Literature for lightweight 70 foot fibre glass construction

[Mikey]

Hello All,
I won't trouble you with questions yet, would be too many. Another year down the line...

What I am looking for is Books, apart from the list below, what other books can you professionals recommend? Especially for lightweight designing and building techniques!

Project: 70*14 foot lightweight fibre glass sailing yacht. Actually rather similar to MacGregor 65, except that the MG65 is too sporty, I more aim at cruiser/sailer than sailor/racer. But weight costs so lightweight with plenty of bulkheads, stringer and reinforcements it still is.

The Elements of Boat Strength - Dave Gerr
Principles of Yacht Design - Larsson, Eliasson
Desirable and Undesirable Characteristics of the Off-Shore Yacht -Rousmaiere
The World’s Best Sailboats – A Survey by Frenec Mate (should I buy part 1 or 2 or both?)

Pro's, I greatly appreciate your valuable input here
Mikey

[Eric Sponberg]

The Principles of Yacht Design is the current recommended reference for the naval architecture of yachts and small craft. Elements of Boat Strength you give you some idea of rules of thumb (not proper engineering equations) to building boats, but if you were to build to its recommendations, you would be building a pretty heavy boat. Desirable and Undesirable Characteristics of the Off-shore Yacht I have found of limited use. It starts out with some pretty dated designs, and for me, the conclusions it draws are pretty obvious. However, you might gain something from it.

To this list I would add another most important book: Seaworthiness, the Forgotten Factor, by C. A. Marchaj (pronounced MAR-ki, with a long "i"). This will give you insights into the proportions of boats for sailing in a seaway (not necessarily calm water). It is not the wind that hurts you, its the waves.

You might also find the books of Steve Dashew helpful. He recommends the long, narrow type of boats as exemplified in his Deerfoot designs. These have generated a class of designs commonly referred to as Deerfoot style. Long and narrow makes good sailing sense, although Steve Dashew likes to load up his boats with lots of equipment. This makes the boat heavy.

To design and build a lightweight boat requires more engineering rather than less. One has to be careful of the trade-offs between strength and safety. That is, generally, lighter weight means less strength, and less strength means less safety. It is possible to be lightweight and strong, but you have to go through the entire structure really well to make sure the details of structure maintain strength. It is easier to design and build a heavy boat, it is harder to design and build a lightweight but strong boat.

Eric

[Mikey]

Eric,
Thanks for input, beginning to wonder if I had been deserted...

Yes, building lightweight requires more, and that's where I lack info. Anyone can build heavy (well, almost... well, no, they can’t actually...). To build lightweight, one must understand at what angle forces come from at different locations so that stringers and frames can be placed at optimum angle and in optimum locations to take up these forces and one must also know how much force. I have a feeling that most boat designers use just that, feeling, combined with guidelines and I somewhat question if that is enough when designing lightweight boats that other people will sail across the pacific.

For boatbuilding as a business in today’s competitive world to make sense (= profit...), the philosophy is simple; total accumulated weight in stringers and frames weigh much less than a thicker than necessary hull and weight is money. S-glass etc. costs too much and is also out. I may be weird but I actually think the more formulas in a book, the better, just can't find any to study...

I would say that ½” of standard non-expensive fibre glass is enough hull thickness for a narrow blue water 70 footer, IF! hull thickness is increased at high load points and stress points and the boat is properly reinforced with bulkheads, stringers, and frames. Do you agree?

Mikey

[Eric Sponberg]

Stringers and frames are always oriented "normal" (i.e. perpendicular) to the plating or hull skin. Usually, you calculate an appropriate size of hull panel, bordered on all sides by longitudinal and transverse frames, and you set up the frames to give more or less equally sized panels. The hull skin, based on the size of the panel, is then calculated for the necessary thickness. Then the longitudinal frames are calculated for their strength and stiffness, and then the transverse frames are calculated for their strength and stiffness. These factors have to be checked all over the boat. So it is not a "feeling combined with guidelines", it is hard core calculations that have to be done, if the boat is to be engineered and designed properly.

The other thing to note is that the load of water against the hull is either a hydrostatic load (the boat is assumed to be immersed in a wave to a certain depth) or it can be an impact load. All loads are assumed to act normal to the surface, because this is the worst scenario. Glancing loads of impacting waves are not a damaging as a square hit. And how big is that hit? Who knows? How big is the wave, how fast is it travelling, and how much power does it contain? It is nearly impossible to tell. That is why we have safety factors, usually on the order of 3 or 4, for our calculated results. We make the boats heavier and stronger to take care of those things we cannot determine. Over time, we develope a degree of certainty that such safety factors work to build strong, safe boats.

Finally, neither you nor I can say, on a forum like this, that 1/2" thick fiberglass is appropriate for your 70' boat. The seat of my pants tells me it is a long ways too thin. And if you are really interested in building lightweight, then you should be considering a cored fiberglass structure. The engineering equations for developing hull structure are available from a number of different sources, most in books, some in computer programs, that we apply to the features at hand. A lot is based on first engineering principles that we learn in college and use every day.

If you want to build the boat right, you should consult with a naval architect or marine engineer who is proficient in composite engineering, design, and construction of boats. It will be money well spent, and then you will be assured that the structure is designed properly, and not have to guess or worry that you did or did not get it right.

Eric

[sorenfdk]

For boatbuilding as a business in today’s competitive world to make sense (= profit...), the philosophy is simple; total accumulated weight in stringers and frames weigh much less than a thicker than necessary hull and weight is money.
Well, yes... and no! Don't forget that the cost of building is also affected by wages. You have to pay the builders for every hour they spend, and fabricating and installing internals is more time consuming than laying up the hull. To maximize profit, you have to find the right balance between how much (and what kind of) material you'll use and how many hours you'll spend using it.

When building lightweight, I think cored construction is the only way to go. And you should also seriously consider monocoque construction, where as many elements of the interior as possible act as stiffening members, too.

[Mikey]

Eric and Sorenfdk,

I am glad that proper calculations are done nowadays. That was not always the case when I as a happy teenager poked my nose around and troubled small ship yards on the Swedish west coast in the 1970’s. The few ones friendly enough to chat did no higher than junior high mathematics...

I have misled you a bit, sorry guys. Lightweight is not the main goal, optimum low cost vs. safety is.
I am interested in building lightweight, but not to all costs. I will happily consider a cored fibre glass structure under the conditions that a). The boat still can outlive me without loosing structural integrity (i.e. slowly rot away), and b). The cost does not go up compared to using solid fibre glass. Can it be achieved?

I am an engineer (not marine though), so I try to use all aspects of engineering design I can think of to create low cost, interior liners stiffing up the construction is in the plan. 1/2" hull thickness may still not be achievable.

Eric, I will certainly hire a naval architect or marine engineer to check my design and calculations and I do expect that corrections will have to be done.

Labour Costs
I was expecting and am glad I got a comment about this. Sorenfdk’s comment is very true. But not as applicable when you are located in Asia and wages are in the region of $2.50 per hour compared with $20 -$25 per hour in Europe and America.

West is West and East is East and never shall the two meet. Not quite true actually, but I would still expect less than 20% of investments here to succeed without very solid local knowledge. No one should rush away and invest in Asia without proper insight (minimum 10 years) unless they want to lose money.

I am assuming that in my location, solid fibre glass is the most cost effective way to build but I may be wrong. Soren and Eric, what would you go for with $2.50/hr labour cost?

Mikey

[Eric Sponberg]

Regardless of the labor cost, for a lightweight and strong boat, I would go with cored construction. But you have to make sure that the people building the boat understand the materials. Cored construction is pretty standard procedure here in the US. I don't know what the knowledge base is like in Thailand. In Taiwan, and perhaps Hong Kong, you might find the right talent.

Probably the easiest cored construction can be done with Core-Cell, which comes in various thicknesses and densities. You can buy it in bead and cove strips, and strip-plank the hull. Build a male armature over which to lay and glue the cored strip planking, then lay up the outside skin first, after which you attach a rolling frame to the hull, roll the hull over, take out the armature, and lay up the inside skin. This gives very secure laminating because you are always working on an exposed surface, first of the core, and then all the succeeding layers of fiberglass. This is unlike female mold building where you work to the mold surface first for the outside skin, but then the core goes against this laminate blind, and you are never sure of the integrity of the bond of the core to the outside skin. You need vacuum bagging materials and techniques, or else go to resin infusion. With strip-planked Core-Cell, laminate integrity is practically guaranteed, provided you have a good laminate design to begin with, and good-quality workers, regardless of their pay rate.

Eric

[Mikey]

Strip-planked core-cell will make more high-tech, stronger and lighter boats. Outer surface won't be as good and will need more work using male armature, but on the other hand fewer layers of fibre glass are needed which will reduce work there.

Which manufacturing process is in your experience faster, cored construction with male armature or female mould and solid fibre glass?

I am still not convinced that a cored costruction will keep its structural intergity as long and as well as solid fibre glass. What's your opinion?

Mikey

[Eric Sponberg]

If you have a female mold at hand, then laying up in the mold will likely be faster because you don't have to build the male armature. On the other hand, if first you have to build a female mold, you have to build the plug first, which is essentially a hull, and then build the mold. So you are effectively building the boat twice first before laying up in a new female mold. That takes time and money, and would be much more expensive and time consuming than building an armature and the hull directly to that.

Solid fiberglass is not very stiff, it is pretty flexible. If you go solid, you will also have to build in frames and stringers to stiffen the hull panels. If you don't do that, then the hull will oil can, and you have to make the hull really thick and heavy to prevent that. Coring makes the hull panels much stiffer and therefore you can eliminate some of the internal framing. Overall, this makes the boat lighter, cheaper, and faster to build.

Cored boats have lasted for decades, and those that have had mishaps, such as collisions with other boats or groundings, etc., have survived very well to sail another day.

For a lightweight 70' boat, I would still go with cored construction. In fact, I don't know of any boats that size in the last 30 years that have been built in solid fiberglass.

Eric

[Mikey]

I thank you very much for valuable insight. I will check locallly manufactured core-cell. If I can't find it made locally, the idea falls. Imports would be too expensive for the project to make business sense.

I'll most likely open up new threads when I know what kind / quality of core I can find here...

Once again, thanks Eric
Mikey

[sorenfdk]

I'm sorry to say this, but you better start looking for something else to use as a core. Core-cell is as far as I know only manufactured in the US.

Another way of doing it, would be to use wood and strip plank the hull. It should be possible to find some locally grown wood that could be used.

And as you originally asked for literature, this one is a must when using wood as core: "The Gougeon Brothers on Boat Construction".

Good luck with your project!

[Mikey]

Sorenfdk,
I don't want to use wood or anything that can rot really, especially not below the waterline. I planned from the beginning not to use anything that can rot even above the waterline but now I am not sure any longer. Wood is definitely out of the question though.

I think we can expect that some owners do NOT use and maintain boats as intended and that water therefore will seep in over time. Is there core material that can be used where something like this will not over time affect structural integrity noticably? I didn't think so before but I am happy if I am wrong!

I hesitate regarding this and also about using male armature. I am planning for several boats to be made so plug and mould will not be the big thing. Outer surface won't be as good and will need more work using male armature, but on the other hand fewer layers of fibre glass are needed which will reduce work there.

I did mislead you all in the beginning. Lightweight is important but optimum low cost vs. safety is the main deciding factor from a business perspective.

No one here that I know of use core, only solid fibre glass so I have no reference. Which manufacturing process is in your experience faster, cored construction using male armature or female mould and solid fibre glass?

I have the Gougeon Brothers book, good one.

Can you advice please... :D

[Eric Sponberg]

Well, in the overall scheme of things, in production boatbuilding, a female mold will offer faster construction for a larger number of boats. That is why so many boat builders use female molds, even for cored construction. But for building just a few boats, you will get at least the first boat to the water faster with the male armature. And with our computer technology these days, we can print out hull patterns very accurately to the inside surface of the core so that the hull will come out very fair. If the design is successful, you can always build another boat with the male armature, and use that hull for the plug to build the tooling. You may even do that with the first boat.

Another thing to consider: For male, one-off construction, epoxy resin would be the best. It is about 3-4 times more expensive than polyester, but it is made up of 100% solids so that you will never have a blister problem. Polyester is 45-48% styrene, and when the resin cures, the styrene flashes off, leaving microscopic voids in the laminate that allow water to pass through. This is why polyester laminates are so porous.

Vinylester resins are in between, both in terms of cost and impermeability. Vinylesters also have about 45-48% styrene, but during the cure, the styrene cross-links with itself and the resin, leaving practically no microscopic holes in the laminate. This is why vinylester resins are used almost exclusively for building tanks that contain dangerous chemicals. Vinylesters are about twice the cost of polyester, but about half the cost of epoxy.

Production boatbuilding uses polyester resin almost exclusively, with minor applications of vinylester and epoxy. This is because of cost and easy workability. Production boatbuilding needs to be cheap, cheap, cheap in order to be profitable.

These are just other thoughts to keep in mind.

Eric

[sorenfdk]

If you treat the wood properly with epoxy, then rot is not a real problem, I think.

To be honest, I think it'll be difficult to design and build a boat, that is both lightweight and able to withstand the abuse from some (irresponsible) owners.

If low cost is the driving factor (and plug and mould not being the big thing), then solid fibre glass is perhaps the way to go. But it won't be really lightweight unless you think very carefully about the layup (fibre direction, stacking sequence etc.), and that requires that you're able to define the loads (magnitudes and paths) exactly. This is not an easy job, but it can be done.

BTW: Have you thought about using aluminium instead?

[Mikey]

It is very valuable to discuss ideas here. Great input! Lesson learned – Correct, complete info is critical.

It looks like I am more or less sticking with my overall original design after all.

Solid fibre glass (E-glass) with more stringers than what normally would be used in a similar construction. When it comes to frames, I don’t mean that this hull would have as many frames as old wooden boats do, but certainly, Much closer between the frames is part of the design, so are 6 full bulkheads.

Interior liners are already designed to stiffen the construction. I don’t want to bond them to the hull as it would mean bringing out the hack-saw if repairs have to be done... I am trying to find ways to bolt them to the hull, in a way so that the actual bolts of course do not take up the forces and I don’t lose the stiffening effect. I should be able to clear this, but precision makes it tricky. Has it been done successfully?

I am considering using one layer aramid on the inside, with a layer of fibre glass on that. Can you please recommend here, is it worth it?, is one layer aramid enough? The necessity of another layer of fibre glass on aramid?

All the above, combined with well optimised layup should generate what I consider a lightweight boat. I know I won’t get all the way with what I plan compared with cored construction or aluminium and, yes, I will still be quite a bit off compared with extreme racers.

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?

Eric, exactly as you write, I plan to use mainly polyester and some vinylester and I hope to be able to avoid epoxy totally, all because of cost reasons. My plan is to use vinylester only for the outer layer of the hull and deck. 2 questions here: Am I cutting it to thin to use vinylester only for one layer (increase to use vinylester for 2 layers?) and I am not planning vinylester on the insides anywhere, recommendation?

Cheap, cheap, cheap? Not only. It will be a very interesting boat to sail too.

Mike

[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]

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 :mad:

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.