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  #11  
Old 11-18-2010
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Quote:
Originally Posted by RichH View Post
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' .... . ;-)
Howdy,

Ok, I'm not sure what you mean by bulwark but the deck has no coachroof. Rather, it's a flat deck with a deck-stepped mast (a challenge with this task) and only the raised pilothouse. The boat was built in 1974 and the builders are no longer in business (after 200-odd years), so if we're going to get further structural checks done, it'll need to be by an engineer.

Regarding the mat, we're looking to use 450g/m2 biaxial cloth, recommended by the West Systems dealer over here.

I'm going to lift a strip of teak on the aft section this weekend and drill a hole in the plywood below to get an idea of what we're working with. At the very least, the plywood will need to be removed as it's soaked & rotten to the core.

cheers,
nathan
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Old 11-18-2010
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Quote:
Originally Posted by poopdeckpappy View Post
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

"Thickness" increases the structural moment of Inertia about the neutral axis, matt requires a relatively resin-rich structure and on a cost ratio basis becomes the most expensive way to 'gain such thickness'. The prime purpose of matting to provide a 'cushion' under gelcoat to prevent 'print-through' of the roving/cloth during the very long term 'post-cure' phase of the polymerization. Stiffness is due to the fiberstress along the outtermost 'skin' of a structure, matting does not have the structural capacity to do this, its a 'cosmetic filler' and not much more.
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Quote:
Originally Posted by RichH View Post
The prime purpose of matting to provide a 'cushion' under gelcoat to prevent 'print-through' of the roving/cloth during the very long term 'post-cure' phase of the polymerization.
My apologies, that makes sense
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Old 11-19-2010
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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.
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Old 11-19-2010
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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.
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Old 11-19-2010
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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
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Old 11-21-2010
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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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