Just curious

Hi everyone,
I have built a few boats in the past but all out of wood. Here lately I've been daydreaming about a theoretical project using glass and foam. What I wanted to know was 1) Is there any kind of marine structural foam out there that can be bought in large blocks and won't give off noxious fumes when hotwired. 2) Is it feasible to build a boat by cutting cross sections out of foam (of course making allowances for fuel tanks, cabin, etc.), fitting all the sections together, and then fiberglass the inner and outer hull. I don't know, personally I think such a method would cut quite a bit of time from a build if it can work.

 

There's no such thing as structural foam, you're just looking for a core to sandwich between two layers of fibreglass or carbon fibre etc. The strength comes from the two layers of glass on either side of the foam, not the foam itself. So long as it is closed cell and won't absorb water, I think you're in the clear. I'm no expert, so I'm sure others will comment.

There must be hundreds of posts and pictures of foam sandwich construction builds on this site, so do a bit of searching and I'm sure you'll find your answer.

Good luck!

 

Hey thanks for the info, I'll take a look around.

 

Yes there is such a thing as structural foam and it is usually rated by the foam density. Light weight foams are used in flotation or light panel construction and heavier density on loaded panels or in monocoque structures.
I think there are soy based plastics that may not be as bad smelling when melted, but I do not know much that does not stink when melted. There are other cutting methods besides a hot wire to look into if the smell is this much a problem.
If building a one off hull, sheet foam bent over a temporary mold will be the quick way for a foam core hull.

 

I've seen foams as light as 1/2" pound (per cubic foot) to over 20 pounds. I know there are high densities, though they may not be common in the industry.

The large block concept has some merit, though you'll be wasting quite a bit of foam when all is said a done and using heavier then necessary densities in some areas. The sheet goods foam as mentioned above, is a better allocation of materials, but you could carve a core from just a big block of foam.

You'll want to use closed cell and you could glue thinner/smaller sheets/blocks together to get the size you need.

Cut the foam with fine toothed wood working tools. I like hack saw and reciprocating saw blades, but conventional hand saws work well to eliminate using a hot knife cutting method.

Of course you'll have to engineer the sandwich laminates for the loads expected, which isn't an easy task. If this is for your planned, stepped 40' walk around, then you should very strongly consider having the laminate schedule and structure engineered professionally. Considering the costs associated with such a project, the fee for having a structure strong enough to accept the loads you plan on imposing, will be a minor consideration.

In other words, foam cored, sandwich construction is a highly engineered structure. Have you completed your Westlawn studies yet?

 

Hi guys,
Thank you for that advice, believe it or not I saw the method I described used in the build of an rc airplane. That just got me thinking of alternate ways of building boats. Par, you're right, the 40' walk around was on my mind when I though of it. I'm was thinking that instead of just cutting out the hull, maybe it along with stringers and top decking could be cut out as one piece. Complicated but with some time, the right laminate schedule and engineering assistance, as you suggest, something like that could possibly work. I actually haven't been able to start the course (read that as my wife and I had a baby) but I'm looking at getting back to it in the new year.

Hunter, I thank you for saving me some research, I just knew that I had seen something about structural foam.

 

It's a lot easier to glue pieces of foam together (or tie or tape, etc.) then to find a big block and cut everything away that doesn't look like a boat part.

You can make a set of temporary station molds and lay a common thickness of foam over it in strips. These can be literally tied, taped or strapped with thread or wire ties to the stations molds, It doesn't have to be especially strong, just hold each strip in place until the outer skin is applied. Once it's 'glassed, the attachment to the station molds isn't relevant any more. Then if you want some stringers, just glue in some lengths of foam the size you need. The same is true of other elements of the build.

Now, this all sounds fairly easy, but it's not. Cutting and attaching the foam is real easy, but that's on a fraction of the work you need, just for a bare hull. The bulk of the effort will be applying the fabrics in the thicknesses necessary to meet the schedule desired for the build. It would be nice if you could just apply a couple of layers of cloth over everything and call it good to go, but this isn't the case. Different areas of the hull will need more 'glass then other areas and some places will need to be a solid laminate, while others can be foam cored, while yet other places may need additional reinforcement. Knowing what these are and where to place them, is the engineering part of the equation and why it's necessary to buy a set of plans with the laminate schedule already calculated.

So, yes your idea of a foam cored build is very possible and commonly done, but these types of structures are highly engineered. You can convert some designs, intended for different build methods to foam core, but again, it's a conversion best left to someone familiar with the issues.

 

Heres a guy using low density foam for boat building over a shaped structure

http://www.bourneboats.com.au/

 

You must know that Boston Whalers have been built with structural foam cores for decades.
In the case of a Whaler- inner and outer shells are layed up in conventional molds and then the foam core is injected.

Here is some interesting info provided with the original patent:
http://images.google.com/imgres?img...&sa=N&start=90&um=1&ei=dGj1SqvSD8vV8AbA4IjzCQ

Here is their bio from the Whaler web site:

"Most boaters are familiar with the fact that you can put a hole in a Boston Whaler hull or cut it in half and it will still float. We've even piled 20 people into one of our 19-footers to demonstrate its remarkable buoyancy. The unsinkability of our boats afforded by our exclusive Unibond construction process is legendary. But the foam provides more than just flotation. Sandwiched between two walls of fiberglass, our high-density, non-absorbent foam provides structural support and sound and vibration absorption in addition to unsurpassed flotation. This means you have a solid hull, not hollow like most other boats. You can literally hear the difference. Pound your fist on a Whaler hull and then on the hull of another brand. If the hull sounds like a bass drum and flexes, imagine what it and you will be subjected to in rough water.

You can literally park a 32-ton dump truck on a Whaler without crushing it that's how strong these boats are. We have a wall of fiberglass on the exterior, dense structural foam in the middle and an additional fiberglass wall inside. We have so much confidence in this construction we offer a ten-year transferable limited hull warranty. That means that when you sell your Whaler to move into a newer or larger model, you can offer the new owner the remaining time on the warranty."

Just shooting from the hip I assume you are trying to avoid the mold work in conventional glass construction?
You may have a broader choice of shape by milling/hot wire cutting the core, but will face having to fair the skins, deal with shape integrity, and have a fussy bonding issue with all the sections cut as well as material waste.

I have used wire cut cores in the past.
CAD driven hot wire cutting can yield amazing accuracy.
I used to vacuum bag rc sailplanes from hot wire cut cores. The wing foils were cut directly from the software. The foam used is Dow 'Hi-Load 60". I used Victor Lazlow to cut the cores and would recommend a talk with him. He will talk your ear off & really knows the business- a great guy. He advertises .005" part integrity.
His web site:
http://www.compufoamcore.com/

The approach is so simple, cheap and accurate that I would recommend a look see to anyone who builds stuff one off. It might serve better for foils and the like than whole boats. In the higher spec hobby world- CNC milled foam is used for the ease of developing compound shapes.

Here is some info on the Dow foams:

Property and Test Method Value
Highload 40 Highload 60 Highload 100
Thermal Resistance(1), per inch, ASTM C518, C177,
@ 75°F mean temp., ft2•h•°F/Btu, R-value, min. 5.0 5.0 5.0
Compressive Strength(2), ASTM D1621, psi, min. 40 60 100
Water Absorption, ASTM C272, % by volume, max. (24hr water immersion) 0.1 0.1 0.1
Water Vapor Permeance(3), ASTM E96, perms 0.8 0.8 0.8
Maximum Use Temperature, °F 165 165 165
Coefficient of Linear Thermal Expansion, ASTM D696, in/in•°F 3.5 x 10-5 3.5 x 10-5 3.5 x 10-5
Flexural Strength, ASTM C203, psi, min. 60 75 100
Complies with ASTM C578, Type VI VII V

Screwed up chart info but go here to see look into the product:
http://www.dow.com/PublishedLiterat...foam/pdfs/noreg/179-02548.pdf&fromPage=GetDoc

From an engineering standpoint, the core and the skins work together to provide the panel strength. The thicker you make the hull shell, the simpler the load case on both core and skin. Local high loads will crush the core so will have to be dealt with in build. Separation of laminate from core will have to be considered and designed for.

There was that crazy Russian guy some years back- designed the thick walled foam/skin unsinkable sailboats & sailed around the world a few times...?
Who am I thinking of??

 

Great to get some product information - it sounds like a solid product.

But ,while I enjoy learning about the quality of the Boston Whaler, and am a fan of foam construction, I think your enthusiastic treatise contains some potentially confusing and innacurate statements.

Great idea, but very labour intensive for the average one off builder, and obviously reduces the interior size. Neither are the moulds "conventional" in the sense that they have to be a very close mirror image of each other, built with close tolerances. They would have to be very robust moulds as well to prevent having the fibreglass deformed by the expanding foam. This is definitely not a job for the "average" boat builder.

This is no different to the foam core technique used for a huge percentage of commercial boats.

I bet you cant name any boat with a hollow hull ! "hollow like most other boats" is a gross innacuracy.

Acoustic "testing" has no relevance to the hull strength. For example, a 40ft 3mm steel hull will echo like a barn, but will cut right through a Boston Whaler at 10 knots in the dark - where you will need all the flotation you can get.

I would bet WHOLE BOAT BUILDING in CNC is neither simple nor cheap! Getting the CAD work done is the first complex item - and that wont be cheap. Getting the milling done wont be cheap either. That web site only advertises small sections.

Do you have any cost examples to support your statement ?



--------------------------------------------------
But aside from these observations, I think that CNC milling and foam in general is a really useful technology, as you have pointed out.

The DOW foam seems to be very similar in properties to the other types of foam coring used for boat construction.


????
One question that intrigues me, does the technique of having the the foam expanded between two skins make it so strong? Would expanding it in a non-compressed environment (like at Bourne Boats, where he also uses a Dow Foam) have any effect on its physical properties ?

 

pull out your electric meat carver and get to shap'n. Fair with grinder with a scotchbright head. Instant artist.

 

Damn, you beat me to it. Whaler hasn't been using "structural foam for decades" nor has it been especially successful for them. They've eaten a lot of boats and had assembly line failures at an alarming rate. In short, their method nearly killed the company. For over a decade you couldn't give away certain models and to this day, if you happen to be unfortunate enough to own one of these models, you're screwed, because the cores have or will delaminate. The foam initially used was purely a buoyancy element and this was their marketing thrust. The hull and deck cap were thick and self supporting. Much later came true cored construction, where the core was factored into the structure's strength.

They've sorted out many of their issue now, but thousands of junk boats are still on the market. Their original foam sucked up moisture like a sponge and production short cuts caused wholesale leaking, which saturated these cores. As I mentioned, they've revised their production practices (many times) to iron out the problems, but honestly, buying into their advertising hype hook and sinker is foolish without some history lessons and back ground.

This isn't to say that foam core should be used, but it is to say that there can be problems if procedures aren't followed or if you don't understand the concept.

 

Pay attention to the " " when I post the information from a builder- don't quote builder statements as my words.
Boston Whaler is a example in the industry that is following the concept outlined by the poster- I would say nothing farther than it is a ideal that has been pursued in the industry and the guy should be aware of it if he is not already.

You actually think I was suggesting that the poster emulate BW when he is not (I presume) even willing to build a hull mold?
Yes- you are confused.

The molds are "conventional" in the sense of being required to the same state of finish and accompanying expense- the substance of which the original poster seems to be wishing to avoid.
As to "enthusiastic treatise"- I provided information and conditional endorsement. The enthusiasm was provided by the builders statement- set out clearly as a quote BTW...

I only post about the Boston Whalers because to my mind the first question when discussing a build technique is to ask if the product of the technique is desirable. Boston Whalers serve as an example of the type of vessel the posters construction approach might provide. Given consideration of these vessels attributes and problems, the next series of questions can be asked:
Is the build procedure suggested appropriate, possible, better than other approaches, etc..

And no I don't think I would build a boat with wire cut cores due to the problems I outlined in my post. But I can clearly see the potential of hot wire cutting foam forms.
I do believe the technique is great ideal for one off structures though and I have used it as I outlined.
It is accurate, cheap, and fast.
By one offs - I refer to parts, not boats.

To pursue the ideal- I would call the company I used....

Par- Boston Whaler patented the technique in the 50's and built their first boat in 1958- this qualifies as "decades". This patent clearly states that the objective of the foam core is as a structural component of the design. The company exists because of the hull construction- not in spite of it. Emphasizing "unsinkable boats" was marketing genius on the part of Dick Fisher. It does not prove that there were not other essential functions provided by the core in the Whaler boats. A core which he also stated was key to the products strength. As to the failures- the original poster should be aware of the limitations I posted as well as yours.

Yes they have- that's why I provided them as an example related to the build concept outlined by the original poster

Did I make that statement?!
No!

Carry on..

 

I'm not going to get into a debate about the evolution of Whaler's production processes, but any simple structure evaluation of their early 60's compared to 70's, 80's and 90's easily indicate the foam wasn't being used in the fashion that is is today, nor was it structural. In spite of well practiced legal language in their patent applications, their laminate thicknesses and the core bonds plainly showed the foam was irreverent until the early 80's and core sheer not addressed until new productions procedures implemented to address their failing reputation in the mid 80's.

It also wasn't a structural consideration as much as it was a cost cutting one, after the "dark years" in an effort to trim the production effort and boost the bottom line. Every manufacture went through this process and all sorts of things were tried, so it's not indigenous to Whaler.

 

Interesting...
That is the first time I have heard this. So we can totally remove the foam core from a vintage Whaler and have the same boat from a structural perspective.... I doubt it.

The foam core in Boston Whalers is structural. It has been since the Whalers were introduced in 1958. If you will only except a structural role since the "early 80's", that's fine but it still leaves "decades" of production of a Whaler with a structural core.

So yes I agree, there is no longer a point to debate.