Weight of new boat?

I am building a prototype of a unique small sail boat for two adults. Initially made of foam and covered with fiberglass. Approximately 11 feet long by 5.5 feet wide. Are they any recommendations of how much it should weigh? I have heard that I should make it as light as possible? Is this true?

 

Yes!

The trick is to make it light without compromising seaworthiness.

Your foam is compatible with fiberglas, right?

-Tom

 

Yes, both the foam and epoxy are compatible. I have made 3 models and all looks good.

I would also like the seats closer to the rear. I will have to experiment with this.

 

One of the first things you need to do when designing a sailboat is establish a target displacement. With this you can shape your boat accordingly and ascertain it's weight. Light weight is always desirable in small craft, but there are material and technique limitations, not to mention engineering requirements, that will pretty much dictate the weight of things.

An 11' boat, being asked to carry a 320 to 400 pound crew load, sounds like it's going to be a quite burdensome shape, so the preformance envelope will be fairly compromised (read fat and slow). The 2:1 beam/length ratio bears this point out rather well.

In short, your building technique will offer up a reasonable idea of the boat's weight, the question really should be, how well suited is the design to the SOR you've established and can you engineer a light, but strong enough structure to meet the SOR's goals.

 

This first boat is designed for simple cruising, not designed for speed.

 

Planning on using 1 pound per sq ft destiny foam. They also have 1.75lb

 

Do the two adults have to be able to lift it? Car-top it? Haul it up a beach?

What are similar boats made from, and how much do they weigh? 3mm fibreglass? 6mm plywood? what internal structure?

You might find that the foam/GRP is not as puncture resistant as you might like, so this might be another constraint.

To my mind, you'd have to have a good reason for wanting an 11ft dinghy for two adults, especially when the dinghy world has so many 12ft to 14ft boats of varying capabilities and performance, at similar weights. What's the unique concept?

 

I think a better question would be what is your understanding of hydrostadics/dynamics, because clearly Simpsons rule and simple geometry isn't part of the equation, nor apparently the appropriate software.

 

Wasaga Beach--now there's old stomping grounds from the 60's (was stationed in Clinton, Toronto and Borden)--Ron I know we all feel pride in designing and building something but to get a good feel for your first build why not purchase a set of plans that closely match with what you want to build. It will be a good learning experience that will teach and give confidence for the second build which you can then have a go from scratch. We all have to start somewhere, my first wood build was a scaled down copy of a set of plans used by an old dory builder in my village. I had helped him as a young teenager build several dories and wanted to have a go at it myself so he obliged me with the plans. I planked her with pine house clapboard. She was a good little craft. My first actual build was a concrete mesh frame covered in canvas -- an origional skin on frame

 

1# foam will be worthless. Don't use styrofoam, it has no strength or stiffness, it might be good enough for floatation.

You really ought to build something well known first, just to get an idea of what it takes.
Viking North gives you good advice.

Watch out for seats in the rear. A boat this small will tip over backwards, fill with water and sink unless you are only sending out children. Even then....

 

Ron, you will encounter some naysayers here, me among them. Do not take negative replies as a personal affront.

It is obvious that you are not an experienced boat builder or designer. The first thing that a designer does is make an SOR. That is a statement of requirements.

In this case you are to consider the place and conditions in which the boat will be used, and this needs to consider worst case scenario. How much can the boat weigh? If it is to be car topped it needs to be as light as possible consistent with durability and structural adequacy. Will it be stored indoors or outdoors? will it be left on a mooring? How much loading do you expect the boat to support while remaining reasonably safe? Will it use auxilliary power in addition to sails? You used the word "cruising". Does that mean splashing around near shore or does it imply overnighting or extended trips? How much do you intend to budget for this boat? You have initially opted for foam sandwich construction. Why? How much time do you expect this build to take? Is there a reason for having selected an overall length of eleven feet? Where will you do this build? In a garage, a workshop, an outdoor shed?

Answer those questions as accurately as you can and then you will have a better idea of what the boat must be. It may not be what you presently want or imagine it to be. DO build the boat but get all your ducks in a row before you cut the first piece of material.

 

Half as wide as it is long ???

If it weighs more than 45 kilos, it is too heavy. (compared to similar sized yachts like a laser http://en.wikipedia.org/wiki/Laser_(dinghy))

How much does it weigh now?

 

How many sqr metres of area x the glass you want to use inside and out + resin + core + paint + undercoat + ? % for bits == your approx weight !!
Measure it accurately and you will get pretty close .
Resin ratio 1.1 to 1to glass . If you are a good clean worker and make a neat job you will come in under that weight guess-timate !!

The hull is really the least of your worries its all the other junk that you think you need thats where the weight rockets and the cost along with it . Dosent mater what size you work on the hull and deck really are stuff all its all the other rubbish you think you cant do without .
Want to save weight ?? go on a diet and sail naked !!!

 

This is the most common phrase quoted by amateur designers when questioned about potential performance. In reality, you probably just like the shape, and got the 'sculptors urge' with little consideration of real life usage ( it happens all the time )


Its also probably designed to be handled and carried by two adults, and float with them with all their gear, and not take all day to travel around the bay.

Its either a sculpture, or it actually works as a boat.