Fin vs Full Keel Revisted

Discussion in 'Boat Design' started by MichaelinSM, May 9, 2011.

  1. MichaelinSM
    Joined: May 2011
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    MichaelinSM New Member

    I’m new to this forum and want to start out by saying that I came to it quite by accident through some google search or other and have been fascinated every since. I am very impressed by both the quality and quantity of the posts and wish to thank you all for making such extraordinary efforts and contributions available to the general public. Truly wonderful!

    I am curious about an issue that has come up many, many times before but, as I look around now, not lately and since technological advances in boat design have come a long way in recent years, I thought I would bring it up again… and that is the debate about fin vs. full keel in the context of seaworthiness. Even Marchaj’s most recent edition of his excellent work, Seaworthiness is now fifteen years old.

    I am planning the construction of a 100 foot cruising sailboat for world cruising under most conditions including the high latitudes and although I’m not a professional designer, I will be involved in the design process and hope that makes it OK to post here. My priorities in the design are seaworthiness, sea kindliness and habitability with safety in mind. I have previously owned a 16 meter, 40 ton steel traditional Jongert classic ketch with full keel at the time that I experienced my first Force 8 gale with gusts to 55 kts and felt amazingly secure through the entire 8 hours. Since then my family has owned a 22 meter modern Jongert aluminum sloop with centerboard that has made only one round-trip ocean crossing under ideal conditions and has never really been put to the test and a 29 meter steel Jongert sloop with a deep fin keel that has circumnavigated and was knocked down once with no damage or injuries. I’m inclined to go in the direction of a full keel classic ketch once more but am being told repeatedly by “experts” that recent advances in technology, materials, etc make it no longer necessary to go with old fashioned ideas to achieve the high level of seaworthiness I desire. I also somehow suspect that a more traditional full keel boat of a size equal to the 29 meter sloop with fin keel might not have been knocked down in the first place. Any opinions on this?
     
  2. Tim B
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    Tim B Senior Member

    The thing that most people forget is that it is only occasionally possible to consider the effect of design changes in isolation. The classic example is comparing the performance of two keels of the same planform and different sections. That's something you can do in isolation, as the relative differences will not change too much as you introduce the hull and bulb etc. However, if you change the planform of the keel can you still consider it in isolation?

    You must also remember that a long-keel is drawn almost as part of the hull, and therefore you have a very different shaped boat as a result.

    So basically the keel is doing three jobs... It's damping the roll motion, it's providing righting moment, and it's producing side-force allowing you to go upwind. Simplistically, the only thing you gain by making the keel longer is damping. By moving toward a fin-keel (not necessarily a racing-type), you can improve the efficiency with which you achieve a given righting moment, and also improve the lift-slope of the keel, meaning less drag from the hull going upwind. Hence a move toward a fin-keel brings performance.

    Your tradeoff is seakeeping and performance. You may want to do some simulation work to find where you want the balance to be, and you may find you need to tank-test three or four designs to get some solid data for comparison.

    Personally I would always go for performance, but that's just me.

    Hope you get the boat you want,

    Tim B.
     
  3. Paul B

    Paul B Previous Member

    It looks like you are in Los Angeles.

    Do yourself a favor and take a two hour drive down the freeway and have lunch with Doug Peterson in San Diego. He has been designing for Jongert and other "Superyacht" builders for more than 30 years. He may have even designed one or more of your previous Jongerts.

    The advice you get from that conversation will be much more valuable than anything you will get from an internet forum.
     
  4. MichaelinSM
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    MichaelinSM New Member

    Thank you both

    Great idea, thank you.... and I'm pretty sure I can guess what he'll say. I was curious about other ideas though so I came here.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    And it was the knocked down yacht? I really wonder why..
     
  6. Stumble
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    Stumble Senior Member

    Personally I prefer a modern fin for cruising. And that is after spending years on a 2/3 keel. The reality is that the modern change is driven by a number of things; 1) Better upwind performance 2) Reducing weight of the boat 3) Modern material science has progressed to allow deep keels 4) I am sure I am missing some.

    To me the idea of a full keel is really an outgrowth of the old system of internal ballast. By shifting the weight out of the hull, more cargo space was opened up, and the boat tracked better due to the keel itself.

    As people realized that as the moment arm was moved lower the righting force was increased, without adding any more weight the keels got deeper, and weight went down. The problem of course is that this puts enourmous stresses on the materials the keel is made of, forcing the keels to be kept shorter than the ideal. Modern materials however have removed this limitation (effectively if not completely).

    I would make some real decisions about the areas I wanted to cruise in, and with a 100' boat those will be somewhat limited anyway, then design the keel depth around those parameters. Though I would never consider a bulb for offshore cruising.
     
  7. viking north
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    viking north VINLAND

    World crusing,: safety: sea keeping qualities and groundings, strength, ease of design and construction, ease of repair, ease of haulout and launch, extra stowage, ability for internal layout to lower the cent. of gravity, draft,the list goes on and on. A challenge: under these headings make a list of pro's and cons, your question will be answered hands down-- the full keel or modification there of, long keel/ skeg still rules as king. In a third colum under each list make a heading, survival for my family, (good)(acceptiable)(poor) While i'm not a designer I am a builder and a sailer with more than my share of rough water experience and while my old school thinking is a factor I still feel the fin keels are for the Dandys and their racing machines. :) (Marchaj is still the bible on this debate, science is still science, gravity and wave mechanics havent changed) ---Geo

    A yacht is not defined by the vessel but by the care and love of her owner
     
    Last edited: May 10, 2011
  8. MichaelinSM
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    MichaelinSM New Member

    I tend to agree but I think this is becoming the minority opinion these days. Thank you.
     
  9. viking north
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    viking north VINLAND

    Fin keels like reverse transoms, and reverse(forward) windshield slopes, have no place in blue water if one has a choice. Just as an example,Two of the most important storm life saving manouvers are near impossible to safely achieve with fin keelers--Running with it and Heaving to.--Geo.
     
  10. MastMonkey
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    MastMonkey Junior Member

    While I too like the features inherent in the full keel, I would think that at the size of the boat being planned, 100 feet, that there is much room for compromise. You could have a substantial, and long, fin keel on that hull. It could be large enough that it might function as much like a full keel in sea keeping and sea kindliness.
     
  11. Stumble
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    Stumble Senior Member

    Viking,

    While it is generally true that modern deep keels are not particularly good at heaving-to, there is no question that they excel at running in front of storms (Just take a look at the Open-60, Open-40, Maxi-Sleds, ect...). These boats can maintain speeds in excess of 15kn for hours at a time, which is actually fast enough to outrun strong storms. Now I certainly don't advocate this type of boat for an off shore cruiser, but criticisms should be factual.

    On a 100' or so yacht average speeds of 300miles a day should be expected in reasonable wind conditions, which are sufficant to run from most large storm systems, and certainly hurricanes. Meaning that the ability to heave-to while perhaps desirable is no longer as critical as it was years ago. In adition as Steve Dashew pointed out years ago, being able to average 300 miles a day provides tremendous options for weather routing.

    By comparison a full keel boat must carry tons more weight in order to have the same righting moment. On a vessel this large the weight difference between a full keel and even a shallow draft fin is likely to be measured in the 10 ton range. This eliminates the ability of the vessel to maintain high daily average speeds, and increases the likelyhood of not being able to outsail storm systems. And this comes at the advantage of? With modern materials (ie aluminium, lead, and steel) there really isn't much of a downside to a fin other than some increase in draft, but even that is pretty marginal.

    I would certainly get in touch with Doug Peterson, but I would also recommend contacting Steve Dashew about Beowulf his 80 ketch that still holds a number of crossing records (even against fully crewed boats) with just him and his wife aboard. And was build beyond any classification society's requirements.
     
  12. viking north
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    viking north VINLAND

    Not really critisims Stumble just an opinion and as I pointed out an old school biased one at that :). Out running storms is great but ya an't gonna outrun them all and with a 100ft.(mind boggling) vessel compared to my average size over the years of 30 to 36ft. it's all relavent. However you have backed up my opinion on the fin equipped vessels in your reference to Beowulf it's a record breaking speed thing and there's nothing wrong with that if it's your cup of tea. I,ve never heard of a long keel breaking away from a hull or dropping off , yet every year there are cases of fins breaking or bulbs falling off. Most long keel vessels survive groundings or kissing a rock with minimilal damage, not the case with fins and if it does there's the big question, what undetected stress damage has occured --just something more to think about mid ocean in rough weather. As much as I try to be open minded on this I'm sorry but the full keel/long keel/skeg just has so much going for it. however who am I just an old school boatbuilder--christ I hate cell phones and computers so take my opinion for what it's worth---Geo.
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    Concur Geo..
     
  14. souljour2000
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    souljour2000 Senior Member

    fin keel vs. full keel

    If I were looking at having a 100 foot world-cruiser then I would look at some of the older clipper-types and schooner types...bigger boats that I have never studied really...some of these were (are) incredibly fast at this size afterall...
     

  15. MichaelinSM
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    MichaelinSM New Member

    I don't think I've ever heard of a keel of any kind falling off a Dutch-built steel or aluminum vessel. Has anyone?
     
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