Ben

It is now 2t on each hull.  The ww hull would be a little heavier if provisioned for cruising.

At present there is only one wheel so both rudders are coupled and nominally align when wheel is centre.  (There could be a couple of degrees offset though because of slack in the linkages)  Sailing normal way around in 10kts the leeway is estimated at 8 to 10 degrees without board down.  Sailing wrong way around it may have been making a little to weather.  We have not sailed enough either way to get proper GPS tracks to compare with headings so can only estimate from land marks or diminishing wake.    

The aim is to have independent operation of the rudders after this week.  Right now the inability to delink the rudders is detrimental to sailing performance.  This is more so since the dagger board brace fatigued and board cannot be lowered as it flogs.

With the harryproa the lateral offset between the sail thrust and the combined hull drag and cabin windage is a larger moment in light wind than the longitudinal offset between COE and CLR.  

Once the rudders are delinked the best sailing normal way around will be to actually create additional weather helm with the leading rudder and then counter this plus the drag-thrust moment with the trailing rudder.  This will need boat speed above 3kts to work.  If there is not enough wind for this then sailing the wrong way around will be the only effective way to go to weather.  

The boat will sail faster using the rudders to offset leeway rather than relying on the hulls.  If rudders are at 90 degrees it will go sideways at 1 to 2kts in 10kts of wind with boom in line with  the lw hull.  This happens when you do not know where the rudders are pointing.  The rudders make reasonable brakes as well.

I really cannot see any reason why not to sail it the wrong way around in light air.  There is no worry with spray until boat is doing over 7kts and no safety concerns with the lee helm until the wind is above 15kts.

 

Rick

On 05/01/2012, at 1:10 AM, bjarthur123 wrote:

 

what is the ww vs. lw weight distribution of this 18m proa? rob has told me that his designs are typically 40:60 when unladen, and 60:40 laden. is that the case here?

i ask because perhaps the rounding-up problem prescribes an upper limit on the fraction of weight desirable in the ww hull. permanently moving some fixed weight (water, gas, engine, anchor, etc) more to lw might be another solution. doing so would decrease drag of the ww hull and hence the yaw torque causing weather helm.

relatedly, have you noticed how much leeway there is? lotsa folks, particularly over on proa_file, worry excessively about COE-CLR balance. in my mind though, in boats without ogive leeboards, weather helm is a good thing, because the aft rudder can then be used to counteract leeway. but only to the extent that it isn't stalled and you're not actually making windway (negative leeway).

ben

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> 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:
> http://www.rickwill.bigpondhosting.com/18m_VPP_As_Built.pdf
> 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

Rick Willoughby

rickwill@bigpond.net.au