Hull structure - newbie question

[Rodz47]

I have found the following statement in a brochure of Islanders:

"- The hull and deck are constructed in alternate plys of hand-laminated fiberglass
- The deck has a ply-core for superior strength and solid thru-bolting of the deck hardware "

Can someone explain in simple words how the hull was built? I understand that the deck has a plywood core.

Thanks

[phallo153]

If I'm not mistaken, it means each layer of fiberglass matting is laid perpendicular to the previous, just like the grain is in layers of plywood.

[sailingdog]

The hull is probably constructed with alternating layers of fiberglass mat and roving. First you spray gelcoat in the mold, then you lay up a layer of fiberglass mat, then you lay up a layer of fiberglass roving, then you lay up a layer of mat, etc.

BTW, the fiberglass mat used in most older boats is chopped strand mat and doesn't have a direction to it, or much strength relative to fiberglass roving or cloth, since the fibers are relatively short and non-directional. The roving definitely has a directional component to it, and was probably laid up in alternating directions to some degree.

[yotphix]

In very simple words, it was built like most boats are built.
They also specify that your decks are ply core as opposed to endgrain balsa, suggesting that it is stronger for mounting deck hardware. What they don't mention is that the ply gives intruding water the ability to migrate from point of intrusion and spread it's damage around.

[sailingdog]

Marine plywood cores are also far heavier than end-grain balsa or foam core materials are.

Basically, marine plywood has the moisture migration problems that the foam core materials have, the rot problems that end-grain balsa have, is heavier, but is probably the best at resisting compressive forces. It is a lousy core material for the most part...

[Jeff_H]

That does not tell you a whole lot about the hull lay-up except that the hull does not have chopped glass. Fiberglass can be laid up in a variety of fabrics (E and S glass, woven roving, mat, cloth, bi- and Tri axial etc) and a variety of resins and orientation patterns. (Phallo 153: the layups are rarely crossed at 90 degrees but were rotated at specific angles intended to trace stress patterns and reduce bias stretch). The Islander brochure tells you almost nothing about the lay-up itself other than it was done by hand.

The one phrase that does catch my attention is "alternate Plys". I am speculating that this refers to alternating between plies of mat and woven roving, a very typical 1960's and 1970's layup schedule. Woven roving was a course fabric that was hard to laminate to itself and so the mat was added to bridge between the layers of roving. We now know that non-directional fabrics like Mat (especially as used in the resin rich, bulky layups of the 1960's and 1970's) greatly reduces the impact resistance of the laminate and also greatly reduces the resistance to fatigue over time.

As to the deck, plywood coring (except at high stress areas such as hardware fastening) makes a very poor choice for a core material in that it is prone to rot and unlike balsa spreads the rot in all directions resulting in a more rapid spread of the core rot. In most, but not all cases, plywood was chosen as a less expensive core material.

Jeff

[Rodz47]

Thanks for all your responses. One supplementary question:

Is it possible that the underwater portion of the hull (just after was hauled out of water for surveying) can give slightly different sound that the above water part during surveying? And still be in a good condition? The above water part as checked by a moisture reader is dry, the other part cannot be measured by obvious reason.

[sailingdog]

Yes, it is very likely that the deck, which is cored, is going to sound somewhat different than the hull, which is solid fiberglass. BTW, a cored deck can read as dry and stil be a problem. One one boat I worked on, the core never bonded with the laminate properly, so the deck had effectively been delaminated from day one. It was dry, but still it was delaminated. Getting a good bond between the laminate to the foam-core was often a problem in the early days of building cored decks and hulls.

[USCGRET1990]

Sailboat hulls get increasingly thicker from gun'l to the keel. If the boat was hauled, a moisture test could have been done below the water line after 24 hours of drying if I remember right.

[sailingdog]

Originally Posted by USCGRET1990

Sailboat hulls get increasingly thicker from gun'l to the keel. If the boat was hauled, a moisture test could have been done below the water line after 24 hours of drying if I remember right.

Unfortunately, IIRC, some antifouling paints prevent the moisture meters from reading properly.