XPS Hull with glass

For test purposes we want to build a 24' open motorboat. We are thinking to have the basic hull shape milled out of XPS (or comparable foam) with high density inserts on sharp edges and for hardware mounting. Finish this shape with glass skin/epoxy resin. Since the foam doesn't serve as sandwich construction I am wondering what thickness to use on the glass. Calculations based on sandwich would probably create a too light solution, however my gut feeling says thickness based on basic FRP calculations might be a bit over sized. Any thoughts on what thickness for the skin and type of foam and density to use?

Despite being for test purposes, the boat should be durable and comply to basic strength calculations. It will be used for sheltered inshore waterways or CE design category D.

What are the major drawbacks of building this way?

 

One of the major drawbacks is that the foam will take a lot of interior space.

 

What is the designed bare weight of the boat ?

 

Indeed, it takes up a lot of space. This is accounted for in the design, I am looking more specifically for major drawbacks construction/strength and durability wise.

The bare weight will be around 600kg. However, This will be dependant on the thickness of the glass skin and the density of the foam. The boat will be fitted with an electric drive, not sure yet if we will use lightweight li-ion type (or equivalent) or more classic type heavy batteries. This of course will also influence the bare weight.

As a starting point I think using a minimum thickness of 100mm of core on outer skins and 50mm on interior parts should be enough. However that is just me and my "gut feeling" Any thoughts on how to determine this?

 

Why are you not using structural foam instead? It will make the boat much lighter. It makes no sense to use cheap foam and then spend a lot more money on fiberglass and resin. You will have to laminate the interior anyway or the foam will fall away in pieces.

 

The hull will be glassed inside and out, no foam exposed anywhere. I know it doesn't make sense, however we want to compare this type of build and explore possibilities. That's why I'm curious if there is any experience with strength calculations using this method.

 

You might contact Hugh Welbourn(Dynamic Stability Systems). Around 5 or so years ago he built a 27' mostly scale model of a hundred footer to test the design which incorporated his patented DSS foil. The boat was built in a similar way to your description of your proposed build. It is still sailing and racing today. Good Luck!

 

While polystyrene looks cheap at first glance it really is not as you will need to use epoxy rather than cheaper polyester resin. Also while a ft2 of a given thickness is a lot cheaper the difference diminishes when you are using a core that is 10 x the thickness of what you would normally use. I have used xps for a core for select things such as berth tops and in ply/foam/ply panels glued with resorcinol but with occasional stringers in there as well. A big problem is that regardless of the resin system used you get very low peel strength and the low density requires a heavier laminate to not dent easy and that would offset any savings on the core. Realistically H80 is about as light a density that is practical to allow a reasonably light laminate.

Steve.

 

Can you tell us the actual reasons/ benefit you think you will get from this construction?

Make yourself a row boat first.
This is an absolutely useless idea.

The foam seems like it will be light, but you will use a large volume, and it will have no structural value - basically.

If you just want it to make a shape, then remove it later.

We did something similar at work for other reasons. The test articles showed the absolute lack of structural benefit.

 

Thanks on the rowboat tip, unfortunately not a rower... Funny to see your reactions where I am asking if any of you have any experience in strength calculations building this way. As explained we are looking to do a one off for test purposes. Not looking for the cheapest or lightest way. What makes sense to you might be ridiculous for me and vice versa, there's been people around having themselves strapped to 100.000 lbs of rocket fuel and have them shot up into the heavens, we refer to them as astronauts. I'd rather go fishing...

Thanks for the lead Doug Lord! I'll definately try to contact him!

 

You need enough strength to get thru your test.

A row boat was just an example of a smaller boat so you could see if the construction will suit your purposes.

I once talked to a co worker about this construction. He was making an aircraft with plywood skins over XPS foam. He did some research and found that there were several fatalities due to structural failure.

There is no secret about the structural sizing. You just need properties for XPS. Plug it into standard formuals.

Do yourself a favor. Make a 6"x 3 ' bending beam with good foam and XPS.
Measure the deflection with a modest weight attached to the end.
Then keep adding weight until they both break.

The if you don't like rowboats, make a 16' power boat. It will cost you a lot less than experimenting on a 25' boat.

Its your money. Good luck.

 

Polystyrene foam seems to exert a strange fascination for people looking for short-cut construction methods, but realistically, if it was a "goer", it would already be well entrenched for the purpose, having been around for a long time, and being relatively inexpensive. It is not quite as insubstantial as "fairy floss", but is heading in that direction ! Any lingering thoughts it might be a good idea, should be dispelled by the need to employ expensive epoxy resin, which should kill off the notion it is a "cheap" solution.

 

Investigate this stuff.

http://en.wikipedia.org/wiki/Polyisocyanurate

https://www.google.co.uk/webhp?sour...n=1&espv=2&ie=UTF-8#q=polyurethane insulation

 

As Pericles says, look into the polyiso foam. It comes in a number of different sizes, densities and polyester resin won't melt it. It's the standard stuff used in commercial roof insulation under tin or hot mopped flat roofs, so you can get it at commercial roofing supplies for wholesale prices, which is way less than at a lumber store like Home Depot.

What are you testing? A hull shape, a method of hull construction or the efficiency of your electric drive?

A major drawback of building this way is you are pretty much on your own, as nobody does it. So unless someone wants to do all the calculations for free, or you want to pay an NA or something, you have to do it by trial and error.

How much laminate you need depends on what area of the boat it is, what the shape of the boat is, (flat areas are 'weaker' than curved,) and what conditions it's to be used in.

Again, it depends on what you're actually testing, but if you need light weight, you can build a boat to what you think might be too light, take it out and beat it, and then reinforce what breaks. Of course what might seem OK in the short term could break down over a period of time and use. The 'friability'/shear strength of the foam will be important, if too little, shear forces will cause the foam to crumble and the glass to delaminate at the interface or somewhere between the inner and outer skin. Once that damage starts, it accelerates quickly.

Laying up glass over a foam shape means a lot of fairing to do to get a smooth surface suitable for a test. On a trial and error prototype that would be testing both the shape hydrodynamics and the structure strength, I wouldn't put any paint or gelcoat on it before testing because then you can see where cracks or damage appear, and if more laminations are needed in weak areas, you won't have to remove all the paint or gelcoat to do it, just rough it up, laminate, fair it again and go test some more.

Of course, if you want to get the boat certified, I imagine you would have to get an NA or Engineer to sign off on it.

 

The basic problem with this approach is you're not taking advantage of sandwich construction, yet eating the drawbacks to it. You're making a cored structure that will need to rely on the skins, for sole load transmission, so the laminate schedule will be understandable heavier, than a true cored structure. The net result is double skinned hull, with the added labor and material costs associated in this, plus the cost of a non-structural element, tossed in just for fun, not to mention the labor to shape it and a heavier than necessary end product. What's the point?