Hull Thicknesses

[Inquisitor]

I am trying to get a handle on “the norm” of hull thicknesses. There doesn’t seem to be any clear cut answer. This article http://www.yachtsurvey.com/Fiberglass_Boats.htm, I read last night, tells me that even full-time professional marine architects don’t have all the answers on this one either. Frankly it scared the hell out of me, that I might buy a production boat and get this kind of crap.

I’ve been reading reviews of productions boats (monos and multis) and recent designs seem to be making explicit claims about the construction as part of their marketing. Almost all were touting that “we” use hand-lay-up cloth. Many were making claims that no core was used below the waterline. These were just the little 2-3 page magazine articles in “Sail” or “Cruising World” not technical briefs.

Would contributors to this forum mind giving out some sample hull thickness that you know? They don’t have to be ones you’ve designed. Don’t feel the need to identify the boat unless you want to warn us away from it like the Sea Ray described in the article. Desirable things (add others) would be.

1. Hull length and beam
2. Three locations (keel, under and over waterline)
3. Material(s) – Metals, wood, composites, foam, concrete… whatever
4. Thickness(es)
5. Your comments – Built like a tank. - or - Do not put your family on this boat.

As an example, here is the one I am designing/building.

SC20 Catamaran
LOA=20’, Hull Beam=20”
Keel: 3 layers of 6 oz cloth, 1” Cedar, 1 layer of 6 oz cloth (going from outside to inside)
Below: 1 layers of 6 oz cloth, 5.2 mm Lauan, 1 layer of 6 oz cloth
Above: 1 layers of 6 oz cloth, 5.2 mm Lauan
Comment: Minimum cost is the goal as this is a research test bed and is not expected to last more than 100 hours of use OR ever to be sold. It will be destroyed like the Phoenix. If it were to rise from ashes, it would use 4mm marine plywood if build one-off (or) an exclusively fiberglass laminate if molds were used.

[Guillermo]

Originally Posted by Inquisitor

I am trying to get a handle on “the norm” of hull thicknesses. There doesn’t seem to be any clear cut answer. This article http://www.yachtsurvey.com/Fiberglass_Boats.htm, I read last night, tells me that even full-time professional marine architects don’t have all the answers on this one either. Frankly it scared the hell out of me, that I might buy a production boat and get this kind of crap....

Most interesting and clarifying article from Davis Pascoe! Really there is a lot of abuse around with this FRP/Composites hulls reinforcement issue, caused, in my opinion, not only because of the desire of cost savings from constructors or low skills, but also because of the actual pursuit of speed and so the needing of lighter structures, as well as some not good enough rules/regulations/calculations.

Classification Societies have Rules about a boat's hull structure reinforcement, both for pure FRP or composites. Properly following them make for safer hulls/structures. Unluckily these rules are not thoroughly applied for boats less than 12 m length.

Some other institutions, be them national or supranational, have rules looking to be not good enough. Have a look at this Report from RINA.

I would like to know what happened with the investigation about a lost keel in a Bavaria 38 in the Adriatic last year. Company claims that the boat's reinforcement was done accordingly to the ISO 12215 rules....

[Robjl]

Inquisitor,
One of the minor details that you don't get on the general description of the various boat construction is panel size.
I'd always been puzzled trying to come up a "standard" laminate thickness by comparing the laminate weights in boat reports...until I had to design my own to the Uniform Shipping Laws here.
My boat, LOA 48' has a max panel size of 2m long x .6m wide.
If I increase the .6m width at all my layup must increase significantly to meet the code.
I had a look at the damaged boats you referred to.....nothing new there...beware of home builts. INEXPERIENCE is a worry.
Regards
Rob.

[Inquisitor]

Originally Posted by Robjl

…My boat, LOA 48' has a max panel size of 2m long x .6m wide.
If I increase the .6m width at all my layup must increase significantly to meet the code.

So if I understand correctly, you have bulkheads every 2m and “what” every .6m… some kind of stringers like in airplanes? How substantial do they have to be to qualify? Does anyone know if we here in the U.S. have similar codes that we have to “live” by? If not, is it possible to get the Australian ones on-line to use as a guide? They’re bound to be conservative.

Originally Posted by Robjl

…I had a look at the damaged boats you referred to.....nothing new there...beware of home builts. INEXPERIENCE is a worry.

I think I might rather take inexperience… at least mine. I’m not feeling so bad about my lack of inexperience… Inexperience will probably add another layer glass cloth “for good measure”… but that Sea Ray photo is criminal! Surely, they have an NA! So is it incompetence on the NA’s part or management rode him into using substandard materials or did he do his job correctly and quality control didn’t catch fabrication’s mistakes? Since there is obviously more than one manufacturer making those mistakes, it might be the NAs were bullied into using some cheaper material that was not fully qualified for strength… particularly impact strength.

I’ve spent at least another six hours on his site and it just went on and on about bad design, bad fabrication, bad QC, bad fitting-out of so many manufacturers. Just plain scary! Although he focuses on power boats, I wonder if sailboats have the same manufacturer issues.

[Robjl]

Inquisitor,
On my boat, bulkhead spacing varies, from the bow, 1.5/2.3/1.2/2.2/1.5/2.0/1.5 + approx metres,
Between the bulkheads,(horizontally) I have stringers, shelves, bunktops, seat sides, floors, bearers and more... all are structurally bonded to the hull and are sized to support whatever hull area bears on them. Does that make sense?
So the max unsupported panel size is 2m (horiz) x .6m (vert).
"Uniform ShippingLaws" are on the net,
http://www.nmsc.gov.au/uslcode_1.htm
but the USL calls up a couple of Aust Standards ...
AS 4132.1-1993 Boat & ship design and construction. Pt 1: Design Loadings.
AS 4132.3-1993 Boat & ship design and construction. Pt 3: Fibre-reinforced plastics construction.
My maths is basic, I worked through it and have plans checked and approved.
The boat will be very strong.
When I referred to inexperience; I had construction in mind...someone said a little knowledge is better than no knowledge at all, in boatbuilding it can be dangerous. In the article we are discussing ALL the photos I saw looked to me like flawed glasswork. The fact that those boats has previously had a working life should give you an indication of the safety factors they are built to. Many people working in the GRP boat construction are untrained in any technical or theoretical sense. They are probably good glassworkers but they need competant supervision. Some of the things I have seen done would make you very cautious about composites..especially GRP .
The problems you refer to are rife in the industry...but it usually takes a smash to expose them.
In my opinion it is always poor construction standards or builders cheating on materials.
Designers just don't do it. Except when the SF is 1.1 and based on what broke last time.... the racing world.
If I am surveying a boat I am always interested in some probing questions to the builder, especially owner first time builders.

Cheers
Rob

[Raggi_Thor]

The new ISO/CE standard also use panel size and stiffness as dimensioning factor, while the loads are calculated from max displacement and speed. Sounds reasonable.