Cutting a Fin Keel

Discussion in 'Boat Design' started by alex folen, Jan 26, 2009.

  1. alex folen
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    alex folen Flynpig

    Pardon me. I have looked through the search and could not find anything relating. I was told by several they cut their fin keel shorter to make it more shallow accessible. On fellow cut his keel 2.5 feet and his friend suggested to add the extra weight low in the hull. The fellow who cut his keel down said he didn’t need any extra weight because it responded exactly like it did prior, or there were no noticeable difference. The bottom of my keel is 5’6 inches form water line. When I pull it is there any way I could shorten the keel and still have the stability as designed?
    thanks a bunch!
     
  2. pat60
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    pat60 Junior Member

    This is a fairly commmon answer to draft issues, check out www.marsmetal.com they have a whole section of their website devoted to it.
     
  3. alex folen
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    alex folen Flynpig

    Thanks for the web site, however I cannot find anything relating to a keel in sail boat. Nice site about metal and weight though! If the draft is a common issue then I’d just add more weight to make up for the draft loss?. Although being that it is a sailboat I would think the longer the moment arm the less weight needed for stability as designed? Reducing the arm would cause more weight concentrated at a fulcrum? Just Dunno now, but thanks!
     
  4. lewisboats
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    lewisboats Obsessed Member

    Be very careful in this...the keel is not simply ballast in a sailboat...it also provides lateral resistance and shortening it can affect the balance of the boat and it's sail carrying ability. You could also experience more sideslip, lee or weather helm and instability issues.

    Steve
     
  5. pat60
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    pat60 Junior Member

    The link was on the bottom of the page it is www.marskeel.com give it a look.
     
  6. robherc
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    robherc Designer/Hobbyist

    From a physics standpoint, I'd recommend making the chord of the keel longer, if you're going to shorten it's length. If you lengthen the chord enough to keep the lateral area of the keel the same, you should be fine for side-slip purposes. Then, you'll have to do some math to se ehow much you should increase the ballast at your keel-tip. Remember your basic fulcrum & leverage principles, if you cut 1/4 off the length of a lever, you're going to have to put twice as much force agianst it to do the same amount of work!
     
  7. pat60
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    pat60 Junior Member

    Thats what they describe at the mars site. They have software that lets them determine the correct additional weight, a split bulb is produced, you cut the bottom off your keel and affix the two bulb halves and fair it in. Simple but probably a little costly.
     
  8. alex folen
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    alex folen Flynpig

    Thanks fellows. I finally found the site under, Services/Stability bulbs at that site. This may be worth the undertaking when I pull it. As some of you mentioned before this is not a racing boat and the modification to the keel would make it more charming. I’d almost attempt to do it myself with further research. This will make me even happier when going shallower. Based on the physics, would I then be adding the more weight, quantified as calculated,and thus having to strengthen the keel trunk, or at least think about doing that? I can do the math.

    tahnks a bunch!
     
  9. robherc
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    robherc Designer/Hobbyist

    I'd look at re-doing the whole keel, honestly. Not only would you have to consider strengthening it to handle the extra weight; but you'll also want to increase your lateral surface area by making it longer (bow to stern length, not vertical length). But hey, that's just my 2 cents ;)
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    Alex,

    I have done this modification a few times, and one story that I have written about is on my website:

    http://www.sponbergyachtdesign.com/Magic.htm

    Magic is a Cambria 44, hull #2. She had an 8' draft keel that we shortened to 7' draft, and took an equal amount of ballast and fashioned it into a bulb and bolted that to the lower aft corner of the keel. This kept the weight low and improved the flow around the keel, so in the end, the boat performed better, according to the owner. This type of modification is fairly easy to do and is not all that uncommon.

    Eric
     
  11. alex folen
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    alex folen Flynpig

    What makes a "Blue Water Boat"?

    Thanks Eric Spon for the informative info?learning something new every day!

    To any, what makes a "blue water boat"? In the past 7 months I've been looking for such a boat and many times have Googled the ultimate "Blue Water Sail Boat". The results are coming up with a West Sail (32), Pacific Sea Craft and several others, in which these boats were said to be "OVER BUILT" in hull thickness. This seems to be the major reason. This leads me to believe the thicker the boat (fiberglass thickness in this case) is the safer one, or more of a "Blue Water Boat"? Anyway, can I just thicken the hull of my Coronado 30 to an inch thick or more thus making a stronger and more "Blue Water Boat"? I should have the resources to add additional thickness to the hull if I want. I'm just curious to know if this has been done and also if new fiberglass has problems sticking to existing fiberglass? Thanks again so much.
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    Alex,

    There is more to being a "blue water boat" than just fiberglass thickness. Virtually everything is a little heftier--rig and rigging, chainplates, keel bolts, anchor, winches and windlass (if you have one), the list goes on. A blue water boat, besides being strong, will support the crew well at sea. Everything works and is robust and easy to maintain.

    Simply adding fiberglass to your hull is not necessarily a smart thing to do. That will add a lot of weight, and maybe in the wrong places. Yes, you can run into problems with secondary bonding of the new laminate over the old--polyester does not stick that well, so you would be advised to use vinylester resin or epoxy resin. But again, I would not necessarily do that. You have to look at the boat as a system and makes sure all the parts of the system are properly sized. As I recall, the Coronados were California boats of the 60s and 70s, probably made of solid fiberglass, not with a core. In some ways, these early boats may have been overbuilt, but in other ways, California boats of that era were really meant for short cruises along the coast. You might want to do a bit more research on the design to see what success people may have had taking the boat offshore. Cruising World magazine has run a column for decades called "Another Opinion" in which people post their reports on the good and bad aspects of their boats for offshore cruising. You might want to consult that resource to get others' opinions on the suitability of the Coronado 30 for offshore sailing.

    You might also want to read Hal Roth's books, such as "Two on a Big Ocean", "After 50,000 miles", "Two Against Cape Horn", "How to Sail Around the World", and "Handling Storms at Sea." Hal was a good friend of my wife and me, and sadly, he passed away in October. He is survived by his wife and life-long sailing companion Margaret. The advice Hal and Margaret give in these books is born of many, many miles of ocean sailing experience. This will give you an idea of what to expect in blue water and the type of boat you would need and how it should be equipped.

    I hope that helps.

    Eric
     
  13. PortTacker
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    PortTacker Junior Member

    Aside from Erics excellent comments regarding overall strength, there's the interior layout to consider. Ample stowage, a galley that works in a seaway, intelligently laid out icebox space, usable sea berths, a head that actually won't kill you in rough weather (been there!) the ability to carry the weight of cruising gear, etc all need to be considered. Will she trim out and balance to be easier for an autopilot to steer? Many things.

    Plus it can mean different things to different sailors. For example, some people demand refrigeration but I think it's of questionable value on a passagemaker. I personally do not like full keel heavy displacement boats whatsoever but others believe they are the only boat to take offshore. I prefer medium displacement (and even a bit lighter) with some performance potential. (My tastes run pretty much exactly with Bob Perry's if you're familiar with his work.) I like my keel deep to provide lift - 'preventing side slip' is not enough - I've experienced a storm close on a lee shore (Oregon coast) and the ability to claw to windward in the scariest imaginable conditions is important to me. I like an ample sailplan - as Hal Roth said, you need a sailplan for the lightest conditions, you can always reduce sail but it's hard to add sailplan.

    Excellent advice re Hal Roth's books. I re-read my copies once in a while.
     
  14. alex folen
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    alex folen Flynpig

    Yeah, I think it?s quick to identify who is in expertise here. Thanks guys?Eric.
     

  15. Brent Swain
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    Brent Swain Member

    The chord or length of the keel has little effect on reducing leeway or sideslip, as it is the depth of the leading edge that does that. Once the water is turned , more keel aft has little effect.
    Brent
     
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