Fibreglass Deck Build

[ausnp84]

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

[Jeff_H]

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

[LakeSuperiorGeezer]

Would fiberglass on top of this plywood work?

[Jeff_H]

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

[Harborless]

How big is this boat? This will be a pretty expensive process

[ausnp84]

Originally Posted by Jeff_H

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

[ausnp84]

Originally Posted by Harborless

How big is this boat? This will be a pretty expensive process

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

[chrisncate]

I like the balsa coring, it comes in small (roughly 2"x2") squares that have a layer of backing. It lays really nice and is easy to cut on site while working with it.

[RichH]

Originally Posted by ausnp84

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' .... . ;-)

[poopdeckpappy]

Originally Posted by RichH

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