up, possibly several hundred feet. When you drill a hole into the the wood it reverts back to it's prior job- pumping water. The air is very handy source for moisture. A very nice humidity gauge can be made by using a wood strip but with the grain running the short direction, hook one end up to a pivoted needle/pointer and fix the other end. Don't paint it.

Liqui-Fly
05-14-2001, 08:01 AM
A wet transom is definitley a very heavy transom. The marine grade will last longer while wet but I'd like to think at that time I'd be replacing before rot ever started. Plus being in the north the boat has only 5 months a year to rot because the rest of the time the saturated areas are frozen hehe. From a quality standpoint I agree Dewey, marine grade is the way but I think there are many ways to skin a cat. I like Eagle One's transom idea the best. David