Plywood beams

[themoose]

I'm hoping someone far more knowledgable than me can help here. I am rebuilding the cabin on my Falmouth Gypsy. It has laminated pine beams.
My question is: if laminated beams are stronger than sawn beams, then if I were to make new beams from laminating strips of plywood, would that be as strong, if not stronger than laminated pine?
Thanks.
Shane

 

[bilbobaggins]

First, the caveats...

Assuming that the quality of the plywood you choose is as good as or better than the quality of the existing pine laminates, and assuming that the adhesive you use is as good as or better than that in the existing beams, and assuming that the build-quality is.....

Your new beams should be stronger, if you use marine grade plywood without voids, you use an epoxy resin system both as glue and as 'encapsulating medium', and you ensure that there is enough material in the right places for the loads to be imposed. The design of beam structures is not rocket science, but it is ( part of ) a science. If at all uncertain - and even then - find yourself a tame structural engineer, cross his palms with Heineken, and ask away. It's surprising how often they can save blood, sweat and tears.....

I'd want to be clear on just why the replacements are necessary, and what problems exist in the boat that has led to the need for replacement.

There are several excellent manuals on this; the Gougeon brothers' 'On Boat Construction' is an excellent place to start, and I would ask for other info-source suggestions on the Classic Boat forum.

/forums/images/graemlins/wink.gif

 

[themoose]

The boat is a Falmouth Gypsy - GRP hull, with ply everything else over pine frames. The ply was epoxy sheafed and had virtually rotted away completely in many places. The cabin leaked like a sieve. So I decided to replace all the ply and a lot of the frame. Unfortunately getting some of the ply off broke some of the beams.
I was going to replace the beams with solid, until I was advised that laminated was better ( the originals are laminated pine). So then I got to thinking that as I had bought stacks of marine ply, why not make the beams by laminating some of that together (using resorcinal). In my way of thinking that should provide the strongest possibility ( the mast is stepped in a tabernacle on the lower roof supported by a couple of beams).

 

[pyrojames]

A warning on ply beams. The grain in ply runs in both directions, which is completely different to the way the grain runs in a laminated beam. Roughly half of the grain in a ply beam will be running across the beam, or the "wrong" way. I think you'd be better off making new laminates in pine.

 

[themoose]

Cheers! That's the sort of information I was hopimg for. Armed with it I can make a decision, laminated pine it is.
Now, any suggestions as to what pine I should use? Looking at what is left of the old beams, it doesn't look like the pine I would get in B&Q, i.e. it is clean not knotty. Would I be better off using fir or even a hard wood like oak or mahogany. My last boat was all grp - so much easier - but this has a bit more character and I kinda like her. For a triple keel she sailed really well and at 24' is easy for me to solo sail.

 

[Peterduck]

Fir would be quite adequate. Whichever you use, look for fine growth rings which indicate slow growth.
Peter.

 

[pyrojames]

Yep, I'd go for fir. It is one of the stronger timbers around, certainly stronger than teak, although not quite up to oak. On a strength to weight ratio is is one of the best.

 

[themoose]

That's great, fir it is! It should bend well shouldn't it? I recall that its quite springy.

 

[kds]

It will bend if you use thin laminations - which will also give it more strength and keep the curve better. Don't think it is a suitable timber for steaming so you will need to think about how you are going to get it to hold its curve after you take it off the former.
Ken

 

[Captain Coochie]

As others have said solid timber will give the best grain direction so will be stronger . As soon as i read this thread i thought douglas fir and again others have said that ( i think )
Now for some of my input ..... Douglas fir where i am is machined to order so is quite pricey . You may want to go to your local timber yard and see what the have in stock and check prices . At the moment Iroko is cheaper than oak and the last time i ordered Douglas fir for a floor it was 7k worth /forums/images/graemlins/shocked.gif
Have a look around at what you can get is what i'm saying .

 

[themoose]

So if I laminate it first then cut it to shape, that would be a better bet?

 

[themoose]

I appreciate that there may be cheaper options than douglas fir, such as Iroko or locally for me sapelle mahogony, but would they make better beams? As douglas fir has a certain amount if spring and as the mast will ultimately be resting on these beams would I want something more brittle, albeit stronger?

 

[Captain Coochie]

Once you laminate it i think you will lose that spring . Timber will be stronger because of the grain direction but i dont think what type of timber you use (within reason) will make alot of differance once you start covering everything in glue .
I dont know what forces a mast puts on the beams so i hope someone who does know will correct me if i am wrong .
When i tried some laminating i found that once glued up nothing moved and i can stand on this tool totes handle with no spring in it .

 

[Peterduck]

The idea with laminating is to bend the timber to the required curve and then glue it all up. You shouldn't have to apply any other shaping after cleaning up the glue.
Peter.

 

[themoose]

Now that photo is interesting, with the laminates one on top of another. I spoke to a local 'shipwright' about beams and he said that they should be made the same as the photo. However the original beams are laminated side-by-side. Another chap reckoned that thes had been laiminated then sawn into shape, that is not bent at all. Confused? Not arf!

 

[Captain Coochie]

What sizes are your beams ?
The point of laminating them is you can bend thinner cuts of timber to the shape you need . By laminating side by side you wont be able to bend the timber without steaming it but it will give a better timber structure . If your going this way then why not steam and bend solid timber . By laminating and then cutting your grain direction will be all over the place .

 

[themoose]

You have me there. I am going by prior advise. If you read my post earlier in the thread, I was going to put solid sawn beams in, but was advised to make laminated for the strength. Someone else told me to laminate boards together then saw the desired shape. I have had that much contradictory advise down the boatyard that I decided I would make this post. Even the books I bought on boatbuilding didn't help. If I wanted to make a hull I have all the info I need. Anything else - go figure!

 

[Captain Coochie]

/forums/images/graemlins/laugh.gif Always the way aint it ! All you want is for someone to say "Do it like this " but no one will commit !
You will find the same on here . Everyone has an opinion and mine is steam solid timber if you can . It's time tested . Glue breaks down over time .
Just to add another point to fry your brain /forums/images/graemlins/grin.gif If you steam the beams out of solid timber the grain direction will be better than sawing the shape out of a solid board , again leaving the grain changing direction .

 

[themoose]

Hmm, think I'll sell this and buy a rib! Never having steamed anything but fish I was hopeing to avoid the steaming route. I want to try and do as much as possible myself beacuse - surprise surprise costs have risen astronomically against me estimates! At this rate I will run out of dosh before she is half completed. Wife's horse don't help eating me out of house and home (horse - not wife! - although......)
/forums/images/graemlins/laugh.gif

 

[Captain Coochie]

Ok then laminate them as the picture shows . Your boatbuilder type bloke says its ok , i think it's the right way ( carpenter and joiner ) and Mr P Duck didn't say it was wrong .
If my picture was wrong beleave me this post would have about a thousand replies /forums/images/graemlins/grin.gif
Now lets talk glue /forums/images/graemlins/laugh.gif Ill say cascamite or extramite as its now called . Let the epoxy v extramite war begin /forums/images/graemlins/grin.gif