Beam shelf advice

Ok folks, just about to glass in a beam shelf on a 25' boat. Planning on 18mm ply at 150mm wide. Now what I can't decide is on how much glass to use. Got a roll of 600g ready. How many layers top and bottom of the shelf to provide ample structure and bonding to the hull, max span for the deck beams will be 2.4m. Deck structure will be 150x75 douglas fir beams and 18mm ply glassed over. Only deck items will be a 900x1200 GRP aft wheelhouse. Here is a pic of the templates being scribed.

 

I’d skip he wood core for the shelf, just use it as a former for the glass.
No bulkheads or stringers?

 

No stringers, and I'll add a couple of bulkheads later one fore and one aft of the engine compartment and maybe a small crash bulkhead up front. my other boats have ply core to the beam shelf, although that's from new. Just trying to work out how much glass to use above and below, don't want to use too little but also don't want to waste any, this build is going to be expensive enough.

 

I can’t make out what you mean by beam shelf. My rorshak is a cleat, but then you want glass on top and bottom, so it looks like you are making a wide cleat above the beam?

The way I’d build things is to cleat under each beam or all with a board flat against the deck and two layers of glass tabbing. Use a laser to line it out.

But maybe this wide cleat for you is a sole seat and you intend to screw the deck beams to it.

In that case, I’d bond it to the hull with a fillet on bottom side and at 150mm wide, add 100 mm to each piece and then a second piece with 50mm. So 250 each side first, followed by 200 wet on wet over fillet on bottom. You won’t need much fillet on top.

Or, as Kapn says, put the board on with ship tape wrapped around it and some hot glue tack points and then glass tab only the top 3-4 layers of 600g, first piece 300 wide, second 250, third 200, fourth 150, then remove the wood. Then the beams come up to it and you cleat under each beam as I wrote first above. Maybe about 150mm by 100 for each cleat under and two pieces of 600. This way the beams cannot ever come down.

 

Why such enormous deck beams? As already asked,how many bulkheads will there be and their intervals will have some bearing on the amount of material appropriate for the job.You don't have ideal conditions for working on the boat in a west country autumn and will need to pick a couple of nice days for the job since water and laminating GRP aren't too amenable and it would take quite a while for the wood to dry out after some rain.

 

I just want to get the beam shelf in before it gets too cold. The bulkheads won't be supporting the deck, there will be one fore and aft of the engine and to the sides, mainly for sound proofing and mounting pints for oil and fuel filters. Once the beam shelf is in I can then get the deck beams cut/oiled and in and start fitting out for hoses/wiring/vents. I don't plan on putting the deck on until next year when it gets warmer.

 

oh, the beam shelf is for the beams to sit on, the frame I have under the template ply is only there to get my levels, I used a few string lines bow to stern to get them and then screwed temp timber in to sit the 4mm ply on to scribe. The shelf will go around all the length of both sides of the hull and the beams sit on top and bolted through. Not my boat but this gives you an idea.

 

It doesn't matter how much glass you put on the beamshelf, it's structural function is minimal. The primary stiffener is the deck, and the deckbeams only reinforce the deck against buckling. Otherwise you would also have to fit hanging and lodging knees to achieve proper bracing.
This isn't a mechanically fastened wooden boat with a caulked deck, so don't use wooden boat techniques to build it.

My advice is to skip the beamshelf entirely and glass the deckbeam ends to the hull. The kind of construction your picture shows, as lovely as it looks, is just an invitation to rot and mold, besides beeing a waste of money and labor.

 

Surely the bolting of the beams to the shelf provides structural stability. That last picture is from an MCA coded boat, they put the beam shelf in from the factory, my Cygnus commercial workboat is built the same, I recently replaced the deck and beams on that and they were original from 1984. I've just never had to replace a beam shelf. I had planned on adding in glassed in frames above and below the shelf moulded over polyethylene foam forms, probably squared off pool noodles which would provide support under the shelf.

 

The only case the shelf has a structural job to do is if the deck isn't bonded to the hull (by design or accident). The deck beeing plywood of the same thickness is much wider then the shelf, and therefore much more rigid (it acts like a bulkhead) an takes all the loads. That's why you don't see lodging knees under ply decks, the plywood itself keeps the beams apart. The beams are glued to the deck over their entire lenght, the bolt to the shelf does nothing at all. The deck is tabbed to the topsides, the tabbing would have to break before any force (in any direction) is transferred to the shelf itself. If thats your concern, it's better to use all that glass wrapping the shelf to reinforce the deck tabbing so it doesn't fail in the first place. Additionally, glassing the beam ends to the topside makes them act like hanging knees.
The only structural function the beam shelf can do is to act as a stringer. It would be needed that close to the deck only if the topside is very thin and flexible.
Anyway, the above answers your question, if you want to use the beamshelf you should tab it to the hull with at least an equal amount of glass as you plan for the deck/hull joint, if not more.

Thing is, tradition is a heavy ball chained to the foot. People often do things a certain way without thinking why, they "improve" on top of the old things. Take the picture above, we can see the plenty of that.
The beams are bolted to the shelf, yet we also see the epoxy glueing them to the shelf and topside. It would take all that glue area to break before the bolt sees any force (a scenario that means the shelf/hull tabbing and beam/deck glue both hold while the deck/hull tabbing and beam/shelf glueline both break). This is way beyond belt and suspender thinking.

The carlins have half notches, but the traditional half dovetails have been replaced by two bolts and glue. This means the notchings only job is to increase the glueing area and/or move it away from the end grain. In both cases a much deeper and longer notch would have been more appropriate. The bolts are also funny, they trust them enough in tension to skip the dovetailing but not enough in shear to skip the notching. Wich is nuts, because here the wood itself is the weakest link, and not the metal. Let's not forget that both carlin and deckbeam are presumably glued/nailed to a plywood deck that acts as a giant lodging knee on both sides of the joint and has to be overcome first.

 

Are you planning on exceptionally heavy deck loads?
I’m wondering why the departure from more conventional stringers, bulkheads and deck shelf?
This could all be accomplished without use of any lumber.

 

This is a pretty standard build for workboat decks. My 40 yr old Cygnus was built like this and I just replaced the deck and beams 2 yrs ago. The beam shelf in that was still ok so I didn't need to do anything to that.

 

I guess I am over thinking this. I'm coming from a place where I would normally be bolting a 2 drum hydraulic winch to the deck and ancillary blocks. This boat will never see that. I'm planning on a lifting gantry and gunwale mounted hydraulic capstan for the occasional lift of a mooring block but that's about it for this boat.

 

If you are using fiberglass and resin, oiling is the worst thing to do.

 

only the douglas fir beams. Few coats of danish oil before being covered over, never to be seen for 30 yrs.