Being fed a line ?

Danny_Labrador

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Just come back from a yacht boat yard where they make new yachts. Salesman explained to me in great detail how the hulls are laid up.

Particular emphasis was made of the fact that they use foam cores - two skins make up the hull with a foam sandwich in between. Inch at most for the foam core.
It was argued that this is better than a solid hull section used by others, (famous names were mentioned). Better impact performance. The same hull single skin would have to be very thick to achieve the same rigidity and then this thickness is in itself a drawback as it wont “give” on impact but instead cracks and breaks?

What do you make of that then? Anyone any experience? Anyone an expert?


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snowleopard

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having owned (and built) 3 boats by this method i swear by it. it gives a lot more stiffness for the same weight compared to solid construction. one drawback is lower abrasion resistance as the skins are thinner. cost per unit area is a lot higher. a well designed boat will have extra reinforcement in high stress areas and will be solid in key areas like keel attachment points

yes, go for it.

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Chris_Robb

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It is true that a foam sandwitch construction will be lighter/stronger than a solid laminate. These have been used for years above the waterline, ie to get stiffenning on a flat deck surface.

Look forward to some 10 to 15 years of heavy use - will the sandwich have remained watertight at all times? will impacts (waves) progressivley detach the laminate over a long period of time? A delaminated sandwich construction has no streght what so ever.

In a power boat sandwich below the water line takes on new problems - - any incursion of water (an accident perhaps) will result in massive hydraulic pressure on hitting the waves, totally destroying the sandwich.

Much of the success depends on the quality of build - run of the mill construction boats may not have this quality. Then consider also - it only needs one minor accident below the water line - which you may not be aware of - and the years go by .........

So it depends what you want - do you want an ultra light boat for performance - or long life for cruising - you takes your choice.

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duncan

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Boston Whaler (fast fishers) have been doing it for years with price the only decenrable downside against a whole lot of upsides (including substantial built in buoayancy even at 1" thickness).
As you say you loose the flex but in the end the key issue here is that the hull construction is as designated by the designer.

New boating playlet " to give or not to flex, that is the question..whether it be sensible to resist or sway with the force of nature........................"

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AndrewB

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Its a line. The main reason cruising yachts are now foam cored, which your salesman 'inexplicably' forgot to mention, is because it has become a much cheaper construction method in mass production, than a solid hull. Certainly cored hulls are lighter than solid, but solid are extremely robust, ask him why so many solid GRP yachts from the 1960's are still around.

I'm no expert, but if you want the view of one who is, look <A target="_blank" HREF=http://www.yachtsurvey.com/core_materials.htm>HERE</A>. It should be enough to convince you that there are arguments both ways.

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Twister_Ken

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It's been the standard method for building some quite spiffy yachts for many years.

When it's done well it produces excellent reults. For my twopenn'orth the major drawback is in the event of extensive hull damage when effective repair becomes much more difficult than solid GRP.


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boatmike

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Foam sandwich construction has been used for years and is common on lightweight designs. It does indeed increase rigidity but not impact strength. (I define impact as a penetrating localised blow by a relatively sharp object like hitting a container) For a monohull cruising boat the best compromise would be to have closed cell foam sandwich above the waterline but solid composite (GRP) below the waterline reducing weight lowering Cof G and making the hull more rigid. It also adds some insulation to the hull structure. The biggest problem with closed cell foam bottoms is repair. There is also an issue with resistance to osmosis but this is related to the quality of layup. One key question to ask the builder is "what resin is used to bond the foam to the outer skin and is this done under vacuum or by hand?" In fact most boats will utilise some foam sandwich construction these days, usually in the deck. Frankly the advantages of foam cores below the WL are outweighed by the disadvantages and I would avoid it unless you are looking for a lightweight racing hull.

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Samphire

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Have to agree with the general opinion,hard to repair,dubious benefits unless you are a racy type.Difficult to repair,if it ever gets osmosis and has to be peeled/shot blasted there is a very thin skin to work with .However my main beef and experience is the problems with water ingress in foam sandwich decks where water gets through the fastenings and where the "end grain" may be exposed, this allows water ingress to spread between the skins.If it is bad on deck imagine the potential below the waterline where transducers and skin fittings go through.

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snowleopard

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there's a common confusion between the two main types of sandwich construction.

there's the balsa core, common in decks of AWBs to give light weight high up and stiffness over a large area. it is also very cheap. if water gets into the core it can rot very rapidly which is a disaster. a carelessly sealed deck fitting can be enough to wreck a deck. balsa has no place below waterline.

true 'foam' sandwich uses a core of plastic foam. there are various types from the ultra-light polyurethane foam which is cheap but structurally useless, to closed cell PVC foam (trade names Airex, Herex, Divinycell) which are more expensive than marine ply but have great strength and don't absorb water if the skin is punctured. Only these latter types should be used for hulls. the manufacturer should be able to tell you which type was used.

repairing a foam sandwich after hull damage is a problem if the foam has been badly damaged as the old foam needs to be removed and a new core applied under vacuum. if it is only the outer skin the repair is very quick and easy.

the common method of construction used today is for a layer of foam to be bonded to the outer skin while it is still wet using a vaccum to provide the pressure. there have been cases where inadequate quality control has resulted in an imperfect bond, the most famous being Team Philips. these are rare and certainly shouldn't happen in a production boat.

to find out how long this construction lasts, a recent thread was asking about Great Britain II, built for the whitbread in the 70's. she is apparently still going strong and has a 100% airex core.

closer to home there is a regular on this site who sails a tri called Aqua Blue (sorry, don't remember his forum ID) who built his boat by this method in the 70's and is still cruising in it. i think he would be upset at the suggestion that his boat should have fallen apart 15 years ago!

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Metabarca

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"a tri called Aqua Blue (sorry, don't remember his forum ID)".
This is: david_brighton

saw the boat - it certainly looked extremely solid!

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Dave_Snelson

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Yes, I have heard this before on certain motorboats that claim to be unsinkable as a result of using this sandwich construction. Good for the first few years, then water gets into the foam via leaks and condensation - and you can't get rid of it. The boat just gets heavier by the year and the foam starts to degrade.

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snowleopard

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we regularly hear stories about people shovelling hundredweights of water-saturated foam out of cavities in 'unsinkable' mobos. the foam in question is usually 'foam-in-place' polyurethane which as i said above is structurally weak, the cell walls break down under as little as 2ft head of water then the material absorbs water like a sponge.

this is foam-filled double skin construction. the foam plays no part in the structural strength of the hull unlike genuine foam sandwich construction.

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billmacfarlane

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Rigidity yes , impact resistance no. It's a common technique being used for a number of years including Sadlers. Two advantage s I can see are sound and noise insulation will lead to a quieter boat down below and a drier one due to less condensation.

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snowleopard

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impact resistance

when building my first boat, i took the cutout from a deck hatch, put it on 4 bricks and set about it with a 7 lb sledge hammer. after about 10 whacks i managed to crack the outer skin but i couldn't make a hole in it. the hammer just bounced back.

one year i was moored fore and aft on piles, two boats to a gap. the fenders popped out in a gale from abeam and we ground against the iron rubbing strake of the other boat. this chewed through the outer skin but did no significant damage to the foam and none to the inner skin. repairing a patch 6 ft x 2 ft took a morning for the laminating and a couple of days for filling and fairing.

bear in mind that this was a racing multihull so the scantlings were very light.

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