Half Scale

Discussion in 'Boat Design' started by Jammer Six, Aug 3, 2014.

  1. Jammer Six

    Jammer Six Previous Member

    I thank all of you.

    Of all the options presented, the strip-built 2.4 appears to be the closest to what I want, and would probably be the easiest to make look like a Skerry. And all the real work has been done-- the shape of the hull below the waterline, the dimensions, etc.
     
  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    If this was a rowing or motor boat that would be true but this is a yacht and due to the poor stability it will often have its decks awash.

    During a sail most parts of the hull will have been below the waterline at some time.

    Unwise changes to the topsides in order to restyle the yacht could make the yacht slow and difficult to control.
     
  3. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    If I recall there is a mini 12 fleet somewhere near you. You should visit one of their outings, see how they work and maybe try one to be sure of your direction.
     
  4. CloudDiver
    Joined: Jun 2014
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    Location: San Diego

    CloudDiver Senior Member

    Jammer, I think you and I have a similar apprectiaon of classic lines while looking for a daysailer of a reasonable size (i.e. not breaking the bank in construction or slip-fees, perhaps trailerable). Check the Dark Harbor 12.5 (20' LOA) and bigger Dark Harbor 17.5 (25'10" LOA).

    http://www.woodenboatstore.com/product/plan_20_Dark_Harbor

    This website is a good reference for many types of plans.

     
    Last edited: Aug 5, 2014
  5. Jammer Six

    Jammer Six Previous Member

    I have the Dark Harbor plans on the way-- I found them a couple weeks ago.

    It's sort of what I want, yes. Not quite as sleek as a Skerry, but still beautiful.

    I will say that WoodenBoat is surprisingly slow at shipping. It took them more than a week from order to shipping for a set of plans.

    P.S. I would be a lot more inclined to build a Deception or Illusion class Mini 12, or even a Millimeter, but I can't find plans for them. The 2.4 is the only design I can find plans for, and the plans I found appear to be from a private party. If anyone knows of plans for a wooden Deception, Illusion or Millimeter, please let me know.
     
  6. John Perry
    Joined: Nov 2003
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    John Perry Senior Member

    I am not sure that your original proposal is as ridiculous as some people are saying. A third scale Dragon is probably going too far, so start by considering a half scale one. (i.e. all linear dimensions halved resulting in masses divided by 8) These are the figures:

    LOA 4.44m
    Beam 0.98m
    Displacement 212kg
    Ballast 125kg

    Assume payload is one crew member, 80kg, that leaves 212-125-80 = 7kg

    7kg for the hull, rig and all equipment - simply not possible. A true half scaling of everything is not going to work. But suppose the hull, rig and equipment could be built to weigh 60kg. I am not saying this is possible, you would need to do some careful calculations to get a true figure, but based on this, the ballast would be reduced to 212-80-60 = 72kg.

    One of the main problems of scaling down a yacht design is that the ability to resist heeling under sail (not counting moveable crew weight, hydrofoils etc.) reduces as the fourth power of linear dimensions whereas for the same wind speed the heeling moment due to the sails reduces only as the third power. This is slightly simplified since it does not take into account that smaller yachts have their rigs closer to sea level so are not subjected to such high wind speed, also smaller yachts may not be expected to sail in such windy weather as large ones. Anyway, this inherent stability advantage of the larger yacht is a large part of why sailing super yachts tend to have relatively small beam when compared to more average sized yachts.

    So, by halving the dimensions of the Dragon, and if we could keep the original ballast ratio (which we cannot) and disregarding the effect of mobile crew weight, in rough terms we have reduced the heeling effect 8 times but reduced the ability to counter that heeling effect 16 times. That does not sound good and it is even worse when we remember that for structural reasons we will need to reduce the ballast ratio from the original 59% to something like 72/212 = 34%. However, all this is forgetting about mobile crew weight. The Dragon is a keelboat that is raced with the crew hiking out as needed, so I assume you would be prepared to sail the half size one the same way. Mind you, you might want to adjust the design of the full size cockpit coamings for comfort at half scale! The full size Dragon has three crew available to hike out, so at 80kg per person that is 240/1696 = 14% of the displacement. For the half size version that ratio is 80/212 = 38%, much better. Furthermore, the dimensions of the human body being the same for the fullsize and half size boats, you would be able to get your centre of gravity relatively further out from the hull centre of bouyancy with the half size boat. I havent done the detailed sums (which would need knowledge of the GM measurement) but my guess is that if you are prepared to hike out you should be able to overcome the stability problem. Mind you, there is a further point to consider which is flotation when no one is on board. I guess that 72kg of low down ballast would be enough to keep it upright (would need a light mast though) but it would float high on the waterline which you might not like since you want to keep the classic looks - perhaps consider water ballast for when the boat is left afloat but not occupied.

    As others have pointed out, the half size version would be limited to a lower displacement speed than the full sized version (sqrt(0.5) according to the usual simplified formula). Hence, for similar wind speed the ratio of boat speed to wind speed will often be lower meaning that the keel surface will often be more heavily loaded for the half size boat. This may not be too much of a problem since the Dragon is a design having a generous lateral area under the water, albeit with a low aspect ratio. The rudder might prove to be a bit small though - one expects small boats to have good maneuverability and that small rudder not very far aft of the total centre of lateral area may not be enough.

    Finally there is the problem of waves. Halving the freeboard will potentially result in a lot of water coming on board and if you keep the internal arrangement of the full scale Dragon your boat will probably swamp and sink like a stone. However, building large sealed compartments into the ends of the hull and having a shallow self draining cockpit should overcome that problem.

    All in all, the boat you are suggesting is not a boat I would want to build or own, but if its what you really want I would not say that it is totally ridiculous. However, for the same overall length you could certainly have a faster and more practical sailing boat.
     
  7. Jammer Six

    Jammer Six Previous Member

    I'm afraid I'm older than that. I might lean out, or I would man the rail on a 40 footer, but my days on a trapeze are thirty years in the past. In fact, cruising or racing cruisers, I make a point of taking at least one crew who is young, strong and obedient along for grinding, mooring and anchoring chores.

    I'm starting to think I need to build two boats. First, a mini-12 of some sort, to race. Second, a Dragon, a Dark Harbor or a 5 meter. It will probably end up being a Dark Harbor, since I already bought a copy of those plans, but it's a Skerry that I really want.
     
  8. WhiteDwarf
    Joined: Jun 2011
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    WhiteDwarf White Dwarf

    An option from Selway Fisher and other thoughts

    You might want to consider the:

    Safe Harbour 12 strip planked daysailer.

    http://www.selway-fisher.com/OtherDB.htm

    So much depends on the builders skills. A strong light structure may enable some of the compromises required by the OP.

    Boats of this size have been bread and butter to boatbuilders for 100s of years. The fact that few practical craft have emerged that embody the aesthetic sought by the OP in the small size he seeks, suggests it is very hard to achieve.

    He could explore some of the plans for 1/2 raters from 100 years ago and modify the rig to his requirements. He might also look at some of the Solent one designs for inspiration, Westmacott's X-Boats and Victories. Tough enduring and at the upper end of his preferred size range.

    By the way, speaking from limited experience, even a full size dragon is among the wetest ways of traveling under sail ever invented. At half the size you would need a wet suit even in summer. Remember, its a day boat, not a 90 minute dash around the inshore marks in a dinghy. She is, I think, a one design based on the 22 sq metre class, there was a 15 sq metre class too I vaguely recall.
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The good news is the cost will be less than half that of the full scale, but it all goes pear-shaped after that ! :rolleyes:
     

  10. WhiteDwarf
    Joined: Jun 2011
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    Location: Sydney

    WhiteDwarf White Dwarf

    Picture of Safe Harbour 12

    The attached was extracted from Watercraft Magazine. They have a detailed article on the build.

    View attachment Nomad.pdf
     
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