Ben
I believe sailing the wrong way around is an effective solution to the "problem". It is simply something I have not thought about before so is a new dimension to sailing a harryproa. I have not done any concrete analysis on sailing this way but it makes sense it goes better this way in light air.
Rudders are already within 1m of the ends so have roughly 16m lever arm. Increasing the area has very slight improvement because the rudder power is a linear function of area but a square function of speed so increasing the area would only make a slight improvement to the low speed end.
Also the aspect ratio of the rudders has a large bearing on the windward performance - deeper and shorter chord the better. Hence reducing the immersion at higher windspeed would reduce pointing ability. In any event this proa has rudders under the hull so they cannot be raised. The owner was keen to reduce the draft by cutting the rudders in half but is now convinced that that would be detrimental to performance.
I did polars for the boat as-built as a means of comparing proposed changes:
This includes the dagger board at full depth with usual provisos - clean hulls, proper sail set and sheltered water. However performance should be better than this without keel providing the rudders can be adjusted independently. Incrementally raising the cambered centreboard off the wind does not improve performance over having it fully withdrawn and relying solely on the rudders. THese polars are based on raising the board incrementally but the boat will actually sail better without the board if rudders work independently.
What I have not included in the polars is sailing the wrong way around but I do know from calculation and observation where the rudders run out of power to prevent rounding up. Sailing wrong way around would alter the polars for the lower windspeeds by increasing the boat speed at the smaller angles of true wind. Note that the 5kt polar has speed at 2.5kts at 60 degrees true. We were getting boatspeed close to windspeed around 40 degrees true in this sort of wind strength. I could put together some data that shows we have done way better than these polars in the sub 10kt range sailing the wrong way around with the dagger board up.
An important understanding is that the controlling blades need to be high aspect if you want pointing ability. There is no point in having a dagger board that has lower aspect than the rudders. Also there is no point in having a dagger board partially immersed if its aspect is then less than the rudders.
One concern I have with the 18m proa is the load on the rudders in wind above 20kts. The trailing one will be borderline for bending strength in this condition. It could bend before the ww hull lifts.
Rick
On 02/01/2012, at 12:26 PM, bjarthur123 wrote:
rick,
would making the rudders bigger or moving them closer to the ends fix this "problem"?
you round up with the lw hull to lw b/c the ww hull is fully immersed causing lots of weather helm. the helmsman hence has to use more AOA on the rudders to counteract. perhaps more than they were designed for, causing them to stall. so make them bigger, and/or move them towards then ends, no?
of course this would add drag at speed. but adjustable dagger rudders could be flexible in this regard.
i'm moving to the chesapeake in march. any proas there need crew?
ben
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> It is simply a practical way to sail the boat in light wind. We were
> holding between 4 to 5kts making less than 40 degrees to true wind
> sailing the wrong way around. (I have the GPS track and wind
> direction from doppler radar but there is a time shift and it is too
> good to be true - that said we were sailing faster and possibly
> higher than a modern cruising cat and 30+ft keel boat.) In the same
> conditions we made 3 attempts to sail to windward with the lw hull on
> the lee side. Speed was between 2 to 3kts but inevitably there would
> be a wind shift or loss of concentration and the boat would round
> up. We gave up beating and just sailed off the wind. By that stage
> the wind had picked up a bit to average about 7kts and we were making
> 6 to 7kts reaching. If the boat could be sailed in both directions
> we would have been better off sailing with the ww hull on the lee
> side on both tacks. However the current rudder set up with chain
> makes bi-directional sailing dicey.