Sleeved luff "wing"-mast

Discussion in 'Hydrodynamics and Aerodynamics' started by Will Fraser, Jan 17, 2018.

  1. Will Fraser
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    Will Fraser Senior Member

    I have just started building a 10ft Spindrift from B&B yachts. The sail has a luff sleeve of 100mm wide, constant from foot to head.
    The plans for the mast calls for a round wood section that tapers from 56mm to 42mm dia. Attached is a sketch that shows what the resulting leading edge will look like at the foot and head of the sail.

    Is there any merit in altering the mast section from a simple round section to something with a shaped leading edge profile, either parabolic or approximating the leading edge of some airfoil? I am only really interested in improving L/D, not so much on boosting CL-max.

    I have the option to either fix the mast to the hull and just let the sleeve rotate about it or let the mast rotate with the boom (as on the Finn).
    Could the blunt diamond section that Arvel Gentry developed for fixed masts perhaps also work with a sleeve?

    I have some glass braid and carbon UD tape so making the mast a little thinner is also an option.
     

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  2. PAR
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    PAR Yacht Designer/Builder

    Drop Graham an email or give him a call. He's approachable and knows the design well.

    A round mast will have uniform bending moments, while a tear drop or other shape will not, making getting uniform sets across all points impossible, if the mast was fixed. If it was permitted to rotate, bending moments will still vary, but because the bend would (relatively) come from the same angles, less a problem. If you elect to re-dimension the mast, you should do the math in both diameter and sectional area, to insure it's strong and stiff enough, with a reasonable margin, but able to take advantage of the weight reduction potential.
     
  3. Will Fraser
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    Will Fraser Senior Member

    Thanks for the reply PAR,
    I realise that there are a number of structural implications, at this stage I just want to establish if the concept has aerodynamic feasibility.
    The luff on foiling moths look like they provide beautiful leeward tangency from the mast round to the sail - do they use profiled masts or is the shape simply achieved through a wide chord on the sleeve?
     
  4. PAR
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    PAR Yacht Designer/Builder

    The Moth masts are all over the place in the approach and methods they employ for orientation and construction. The SpinDrift you're building wouldn't be able to take much advantage of an improved leading edge treatment. A rotating mast with the sail on a slide or a fixed mast with a luff sock will do well, given the limitations of the design. I'm fairly sure Graham intended the mast taper to save weight more than entertain sail top washout or other bend and set considerations.
     
  5. CT249
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    CT249 Senior Member

    I've been sailing with sleeve luffs since the '80s, as well as with wing masts. On a boat like the Spindrift, the amount of performance you'd gain by getting a luff sleeve working perfectly would probably be about as much as you'd gain in the 26th week of doing hill climb repeats on a bike to improve your hiking, or during your 18th session of intensive roll tacking training if you were sailing 5 days a week for 6 months. In other words, the aero improvement would be so minute that you would only notice it if you were sailing the boat better than about 98% of people had sailed when they were having the best race of their lives in a class they had been sailing for a decade.

    A week or two ago at the nationals, the mid-fleet guys in the national titles of the world's biggest foiling Moth fleet seem to be finishing about 1.6 laps when the top guys are finishing 3. Many of them are using the same rigs and boats - and the mid fleet guys would have been training. Obviously the Moth magnifies skill difference enormously, but the point was that the aero efficiency of such rigs would normally be basically completely submerged by the skill difference of the users.

    In Raceboards, which are the most popular class that uses big sleeve luffs, you'd probably have to be comfortably in the top 30 in the world to have the faintest idea of whether any performance gain or loss was down to your sleeve luff or your sailing.

    I think the Moths, like the Raceboards and Formula boards, just use a small diameter round tube, but I haven't sailed one for eons.
     
    Last edited: Jan 18, 2018
  6. Erwan
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    Erwan Senior Member

    If carbon is not prohibited, according to the diameter involved, try to look at windsurf masts
    typicaly they have 55 mm diameter at the bottom and around 30 at the top for a 5.20 m model and weight less than 2.2kg, second hand one are available for cheap.
    Just an idea, in addition old windsurf sails could be recut and used for your boat.

    Best wishes for your project

    Cheers
     
  7. Will Fraser
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    Will Fraser Senior Member

    Does the Spindrift design perhaps offer any other "low hanging fruit" that could benefit from some refinement to improve L/D, either aero- or hydrodynamic?
    It is not speed I am after but the ability to point really high. I don't race but sail in a river that requires lots of tacking.

    Thanks for the suggestion about the carbon mast - I happen to need exactly 5.2m - it would be nice to have something that light. I estimated the solid spruce mast as well as the 3-piece ali option to weigh around 4kg.
     
  8. tom28571
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    tom28571 Senior Member

    I see no advantage and possible disadvantages to a tear drop mast as PAR said. Unless the mast and sleeve combination approaches a true wing mast shape, the simple round mast is a good one. Aluminum Spindrift masts are specified in a smaller diameter upper section so that does tend toward a taper.

    I built and raced one of the first Spindrifts in frostbiting and many other events, usually racing against Graham. It is a great little boat and the perhaps the smallest dinghy that sails well with an adult male aboard. Although we sailed against many of the other 10 footers and some purposely built to beat us, neither of us ever lost a single race to them. Needing to carry up to 200lbs, it is not optimum for either speed or pointing ability. A bit too fat for either so I consider it folly to spend lots of effort to mess with a good boat. While you are thinking about the minutia, you will miss tacking on a header and drop way back in the fleet.

    Build it as designed, shape the foils properly and go sailing.
     
    Last edited: Jan 18, 2018
    Tom.151 likes this.
  9. Erwan
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    Erwan Senior Member

    Try to find a North Platinium serie available in 100% carbon for 2.2 kg.
    But for a little give-up in weight, you should find some cheaper one with only 60% carbon.
    I am sure the top of this North 2 parts mast weights only 785 g for 2.60m lenght because
    I got one just in case I find a good idea to use it for an A-Cat wingsail.
    And I made some flexion tests with it, by suspending a bunch of water bottles in order to mimic an elliptic loads and mesured the tip deflection (around 6 cm for 150N load elliptically distributed).
    Cheers

    Would you post a little pic of this famous Spindrift, it could be helpful to trigger suggestions.

    Cheers
     
  10. PAR
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    PAR Yacht Designer/Builder

    Erwan, are you remotely familiar with the SpinDrift 10? If you are, the idea of a windsurfer, carbon or other mast is simply nonsense. This is a well thought out little boat, but not to be confused with anything close to a typical Moth. She's well shaped, but on the burdened side given her intended role.

    If you want to point as well as practical, make the rudder with an appropriate XX section for it's chord and the same for it's centerboard, though a 63A series may do better, I doubt the hull form can take much advantage of it, so the XX section would generate more lift over a wider incidence range, if a little more draggy..
     
  11. Will Fraser
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    Will Fraser Senior Member

    Tom I am glad you chimed in. Only 200lb, you say? I am 230, so now you have me hesitating. I had just marked out the hull bottom and topsides earlier today. I will be building out of honeycomb sandwich, so I hope to knock off around 20lb of the stock weight.
    I chose the 10ft over the longer options because the beam fits nicely on my roof-racks. For me the sensation of speed at a high-ish Speed-Length ratio gives me just as much thrill as outright speed, so a short hull does not bother me in principle. But if I am too heavy for it to ever really get going then perhaps I should consider something longer.
    I had already started a thread on the Messingabout forum about a 13-14ft Core Sound option and Graham has indicated that it is worth looking at, but that will be a bit more of a cruiser. I still want a light minimalist dink with the option of car-topping, so the beam is fixed (1.28m). I already have the sail, so there is no point in adding more sail carrying capacity. It also means SA/Disp is fixed at 68sqft and roughly 330lb. How much benefit would and extra foot of length be?
    I have a very complete CAD model so I can stretch, scale and tweak the hull shape as required. I guess now is the time for me to dust off all those articles about important ratios, pull the calculator closer and sharpen a pencil...
     

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  12. Will Fraser
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    Will Fraser Senior Member

    At 330lb, these are the vital stats as measured on my model:
    LWL: 2.8m (9.2ft)
    BWL: 1.13m (3.7ft)
    Waterplane area: 2.4 sqm (25.8 sq-ft)
    Wetted area (clean hull only): 2.54 sqm (27 sq-ft)
    Wetted area (with foils, skeg): 3.2 sqm (34.4 sq-ft)
    Half-angle of entry: 28deg
    Quarter-beam buttock angle: 4deg (buttock tangent at transom: 6deg)
    Deadrise: 9.9deg (midship), 10.4deg (transom).
    Prismatic coefficient: 0.63

    s10 hydro side.JPG
    s10 hydro top.JPG
    s10 hydro1.JPG
     
    Last edited: Jan 18, 2018

  13. Will Fraser
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    Will Fraser Senior Member

    The only dinghy-specific metrics I could find was more focused on sail-carrying capability, max beam for hiking power and low wetted area. It had no metric that took lwl into account, so clearly it is aimed more at hardcore racers with big asymmetric spinny's etc.
    I tried to compare SA/D and D/L ratios with the likes of the Laser, RS Aero, RS Tera and my own heavy 13ft Gypsy (a local class).

    With me on board, the Spindrift has a SA/D of 22.3 and DLR of 190. The short LWL makes it impossible to even approach the DLR values of all the longer boats. If I go on a bit of a diet the DLR will match that of an RS Tera Pro with a crew at the heavy end of their recommended weight range, although SA/D for the Spindrift will actually be better (the Tera Pro with 70kg crew has SA/D of 18.7)

    Another interesting metric that I found in an article from BCA Demco is called the Hull-planing-index = weight [kg]/(LWL[m] x Waterplane Area[sqm]). A value of less than 15 is desired for easy planing. The company is a key player in generating designs for the Italian Diecipiedi (i.e. 10ft) class. The point he makes in the article is that it is very difficult for a 10ft-er to plane with an adult. The Spindrift's index is 22, compared to 14 for the Tera with a 50kg crew and 10 for a Laser with a crew of 70-80kg.

    I think I will build the Spindrift as is. If it is a real dog with my weight on board (which I doubt, given our typical wind strength) I can always use it as a club trainer for the kids to sail two-up. It will also make a perfect intro boat for ladies, the sail even has a reef-point to keep things docile.
     
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