shaft strut backing plates

Bermuda Brother · Post by **Bermuda Brother** » Jan 25th, '09, 14:48

Hey guys and Pat,

I was wondering if the use of pvc board as the core material for the strut backing plates would be acceptable in place of plywood?
If so what thicknes board, how many layers and what size/area plate to use?

JJ

CaptPatrick · Post by **CaptPatrick** » Jan 25th, '09, 15:08

JJ,

NO! This is one place that a properly installed and epoxy glassed plywood will, at least in my opinion, be the best choice. PVC Foam board doesn't have anywhere near the strength & bonding characteristics of the ply.

Br,

Patrick

Tony Meola · Post by **Tony Meola** » Jan 25th, '09, 20:41

Not sure if they ship overseas but you can also use 1/2 thick fiberglass sheet that you can order from Mcmastercarr. That's what we used.
www.mcmaster.com/

bob lico · Post by **bob lico** » Jan 25th, '09, 23:24

or titanium from yard metals.

bob lico · Post by **bob lico** » Jan 25th, '09, 23:26

actually yarde metals 999 motor parkway ny

CaptPatrick · Post by **CaptPatrick** » Jan 26th, '09, 05:21

Sorry, still just my opinion, but for cost, material availability, shock absorption, and strength, epoxy glassed plywood, is still king for the strut pads....

jspiezio · Post by **jspiezio** » Jan 26th, '09, 07:39

CaptPatrick wrote:Sorry, still just my opinion, but for cost, material availability, shock absorption, and strength, epoxy glassed plywood, is still king for the strut pads....

That is one important consideration. The Capt is correct, if you use titanium, or any other metal really, you will have, for all intents and purposes, zero shock absorbtion or flex. That means all stresses, when they occur, will tranfer to your hull. Meaning the hull will flex much more. Even worse, they will be flexing where the edges of the block contact the hull, resulting in small forces having substantial leverage.

But if you really want to use ti you can get random pieces on ebay. Also several distributors that sell "surplus" ti on the net.

CaptPatrick · Post by **CaptPatrick** » Jan 26th, '09, 08:40

Just because metal is harder, (denser), than plywood it is not as strong as plywood in relation to a strength to weight ratio... Metal also isn't going to bond as well with epoxy as wood because there is no penetration of the epoxy beyond the surface.

Then there's the issue of memory. Unless the metal has been sufficiently hardened, either by work or heat, an impact will move the metal, but the metal will resist the effort to return to it's original placement, (malleability). Metal is fine to use as a backing plate so that nuts and bolts can be torqued down without depressing into softer materials, (load spreading).

While more bondable through the use of epoxy, solid fiberglass plate will have more tendency to fracture under high impact than plywood, is far heavier than plywood in the same thickness. Solid fiberglass is still a better choice than metal.

Remember, the whole purpose for changing the original strut pads, as I have described, is to:

1. Increase the the thickness over that of what Bertram installed. The thicker a composite becomes, (in this case the distance between the strut plate and the backing plate), the more rigid it becomes. Simple compression and tension theory for any composite structure: Basic Composite Structures

2. Increasing the surface area over that of what Bertram installed. Increasing the surface area decreases the load of an impact by allowing the larger reinforced area to dissipate the shock further.

3. Increase the bonding strength between the pad and the fiberglass hull by using epoxy, (over Bertram's Bondo Blob technique).

4. Based also on #1, two layers of 1/2" plywood, bonded together with a layer of biaxle or even 0/90Âº roving will be stronger than one piece of 1" plywood, (just glueing two 1/2" pieces together).

5. Accomplish this modification with materials and techniques readily availabe, and as cost efficientlly, to anyone, anywhere, in the world.

My story and I'm stickin' to it...

bob lico · Post by **bob lico** » Jan 26th, '09, 08:53

the titanium was not meant to lay directly on the bilge floor i would first put unthicken expoxy on all sides of 3/4" coosa board then laminate to ground until green floor. then expoxy/cabosil/mill fiber sandwich.then the titanium goes on there would be no way in hell i would attemp to waterproof direct titanuim to bilge floor.

Bermuda Brother · Post by **Bermuda Brother** » Jan 26th, '09, 16:41

Hi Pat and gentlemen and thanks for the resposes.

So Pat, I will use the ply as I am not a titanium kinda guy!

Would 2 pieces of 1/2 inch ply bonded with biaxial be ok for the strut pad?

Also how big do the pads need to be minimum?

Take Care,

JJ

CaptPatrick · Post by **CaptPatrick** » Jan 26th, '09, 17:00

JJ,

Follow this link and all will be answered: http://bertram31.com/proj/struts/strut_backing.htm The first part encompasses the repair on Richard's boat after the strut was driven through the hull. You won't be doing that; just pick up where the strut pad installation starts...

Forget about minimums, the bigger you make 'em the better they'll work. Twice as large as Bertram did.

Tony Meola · Post by **Tony Meola** » Jan 26th, '09, 21:47

Capt. Pat never considered the shattering view point of a fiberglass pad when we did ours. Trying to move away from potential rot and figured if they did it at the factory today, they probably would have used galss of some type. But your point is well taken, and I am too far along to grind it out and start over. It might shatter, but I don't see the strut comming up through the hull creating a sinking situation. Not sure if you can ever completely protect against that happining.

CaptPatrick · Post by **CaptPatrick** » Jan 26th, '09, 22:16

Naw, Tony, you're fine with the way you did it. Don't even consider back tracking. You certainly better off than wearing those worthless Bertram pads...

Actually, you'd be amazed at the amount of plywood still being used in large boat plants today. The real difference is in HOW they are using it compared to the dark ages of the B31.

Tony Meola · Post by **Tony Meola** » Jan 26th, '09, 22:51

Capt. Pat

Thanks takes one more worry of what I might have done wrong off my list. Trying to follow your pattern as best as possible.

Whaler1777 · Post by **Whaler1777** » Jan 27th, '09, 02:14

Bermuda Brother wrote:Hi Pat and gentlemen and thanks for the resposes.

So Pat, I will use the ply as I am not a titanium kinda guy!

Would 2 pieces of 1/2 inch ply bonded with biaxial be ok for the strut pad?

Also how big do the pads need to be minimum?

Take Care,

JJ

If you would have read what bob wrote, you would have realized that the titanium is a backer that goes over the plywood, not a total replacement for the ply...

jspiezio · Post by **jspiezio** » Jan 27th, '09, 07:22

Whaler1777 wrote: If you would have read what bob wrote, you would have realized that the titanium is a backer that goes over the plywood, not a total replacement for the ply...

Whaler, do you just troll here to hassle people?

Bermuda Brother · Post by **Bermuda Brother** » Jan 27th, '09, 11:44

That's all cool guys. No offense taken just part of the cut and thrust of the sandbox. I appreciate all of the contributions and really respect Bob for what he's done. In my case I don't feel like doing all of the modifications to optimize the 370's and will keep shaft angle the same and just under utilize the top end a bit.

All is cool and the engines might even last longer.

All is cool.

JJ

jspiezio · Post by **jspiezio** » Jan 27th, '09, 11:56

JJ where are you in Bermuda? I have some friends down there. Been twice and enjoyed it, stayed at The Reefs.

Can't wait to see pics of the boat.

ed c. · Post by **ed c.** » Jan 27th, '09, 21:39

We did ours as Capt. Pat recommends. I was going to use the NY Telephone directory but it was just too big. Follow Capt. Pat's advice.

Hueso · Post by **Hueso** » Jan 28th, '09, 09:11

I'm love to work with PVC....in fact I replaced all the wood inside my boat with PVC covered in formica......completely trust Capt. Pat on this one...........

Bermuda Brother · Post by **Bermuda Brother** » Jan 28th, '09, 19:10

Hey J,
I am in the western end of the island though this is a bit of a joke as Bermuda is only 22 miles long!

I will start taking pics of the project and post them if this is easy enough.

Gonna spray her all Oyster White but in Imron! Resoring her to as original spec as poss. gonna be a sweatie!

JJ

shaft strut backing plates

shaft strut backing plates

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