Difficult going up headwind and lots of rolling downwind with bilge keel ?

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  • 27 Feb 2019 22:07
    Reply # 7190742 on 7185591
    And also, the Macwester 27 looks very intersting as well.  About the french style, RM and others, to me I feel that the interiors are really not nice in french sailboats. To much plastic.  And it is not getting better beccause today the interiors are getting worse and worse. There is no  athmosphère in boats like that.  Sure, they perform well but the atmosphere is important as well. 
  • 27 Feb 2019 21:54
    Reply # 7190715 on 7185591

    Many thanks to all of you for all this very interested reading.  Since speed is not of any importance to me, the twin keel seems to be the way to go. But loosing one keel while cruising is of course scary !  But I suppose that one could make some work to better fix the keels on a Centuar 26 or would this not be possible ? 

    And I agree with David about a caravane and since I measure 1m78, being able to be somewhat upright is appreciable. But in a storm, I probably read to many books about storms and how to survive LOL, but it is true that although rare, it can happen, and if one happens to be in a storm, wheather "heaving to" or letting out the Jordan Series Drogue on the aft or stern, the very high freeboard of the Centuar and any type of sailboat with such a high freeboard is not very safe.  I saw that the MINGMING was sitting very low in water, and with a tiny cockpit, obviously a big advantage to ride out a storm. 

    But as far as twin keel is concerned, thanks to this input, there seems to be as far as I am concerned, only advantages since I intend to do slow sailing.

  • 27 Feb 2019 11:07
    Reply # 7189498 on 7187957
    Anonymous wrote:

    For something that looked like a caravan, 

    When you are at anchor or living aboard on a berth, ie the large majority of the time aboard for the large majority of boat owners, a floating caravan is good. 

    From memory, there were three interior layouts of Centaur:  the B layout included a quarter berth and was more traditional for a small sailing boat and was I think the most favoured by those more interested in sailing ;  the A layout included a long mid ships galley which was more favourable to living aboard. 

    Also, it may be worth noting that a few were made with a fin keel and I think from memory they were the Chieftain.    

    Westerly made boats in order to make money and to do that they provided a cost effective answer to the public's requirement.  And they made them solidly.  (The weakness of the keels was if anything a design fault.)  

    have had many happy days on a Centaur.  The yacht club of my Corps in the Army, REME, owned one and it had the considerable advantage that it was "soldier proof".  

    Last modified: 27 Feb 2019 11:11 | Anonymous member
  • 27 Feb 2019 08:07
    Reply # 7189476 on 7185591

    One factor that's not been mentioned is that asymmetric, angled twin keels should also be toed in a few degrees. Then, when you put some numbers in and analyse the vectors, there does seem to be a slight positive effect. When you also put in some extra drag due to extra wetted area, it's cancelled out again, but the main point is that twin keels needn't be markedly worse than a single keel, if done right.

    My views on the rolling question:

    Narrow, deep boats (eg Vertue) roll slowly through a large angle, which can be uncomfortable.

    Boats with a circular cross-section roll easily through a large angle, but are relatively unaffected by waves.

    Beamy, flat bottomed boats roll quickly through a small angle, reacting to waves faster. This can be uncomfortable. Multihulls do the same.

    Chines dampen down any tendency to roll.

    High, heavy masts slow down the roll, but do not affect its amplitude.

    All kinds of keels dampen the roll, whatever kind of hull they are attached to. The greater their area, and the more there are of them, the more the damping.

    Unsurprisingly, I come down in favour of Tystie's and SibLim's more or less V section hull with chines at the waterline, for seakeeping and seakindliness, combined with bilgeboards or twin keels for lateral resistance and roll resistance. When I was in an anchorage where a swell was making her roll, I would put one or both boards down to dampen it.


    Last modified: 27 Feb 2019 15:57 | Anonymous member
  • 27 Feb 2019 03:03
    Reply # 7189300 on 7189104
    Graeme Kenyon wrote:

    What we know in practice is that apart from the other advantages and disadvantages of twin keels, this configuration can, as David has informed us, be quite comparable with a single fin keel both in terms of windward ability, and the ability to stand up to sail.

    By the way,

    I don’t think anyone has answered the last part of Raymond’s question “… lots of rolling downwind with bilge keel?” I am given to understand that rolling is not induced by having more than one keel, and would have thought that bilge keels might actually damp down any tendency to roll. Again, this is probably a somewhat spurious over-simplification, calling for an answer based on practical experience.


    There are not many boats being built these days with twin keels, no longer known as bilge keels it seems. Alan Wright was a New Zealand designer of bilge keel yachts, and those performed very well. From memory his boats had symmetrical keels (?). The modern twin keel is a very different keel than the more traditional 'bilge keel'. My favorite boat manufacturer, RM Yachts do twin keel versions of all of their range. These are deeper foils with bulbs, canted outwards a little based by photos and drawings, and as far as I can see symmetrical in their shape. Judging by the look of these keels I imagine the yachts would sail to windward very well. I have met the owners of an RM twin keel yacht, they had cruised from Spain to New Caledonia via Patagonia and all other sorts of interesting places. They had nothing but praise for the performance and all other aspects of their twin keel yacht.

    On the question of symmetrical vs asymmetrical twin keels, I do note that the vast majority of modern catamarans, whether they have twin keels or dagger boards, have symmetrical foils, and go to windward extremely well, even with low aspect ratio dagger boards. The designers of these yachts must know what they are about. Are asymmetric foils, (twin Keels, boards), even needed for windward ability? Look at all the highly windward efficient yachts that are around with only a single symmetrical keel, they climb up to windward very well. Windward ability is not only a function of the keel shape and aspect ratio, but also involves efficient sails and many other factors.

    I think a yachts tendency to roll, whether downwind, or at anchor is more a function of hull form rather than keel configuration, based on practical experience with various boats I have owned.  

    All of the above comments are based on actual observation and experience. 

    Last modified: 27 Feb 2019 06:46 | Anonymous member
  • 26 Feb 2019 23:26
    Reply # 7189104 on 7185591

    Arne, I was at first not convinced by your argument, since the two force vectors arising from the asymmetry of the two canted keels are what they are, however you care to resolve them, and they sum to a single vector acting approximately downwards (in the direction of the hull's vertical centre line.)

    You can rotate the diagram, and resolve the individual forces any way you like, this does not change the above statement.

    However, I thank you for forcing me to think a little more deeply on the question, and after “wasting” half a morning I now realise that at an angle of heel, this resultant “downward” force (along the hull centre line) does resolve into a useful horizontal component in the direction of windward, and a component vertically downward which might add to the righting moment.

    The lift from a single fin keel, on the other hand, is perpendicular to the keel, and at an angle of heel this resolves into a horizontal component in the direction of windward and a component vertically upward (along the hull centre line) which might subtract from the righting moment.

    The numbers will be quite different and there is still the question of drag and, of course, many other design factors, so what this small theoretical discussion means in practice probably does not amount to much. I still think the double-advantage claim of twin keels (that one keel is pushing to windward while the other is simultaneously helping to keep the boat upright) is fallacious if for no other reason than that is an over-simplification, but I do finally understand now that my analysis was also incorrect.

    What we know in practice is that apart from the other advantages and disadvantages of twin keels, this configuration can, as David has informed us, be quite comparable with a single fin keel both in terms of windward ability, and the ability to stand up to sail.

    By the way,

    I don’t think anyone has answered the last part of Raymond’s question “… lots of rolling downwind with bilge keel?” I am given to understand that rolling is not induced by having more than one keel, and would have thought that bilge keels might actually damp down any tendency to roll. Again, this is probably a somewhat spurious over-simplification, calling for an answer based on practical experience.


    Last modified: 27 Feb 2019 00:28 | Anonymous member
  • 26 Feb 2019 18:05
    Reply # 7188501 on 7187957
    Anonymous wrote:....that said there was a problem with the twin keels being not well attached - one of his fell off when moored on a mud berth! 


    That is a notable defect in Centaurs. The keels are splayed out at such a wide angle that settling down into a mud berth forces them apart. Good thing they have a spare keel!
    Last modified: 27 Feb 2019 08:38 | Anonymous member
  • 26 Feb 2019 13:19
    Reply # 7187957 on 7185591

    A friend had a Centaur a while back.  For something that looked like a caravan, it actually sailed very well.  There are lots around, so not expensive....that said there was a problem with the twin keels being not well attached - one of his fell off when moored on a mud berth! 

    Of course all boats will have a wind speed when they will not longer go to windward, the drag on hull / rig exceeding the forward drive.

  • 26 Feb 2019 12:28
    Reply # 7187895 on 7185591
    Anonymous member (Administrator)

    Graeme,
    I suggest you heel that drawing over, say 20-25 degrees, and then take another look at the vectors and their horizontal and vertical components...

    Arne

  • 26 Feb 2019 11:17
    Reply # 7187838 on 7185591

    Many years ago, in Auckland,  I had the privilege of meeting Lord Riverdale and being invited aboard Bluebird of Thorne. I have been very interested in the evolution of twin keel designs since then. So, the following is not intended to disparage the concept, but to take issue with just a couple of commonly believed key points which appear in the articles for which David provided the links.

    https://uk.boats.com/boat-buyers-guide/choosing-a-yacht-bilge-keels-vs-fin-keels/ “…the immersed keel would provide lift that would tend to ‘suck’ the boat up to windward….”  and http://www.brayyachtdesign.bc.ca/article_twinkeels.html   The windward keel is working more horizontally creating downward lift that increases righting moment giving more power to carry sail…”

    I could be wrong here, and if so, would welcome a correction, but I don’t see how the above two claims can both be true at the same time.

    Both keels are attached to the same hull, and if we draw a vector diagram and consider the combined effect of these two canted forces, it seems to me that the resultant is a small downward force in the direction of the boat’s centreline, which is of little, if any, benefit. The claimed useful components of lift (windward lift from one keel, with simultaneous downward lift from the other) are self-cancelling. The associated drag, however, is not!


    It seems to me that there is value in canted asymmetrical foils which can be lifted (as in, for example, the SIBLIM design) because one foil can be raised, which would enable the other, unbalanced, to provide useful hydrodynamic lift. However, the claimed serendipitous paired advantage of fixed twin keels would seem to be a fallacy, unless the vessel is heeled so far over that one keel is no longer in the flow of water.

    Yet designers continue to make these fixed canted keels asymmetrical, which to me just means increased drag.

    I have often wondered about this. Have I missed something?


    Last modified: 26 Feb 2019 11:52 | Anonymous member
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