Help needed with fiberglass for a custom hull

I am a student building a hull that my group designed for a competition we are entering. Approximate dimensions are 16'L x 40"W x 24"D. We had planned to make a plug cover it with a first wall of fiberglass then put in thin balsa ribs filled in with expanding foam then a final level of fiberglass. Like every project we want to keep weight low and cost low.

I am trying to figure out how many layers of glass we need for the first and second layers along with which types cloth and resins will work best. I am leaning towards epoxy for good adhesion to the ribs along with flexible crack resistance.

I talked to a supplier about how much we might need to get the job done but I couldn't tell him how thick we wanted each of the two layers to be. Help and advice on any part of the project is welcome and appreciated.

 

How long is the boat to last and how will it be treated? You said it is "for a race" but also talk of resisting cracks...what type of boat? Does it have to be molded fiberglass?

 

Longevity and care

My hopes is that It will last for a few years. Meaning a month or two of testing along with a few weeks of competition per year. max speeds around 30MPH for short periods along with 2 hours plus at about 15MPH.
My hopes is that it will be cared for gently but it will be handled by students so some abuse can be expected.

There are no regulation on materials but fiberglass was chosen because it seemed like it would be easier to get the shape we wanted. The hull will resemble something like this

 

Nice looking boat - What is the power?
Consider glued lapstrake construction using Shellman Marine okuome 5mm plywood http://www.boulterplywood.com/ . Problem - it will last too long (a lifetime with care) but the 5mm has two more plies than the four and is worth the small weight penalty. Do not use any other plywood. This stuff has VERY few and very small voids, is light (okuome - one step in weight above balsa) and is made with a fungicidal glue. No fiberglass. Check out this book -



If you do not have confidence in this technique, wait for Rick Willaby's response, but I think he is going to tell you 1/4" end grain balsa and some light layup of epoxy/glass with carbon transverse stiffeners in way of the engine. Might be a perfect use for Rick's strutless shaft, as well. Some guys he probably knows down in Aus. are using balsa planks but, IMO, that would be a truly short-lived vessel. One scratch to the core and you then have a very heavy balsa/water vessel. That is the conumdrum with light weight - if you sheath the lightweight core with enuf glass to protect it, it then becomes heavy. Even a glass sheathed plywood boat is lighter than a foam one if you want the thing survivable in the real world.



One final thought on saving weight...by far, the easiest weight to save is in the crew. I have learned this in everything from boats to motorcycle racing to cars. I am building a "GMW" (Ls-3 Vette rolling chassis in a 300 series BMW) and I hear others doing the same thing saving a few grams with a carbon fiber mirror or whatever... and they take down pizza and beer the night before a race (and the years before a race). Do you have a weight handicapping system in your race? If not, the tiniest guy you can find to drive it will win with equal boats in calm water.

 

Skill/Timeline Concerns

That picture is of another schools boat that has already competed. Our Boat will have about 15Hp when maxed out(electric power)

I have a few concerns with the method you mentioned. We are not the most skilled craftsmen, and we don't have many tools or funds to buy too many specialty ones. We liked the method I talked about because it seemed somewhat simple and did not require much in the way of tooling.

Do you know of any books that talk about processes similar to the one I described?

Also how do the costs of the glued lapstrake construction compare to a fiberglass foam sandwich like I described?

 

It would be cheaper.

In fact almost every proven method is cheaper than the foam sandwich.

Use Kiri wood, strip planked and a single sheath Glass / epoxy of about 200gsm. That would be my method for such a vessel.

Regards
Richard

 

Loading

The boat will be carrying 100lbs of batteries, 300lbs of solar panels, around 175lbs of pilot, around 125lbs of motors and maybe 60lbs of miscellaneous other items.

Could the lapstrake construction handle almost 800 pounds of loading?

 

Building methods and load carrying abilities are not directly related.

My recommendation for a strip plank method, using the lightest (strong) wood available in AU, "Kiri", was given with the ease of build in mind.
It produces a very strong though lightweight structure.
The capability of carrying a payload has to be designed in the hull shape. Of course there is a relation to the material used.

edited:

You may go here:
http://www.newfound.com/
download the "construction notes"
You will have a good tutorial on wood epoxy works!
The described method is the one I recommended.

Regards
Richard

 

Strip Method hull

We had looked into the strip method construction but moved on because of the tooling and skill required. Any methods that are maybe not as effective but easier to accomplish?

 

Strip planking is the second easiest way to build a boat!

The easiest is to hollow out a log (and thats not so easy too).

Although methods like "stitch and glue" and the derivatives are often advertised as the easiest building methods, that is quite not true. (and in your case no option anyway, because you want a performer, not something flotsam)

Tools and skills requirements for strip planking are really minimal.

Regards
Richard

 

Literature

I was going to go pick up that Ultralight Boat building book that was recommended earlier in the thread. Any other books about the strip and glue method that you think would be good to look at?

 

Strip plank,

not strip and glue!

No, you do´nt need a book for that.
Though it might be a good lecture, I do´nt know it.
But the "Gougeon Brothers on boat construction" goes quite deep into cold moulding and strip planking.

You can download another tutorial for wood epoxy building methods in general from the "west system" website. (Basically the excerpt of this book)

And take care, when you go for a "Ultralight" boat which today is commonly misunderstood as "Carbon composite" you would go for about 5 -10 times the cost and tooling and skills!!!

One comment on the figures Mark provided.

Kiri is the lightest next to Balsa, not Okoume but you could use that too.

Remember when choosing the thickness of a batten or the ply, that the strength of the plating goes up dramatically with every additional millimeter. (to the cube)
A 4mm ply has a strength of 4x4x4 = 64 (x properties) where a 5x5x5 = 125 !!! That is twice the value.

Regards
Richard