Skin thickness

Still partying into the New Year RX??

Post #12 here

Slip of the finger or the bottle of Tanduay on the keyboard??

 

New Year's hangover.

 

Yup. Cored bottom too stiff. Calcs showed that on 1m x 0.5m panel with just 20 mm of A500 core, is barely deflecting at 1.5 mm. The outside skin thickness assigned is 1.6 mm only, easily punctured by a rock. The only way it would deflect 15 mm is to use a 2m. span.

Seems single skin is the way to go. Will try to come up with something.

 

Tungsten,
Try to make a panel 1m x 1m x 6 mm thick with a center girder and a keel reinforcement.

The keel reinforcement is to be 350mm wide, 1.5-2 mm thick. The tophat girder not lower than 35mm deep, not more than 50 mm wide with a low density foam core such as A500. Overlap the cap layout or insert a couple of Unis on top as this girder is highly stressed.

All laminate hand layup with Vynil Ester. You can insert Biax in the WR layers as long as it is not more than 40% of the total. Keep the laminate balanced.

Fixed the ends to a board and try to test the panel by standing on it, dropping a log or even a sledgehammer.

I tried a higher glass content consistent with resin infusion. The reduced thickness created a stress on the outer laminate and showed ply failure. With the added stiffness, the only solution was to increase the span or add more layers to decrease skin stress.

Seems hand laminate is more flexible and gives the right amount of deflection.

 

Thanks again Rx,
Hand layup is out,too many layers to try and wet out on top of that no experience with this resin/method.

Infusing seams to be working for me so going to stick with that.

You mentioned no more then 40% DB when mixed with WR on hand layup.I did some test pieces(infused) that alternated DB with WR with the DB being the outside layers.Tests are showing good results with flex and durability.

3 layers 600 WR with 4- 400 DB Infused I'm getting 2.5mm thick.

 

If you want to go infusion and that is where you have the skill, go for it. I am just saying that with infused panels, there has to some re adjustment of frame spacing (for which, up to now is arbitrary) to benefit from the method.

A WR600 with 0.50 glass content is 0.734 mm thick. With infusion, you can get up to 0.65 glass content and the ply will only be 0.50 mm. thick. DB 400 with 0.50 Glass content is 0.49 mm thick, 0.336 mm. thick only if the the glass content is 0.65.

If you rotate the fiber from the axis, you lose axial strength/modulus in that direction(longitudinal). Not necessary "losing" it as you will regain the strength as you rotate your stress in the direction of the fibers. That means, if all your fibers are aligned along the length of the boat, the strength or stiffness is at maximum in the longitudinal direction but very little in the twisting department. That means a boat that is stiff along the length but twist a lot (or break) when a rock is wedged on the side at 45 degree.

Attached is the fiber angle strength from my book of Quality Control for resin infusion.

 

I've built many of these boats, don't stress too much over it. I don't use a raised floor in mine, some people do like them though, most of my customers that make Drift boats have a raised floor as an option and most of their customers get them.

On my first build I made the hull and put it in the water before adding all the glass to see how much flex there was, even thinner than I had planned there was little flex unless going over rocks, so I left it as is. When walking on the hull no movement is felt. I could have added more glass after testing if needed.

1/4" on the chine is thin, most are about double that with the laminate tapering off by staggering the glass widths to about 6" from either side of the chine. This adds stiffness to the hull and sides.

Two of my customers use foam core in the hull bottom, the outer skin is thicker to deal with impacts, not really much thinner than a hull without a core. The inner skin is only there for stiffness, it doesn’t create a balanced laminate, but it is durable. I haven’t built one of my own in about 20 years, so I don’t have any laminate schedules or thicknesses to give you. I will say that none of my customers build them in the same way, they are all different, and every one of their methods work fine.

 

Tungsten,

The bottom panel schedule is designed with a stress ratio of 0.25. It means only 1/4 of the stress is applied for design purposes, typical of most bottom plate design. It also centered around the premise that it is a workboat, subject to a lot of abuse. The 6mm thickness has all the safety factors built in. You can probably reduce the thickness if the boat is not abused that much.

The side panels receive a lot of shear, so the gunnels play an important role. Too small (low modulus) and the bottom panel takes much of the load. The gunnels determines where the neutral axis will be in relation to the midship section of the boat plus it should take up the banging associated with berthing or getting close to another boat, or just simply being tied to another boat.

 

Gents,thanks for the input.