Forward Keel Location & Angle Questions

Discussion in 'Boat Design' started by viking north, Apr 9, 2011.

  1. viking north
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    viking north VINLAND

    In reference to my doodle drawing:nts&ntd (1) How important is the forward starting point of the keel. Would one foot one way or the other play a big factor when the area involved is but a small ratio when compared to the overall keel and underwaterprofile. (2) I read somewhere in keel design, there is an optimum sweep back angle for the leading edge, how critical is this angle in that, depending on the starting point of the keel I might have to alter it to re position my CLR in order to be flexiable on my mast's fore and aft location a little. (i'm trying to set up the rig with the forestay on a short anchor platform.) I used GFE(gut feeling engineering) on my other conversions and went with what looked good should be reasonably ok.(can't believe I just said that.)
    I know there's no easy answer to these questions as it involves just so many factors, Hull shape, Canoe hull (without the keel) under waterprofile, CLR , Cutaway fore foot verses coming about, and probably a few more that not even aware of. Like I posted on another thread (Can Anyone Design) a little knowledge is just as bad as too much- both strive for the impossible especially as far as boats are concerned. I know i'm over thinking here for a motorsailer conversion build but humor me and pretend it"s a racing motorsailer :D ( a new racing class) Ok here goes the post,Thanks--Geo

    P.S. R.E. "Can Anyone Design" new answer, "ya they can try'"

    A yacht is not defined by the vessel but by the care and love of her owner
     

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  2. BYDE
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    BYDE Junior Member

    what's the function of the 'cut out' in the keel? other than collecting some plastic bags... :)

    (1) & (2). no i wouldn't worry much about them as it's such a low efficiency design that will produce very small lift/drag ratios anyway
     
  3. philSweet
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    philSweet Senior Member

  4. Tad
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    Tad Boat Designer

    Oh boy.......What you're really asking (and I think you understand this) is how do you integrate a thousand variables into a cohesive working design? It can't be answered in one sentence or even a few........It's the "Black Art" of boat design......Some things work together and others do not.....you cannot change one thing (say the keel) without affecting the rig, and the rudder, and these are dependent on the hull shape, which controls where the ballast goes, which affects the keel shape.......round and round......

    To answer your questions....

    1) It's important yes......One foot either way is a big deal, yes......

    2) Read this... View attachment Keel and Rudder Design.pdf
     
  5. viking north
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    viking north VINLAND

    Byde-Got it on the low efficiency keel. Re the cutout, On my former builds, i built two very similar designs, both motorsailers that as typical are bit--es to tack. My first had a full keel(no cutout) and would come about only under ideal conditionds, most time i had to jibe tack. I did tried several mods/techniques and improved her ability but never to my satisfaction. On my second build i reasoned that if i provided a break in the aft section of the keel for the water to shoot thru there should be less resistance for the stern to kick around, thus easier to tack.It worked, the second vessel tacks much better. In my thinking it couldn't get worst and if it turned out to be a disaster i could easily fill it in, much easier than cutting it out. :) Just another bit of GFE (gut feeling engineering)
     
  6. viking north
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    viking north VINLAND

    Ya TAD knew that can of worms was there, gonna be alot of speed reading and seperating the chaff from the wheat i'm afraid mainly because i suspect i'll be dumfounded on what the he-- they're talking about but i'm sure the basics will find the grey matter--TNX. Geo
     
  7. viking north
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    viking north VINLAND

    Phill Always amazed me the Fishers could go to windward at all, almost no keel. I always figured their underwater hull form played a big role. I made an unsuccessful bid on a fire damaged 37 with the intention of installing a deeper keel and lower superstructure. Making her more of a sailer, would have been a good experiment. I'll read up on your reference-Tnx. Geo
     
  8. Tad
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    Tad Boat Designer

    As a start (preliminary design) I would relate the keel leading and trailing edges and the rig position to the ten station divisions of the waterline......but you don't know your waterline length because you don't know your final weight or volume?

    As a rule the mast in a masthead rigged sloop goes at about station 4, the root of a high aspect fin will go about there as well (station 4). A longer, lower aspect fin will have the root start about station 3 and the tip start at 4 or slightly aft of there.

    A ketch will have the mainmast at station 3 and the mizzen about station 8.

    Calculate CE with a geometric fore triangle loaded to 1.7 * 100% foretriangle area (I*J), CE should land somewhere between station 2.8 and 3.6 (roughly).

    All stations are equal, outboard rudders not included, zero is the intersection of the stem face and DWL.
     
  9. BYDE
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    BYDE Junior Member

    to gybe tack, haha nice :)

    ok, i understand your issue.
    with the cut-out you also lose roll-damping capabilities, one of the few advantages of such a long keel, but maybe that's not important for you.
    it also makes a lot of turbulence, hence drag and a less clean water flow to the rudder.
    if you really want to do a cut-out, then I'd do it at the aft end (not in the middle) of the keel. Because further from centre will be more effective in easing the tack.
    You'll need to re-think a bit of you your rudder design, but nothing difficult to do. Moreover being more effective you'll need a smaller cut-out
     
  10. viking north
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    viking north VINLAND

    TAD I'm fighting for a final displacement of 10,000 but can live with 12000max. I have hired a designer and his wonderful computer program to help me with the difficult calculations of the magic numbers as i call them, CB, CLR, and so on but I have to tackle this keel design and supply the dimensional info to him. Basically it is suppose to be just a full slab sided keel but I'd like to make the most of it, He is going to provide me with three waterlines, a working, one at 10,000lb and one at 12,000lb diplacement.Once i get that info and combine it with the info you have posted plus other posts I should be able to work this thing thru. My hope is to put a bit of foil into it and i can only do that by breaking it up and incorporating some sort of skeg into it. Right now i'm just happy to get a starting point, one step at a time, --appreciated--Geo.
     
  11. viking north
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    viking north VINLAND

    Byde- that was just what i call a doodle drawing as on a napkin at a restaurant, but byde awhile :) i'll need more detail input on that shortly as you've probably already surmised on my reply to TAD. thanks--Geo.
     
  12. PAR
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    PAR Yacht Designer/Builder

    The hunk out of the after section of keel is what Ted Brewer calls a bite. It would be more effective if the bite was turned into a skeg, possably with a bridge at the top for a shaft. It would also be nice if you could place the prop in front of the skeg, rather then an aperture. To be honest, set your SOR up with your designer and let him make the decisions about appendage entry angles and shapes.
     
  13. viking north
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    viking north VINLAND

    PAR the interior layout,(engine location) the existing shaft log, all dictate the prop in front of the rudder but it's looking good that i will not need a cutout. I agree 100% on the bite being closer to a skeg and thats what i'm working toward. It will allow me to possibly develop some foil shape in the keel . I know for a conversion into a motorsailer this is alot of overkill but maybe be my last chance to play and i'm enjoying it thanks to all. It would be easy to pass it on to the designer but then i wouldn't have the fun and learning. He's a great guy and understands i want to have a go at this part of the project. The great plus is this is not a high teck machine and I'm not building for a customer. If I don't get it 100% right I probably will never know the difference,hey I can't get too far off base I have access to top notch people here, just can't to be shy or too proud about asking questions. :) Thanks -Geo.
    P.S. Mentioned to my good wife a few minutes ago, We've been in Eustis on about 5 to 10 ocassions (antique bike market) possibly drove past your place, small world- My sister lives in Deltona did some boating on the St John River.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Yes, a surprisingly small world. Next time you're in town give me a yell, I'll buy you a beer.

    Don't bother with foil shapes on such a low aspect appendage, you'll not see significant enough advantage to warrant the trouble making them. Do your best to dived the appendages into separate skeg and fin arrangements. Maneuverability will be greatly rewarded with the bother.
     

  15. viking north
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    viking north VINLAND

    That sounds like good advise to me and the road i'm most likely going to be forced to travel due to the dimensions of the existing keel surface i have to join up with. Some experimentation on blending in will be in order. I'm going to play with it over the next month and see what evolves, the kiss principal will be my guiding light--and by then i'll need a break and a beer :) tnx. Geo
     
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