Would a tiny mastless multihull be sufficiently stable in heavy seas?

Discussion in 'Multihulls' started by NeilG, Jan 16, 2014.

  1. NeilG
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    NeilG Junior Member

    I'm envisaging an sea-/ocean-crossing pedal-powered boat. Pretty much all of the solo versions of these boats to date have been monohulls, and for stability have often dragged a heavy keel along in the bottom of or under the hull.

    I can see some clear advantages to a multihull. (In particular, I'm thinking of a proa (or maybe a trimaran).

    Advantages
    - Propeller fitting could be between the hulls straight into the water, so would have no need to risk compromising the hull (for it to be fitted through the hull).
    - Improved comfort.
    - Lighter weight boat with narrower hulls should be somewhat faster.
    - Better ability to land on beaches (I'm thinking of a mediterranean passage in this case, where plenty of islands would be passed) - assuming that either the propeller is not too deep or can be retracted for landing.
    - Relative unsinkability; in a holing or capsize the boat would still provide a life-raft of sorts.

    BUT I'm very concerned about the risk of a capsize. Size I envision in the range of a main hull 7 to 9 m (23 to 30 ft) in length and around 0.9 to 1.2 m (3 - 4 ft) beam. Hull separation I'm not sure what is best but at a guess it'd be around 4 m (12 ft) for a proa, if physics don't dictate a better distance.

    I can't see a good way for a proa to be self-righting without compromising the design. I can see it might be possible to get it to right from outside, but the last thing I'd want to do if I've capsized in heavy sees is get out and try to right the boat (of course, it might be feasible once the weather has passed). There's also the added expense of having an escape hatch for if inverted.

    Is there a real risk of capsizing a mastless proa or tri of these kind of dimensions just from the action of the waves in typical sea or ocean storms? How great a risk? What design elements would help reduce the risk of capsize.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    For your purposes, catamaran hull slimness is important, and 4 feet would be way too much, you could think about half that. Separation matters because of wave interference factors, in a slow conveyance like this the distance would need to be substantial, at least as much as sailing cats of like size. For me, the novelty of pedalling would have worn off within sight of the starting point, but each to his own amusements ! Stability is probably the least of your problems.
     
  3. NeilG
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    NeilG Junior Member

    Thanks; I understand thinner is better. I'd want to be able to sleep within the cabin of the main hull, so shoulder width plus wriggle room will be the limiting factor for thinness. Sounds like I want it as thin as possible within that constraint. (The pedalling station needs to be wider but would be above and between the hulls.)
     
  4. jehardiman
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    jehardiman Senior Member

    Yes, with capsize/pitchpole being more a function of plunging/jetted crests (i.e. wave steepness and windspeed) than wave height. This is true even for masted cats/tris and monohulls, i.e. it is wave surface, not mast loads or static stability, that cause many capsize/pitchpole.
    Depends on where and when you are. Overall, the risk is "small but real". Every year cats/tris much larger are capsized and monohulls rolled by waves.
    If you want it to be high efficency, make it a monohull with a heavy keel designed to be boarded and rolled. Really, there is a lot to be said for a passive self-righting hullform. Though there are hullforms that can't be capsized (think spar buoy), they are not ameniable to propulsion or low windage.
     
  5. keysdisease
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    keysdisease Senior Member

    Any multihull can be capsized, in fact many multihull peeps use the term "stable one" for upside down and "stable two" for rightside up because multihulls are more stable upside down :p:p

    I believe conventional thinking to improve stability is to make the vessel wider and keep the c/g as low as possible (no mast=good)

    If you plan to make long passages with no outside support I would recommend a process to right the vessel in case of capsize.

    I believe the consensus on length to fineness ratios of hulls for max performance is around 16:1, and then there are other considerations like hull shapes (semi circular , soft V ) and rocker.

    Good luck :cool:
     
  6. upchurchmr
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    upchurchmr Senior Member

    There is already a widely reported trimaran rowed transatlantic boat. I have no links.
    look at the large number of sailing cats and trimarans where the waterline is very narrow, but a short distance upward the hull widens for internal volume. You can have both habitibility and low drag.

    I assume you have been looking at the HPV sites?
     
  7. Corley
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    Corley epoxy coated

    There is a trimaran design by Craig Loomes of LOMOcean design for a rowing trimaran that is unstable upside down in other words self righting. It achieves this with the shape of the coach roof.

    A small multihull can be quite stable but considering the tiny amount of power that a human can apply you want it to be narrow in beam to reduce windage.

    You can see a video on the boat at the following link:

    http://www.3news.co.nz/World-first-...ecord/tabid/309/articleID/254230/Default.aspx

    and the rower Danny Sunkel's page here, he didn't feel he was adequately prepared to leave within the time frame on the video (had some knee problems apparently):

    http://www.dannysunkel.com/
     
  8. jehardiman
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    jehardiman Senior Member

    That's not a tri, that's an ocean shell with training wheels.
     
  9. Corley
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    Corley epoxy coated

    It has three hulls, it's a trimaran.
     

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  10. Doug Lord
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    Doug Lord Flight Ready

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  11. Richard Woods
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    Richard Woods Woods Designs

    so can any motor boat. Throughout history the only vessels that are self righting from more than 90deg are externally ballasted monohulls (and not even all of them)

    If you are pedal powered you can put out maybe 1/2hp. A 9m long multihull will probably need 8hp to be an effective cruiser. So basically you are only going to be moving in calm conditions. The rest of the time you will be drifting with wind and waves (which is why they can row the Atlantic)

    Richard Woods
     
  12. NeilG
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    NeilG Junior Member

    0.5 bhp is generous, sustained is more like 0.25 bhp. It's generally essential to plan a route with the currents.

    Nevertheless, pedal boat Moksha was a heavy monohull of 26 ft x 4.5 ft carrying 300 man-days of provisions, yet could maintain around 3 knots. I believe a well-built sea-going pedal multihull should be able to beat this.

    Yes, in heavy seas Atlantic rowers put out the sea anchor and take refuge in the cabin - that goes with the territory of human-powered sea boats. A passage of the Med may, with land often closer nearby, offer the additional possibility to reach a beach and pull the boat up on the shore to sit out the storm.
     
  13. NeilG
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    NeilG Junior Member

    Anyone know how the Hallin Marine rowing tri was designed to deal with capsizes? (and whether it had any?).
    http://www.teamhallin.co.uk/wp/?page_id=1390

    I can't find much about the boat design or crossing online, though I just messaged them about this issue through the contact form on the site.
     
  14. NeilG
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    NeilG Junior Member

    Some small human-powered multi-hulls

    I had a reply from the Hallin Marine rowing skipper:

    "The boat was very wide with the amas [supports] spaced far from the main hull - this made it easy to row and made the boat harder to capsize - the amas were also light and strong and very buoyant they were built from Carbon rather than GRP as the main hull was.
    We were at sea for 31 days and experienced all weather types and sea states from flat calm to quite rough - no we did not capsize."

    A description of the boat is here:
    http://www.teamhallin.co.uk/wp/?page_id=1390
    The Hallin Marine was built for six crew (three rowing at a time) with the main hull 40ft long and a maximum of 3ft wide. They set a new Atlantic rowing record (Tenerife to Barbados in a shade under 32 days).

    I'm guessing though that if freak circumstances had caused a capsize, they would have had no choice but to signal and wait for assistance.

    ----

    The Microship is an interesting project (pedal power, sailing and a backup trolling motor), but doesn't appear to have been designed with crossing open seas in mind.
    http://microship.com/microship/index.html

    -----

    but it seems several ocean rowing boats have capsized and not self-righted (frequently because carelessness or a freak wave caused the cabin to fill with water, removing the buoyancy and swamping the boats. No hull is 100% safe, but I'm getting increasingly convinced that the multihull approach is at least as safe, and definitely more comfortable.
     

  15. nimblemotors
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    nimblemotors Senior Member

    I'd build a boat that has no 'topside', and if 'capsized', it is the same as if it was not capsized.
    Build essentially a round tube vessel with the inner quarters that rotates inside the outer shell. The weight of the contents keep it rotated to 'rightsized'.
    Similar idea to 'zorbing', or being inside a sphere on land.
    https://www.youtube.com/watch?v=lRLPLwVQYjs
     
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