Low-speed sailboat hull

Discussion in 'Hydrodynamics and Aerodynamics' started by laukejas, Oct 12, 2014.

  1. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Hello,

    Over time, I've been studying hydrostatics and hydrodynamics of various hulls, experimented with DelftShip software to create different shapes for different purposes, took some existing, proven designs, and analyzed them to see what makes them so good.

    Now, I have an interesting challenge. Being a wannabe sailor, I theorize on the next sailboat I could build next summer. Since I have very strict requirements, I can't find an existing design to buy or one that would fit my budget. Of all the options I have thought of, I'm working on this one now: 1 or 2 person ultra-light skin-on-frame dinghy, probably standing lug-rigged, 4 meter LOA, displacement mode, designed for cruising. Since I'll be primarily sailing in lakes in my country, we often have near-dead winds. So I need a boat that is very easily motivated, at the expense of top speed.

    I expect this boat to weight around 20-25kg. Add 10-15kg of equipment, 80kg of a sailor, and we have a displacement figure of 120kg. With two persons, that's 200kg.

    The main thing I'm looking at is prismatic coefficient. I know that for easily motivated hull, I need it low. As I mentioned, I don't care much for top speed. This won't be a racer. I need this boat to move when all other boats are becalmed.
    I have experimented with various shapes in Delftship, and managed to get a CP as low as .4. However, for practical reasons, I guess that is way too low.
    I know that most sailboats aim for .56-.6. Since I'm designing this for particularly low speeds, I aim for .54. Do you think this is low enough?

    Another thing is that I need same prismatic ratio at both 120kg and 200kg displacement. I noticed that as displacement increases, so does the prismatic ratio. So what I'm trying to do is make this hull full-ended at shallow draft, and fine-ended at deeper draft. So far, this is proving very difficult to do.

    Also, I need to maximize waterline without submerging the transom (I know it will produce a lot of drag in these low speeds).

    My question is - do you know any existing designs that fill this criteria? Low-speed oriented displacement hull with same CP at different displacement figures, full-length waterline without submerged transom?
     
  2. HJS
    Joined: Nov 2008
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    HJS Member

    show us what you've done so far
    a picture says more than words
    js
     
  3. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    Okay. In .fbm format or just photos and numbers?
     
  4. philSweet
    Joined: May 2008
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    philSweet Senior Member

    The trouble is the weight issue. For good light air performance, you need a lot of sail area compared to the hull's wetted area. This translates into heavy, beamy, solid little boats. D/L in the 350 - 425 range for a 16 LOA, 12.5 LWL footer. Mast about 24 feet off the water. The hull isn't the issue for low speed, holding the rig up is. Look at the proportion of some of the small Herreshoff boats designed for the light winds of Long Island Sound and Narragansett bay.

    http://sailboatdata.com/viewrecord.asp?class_id=3583

    Things begin to change when you get a bit more length on it, but this is the way to get a 16 footer to ghost along. A light weight boat is just a pain to be in in these conditions. You could build the whole thing out of concrete if you are desperate to save money.

    Also take a look at Shorty Pen's Pocket cruiser site for lots of ideas.

    http://www.shortypen.com/sailboat-guide/pocket-cruiser/

    For me, camp cruising meant about half a ton of gear, so I could get away with a light hull as long as it would carry 1500 pounds of load through a small surf to the beach. But I sailed in a notoriously high wind area. Anything less than 15 knots was uncommon and probably meant I was riding my motorbike, not sailing.
     
    Last edited: Oct 14, 2014
  5. laukejas
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    laukejas Senior Member

    Thank you, Phil, but what we're talking completely different boats. What I have in mind is a boat so light that it can be carried with one hand.

    When I said cruising, I didn't mean taking half the house with me! :D Just some food, some clothes, maybe a tent. That's all.

    And here, sometimes winds don't go up more than 2 knots during whole day. That's one of the reasons why I want a light boat: to be able to ghost along in these conditions with minimal load. In my case, wetted surface is 2m^2 at 120kg load, and 2.5m^2 at 200kg.

    I calculated that with 200kg displacement, my D/L ratio would be 87.

    I will take a look at Shorty Penn's Pocket cruiser site, but in the meantime, could you suggest some ideas for this specific boat type I'm aiming for?
     
  6. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    " I need a boat that is very easily motivated, at the expense of top speed"

    ... is a contradiction in terms.

    The very things that make a boat good in light airs, are what makes it good in top speed. This includes efficient hull form, good sail performance and light weight.
     
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    The lightest way to sail fast in light wind is a multihull.
    A trimaran would be best.
    As has been said, stick up the biggest effecient sail you can. But you will be in real trouble in heavier wind.
    An alternate rig (shorter/ smaller) would make it easy to sail both heavy and light winds.
    You might look at the Sailrig for a kayak on clcboats.com
    http://www.clcboats.com/shop/boats/wooden-sailboat-kits/clc-sailrig-kayak-canoe-sailing-rig.html
    A double kayak would help with a passanger and some gear.
    The sail area could be increased with caution. Due to the lack of freeboard this rig is suited to lighter wind/ calmer seas.

    Or try the Trika 540. http://www.dixdesign.com/Trika_540.htm
    As drawn it does not have a huge sail, since it is an unstayed mast you might have to make some changes to go larger.

    In neither case is the weight TOO high, IMHO.
     
  8. laukejas
    Joined: Feb 2012
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    laukejas Senior Member

    I understand. What I had in mind was derived from prismatic coefficient principle: lower coefficient gives an easily motivated hull which, however, struggles in higher speeds, and will require a lot of power to achieve hull speed, while higher coefficient allows higher top speed, but requires more power at low speed. Same with planning hulls (compared to displacement hulls, they produce lots of drag at low speeds).

    At least it's how I understand it. Correct me if I'm wrong.

    upchurchmr, thank you for your suggestion. I have done some research before posting this, and many people indeed recommend trimaran for this purpose. However, since I'll need to car-top it single handed, carry to very inaccessible launch spots, and do it often (anything that is left afloat overnight is stolen in our country), so I'd guess Trika will be too heavy for this purpose, although it's car-topable.
    Another consideration is budget. I can spare 800-1000$ for this project, all included. I doubt trimaran would fit the budget.
    Another thing is, I'm learning. Many sailors told me it's better to learn in monohull, because it's more sensitive to crew weight shift.


    Of course, I'll try to maximize sail area, that's understandable. But right now, I want to understand which hull would be best for low-wind sailing. Low CP, transom above waterline, fine entry on bow.
    Anything else?
     
  9. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    That seems to be correct eg.

    "A low or fine Cp indicates a full mid-section and fine ends, a high or full Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp."
    http://en.wikipedia.org/wiki/Hull_(watercraft)

    Also, you are correct in anticipating the complexity of multi-hull construction and transport, as well as load carrying limitations. ( Try stuffing supplies into an inspection hole in a narrow hull )


    But I think you are over complicating the thinking process.

    With your budget, and size limitations, just creating a hull that wont tip you out while getting in, and while sailing, and light enough to throw round on car tops - makes any kind of co-efficient considerations pretty minor.


    having said that, a good solution is found here

    a SOF sailing boat

    http://gentrycustomboats.com/Annabelle.html
     

    Attached Files:

  10. laukejas
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    laukejas Senior Member

    Rwatson, thank you, Annabelle looks beautiful. It might just fit the requirements. Do you have any estimate on budget to build such a boat?

    By the way, I got the idea of ultralight boats from here: http://gaboats.com/
     
  11. rwatson
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    rwatson Senior Member

    They sell the kit for $US750, so if you couldn't buy the materials for under $700, you wouldnt be really trying.

    I suggest you buy the plans, and substitute the suggested materials with whatever is closest to materials where you are. Having a plan will save you lots of money in wasted and unsuitable materials.

    eg. They might suggest nylon for the skin, but light canvas would do the job.

    You would end up with a useful, great performing boat you could be proud of.

    GAboats is inspiring, but that canoe he was holding would probably use up your entire budget in one go.
     
  12. laukejas
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    laukejas Senior Member

    I'm really considering buying these plans. Building it would be a real challenge, but very rewarding. You've shown me a gem here.

    How do you think this boat would perform under very light winds? Also, the hull seems like it was designed to be planing (because of flat, wide aft shape). Is that correct? As an experienced boatbuilder, could you analyze this hull from pictures, please?

    Why would that canoe be that expensive to build?
     
  13. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    This design would give you as good a light wind performance as any boat that size - and its really light. Flat ,wide aft is for stability in a gust, for holding you up out of the water when you unwrap weed off your rudder, and for you to keep out of the water in lights airs like in the attached photo. (Remember that advice from your first project about trim ? )

    Given the experience you have had on your previous projects, I think you would find building it little trouble at all.

    I was going on the sailboat equivalent at
    http://gaboats.com/boats/blivit13.html

    The rig alone would set you back $400, and the material they use for the skin
    http://gaboats.com/construction/

    (Dacron. ....it heat shrinks. It is a super-weight, airplane wing covering type of fabric, .)

    with the internals
    " braced with triangulated KEVLAR® roving strands"

    and

    "Q: What is used to seal the Dacron skin?

    A: There are a lot of options. A couple of coats of exterior acrylic latex paint or water born varnish are the simplest, The varnish has been popular to render the translucent see through geodesic framework "

    http://gaboats.com/faq.html#q4_2


    would be another $200 + , or more depending on where you live in the world.
     
  14. Nick_Sinev
    Joined: Aug 2014
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    Nick_Sinev Junior Member

    Re: weight around 20-25kg

    Inflatable catamaran?
     

  15. Ben G
    Joined: Jun 2013
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    Location: Brisbane

    Ben G Junior Member

    Hi,
    I sail a 12' skiff (that I designed and built). Take a look at the 'Woof' hull design (google images - 'woof 12' skiff'), it would probably do well due to its U-shaped sections, add some more rocker and more WL beam etc. 3.7 metres long.
    If you do build a nice light boat with large sail area you will find it hits hull speed very quickly. I'm guessing we would often do 5 knots in 5kts TWS at most angles. But this is not comfortable sailing.
    From this point of view I'd assume you will reach hull speed without much trouble. Now if you're not going to try and plane, the next best thing is to be able stay at hull speed for as long as possible with minimal effort and maximum comfort.
    For example, a 3 knot gust on a 3 knot base wind will capsize you immediately..
    Jumping all around the boat will shake the wind out of your sails. You might like to sail at a slight heel to let the sail fall to shape. etc.
    In summary I think the practical aspects of sailing well in light breezes will outweigh the finer details of the design.
     
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