Gardner
I have to estimate a span efficiency for all the foils including the sail rig. Remember these values relate to the whole rig. I was actually using 1.5 with your schooner rig previously because of the mast separation - I based that on what I have read on biplanes. So 1.3 is possibly a bit lower than what you should get with this rig as you have laid out. That would be the value for your original sloop rig.
When you are sailing close to the wind the booms will be sheeted along the hull or close to it. The leading one a few degrees freer than the trailing one. As noted for the 50T the apparent is 28 degrees. You can use JavaFoil to play around with the sail angles to get the best L/D when the boat is at this angle. The one I posted in your files was reaching with apparent wind at 40 degrees.
With my masthead sloop rig I got the best windward performance with the main boom sheeted a few degrees to windward. It could be similar with the trailing boom on your schooner.
It takes as much fiddling in JavaFoil to trim the two sails to get best result as it does on a boat. You need to adjust the angle of the sails relative to each other as well as relative to the wind angle.
Rick
On 08/06/2011, at 2:43 AM, Gardner Pomper wrote:
Rick,I am a bit unclear. Did you just add the span efficiency to the model and sailplan you already have for ToyyoT, or did you change the sailplan to match the Visionarry sailplan? I am all in favor of being able to match the numbers you have listed below! 12.2kts into a 15kt breeze would be awesome!Can you tell where the boom would be positioned for a 50% true wind angle? If it is over the lw hull, your idea of a tarp joining the hull to the boom to increas the span seems doable. Not for short-tacking, but if there is a long upwind run while island hopping in the Bahamas, it might be worth the effort to get that extra 1.4 kts of speed. If it is angled out over the water, that is more problematic.
- GardnerOn Tue, Jun 7, 2011 at 4:40 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Arto
Using your numbers with a sail area of 72sq.m the L/D improves from about 21 to 26. These values are quite high because the rig is high aspect.This does not mean much unless you are looking at the VMG to windward.I have not yet made a VPP for the Visionarry design so I have used Gardner's ToyyoT as an example. Just changing the span efficiency to the values you have provided gives the following data for his boat:With wind of 15kts at 50 degrees true the lower value gives boat speed of 9.8kts while the higher value gives boat speed of 12.2kts. So significant gain in VMG. This is about as high as his rig will get as they correspond with 31 and 28 degrees to apparent wind. I expect a single wing will point higher but have not checked it.As the boat bears away the benefit diminishes. For example with wind at 15kts, 90T the respective speeds are 14.9kts and 15.9kts.Sailing at 50 degrees to the true wind does not sound too good but remember the boat is going fast so the apparent wind swings a long way toward the nose. Also sailors are used to making observations of the wind direction from the boat - in this case it would seem the boat is capable of tacking through 56 degrees viewed from the boat when in reality it is 100 degrees or a bit less if you look back at the tracks. It gets more realistic looking at what the angle between two windwards tacks look like. Not many boats get within 90 degrees particularly if they are capable of achieving wind speed with boat speed.With the higher span efficiency the boat will pinch up to about 42 degrees true while it goes out to 48 degrees with the lower value. In both case the speed is around 8.8kts. I have not checked if these values give better VMG.In all these cases I have not fiddled with any sail trim to see if there are better options by tweaking the sail set. As they would benefit both cases in similar ways. First thought is that flattening sails may benefit at higher wind speed but ToyyoT is not overpowered in 15kts.Rick
On 07/06/2011, at 5:14 PM, Arto Hakkarainen wrote:
From: Rick Willoughby <rickwill@bigpond.net.au>
Subject: Re: [harryproa] Span Efficiency - Another Harryproa Opportunity
To: harryproa@yahoogroups.com.au
Date: Tuesday, June 7, 2011, 9:40 AM
ArtoBasically your last statement is true.Tom Speer has published some charts on it from 3D modelling on his web site.The span efficiency rapidly drops from near 2 at full end plate to about 1.3 with a gap that is 10% of the span above an effective end plate like a deck or water level.Improving the span efficiency has a small impact on the coefficient of lift of the rig but a direct inverse correlation with the induced drag. So doubling the span efficiency halves the induced drag. It is really significant in improving the windward performance of low aspect rigs.Rick
On 07/06/2011, at 4:16 PM, Arto Hakkarainen wrote:
From: Rick Willoughby <rickwill@bigpond.net.au>
Subject: Re: [harryproa] Span Efficiency - Another Harryproa Opportunity
To: harryproa@yahoogroups.com.au
Date: Tuesday, June 7, 2011, 2:04 AM
ArtoAnything that creates flow resistance from the pressure side to the low pressure side helps with span efficiency. Adding bits to achieve it gives diminishing return because the added bit adds drag. The deck of the lw hull is there anyhow. Bringing the boom down to the hull does not add more drag area.Rick
On 06/06/2011, at 11:52 PM, Arto Hakkarainen wrote:
From: Rick Willoughby <rickwill@bigpond.net.au>
Subject: [harryproa] Span Efficiency - Another Harryproa Opportunity
To: harryproa@yahoogroups.com.au
Date: Sunday, June 5, 2011, 5:50 AM
Looking at the mast and boom on Peter's boat yesterday the topic of span efficiency came up.Also when I was working on the parameters for Gardners schooner rig to plug into the VPP I had to take at stab at the span efficiiency of his proposed sail plan.This is something that I have discussed with respect to rudders and daggers but I don't think it has been discussed with sails. Although I have referred to aspect ratio rather than span efficiency. In effect the induced drag is halved if the blade is effectively end plated against the hull or if a surface piecing blade is at high speed and non-ventilating. End plating is equivalent to doubling the aspect ratio or span efficiency is 2.Anyone who has sailed on a sloop rigged keel boat knows how the pointing ability improves when the genoa is sheeted down against the deck. This increases the span efficiency of the rig. There is little or no air spilling under the sail.Another good example is Macquarie innovation. They went for a low aspect wing but were able to improve the span efficiency by setting it close to the water:Wing end plating is something I proposed for my beach proa. It can also be achieved with the schooner rig Gardner is proposing.The sail efficiency is most important when sailing close to the wind and in these angles just the lw hull provides reasonable end plating.Gardner - For the VPP I produced I selected a span efficiency of 1.3 based on what I have read about bi-planes. If you "end plate" at the bottom the span efficiency lifts to 2.There is some work being done with winggrid winglets that get span efficiencies above 2:Rick Willoughby
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby
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