Are keelboats for cruising dead?

Discussion in 'Boat Design' started by Vineet, Jun 6, 2021.

  1. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Sharpii, you should be much more differentiated in your opinion. Just some facts to consider:
    Long and low aspect ratio fin keels can have bolt on, or encapsulated ballast. It comes down to the orginal engineering and construction if you loose a keel or not, because it all depends on the underlying structure, just as with a high aspect ratio fin keel.
    Riding over an obstacle depends on the angle at the front of the keel. Plenty of high aspect fin keels are not arranged vertical, and don't have T shaped bulbs, giving them the same ability.
    Skegs are a very difficult piece of engineering because they are cantilevered from the hull skin, and take the bigger portion of the rudder pressure. Spade rudders are a simple proposition, you have a rudder stock supported by two points, it's simple to make it withstand any force you choose. The only advantage of a skeg hung rudder is protection from floating lines wich could wedge themselves between spade rudder and hull.

    If you are really concerned about impact and grounding your best bet is a full centerboarder with kick up rudders. If you hit something the appendages retract and the shallow dinghy like hull rides over the obstacles. If the boat is metal and has sufficient thickness in the bottom plate you can skid over granite and keep sailing. Most have full internal ballast and in the medium displacement group, wich makes them quite sea kindly, plus they retain all the advantages of having twin rudders and a good upwind foil.

    Just to be clear, I am not advocating for one form of boat over the other, all have strengths and weaknesses. If someone has design points that are specifically important to him, he should prioritize them and see where the SOR takes the design and what compromises he is willing to live with.
     
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  2. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    Much of the fin keel issue is from builders cutting unseen corners or edgy racers. A moderate, well built fin keel is seaworthy. Badly built keels that fall off can come from budget Beneteau or expensive Oyster.
    Skegs have proven to be something of a security blanket. They look safe but can't really contribute any strength. If they had a spar going through the skeg up to the deck they could but they don't.
    There are plenty of clam crushers that aren't ocean worthy. What really matters is a moderate, balanced, competently engineered and built boat.
     
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  3. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    A deep, high Aspect Ratio keel has a very small support area on the hull.

    It's not just a matter of fore and aft loads, when the bottom of the keel strikes an obstacle. It can be vertical ones too, when the top edge of the keel wants to push its way through the bottom of the boat.

    The designers try to take this into account by assuming a collision speed of around 3 to 5 kts. Unfortunately, these boats sail much faster than that, and it's not always obvious where these obstacles lurk.

    For a time, I was following a YouTube video of a couple who had purchased a boat with this sort of keel as salvage. It had suffered a keel strike, and the insurance company had written it off.

    The couple had found the damage more extensive than they originally thought, but went ahead anyway, doing the work themselves. They had to cut away much of the floor structure, which consisted of composite box beams, and rebuild it.

    Yeah, good engineering can overcome to some extent the the inherent vulnerability of this hull/keel type, but never totally.

    Spade rudders can be massively built, with large diameter shafts. I saw a Pearson 26 which had suffered a rudder strike. The shaft was bent maybe 5 to 10 degrees. But the rudder stayed on the boat, and it was still steerable.

    I also once saw a pretend skeg on a 27 ft Dufour. It was short and deep. And it had a very bad habit of breaking off.

    The owner had it replaced at least twice. But, once broken off, It affected the boat's performance insignificantly. The yard manager recommended not replacing it after the 2nd time it went AWOL. It's jagged stub was ground down and glassed over.

    But a real skeg is a major structural member of the hull. The kind I have in mind are as long at the top as they are deep and have a layup schedual befitting their role.

    I see a definite trade off between speed/windward performance and general durability.

    I would never argue that a well made high AR Fin keeler is inferior to a poorly made (or designed) crab crusher. But I would argue that one of equal quality is far more likely to survive a grounding mishap.
     
  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    We seem to be digressing into the realms of designing adequate internal structure.Which some builders of production boats do better than others.It is the case that a short, high aspect ratio keel will impose greater local loads than the long keel and like most things in life there are compromises between a desire for performance,access to shallow waters and the ability to look after the occupants of the boat if things get really wild.We may choose which corners of the performance envelope we would prefer our boats to occupy and we have to hope the designers and builders did a thorough job.It helps if we are honest with ourselves when making the choice since any number of us might dream of a life of ocean voyages and few of us actually do it.Its certainly something I have no wish to indulge in.
     
  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I think there are a few points from SEAWORTHINESS: THE FORGOTTEN FACTOR that are relevant to this discussion. The two main points in the book, which advocate for long keels boil down to:

    1.) directional stability. The argument is that, with the area of the keel spread out over the length of the WL, the boat is less likely to turn while having a wave at the bow and at the stern. The wave at the bow has orbital currents that run the opposite direction the wave is going. The wave at the stern lifts the stern of the boat, causing the boat to slide forward. This, the book says, can cause a considerable turning moment, which the long keel dampens.

    In retrospect, I'd say this is true, but it may not matter if the boat is moving with some speed. Then, maneuverability is what counts. It is here that the long keel quickly becomes a liability, because the very turn dampening that may keep the boat from broaching, while going slow, may actually cause it to do so, while going fast. The boat may get off course and turn too slowly to get back on course before the next wave hits and knocks it even further off course.

    2.) roll dampening. The long keel may have the same Vertical Center of Gravity as the short, deeper fin keel, because the ballast has more length to be distributed in, but the short, deep keel has far less roll dampening than the long keel does. In fact, the book claims with some evidence, the short, deep keel may actually induce rolling, as the boat is lifted an lowered by passing waves. The long keel, mainly because of its greater area, and that this area is distributed more towards the ends of the boat, making it more effective while the middle is in a wave trough, tends to dampen such rolling. This can be even more true if a wave hits the side of the boat. The book claims that the boat with the short, deep keel will roll more than the one with the shallower long keel. The long keel, not only dampens the original roll, but dampens the recovery roll as well, giving the boat an easier motion. This, the book says, is especially true if the mast is lost (the inertia of the mast also dampens rolls due to wave hits.).

    The book gives a chilling example of a fishing trowler that was built without a keel and was hit on its side by a medium sized wave, which rolled it completely upside down. It's anyone's guess if the same wave would had flipped if it had a long keel. The book implies it wouldn't, which now leaves me a bit skeptical.

    Those were the two main points of the book.

    My take now is that the long keel boat is safest when stopped in a storm, while the short keel boat is better off if sailed at some speed in such conditions.

    I also don't think the '79 Fastnet race disaster proved anything except how distructive the sea can be if you're caught in the wrong place at the wrong time.
     

  6. rangebowdrie
    Joined: Nov 2009
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    rangebowdrie Junior Member

    We see this kind of construction in Pacific Seacraft.
    It's also a staple in many of Ted Brewers designs.
    I first observed it on the Peterson 44s from the '70s,, kind of a "cut away" full keel.
     
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