Forums: Whisstock Designs: Composite Skin New Topic Post Reply
Author Post
Posted: Feb 5th, 2010 at 02:38   |   Subject: Composite Skin
Hi George,

I've been intrigued by your designs so I purchased Design No. 55 so that I might study it. No plan to build yet but who knows, I've 4 boats in various stages of construction. I love building boats and I think should build at least one traditional boat within my lifetime.

My question is what are your thoughts on replacing the 9mm lapstrake planking with a 5mm 5-ply plywood planks with fiberglass laminated on each side. The reason I ask is because I'm here in the tropics and I believe it is a good idea to really protect the wood lest it rot. Putting fiberglass on the 9mm seems too much for a 15-footer. Would 200gsm cloth work?

I realize there is some issue on how to get the fiberglass on the inner side and I'd appreciate your thoughts on this too.

By the way, I'd like to compliment you on having put together this really good website. The organization and content is great. I especially like the "user's design portfolio" feature, fantastic idea. I also think your instructions and tutorials are great. Just one request really: please put more pictures.

Posted: Feb 6th, 2010 at 15:47   |   Subject: Composite skin
Hi Louis

Thank you for purchasing a design; I hope you do get to build it, but you sound as if you are pretty busy already!

If a wood/epoxy boat is build carefully and properly, using good quality materials, and given a reasonable minimum of maintenance and care, it should not have any decay problems. There are lots of wood/epoxy boats we built in the late 1970's and 1980's all over the world and still going great without problems. There are a few others which for some reason became sadly neglected over quite a period of time, which have suffered problems - no unfixable problems, but still problems of decay.

There are a few reasons why a wood/epoxy boat may start to rot:

1. Bad or careless construction; poor, careless or insufficient application of the epoxy systems.

2. Unsuitable epoxies or timber species. Traditional boatbuilding timbers such as Teak, Oak, Pitchpine are not suitable for epoxy construction because they contain too much oil, tannin, resin etc. Timber with too high a moisture content is also unsuitable. Unsuitable epoxies really means epoxies not designed for timber construction. I specify WEST™ system materials because I know them well and know the to be suitable for the purpose – but there are many other makes that are also equally suitable.

3. Building conditions. This is really related to the epoxies used in the construction. Build conditions can vary widely from very hot to very cold, and from very humid to very dry. You do need to select an epoxy that is suitable for your conditions otherwise they will not be as effective as they should be and you won't get good results, not structurally nor with regard to longevity. Most brands of boatbuilding epoxy make different grades of their epoxy/hardener combinations to suit differing humidity and temperature conditions and you do need to take this into account when choosing an epoxy. Hot and humid are perhaps the most difficult to deal with. Very cold can usually be fixed by an appropriate building and appropriate heating systems (specifically ones that do not dump lots of moisture into the building shed). Ideal conditions are a temperature range of 10ºC - 20ºC (50ºF - 70ºF) and humidity of about 45%, with timber moisture content 12% or below.

4. Damage to the epoxy coating. This can short term damage caused by impact, abrasion and so on. It can also be long term degradation caused by ultra-violet in the sunlight. Damage should be fixed as soon as possible. Ultra-violet degradation can be largely avoided by using pigmented coatings rather than a clear finish; and in clear finishes, using an ultra-violet resistant varnish and renewing it regularly.

So we need:

Suitable timber in a suitable condition.
Good quality epoxies, suitable for the building conditions, properly applied.
Building conditions controlled within the specifications of the epoxies.
Careful and proper building techniques.
Sensible maintenance and care.

Then you will get a long-lived, low-maintenance boat that will give many many years of service under most all conditions, tropical or otherwise.

Adding glass cloth will not make up for getting any of the above wrong. And badly applied, or applied to cover up other deficiencies, it simply makes matters worse.

Having said all of this (and I seem to have said rather a lot!) properly applied glass cloth, can add abrasion resistance, strength, and impact resistance to the structure and is an excellent move.

With small boats, such as design 055, abrasion resistance is probably the most important thing that glass cloth can add. We are not really looking for additional strength or even impact resistance. 200 g/sq m cloth would be entirely suitable for this. The surface area of the hull is about 10 sq metres, so we are only adding about 2 kg to the weight of the boat, which is not significant.

If you glass cloth the exterior carefully, usually incorporating the cloth in the second epoxy coating, allowing it to wet out well and really drape into the shape of the boat, you will get significant benefit. On lapstrake construction, you would fillet the plank lands, carry out any filling etc. before applying the glass cloth.

I personally wouldn't try to glass cloth the interior. I think you would be better served applying a fourth coat coat of epoxy to everything to be sure that the timber structure is properly and fully encapsulated. And take great care with any fastenings that pierce the epoxy coating.

If you do wish to have a glass clothed interior and exterior, then it is best to build the hull (strip plank or lapstrake) on temporary moulds. Then, once the hull is complete, remove the moulds and glass cloth inside and out. Then retro-fit the various structures (frames, thwart assemblies etc.).

If you increase the weight of the cloth to a structurally significant weight (say 450 g/ sq m or more), and glass inside and out, then you could reduce the thickness of the planking. I wouldn't go thinner than 6mm however. Provided it is properly wetted out, thicker cloth can drape quite well and provide a nice finish to the structure.

I hope this helps. I seem to have run on a bit! I am not adverse to glass cloth at all – it just needs to be well done to be effective.

I'm glad you like the website. I really have tried to make it easy and clear – though boats themselves are quite complicated things! I would love to add more photos. All my photos got completely lost some years ago – when I first decided to have a website I stupidly sent the whole portfolio to the people who were building the site for me and they moved office and lost the lot. That's probably when I decided to learn how to code and make my own website! I've been building websites as well as designing boats ever since. So I am horribly short of photos even now.


Posted: Feb 9th, 2010 at 23:32   |   Subject: Re: Composite Skin
Thanks George for that exhaustive answer, wow. Actually, I did try to reply earlier, three times in fact, but for one reason or another the system dropped my replies. Just a suggestion, but maybe you could make replying more foolproof.

About the glassing, I was actually thinking of glass each individual plank as I go along. After spilling a plank, I'd first glass the inner surface and then when the epoxy is green enough I'd attach it to the hull. Finally, once the plank is firmly set I'd then glass the outer surface before proceeding to the next.

What are your thoughts on 6mm 5-ply plywood with 400gsm biax on the inner surface and 200gsm cloth on the outside?

Would the above also work for Design No. 146 18'6" 3-Berth Yawl? I got those plans too :).
Posted: Feb 11th, 2010 at 15:26   |   Subject: Composite Skin
Hi Louis

There is no reason why you shouldn't proceed as you suggest, glassing the inside of each plank separately. That should work very well. I hadn't though of doing it that way. Might be a bit time consuming, but you should get a good interior finish. You will still need to fillet the inside lands (the corner formed by the join of one plank to the other) as this is an integral part of the structure.

With regard to the outside, you can do it as you suggest and then fillet the planks as usual. I think however that it would be preferable to glass the whole of the exterior after all the planks have been fitted and the exterior filleting done. The glass will drape easily into the filleted lands.

On Design 055, you can use 6mm ply with this system. I guess you could use 5mm, though it is getting a bit thin.

On Design 146 (which I saw that you had bought!) I really don't think a 6mm ply skin will give sufficient strength and rigidity. I would stick with 9mm with or without glass.

Part of what we need on a hull skin (once you move away from canoes, dinghies and small open boats) is actual skin thickness for rigidity. Wood is an excellent material to achieve this modest weight. Glass is heavier than wood, even when considered strength for strength. So a thin timber skin with sufficient glass to restore the rigidity of the thicker timber skin, will be heavier. That is why, for core construction, quite a thick, but very light, inner core (Balsa or foam) is used, with relatively thin glass skins.

I'm sorry you had trouble posting on the forum. I don't know why this should be so. I do have a simple time safeguard to prevent spamming, otherwise a forum just becomes unusable. I will have a check to see if I can discover why you might have had a problem.

Posted: Feb 12th, 2010 at 06:53   |   Subject: Composite skin
Thanks again George. I'm getting really warm about building your 15-footer. I might just start lofting the frames on to the backside of my current lofting. Plus I do have some 1x8x16' boards that seem to be begging to be made into the necessary strips.
Posted: Feb 12th, 2010 at 11:43   |   Subject: Composite Skin
If you do, photos would be great! She is a really very nice craft to sail. Enjoy the building process too. I think there is little more satisfying than seeing a boat grow from lines on paper or a floor to the real thing. George