Dinghy Cruiser /Racer Design

Discussion in 'Sailboats' started by Doug Lord, Apr 15, 2004.

  1. Doug Lord

    Doug Lord Guest

    Hopefully, this thread can pick up from the "Sailing Dinghy Design" Thread with more of a emphasis on cruiser /racer dinghies.
    Seems to me that a possible solution to a small- 16'(4.8m)-20'(6m)?-cruising capable dinghy that could also perform well might be the addition of a manually controlled retractable canting keel: it could be locked for cruising and unlocked for maximum power.
    Looking forward to hearing what others think....
    Last edited by a moderator: Apr 15, 2004
  2. spoedvraat
    Joined: Feb 2004
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    Location: South Africa

    spoedvraat Junior Member

    Hi Doug,

    Myself and Ian were discussing a thread exactly like this yesterday, so thanks for the effort, if you don't mind us crashing the party. For posterity's sake, I'll put down my requirements again.:

    Requirements for the Dayboat:

    The boat will be sailed on inland and fairly rough coastal waters. The worst case scenario is probably the same as for an oceangoing boat.
    I will try to apply sensible safety factors when designing critical components.

    The boat will be singlehanded often, but will have to carry 4 to 5 people on day-sails.
    This means I must be able to launch, rig, de-rig and retrieve the boat alone.

    Various options exist for construction, I prefer strip-planking and glass/epoxy reinforced approach currently. I haven’t done this before, just stitch-and-glue. I have enough basic knowledge of composites to feel confident I can do the necessary work.

    The boat will most often be used for daysailing during the weekend, and might have to do a three day trip once a year.

    Broadly stated imaginations……:

    I want to have a boat which can be used to chase the local racing dinghies, which are mostly 505’s, FD’s, some local classes, and a smattering of 49’ers (which I won’t try to catch.) I thus require a light boat which is powerful enough to plane quite easily, with crew on trapeze if required.
    However, come the summer holiday I want to use the boat as a basic cruising dinghy, on which I can cover some distance in marginal conditions, which means high stability, adequate freeboard, predictable (even docile) performance, reef-able sailplan and enough storage for the “stuff.”
    The concept I want to attempt is probably not a new one, but currently it revolves around the idea of having two options of centerboard/keel, and two of rig/sailplan, thus:

    Sailplan/Rig: Keel/Centerboard:
    Racing mode: Powerful, large roach Unballasted foil
    Cruising mode: Smaller/docile Ballasted Keel

    In cruising mode there must be a headsail furler, lazyjacks and reefable main.
    The boom is higher up the mast, and the roach of the cruising main is much less. The mast can be raked back to balance the helm, or we go to decreasing the size of the furling headsail as well to balance.

    The “furniture” inside the boat to make it nice for cruising must be removable for racing, and this is probably one of the hardest things to do elegantly.

    Further ideas:

    I like the concept of an open boat which has a double bottom, so no self-bailers etc required. Water coming on has no option but to disembark, pronto. This is nice and safe.

    Double rudders are probably equal to singles all things considered (except cost…,) and leave a nice space for outboard engine mount in the center of the boat.

    A bimini is non-negotiable in the summer down here.
    An easy arrangement for a boom-tent would be very convenient when in cruising mode.
    Further we have thought of using rolled-up (lengthwise) inflatable mattresses as buoyancy under the seats, to double as beds when unstowed and unrolled for the night.
    A battery is required when cruising, as we need to drive the VHF, GPS, tillerpilot, running lights and fishfinder as a minimum.

    And as I start churning numbers, the following seems to emerge:

    Loa 5000
    Beam 2000
    Sail area 14 m² (white sail area) (main 9.5 m², jib 4.5 m²)
    Mass 120 kg (all up, excluding crew, racing mode)
    Mass 80 kg (hull only, racing mode)
    Construction: 6mm Cedar strip with 1 layer 300 g/m² plainweave either side. Resin:glass = 1:1

    Cp 0.6
  3. PaulCoffin
    Joined: Aug 2003
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    Location: Maryland, US

    PaulCoffin Junior Member

  4. Doug Lord

    Doug Lord Guest


    That's a great looking design!
  5. Ian
    Joined: Apr 2004
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    Location: Camden, Maine, USA

    Ian Junior Member

    I hope no one minds my copying my previous post to this thread. This does seem the best location for much needed advice.


    I am working on a 16 foot cruising dinghy. Twin side by side center-boards fascinate me. Tandem (fore and aft ) boards and keels get a lot of press but not the alongside arrangement. It's extra work to build, but it would allow me to get the case out of the way. I could put the cases on either side of the well and built into the seat fronts. I would be planning on using both boards at once rather than alternately, as in lee-boards. It seems that I could get shallower draft for a given aspect ratio. Also, I suspect the heeling moment would be reduced as the center the combined lift would not be so far below the heeling axis. Is this true? It seems obvious, but what is obvious is not always what is true.
    Side by side boards would not interfere with each others hydrodynamics like fore and aft boards. I can see that ventilation at the root of the windward keel might be a problem in a keel boat (I suspect that bilge keel boats use a little extra area and the more vertical position of the lee-ward keel, due the splay, to make up for this) but I do not believe that a dinghy, which would be sailed more upright, would encounter this problem. I would not consider splaying the boards for practical reasons.
    I do, however, worry about not having a nice long single center-board to use as a righting lever should I turn turtle. My other plan is to have a single offset board.
    I was planning on having a considerable amount of water-tight storage fore and aft and in the side benches. I have since decided to make the starboard bench mostly free-flooding. It seemed to me that I might be designing a boat that was too stable up-side-down. The starboard bench would have wet storage. I feel that, if I can use my weight to easily submerge one side of the inverted boat, it would be possible to roll her back up. I noticed that the Drascombe boats have very slim, low bouancy side benches, perhaps for this reason.
    I would like to right this boat very quickly in adverse conditions as I intend fairly extensive cruises in very cold water. She will not capsize easily--probably never (except for testing purposes) but it would be unseamanlike to be unprepared.
    I would appreciate any comments whatsoever. I am not very experienced with sailing dinghys though I have had a fair amount of sea time in on very large yachts. It is not at all the same thing. I have had no experience righting dinghys any larger and heavier than a laser. Go ahead and pile on the suggestions! Thanks.
  6. Guest

    Guest Guest

  7. Doug Lord

    Doug Lord Guest

    mini Melges!

    That is the coolest little daysailer/ weekender I have EVER seen. What a great job! Too bad there wasn't more info on the site...
  8. Guest

    Guest Guest

    I saw it in a copy of Australian Amateur Boatbuilding magazine a few months ago. It is quick by all accounts and it sounded like it fitted the design brief well. There is a bit more information available on Colson's web site http://www.colsonyachtdesign.com
  9. Ian
    Joined: Apr 2004
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    Location: Camden, Maine, USA

    Ian Junior Member

    Carlson trailer-sailer

    Boy, that's a great design. Not for me though--more what Spoedvraat is working on. Working as I do with a bunch of conservative woodenboat builders, I incline more towards a traditional designs--for this boat anyway.
  10. SailDesign
    Joined: Jan 2003
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    Location: Jamestown, RI, USA

    SailDesign Old Phart! Stay upwind..

  11. Doug Lord

    Doug Lord Guest

    24 footer

    Great design Steve-and terrific design philosophy as well....
  12. SailDesign
    Joined: Jan 2003
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    Location: Jamestown, RI, USA

    SailDesign Old Phart! Stay upwind..

    Thanks, Doug. That is/was a great little boat. I kind of lost track of her after Phil sold her on, so if anyone knows where she is now, I'd love to hear..

  13. FoveauxSailor
    Joined: Apr 2004
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    Location: Southland, New Zealand

    FoveauxSailor Junior Member

    Foil Areas (rudder and keel)

    Hello Guys

    I too am working on the design of an 5.6m "Expedition style' row/sail dinghy, something that will foot it with Ian Oughtreds Ness Yawl/JII designs in terms of "tradition" but able enough to give some of the modern one designs something to think about. She will use the rig from a 470 dinghy (12.7m^2) in bermudian mode and a balance lug (about 14m^2) in traditional mode.

    Incidentally the deck plan will be based on the Drascome lugger.

    Currently what has got me stumped a methodology to determine optimal areas for the rudder and daggerboard. I know keelers are based on sail area, starting at 1.5% for the ruddr and 3% for the keel. However looking at any dinghy plans jsut gives me a headache as they vary so much. For my boat I paln to use a Naca 0012 for the rudder and probably 63015 for the daggerboard.

    Any thoughts appreciated

    Thanks - Foster
  14. Ian
    Joined: Apr 2004
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    Ian Junior Member

    Hi, I am glad this thread is attracting some attention. I used "Principles Of Yacht Design" for my final decision on area. It has 3.5% with a .75% spread for keels (down to 2.75% for outright racers) and 1.4% for rudders with a lower limit of 1%. I went for 3.5% and 1.4%. My aspect ratio will be about
    0.3 and I will use a eliptical planform of the type that is based on a straight quarter-cord line (out of the same book). It looks a lot like a spitfire wing. I believe that I will use the same sections that you are. 12% maximum cord. My sail area is 13sq.m. split into standing lug main,jib and mizzen. Thats at the high end for pure cruising dinghies--but low for a serious racing dinghy.

  15. FoveauxSailor
    Joined: Apr 2004
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    Location: Southland, New Zealand

    FoveauxSailor Junior Member

    Hello Ian

    Have you sailed a standing or balance lug before ?? The seems to be a bit of a lack of people who have really worked with these rigs to extract maximum performance with modern low stretch line and really good sails (I know there are quite a few designs done, like Oughtred and Irens but I mean actual sailing to determine the tricks/gear needed to get them to optimal trim etc)

    The best I've come across is Tom Dunderdale at Campion Design, but I don't like bugging the guys who have to make a living at selling plans etc (well actually I love talking with them . . . but it isn't really fair if you know what I mean)

    Anyhow with a balance lug He uses a kicker (or vang) a downhaul (plus an "emergency" backup loop, and a kicker on the part of the boom in front of the mast. He gets very nice sail shape. But some photos I've seen appear to get as good a shape without the extra bits ???

    Any thoughts guys
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