Fiberglass hull thickness

[AndamanSS]

Is there a standard formula for calculating the hull thickness?? Given it will be a 22 ft boat for use in sheltered water(lakes&river) and capable of 35knots.... Any help would be greatly appreciated.

[Eric Sponberg]

In a word, no, there is no simple formula. This is because boats are actually fairly complicated structures, and they are designed for the internal structure, the shape of the bottom, and the speed of the boat. It takes engineering analysis and a series of calculations to figure out a boat laminate.

Having said that, however, a fairly good guideline to refer to is Dave Gerr's book "Elements of Boat Strength." This is not an engineering guide per se, but it will illuminate some of the science that goes on in engineering of boat structures.

Eric

[AndamanSS]

Thank you eric for your reply. i will try to find that book. hope its available here in Thailand. I have one more question which was related to the first queries. How accepted is wood/ composite construction in North America. Our current boat uses 10mm thick cedar strip planks with 2 layers of fiberglass cloth on both sides,and West system Epoxy on the outside. adding about 7-8 mm to the thickness. The Duflex Balsa composite board bulkheads are 22 inches apart from each other. In Australia this construction is widely accepted but i am worried that this is not the case in North America. One would argue that our fiberglass is too thin, but our trial test in the water have proven otherwise.

So I am trying to make a test panel to compare the impact strength between the panel that reflects our current construction and the Fiberglass panel. But i am just not sure how thick the fiberglass panel should be..

[Eric Sponberg]

*******SS,

You can get Elements of Boat Strength in many places, one being www.amazon.com which is an on-line book seller here in the US. They will ship anywhere. That book will likely give you enough guidance to compare laminates. Another resource that you might consider is High Modulus in New Zealand. They supply composite materials for many composites industries, including both production and high-tech custom boats. They can probably give you a closer idea of a suitable fiberglass laminate that would work in your boat based on a description of the boat.

Here in the US, wood-epoxy is very well known, although you would most likely find it in custom boats, do-it-yourself boats, or limited production boats. The vast majority of production boats here in that size would be solid fiberglass on the bottom and perhaps cored fiberglass on the sides and deck. Foam core is probably the most popular, although balsa is very common. On my CC20 speedboat design for Cherubini Yachts, the whole boat is solid fiberglass with the bottom being about 5/16" to 3/8" thick (8-10 mm). It has a mix of mat on the outside, and various layers oriented at 0/90 deg and +/-45 deg. Boat speed is mid 40 knots, although one version has been up to about 60 knots. Fiberglass thickness depends a lot on the speed of the boat and the size and orientation of the internal framing--we have a lot of structure in the bottom of the CC20.

Eric

[Nojjan]

Check the worksheet location of this site http://www.boatdesign.net/forums/downloads.php?do=cat&id=1
I have loaded an example there (scantling calculation). I still suggest you buy the book but a finished sheet is still helpfull. / N