Foam Cored

[purplerobbie]

What are foam cored boats all about?
I looked at one for sale and the broker said there is water in the core? How would you go about getting it out?
He implied its like a second skin and the water is inbetween them?? He said that it is fiberglass foam then ply? and that some have a fiberglass skin on either side of the foam?
Would that make them unsinkable?
Is it repairable?
I dont think i'll be buying it but was just wondering?
Rob

 

[Bajansailor]

In a foam cored hull the inner and outer fibreglass skins are seperated by a foam core. This is often called 'foam sandwich' construction. If there was no core you might call the fibreglass lay-up a 'single-skin' construction.

The foam basically functions in the same manner as the vertical web does on a steel 'I' Beam to separate the two flanges of a typical 'I' beam. If the two flanges of the 'I' beam were simply welded together without the vertical web separating them they would have very poor resistance to bending because their moment of inertia is much less.

Unlike the web of the 'I' beam, the foam in a foam sandwich hull is considered to not have any strength as such - its function is simply to act as a spacer between the fibreglass skins, to increase the moment of inertia (hence strength) for very little increase in weight.

Most decks are built using sandwich construction, although they are usually balsa cored rather than foam. I think that balsa is probably relatively cheaper and stronger, however it is heavier, and also prone to rotting if it gets damp.

Why are'nt all hulls built using foam sandwich construction then if it is so wonderful?
Ummm well, foam sandwich hulls can hide a multitude of sins very effectively, and if there are problems they are usually serious - eg the fibreglass skins seperating away from the foam (or balsa), or the core soaking up water.

And because foam is rather expensive, it is usually cheaper to specify a single skin construction with additional stiffening inside the hull - although this usually means a heavier hull weight.

If the foam is not closed cell, and it has absorbed water, it would be very difficult to get it out. I have seen Boston Whalers that are supposedly 'unsinkable' because of the foam buoyancy between the inner and outer skins with water logged foam cores - the only answer there really is to dig it all out - a very big job....

Any foam core would only render a boat theoretically 'unsinkable' if the buoyancy provided by the foam is greater than the immersed weight of the boat itself.

If plywood is involved in a sandwich construction, this is usually just in way of where fittings are to be attached, eg cleats on a deck. Although this is still the cheap (and ultimately nasty!) way of doing it, as eventually the plywood will probably rot away - it would be better (but more expensive!) to locally use a very heavy foam, or a beefed up area of single skin fibreglass in way of said deck fitting.

All fibreglass is repairable! The bottom line is if it is economically feasible to repair it or not......

 

[cliff]

How does the "broker" know there is water in the core?.

Many boats were/are made of foam core construction, for example Sadlers and Etaps. Foan cored construction leads to a warm boat. The problems (if there are any) are mainly with the decks delaminating from the foam core usually due to the deck becoming too hot so foam cored decks should not be covered with dark paint or other covering. Delamination of the deck is not a major problem and is easily repaired by epoxy injection.

The foam cored Sadlers are "unsinkable" and this was demonstrated to the DoTI in, IIRC, 1985 or '86 to substantiate the "unsinkable " claim.

Do not take the word of any "broker" on technical matters - get a professional surveyor in then decide.

Personally I am very happy with my Sadler as are other Sadler owners I have met. Todate, I have not heard of any problems with either them or the foam cored ETAPs although some time ago there was a rumour of the foam turning to dust but this was never substantiated. A few years ago I had the deck in mine epoxy injected but before that several circular cores were removed to check the condition of the foam - no problems at all - the fibreglass had just come away from the core in a few places.

 

[purplerobbie]

He said the boat had been surveyed and the findings are there is water in the foam in the deck and hull.
He said that an anchor windlass had been fitted badly and has leaked for a long time
Rob

 

[cliff]

Depends on the extent of the water penetration but should be easy to repair by removing the windlass and drying out the area followed by epoxy injection. It certainly would not put me off the boat having been through the experience of having my decks done (I was doing other owrk on her while the lads did the epoxy injection and they walked me thought the process step by step along with the surveyor who certified the work) The only problem would be keeping her under cover while drying out.

 

[Birdseye]

wouldnt go near the boat - there are lots out there that dont have problems of water in the foam. there are lots that dont have foam at all. why buy a problem?

using a foam sandwich allows you to get a more rigid panel which is lighter and cheaper than doing the same thing with a solid laminate. Fine as long as the surface GRP isnt breached, but then all boatbuilders go and put fittings through the sandwich, often quite badly. and previous owners DIY.

For example, just look how many self tappers are used to secure fittings like sprayhoods. Often the self tappers go right through the outer skin, load on them breaches the dab of sealant the boatbuilder might have used, and in comes the water.

 

[Topcat47]

I know from bitter personal experience that once water gets in alongside the foam, you're going to have problems and the problems you have will depend on where you are and the nature of the foam.

Making decks with closed cell foams is relatively expensive. Sadly it is less common than many people think. With a closed cell foam, in "cold climates" delamimation occurs when water freezes and injection of epoxy may cure a localised problem but there's no guarantee.

When "spongy" foam gets wet a lot more water is absorbed and when that freezes it destroys the integity of the foam, allowing more water to leak in until all you're left with is a sort of muddy mush with no structural integrity at all.

In hot climates water penetrates just the same but in the heat of the day tries to vapourise. The GRP is not "Breathable" so the vapour just pushes deeper into the foam where it can, or causes delamination where it cannot (closed cell foams).

In all cases, drying the area before attempting remedial work is the biggest problem. I'd walk away.

 

[bigwow]

[ QUOTE ]

using a foam sandwich allows you to get a more rigid panel which is lighter and cheaper than doing the same thing with a solid laminate.

[/ QUOTE ]

You have not bothered to check the cost of closed cell foam then!
/forums/images/graemlins/smirk.gif

 

[snowleopard]

There are two types of foam commonly used in foam sandwich construction.

a) Polyurethane foam. This is cheap, lightweight and utterly useless. If water gets into it the whole thing becomes saturated and impossible to repair unless you open up the moulding and scoop it out. The cells of the foam are theoretically closed cell but the structure is so weak it can be crushed by the pressure of 3 ft of water! If faced witha a boat made with this core, run, don't walk.

b) PVC foam, e.g. Airex. This is tough, completely closed cell and doesn't absorb water. It is also as expensive as the best quality marine ply. A boat made with this foam is likely to last extremely well, in fact I am in touch with the latest owner of a boat I built in 1975 using it. There is a possible fault where the outer skins of GRP aren't properly bonded to the core as a result of poor technique in the laminating. These are floppy and weak, a classic example being the hulls of Team Phillips.

Balsa core provides the same stiffness as foam core but has to be kept totally watertight or it will rot. Balsa is used by volume manufacturers because it is cheap and, when the boat is new, you can't tell the difference. Balsa should never be used below the waterline.

If a surveyor has raised a question mark over the boat the easiest thing to do is walk away. You can never dry out a core without separating the laminate which is virtually a re-build job. OTOH if the boat has a PVC core I would first find out if the surveyor understands the material. Most don't. If you want a survey done yourself, try Jim Pritchard who advertises in the back of YM. He has worked with foam sandwich over many years.

 

[vyv_cox]

<< a) Polyurethane foam. This is cheap, lightweight and utterly useless. If water gets into it the whole thing becomes saturated and impossible to repair unless you open up the moulding and scoop it out. The cells of the foam are theoretically closed cell but the structure is so weak it can be crushed by the pressure of 3 ft of water! If faced witha a boat made with this core, run, don't walk. >>

Something of a generalisation. Industrial closed-cell polyurethane foam is totally waterproof and very strong. We immersed a sample cut from the hull of our Sadler 34 in water. Months later it had not absorbed at all. It appears to be as strong and rigid as Kingspan P/U insulation that I have in my roof. It appears to have similar properties to the stuff that my colleagues sprayed onto LNG tanks. If it crumbles due to mechanical action it can't be expected to be much use, but otherwise no problems.

 

[Sans Bateau]

[ QUOTE ]
a) Polyurethane foam. This is cheap, lightweight and utterly useless. If water gets into it the whole thing becomes saturated and impossible to repair unless you open up the moulding and scoop it out. The cells of the foam are theoretically closed cell but the structure is so weak it can be crushed by the pressure of 3 ft of water! If faced witha a boat made with this core, run, don't walk.


[/ QUOTE ]

mmm, better tell ETAP then, they have been using closed cell polyurethane for 37 years of successful boat building.

 

[willincarib]

i am interested in any infomation anyone here can give me on what is the correct method of repairs to foam hulls,as in foam to use,fibreglass,resins,and does it have to be gel coated afterwards,i am new to the site and value all your knowhow,my main interest lies on ,if a yacht had the outer skin peeled, reglassed and foamed,what test examination can you do to check,if the foam is glued correctly,does the outer skin of fibreglass have to have any coatings to make the skin more resilient to water.thanks to all who reply