Robert
This boat is a 1/4 scale of something to suit two people. Those two people would be 70% of the boat displacement and both ballasting the windward hull. On the model the centre of mass is too far to the lee for the cant we have to get anything like the force alignment needed to get lw hull to lift first so it is very tippy because the sail area is going up as it heals. We plan to reconfigure the sail with greater cant to get closer to the force alignment on the model that we were after on the full scale. Also improve the rigging to hold the sail flatter.
The increased cant will be achieved by increasing the length of the booms and lowering the head. Canting the mast to windward required a change to the ww hull that would take more time. This will be considered in conjunction with geting the required canting angle on a practical beam. The planned beam is 4m so already quite large relative to length.
The model will also be changed to use a boom and vang system that will have a means of instant depowering. On the full scale the sheeting system would be different so gives an alternate means of rapid depowering.
With the model as is, it was possible to depower by bearing away in the gusts to reduce apparent wind but that was not always possible due to lee shore. We tipped it a few times.
At full scale the sail could be reefed as well as the cant angle changed to suit the load and wind conditions.
Rick
On 17/03/2013, at 8:59 AM, robertbiegler wrote:
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> The aim with the canted bi-directional rig is to get better
> alignment of drive and drag forces in 3D.
The effective angle of attack of a sail depends on its cant angle. If you start off with a vertical sail, and the boat heels to lee, the effective angle of attack decreases. If you lean the rig to weather, but you fail to sort out the force vectors to prevent heeling to lee, you increase the angle of attack. Windsurfers demonstrate that, with quick enough control, this can be dealt with. However, the bidirectional sail will have its centre of effort forward of the axis around which the sail rotates. You can't depower by sheeting out, you have to depower by hauling in a brace at the forward end of the boom. That will take more time.
> the mast could be canted to the windward side rather than being
> vertical to give more vertical component to counter healing.
That would also move the vertical component more to windward. You would be better off moving the foot of the sail leeward rather than the head windward.
If you want benign behaviour in a gust, you need to go all the way to non-heeling or heeling to weather. Sailrocket demonstrates how wide a boat you need to achieve that, at least if you want your sail upright enough to produce reasonable drive for its area.
Regards
Robert Biegler