I first built foam sandwich boats in the late 1970s when working for Derek Kelsall who was the pioneer of the method. Many of the foam boats home built in the 1960's are still sailing.

Sandwich construction is also used in race boats and fighter aircraft and there is a desire to impress the public with how cutting edge and high tech it is. Which it can be, but doesn't have to be, when building a cruising boat.

For example, vacuum bagging only became widespread in the late 1980's and infusion about 5 years ago. Yet, as I say, many 40 year old foam sandwich boats are still structurally strong and still sailing

The way foam sandwich boats were built in the 1960's was to make a "male mould", essentially a stringer/frame hull with the "planking" being the foam sheets. Those sheets were temporarily sewn or screwed to the mould and then the outer glass layer laminated. After curing the hull was removed from the mould turned over and the inside laminated. This is how the round bilge home built Flica, Nimbus etc designs were built.

There were two problems with that approach. First the outer surface was naturally very rough so it needed a lot of filling and sanding. Second, the hull was very delicate and easily distorted when right side up prior to laminating the inner skin. 

So the next stage was to make a complete "planked" hull. Usually using MDF strips covered with packing tape or cling film and then release agent. The inner skin was laminated first. Then the foam applied and finally the outer layer. That solved the floppy hull problem, but still meant a lot of filling and sanding to get a smooth outer layer.

It is easy to glass onto foam, but hard to make foam stick to glass. Initially people tried sandbags and weights to hold the foam in place, but that never worked well as it was impossible to get an even, heavy load. So  vacuum pumps were used, (often large vacuum cleaners used by chimney sweeps or even cow milking machines). In essence a large bag is placed over the whole hull and the air sucked out. 14lbs/sqin air pressure is a lot of weight, easily applied evenly across the hull. This method was actually first used on "hot moulded" parts for the plywood Mosquito aircraft used in WW2, and then, in peacetime, to build the Fairey dinghies and keelboats.

But it took several years for the details to get worked out. In particular ensuring that the air was sucked out of all the underside of the bag. Initially this was done using plastic garden fencing as it made lots of air channels. Then suppliers developed special bleed cloths and plastic bags. Which all end up being scrapped of course. Poor taping all round to seal the edges also caused problems as air leaks destroy the vacuum. So more developments there.

To avoid the filling/sanding and vacuum bag problems people started using a flat table to make panels, although obviously only on hard chine single curvature hulls and decks. But it meant that gelcoat could be applied to the table, then skin mat, then outer glass laminate then foam, then inner laminate. The result, a finished panel with a smooth gelcoat finish.

But there was still the problem of joining two adjacent panels. Both sides of a joint need to be glassed, so that meant that the outside joints were rough and proud of the hull surface. So filling and sanding was still needed, albeit in a much reduced amount. This is used on the foam sandwich Windsong, Mira etc.

So the next stage was to make rebates round the edge of the panel on the flat table (usually using thin hardboard taped in place). This meant that the external joints would be flush so there was no bump in the hull. But it did mean the hull panels would need to be drawn reasonably accurately so the rebates were in the right place. This is the system used on the foam sandwich Gypsy etc.

That method works well, except that the joints add to weight and build time. So the final stage in development was to make temporary half female moulds using melamine faced MDF. The result is essentially the same as a production boat, except only one or two releases can be pulled from the temporary mould rather than the 100+ from a true production mould. This is the system used on the foam sandwich Skoota powercats.

I still prefer to use the low tech foam sandwich methods from the 60-80's. Which is similar to the way the production Flicas and Banshees were built, over 30 years ago, and yet are still structurally sound. In 2019 we were on a Flica 37 in New Zealand that was launched in Cornwall in 1989. It felt really solid, no creaking or movement anywhere.

Instead of vacuum bags and infusion what I use is scored or "contour" foam, which in use is a bit like end-grain balsa without the rot or weight problems. Basically a sheet of normal rigid PVC foam precut into say 25mm squares with a scrim backing holding it all together. It is fitted onto the cured skin using a bonding paste. This is a tough, thus slightly flexible, lightweight thickened resin. In use it is maybe a bit like applying runny honey or icing a cake. 

In practice put a resin drum on its side, thus forming a curved surface. Lay a sheet of polythene on it. Then put on a sheet of contour foam (normally about 1sqm, 1.2m x 0.8m, approx) scrim side down. The curve forces the squares to open up (a bit like cutting a mango or avocado). Then scrape on the bonding paste. Turn the sheet 90degrees and repeat, so the bonding paste goes into all the cracks as well as over all the surface.

Then apply to the main laminate pushing it down by hand, or even standing on it and shuffling to and fro (plastic bags on shoes of course!). The bonding paste is normally coloured (eg red) and the foam is green or blue. So it is easy to see the paste oozing through the cracks. Not only do you then know that the bond is complete with no air gaps, but the paste goes through the foam and thus helps bond the two glass layers together. The paste normally cures in about 30 minutes.

Once cured a light sand to remove any excess paste and you can lay up the final inner surface. That can be several days later, as not even a production boat is built in a day, it always takes several days, at least, so there are no worries about bonding onto a cured laminate. 

I agree that using a vacuum bag will reduce weight and resin use, and that infusion is less messy. But the weight saving is minimal (maybe 10% less resin), as the glass quantity remains the same, you use the same weight rig, engines, galley cooker etc. So the overall weight difference is irrelevant for most cruisers.

Lots of professional builders have had issues with infusion, and it can be very expensive when things go wrong, as you normally have to scrap the whole moulding and start again. It is also complicated and time consuming to set up, so most builders don't use infusion on small parts, just the main hull mouldings.

Both vacuum bagging and infusion have a lot of waste, bags, hoses etc, never mind the vacuum pump in the first place, so what you save in resin you spend in disposables.