Sorry, none of this make much technical sense. Later more on this.

But also the reasons for having a max luff limit are the same for both boomed and boomless sails.

Booms only "smack over" on slow mono's as the boom only comes over very gently when a cat at normal boat speed is gybed. Remember which direction the apparent wind is coming from on a cat.





That still doesn't allow the rig to weathervane or even to achieve large boom angles as the rods will hook the sail then at 45 degree angle. This max angle could make the boat hard to control on courses between beam reaches and broad reaches in a blow.

In addition this flap makes no aerodynamic sense. Many believe that it does, but it really doesn't.


- first technical reason

All area used up by the flap needs to come out of the total sail area. So low above the watersurface the windspeed is only a fraction of the windspeed higher up the mast. As such the same area is MUCH more effective when used higher up in the sail.


- second technical reason

Because of the relatively low windspeed so low to the watersurface and the fact that the flow here is largely disturbed or even blocked by the hull makes any lift gains here negligiable and any drag looses when not having the flap to small too matter. The loose flap and felcro are more work for the sailmaker (more labour costs) and more stuff to get broken without any measureable benefits in performance or simplicity of design. I've used a similar setup on the 49-er I sailed for a while and I would have gladly removed that whole flap. It was the first part of the sail to crack up and once the velcro is gone it undoes itself and flutters in the wind. By the way, Nothing is as draggy as a fluttering piece of cloth, even if only the leech flutters then that increases airfoil drag by 20-40 % No amount of sail drive of the same piece of cloth can every correct for that.


- third technical reason

For an airfoil to work in unstable flow (as we see all the time while sailing) it needs to have a sufficiently large cord. Wind is basically an unstable airflow that is made more unstable by waves, chop and macro turbulance induced by the wind rubbing against the ground/watersurface or passing buildings, trees, hills, dunes, other sailboats and whatever else was in its way.

Very short cord foils stall very easily and have trouble reattaching the airflow. The flap is basically a very short cord foil unless it reaches back very far along the boom. This can of course not be had for other reasons.

A short cord foil also stalls at significantly lower angle of attacks then a longer cord airfoil. Of course you'll set the boom angle for the lower portion of the sail (= long cord) and the flap (short cord) is set accordingly as well. By any analysis it will be suboptimal. If you are lucky (!) its lift will cancel out its drag and the drag of the mast (but still lower the effective sail drive area of the sail that is above the boom, probably losing some performance)


- forth technical reason

Alot of people think that the flap will smooth out the flow around the mast and thus lower the drag of that part of the mast.

Even if it does that (and it most probably don't) then the drag of that part of the mast must be sufficiently large to matter, which it most definately is not. A 50 mm diameter round tube of 0.750 mtr length (my exposed mast below the mainsail sleeve) produces at max 0.50 kg of drag when sailed at 15 knots on a 45 degree upwind course in 20 knot of winds. It is probably significantly less as the wind so close to the watersurface is travelling significantly slower then the windspeed readings that are officially taken at 10 meters altitude. Additionally you can never remove the total amount of drag of this portion of the mast, even with the flap a portion will remain. So the total drag amount is less then 2% of the total already.

But this assumes that the flap actually smooths out the flow over the mast here. If a round mast with a sleeved flap is used then this won't happen to any significantl extent. Even if the flow was laminair before it reaches the mast (unlikely) then it will become turbulent and detach from the mast+flap BEFORE the side of the mast is reached. When teh flap is properly aligned then the flow will reattach some distance behind the mast. If the flap is too small then flow will not reattach at all and the flow pattern around the mast will be the same as with the flap (foil) completely stalled. In that situation there will be hardly any lift created and the drag of the mast+flap setup will not be less then just the mast alone. In fact, it will mostly likely be more because as a due to the angle of attack (20-25 degrees) a larger portion of the flow will be disturbed.

In effect it is better to have a very small area operate at a bad drag coefficient then a large area operating at a mediocre drag coefficient. A flap is alot larger in area then just the round tube of the mast.


It is interesting to note that flapped mainsails are not found in EU. Any design that had them like the older Nacra's and supercats had these flaps removed because the measurement based rating systems like Texel and SCHRS counted this area with the same weight as area higher up along the mast. It was found that it was better to remove the flap and make the sail above the boom a little larger.

Other area's using yardstick systems still see these flaps as here these systems don't not look at the total amount of sail area just at the performance that is achieved.


For these reasons I see no technical reasons to have flap below the boom to try to maximize luff length. It is either a full sail going down closer to the mainbeam or just cut off the foot along some horinzontal. The jury is still out on the "angled upward booms", but I'm personally skeptical of them as well.

Wouter