Sailing characteristics of twin bulb keel designs

Discussion in 'Boat Design' started by aramisboat, Jan 31, 2017.

  1. aramisboat
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    aramisboat New Member

    Do any of you learned guys and girls have theoretical or practical experience of the sailing characteristics of a 40´sailboat with a twin bulb keels?

    I have been told that such designs have a different feel at the helm and load up in a somewhat different way to boats with a single bulb keel. I appreciate that the devil is in the detail of the design, but are there any general characteristics of such designs worth noting.

    I ask because I have never sailed a twin ballasted keel design and have identified such a boat that ticks all of my boxes.

    The boat is a Mercator 40. It was designed by a respected French naval architect Guy Ribadeau-Dumas, so I am sure that it works. I am just trying to understand how it works.

    This particular boat is constructed in aluminium (that´s aluminum for those whose Queen´s English is a bit rusty;)) and it is hard chined. The hull between the canted keels is flat.

    I am attracted to this particular boat as it is aluminium and can dry out without any additional supports. I am favouring the twin keel arrangement over a lifting ballasted keel for reliability and for the “theoretically” better AVS/STIX compared with the other option - an internally ballasted centreboard design.

    Thanks and regards
     

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  2. DennisRB
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    DennisRB Senior Member

    Check out RM yachts.
     
  3. aramisboat
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    aramisboat New Member

    Rm

    Know them and love them!

    I am not sure about the effects of a few tons of water on those large acrylic panels!

    and the condensation...and I have just spent 12 years with a wooden boat.

    Other than that, if I won the lottery an RM1270 would be on the list
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    It's the same boat of this announce?
    http://www.ayc-yachtbroker.com/mercator-40

    Bi-keels have a small but steady group of fans in Europe, specially in the places with high tides like Brittany, Solent, North of France, Holland... a bunch of desolate places with rain, cold climate, harsh seas, tide currents, more than 5 meters tides than leave most of the small harbors and moorings dry and the boats on the sand during long hours. That's the place for bi-keels. Boats for people who do not want to pay for marinas. Against all odds there are plenty of sailors and sail boats of all kind.

    Bi keels are now very mature but remain rather specialized for big tide seas and dry moorings at low tide.. There is nothing peculiar about sailing them. The owners words say practically all you need to know. 50 degrees angle upwind doesn't seem to be very good at first sight but that means that is the good angle to get get a good ratio speed/angle, with the boat comfortably sit on its chine. It's pretty wide so heeling to much is counterproductive. Better to let it run than struggling to go against the wind.
    It's a boat looking to be spacious and comfortable, not to be fast.

    Bi keels have great advantages in the desolate places I have described, with a few inconveniences; a litle loss of upwind ability, and overall speed (not always). In Mediterranean a bi keel has none advantage, but may lack cruelly of spedd in light winds.
    For transoceanic voyages a ballasted centerboard is far better, being faster, and more adapted for travelling to the Pacific Islands for example with the long downwind runs, and able to pass over a reef in some desperate cases or go up in a river.
    They are also surer in very high winds. That seems counter intuitive but with with the centerboard up and almost no sail, just to enough have speed to keep the rudder working, a dériveur lesté slides with litlle heel and with soft motions. If you have enough water to run it's the most comfortable way to let pass a tempest.
     
  5. aramisboat
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    aramisboat New Member

    Mercator

    Hi Ilan

    The ad. is indeed the one - a rather rare beast!

    Thanks for your comments.

    The designers web gives some insight in to his brief:

    http://www.ribadeaudumas.com/naval/...14&PHPSESSID=c2a96aba14f24b800761369a68c50b42

    I started this whole process looking at the Atlantic Yacht aluminium designs from the Netherlands. They offer the option of centre board interior ballasted, ballasted lifting or fixed keel. Unfortunately they are very (extremely) expensive but no doubt worth it. Then I came to the Allures 40 or 39.9 dériveurs. However, these are too young to be on the secondhand market (I guess that they are all still out on their 5 or 10 year sailing projects - lucky them!)

    I have looked at OVNIs (again dériveurs) but they lack the homely live-abord warmth that I am looking for, except for the few that were fitted out in merisier. The OVNIs are also stupid money secondhand even after the paint has fallen off!

    I don´t have a problem with the internal ballast on the OVNIs/Allures et al - you've just got to keep sailing.

    I was curious about the twin bulb keel arrangement as an unknown to me. Certainly looking at the RM (Fora Marine) range the performance difference between the single and bi-quille is not so remarked, however, the RM hulls are really designed for speed (at least off the wind). The Mercator hull seems to prioritise easy of manufacture, hence the flat bottom.

    Someone did say that if you round-up in a bi-quille, they stall in a very different way to a mono-keel and then sort of fall back in to the slot. Hopefully (bank permitting) I shall find out soon.
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    The price of the Mercator 40 is rather good. It seems in nice shape. The owner talks of good insulation, a very important thing on a metallic yacht. If the inventory is good it can be very interesting. Yes the boat has very simple lines, a bit in a 1970 style. The lone way to find if it's not too boxy is to see it from all the angles. Pics can be very misleading on chine boats...The NA describes a normal voyaging boat like the yachts the French shipyards made by thousands in a 45 years period. Ribaudeau Dumas knows his job, he began to draw boats before the extinction of the dinosaurs...
    The small flat bottom is not a problem at all. It's even an advantage: when the boat heels upwind at the good angle, the chine between the bottom and the lower panel becomes a keel that cuts the water, and improves the hydrodynamics.
    Also in a aluminium boat the welds can be done easier and with better quality. That makes the bulkheads and floors easier to build and the installation of the keels stronger. Lone disadvantage of a flat bottom is to be a bit more difficult to keep it perfeclly dry at the bottom, in a aluminium boat is not a problem.
    The Dutch yachts are over priced, like the German car they have a hype, not always justified. Plenty of French shipyards are simply as as good in a simpler style.
    There are also plenty of good second hand polyester boats. Those made by Wauquiez are worth to be examined. They are of excellent quality and very durable, 40 years boats in good shape are common. The lone thing I cannot stand up with polyester boats is the smell of styrene. Even when 20 years old they do smell...
    So I prefer wood/epoxy or aluminium. Steel is not an option on yachts, the maintenance and perpetual fight against the rust can become a nightmare.
    A bi keel is very interesting in the Channel, North Sea and some similar places in the world. As the the bi keel becomes av very blatant advantage in "dry" harbors and moorings. They can be left with no risks for months. With a keeler you'll need crutches that can slide. letting fall the boat on the ground. A perspective that makes nervous.
    It seems you are going to buy the Mercator, I wish you plenty of nice navigations.
     

  7. DennisRB
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    DennisRB Senior Member

    Price is good? It seems insane. You could get a catamaran with more space speed and comfort and the ability to dry out for that price.

    I actually like a lot of these twin keelers, but the the centerboarders are even better with even shallower draft. But when I look at the price they are too expensive. 3 times that of a regular fixed keel boat so it makes sense to buy a catamaran. If I could find a centrerboarder with a similar price to other monohulls of the same size I would buy one. The price does not align with the benefit IMO.
     
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