Designing a sailing dinghy for expeditions

Discussion in 'Boat Design' started by BoraBoats, Feb 25, 2019.

  1. Doug Lord
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    Doug Lord Flight Ready

    Emilie, I'm really impressed with your initial drawings and concept. I think Dolfiman and John have some good suggestions as do others. The idea of adding a bit of performance under sail to this concept is innovative and certainly worth exploring further!
    I did a boat in the mid- 70's that had a molded-in cooler and large molded-in stowage compartment.
    The boat also had molded-in leeboards that worked very well. Here are some shots-a couple taken after prolonged storage but you'll get the idea:

    Tantra + Frank.JPG

    Tantra with "US 1" sail:---------------------- Tantra with experimental rig:
    Tantra_with_US_1_rig_002.JPG Tantra_X-rig.JPG

    after prolonged storage:
    Tantra_circ_cockpit_stowage_cooler.JPG Tantra-molded_in_leeboards.JPG
     
    Last edited: Mar 2, 2019
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  2. sailhand
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    sailhand Junior Member

    If as originally posted the design is to be used for exploring and expeditions and your primary focus is ergonomics aesthetics and user friendliness then by all means include performance however not at the expense of your primary focus. Sailing monohull dinghys is both exhilarating and rewarding and requires almost constant attention, a degree of physical activity and skill. If expeditions exploring ergonomics etc are your primary focus then ease of sailing and long term comfort and the ability to carry that which makes your journey comfortable would necessarily be your main focus. The benefit of stability being provided by the platform you are on rather than having to constantly position your body to provide stabilising ballast would allow for long term "user friendliness" more effectively. To my mind that therefore implies stability provided by ballast or bouyancy associated with the platform rather than the sailor constantly shifting to achieve a level of stability. If we then consider beaching launching etc then bouyancy is far easier to move around than ballast. The idea of being seated in a central hull and letting the amas do the work seems more user friendly to me. A simple cat rigged boat with easy to hand sail and steering controls would be my ideal platform. The downside of the multihull approach is of course load carrying capacity. Perhaps the weta trimaran could supply insight into how to achieve both load carrying ability and stability. Some might categorise the weta as a stabilised dinghy/skiff rather than a trimaran. It would certainly have more room for accommodation and carry heavier loads. The larger amas would also provide a greater degree of both directional stability and righting moment. And just for the sake of another hat thrown in the ring there is bernhd koehlers duo 480.
    Kohler catamaran Duo480 Jigsaw https://www.boatdesign.net/threads/kohler-catamaran-duo480-jigsaw.52629/
    Maybe a smaller single free standing mast version with enough beam to sleep athwartships in a cabin.
     
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  3. gonzo
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    gonzo Senior Member

    The rule of thumb for oars is that they are 1.5 times the beam. Depending on the freeboard, that dimension needs to be adjusted.
     
  4. markstrimaran
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    markstrimaran Senior Member

    20170708_173158.jpg
    Sound like something I would like, this but maybe with carbon fiber and titanium.
     
  5. sailhand
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    sailhand Junior Member

    Max hull weight 60kg- trimaran with three hulls can be separated for launching etc well under 60kg per component

    Max 2 people- look to tandem hobie, these are rotomolded and therefore extremely heavy for size, could be made much lighter.

    30-40kg cargo capacity- no problems

    Two people sleep aboard- easy with trampolines on each side or maybe cabin fore and aft similar to angus rowcruiser.

    Seaworthiness- mast top float and folding amas for capsize recovery, cabins sealed water tight and bouyancy up high. Sail reefs on mast rotation, very simple. Rig easily removed at sea and secured to amas, hobie rig just pops out. Hard to flip a tri with no rig. Higher cabins also provide protection from elements, maybe some type of sliding canopy over seating area.

    Beaching boat- refer video for wheel system, can leave admiral in dinghy and wheel up beach no problem at all.



    Cost- simple low cost build should make it affordable, use existing high volume rigs etc where possible, as large scale production lowers costs.

    Production series- refer previous question

    Style- thats up to you.

    Speed- tris are very fast. Refer round the world solo records. Two handed atlantic race etc etc. If seated near to water then impression of speed and excitement much greater than when seated higher, ditto fun factor.

    Human powered- oars are large and cumbersome when space is at such a premium, also requires a rear view mirror to see where you are going. Pedals are more compact and uses larger muscle group and facing forward, thin trimaran central hull facilitates kayak style paddling, again facing forward, no need to change seating arrangement for rowing. No need for sliding seat and outriggers etc.

    .
     
  6. Dolfiman
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    Dolfiman Senior Member

    I have continued to investigate the bi-convex hull option for a monohull dinghy. Details in the document attached, the results in short :

    *** With a beam overall of 1,80 m (instead of 1,50 m for the conventional U hull), all other things equal inc. the waterline beam 1,07 m, this option leads to an extra mass of about 7 kg, so + 9% of the light weight equiped boat.

    *** For the usual sailing configuration with a payload of 2 crew/150 kg just sit windward, i.e. the dinghy upright with more or less 5° heel angle, the Bwl, the waterlines and the wetted surface are similar but the RM is better by + 20% (due to more overall beam). In case of gust of wind leading to an over heel of 20°, the extra RM is + 40%.

    *** For the stability during a tack or a gybe, when the 2 crew are squatted in the boat center to go under the boom, the GM increases with heel angle instead of falling dangerously in the case of an U hull : at 15° the GM is twice (35 cm instead of 17 cm), at 25° the GM is 4th (32 cm instead of 8 cm).

    In my opinion, this bi-convex option, at the cost of just 7kg more weight, with no or marginal extra drag, offers significantly more RM and stability margin, relevant features when envisaging a dinghy sailing in wind force 6 / sea state 2m conditions.
     
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  7. Angélique
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    Angélique aka Angel (only by name)

    There's also a part 2 video.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Emilie, how's it going? Hope all is well.....
     
  9. BoraBoats
    Joined: Feb 2019
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    BoraBoats Emilie

    Hi! It is going a bit slow, but hopefully I will be able to post an update later this week.
     
  10. BoraBoats
    Joined: Feb 2019
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    BoraBoats Emilie

    Project Update 5 – Concept 1.2

    Finally, here is a project update. I have been working on making the concept I posted earlier more realistic.

    c1_2_1.jpg c1_2_2.jpg c1_2_3.jpg c1_2_4.jpg c1_2_5.jpg c1_2_6.jpg c1_2_7.jpg c1_2_8.jpg c1_2_9.jpg
    I am struggling a bit with deciding what kind of rig the boat should have. I want it to be simple and user friendly both when sailing and rigging. For now, I have chosen a boomless furling mast like Hobie Island. I have also chosen to have a gennaker, and not a jib, after the recommendations from users that I have talked to. One problem that I have yet to solve is the placement of a gennaker sock/ compartment. Initially, I was thinking to have a rigid plastic compartment under the deck. A conventional sock under deck can be a problem if the user packs the boat full of gear which might cause the gennaker to get stuck. The maximum length of a compartment under the deck is 92cm. A gennaker with a luff of 4.9 m, which would be fitting for this boat, would need a sock/compartment that is around 170 cm long. So, unless I figure out a new way of folding the gennaker, a compartment under the deck will not work as a gennaker sock. One solution is that the user has to stuff the gennaker into the compartment manually. Thoughts and suggestions for the rigging are welcomed.

    I need to work on the mechanics of the hinging/connection between the hull and the wings, storage possibilities, rigging + a lot of detailing. Hiking straps, tiller extension and the running rig will also be included later. I will look at the possibility of having extruded aluminum rudder and keel with plastic caps. I have seven weeks after easter to finish the project. My plan is to use a couple of weeks to improve the concept, then make the final CAD model and a physical scale model.

    So, what do you think? What is good? What could be better?
     
    Last edited: Apr 14, 2019
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  11. Angélique
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    Angélique aka Angel (only by name)

    Rotational molding of polyethylene is nice for mass production, but it gives pretty heavy products, did you calculate the weight of it yet for your expedition dinghy ?

    Note the little wheel on the aft end of the Walker Bay dinghy keel, for ease of dragging it around on the hard, and to prevent wear on the hull.

    little aft wheel Walker Bay dinghy.jpg

    The little permanent aft wheel is shown in this video from 0:07 to 1:09, couldn't find it on the Walker Bay website, of course it will cause some added drag plus weight and costs, but it could be considered for your dinghy.

    Good luck !
     
    Last edited: Apr 17, 2019
  12. Doug Lord
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    Doug Lord Flight Ready

    Have you considered a roller furling gennaker/asy spin? Then you wouldn't need any space below.... You're doing a terrific job!
     
  13. Angélique
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    Angélique aka Angel (only by name)

    - Is this a 2 people boat ?
    - What is the max carrying capacity ?
    - Where is the camping gear plus food and fresh water to be stored ?
     
  14. BoraBoats
    Joined: Feb 2019
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    BoraBoats Emilie

    @Angélique, good idéa with the wheel! I haven’t calculated the weight yet because I do not know what material thickness is necessary for a boat like this and I don’t know the density of the polyethylene that is used (that might be easy to find out). Do any of you know? If I had the density and thickness of several material options, I could do some calculations and compare the results.

    @Doug Lord Thanks! I have briefly thought about a furling gennaker. That might be the solution. I will look more into that.
     

  15. Doug Lord
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    Doug Lord Flight Ready

    Just an idea: you appear to have a lot of space just ahead of the rudder and under the tiller. You could make a removable box ("campbox") to fit that space so for performance sailing you didn't have to carry it. You might raise it a little above the cockpit sole for drainage?
     
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