Ultralight Downwind SUP Board Design

Objective to make a Sub 10 lbs Downwind SUP Foil board!

Final Weight 9 lbs 12 oz or 4.4 KG: 7’ 9" x 21" 6" @ 128 Liters

My design was constrained as follows:

• XPS Waterproof foam with Density 1.5 lbs per foot cubed.

• Board designs are changing so fast you might pay $1,500 or more and get a board that is obsolete in 6 months

• Figure out how to minimize the investment in ‘Unnecessary’ infrastructure. This includes:

  1. Material 2) Cost and 3) Weight

• The board does NOT have to last 10 years

• 100% Vacuum Bagged at every Step: Full 20+ in HG. The XPS foam that can handle 25 psi, eg. full vacuum pressure, resulting in better and tighter final laminations.

• Invest in the Foot area and the TRACKS. BUT Don’t over index on the TRACK reinforcement. I have been a Tuttle advocate since Day 1. For 30+ years Tuttles were the only boxes that wouldn’t rip out of windsurfers. Tuttles go Deck-to-Deck. There has to be some substantial material extending from Deck to Deck. BUT there could be a very light Matrix of foam and carbon rod, with solid top and bottom carbon layer for example that would suitably hold the load for dual tracks.

Additional thoughts: XPS is closed cell foam. It will NOT absorb water. It weighs 1.5 lbs per cubic foot. EPS is lighter but when you get a ding it will absorb water quickly.

There have been several prototype boards in the neighborhood made with XPS. One board did NOT even cover all the XPS but left exposed foam in Non-Critical areas. After 2 years the foam, with little care, started to flake off from chips and/or dings.

I have been testing another approach that would be to GORILLA glue the DRY fiber to the XPS. The objective would be to provide a lightweight jacket to the foam. It would carry load in TENSION only, leaving the fiber attached to the foam but NOT fully wetted out.

IT IS POSSIBLE THAT GORILLA GLUE IS SUPERIOR TO EPOXY AT BOTH ADHERING TO FOAM SUBSTRATES AND HOLDING FIBER IN PLACE FOR TENSILE LOADS. The only hesitation is Gorilla Glue’s ability to handle compress ive loads as part of a Composite Matrix.­­­


  1. Ding Prevention. The exposed XPS dings with a fingernail. A test piece of foam with carbon was very durable. Another test piece with 3.7 oz S-Glass fiber worked very well and was resistant to puncture, even good with a hammer test, but lighter than the carbon sample.

  2. Compressive loads would have to be carried by the XPS foam. But the shell of fiber would still be stiffer and more resilient than foam alone

  3. The skin material is porous. Some people have experienced delamination related to XPS foam. Because the foam is closed cell ANY moisture that is trapped between the foam and an impermeable skin is at risk of evaporation and delamination. IF the fiber is porous this would significantly reduce the risk of delamination. PS: After building I suspect many delaminations were from either A) Overheating in the Sun or B) NOT using a vacuum to laminate the Board


UPDATE: I have been using this board for 2 weeks now. This has included 15 sessions:

• 2 Downwind Sessions. Each 2.5 miles. Both Bay sessions, Coyote Point to 3rd Avenue San Francisco Bay: First session was blowing 35 gusting to 50, second was blowing 40 gusting to 60 mph. Not mellow.
• 6 Winging Sessions. One started out light, I was on an 8 Meter CWC, then the wind picked up to 15-20 mph. Other Sessions have all been on a 5.5 meter wing. I am realizing that this board may NOT need anything bigger than a 5.5 meter (all on Axis 1300 except one time with Axis 1099). Several sessions in the ocean inside Mavericks. A few where I was worried if something went wrong everything would break. Waves from 5 to 12 feet. Tried to avoid whitewater.
• 7 SUP Foil sessions. I have been committed to this board with the Axis 1300 with 450P tail. I have been taking this out in BIGGER conditions than I would like. Examples 10 feet @ 13 seconds and I would just go out and try to catch and fly on smaller, 5 to 7 foot faces. Winter in Northern California IS abusive.

The board has held up despite relatively extreme open ocean and hurricane wind conditions: I have 2 tiny pinpricks in the 1.4 ounce glass in the tail. The rails, forward of where I stand also started getting abused from my legs when I would paddle sitting down. I added a 10” x 2” carbon tape extension. I keep the board in a padded bag, like the old days of windsurfing. The greatest risk to the board is from travelling in cars, on bicycles and on the beach. Otherwise, the thin protective shell is working well. No weight gained, and no delaminations thus far.

Learning to SUP Foil a 21” wide board is another topic. This has taken a great deal of patience, and stretching my lower back. I am just starting to be able to stand semi-comfortably mostly-square stance. I will then wait for a wave and paddle square until the last second then get into ‘surf’ stance. However the conditions have NOT been clean EVER. The ocean chop is arguably harder to stand in than the Bay when it was blowing 50 mph. I am looking forward to some smaller days to practice.


This is a great project and I enjoyed reading your thread on standupzone.

What are your thoughts on vertical rails, as you appear to have here, in terms of stability?

The ‘dead vertical’ rails seem to be helping for stability. I have another downwind shape that is 25" wide at the top but slopes to 17" wide at the base. This board is 21" wide at the top and 21" wide at the bottom and appears to be as stable but faster through the water.
What I have found with making a board is that the “IF” statements add indefinitely to the design and ultimately the weight. IFI want to turn . . IFI get hit by shorebreak . . .IF I put it down on a pebble . . . IF I want more stability . . . IF I want to flatwater start . . . .
This board now has 3 sister boards. Made with the exact same template. 2 were built with EPS, and now one with XPS. They all have chines, mine does not. I left a square rail. The 1 problem with the square rail, and light fiber, is that any bump concentrates on a right angle. The next board I will build would have slightly more rounded edges. Somewhere between a pencil and a ‘sharpie’ pen rounded ~ 3/8" diameter.
PS: One pro-level winger stated that the square edges would catch on touchdown and “track uncontrollably.” But what we are finding with these shapes is that touchdowns almost don’t matter. The board touches, and its nearly unnoticeable, they just keep moving. No skip, no drag. The Gong guy described it as indiscernible between glide on the water and flight.

1 Like

Love it ! @Beasho
Lets see if it works in Rhode Island!

Thanks very much. Very interesting.