Scale Model Testing of Sailing Rigs, ...Outdoors

Earl Boebert · Aug 10, 2017

Well, there's a lot more about the Claudio gadget than that one video.

A lot of EC 12's run Carr sails (he helped establish the class). Drop by an event and check out the boats.

Cheers,

Earl

brian eiland · Aug 10, 2017

I'm going to attend an event in Jacksonville when I return to FL this fall. I understand they have a very active fleet.

brian eiland · Aug 10, 2017

Sail Draft
i recall reading this analysis some time ago, and had lost track of it when my laptop hard drive failed recently. I just found it again, and figure I will quote it here so I will keep track of it

Sail draft http://www.onemetre.net/Design/Draft/draft.htm

There is some debate on the position of maximum draft needed in an IOM jib and main. Some skippers insist that maximum draft should be about 45% of chord, while others say 30%.

Draft position

I was interested to read what Frank Bethwaite had to say on the position of maximum draft in his "Tasar" dinghy class -- he was comparing maximum draft at 25% against 50% of chord:

"My authority for [my] statements are experiment and observation but primarily model aircraft glide tests with different wing sections. The dynamics of sailing to windward at max VMG in lighter wind are the same as the dynamics of a glider which is looking for minimum sinking speed. A test model with a wing with a thin section and max camber at 25% from the leading edge was efficient at one particular speed. With a wing of identical shape and weight but with max camber at 50% (ie. a circular arc section) it was as efficient as the “knuckled” section [ie max draft at 25% of chord] at its best point, but it maintained this efficiency over a wide speed range. In smooth air the two sections could glide and lose height as well as each other. In rough air the knuckled section became unstable and the model sank faster, while the circular arc section continued to fly steadily in the rough air, the model remained stable and sank more slowly."

This was interesting on two counts. One was that Graham Bantock uses an aerofoil section on his rudder which he tells me is pretty much the same as that used on competition (full size) gliders. Both model yacht and full-size glider operate at similar Reynolds numbers.

The other was that Frank Bethwaite was clearly saying that draft forward was not advised.

This is only half of the story, however. One of the key things about draft amount and position is that they affect the entry angle -- and it is the entry angle which is so important while beating.

Entry angle depends upon draft amount and position

Will Gorgen sent me some comments on entry angle and its importance:

However there is another principle that you need to keep track of. That is the incidence angle at the leading edge. As you pull the draft forward, you change the angle of the leading edge relative to the incoming air flow. This causes your angle of incidence between the leading edge and the incoming flow to decrease. In the extreme, this will cause the front of the sail to luff and will effectively force you to sail lower (foot) in order to keep your sail from luffing. When you sail lower, you increase the overall angle of attack of the sail and move yourself closer to stall.

Thus, moving your draft too far forward can cause your sail to stall, and moving your draft too far back can cause your sail to stall. There is an optimal draft position in between these two competing forces that will determine your optimal draft position. This is often determined by your individual sailing style. For people who like to point high, you generally need the draft of the sail further back in order to keep the entry of the sail at a positive angle of incidence and prevent the sail from luffing. For people who like to foot, you can tolerate the draft further forward and sail with a higher overall angle of attack and thus carry more force in the sail. Of course optimal VMG depends on carefully balancing the lack of windward progress with the extra speed that you get when you foot.

Moving your draft further forward allows you to generate more drive in the sails and tolerate a higher angle of attack, but at the expense of pointing.

The "Entry and exit angles" page and associated spreadsheet explores these angles in more detail.
Click to expand...

I like this 50% draft idea, particularly for a cruising boat like I am hyping, and one that has the interaction of the 2 headsails as mine relies on for light & moderate sailing conditions.

brian eiland · Aug 10, 2017

Another reference from that onemeter website,...

Books http://www.onemetre.net/OtherTopics/Books/Books.htm

Robinson, Larry. Making Model Yacht Sails, Parts 1 & 2.
1997 & 1998 Plastic comb bound paperback, 28cm x 22cm, 43 & 46pp

Part 1 deals with Larry's basic ideas about sails and sailmaking. Although he emphasises these are derived from the EC-12 class, the ideas apply to all classes regardless. Chapter 1 concerns the camber distribution required in a sail, where Larry puts his view that we need more draft at the head, not less. Chapter 2 introduces the simple sail blocks that Larry uses, while Chapter 3 shows how to make the blocks to meet desired specifications. Chapter 4 discusses various sail materials from the point of view of strength, flexibility, and durability. Chapter 5 discusses the topic of where and how to place the broadseams, and in particular Larry's view that the bottom part of the sail probably doesn't need a seam. Chapter 6 discusses cutting the luff and methods of measuring the luff allowances. Part 2 deals with making "string" sails, and is probably most interesting for its appendices offering comments on sail shape. If you are serious about your model yacht racing, even though you may not want to make your own sails, the booklet provides a valuable introduction to relevant sail theory, invaluable advice, and thought-provoking opinions. E-mail Bob Wells at "bob" at "islandinet.com".
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markstrimaran · Aug 11, 2017

Trolling with a 6' reef on the roller boom furler. In 5 mph winds, for a 2 mph trolling on a broad reach. Camera is on the lead stay sail halyard.

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Scale Model Testing of Sailing Rigs, ...Outdoors

Earl Boebert Senior Member

brian eiland Senior Member

brian eiland Senior Member

brian eiland Senior Member

markstrimaran Senior Member

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