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Foam Construction Update

By VictoriaPDRClub1 #901 "Victoria2" (3ar)

Hi, Shorty,

Just wanted to give you an update on Victoria1 and Victoria2. Victoria, BC has now had its first PDRacer class race at our local Elk Lake, with Victoria2 winning over Victoria1 by wide margin - although I would mostly credit that to sailing expertise rather than the differences in construction and rigging.

I have confirmed that Victoria2 is within class measurements - I had been a little concerned because of the thickness of the foam. I like to work in metric, and EPS foam often comes in metric, so I'd like to relate some metric measurements for anyone else interested in this construction technique. I did extensive testing of different thicknesses of foam to find out what I needed in terms of strength. I believe I-beam and sandwich construction usually takes strength to increase with the cube of the thickness. To take my 220lb weight across 12" centres, it took 3" or 75mm to provide sufficient compressive strength. So Victoria2 is made entirely of 75mm normal white 'beady' polystyrene foam.

However, if you cut the *sides* only according to the metric hull definition, the 75mm thickness changes the location of the hull-to-bow-angle chine towards the stern, and you're no longer class-legal. So you must cut the sides to account for it, in advance.

That chine point, according to the metric diagram, is 152mm up and 2287mm from the stern. As I prepared to run a spline between the stations drawing on the sides, I moved that point forward to 2310mm, and added another station at 162mm up and 2350mm from the stern. I drew the rest of the spline as usual, stopping at my new 162mm at 2350mm point, where I commenced the bow angle. For Victoria2, I rose to 312mm at a total length of 2425mm.

I then glued the 2350mmx1200mmx75mm bottom onto the two sides (it took a huge amount of weight - perhaps 200lb - to bend the foam into shape while it set) using a mix of portland cement and PVA glue - a slow-setting filletting-filler epoxy would have been better, although significantly more expensive. The bow and transom fit between the sides, each from a 1050mmx235mmx75mm 'blank' that I shaped to match into the bottom.

Then I used the bow as a flat surface to guide cutting the bow angle through the excess foam back down to the bottom. Same at the stern. And, blissfully, the resulting shape was within tolerances.

To sum up the measurements:
Victoria2, all from two standard sheets of 2430x1215x75mm polystyrene foam.

Length 2425mm
Height 387mm
Width 1200mm

Bow from 1050x235mm
Stern from 1050x235mm
Bottom from 2350x1200mm
Sides 2425x325mm

Metric bottom profile spline-drawn onto sides as usual, except for moving the bow chine station forward plus adding an additional station:
Modified bow chine station: 152mm up, 2310mm from stern
New bow chine station: 162mm up, 2350mm from stern
and commencing the bow angle at that additional station.

All foam is glued together using a wet paste comprised of PVA glue and Ordinary Portland Cement. All outer surfaces are glassed in 4oz fibreglass cloth, using TitebondIII waterproof PVA as the resin. The interior is just bare foam. Unrigged, I estimate her weight at 6kg. Hull materials cost a bit less than $250: $140 in foam, $40 in TitebondIII, $40 in glass (5 yards at 60" width, if I recall), $10 in Portland, plus sundries. The build took about 8 hours' work, but I gave the cement a week to dry under weights, between gluing the monocoque and glassing it all.

Victoria2, like Victoria1, is currently rigged using a windsurfer rig. Both Victoria1 and Victoria2 use 4 skegs at the 4 corners for lateral resistance and tracking - although Victoria1's are cedar, while Victoria2's are steel plate. The windsurfer rig needs no rudder, since you use the sail to steer. Currently, the mast feet are mounted at the exact bottom centre of the hulls, but I'm going to move that ~20cm for'd to give less 'weather helm' and get the mast more vertical while close-hauled.

Victoria2 has about 6 hours of sailing in her so far, but is starting to ship a little water in at the very bottom of the joint between the sides and the hull - I think I might chase the inside joints with some angled foam chines and fibreglass to give the joints more strength and reduce flex. The fibreglass skin is eggshell-thin and can ding very easily - so I don't run her onto the beach. I also have to be careful on the beach if the wind is >10kts, because she can blow away, being so light - I have to leave her upside-down and bow-to-wind.

Cheers,
Eric.