Multihull ocean rowing vessel

Discussion in 'Multihulls' started by sibosun, Apr 16, 2009.

  1. science abuse
    Joined: Nov 2009
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    Location: Cincinnati, Oh

    science abuse Junior Member

    I tossed this together a few years ago for the great lakes:
    [​IMG]

    outrigger spacing was based on two things:
    A) Where I wanted the oar lock to pivot
    B) Keeping them out of the water (no drag) when not needed.

    Succeeded in both, and it was a easy boat to row... Then the waves came.
    Actually, I went out after them. By ocean standards, 4ft breakers and 6ft swells are nothing, but keep in mind that the freshwater wave interval is shorter than saltwater, the waves are steeper, and it was a decent knocking around.

    Results; I was dissapointed to see that my hard work was basically unnessesary. My balance and the 43" beam of the boat was more than enough to keep her upright taking the cresting waves to her side.
    In the break, the boat was rolled equally both with or without the outriggers. If the waves were big enough to roll one, they could roll the other. from that I took away some thoughts:

    1) If waves are big enough to roll a small boat, they are going to roll your small boat, no matter what you stick to the side. The margin beween multihull and monohull is so narrow that it won't matter.
    2) Given that they only help with very few waves, they aren't worth the weight and potential drag. They also represenet one-more-thing-to-break. I'd always heard rule 1 in boatbuilding was "keep it simple".
    3) On a trans-ocean row, you're probably going to get rolled, it's just going to happen. It seems safest to go for the boat that is easiest to right. Getting rolled 20 times and righting the boat 20 times is better than being rolled once and not being able to right her at all.

    I'm planning a row to Isle Royale on Lake Superior (90+mi round trip), with a warm-up crossing of Saginaw Bay (40mi round trip) this summer. This particular bay does funnel the chop and I will be prepared for weather and rolling waves, and Lake Suprior is known for swallowing properly large ships. I'll be leaving the outriggers off and focussing more on righting and water removal; positive boyancy used to get it as close to self-righting as possible.

    But that's just me. :)
     
  2. delphic
    Joined: Dec 2009
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    Location: Australia

    delphic New Member

    Hi All,

    I'm James from Australia. I was very very lucky as I was coached by James Tomkins and rowed with Drew Ginn (If those names mean anything to you?)

    I'm after something so simple but I've searched for hours and cannot find. I was after a 2D 'over head' & 'Side on' illustration of a M8+ and a M4x. It's just for mock ups and working through different geometry ideas with rigging. Any help would be very much appreciated.

    Cheers
    James
     
  3. Guest625101138

    Guest625101138 Previous Member

  4. delphic
    Joined: Dec 2009
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    delphic New Member

    Thanks Rick

    It's crazy,

    between the two clubs I coach we have many Sykes shells but no-one wants to give out even the most simple diagrams. Thanks for the lead.

    And I'm very happy to have found a Rowing forum. Now fat and balding I coach, like most .I'm an Aerospace Engineer and fluid dynamics are different to aerodynamics, but not by that much. Eg the good old 747 is a pretty close match to a men's 8+ or 4+, but any strike fighter way off the mark.

    I look forward to picking all of your brains!

    cheers

    James
     
  5. Guest625101138

    Guest625101138 Previous Member

    James
    There are a few differences between water and air. The difference is most pronounced in displacement craft like rowing boats. Here there is skin friction like an aircraft but you have wave drag rather than the drag associated with lift in an aircraft.

    The work that Leo Lazauskas has built on to create Michlet is very useful for analysing wave drag. This is the biggest factor in coming to grips with surface vessels as compared with aircraft.

    In the table I referenced in the above link you will see there is a hull referred to as Godzilla. This is a hull optimising routine given various hull constraints. For a rowing 8 you can develop the lowest drag shape from the total displacement and the combined power output of the rowers with allowance for rowing loses using Godzilla.

    So if it is difficult to find the hull shape for a rowing 8 you can create the lowest drag hull using Godzilla. Takes a bit if knowledge to use it.

    Rick W
     
  6. delphic
    Joined: Dec 2009
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    delphic New Member

    Rick youhave exposed your mind!!

    Rick,

    You would not believe that was first on my list to get my head around!!

    I'm still pretty raw to Fluids so I'll try and use the correct terminology:

    Resistance that yeilds the boat moving forward is made up of:

    1. A direct correlation of water Displacement created by the hull to that of the weight of the boat and crew so the don't all go swimming...
    2. water drag against skin
    3. Wave drag, whereby the shape of the hull 'bends' and conversly creates a wave thereby using a % of the oarsmans' energy that will not be 'given back' in a useful form. Or if the water is not glass maybe?
    4. The stern of the boat creating a lowpressure system of water as it moves on and that low pressure water system 'sucks' it back for want of a better term
    5. Water aside there is actually aerodynamic resistance unless you have a tail wind

    These are absolute stabs that I'm trying to transpose my Aero knowledge into water.

    I know somethings that we do with fighter aircraft have been deemed illegal by Fina. We're still mucking around with drill holes vertically down through the leading edge of aircraft above about Mach 1.7 in an effort to reduce drag to air over air instead of air over surface.

    The other relavent point that maybe of interest to you that I could not see working with a rowing shell is that - again - on fighters' there will be various points on the static frame were we induce turbulance as (back to the low pressure water area at stern example ibidem) some vector thrusting turbines or 'ugly' bits that have to be there to hold weapons can move through turbulent air with more ease (or less energy) than air that is still. I have no doubt you probably already know that one too though!!

    But help on my stabs above would be greatly appreciated.

    Best Regards,

    James
     
  7. Guest625101138

    Guest625101138 Previous Member

    James
    You have got the factors nailed reasonably well. Planing hulls are closer to aircraft. Foilers are basically aircraft with a little wave drag thrown in.

    With easily driven hulls, particularly things like single sculls, the windage can be a significant portion of the drag. There is a real advantage in improving aerodynamics for windy conditions. It is not so critical with a rowing 8 because each rower gets some benefit from what is in front and what is behind.

    Also with rowing the speed fluctuates with the stroke. The power is a function of speed cubed so there is a cost in fluctuating speed compared with steady speed. Leo has written papers on this. My own work is on pedal boats that overcomes this problem.

    Another loss comes about with wind waves causing the boat to pitch and roll. In a rowing boat the loss of rhythm is likely to create greater loss than the actual increase in friction.

    With an optimum rowing hull the wave drag is about 10% of the total drag. This is achieved by the long slender hull and avoidance of a bluff transom. So the real gains are in reducing skin friction. There are some proven methods that give small gains. The best proven method are prohibited for environmental reasons. There are probably ideas yet to be tested. Air micro bubbles lower drag but the energy to pressurise the air is more than the saving.

    My view is that skin friction is an area needing more research. It costs a huge amount of energy considering seaborne trade.

    Rick W
     
  8. delphic
    Joined: Dec 2009
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    delphic New Member

    One more q and then I will leave you in piece??

    With regards to skin friction, the most successful method 'we' found on a metal skin, after the usual heat smoothing on curvature then a very flexible layer of poly*, the final layer was an non-inert compound that was only a liquid when mixed with the lightest of para-hyrodo carbon dirivatives ( a lot lighter than ethonal, I know you get the drift); so light we brought down the bake room to about 4 deg c and then it would be airbrushed over the poly* by a robot for consistecy. The para would evaporate on contact leaving the compond to bond with the previous layer. We confidence limits were 20 - 25 microns. I could never get my head around it's properties to be honest as it had to flex as it was now bonded but getting but would not be warped by the atmosphere. And obviously the higher the cruise altitude, etc. Have a think about that approach maybe?

    All the best,

    j
     

  9. cristofa
    Joined: Oct 2004
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    Location: Cornwall, UK

    cristofa Junior Member

    I have just come across this post ... while, recognising your vast knowledge on the subject, I accept your right to express your opinion about the passage of boats through the water, but you are totally out of order declaring that my customers were disappointed with the performance of their ROCATs! Did you ask them? That's like saying that people who have bought a Marin mountain bike are disappointed that they cannot use their purchase to take part in the Tour de France - you have completely missed the point of the ROCAT. Having rowed ROCAT prototypes for hundreds of hours in all sea states, I am absolutely confident that there is no rowing boat on this planet that is as seaworthy - what's more, the easily driven hulls enable the ROCAT rower to make good progress into gale force winds like this



    ... would you take one of your pedal boats out in that?

    I never claimed that a ROCAT is as fast as a smooth-water sculling boat, but it's a damn sight faster than a passibly seaworthy tub, and it's much easier to transport too. It is the rowing equivalent of a mountain bike - you can decide to go out for a row without being constrained by the conditions. Two hulls may be a bit slower than a skinny scull, but they are absolutely stable, which can be handy.
     
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