RowCruiser in foam core sandwich instead of plywood?

  • Just curious, has anyone ever built a RowCruiser using foam core sandwich method?  This would be a core of dense PVC foam (like Divinycell or similar) sheathed on both sides with woven roving or biaxial glass and epoxy.  Has that approach ever been discussed, and any potential downsides other than the cost?

    EDIT - Update 6.16.2023:
    After a bit of research and consulting a foam core expert, the feedback is that it's almost certainly not worth it to translate the RC/RCS into foam core.  Short version:

    • If the boat is already specified in 4mm Okoume; then gains will be very hard to come by (only 2 sheets of the RC is 6mm, everything else is 4mm)
    • Foam boats typically have a practical cutoff around 18-20 feet; below which point not a lot is saved.

    Longer/complete version of the feedback in the comment thread below.

  • That's a great question, and one that I receive a lot.  As far as I know, no one has yet built a RowCruiser using foam coring.  I'd love to hear from anyone who has built a boat of any style this way, and what their experiences are.  Here are the first questions that jump to my mind:

    1) Would it, indeed, be stronger for weight?  Using foam cored construction ends up being the lightest way to produce a hull with standard molded construction and vaccum infusion, but would you actually end up with a lighter  boat if you did it with stitch and glue construction?  The thing to remember is that okoume plywood sheathed with fiber glass on both sides is a form of cored construction, and is remarkably light.  It is lighter and stiffer than non-cored molded equivlaents.  It would be really interesting if someone did a weight-to-strength test to compare the two materials - a piece of foam with hand laid glass on both sides, and a piece of okoume wood with glass on both sides.

    2) It is the okoume plywood that creates the shape of the boat when it is all wired together. It is the rigidity and bending properties that allow the precise shape of the hull to form. Would foam have enough strength  and stiffness not to bend, sag, and be able to hold its shape until the fiberglass is applied?

    3) Some of the hull pieces really need to be cinched together tightly to close the gaps.  Would the wires simply pull through the foam?

    4) Would the outer sheathing puncture more easily than with a wood core, since the foam would provide a less rigid backing than plywood?

    As with you, Eric, I'd love to hear from anyone that has experience doing this, so if anyone has anything to share, please do.  I've had more than a dozen people asking me about this, however, judging by the lack of information on it on the internet, I'm wondering if there may be some challenges that keep the concept from being popular.  My biggest concern is if you'd really get a stronger boat for weight.  There's a big difference between vaccum bag infused cutting edge composite processes compared to hand laying glass over the foam.

  • >Would foam have enough strength  and stiffness not to bend, sag, and be able to hold its shape until the fiberglass is applied?

    I think it would not hold its shape.  Based on some limited studying of how other foam core boats are built, you have to build a sort of jig/plug with narrowly spaced slats to support the foam.  You can:

    • build a positive jig, foam around it, and glass and bag what will become the bottom.  Then, when the glass sets up, remove it, epoxy/fillet in the bulkheads (cut from pre-laid up flat pieces of foam sandwich), and then glass &bag the inside


    • do the opposite.  Build a negative jig, foam inside it, glass and bag what will become the inside, then when the glass sets up, remove it from the jig, epoxy/fillet in the bulkheads, then glass &bag  the outside.


    I agree, @Colin Angus , the ecosystem of foam core boat construction (and the requiste DIY vacuum bagging needed to make it worth while) is a bit opaque at the moment.  It can't be that hard, but I think there are some fine details that make all the difference, like wieght and orientation of glass layers, etc.  Black Magic / Trade Secrets.  I'm going to try to find an expert and run it by them.  I'll report back...

  • It sounds like you've put a lot of thought into how it would all be done.  That all makes sense, and in theory I think you could make a decent boat going this route.  You're so correct with regards to the "Black/Magic/Trade Secrets"  - having spent time doing more advanced composites with Open Ocean Robotics, it makes you realize how many tricks and processes have been developed within the industry that are kept hush - that even educational institutions aren't aware of.  I think a foam cored Rowcruiser could be done well, but it would need to be done by someone who understands the processes very well, and has an eye for detail.

    And then, of course, there's always the option of simply molding it - create the overall hull shape with cheap ply, coat it with primer, sand and polish it, and then use it as a plug to create a female mold.  Or just use it as a male mold and sand/paint the exterior.

  • Relevant, great article:

    What's interesting/compelling is (for 3/8″ X 2′ X 4′ flat panels as an example), epoxied Okoume is 8.5 lbs (only 1lb lighter than standard marine ply, which is suprising, right?) and, that's not glassed, just epoxied!  So, glassed and epoxied Okoume would be a bit heavier than the 8.5 lbs.

    foam core with 2 layers of glass/epoxy of the same dimensions comes in at 3.85 lbs. So that's (potentially) a 55% weight reduction! (even more, after glassing and epoxying the Okoume in an RCS build)

    Not exactly a scientific comparison, since many sections (bulkheads/transom for sure) will require more than 2 layers of glass and the associated resin, probably in very specific weave/strand orientations relative to the loads.  But still the potential is there for sure.

  • Here's the complete answer I got from someone more familiar with the tradeoffs between glassed ply and foam core construction.

    (FWIW: the references to R2AK below are inferred from his own googling of the RowCrusier.  I don't have any plans to participate in the R2AK in the near future)


    Well, it would be pretty complicated, Eric,

    The weight gains would be incremental and only if you drove some cost and did it in infused or wet bagged carbon. The issue is to make a puncture resistant foam; you need to add enough glass and glass is heavy. The way to reduce the weight of resin in glass is via vacuum.

    Assuming the boat is specified in 6mm plywood with 6 oz skins now; you could probably make the thing in 4mm okume plywood with 5oz carbon skins and vac bag it. Weight 4'x8' panel is 12# plus 7.1yds time 5oz or 35.5 ounce skins and call it 20 oz resins for 55.5 oz sheatbing or say 3.5 pounds for 15.5 pounds. And you have a flimsier boat, but saved 6 pounds or more against assumed specs of 6mm ply and some heavier glass skins.

    12mm 4# foam is 5 pounds per sheet which allows you about 10 pounds for glass and resins. 12 oz glass at 7.1 yds is 85.2 oz plus say 50 oz resins is 135 ounces or 25 ounces under the 10 pound constraint. So you make it but the savings is under 2 pounds. So then you get into using carbon skins on foam to save weight. And things get very expensive ito time and money to save a few pounds and make a boat that is not as R2 Alaska capable, perhaps. Lotsa assumptions here..but the issue is incremental gains for some added risks and a sort of experiment to build a tortured panel foam panel that is stiff enough for the ?jig while saving a couple pounds per 4x8 sheet.

    Then as pointed out above; the hardpoints and beams all need to be made in something stronger with improved attachments vs all of a sudden; these incremental gains start to get wiped out by the akas and attachments for masts, etc. And all of that stuff needs to be embedded in the foam core before skins are applied; generally. And, because you are in a sort of risk mode; you also need to take risks with bonding. Bonding, or tying foam panels together requires more glass tapes than a stiffer ply panel; generally, as well. This adds back some weight, and can also wipe out gains.

    In a small boat; do you want to make it weaker to save 10 pounds net?

    I spent more time drafting this contribution than I care to, but hopefully you understand; this is the business of hair splitting and taking risks against safety margins; not simply following Gerr or others guidance for laminates which would be too heavy for a return of effort.

    For this reason, foam boats typically have a practical cutoff around 18-20 feet; below which point not a lot is saved. Now, that said, I worked on a foam dinghy design with some others here that is in foam and we got the weights down, but the thing is not made for hard use either..but light enough to be pulled up onto a catamaran by the bow...but one not built yet... Every ounce gets considered.

    So this type of thing can be done, but it becomes a lot of effort for small margins of weight savings and usually some tradeoffs in safety.

    If you are serious about such an endeavor; you need to understand the amount of square footage of hull and deck in the boat, the original layup plans and see whether there is any room for gains. If the boat is already specified in 4mm okume; then gains will be very hard to come by..

    Should add, changing things like the crossbeams to carbon can be done, but the risk reward is not great for a typical user.. maybe if you wanted to do the R2AK

  • That all makes sense.  The beauty of stitch and glue is it is a relatively well-trodden road, and it does make for a light and strong boat.  I think to make a foam cored boat lighter (and equally strong) as stitch and glue, you'll need to be pretty capable in the realm of composite construction, and the weight savings will be marginal despite a lot of extra work and thinking.

    Your calculations above stating the potential 55% weight savings isn't entirely correct.  What's missing is that the okoume plywood is also contributing an immense amount of strength and puncture resistance, while the foam adds very little.  I wouldn't be surprised if the piece of glassed okoume was twice as strong as the glassed foam - in particular if it was just glassed on one side.

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