Twin Keels

Discussion in 'Sailboats' started by Trevor P, Jan 17, 2017.

  1. Trevor P
    Joined: Jan 2017
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    Trevor P New Member

    Hi
    I am new on here and hope it will be of some help.
    I year ago bought an unfinished steel Bluewater yacht from a company that went bankrupt. At the time I had a very good job. This contract has now ended. I now find it difficult to finance the work on the yacht. Hence my registration on boat design.net.
    I am planning on adding twin keels but, it was designed for a single fin keel and already has the stub keel.
    I understand it is wise to keep the wetted surfaces to a minimum but the small loss in speed is more than made up for in practicality.
    I also joined the JRA and am now going with a 3 masted Junk Rig as this has so many benefits (less draft, shorter masts, easier solo sailing, beautiful looks "if done right" etc).
    I have two questions: Assuming the Junk Rig is used, is a 5.5 ft draft and 7 ton ballast sufficient? Boat is 60 ft, 17 ft beam, and will weigh aroun 30 ton (steel)?
    I know about ballast ratios with a bermudan rig with 33% being the ideal but, this is a junk?
    Also can i engineer the unstayed masts to have a join, either at deck level or around 5 ft above deck to allow easy demasting to enable the odd Euro canal trip?
    Regards...
     

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  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    You've made a few inappropriate assumptions about ballast ratios, draft, etc., making me think you're not really qualified to "engineer" this types of changes. I'm not trying to insult you, but 33% isn't any ideal for any boat, though many do fall into this range, it's just a number, nothing else. I personally own a Bermudian sloop about the same length as yours (65'), with a 50% ballast ratio.

    Free standing masts can be placed on tabernacles, but it's some serious engineering to accomplish it. Simply put, you'll need the help of a designer or NA to sort through the various issues and concerns, from the shape, placement, cant and toe of the bilge keels, to the rig, ballast and it's placement, etc. You can perform much of the physical work, but you'll likely have to pay for the math to get it right.
     
  3. CT249
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    CT249 Senior Member

    I'd be wary of the JRA's claimed "benefits". For example, one of their members post photos of bermudan rigged boats he sails past, but looking at those photographs it is apparent that the boats have small sails up in light winds. Therefore to use those to imply that junk rigged boats are fast, as he does, is rather misleading - especially as the bermudan rigs with small sails up don't have to reef when the wind picks up.
     
  4. Trevor P
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    Trevor P New Member

    Thank you.

    These inappropriate assumptions were not assumptions. They are statistics that i have read in numerous articles and were relevant to craft of more recent years. A link to one such article from a well regarded source is pasted below. I came on here in the hope i would receive some professional advice not, "they will have to be substantial tabernacles"? Would you not stop to think that maybe he knows they would have to be substantial? I am looking for advice on construction of such tabernacles or joints and also the calculations to back up the design? Also the benefits of the Junk Rig are well documented (not just the JRA) by the best selling yachting publications with numerous tests and races to back it up. One such test by Yachting monthly claimed the Junk wins on all points of sail bar 1. I dont need pointless advice, just educated professional advice and if i dont get anywhere i will either model the keels on proven designs with a 1 to 3 degree toe in and overbuild the Tabernacles or pay for a professional design. Opinions are completely irrelevant. http://www.yachtingmonthly.com/yacht-reviews/understand-boat-statistics-30154
     
  5. PAR
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    PAR Yacht Designer/Builder

    Okay, you're right and the 90+ designs I have in the drawer and the new one about to go into production later this year, simply means I'm guessing. There's no ideal ballast ratio, there's no ideal D/L ratio, nor any other geometric or hydrostatic figure you might offer up, in spite of the bla, bla, bla from Nigel, whom I respect. I also understand the article and it's not a tell all, just a way to use some of the figures for comparative purposes, one boat against the other. Ballast ratio do fall in the 35% range, but this is more a function of practical engineering and construction (to a price point) than any form of ideal ballast ratio. Don't even start me up on the junk rig, let alone a 3 masted one, with low aspect bilge keels. There's a good reason the Bermudian sloop/ketch outnumbers the junk in the lands they once dominated. Lastly, all Nigel, Chris Beeson or any other NA or designer has to offer is their opinion. An educated one, but little more, once you get over the actual effort. If this wasn't the case, the results of the last America's Cup would have been quite different and NZ would have the mug.
     
  6. endarve
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    endarve Junior Member

    Trevor, I've been sailing a small English twin keel for nearly 20 yrs and can appreciate what you are doing. I will offer this up but you may already know it. Tom Colvin has gone to greener pastures and he was THE expert on steel and junk rigs. If steel and for a boat he designed it, made it or showed someone how to do it. His boats are "out there doing it" while others are tied to the dock. I believe his wife is still alert and it may be worth a try to locate her in Alva Florida. He also has sons, one is Kevin and the other I believe is Josh.

    I had the good fortune to anchor next to Tom and wife Jean for several months when cruising the Bahama outislands back in the 1970s. It was a humbling experience to sit on the fantail of his aluminum junk, drink coffee and talk boats. The salt of the earth kind of family and will bend over backwards to help. It was a sad day to hear of his passing but I'm sure his workings are still available and the family may be able to help you. Sorry I don't have his contact info any longer but you should be able to find it online.

    Good Luck
     
  7. Tad
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    Tad Boat Designer

    Trevor, understand that those statistics are taken from published data which is a mix of assumptions, hopes, and lies. Real designers and builders closely guard actual sailing weight of boats(and thus actual ballast ratio) because it is always different than the published numbers. And rarely in a good way.

    If you want to move forward you'll need to proceed with engineering rigour. First you need to think about the boat as not a collection of parts but as a system where all parts (keel, hull, rig, rudder, ballast) work together and are dependent on one another. The keel design is dependent on required volume and centre of gravity, also on required side force and location (for balance under sail). The rig design is dependent on the keel design, hull design, and the weight and righting moment of the boat.

    For anyone to help with the keel design will require you posting some drawings of the boat's profile, plan, and sections, plus the rudder.

    To proceed properly with spar and tabernacle design you need to know the righting moment of the boat. At this point no-one knows what the boat will weigh when she's launched. So you need to develop a comprehensive and realistic weight study, centre of gravity, and from that a righting moment curve.

    Post that data here and then ask your questions.

    Good luck.
     
  8. Trevor P
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    Trevor P New Member

    "Arguing with a fool, only proves there are two fools"
    I am not a boat designer neither am i an engineer. The 33% was merely a ball park in a world of designs yet, the engineers/designers have chosen to find fault? If i said 20% or 50% i would have been even further off the mark so i think this was a fair figure.
    Endarve, thank you for the useful information. I know a couple who sailed around the world for 5 years + in a Tom Colvin designed Junk rigged yacht. A beautiful boat. And they could see where they were going!
     

  9. Trevor P
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    Trevor P New Member

    I just sold an 18 ton steel bluewater yacht which had 4 ton of lead ballast so we all know of the differences.
    I bought this from a marina full of "experts" that weren't interested in the vessel and i made a £12K profit from 3 days work.
    Tad, i do know i require all of this but, without drawings this is difficult. I have used some calculators for various moments etc. Thank you for the advice though.
     
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