That's what i mean: the spinnaker halyard is pulling forward on the top of the mast, causing a bow-down pitching moment. This is partially countered by the downhaul pulling upward on some point forward of the LCB. But in any case if the spinnaker is propelling the boat forward and the CofE of the spin is above the waterline, you have a net bow-down pitching moment.
With spinnakers I have used the halyard is close to horizontal when the spinnaker is flying - little or no downward component. The pole needs a topping lift to avoid it dropping if the spinnaker loses air - again demonstrating that there is uplift. This image shows what I mean about the halyard:
http://ffden-2.phys.uaf.edu/104_2012_web_projects/Benjamin_Renshaw/img_1/spinnaker2.jpgAnother example of the uplift a spinnaker can provide:In a knockdown situation I have seen an RL24 release spinnaker sheet, brace and halyard to their limit with the spinnaker going up like a kite dragging the boat on its side through the water. They had to cut one of the lines to get it down. There was plenty of uplift from the spinnaker throughout. This was on Lake Macquarie where a "bullet" hit them on the lee side of Wangi Point. It was too quick to react and I guess they released all lines at once rather than just spilling air. Once the spinnaker was flying high it was no longer shielded by the cliff and was in the full wind force of about 30kts and boat was out of control.RickOn 24/10/2012, at 10:09 AM, Micha Niskin wrote:The pole downhaul might be pulling upwards, but that's not telling you much if you don't also measure the halyard tension. The downhaul is producing a pitching moment that is opposed by the pitching moment due to halyard tension.
On Tue, Oct 23, 2012 at 4:57 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:Ben
Conventional spinnakers certainly provide considerable lift as guaged by the pole downhaul. A gennaker adds sail area to the rig so little extra lift although the shape of the foot would have some influence.A traction kite is going to provide lift but will have a narrow range of operation for wind angle and windspeed. I doubt that there would be circumstances where you want to shunt it while set. You would want open water to work a kite and in these conditions a small boat will be affected by the size of waves. I have used a spinnaker in 35kts on a trailerable yacht in 2 to 3m seas and we were easily driving over the waves but it is literally white knuckle sailing - we averaged 12kts over 18nm including a 4nm upwind component. The waves were not fully developed - if fully developed there would be something like 8m waves and I expect that I would be in survival mode before that in a cruising boat.The harryproas have hulls that enable achieving boatspeed above windspeed but it requires enough sail area. The more sail area means the earlier you have to reef.Cannot recall if I already mentioned this but the ORMA 60s can cant their rigs to windward to get lift. Again this is a feature that reduces the likelihood of pushing a bow under but once the boat is slowed causing the wind to move aft it does not prevent pitchpoling.If you are broad reaching and induce bow-up trim then the rig will have a lift component. Obviously the reverse is increasing the risk of pitchpole.RickOn 23/10/2012, at 11:52 PM, bjarthur123 wrote:
i was thinking of an asymmetric spinnaker, like smyth has popularized on farrier trimarans.
and yes, i totally agree that broad reaching to the true wind, keeping the apparent abeam, is way faster than running. though you'd never know it from my performance. i suck so bad at going down wind :( last weekend i was first to the weather mark in my weta against four hobie 16s. and last to the leeward mark. three races in a row. one of these days...
rick-- what's your take on spinnakers lifting the bow?
all the accounts i've read of harry proas claim to achieve windspeed in the summary, and then later on when they give numbers it's only 3/4 of it or so. a bit of an exaggeration that's always bothered me. the numbers aren't so different though than a weta or a cruising farrier.
by limited range of use of a kite i presume you mean only suitable for running. i would agree with that. however, the windspeed at their deployed height is much greater than at sea level because it's out of the ground shear. and if they lifted the bows so you could just plow into waves with abandon, that seems like a really fast combination.
kiteship sells a 10m^2 trainer. i'd give that a go just for kicks. could girth hitch it to the forward beam. my only concern is how to shunt it around the mast.> If you are in a high performance multi-hull it is unlikely you would
> want a spinnaker - it limits you to windspeed. If a boat can get
> boat speed above windspeed it will be better off tacking downwind.
> Even kites have limited range of use on a high performance multi-hull.
Rick Willoughby