You probably know that the induced drag on any foil (water or air) is proportional to the aspect ratio (actually, in some cases this is a logarithmic relationship-i.e., you get big changes in induced drag for small changes in aspect ratio).
Keels, and to a lesser extent rudders, can be end-plated by the hull. This is why a rudder suddenly quits working on some boats with heel-as the end plate of the hull, sealed by the sea water, is lost past a certain point of heel.
Sails are subject to the same rules. And the lower the aspect ratio on the rig, and the less efficient the keel and rudder, the more critical is the effective aspect ratio of the rig.
Triangular sails, i.e., those with pointy tops and minimum roach, are extremely inefficient in this regard. That’s why the big roachy sails we’ve been using for so many years are so powerful. That elliptical tip shape doesn’t bleed pressure like a triangle, so the effective aspect ratio is much better. Less drag and more usable power is the result.
But what about the bottom of the rig? On racing boats the upwind headsail is always sealed to the deck. This is usually impractical on cruising boats because you want the clew lifted to keep ocean waves out of the sail, and deck sweepers are not very efficient reaching. The main (and mizzen if there is one) are left well above the deck and they bleed pressure like crazy around the boom.
In the 1960s and 70s we addressed this issue on our racing catamarans with deck-sweeping mainsails. We found a good seal was worth at least five degrees in tacking angle with no loss in boat speed. If you lifted the seal just six inches, this advantage would be lost.
So when we first built BEOWULF we first thought about sealing her main and mizzen, but did not get around to trying this out until this past summer.
Main deck seal.
We had Dan Neri and the North Rhode Island loft make a couple of seals which attached with zippers on the foot of the sails they had installed when the sails were new. These seals had vertical battens to help keep them in place along the deck, and are connected to the base of the mast and to the sheet travelers. They add 137 square feet of area-almost a six percent increase in total area. Of course they are down low, where winds are light, but if the end plate effect is working, they should make for a substantial increase in boat speed.
Do they work? You bet! Upwind, in 13 to 14 knots of breeze, BEOWULF now averages between 9.7 and 10.00 knots, making 95 degrees between tacks including leeway (based on the SetSail/MaxSea software track she leaves on the computer). This is five to seven degrees closer than before, at a speed improvement of around two percent. That is a huge difference.
And reaching, we seem to be going five to seven percent faster than was previously the case.
If you want to think about deck seals for your own boat here are a couple of considerations: First, you need to think about visibility. This is definitely compromised. In our case we can see OK ahead on the weather side but visibility to head and to leeward is cut off-so we do not use the seals in traffic.
The second issue is how to furl them. We have webbing ties sewn every couple of feet, so when the seals are not in use, they are rolled up and tied along the foot. We used a heavy-duty zipper for attachment, but will change to double Velcro the first chance we get. Finally, you need a system for controlling the lower aft corner of the seal. We now tie this to the traveler-which works well on BEOWULF because we have full-width travelers on the main and mizzen. For most boats this would be impractical. Another approach is to have a vertical strut attached to the boom, and then “sheet” the aft corner of the seal to the strut.
If you can fit a seal it will give you more increase in performance for less cost than anything else you can do.