Roberto barros MC28 - minimizing draft

Discussion in 'Stability' started by Damage, Nov 13, 2014.

  1. Damage
    Joined: Nov 2014
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Netherlands

    Damage New Member

    Hi guys,

    just new to this forum although I've been reading here a while.

    The folllowing is the case, I'd like to built a sailboat myself (for the hobby of building), but when finished I'd like to sail it as well. There is this one thing I can't really figure out.

    http://www.yachtdesign.com.br/ingles/projetos/mc28/desc28-2.php

    I really really like the design, only one problem here, where I live (Netherlands) a lot of the lakes are quite shallow and a big interesting part of the coastal cruising grounds are too. Therefore it would be preferrable for me to bring back the draught from 1.55meter to let's say 1.25 max. 1.35 meter.

    In my opinion there are three options,

    1. Making a longer keel (at rooth) for instance, but shorter with more of a bulb at the bottom.

    2. Making a double keel (will this even be possible for an excisting design?).

    3. Make a lifting keel. In this case I am quite worried of inside space (?).

    It would be nice to be able to beach it on low tide (this would definitely be a big plus).

    Basically all options come with the question of stability/sailability; will it be feasable on this excisting design? What would you think would be the better option and why? Because if not, I will have to search for another design to my liking.

    What do you think?
     
  2. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Since you will be building from scratch perhaps you could talk to the designer about using a Sheel Keel, if you are not familiar do an online search and you will find out more than i can tell you. Another option is the tandem keel, a British design which the Hills used on the Benford dory, Badger. Again, online search.

    Steve.
     
  3. Damage
    Joined: Nov 2014
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Netherlands

    Damage New Member

    Thanks for your reply! It helped me to understand more possibilities that might be possible.

    But do you think it is possible to switch to a different keel type?

    In other words, is it almost as simple as it seems or is there a part that I'm not understanding completely?

    Especially the idea of lengthening the keel at root and thus creating a keel with the same area seems like a feasible way to go as far as I can tell, but is this true?

    I will contact the designer if I where to be very sure that I want to built this particular model, but more models that I like have this very same issue and that's why I'm asking on this forum.
     
  4. Tad
    Joined: Mar 2002
    Posts: 2,302
    Likes: 182, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    The standard keel is cast iron, reducing draft by a foot can be done by designing a new modern foil with a good sized lead bulb/wing at the bottom. You should be able to lower CG by going to external lead, not in a steel box as the designer suggests.

    Just making a longer and shallower fin will degrade performance.

    Twin keels will degrade performance.

    A lifting keel (ballasted) could improve performance, but it's going to be complex and more expensive, plus taking up interior space.

    A Scheel will really degrade performance. The Collins tandem less so but no one makes them any more AFAIK. (they are cast iron and rather complex).
     
  5. Damage
    Joined: Nov 2014
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Netherlands

    Damage New Member

    I think you are pointing towards something like the red ones in the following picture?

    [​IMG]

    Thanks, that could also be a good option if possible... I assume normally, when you change the keel for minimizing draught, the rudder should be alterd as well? What could be the options for that change?
     
  6. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    The problem with a shallow draft keel with a bulb is that it will not get upwind very well either, fact is any shoal draft option is going to degrade performance but sometimes you have to compromise. Yes, you guessed it, a shallow rudder is going to degrade performance too. Perhaps a multihull would be a good choice for your area. Bernd Kohler has some good designs.

    Steve.
     
  7. Damage
    Joined: Nov 2014
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Netherlands

    Damage New Member

    Will this problem with not sailing upwind that well continue to persist when the keel isn't only equipped with an bulb at the bottom, but is also longer at rooth?

    Reduicing draught to 1.25-1.30meter would dramatically increase my sailing grounds. I think, since I learned a lot since I started this thread, that I have to rephrase my question: How bad will performance and sailing upwind be affected if reducing draught with a well designed keel?

    I too have thouht of a catamaran, but when sailing I prefer monohulls more so I'm sorry no multihull for me.
     
  8. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Unfortunatly, all other things being equal keel span does more to get you upwind than anything else so when you reduce the depth of a keel and then put a bulb on the bottom to try to reclaim lost stability you suffer the double indignity of first reducing the span of the blade and then reduce it even further by the depth of the bulb itself so you are left with not much to prevent leeway. If you look at a lot of older boats where they offered a deep draft and shoal draft version of the same boat before the popularity of bulbs and wings, without fail the deeper keel was much better upwind. I would think that a well designed winged appendage would work better than a bulb on a shallow blade due to the end plate effect but beware, I did a wing on a fairly deep short chord blade 25yrs ago and while it was clean through the water and worked well when we ran aground in the harbor which happened a fair bit it would dovetail into the sand and was difficult to extricate, we changed it to a bulb for this reason but we also lengthened the blade. I don't really know the answer to your situation as having shoal draft and being able to actually sail in shallow water are two different things. Twin keels for example have quite shallow draft while upright but it increases as you heel while the opposite is the case for all centerline configurations.

    Steve.
     
  9. WindRaf
    Joined: Oct 2014
    Posts: 343
    Likes: 5, Points: 0
    Location: Italy

    WindRaf Senior Member

  10. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Yep, that's the Collins tandem keel that I suggested in post #2, it may well be the best option if you can get a design for one, Tad said that they were cast iron but it may be possible to cast one in lead with a high antimony content and some steel structure cast in or even a lead filled steel fabrication if you have some fabrication skills or are willing to pay for it. It has a pretty big wing on the bottom which would probably cause issues in a grounding.

    Steve.
     
  11. WindRaf
    Joined: Oct 2014
    Posts: 343
    Likes: 5, Points: 0
    Location: Italy

    WindRaf Senior Member

    Steve W,
    yes.

    For an amateur the builder easiest and cheapest way is to use thick steel plates, weld, and then finish the Naca profiles with the ferrocement system.
     
  12. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    There are many ways to build a keel but the shallower you go the more important it is to use as much lead content concentrated as low as possible so i personally wouldn't use FC. I have actually built in FC and they typically use steel punchings and cement but they usually are heavier displacement boats with a low ballast ratio and fat built down keels with enough volume to use a lower density ballast material. I have cast keels up to 24000lbs in lead, its pretty easy to do but pouring into a fabricated steel shell is also a good method and a lot of folks have steel fabrication skills.

    Steve
     
  13. WindRaf
    Joined: Oct 2014
    Posts: 343
    Likes: 5, Points: 0
    Location: Italy

    WindRaf Senior Member

    Steve W,
    maybe I have not explained well.

    The structure of steel blades very thick, the sole is a box of steel filled with lead, and only the external finish in ferrocement for the right profile.
     

  14. Steve W
    Joined: Jul 2004
    Posts: 1,809
    Likes: 57, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Yes, but in a perfect world the entire keel would be lead, which is also probably the easiest to make, but sometimes an all lead keel may not be strong enough, the Collins keel may be such a case because of the large end plate and smallish struts. My point is though that lead is the most dense material we have available for a keel, then steel, anything else in is substantially less dense and especially with a shoal keel you want it to be as dense as you can get it to get the cg as low as possible unless of course the boat was designed from the outset with a lower density ballast material, such is the case with most FC boats.

    Steve.
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.