Too stiff ?

[Daidohead]

Hello All,
Is it possible to make a boat too ridged ? I have a 17' fiberglass ski boat with a 100 hp outboard. She has a shallow "v" hull with a rather long front deck ( 7 1/2' ). I am looking to install a bulkhead under the dash and some extra support under the gunnels to help with her 25 year old shakes and rattles. The front deck currently has no support other than 2 ribs glassed onto the underside of the deck.( I'd be afraid to stand on it as it sits).

My question is; Are boats of this type designed to flex ? and if so what will i be in for if I add to much support ? Thanks, Mark

[moTthediesel]

To answer your question literaly; no, you cannot make a fiberglass boat too ridged. A fiberglass boat should not be allowed to flex, every time it does, the glass fibers within the resin matrix are broken. What are commonly called "stress cracks" that appear in gelcoat are the result not so much of stress, but of flexing in that area. In fact, it's well known that many one design racing sailboats built of fiberglass become less competitive over time because the flexing of their light and thin hulls tends to "soften" them over time and use.

In your case however, what you need to think about is that when you add bulkheads or other renforcements to your hull/deck structure, you have to take care not to form "hard points". Flexing that is spread out over the surface of a large fiberglass panel will very gradually degrade it's stiffness. But adding a inflexable "hard point" somewhere in that same panel will tend to concentrate all the flexing in one area, and can lead to cracking. Wide "tophat" attachment points and/or large fillet corners will spread out the stress over large areas and greatly reduce the formation of these hardpoints.
moT

[SeaSpark]

The flexing of a hull wastes energy that otherwise could have been used to make it go faster.

[Daidohead]

Thank you. Mark