Doug
When sailing in light wind a HP has weather helm; it wants to turn up into the wind where sails will luff and it will sit in irons unless shunted out of the situation. The drag on the windward hull is screwing the boat to windward because all the drive is over the lee hull.To hold a straight course the weather helm needs to be countered. Using the trailing rudder means the lateral force from the rudder reduces the leeway. Using the leading rudder means the rudder lateral force increases leeway.
The big rudders mounted off or near the beams in the standard HP design generate a lot of lateral force for the steering moment created because the moment arm from the rudder to the hull CLR is not very much. Rudders, so placed, essentially offset the sail lateral forces when the boat is on the wind and the trailing rudder is used to counter the weather helm and hold the course.
On the 18m proa, when the board is lifted and we are tight reaching at less than 8kts the trailing rudder is cranked on close to stall. That reduces leeway to very little. The front rudder operates around the in-line position to hold course. This is the fastest way to sail with board up because the trailing rudder is reducing leeway. It is far better to use efficient foils to resist lateral loads than allowing the hulls to crab.
A cambered foil is just the shape of a typical aircraft wing. It is the most efficient way to produce lift; meaning the lateral force on a dagger board. A sail is a cambered foil that produces lift perpendicular to the wind flow that has a component of drive and a lateral force creating the healing moment as well as producing leeway unless countered.
The objective is to eliminate leeway or crabbing of the hull through the water. Leeway of a hull adds significantly to drag. A high aspect foil will generate lateral forces with less than a tenth of the drag of a hull that is crabbing to generate the same lateral force.
Rick
On 27 Mar 2019, at 1:27 pm, doha720@yahoo.co.uk [harryproa] <harryproa@yahoogroups.com.au> wrote:Can i just ask:
You say lift, do you mean the water flowing over the rudder will be at a slight abgle, due to yhe leeway of the boat or because of the balance or something, and so there will be little bit of "sailing" done by the rudder to windward.
Can't imagine thst would be a very significant amount of gain.
And, cambered foil, you mean like curved on one side, flat on the othef and so get more lift?
Must be some loss from the less efficient streamlining shape.
Doug
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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