Originally Posted by Robertcateran
Hi Big Cat,
Have you looked at the impact tests done on polycore polypropylene honeycomb against that of balsa core
. Balsa core is stiffer but does not resist impact as well. I agree with you about fire retardant resin, especially on the inside. I have also been looking at bamboo fibre as it is stiffer and lighter than glass but am having trouble sourcing it in smaller quantities than a container load. I will probably use fire retardent vinylester on the inside and epoxy
below the waterline outside. The strenght of the laminate seems pretty close with either but there is a very slight water
penetration difference- probably not enough to worry about.
don't know why you want to go to solid below the waterline. A fair bit of extra weight and not necessarily any better for puncture proofing. Polycore polyprop seems to be up to it.
Hi, Robertcateran - Several experienced boat builders have expressed concern about the ability of polypropylene honeycomb to resist sheer stresses, stating that if you take a piece and push the skins in opposite directions with your hands it is easy to collapse. Balsa and foam are, of course, also much easier to infuse than honeycomb.
I am infusing my laminates, so having different types of resins is not an option in a given panel, as both sides are infused at once. The Jamestown boatyard reckons that vinylester is actually better than epoxy
at resisting water
penetration of laminate: See http://www.jby.com/OSMO_web.pdf
In any case, it certainly is blister-proof. See: www.zahnisers.com/repair/blister/blister1.htm
With a solid bottom, hull penetrations caused by impact can't lead to core problems, and you have better penetration resistance to highly concentrated loads. A solid bottom is best if you hit something like coral
or rocks, where you can get highly concentrated loads that would pierce the thin skin of a cored laminate. When solid laminate has been damaged, it is easy to identify the extent of the damage, and that is not always the case with core.
I can afford the weight of solid bottoms because with the 'little big boat' philosophy, I already have a very, light boat, with a D/L ratio of 61 lightship and 78.5 in cruising trim with 1000 gallons of liquids aboard. Amusingly, that is about half the D/L ratio of a (so called) Fastcat
435, and that is possible because the boat is so large. (64.5' x 35.5') Everything Gideon says about the relative weight and strength of materials is completely wrong, as my much lighter D/L ratio so clearly demonstrates.
I am building to the ABS Rules for Offshore
Yachts, and the high strength of balsa lets me satisfy that rule
and still build a very light boat. By the way, the claim that balsa is lighter than foam is outdated, as light density balsa (6 pounds per cubic foot) is the same weight as light foam (80 kilos per cubic meter.) Balsa comes in three densities, light, medium, and heavy. I am using medium density (10 pounds per cubic foot) in my topsides, and light balsa elsewhere. For that matter, claims that balsa soaks up a lot of resin are also outdated, as it is normally used precoated now.