resin bonding issue Apoxy vs Vinyl vs Ploy

Core shear

Cebu,

I did some number crunching with the boat dimensions you have given earlier.

With a waterline of 22', and a Cb of 0.44, draft of 10", you will have a displacement of 1.5 tons in fresh water.

Travelling at 8 knots max, bottom shell pressure will be about 3 lb/in2.

Designing for a bottom panel size of 36" x 18", you need a core about 3/4" thick. With fiberglass skin of 1.5 mm. inner, 2mm. outer, the core should have a direct shear strength of 30 lb/in2. Allowing for a safety factor, you need at least 80 lbs/in2. A typical 80kg./m3 H80 Diab foam core will fit the bill. Sorry about mixing up the units but most of my data is in in metric.

If your treated laminated core at 3/4" can receive a max of 80 lb/in2 core shear, I will design the laminate schedule of the boat for you.

No problem at the moment as there are still too many variables. The core is receiving much of the stress at 3/4" thickness. The skin, even at 1.5 mm. and 2 mm. is barely stressed. The panel is fairly stiff, barely deflecting at 1 mm. I can reduce panel size or increase lamination thickness of the skin to take up the slack in core shear deficiency. Or we can go for a much thicker core.

Rx

Whar Whoop,

Thanks.

 

How did you arrive at a draft of 10 inches

A Jon boat usually has a draft of 2 to 4 inches.

CEBU

 

From the formula length x breadth x draft x Cb. I was guessing only the draft to arrive at a reasonable amount of displacement of 1.5 tons. The Cb is also a guess as I do not have the boat midship section.

I just have to start somewhere to get the bottom pressure. If you are willing to give more details, I can be more accurate.

 

What is Cb?

If I take dimensions of 7' x 18' x 10", I get a displacement of around 6500#, a little more than 3 tons. The 18' is just a rough guess of the averaged out waterline length accounting for the curve of the front.

 

Block coeffiency. Your numbers being like the block is calculated (lwl x beam x draft) and 0.44 the amount of the real displacement of the block..

 

Why would it be so low? Jonboats are about as close to a block as a barge is. How is .44 arrived at, a guesstimate of some kind?

 

It is an emprical data for a lightly loaded boat with a V shape bow, U shaped midsection, becoming flat towards the stern, perhaps with a rocker. This is the part where all the curves are underwater. The clue is the boat has 2 feet of sides with only 4" (I used 10") of draft. Waterplane coefficient would at best be 0.65 to 0.75.

If the boat is loaded to 50% or 60% of the depth, with all the sides now becoming fairly constant, Length waterline lengthening due to the rake of the bow, Cb is about 0.7, typical of a workboat.

But if the boat is built like a barge, all flat, short and fat, bow nearly vertical, Cb would be about 0.8 at fully loaded condition.

 

It's not that low.. for a boat something less around 0.3sh is more common..