Rick,
I am not sure I understand what you mean by winglets. I was just envisioning a flat plate on top, like the plate at the boom. Do I just have my terms mixed up, or is a winglet something different?
And I am still confused by the "off the wind" idea. If my mast rotates, isn't the sail always hard on the wind and has the same performance characteristics? I thought that was one of the advantages of the easy rigs; you did not have to change the sail trim based on the relative wind direction. You might want to ease the outhaul to give more belly in light wind, but the sail is always trimmed for close reaching.
I am never sure if I am bringing up valid points, or just exposing my ignorance <grin>. In either case, I appreciate the explanations.
- Gardner
On Tue, Jun 7, 2011 at 6:51 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Gardner
End plating is common practice with keel boats using large genoas.I provided additional values for true wind at 90 degrees. The performance advantage diminishes as you come off the wind.I do not know if anyone is playing around with winglets on a boat. Monohulls do not like a lot of lift from the upper part of the sail. In fact Tom Speer has shown that there is an advantage to reversing the AoA at the top of the sail to improve the righting moment - hence wing twist.In your case where you have a limit on the mast length for transport it could be a distinct benefit.A clean deck where the sail sheets hard to it is an effective end plate. I can really only make a guess at the benefit from certain size winglets - there is published data on them. I have played around with winglets on prop blades but did not get any concrete data. Ducted fans on aircraft or Kort nozzles on tugs achieve the same thing in a qualitative sense as winglets but the former eliminate induced drag from the blade tips or very close to it. Winglets do not.One potential feature of a winglet is use as a float for limiting complete capsize.The pictures of the Flettner rotors mounted on ships all show upper flange. These are circular winglets. With the very high lift coefficients possible from a Flettner rotor there is significant benefit in reducing the induced drag because it is a large proportion of overall drag.RickOn 08/06/2011, at 3:59 AM, Gardner Pomper wrote:Rick,I have been trying to poke around to see if anyone has ever tried putting endplates on sails; both the foot and the head and I find a bunch of discussion, but no trials, even theoretical ones. From your explanation of how much using the deck as an endplate helps when close to the wind, it made me wonder if it wouldn't help on most all points of sail, once you are dealing with a rotating mast. Won't you always have the sail trimmed for close reaching, and it would then also benefit from the endplating even on a beam reach. Does it actually matter which way the hull is pointed, or is it just effected by sail trim?The only actual trials I have seen are on the wing tips of RC planes, where just a square tip made a big difference. Wouldn't it be possible to just attach a 2'x6' piece of laminated foam to the top of the sail? That would only weigh about 5 lbs.So, I guess the questions are: how big does a boom or head plate need to be to get the span efficiencies up, and how much drag would they add? (assume for arguments sake that we use 9mm foam glass on both sizes, so 8oz/sq ft at most for weight).- GardnerOn Mon, Jun 6, 2011 at 7:04 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
Arto
Anything 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 WilloughbyRick Willoughby03 9796 24150419 104 821Rick Willoughby03 9796 24150419 104 821
__._,_.___
.