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Fibreglass Deck Build

2K views 16 replies 8 participants last post by  ausnp84 
#1 ·
Hi guys,

We're looking to replace our teak decks with fibreglass and I'm after your thoughts on what the thickness should be.

The current construction is steel cross braces, plywood panels, then the teak deck.

We'd be looking to replace the plywood panels like-for-like, then coat in West Systems products.

Any thoughts on how thick / many layers of mat we should lay down?

cheers,
nathan
 
#2 ·
You really have not provided enough information for someone to give you a meaningful answer since the scantlings for a deck are somewhat dependent on the size of the boat, curvature of the deck (larger amounts of curvature in the decks require multiple laminations rather than a single thickness of plywood) and the spacing of the framing.

But also with an unusual construction like your former construction ( steel cross braces, plywood panels, then the teak deck.) you should probably see a yacht designer because the connections to the framing will be more difficult and may impact the correct answer.

Jeff
 
#6 ·
You really have not provided enough information for someone to give you a meaningful answer since the scantlings for a deck are somewhat dependent on the size of the boat, curvature of the deck (larger amounts of curvature in the decks require multiple laminations rather than a single thickness of plywood) and the spacing of the framing.

But also with an unusual construction like your former construction ( steel cross braces, plywood panels, then the teak deck.) you should probably see a yacht designer because the connections to the framing will be more difficult and may impact the correct answer.

Jeff
Hi Jeff,

Appreciate the points. I've emailed the surveyor we used to see if he had any thoughts also, but I figured it never hurts to do the scatter gun approach.

The horizontal stringers are 1ft apart (and about 2" wide), there is minimal curvature to the deck and the plywood is bolted to the steel stringers, then the teak screw to the plywood.

cheers,
nathan
 
#4 · (Edited)
The problem with simply fiberglassing the plywood, without knowing the design intent would be that you don't know whether the boat was designed with the teak considered as part of the structural spanning system such that the teak and plywood acted as a composite with the decking spanning between the frames and the plywood simply acting as a sheer membrane.

Jeff
 
#7 ·
The boat is 40ft with a 10ft x 10ft deckhouse. I've priced it up and the cost won't be ridiculous - we've taken this steel boat on as a project and having stripped the entire interior once, I don't plan on having water ingress thru a teak deck and having to do it again :)

cheers,
nathan
 
#9 ·
Any thoughts on how thick / many layers of mat we should lay down?

cheers,
nathan
For structural 'surety' you need to exactly define the dimension of the deck, the dimension from the 'bulwark' to the coachroof being the most important.

A laminated decking needs to be very carefully engineered, as it is essentially a flattened or 'flat-plate' box-beam laminate structure with fixed 'ends' .... quite complicated to design 'well'. A good structural engineer can easily 'back calculate' the original 'intent' of the structural load bearing ability of the deck. So for an amplification to Jeffs recommendation Id further have the OEM deck analysed first, before taking it to a 'boat designer'. That will establish the strength characteristics/function ... and 'validate' the original designers/engineer's (important) FACTOR OF SAFETY. The factor of safety is a 'contingency factor' that is included in structural design so to withstand 'unforseen but probable' maximum loads. Most 'blue water boats seem to 'back calculate' to a Factor or Safety @ 3 or more, 'coastal' boats at 2 - 2.5 FS. etc.

BTW - you DO NOT want to use "mat" for this reconstruction, as 'mat' is only for 'cosmetic purposes', and is essentially non-load bearing form of 'fiberglass'. For this rebuild you want either heavy weight cloth or woven roving .... then maybe covered or overlayed with 'mat' as a cosmetic 'leveling or smoothing' layer. For an ocean capable boat the deck must withstand the impact forces of a large 'boarding/breaking wave' .... . ;-)
 
#10 ·
BTW - you DO NOT want to use "mat" for this reconstruction, as 'mat' is only for 'cosmetic purposes', and is essentially non-load bearing form of 'fiberglass'. For this rebuild you want either heavy weight cloth or woven roving .... then maybe covered or overlayed with 'mat' as a cosmetic 'leveling or smoothing' layer. For an ocean capable boat the deck must withstand the impact forces of a large 'boarding/breaking wave' .... . ;-)
This is not entirely correct, it doesn't have any load bearing value but it's not for cosmetic purposes; CSM will give you thickness and stiffness but it's the cloth ( E,S or biaxial ) that will give it strength and is the last layer ( excluding the gelcoat ) in the finish product, that is why they use 4 or 6oz cloth on surfboards, strength and appearence
 
#14 ·
You speak of steel deck beams... Is this a steel vessel? If she is then the designer went for ascetic looks with the teak deck.
You can pull all of the old decking up and then using high quality marine plywood, sealed with expoxy and install your first layer of the deck... Use the same thickness of Ply or a tad thicker. Then you can apply the top layer of decking... And there are quality decking material that is just as good in looks and is a tad less expensive then teak and has a good feel under your bare feet.
Hopefully you have measured where all of the deck jewelery was on the old deck.
 
#15 ·
Just wanted to add:
Steel and fiber-glass are not a good mix. Especially after the boat has been in rough weather a time or two... The seams where the two materials join will probably open up.
If you do go with an under layer of plywood, Then use two layers of ply Each about 3/8 to a 1/2 inches thick. Ensure that the seams & butts over-lap of about half the width & Length of the plywoods. This two layer will strengthen the deck. Then you add your top layer.
 
#16 ·
It is not all that unusual to combine glass over plywood decks with a steel hull as a way of reducing the weight above the roll center. The key is designing the structure one of two ways.
-Either you engineer the decks as a composite with the rest of the boat and so that the decks and their connections are detailed sturdy enough to take superimposed loadings, or else
-You design the hull and deck framing to take all of the sailing loads and the decks only take the loads that are necessary to keep the water out.

I know the a lot of steel guys like steel decks, but glass over plywd decks make so much sense in terms of low maintenance, noise and comfort under foot.

Jeff
 
#17 ·
Hi guys,

After speaking with our surveyor, I think we're going to go 3mm steel instead. This will add weight to the boat, but it's a.) relatively easy to install and b.) going to provide a solid deck / hull join. Our surveyor recommended it and having done the figures / spoken to a local welder, we should be able to get it fitted relatively easily then weld all the deck fittings back on.

Thanks for your thoughts on this - next on the list is whether to or not to foam the steel hull.....

cheers,
nathan
 
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