Oday 39 racing

[DrJudyB]

@DrJudyB I have a sister ship to the OP, and I have heard this as well, I am curious what makes that determination? We previously had an O'day 34 and specific rig tuning information for either boat is almost nonexistent.

Scott

The geometry of the rig determines the general approach to tuning. If the designer did the job of combining the right size wires and mast in the right geometry, we can deduce the design and engineering by observing whether its's a masthead or fractional rig, spreaders are in-line or swept, and where the side shrouds are anchored, etc. Then we know how to tune it.

[ If you want an introduction to rig design, and you like trigonometry, I can recommend a book by Brion Toss The Complete Rigger's Apprentice. Read Chapter 5 and Chapter 8. ]

If you want to learn how to tune a rig based on the geometry of the rig, down load a copy of Selden's Hints and Advice .
Read Pages 42, 43, and 44 for how to tune a masthead rig with multiple inline spreaders and forward and aft lower shrouds. also read pages 1-33 for important introductory information.

See below for pp 42-44 . You'll notice that the instruction tell you to put "a slight positive prebend" in the mast using the forward and aft lower shrouds. prebend on a masthead rig can vary from 0.5 to 2 inches of bend on a medium size boat, depending on the mast extrusion. If it's a telephone pole, all you need is less than an inch.

There is no advantage using more bend than is needed to a) prevent pumping and mast inversion when the backstay is eased downwind b) prevent inversion when the spinn pole is used. If you put too much bend in the mast of a MH rig, the forestay will sag.

. A typical masthead rig will have between 0.5 and 1.5 degrees of rake if it's well designed and floating on her lines fore and aft. With most boat's the required rake is usually closer to 0 degrees than 1.5, so I'd start at 0 or 0.5 degrees and test for 3-4 degrees of rudder angle close reaching at 15-20 degrees of heel.

Use a plumb bob to measure rake. Hang it from the masthead using the mainhalyard. Measure the horizontal distance from the back face of the mast to the plumb bob. (Correct for position of the main halyard exit at the masthead compared to the back face of the mast) To get 1 degrees of mast rake, multiply the luff length (P) in inches X 0.017 . For an Oday 39: 0.017*44.28' * 12"/ft = 9.03 inches measured at the boom. For a half degree of rake, use 4.5". etc.

Judy B

 

[Davidasailor26]

On the day of the pic I was running the main pretty loose to get more power and speed out of it (and I don't think I was doing to bad). It seems if I run it tight (outhaul and sheeted flat) I really loose both is this because the shape is already bad? From my experience (please correct me if I am wrong) if the sail isn't luffing and I am not over powered keeping the sails as loose as possible seems to work best, in about the 10knot range, after that you can either add twist or flatten to depower. Now in sail as loose as possible I do loose some on pointing but the speed difference between the couple degrees gained is a big loss.

Tightening the outhaul too much will certainly depower the sail and slow you down in lighter winds. You need to have some draft to get power. when you’re adjusting the sheet you are adjusting the twist (depowering when you let out), but you’re also adjusting the angle of attack. Tightening the sheet too much, especially if you have a traveler that lets you bring the boom above the centerline, will probably reduce the AOA to the point where the main stalls and gives no lift. Do you have telltales on the leech of your main? You want to let the sail out just enough that those telltales are streaming back most of the time. In too tight and they’ll curl forward behind the sail. Out too lose and they’ll stream back, but the sail will luff.

 

[DrJudyB]

In addition to what I wrote above: For a mast head rig with a stout mast, I'd recommend you tune your rig so there's minimal bend (less than 1" on a medium boat) when you order your sails. Your sailmaker should tell you that and measure the mast bend before doing the design.

If you start with minimal prebend, you can increase it over time as your (dacron) sails stretch and thereby compensate for the deepening draft. Dacron sailcloth stretches over time (although some elements of construction can shrink) , and as it does, you can increase the amount of mast bend to compensate for the draft getting deeper. However, there's not much you can do to keep the draft from moving aft on stretched out sails....

Laminate sails mostly shrink over time.... so that's a whole different animal.

Judy B

 

[Sefuller]

@henrykorous I read that you got new sails this year, we need to order a new genoa for our 39 for next season, and probably main the year after. I have been trying to get my rig tuned properly prior to having sails measured, and am curious if you have had any rig tuning done on yours? All of my wire rigging is 5/16". I have about 10" of rake on the mast, and maybe 2 - 3 inches of bend as I have been told these rigs are meant to be pretty straight. I have the backstay at about 20% breaking strength in order to keep the headstay from sagging off more than a foot when on the wind in 12 - 14 knots. When I set it to 15%, I get close to 18" of sag to leward when on the wind. I have the cap shrouds at around 12%, and with the intermediates I have run them between 6 and 10%, but the problem I have is the top of the mast from the second spreaders up falls off to leward when we are on the wind. The Selden tuning guide says to loosen the windward intermediate in this situation so that the mast falls off in column. At anything below about 8%, the leward intermediate flops around like a noodle and even at 10% it is pretty loose - loose enough I can easily adjust the turnbuckle by hand. Not sure exactly what do, and unfortunately we really don't have any riggers in our area.