bertram31.com

Subject line just about says it all...wondering what the purpose of the two sheets of 1/4" ply are that sandwich the center hardwood on the engine bed sisters.


I had some thoughts...but each kinda fails.

-I thought they were laminated/glued/adhered to hardwood to keep weather out...but they are not attached.
-Thought if laminated they would act as plys of plywood for integrity, stiffen and support the length...but not attached to hardwood.
-Thought maybe as a large washer for the through bolts...but then why the inner piece.

Or maybe once long ago they were adhered and were laminated and did do...


- - Last few years it looks like the side plys held pockets of water to promote dry rot...



In the rebuild section Capt Pat just laminates the outer side of the hardwood, so whatever it did or was supposed to do I am thinking the top U-Channel is taking care of it...hope so.

not to be flip, what would 1/4" ply do against the load of 1000# of engine weight. In my opinion not much.

If you need more support in this area, I would be looking at reinforcing materials at a much greater scale.

KTM-

No worries on being Flip, I go there myself at times.

My question is why did Bertram go through the trouble of putting 1/4" of ply on both sides of the hardwood sisters.

Personally, like you I cannot see how much of a difference it would make...but there it is on all four of the engines mounting points.
I doubt Bertram just said ...well we have all this 1/4" plywood crap...find a place to get rid of it.

Best I come up with is it was once laminated to the hardwood and acted to keep the hardwood straight and avoid sagging. Kind of like a fiberglass deck where you have a top layer of glass, a center filler and a bottom layer of glass. Top and bottom very weak and unstable...but tie them together and one works in tension and the other in stress making for a strong unit. Only now, when I removed the ply is firmly unattached to anything.


As for my support...I'm capping the top of both in alum U- Channel.

Carl,

B31 stringers are not that tall so plywood shouldn't add much of any strength in opposition to up and down load as there are only so 1-2 layers of ply going in the horizontal plane. I would agree with you that the ply might be used to keep it straight or to bridge 2 pieces of inner core to make it act as one piece. If they are not all attached / glued is kind of odd. While you have it opened maybe you could take one side off at a time, re-create the ply panel with thicker stock epoxy glue it together. I'd also want to cover the whole assembly with a few layers of glass to make it more difficult for water to get into the inner wood structure.

I am going a different direction in beefing up the engine beds...I'm not worried about the plywood.


I'm just curious as to the actual reason for it being there in the first place.

Are the sisters also bolted to the stringers? If, probably, yes, then someone might want to achieve the thickness of the total wood for the nut to catch?
Were washers even invented back then?
Were perhaps the engine supports wider than the hardwood alone?
All of the above? None of the above? Lol!

Yannis-

It's one big sandwich all thru bolted.

It is stringer, thin ply, thick hardwood and thin ply. Several carriage bolts going thru with washer and square nut to hold together.

Yes the ply does make wider...but 1/4" isn't going to hold up much of anything. I can see maybe they wanted to shim the hardwood to center the motor mounts...but that would only account for the inner ply

Are you sure the carriage bolts have turns all the way, or just (usually) in their last cm or two? Cos if they only have that screwing capability (funny mechanical term!) then the plies give the combo enough thickness for the nut to catch.

Carl

Strange, I am trying to remember but I don't remember any plywood on my engine bed sisters when we repowered. After we cut ours down we capped them with Aluminum made into U Channel and then bolted through that. Than L brackets bolted through the front and rear bulkhead. That was to make sure nothing would twist.

Carl,
2 reasons I reckon...1)Cosmetic to provide a smooth finish instead of the rough sawn beds and 2) Cosmetic so the engine mounts did not hang over the bed edges and look bottle boat

Carl
When we pulled the sisters on my boat the ply was still attached. I will look in the AM and see if can tell how. I assumed it was laminated for strength as well. but maybe it was just used as a spacer.
Though the wood was pretty dang solid, we removed it and replaced with composite. He used 1 layer of 2" laminated with 3/4" on each side. Then glassed with woven. He used the old ones as template but they ended up being 3.5" thick and a little tall. Which actually is making the motors a tight fit. We have had to grind down and notch them in a few spots for the starter and the oil filter. We are capping them with aluminum channel or angle and epoxy studs into the core like in the building tips. In the one picture of the the engine the studs are just in the aluminum not into the core yet, which is why they are so tall. all just test fit, nothing bonded down yet.
If something looks wrong, someone speak up!

carl
they were on the sides of my engine beds also