# fiberglass weight

Discussion in 'Boat Design' started by david allan, Sep 2, 2012.

1. Joined: Sep 2012
Posts: 2
Likes: 0, Points: 0, Legacy Rep: 10
Location: PA

### david allanNew Member

Can someone tell me the typical weight per square foot for 2 layers of fiber glass, thickness suitable for small boats, epoxied together? I am designing a light weight sailing pram in which the fiber glass, 1 or 2 layers will encase a woven bamboo frame.
Thank you,
David Allan

2. Joined: Oct 2007
Posts: 4,603
Likes: 174, Points: 63, Legacy Rep: 2484
Location: Colonial "Sick Africa"

### FanieFanie

Hello David,

Different weights of glass exist, the strongest is woven roving. To make stiffness and resistance to twisting and bending, the glass has to be layered in specific orders, submissive to the design criteria, it is not a matter of plonking a bunch of glass down and there you go, this can be costly and dangerous.

Find someone that can help you and discuss your ideas, I guarantee you there will be a bunch of other unforeseen little problems coming out as well.

One layer of 450g woven comes out at around 0.35mm. Even two layers will leak because it will not seal properly and it will be very soft and flexible. I'm not inspiring you to set off and attempt something here, get proper help.

3. Joined: Feb 2007
Posts: 368
Likes: 20, Points: 18, Legacy Rep: 247
Location: Anacortes, WA

### jim leeSenior Member

Weight of glass (oz/yd)/9 gives your weight in glass per sq foot. You must choose the oz/yd for your application.

Take this result and x 2 to get two layers. Now you have total glass weight / sqft

From total glass weight a rule of thumb is 50:50 glass:resin, so multiply the total glass weight x 2 to add in the resin weight and Vola! You have your answer. But of course, like I said before, YOU have to choose the glass weight you plan on using first. Then rattle through the calculation.

Good luck, I hope this helps!

-jim lee

4. Joined: Oct 2007
Posts: 4,603
Likes: 174, Points: 63, Legacy Rep: 2484
Location: Colonial "Sick Africa"

### FanieFanie

The optimal glass to resin ratio is about 2 : 1. The additional resin does not add to strength, weigh 25% more and is extra cost. With hand layup it may be possible to roll the access resin out of the cloth with an aluminum or polyurethane coated roller before placing it, depending on how big the job is...

That is exactly what vacuum forming and vacuum bagging does, it sucks the excess resin out leaving you with lighter stronger end results. If the weight is not a concern then rather add more glass that will add stiffness through thickness and more strength.

5. Joined: Jan 2006
Posts: 3,156
Likes: 321, Points: 83, Legacy Rep: 1279
Location: Lakeland Fl USA

Woven bamboo? ? ? Your fiberglassed surfaces, it seems to me, are going to be kind of lumpy if you weave bamboo (as in a reed chair bottom). In order to make it smooth you will use generous quantities of resin which will make it much heavier than if you do it over over a smooth armature.

6. Joined: Sep 2012
Posts: 2
Likes: 0, Points: 0, Legacy Rep: 10
Location: PA

### david allanNew Member

Thank you all for your thoughtful replies

Remember the somewhat flexible dinner mats of plastic encasing an open mesh of 1/2' by approximately 1/16" bamboo strips? My idea is to create an
8' pram shaped hull out of bamboo and cover with fibreglass. Ripples in the hull are OK. I'm concerned about lightness. We 2 passengers would sit on boat cushions. David

7. Joined: Oct 2007
Posts: 4,603
Likes: 174, Points: 63, Legacy Rep: 2484
Location: Colonial "Sick Africa"

### FanieFanie

David, you okes that want to make these small thingies scare me... are you going to play Romeo and Juliet and never come back or what ?

Look for an old hobie cat and fix it up. You just need the hulls and the frame and the trampoline. You can put a very small cheap motor on it and it will still go well.

8. Joined: Jul 2010
Posts: 365
Likes: 38, Points: 28, Legacy Rep: 233
Location: N.W. England

### latestarterSenior Member

9. Joined: Nov 2003
Posts: 19,133
Likes: 484, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

For an 8' pram, concerned about lightness, 'glass and bamboo should be well down on the list of considerations. Typical 'glass laminates are heavy for their strength and stiffness. This is why small wooden racing dinghies have to carry lead weights, to allow the same design produced in 'glass to remain competitive. If you look into composite structures you can get lighter, but materials cost soar. Reevaluate your material choices if light weight is truly a goal.

10. Joined: Feb 2007
Posts: 368
Likes: 20, Points: 18, Legacy Rep: 247
Location: Anacortes, WA

### jim leeSenior Member

You going to make it clear so you can see the weaving?

-jim lee

11. Joined: Jul 2011
Posts: 172
Likes: 6, Points: 0, Legacy Rep: 91

### river runnerbaker

fairing compound

You could smooth out the hull by first smearing fairing compound over the hull and then sanding, but this would add even more weight. I think I understand the thinking that led to your idea, but a more practical design along the same line of thinking would be a skin on frame boat. That is, a laticework of wood covered by a tight skin. That is how native amercans built canoes and kayaks for centuries.

12. Joined: May 2008
Posts: 2,456
Likes: 266, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

As you describe it, you are using the bamboo mat (which is rather heavy) as a coreing material that will also provide the shape. The mat thickness should be about 1/8 inch or a tiny bit thicker. Then fill with a thickened and bulked epoxy putty. Use "bubbles" to bulk and fulminated silica to thicken. Don't use any more silica than necessary because it is difficult to sand. A very rough guess at a recipe would be about equal volume of bubbles and silica and rather less epoxy for a density of around three or four pounds per gallon. Let cure, sand smooth, and put a layer of six ounce weave on both sides. You should be able to bring this in around 90-95 pounds. As others have mentioned, plywood is probably lighter and better suited to epoxy coating. The glass has to bear 100% of the load. The bamboo is just taking up space. Basket boats usually have only a tar/pitch sealant and can flex over a wooden frame. Can you show us a photo of the bamboo mat, it's hard for us to guess about the interaction between the glass and the mat. You may need to go heavier than six ounce glass if the bamboo mat is thicker than 1/8 inch. If doing it this way, it is important that the glass weave lays smooth and isn't following the texture of the mat.

If the mat is coarser and you mainly want to seal it, you might want to use Dynel instead of glass. It is strecthy and will let the bamboo carry some of the load.

13. Joined: Nov 2003
Posts: 19,133
Likes: 484, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

If he uses Dynel or Xynole, the bamboo would have to carry all of the load. I have a 9' dink design, all plywood that comes in at 60 pounds; less if you are frugal about the plywood species and solid stock employed., so, the 90 pound bamboo pram seems like not the wisest way to go.

Maybe some shrink wrap of unusually thick film.

14. Joined: May 2008
Posts: 2,456
Likes: 266, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

60# suitable for sailing? He wanted a sailing pram. I didn't ask him how he planned to secure a mast to a basket boat.

Thinking about it, my strip plank canoe is nearly a basket boat. It has 1x1/16 ribs on 2" centers and 1/16 planking glassed with 4 oz weave on the outside. 12'6" long and about 40 pounds. So maybe I was a bit heavy on the bare hull. But I figured the basket boat required a full frame if it was going to sail.

15. Joined: May 2008
Posts: 2,456
Likes: 266, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

Then again, I just went back and reread post #1. He said a woven bamboo frame. Maybe a fiberglass boat with a bamboo frame. That makes a bit more sense to me. If so, total fiberglass thickness needs to be at least 1/8 inch everywhere. A bit thicker where extra strength is needed such as mast support and keel.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.