SUP with sail - solved!

This is a reliable and repeatable method for paddling with the sail. I've paddled 10-20km distances like this, have used it to get back to land, under bridges etc...

The challenge is to prevent the sail dragging in the water, whilst keeping it rigged so that the transitions from sailing-to-paddling and paddling-to-sailing are quick and easy.

This offers two big advantages:

1. In a tolerably flat sea, cutting corners and being a long way from land when the breeze is uncertain is no longer such a cause for concern. If the breeze fails land can still be reached.
2. In a tolerably flat sea, progress can still be made on calm days.

A working solution

The boom rests on two foam blocks attached to either side of the board. I cut foam 'bricks' from a kids body board and glued them two or three high with bits of carbon batten tube inside as stiffeners. Loops of rope go round the batten tube as attachment points. The underside of the lower bricks are shaped to match the deck of the board.

I drilled the wings of the board to help with attachment points. The barrel carrier bar and footstrap inserts are also used. There would certainly be other and better ways to build the bridge, but this works, is very light weight, and can easily be removed. The bridge just needs to be firmly held in position.

Now allow the sail to fall back onto the bridge and...it will jiggle off balance pretty soon and the sail will drag in the water. A stabilization method is required too.

I found a single central tie down to be insufficiently stable but have excellent results with a dual tie system. On the clew side a line is attached to the boom behind the harness line, routed so as to pull down, and then led forward where it is easy to secure. On the luff side a short line from the front footstrap loops around the adjustable downhaul lines and is then led forward to where it is easier to secure.

Find a good system and the rig will now be very stable. So stable that if the wind does pick up and catch the sail it will attempt to capsize the board, so make sure the tie-down system is easy to quick-release even when under tension!

Performance

Normal (sustainable) paddle speed 2 - 2.5 knots. 3 knots when being filmed! In a very light headwind, especially against a current, it makes more sense to paddle than sail. Downwind if there is any wind then sailing offers a better return on effort. In zero wind the paddle wins in all directions.

This system works well in flat water and can cope well with a bit of ground-swell (longer, smoother waves). With chop it is less efficient. With a genuinely choppy or rough sea it isn't really viable.

Anything that drags in the water needs to be removed for normal sailing - the two elastics used in my system really drag at the board and slow it down once there is decent breeze.

Float method (rejected)

I also experimented with inflatable floats attached to the boom, forming a kind of trimaran. Stability was excellent but the floats created too much drag and made paddling very inefficient.

Big thank you to Avre Johansen for the filming :)