keel fin design,foil shape and surface area for a given amount of sail area.

Discussion in 'Hydrodynamics and Aerodynamics' started by RumnCoke, Jun 5, 2018.

  1. RumnCoke
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    RumnCoke Junior Member

    I did check the discussions and found a handful of them on the subject.
    I now have a US1m design completed. The boat is not focus of the topic here, but I will be designing my own fin/keel for it.
    You intelligent gentlemen are able to discuss many topics with Math and Algebraic equations, I am not, for lack of education in those areas. I can discuss my perceptions and ideas on a lesser level.
    I can have some aluminum plate CNC cut to achieve my profile shape, foil shape and thickness ,and use a mechanical pressure process with composite materials. It would be somewhat easy to just buy some of the current fin offerings for models, but they do not offer specifically for this boat class so it would be a compromise at best. In decades gone by there were men who made keel fins for this boat class and several boat designers and builders. They have since sailed away into Heaven.

    The boat is one meter in length, max sail area allowed is 600sq in divided between main and jib. Max keel depth from the keel line to the bottom of the Bulb is 14.25 in. Back in the day, the general consensus was 32 to 34 sq in. of surface area on the fin from keel line to the top of the bulb. The boats generally compete in winds from 1 to 10 knots. Anything more and we just pull our boats from the water.

    Now, the first objection for discussion. Todays fin offerings have nice efficient NACA profiles. They are made from Compressed Carbon Composites, but that is where my agreement stops. They are made for an International One Meter boat that can carry 800 sq in of sail area and a lead bulb of around 5.5 pounds minimum. The surface area of the fins are around 39 to 42 square inches. They are also held to the 14.25 fin depth from keel line to the bottom of the Bulb. They are simply a rectangular chunk of carbon, dimensionally the the same from top to bottom. Since getting back into this development class and talking with some of the knowledgeable skippers, I have decided that their current perceptions are not optimal for a few reasons. At least my reasons.

    Okay, most everyone here knows how a boat is propelled through the water, its not a true course. The fin acts as a stabilizer/foil against the side force of the sails on the boat. The boat still has leeward direction. True course vs course sailed. This is where I disagree with the current side profile shape. A rectangle with a bulb on the bottom vs a trapezoidal shape with a longer chord at the keel line and a shorter chord ending down at the bulb. Two things. It seems to me that having more surface area up at the keel line where it can interact with the hull as a deterent of leeway and to a better efficiency against healing moment. Having a smaller area down at the bulb to prevent fin lift dynamics from defeating the weight of the bulb and hence leverage against the sails. This will raise the center of lateral balance into a higher position and create better performance ( lesser healing moment by degree). For those of you who would argue that it doesnt matter. Having more pressure up at the hull and less pressure down at the bulb does create better leverage and a more vertical sailing angle. Who cares if the fin stalls near the bulb. They dont really "stall" anyway. Im hoping for an expansion of the discussion and more clarity. From my sailing experience, changing bulb weights with the same fin on the same boat with the same sails is going to increase/decrease the healing moment for a given wind velocity. Having a longer chord at the bottom cannot be an advantage. Prove me wrong or agree with me or something in between, I am here for more understanding. That is my first objection to current status quo in this class.
    Second objection would be that the actual frontal thickness of the fin, usually at the max chord depth is also way overdone for this class. Also there is very little taper if any at all in the current fin offerings.
    They start at around .250 in at the top and may taper to around .200 or .190 at the bulb. The idea of the importance of deflection has been (at least to my experience) over developed. The idea of a static weight placed somewhere mid length of the fin does not accurately define the characteristics in water under way. In practical application a small amount of deflection vs no deflection has less importance. The US1M will carry a maximum weight bulb, at around 4 pounds and not many instances of sailing require that much ballast. The boat design has developed in that class, if you can call it development has been to create "missile" or "torpedo" hulls that can barely carry the max sail area allowed, the newest designs cannot. They are simply overpowered and have to reduce sail area by around 25 sq in. The beams are (to me) too narrow and the hull displacement is so small that they cannot carry enough ballast in the bulb to keep the boat in a good performance envelope. The Idea of less hull and deck = less weight, I think is has gone too far. On the other hand they do very well downwind in light air, but any moderate or heavier air the bow dives and the boat stalls. Not a good result, since it is repeated over and over on the downwind leg. Meandering, sorry.
    The fins do present a frontal profile drag and overall surface drag. Their depth by scale to large boats is very deep. If you have more surface drag down at the bottom and overall more frontal profile drag, this will add to the leverage of the sails forcing the bow down into the water on the downwind leg and just more drag overall. So, my solution is to just go ahead and pay for the CNC work for a compression mold and make my own fins for this class. Just as easy to make a trapezoid profile as a rectangular one. From my experience, something starting at .200 at the top frontal and tapering to around .170 at the bottom. Side profile, top 3 in., and bottom 2 in. This should give me around 32.5 sq in surface area considerably less than the fin currently being adapted from the International One Meter class at 39 to 42 sq in of surface area. There will be more flex in the fin but there will also be considerably less weight in the bulb.
    I am asking for opinions, clarity, or additional info.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Have you studied Lester Gilberts Radio Sailing site? It is one of the best technical sources for RC sailboat design. Lester Gilbert's Radio Sailing http://www.onemetre.net/
    You might also checkout Graham Bantocks site-Sails etc. Bantock once told me that the most critical dimension of an rc fin keel is the T/C ratio(thickness chord ratio). He said about 20 years ago that the thinnest is best-around 6%. He said then that if the chord had to be longer to make the fin strong enough in bending, that was ok because the gain from a thin fin was huge. In my personal experience thin seems to be the way to go......
    SAILSetc http://www.sailsetc2.com/store/ (Note the Downloadable Documents section)
    Good Luck!
     
  3. RumnCoke
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    RumnCoke Junior Member

    Well thanks for your input Mr Lord. I believe both are Europeans and design for the "International One Meter". I have met and purchased materials many times from Graham Bantock over the years. He is a huge credit to the sport. I will check out Mr Gilberts site just for curiosity sake. I have used thinner fins in the past and this experience is what tells me it is a larger factor than a small amount of deflection given up. Six to seven percent on the chord is the consensus these days. Guys are still testing. My main beef is for thinner than is being used and for a trapezoidal profile vs the rectangle now in use. A lot of difference between the two classes.

    To me the IOM class has some big problems within its rules. They stick participants with using antiquated materials for hull and mast/boom stock, under the guise saving money and making the class more accessible . Rules drawn up back in the 80s I believe. They should not be etched in stone, to my reckoning. The irony these days and for more like 20 years, or longer, is the fact that the boats are horrendously expensive to buy which by the way, is what most people do. The aluminum masts cost the same as a Carbon Composite mast. Boom stock and hardware the same. They use the same advanced layup techniques regardless of materials. The minimum weight on a 1 meter length, is like trying to make a bowling ball fly, eight and a half pounds. The rules committee is about as flexible the PGA rules committee, less in fact. Until they get over themselves and step into the 21st Century, I know I will not sail the class.
     
    Last edited: Jun 5, 2018
  4. RumnCoke
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    RumnCoke Junior Member

    Ok, Mr Lord, I do apologize for any brashness in my comments. It seems Mr Gilberts site has some interesting discussion very close to the actual point of the question I wanted to raise. They discuss keel chord shape as related to surface drag and percents of Leeway. It was the added comments at the end of the discussion brought my query closer to light, but did not proceed further. Given that any one achieves their optimal compromise related to fin thickness and some NACA chord profile I would then extend the point of efficiency to include surface side profile, not just chord profile. It was discussed briefly as a trapezoidal or the ever conceptualized elliptical shape as an optimal profile for loading. It seems to me that raising the longitudinal center of effort in the fin and having more lift up closer to the keel line of the boat would be a lot more optimal. Again, a simple diagram with a simple mathematical equation in the Documents section at Sails etc. aids to my point of discussion and gives me confidence in defining my own shape conclusion. Since I am building my own keel, I can make myself happy to build it 'my way". As we all know, it requires a successful marriage of many dynamic factors to achieve "the winner". Thanks for your input Sir.
     
    Last edited: Jun 10, 2018
  5. Doug Lord
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    Doug Lord Flight Ready

    Thanks. No apology necessary........
     
    Last edited: Jun 10, 2018
  6. Doug Lord
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    Doug Lord Flight Ready

    Thanks! Looks good-have you raced it yet?
     
  7. RumnCoke
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    RumnCoke Junior Member

    No, I only raced its predecessor, from 25 years ago. I do have a box full of trophies, plaques and ribbons to massage my pride lol.
    I kept as much of the parabolic power curves as I could. Therefore the shape in general is very similar. The displacement has been reduced from 7.5 lbs to 5.75lbs. The beam has been reduced from 8.0 to 7.0 and the aft section in the last 30% has been reshaped. The boat had been originally designed to plane up on its wake during optimal speeds. Unfortunately, the weight, hull length, and sail area were not optimal for such a design. The power curves aft, have been improved to generate much less drag. Approximately 8 percent less drag in flow tests. I need to get a precision mold made that will facilitate the vacuum infusion process. I can hand them a thumb drive with the correct file type to work with CNC. Let them figure everything else out.
     
  8. Doug Halsey
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    Doug Halsey Senior Member

    Where?
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Thats weird-it was there yesterday!
     
  10. RumnCoke
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    RumnCoke Junior Member

    LOL, Doug, I took it down. I suffer from RC Design Paranoia.
    Today you can take a picture of something and the computer software available today can recreate very accurately. With the Schematics I were showing, it would be a snap. One meter down the centerline and all the other contours fall into shape and definition.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    David, I'm sorry you felt you had to do that. I seriously doubt anyone would steal your lines plan-at least on boatdesign.net. Oh, well......
     
  12. Doug Lord
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    Doug Lord Flight Ready

    David, trust your own judgement! I don't think anyone would steal your design but, again, its not my work so.........
     
  13. TANSL
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    TANSL Senior Member

    I think I've seen those forms before in this forum. Is that possible?
     
  14. Doug Lord
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    Doug Lord Flight Ready

    =====
    Unless it was the day before yesterday, probably not.........
     

  15. RumnCoke
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    RumnCoke Junior Member

    @ Tansl, I believe I sent you the calculations for my original design, but you had encountered some difficulty in the translations. I was approached by a few others and one in particular was able to accomplish what I was looking for. What you see is the now reworked version of those old lines.
     
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