From winged keel to bulb keel

Discussion in 'Sailboats' started by Mad for sail, Nov 15, 2011.

  1. Mad for sail
    Joined: Nov 2011
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    Mad for sail Junior Member

    Hi Mikko, it´s been a while!
    The data in the table came from the Wing32 / Vacanti SW which I think is Ok for Hidrostatics but the L/D calculations are based on a kind of "average chord"so it´s very likely that my L-keel has not been "fully understood" by the software. I read carefully a Thesis from Chalmers University written by Kasper Ljungqkvist named " Shape optimisation of an integrated bulb-keel " where he describes how he has improved a given L keel aided by CFD analysis like you did for me in the past. Unfortunately several details about the geometry are not available so although my keel "seems" to follow m,ost of the guidelines it´s very difficult to assure that the performance will match the Vacanti´s simulation. I have been making a benchmarking on sailboats the same size as mine and there are some common values I have used to define my keel. What I got from Ljungqvist work is the use of 63xx serie on root chord, 65xx on the bulb tho allow the bever tail and a rounded bottom which was actually one of the main keys for the performance improvement. If I confirm the shape I have them to perform some finite elements analysis to check the robustmness of the solution. There are 3 main possibilities: full lead, ful lead with steel strut core and steel fin plus leab bulb.
    I have the IGES file available so any chance you simulate this keel on CFD like you did with the other ones?
     
    Last edited: Feb 16, 2012
  2. Mad for sail
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    Mad for sail Junior Member

    Just in case somebody wants to see it in detail ...
     

    Attached Files:

  3. Andy
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    Andy Senior Member

    How about some pics of the boat? Btw, a couple of boats up here have improved their light air performances significantly by spraying the bottom of the hull, rudder and keel with hi-build epoxy primer and longboarding the lot to a very good finish, then buffing a hard antifouling over the top. It's always worth fairing foils (esp on older boats) to templates as very often they just weren't very accurately made in the first place...loving this project though!
     
  4. Joakim
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    Joakim Senior Member

    Your new keel design looks good to me. I would choose the full lead option for several reasons:

    1. Lesser change of displacement
    2. Lesser change of righting moment (the lighter L-keel likely has lower RM than the original)
    3. No problems with rust
    4. Tolerates groundings better
     
  5. Mad for sail
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    Mad for sail Junior Member

    I fully agree. On top of that it´s far easier to build it with lead than in Iron. Next step is to proceed the strut analysis as the new keel has shorter and thinner root chord . All I know about the flange is the bolts position but thick, wide and length are totally unknown. The risk area will be the joint between the fin and the flange so the idea would be to finish the keel mold only when the current keel is disassembled.
     
  6. Mad for sail
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    Mad for sail Junior Member

    Andy, she was built in the 90´s so when I bought her the hull had blisters which required removal, insulation with epoxi and later fairing and anti-fouling preparation. The area where I have my boat here in Brazil has weak tides and high average temperature during the year so the "sealife" grows very fast requiring hull cleaning every 5 weeks. Attached some pics from the prep.
     

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  7. Andy
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    Andy Senior Member

    Nice boat - was she meant to be a larger sister to the Spring 25? Looking at the hull, there are a few things you could do there to get her slipping along more easily. I'd move all those skin fittings to above the waterline and plug the holes. Then get rid of the permanent anodes and use hanging anodes (you chuck them over the side when you're in the marina, and take them aboard when your racing). I can't really tell from the photos what the surface fairness is like, although small wiggles can make a big difference. A good folding prop is always helpful (can't see what you have?) and the aft end of the rudder should be fairly fine with a flat of only a couple of mm. All these things help a boat in light airs, but your new keel looks good for performance, and as Joakim said it would be best made from lead. Looking forwards to seeing this done!
     
  8. Joakim
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    Joakim Senior Member

    I don't think skin fittings are a real issue and worth the trouble moving them. I made some calculations last year. Unfortunately this is in Finnish, but you can see the pictures and values: http://www.avomeripurjehtijat.fi/index.php?option=com_docman&task=doc_download&gid=395&Itemid=250

    A rather big skin fitting has about 1/10 of the added resistance of a 7 mm thick trailing edge a keel and less than 1/1000 of the total resistance of a 35' boat at 6 kn.
     
  9. Mad for sail
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    Mad for sail Junior Member

    Have you ever tried to compare a sail drive and the traditional axle+propeller (I don't know the names in english)?
     
  10. Joakim
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    Joakim Senior Member

    No, I haven't looked at them at detail. However the ORC rule has formulation for drag of those depending on dimensions and based on measurements. If the shaft angle to the flow (local keel angle in the rule) is small, it has about the same resistance as a SD.

    Look for the formulas for PIPA and then drag: http://orc.org/rules/ORC VPP Documentation 2011.pdf

    Propeller is another issue. The drag of a typical fixed propeller is much more than the formulas give in order to avoid using drag optimized fixed propellers. Also some folding propellers have much more drag than the formulas predict. E.g. Volvo Penta curved blade propellers have a huge drag for a folding propeller and all three bladed (folding or feathering) propellers are bad for a shaft installation, which always has an angle to the flow, thus it is impossible to streamline three blades.
     
  11. Mad for sail
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    Mad for sail Junior Member

    What abiut two rudders instead of one? My boat has two short rudders so how the position of them out of the centerline afects the performance? In other words how much of the rudder performance is lost because rudder is aligned to keel and propeller? Additionally what difference it makes to pisition the rudder under the hull or out on the transom?
     
  12. Joakim
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    Joakim Senior Member

    That's a difficult question and there are certainly better knowledge than mine on this forum about it.

    The rudder needs to take part of the total hydrodynamical lift force and it must be able to steer the boat well enough. Thus it must be able to produce much more lift than needed for sailing straight ahead in (almost) any situation. For that purpose a deepest possible rudder is the most efficient one and the deeper it is the less surface area (and drag) it needs. Thus for upright sailing one deep rudder is clearly better than two shallow ones.

    But things get more complicated when the boat heels. Having the whole rudder in the water is more efficient than having part of the rudder in the air. It is even better to have the rudder sealed by the surrounding hull, which eliminates air entrance to the suction side of the rudder and acts as a mirror, which doubles the effective span of the rudder.

    Also heeling may lift the other rudder out of the water and thus there is no drag from it.

    For these reasons more traditional hulls have one deep rudder and very wide transom hulls have two not so deep rudders. Your boat is a more traditional design and it probably has two rudders just for the sake shallow draft and maybe even for balancing the boat while standing on the ground during low tide. It most likely would benefit from one deep rudder.

    You must really like your boat for even thinking about all these big changes. Wouldn't it be easier just to sell the boat and buy one with a deeper keel, one rudder and a SD?
     
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  13. Mad for sail
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    Mad for sail Junior Member

    You must really like your boat for even thinking about all these big changes. Wouldn't it be easier just to sell the boat and buy one with a deeper keel, one rudder and a SD?[/QUOTE]

    Well I actually love this boat but the reason for all this questions is that I still have 10 years to my retirement and definetely this won´t be my last boat so I really want to have all my doubt´s cleared before moving to the next.
    According to my calculations a new keel won´t be that expensive as I intend to build the mold and por the lead all by myself.
    About the rudders and shaft I can assure you I won´t change it! :cool:
     
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  14. Mad for sail
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    Mad for sail Junior Member

    some more pics from my keels ....
    I love those 3D printers!
     

    Attached Files:


  15. Mikko Brummer
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    Mikko Brummer Senior Member

    Nice! What size are the models, like how many cm is the deepest keel? Where did you have them printed & how much did they cost each?
     
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