Designing a sailing dinghy for expeditions

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

  1. BoraBoats
    Joined: Feb 2019
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    Location: Norway

    BoraBoats Emilie

    Two people, yes. The displacement at the waterline is 227 kg. The displacement at the waterline + 0.7 cm is 247 kg. But remember that I'm not a naval architect and this is just a concept. A naval architect would have to do the final hull design if this was going to be produced.
    Right now it's only a stowage place at the bow, under the deck. I have to work on finding solutions for strapping on gear several other places, so that the weight can be balanced.

    (edited)
     
    Last edited: Apr 14, 2019
  2. Angélique
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    Angélique aka Angel (only by name)

    As an alternative to heavy rotational molding, hand lay-up is not so expensive in low labor cost countries, here it says in Uganda it's a shocking minimum hourly rate of $ 0.01
     
  3. BoraBoats
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    BoraBoats Emilie

    I'm thinking rotomolded polyethylene is a good choice because it can handle rough treatment, like geting pulled up on a rocky beach. If you get a scratch, it is not so visible. It is impact resistant. Also, it can be recyceled, which is a pluss. But, of course, the weight is important and I need to do some weight estimations before deciding.
     
  4. Angélique
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    Angélique aka Angel (only by name)

    If the complete boat turns out to be 60 kg and the crew 150 kg, than that leaves 37 kg for camping gear plus food and water.

    Might want to increase the beam a bit if the empty but fully rigged boat turns out to be heavier than 60 kg.
     
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  5. Angélique
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    Angélique aka Angel (only by name)

    High-pressure injection molding has higher initial costs, but I think that's not a big problem for very large production series, it can be stronger and a bit lighter than rotomolded I believe, and it's also 100% recyclable. E.g. the Walker Bay rigid dinghy hulls are made by high-pressure injection molding.


    P.S. - I don't guarantee any dinghy survives this kind of treatment, but Walker Bay itself did put up this promotional video, but note the limitations they've put in the video text.

    P.P.S. - hull integrated double stern wheels are shown in the Walker Bay Genesis video at 2:38.
     
    Last edited: Apr 15, 2019
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  6. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    Rotomade web site shows the process, in the video in particular. Density 960 kg/m3
    Rotomolding process - Rotomade https://rotomade.com/en/rotomolding-process/
    Examples with similar sizes are the RS Feva, Zest and Quba, built with :
    • 3 Layer rotomolded RS Comptec PE3 construction
    • Tough outer skin, foam core for stiffness, strong inner skin
    • Thickness added in high load areas which enables reduced overall weight
    >>> bare hull masses :
    Feva (L 3,64 m x B1,42 m) >> 63 kg
    Zest (L 3,59 m x 1,47 m) >> 68 kg
    Quba (L 3,48 m x B1,37 m) >> 58kg
    RS Feva XL (RS Sailing) - Fiche technique de voilier sur Boat-Specs.com https://www.boat-specs.com/fr/rs-sailing/rs-feva-xl
    http://www.zouintermarine.com/boats_rs_ZEST.html
    http://www.zouintermarine.com/boats_rs_QUBA.html

    ...to compare with the RS Aero built in composite glass and carbon + foam core, given (bare hull) for just 30 kg :
    https://www.rssailing.com/wp-content/uploads/2017/06/RS-Aero-Brochure.pdf

    By LxB ratios, you can have a first estimate of your bare hull mass with the two options.
     
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  7. BoraBoats
    Joined: Feb 2019
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    BoraBoats Emilie

    @Dolfiman, thank you! I will calculate the weight when I get home from easter holiday. Right now I’m at a cabin and I don’t have access to the CAD files.
     
  8. Angélique
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    Angélique aka Angel (only by name)

    There are more materials to choose from for injection moulding than for rotational molding, so I think there's a challenge to get the strongest and lightest mass produced end product for the price that you have in mind, which I think should be competitive with what's on the market already.

    Some Wikipedia links:
    - Injection moulding Examples of polymers best suited for the process (named per group)
    - Rotational moldingMaterials (individually named)

    You could inform some outsource subcontractors of both systems about your master thesis, and ask if they can advice a material, and if they can name an estimated weight and price per hull, the latter will require that you specify a quantity per production run. And both systems will require that you have a specific mold build, so I think you'll need an estimated price for that from a mold-maker/toolmaker/machine-shop. They like to sell their products, so best give them a chance to advice and to think with you . . :)

    Maybe you can 3D print your physical scale model and all its parts from the CAD model files, there must be such an facility available for a master thesis at the Norwegian University of Science and Technology . . ?
     
    Last edited: Apr 16, 2019
  9. Angélique
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    Angélique aka Angel (only by name)

    Forum member Raggi_Thor from Trondheim used to be active on these forums, if he's willing then you could have a good chat with him about your project and about local production facilities. Not that production needs to be local, but to get addresses for local first hand production information and prices might be handy for a master thesis. So you could ask Ragnar about it — M BoatsDesigndata AS
    You don't want to have them filled with water over time, so if made from extruded aluminium profiles with plastic caps then best have them filled with lightweight closed cell foam in the production stage. For mass production I would have all rudder and dagger/centerboard foils complete injection moulded from strong lightweight plastic though.

    Ragnar also has some marine quality extruded aluminium NACA profiles for rudders plus daggerboards and centerboards on his website at the hardware* section for DIY builders and small series, so you could have a chat with him about that too, if he's willing to do so . . :)

    * note he shows the NACA profiles foam filled there

    Good luck !
     
    Last edited: Apr 16, 2019
  10. BoraBoats
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    BoraBoats Emilie

    Thank you for your tips, @Angélique . I will try to get in touch with Ragnar. My plan for the scale model is to use cnc milled foam for the hull and 3D print the smaller parts.
     
  11. Six Pack
    Joined: Apr 2019
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    Six Pack New Member

    Hello Everyone!

    I've built a few boats / floating vessels out of PE and have been around rotational moulded items in rugged environments a bit, so here my 2 cents:
    • One of your main objectives was for the product to be light, and that is the thing that unreinforced thermoplastics are really bad at.
    • You can get it super rugged with PE, but then it will be - very - heavy. They make great workboats out of PE sheet (these are Fabricated - welded - out of semi-finished products), but they are 8mm or so wall thickness.
    • To get it light, you can do that only by making it thin - and then it will be no good for the outdoors environment you are envisioning (or anything at all...) Landing on rocky beaches? Definitely not.
    • The most rugged and affordable thermoplastic is ABS (Snowboard Edges, etc...).
    • Density of PE is just under 1, so you have positive flotation there, which would be a plus.
    • Rotational moulding is bad at putting the material where you want it in general. Edges and corners will be thicker than the rest.
    • Repairability of PE is generally good, but you need a tool / electricity. While it is possible to do it on a camp fire, you have to be McGyver to get it (the temperature) right.
    • Long story short, if you make it out of unreinforced PE (no matter the fabrication process) and want it light, you will end up with something that your users will drag over the rocky beach, get a hole in it, and can't repair it where they are at.
    More thoughts:
    • Wheels are great, on boat ramps. And only there. Definitely not on beaches or rocky Norwegian shores.
    • Injection moulding is really expensive tooling wise, I am pretty sure that's prohibitive for your product
    How about using FRP - consider Aramid! - and using "PE trailer skids" (google...) for the bottom? That would end up light and very rugged. A worthy product, the user of which shall fear no rocky shoreline. And who would bring along some patches and epoxy for repairs underway.
     
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  12. philSweet
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    philSweet Senior Member

    While I like the design of your boat, I don't see what it has to do with exploring anything by boat since I appears to have about a 2 hour voyage endurance as shown. I've done quite a bit of minimalist coastal exploring and the departure cargo weight is usually around 700 - 800 pounds for a typical 2 person trip.

    Propulsion - You absolutely need a little outboard - it is just plain stupid not to have one. People need to be able to keep a schedule and arrive at their destination on time each night. This is crucial. The only way a small boat can do that is with a little outboard. So a 2 hp outboard and 12 gallons of gas gives you the ability to motor at 3 knots or so for 100 hours or more. This is what it takes just to begin, because pulling permits for camping, even if you book a year in advance, is fraught with challenges, like a hurricane took three of your sites away over the winter, or the bloody clerk left one of your stopovers off the permit and now you have to do a 50 mile day in the middle or not go. Or prudence dictates starting a day later due to weather and you have to do 40 miles on your first day. The reason I was able to do so many trips is that I was at least able to begin every one of them. Call it 150 pounds with the engine mount.

    Water. My typical water load was 4 x 7 gal hard sided jugs + 4 x 5 gal collapsible jugs = 48 gallons of water. Call it 410 pounds with the jugs.

    Food - about 45 pounds for 2 people for 2 weeks. Mostly dry/dehydrated, but some cans and jars. 2 backpacker stoves, a grill, pots and pans, and 2 gals of whitegas. All stored in steelclad Coleman coolers (not bear-proof, but you'll sure hear them going to work on them). Call it 75 pounds.

    Camping gear. Usually an expedition-rated 3 man tent plus jungle hammocks or bivys and the usual kit and caboodle. 40 pounds for 2.

    Electrical - boat should be night capable with a decent nav system and possibly an autostick. It needs a battery, solar panel or small gen set, small inverter, laptop, gps dongle or portable gps, electocompass, mag compass, Q-beam, vhf radio and autostick. If solar, you'd want about 200 Watt rated. There are also a couple tiny outboards that can generate a little juice.

    Ground tackle and mooring gear. I carried two anchors, a sounding lead and logline. I carried about 1000 feet of 3/8 hard nylon for hauling the boat. Look up "flip flop winch" for how to drag your boat over problem areas. You'll need a saw to make the winch and cut rollers. I only had to do this three times in 25 years, but I was the only camping party that still had a boat after 2 of those times. You need the ability to lift your entire boat up vertically as well. This saved my hide once. Always carry a pair of handy-billys made up. Basic tools and hardware to patch boat - like a carpet square and a quart of epoxy, are important. My first aid kit weighs about 20 pounds.

    Hobbies? ham radios, camera gear, telescopes, binoculars, artist pallet, books, guide books, fishing gear, crab pots, snares, guns, snorkle gear, pets. My travelling partner tended to abuse this part of the weight budget.

    If your going to head out there any do some exploring, don't cut things so fine you can't render aide to others. You should always keep enough reserves to help out others, including towing bigger boats and helping out stranded parties. Also consider that people are getting bigger. I've had an invited guest show up with another guest that weighed 350 pounds. No problem, pile in and let's get going.

    So I think you need to look at where all this stuff is going to go. My dinghy wasn't slow. She ended up as a pot tender with a 25 hp Yamaha on the back after 25 years as a sailing dinghy. I built her specifically as a camp cruiser for the Everglades. It's predecessor did weigh about 130 pounds and I used it until I was about 25. But it just was too small and a bit fragile. It was a 14' 50's Mac Dinghy made of 4mm ply and red paint. Some photo's of it @ looking for the right boat https://www.boatdesign.net/threads/looking-for-the-right-boat.54096/#post-749289. My next one was 16', and a bit heavier.
     
  13. Clarkey
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    Clarkey Junior Member

    It is interesting how far the concept of 'minimalism' can be stretched.
     
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  14. Six Pack
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    Six Pack New Member

    Talking about stretching... Here is the setup of an Jean-Christophe, who came by our island a few years back:

    [​IMG]

    He went around the islands with this for around 3 weeks, his blog is here. Pretty cool Pictures and story.

    To get back to the topic, I imagine we are talking about something roughly in between the 2 extreme examples above?
     

  15. BoraBoats
    Joined: Feb 2019
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    BoraBoats Emilie

    I think you are picturing a different boat than me, @philSweet . This project has greatly been inspired by Magne Klann who sailed Norway from north to south, about 1000 nautical miles, in an RS Aero. He brought with him about 30kg of equipment. forum | General Discussion | NORWAY TO PORT! | RS Aero Class Association https://www.rsaerosailing.org/index.asp?p=forum&fid=1&tid=8067
    Comfort is not the top priority of the user I have in mind. It is rather to get a challenge both physically and mentally, and to get a sense of achievement.
     
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