Polyurethane as Core Material

Dears Sirs.

I would appreciate if you could tell me if there is any boat whose hull,deck or bulkhead was built with Polyurethane (PUR) as core material (sandwich construction method).
My doubt is regarding the fact that PUR characteristic is that it usually deteriorates with time.

Thanks in advance.

Victor Lemmi

 

Polyurethane foam is an unsuitable material for boat building. It has not enough resistance and it destroys itself as aging. There is not cheap alternative to the expensive boatbuilding foams.

As you're in Brazil where the marine foams are very expensive, the better to do is to change of material...

 

Dear Ilan

Do you , or whoever you know,have any experience in using Polyurethane as core that really did not work out? :?:
I would really appreciate to know exactly the material in order to have the mechanical characteristics.

I intend to test (tensile, compression and shear) some samples to compare with other types of foam..

Thanks for your attention.

By the way, are you a designer or builter?

Regards

Victor Lemmi

 

About polyurethane

I'm naval engineer and have worked with shipyards, mainly for warships and wooden fishing boats. I've worked also in high tech composites for racing sail multihulls. This last domain gave me a lot of experience about sandwiches and high strenght fibers.

Polyurethane foam has been heavily used in boats as filling foam, and base for laminating stringers in polyester boats. It doesn't age well as it tries often to returm in powder in a few years, after a very brittle state. It is not stable in most cases. It sucks a lot of water also.
On warships its flammability and toxic fumes caracteristics forbide totally its use, besides it's not situable for structural purposes.

It has been used also in house building. The results are very mitigated. For filling and insulation it works at least the first years; after the problems of ageing begin in a great number of structures. As structural foam as some sandwich roof panels made in the seventies, it fails after a few years unless the panels have redundant internal beams and stringers.

In european professional shipyards it has been never used as its caracteristics are to low to be a suitable structural foam for sandwiches, so it has been rejected by the engineers and the classification societies.

Do not waste your time in making samples and testing it, it has been already done 30 years ago... You're reinventing the wheel.
Make a search in internet, I remember that the french institution CETIM made studies about this group of foams, I think you'll find some information. Think that after the usual caracteristics' ciphers, you have to take in account a lot of other caracteristics: degazing under vacuum, absorption of resins, resistance to resins, solvents and water, behaviour in tropical temperatures, ageing, chemical stability on a long period of time etc...

By experience I can say that sandwich composites need a very good engineering, happily well known today after some spectacular failures at the beginning of this technic. I know also that sandwiches require the best technical level and care during bulding or the results will be from bad to the very worst (make a search in internet about yacht surveyors sites, you'll learn a lot about bad boatbuilding with not suitable technics and materials). On female molds vacuum is practically mandatory and it's very expensive because of the price of expenditures like peelply and others.

The foams (generally PVC based) able to fill all the requirements needed in boatbuilduing are not numerous, after a lot of trials on 40 years with dozens of foams.
For example Airex in Europe has long good records when the density is at least 75 kg/m3 , the racing multihulls made with this foam last as some have more than 15 years of hard work and remained structurally sound, but it's very expensive and has to be made by very highly skilled professionals.

There is a lot of alternatives to the very costly marine foams, specially in countries like Brazil where the custom taxes are high: phenolic impregnated paper honeycomb (Tricel), polyethilene honeycomb (Nidacore), light woods (some tropical "cedars" as the cupressus lusitanica, and Brazil has so many woods), etc...

That depends of the kind of boats and if it's one off or serial boat building and the resins you want to use. Even the paper honeycomb used in light doors is more convenient than polyurethane foam when impregnated with epoxy or phenolic resin for making decks.

A good alternative is also to make a monolithic bottom (all the underwater part of the hull) with flanks and decks in sandwich, where savings of weight are important for the stability. I'm also in a third world country, with all the problems you know for living with them; for the one off small boats I stay with monolithic polyester with some nidacore in the decks or wood strip plank or plywood with epoxy resin. These technics are simple, don't need a lot of tooling and are forgiving of small mistakes. Sandwich would mean to spend one year giving formation to the workers and any mistake is unaffordable.

I'm afraid that a boat in sandwich with polyurethane foam as core will be very hazardous project and a costly mistake. As naval engineer I would never work on a such project, even with a liability discharge, as I'm sure at 99.99% of its ill fate.

Best regards.