Seems like there is quite a bit of discussion on playing the downhaul in cat sailing, much more than in keelboats. My experience on keelboats has always been that you tension the downhaul to flatten the sail going upwind (once you have the sheet fully tensioned), then leave it until you let it off at the mark rounding. So, what's different about playing the downhaul on a cat?

Big difference. Think of the down haul similar to your keel boat's back stay adjuster. It acts to depower the sail plan or power it up, depending on situation.

Be careful and spend the time to actually learn its capabilities. It is not an on or off thing. You can actually work the down haul instead of the mainsheet going up wind, depending on the boat and design.
Best to set the boat up on the beach or trailer and then watch the sail as you go from minimum tensions to full tension of the down haul. The body and top of the sail will likely change dramatically. Of course I am assuming a rotating mast on a typically stayed catamaran.

Good luck.

hmm. interesting. okay, so we can use the downhaul to depower in a puff and also to rake the mast back and add some tension to the forestay? do you play it like you would the backstay on a lighting (which is usually adjusted before the sheet)?

If you were to go to you local sail loft for a cat, it would be laying there perfectly flat. The batten pockets also would all be flat.
The luff of the sail would also be flat but from top to bottom there would be an arc. So, When you put that flat sail on a straight mast that extra arced material is the fullness and makes the battens bow out -- for a full sail, great for lighyt air.
As the wind picks up and the boat flies a hull, you first hike, then trapeze, keeping the powered up sail.
Now more wind. And now, despite your trapezed ballast the boat is still trying to fly a hull, apply the downhaul, (which bends the mast thus absorbing that arcing material) a little bit until it becomes controlled healing. If the air lightens a bit and you see the windward hull is no long just out of the water, ease off the downhaul. That straightens the mast again, getting you more fullness and more power.
If the wind continues to build, apply more downhaul until under control again.
etc., etc., etc. until you run out of downhaul. Then you might have to let the traveler down an inch or so at a time.

Best to have something like 16:1 or 18:1 downhaul. Not that it takes that much to sheet it in, but with those ratios when the air lightens up you can flip the control line out of the cleat. Lower ratios and you can't, meaning the crew has to get back into the boat to ease the downhaul.

On the Laser the most important tool is the vang. It does the close to the same thing -- flattens out the mainsail.
Rick

down-haul doesn't "rake" the mast, it bends the mast at the top, thus making the sail flatter (assuming the sail was correctly cut for the mast track/curve)

It's all about power level vs sail plan drag-related efficiency;

Phase one of depowering is use the downhaul and don't touch the mainsheet. Once the wind / gusts get beyond where the downhaul can manage it, it's time to start working the mainsheet and average the downhaul position to a manageable level (unless the two of you are incredibly in sync and can work both the main and the downhaul in sequential concert without trading for an unsteady helm).

Once the crew starts to be able to anticipate the puffs with the downhaul, you'll start to feel the boat make little leaps forward every time you hit it just right. That magic happens when a puff hits, the helm doesn't change much at all, the boat heal remains the same, and speed increases through the puff. If anything, you can sail slightly higher at this moment if you need the height. If the crew can then ease the downhaul and power back up before splashdown - you're hauling.

Besides the obvious of flattening the mainsail and moving the draft forward a little, the downhaul (more importantly, in my opinion) lets the head of the sail twist off while still permitting the mainsheet to maintain tight leech tension. Downhaul reduces power in the sail, makes the center of effort lower (now that the head is not producing as much and the bottom is still sheeted tightly to center) but efficiency increases, due to the slightly flatter shape, the twist in the main, and the maintained leech tension. If you are just easing the mainsheet and not playing the downhaul, you're depowering but not reducing drag.


I don't know why monohulls do what they do but a lot of the way we use downhaul has to do with square head mains most of us are running...and so few monohulls use fat heads, it doesn't really relate.

Common wisdom is that adjusting the downhaul between marks gets you almost nothing on a Hobie 16. Good idea to bring it in upwind, and let it out downwind, but it's not something you are likely to adjust on a single beat or run of the race.

It has far greater effect on the larger cats, as mentioned above.

Mike

Plus on a H16 you can't adjust the downhaul from the wire.

downhaul worked just the same in the 49er and even the 505