Subject: Re: [harryproa] Re: rig-to-windward polar?
From: Rick Willoughby
Date: 7/15/2012, 9:55 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Ben

I would defy anyone to tack this boat in any condition without using the motor.  It has a generous cabin that is hard to push through the wind.  I doubt that it is possible to maintain enough momentum to tack through 120 degrees while keeping speed above 2.5knots.  If speed drops below that the steering is doubtful.  Does not matter which way the boat is sailed, getting the cabin through the eye of the wind is a challenge.  It can be challenging getting it turn through the wind with a 60HP outboard in wind over 25kts and a bit of joggle.

The other point is that if you want to keep the ww hull to the lee all the time in light wind then you must shunt anyhow.

My mind still boggles at how a boat having such a large deck area, voluminous cabin and small rig can achieve the speeds it does with such little effort.  Once you are familiar with shunting it is not difficult to get out of what might be tight situations where you would need to tack on non-reversing craft.  We are not racing the 18m proa but there is a bit of traffic and other sailors get a surprise when we simply shunt away from their line.  Compared to tacking shunting is foolproof.  Release one sheet and pull up on the other.  At worse you go back over the ground you have just covered.  The art is doing it quickly going upwind so you do not lose much ground going downwind.  When steering I tend toward caution to make certain the boat has speed before coming on the wind again to avoid irons.  The tracks of my shunts are awful when looked at closely.  You do not need this caution when sailing ww to lee because you can always bear away.

The design input from this observation is to be aware of windage and how it impacts on the overall performance. It is another factor that limits windward performance in particular.  I expect that a harryproa could be tacked provided the windage was low; the rig was efficient and the underwater control surface were also efficient.  I have given up trying on the 18m.

To tack reliably I expect you would want to have a boat that could maintain helm response to maybe 30 degrees off true wind.  This means efficiency - low frontal windage, high aspect sails and control surfaces, rudders well-separated etc.

Rick

  
On 16/07/2012, at 10:58 AM, bjarthur123 wrote:

 



thanks for the info rick.

i understand how sailing with the rig to windward keeps the boat keep out of irons when the rudders are on the verge of stalling in light air. the drag of the ama counterbalances the weather helm caused by the aft COE of the sail instead of adding to it.

it seems to me though that, even if VMG is not better sailing with the rig to windward, that there is another benefit to having a boat which can sail the wrong way around, specifically in (1) short-course W/L buoy racing and (2) long narrow channels.

in light or heavy winds you don't have a choice, and would generally want to always sail with the rig to windward and leeward, respectively, and hence shunt. but in moderate winds one could tack/gybe ones way up/down the course/channel instead. i would have to think that that would be faster, as one doesn't completely lose all forward momentum during a tack or gybe as one does during a shunt.

does it make sense then to design the boat from the start to be able to sail in light to moderate winds with the rig to windward? and if so, how would the design be different from a boat that was meant to always sail with the rig to lee, if at all?

ben

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Ben
> I did it but cannot recall if I published the comparison. There was
> not much difference in the polars. VMG does not change measurably.
>
> From a sailing perspective having ww hull to lee is significant
> benefit in light winds because the boat automatically bears away when
> the rudders stall so it is self-correcting. Sailing ww hull to
> windward the boat rounds up when they stall and there is no way to
> recover other than shunting out of it.
>
> The rudders stall around 2.5kts sailing either way around. In 5kt
> wind that occurs at 50 degrees to true wind. In present form with
> the two 900mm deep rudders the best VMG is at 55 degrees. So there
> is 5 degrees to play with. Momentary lack of concentration, wind
> shift or wave can cause it to round up.
>
> In practice it is very easy to sail it at best VMG with ww hull to
> lee. With ww hull to windward it needs to be sailed freer than best
> VMG to avoid rounding up.
>
> The polars show where the rudders stall and the angle for best VMG
> but they do not show the consequence of stalling the rudders.
>
> The other factor I have not yet included in the VPP is the loss of
> span efficiency when the rudders lose the end-plate effect of the
> hull due to it being round section. This is most noticeable when
> sailing faster because there is a position of instability compounded
> by backlash in the two gearboxes in the steering linkages.
>
> I have done a number of comparisons for different rudders and
> concluded that going to 1.2m deep but shorter and thicker would
> improve windward performance. However that does not completely
> overcome the rounding up issue but it occurs closer to the wind.
>
> The rounding-up issue is simply something to be aware of. We have
> made lots of "Z" patterns over the Bay trying to sail to windward by
> getting in irons and needing to back out. It is nowhere near as
> inconvenient as trying to get some beach cats to tack through the
> wind - they are not intended to go backwards. This nuance with the
> proa is a blessing in light winds with ww hull to lee because the
> boat essentially sails itself.
>
> Rick


Rick Willoughby




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