single skin vs. sandwich fiberglass

Looks like I have 2 options when I build my speedboat from Bruce Roberts plans. Single skin or sandwich with foam. What are advantages?

 

It should not be triple weight, it should reduce it. It works like this, if you have a single skin, lets say 3/4" thick. Much greater strength and less weight, can be had by two 1/4" fiberglass skins and 1/4" foam panels.

 

Which design is it you are looking at ?

I note that in his discussion on 'single skin' building, he says "This construction is generally limited to vessels up to 40’ in length. "

Mydauphin is spot on in the advantages of foam core, but also the type and size of the vessel may require solid glass in some areas of say, potential for high impact.

 

Along with greater strength (stiffer less flat area and torque flexing)to weight ratio you will also have built in insulation resulting in little to no condensation (sweating)on the inside surface of the hull.

 

A foam cored builds will have sections of solid laminate built in and as mentioned, typically in high stress areas, around corners, etc.

The difference basically is a foam cored build; you have to laminate it twice and fair it twice, once for each skin, with a single skin build; you laminate it once and fair it once. Considering the amount of fairing involved in a backyard build, cutting down the amount of laminating and fairing as best as you can, should be primary goals. Unfortunately, most novice builders don't know this, so they learn the hard way.

 

The foam sandwich panel you have described is slightly weaker than the single skin assuming the same grade of fibreglass is used.

The overall thickness of the foam sandwich panel has to be thicker than the single skin to compensate for substituting weak foam for the loss of strong fibreglass.

As the thickness of the the foam increases there will be rapid improvements in strength and stiffness.

 

Simple answers, but reality is different, techniques, materials and circumstances are different.
When looking to build a hatch for example is different than a transom. Some panels are gussetted for strength and yet the panel is not full full thickness. Doing a panel single skin for stress is not exactly correct, it has to do with layup technique. How you lay glass, joints and type of glass have many options. There are many good books on issue, including navy tests. There should not be to much more fairing in foam sandwich than single skin, you have to have better techniques however. My typical hatch consists of a Peel ply,2 biaials,1/2 foam, 1 biaxial, and then stiffeners made of strips of foam, reglassed if needed. Developed this technique for 400lb customer that kept breaking most boats.

 

Note most single skin boats have a structure, a foam laminated Hull is part of a structure.

 

If weight counts the foam cored is the choice.

BUT the weight savings come at a high cost as quality foam core is really expensive.

A small cruising sailboat is probably not worth the cost and effort.

If you are talking a 50K motor cruiser , foam is worth it, just in the price of smaller engines required.

FF

 

If you need strength like hatch, transom,bulkhead,etc..., you don't a choice but sandwich , marine plywood is not cheap either. Foam is not only choice for sandwich, neycombs, metal grid, balsa, wood. It is like making a cake, different recipes for different effects. For mental experiment. Take a 3/4" solid fiberglass panel, or a foam sandwich totaling 3/4 or marine plywood 3/4 sanwich, they all cost about same. Solid heaviest, foam lightest, plywood strongest in a impact because it wont shatter.

 

There are a number of different types of 'strength', single skin and cored laminates have varying combinations of this one or that one. Choice of materials changes everything. You need some reputable books to get a grasp of the situation.
You might try here as a start, at their 'home' site they lightly explain the differences between the methods.
http://www.bruceroberts.com.au/allegations.htm

 

I dont see 3/4" fibreglass shattering, unless you havnt put any fibre in it.

If you want the greatest impact resistance, there is nothing as strong as solid, well designed glass and fibre out of those three.

If weight matters, then you have to think about other designs.

 

For a smaller motor boat (12-30ft) the cost of a good foam core might be justified by the building ease. No mold required.

Sure fairing is no fun in a boat created on a plug , but a small boat is no big deal.

FF

 

Wrong, give me an ax. Show me solid core, foam sandwich, and plywood sandwich. Same amount of fiberglass, even more in solid if you want.
Solid one wack, foam two wacks, plywood at least three wacks.

 

strength is a relative term and also has many definitions -i.e. there is tensile- shear-impact-compression etc...what ones are important to you?

The core density has a lot to do with strength- a 5 lb density is not nearly as able to withstand impact as 8 lb density etc... however a single (multi)laminate skin 3/4 thick WITHOUT csm, would be very strong indeed, it would almost be bullet proof but heavy. and resin rich as well, hence more costly than a core with two light skins. of course it equals out when you add in the core costs--single skins are also less costly.
For what i am building- impact resistance is a very important factor as is stiffness. so for me a singkle skin or a dense foam would offer this--if weight is a concern well--rule out single skins unless you dont mind the oilcanning that will happen in a chop.

find out what you want your boat to be used for--then build based on that...just my two cents..good luck!