Questions On How Keel Design Affects Lying Ahull Or Heaving Too

Discussion in 'Boat Design' started by viking north, May 2, 2011.

  1. viking north
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    viking north VINLAND

    I'm into the actual building of the male mold for my keel and still have time to modify the forefoot and cutout sections. While working on this it suddenly dawned on me that these two design features not only play a role in less wetted surface and handling but also in the two important storm tactics of "Lying Ahull" and "Heaving To." Two storm options very important to long distance deep sea sail powered crusing vessels. So: Query#1, How much does the size of the cutout between keel and skeg affect, (A)"Lying Ahull" and (B) "Heaving To". Query #2, How much does the cutaway forefoot affect (A) "Lying Ahull" and (B)" Heaving To."-- Tnx. Geo.
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    #2 (A and B) Deeper forefoot the better, that is IMHO
     
  3. viking north
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    viking north VINLAND

    Thanks TeddyDiver-- had a suspicion of such- going to hang by here and see what develops in further postings--Geo.
     
  4. tom28571
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    tom28571 Senior Member

    Having hove to in full keel, 3/4 full keel as well as fin keel plus separate rudder with and without skeg, there is a lot of difference. Full keel is the best in allowing the bow higher into the wind. Fin keels are not so good with some heaving to with the bow well off 90 degrees and drifting much faster than full keels. I'd say the gap between skeg and keel is less important than the area of the keel and its relative position. Relative longitudinal position depends on the sail plan CE and mast position of course and which sails or none are employed when heaving to.
     
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  5. viking north
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    viking north VINLAND

    Thanks Tom- I suspected that that would be the case and there's nothing like info from a person that has hands on experience of performing the same manouver with the three standard keel designs. I've searched everywhere for basic design criteria on these modifications but it seems to be all hit and miss with no "general" rules. These are the studys i'm looking for:
    Example (A): Forefoot Area and Modifications: (sloop) If one were to draw a straight line from the natural run of the bow to the beginning of the deepest part of the keel a forefoot area would be enclosed. Let this be our reference area. Any downward curvature added below this line would be classified as "Positive Forefoot" and the amount can be determined as a ratio to our straight line Reference Area. Like wise any area cut away reducing the straight line Reference Area could be classified as Negative Forefoot and again labelled as a ratio. Thru experiments using the same vessel and rig one could determine the pro's and con's of each.(lying ahull and heaving to)
    Example (B): Keel-Cutout-Skeg ratios, again one could set up a Reference Area, in this case an uncut full keel on a sloop rig and work the experiments from there developing the pro's and con's of their affects on the same.
    Is anyone aware of such studys having been carried out. I can't believe there hasen't been in the evolution to modified full keels.-- Geo.

    P.S. Likewise I still can't find any info on a prev. query on the pros and cons of full keels with and without taper in their "cross section" from a previous thread. One would think detail info on this and the above query would be a part of basic study in boat keel design.

    A yacht is not defined by the vessel but by the care and love of her owner.
     
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  6. tom28571
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    tom28571 Senior Member

    I suspect that the only people interested is such criteria would be those making long voyages offshore with a short crew list. Much more attention is paid to the areas of speed and other sailing qualities. Some dependence on drogues off the bow may be uses instead of greater forward keel area. I've never been in survival mode offshore and only heave to for rest or other short term single hand uses like repair or sail changes or eating lunch between races. My current small cat ketch with deep centerboard and transom rudder heaves to with the bow pointed up wind much higher than the normal tacking angle. Aft drift is probably faster than abeam mode but the control is amazing.
     
  7. viking north
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    viking north VINLAND

    Exactly Tom, long voyages- short crew- the prime definition of a "Cruiser". Somewhere some place there has to be info on this as I can't believe keel modification guys like Herreshoff, Stephens, Brewer and a few more didn't experiment and keep records in some detail. Designers and N.A.'s must use some references in their work not just educated guesses. I have researched everything I can find on the subject and the info is little to none. Brewer comes the closest with a little general info but certainly nothing that a qualified person in the field would use as a reference. Someone out there has this old reference book stuck in their shelf and lo and behold there are the scribblings of full keel design and evolution from full to modified keel experiments -- Ok, who has this book, own up :) ---Geo.
     
  8. tom28571
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    tom28571 Senior Member

    Well, maybe you should just ask Ted what he thinks. He is very helpful. Since heaving too has mostly everything in stall mode, the fore and aft balance of areas above and under the water should give you a decent estimate of the result. Everything counts, sails, masts, rigging, hull both above and under water as well as the keel and rudder. These things may have different weighting depending on the shape as well as area. Most rigging and halyards have very poor aerodynamic shape and thus, a lot of drag, more than we might suspect.
     
  9. viking north
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    viking north VINLAND

    TOM, Possible I am overthinking the forefoot modification (would be typical of me) so based on TeddyDivers post, Your experience and the info in Ted Brewers, "understanding Boat Design" 4th edition page 38, I have continued with the mold build maintaining what I would classify as a traditional looking forefoot as one would see on a typical full keel vessel. I understand it will increase wetted surface but the trade off for good "Heaving To" qualities is well worth it. However I'm still curious if there have been actual detail studies done on this, it would make interesting reading. Thank you all for taking the time to post replys that help an inquisitive mind come to logical decisions and of course Ted's book included. :). As for the keel, cutout, and skeg ratios, after more research reading and talking to myself constantly I have come to the conclusion it might have to do more with wetted surface, volume for ballast, aspect ratio , and 100 more design criteria rather than actual size ratios, so I'm just going to make a nice long keel for good directional stability, an ample cutout to allow easier tacking (getting her stern around) and a moderate skeg. Hey maybe i'll invent a new underwater racing profile :p Thanks again Geo.
     
  10. u4ea32
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    u4ea32 Senior Member

    I think deadrise has more impact than keel and rudder area. More deadrise is a good thing in survival conditions. No deadrise is a bad thing.
     
  11. viking north
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    viking north VINLAND

    My hull has low to moderate deadrise, however within the full keel family of appendages it should be a reasonable survivor.Hope the he-- i don't have to find out too often but being a past open atlantic small boat fisherman i sort of take it in stride now. As i've told members of my crew in the past, panic is not an option, praying is acceptiable, :)--Geo.
     
  12. tom28571
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    tom28571 Senior Member

    There is certainly a case for that view but there are also many who will argue for shallow draft and less area down below in survival conditions. I listened to Sven Yrvind expound on the shallow draft view as a survivor of numerous difficult voyages in small boats. The idea is the the boat should be able to slide rather than attempt to stand up to a hard blow. Of course that is not too useful against a lee shore.
     
  13. viking north
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    viking north VINLAND

    The theory is that by sliding the hull is less subject to being tripped by a beam sea. I did have some experience of this in my alum. 26ft. standard ships lifeboat full keel conversion.Got caught out in the tail end of a hurricane running 17 to 20ft breaking seas. Was doing great on storm jib and engine just ticking over but had to make a port tack to head into the bay entrance and got nailed by a big confused sea on her beam before i could get on my new heading. next thing I knew she slewed around and i was heading from whence we came. :) I think most deep full keel boats would have been tripped but I really feel that broad beam and flat bottom saved our bacon. Will not be so lucky with this new build she is old style long, narrow and not flat bottomed-The difference between a standard lifeboat and a surfboat style, however I'm making every effort that she'll be much better under sail ( to windward)
     
  14. tom28571
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    tom28571 Senior Member


  15. viking north
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    viking north VINLAND

    Thanks, gonna have a read on that tonight. Did some good research last night(like a dog with a bone) dug into my clipped magazine page collection on keel info and came up with a David Vacanti article that factors in" Keel Taper". Finally! I knew it played a role but man it was elusive. It's looking like my suspicion that "Taper" plays a role in lift and drag regardless of aspect ratio is on the money. I will present the formulae and graphs tonight and see if my reading of them is correct--Geo.
     
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