Slamming area laminate

[Bluedesign]

Hi,

Designing a 29ft sailing boat in balsa/GRP laminate we are a bit struggling on the slamming area. In fact due to complicated shape, putting the balsa core is not so easy. Moreover in the high pressure area single skin laminate (or high density core) need to be used (such keel area, shroud plate...). One of the reason for that is the shear force acting on the core of the sandwich.
If i need to change my sandwich to a single skin laminate, i have to, according to ABS, for the keel area for example, to have a laminate (single skin) thickness equal to the total sandwich thickness, which is here, probably 20 mm (16 mm core). This thickness looks quite huge, but i guess this is acceptable for the keel area. Nevertheless i can't see such a laminate for the slamming area (positionned at the first 1/4 of the length of the boat from the forward perpendicular).
The boat is a offshore sailing cruiser/racer of 8.9 m length.

Any comments on what i said is really welcome. I might have said some things wrong, i might have not understood properly some things. If needed could you tell me if so. Moreover could you tell how people or shipyard use to deal with the slamming area.

Thank you very much

[Roly]

As a comparison, using formulae from David Gerr's "Elements oF boat Strength".
General rule-of-thumb.....briefly...
Estimating your boat LOA= 29', beam 10',Height amidships keel to sheer 5',
Scantling number (Sn) = 29'x10'x5'/1000 =1.5 approx.
Solid glass thickness (lower topsides) =6.35 x Cube root of Sn (6.35x1.5v3 =7.33mm) switched to metric as Sn is only a factor.
Keel region glass thickness is factored by 1.5, (1.5x7.33) = 11mm.
According to Gerr.........
Core thickness = 2.2 x solid glass (L.Topside thkness) =16mm
Outer skin = 0.4x solid glass thkness.= 3mm
Inner skin = 0.3x solid glass thkness. =2.2mm
These laminate thicknesses include csm/polyester.Using epoxy you can gain higher strengths by using non woven fabrics for the same build thickness and delete the bulking csm. Or, lower the build thkness by 10% if you want to optimise.

I liked the simplicity of his book. The above should be qualified in the context of the whole pubilication.

Hope this helps.

[Bluedesign]

thank you very much for the quick answer. This will be quite usefull... and i guess a bit of study thanks to this book will be really welcome. Nevertheless, we had the impression that the laminate needed to be changed in the slamming area ( first 1/4 in length) in the bottom of the hull. Aparently not ?!

Regards

[Roly]

we had the impression that the laminate needed to be changed in the slamming area ( first 1/4 in length) in the bottom of the hull. Aparently not ?!


I included a woven 300gm kevlar sheer to sheer (inside) in the slam zone & chainplate area.
More because I had it, than helping the numbers.Also outer and inner skins around the mast.Lot of effort, handlaying, for dubious gain.
Do not be surprised if you do not get a response from a laminate engineer.
As you know, it is a complicated science and all the parameters of the design are needed,
including the expectations of the owner. How long is a piece of string?
Who needs the responsibility? Also,kinda like asking a tradesman to work for free.
I have had the odd terse reply.:D And, some excellent advice from friendly B/Builders's.

Being a cheap skate,I Bought Tom MacNaughtons scantling rules and Davids book in lieu of following ABS (which I considered conservative/dated) or buying a laminating schedule from an NA/Engineer.
Horses for courses.:cool:

[waikikin]

Bluedesign, the forward1/4 of my hulls have an increase of core thickness from19 to 25mm or more longtitudinals could have been added but more work. Often on single skin layup of hulls the laminate overlaps the centreline by a good margin & interleves from port to stb' kinda doubleing the thickness there, also up forward the topside panels can be largish & kinda flat & really cop a pounding when heeled & landing off a backless wave with hull shuddering crash- Invest in some design pro fees from some one experienced in the style your building in & proceed with peace of mind & confidence in the build. Regards from Jeff:)