Literature for lightweight 70 foot fibre glass construction

[FAST FRED]

AIREX over a male plug is very rapid and strong.Proven for 40+ years .

The usual complaint is the cost of fairing to "Yacht" standards takes a good deal of manhours. $2.50 labor would be GREAT !!! at solving this.

LLOYDS has a book on GRP construction that would give skin thicknesses for yachts, and the AIREX literature will give methhod to converet to cored scantlings.

A cored hull is far the best for cruising as the Airex is an insulator of both heat and noise. And never has the incredablre problems that Balsa cores do (they ROT!).

No cruising boat will be built so close to the edge of failure that epoxy would be needed in the laminate . Costs far too much for such a tiny bit of streignth.

Most cruisers will be slightly overbuilt,

" Someday you tie up next to the tugboat,
Somedays the tug boat ties up next to you".

Strong is good & need not be heavy,

FAST FRED

[Eric Sponberg]

For a solid fiberglass hull, I would not bother with the Kevlar. Your hull thickness on a solid (non-cored) hull will be sufficient protection against impact. Thick laminates are hard to break, and I bet that you are going to be thicker than 1/2" on a 70' boat.

Kevlar is best used on a cored laminate right next to the core on the outside skin. One layer of tight weave will do. This protects the core like a bullet-proof vest. On impact and abrasion, you don't want the core to get breached. Kevlar also is very good in tension loading, but terrible on compression loading. Therefore, it works best on the outside skin where the tension loads are highest. On the inside skin, where compression loads are highest, it won't contribute much of anything to strength.

One layer of glass over the Kevlar is not enough, because on impact and abrasion, the outer fiberglass layer can break through, exposing the Kevlar to impact and tear. Kevlar is very difficult to repair because when it is impacted and torn, the fibers at the cut fuzz up, and you can't easily get them wet out and laminated over. Also, Kevlar is sized (coated with a binder) to accept epoxy resin, not polyester or vinylester. The bond of the polyester resin to the fiber will not be very good.

Eric

[sorenfdk]

Originally Posted by Mikey

Stacking sequence is no problem but to optimise fibre direction at all locations, I need more information but have problems to find it. Have you any suggestions on where??? / how?

Stacking sequence is usually not a major concern, but if you're going to use a number of plies in your laminate, then you might as well place them in the best possible order.
Let's say you choose to use four plies of UD for your boat. In this case, you should choose 0/+45/90/-45 degrees instead of 0/+45/-45/90 or 0/90/+45/-45. In general, the variation in fibre direction from ply to ply should be as little as possible. The difference may not be big, but it's there!

The fibres should of course run parallel to the loads, but this is not always practical, since they should also run parallel to the short side of the panels. The true direction(s) of the loads can only be found using FEA. This is expensive, so it's probably better to take a look at fig. 12.14 and 12.15 in Larsson & Eliasson (2nd edition). The figures only tell something about the direction and nothing about the magnitude of the loads. In general, the fore-and-aft and the athwartships (sp?) loads are the biggest. It's impossible to say anything about the shear (or torsional) loads in general.
So, going back to the example above (and letting 0 degrees be the longitudinal direction), the first and third ply should be heavier than the other two. And if your stringer spacing is smaller than the frame spacing, the 90 degree ply should be a little bit heavier than the 0 degree ply (and placed on the outside). If the frame spacing is the smallest, it's the other way round.
With large panels, it may be better to forget about optimum stacking sequence and move the 0 (or 90) degree ply closer to the surface.

I sincerely hope this makes any sense!

[Mikey]

There is one thing I worry about.

Labour Costs
I hesitated very much to put in labour costs in this forum, but I decided that I had to, to be able to get accurate advice taking that into account.

To all you guys reading this, Asia is not a gold mine for cheap manufacturing of boats. Anyone doing it must have very solid local knowledge first.

Imports of basically anything except parts of rigging and some sailing equipment are a no-no cost-wise because of 30-40% taxes on import. Quality of what you get delivered from local suppliers can vary from unusable to good, with sub-standard by American and European standards being the normal. Good quality is difficult to find and needless to say, without very solid local knowledge, sub-standard is what you would get.

When you get sub-standard quantity delivered, there is almost nothing that you can do about it, even if quality is much lower than the samples that you still have to compare with. Sue? Yes, by all means, be my guest, but the way the legal system works in most Asian countries, it would take a year and it would always cost more than what you get.

And suppliers talk to each other so you would probably end up on the "black-list" and most suppliers would refuse to sell to you. Yes, there is nothing to do about that either. No supplies, no boat building.

I actually don’t see designing and manufacturing as the most difficult parts of a project in Asia, finding good, reliable suppliers definitely is. I have 13 years in the country, the others 20 and 10 years respectively and every minute of that will be needed. Have less than this and 90% would probably walk back empty handed.

It is not a black picture I am painting, just a realistic one. You generally get nothing for free in this world, it is always give and take. I am turning bald, been tearing my hair of my head in frustration over the years. But I have learned.

I have been "green" here, and there has been many times when I thought I wasn’t any longer but I was.

East is East and West is West and never shall the two meet. True until west understands that one must meet half way and that means that west must be able to understand east first. Don't underestimate this, please.

Mikey

[Mikey]

I don’t know if Airex or some sort of Airex substitute is manufactured here. There is a whole, whole lot of hull in a 70 footer, don’t think that I like the idea of male armature regardless of labour cost. Would feel too sorry for the guys...

I will stick with mainly polyester and some vinylester, no epoxy. I know of no one who use cored construction here, only solid glass so skilled labour would probably be a problem to find. I will stick with solid glass in the hull.

Aramids
My mistake, I meant on the outside, not the inside. After your advice, Eric, I think I can just as well skip it totally. More than ½” solid fibre glass will be enough, as you say. Thanks Eric.

Soren, what you write about fibre directions make sense but also generate more questions... I will talk to Martin here and... it would not surprise me if I have more questions tomorrow.

Thanks All
Mikey

[Mikey]

Sorenfdk, your explanations are great and logical.

I have not finished calculating by far so what I write now comes out as I write it, nor have I decided if I should use 7 or 9 stringers. I assume 7 for now.

Frame spacing will be something like a foot or so frame spacing will be smaller than stringer spacing. Torsional loads, I believe that 6 full bulkheads and a couple of more half ones and my deck design will be sufficient.

Frame spacing smaller => 0 degree outside and heavier than 90 degree, +- 45 a bit lighter than that as you write. Would this do?

I am assuming one layer of finer say 8 oz ply on the outside, then a 0 degree 24 oz, then say 18 oz of +45, then 20 oz 90 degree and last an 18 oz -45. These layers will not be enough to achieve the more than ½” hull thickness that is needed but it is just to continue following the rule - as little angle difference between fibre directions as possible.

These are the general fibre directions in the hull, they will of course be different where forces from e.g. stay’s come into play.

A question regarding CSM:
I have never used mat, only woven fabric. Larsson gives 50% CSM, 50% WR for their 40 footer, 50% CSM sounds much to me.

The actual mat is quite a bit cheaper than woven fabric, but on the other hand, CSM requires much more resin and resin is expensive. And it is “non-directional”.

At the end of the day, is it really cheaper to mix in CSM taking the resin into consideration? Depending on the difference, a little bit extra weight may be accepted.

If so, is there a sequence that can be recommended?

Thanks
Mikey

[sorenfdk]

I think a frame spacing of a foot is overkill. Since the only yachts of this size we've designed were aluminium yachts, I haven't got much experience with GRP yachts of this size (well, we have designed one 131 feet yacht that is currently being built, but tha sacntlings were calculated by High Modulus). I think I'd go for a frame spacing of app. 7 feet.

I can't comment on your plies, because I'd have to convert oz into g/sq.m and then do some calculating, and I haven't got the time for that right now!

Don't forget that you don't have to use the same laminate all over the hull. There is no reason why the aft topsides should be as heavy as the slamming area.

A rule of thump says that you should increase the laminate by 40% at chainplates etc. You should add these 40% to the 90 degree and the +/- 45 degree plies.
In our designs, we often use a bit of CSM between each ply of WR (or UD) and on each side of the core if it's a cored construction. The idea is to have a resin-rich layer that better binds the plies of WR together. Also, it is a very good idea to let the two outer plies be CSM (450 and 600 g/sq.m). This gives better puncture and abrasion resistance.

[Mikey]

Frame spacing
As labour cost has such a small impact on final cost here, I figured I can just as well have plenty of frames and stringers in the design.

I have 5 - 10 foot between my bulkheads. Bulkhead 1 => 220 cm (full, watertight), 2 => 440 cm (full), 3 => 590 cm (to deck only), 4 => 740 cm (full), 5 => 940 cm (full), 6 => 1240 cm (now half but must be full, problem is that it eats headroom right in the middle of the saloon), 7 => 1390 cm (full), 8 => 1690 cm (full, watertight).

OK, how about 2 foot between frames then? Still overkill? Is there a special reason for not choosing 7 foot as you suggests? A bit strange but we can almost forget about labour costs...

You are giving good and clear advice Soren, thanks for your time.
Mikey

[sorenfdk]

The answer to the question about how many frames and stringers a boat needs is: "Enough to make the structure sufficiently strong and stiff". It's as simple as that

There are two schools of thought here. One believes in having a lot of frames and stringers giving the structure its strength and stiffness. The shell is - roughly speaking - only there to keep the water out. The other school believes in the opposite: Reduce the number of stiffening members and use the internal structure. The global strength and stiffness lies in the shell, which therefore has to be thicker. This often requires a cored construction.

Your design belongs follows the first school, which is fine. There is, however, one thing you must keep in mind: Puncture resistance. The thinner shells do not have the same puncture resistance. This is one thing which is not covered in detail by the scantling rules (Lloyd's, ABS, GL etc.) and it's not an easy thing to calculate.

So the overkill may not be in the number and size of the stiffening members, but in the hull shell thickness (i.e. when the necesessary thickness to prevent puncture is larger than the thickness required by the scantling rules).

The frame spacing I mentioned should really be the bulkhead spacing (but not over the entire length of the hull, though!). Why 7 ft? Because with this spacing it's easy to fit a nice bunk between two bulkheads. So it's not so much a matter of strength as of liveability!

You can of course vary the frame spacing along the length of the hull, but everything is easier if you keep it constant. This means that the bulkhead spacing mentioned above should be a multiple of the frame spacing, so you can choose for example 3.5 or 1.75 ft., but I think (and I did say think - I haven't done any calculations!) 3.5 ft. is enough. This results in a bulkhead spacing of 7-10.5 ft. instead of your 5-10. That shuoldn't be a problem.

Glad to be of any help, so keep the questions coming (and when eventually you run out of questions, keep us posted about the progress of your project).

[Mikey]

Sorenfdk,
I decided early that being in a low labour-cost country, the first school of thought should be the most cost-effective one to use. In Europe, it would most likely be the opposite.

Interior liners and bulkheads
It’s difficult to utilise space efficiently in a less than 14 foot wide boat. It’s even trickier when port and starboard sides must be of equal length everywhere because of the interior liners. The bulkheads are where they are because of liveability reasons. I have "natural possibilities" at those distances so why not put in bulkheads there? I would rather vary frame spacing than bulkhead spacing. As you write, 220 cm are bunks, 150 cm is toilet on one side and 2 wardrobes on the other, etc.

Spreading out frames evenly between the bulkheads, I believe that 2 in the 150 cm to 220 cm areas and 4 in the 300 cm areas will give a very strong hull (and that’s still fewer than I originally planned).

I don’t know if I will reach where hull thickness can be decided by its puncture resistance and it’s probably not a good idea to have a hull that thin on a cruiser anyway but I actually still think that 1/2" hull thickness will be enough (except slamming areas, other high load areas of course) in this construction. I have never tried but I bet I would hurt myself pretty badly if I tried to kick a hole in a ½” thick solid fibre glass hull

Calculations will show what hull thickness needs to be. Considering that this is a cruiser, not a racer, what would in your opinion be the “no less than” hull thickness you would recommend?

My question earlier about adding a layer of Kevlar was to increase puncture resistance, I assume that all kinds of Kevlar needs Epoxy though and having to use a third type of resin is no fun.

One reason for adding CSM to the hull was resin rich layer that binds better. I see all stringers as made of directional roving and do not plan CSM there. Recommendations?

The books I ordered from Amazon has finally arrived, $250 of interesting reading to look forward to

Mikey

[Mikey]

It's about time that I return something to this forum, not only ask, ask, ask. Anyone who wants advice on the culture of running a business in Thailand, please open up a thread and shoot me a note too so that don't miss it!

Mikey

[sorenfdk]

You can of course use as many stiffening members and space them as you like. It'll probably take a little longer, but with the wages you'll pay who cares?

I can't really give a "no less than" hull thickness. First of all, it depends on what part of the hull you're looking at (slamming area or upper topsides aft). Secondly, that's design work for which I normally charge money

You don't have to use CSM in the stringers because they only seldomly delaminate. I'd use a mix of UD and WR. The UD should run lengthwise to give the stringer bending strength and stiffness. The WR should run +/- 45 degrees to give the stringer torsional strength and stiffness. The WR is also used to bond the stringer to the hull.
The bending strength and stiffness is the most important, as stringers rarely are loaded in torsion (only exception may be longitudinal girders that are connected to the floors that take up the load from the keel). Therefore you should use more UD than WR, especially in the flange area of the stringer.