Ocean rowing boat stability calculations

Discussion in 'Stability' started by Tom Robinson, Sep 22, 2020.

  1. Tom Robinson
    Joined: Sep 2020
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    Location: Brisbane, Australia

    Tom Robinson Junior Member

    I am an apprentice boat builder in Australia. I am currently designing an ocean rowing boat for a planned Pacific crossing.
    I started by carving a half model then took the lines off. I now have a full lines plan and general arrangement including relatively accurate weights. The boat is 24’ long and 6’ wide, she’ll be built glued clinker ply. Her lines are very similar to a whaleboat, although she’s a few feet shorter.

    I now need to design the cabin so it has sufficient buoyancy to make the boat self-righting from 180 degrees. I would also like to consider a ballasted keel, or a combination of the two. Is there any way I can accurately calculate this without computer programs? If not, are there any boat designers here that would be interested in doing the calculations, for a fee of course.

    Any help would be greatly appreciated.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You could make a scale model of it, I guess. The hydrostatics would scale perfectly. As for rowing across the Pacific, is that for yourself or someone else ? I thought only the poms were mad enough for that type of thing ! (Sure to be one or two rise to that bait). :D
     
  3. Tom Robinson
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    Tom Robinson Junior Member

    I've considered a scale model, I like the simplicity of it, but it seems like a lot of work. Yes the boat is for me, those poms know whats good...
     
  4. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Tom.

    A very effective version of your boat already exists - have a look at
    Pacific Rowboat https://www.ericwsponberg.com/boat-designs/pacific-rowboat/

    Jacob started off rowing across the Pacific 2 years ago, and arrived in Australia almost a year later.
    Here is some info on his website -
    The Boat — Jacob Adoram https://www.jacobadoram.com/theboat

    Eric Sponberg and his wife Arliss were on hand to meet Jacob when he arrived in Australia -
    Waiting for Jacob – Arliss Ryan https://www.arlissryan.com/waiting-for-jacob/
     
  5. Heimfried
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    Heimfried Senior Member

    A scale model will not automatically provide the correct center of gravity and weight/displacement. It has to be calculated.
     
  6. TANSL
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    TANSL Senior Member

    In my opinion, if you want to make an accurate calculation of your boat's stability and self-uprighting capacity, you have no choice but to use computer calculations and reliable software. The calculations are complicated, time consuming, and correcting the shapes based on the results can be extremely slow, not to mention the possible mistakes made, some small but very important, which are always difficult to detect. Making a scale model will not solve all the problems that arise and, in addition, it will be more expensive than what a preliminary study by a boat designer could cost.
    I would be happy to take a look at your boat and what you need to achieve with it, its SOR, but talking about free calculations would not be possible without having a more complete idea of the complexity of the work. I am used, and I am proud of it, to help many people for free but you will allow me that, at this moment, I cannot assure anything. If you want to exchange more information with me, in a more private way, send me a personal message through this forum or give me an email that I can address.
     
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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I realise that, but if made of like material, and scaled down, no great problem.
     
  8. Heimfried
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    Heimfried Senior Member

    Hi Tom
    Yes, but it is very laborious. If you use a computer spead sheet and calculate the co-ordinates of the hull skin (and deck/cabin) out of your lines plan, I would load it up and you could look at the hydrostatic behaviour of your boat online. Loading/unloading, shifting weights and so on. This would be free.
    example here: Berechnung der Schwimmlage von Booten http://www.bootsphysik.de/index_x3.php
    Button "English" top right. (Because I completely reworked the site, the i buttons for info are not working at moment.)

    This could only be a start to check your design, test it, reshape it, test it again ...
    If it seems to work as required, a professional has to doublecheck it.
     
    Last edited: Sep 22, 2020
  9. Tom Robinson
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    Tom Robinson Junior Member

    Wow what a cool place this is...thanks for your replies so far. Sounds like software is the best way to go.
     
  10. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Just a thought - re your proposed clinker / lapstrake method of construction - all else being equal, wouldn't this have a bit more resistance than a smooth / fair round bilge design of similar size and with similar design coefficients?

    Re Jacob's boat, I remember reading on his blog about how conscious he was of the extra drag from hull growth, and how he occasionally had to go for a swim to scrub the bottom.
    A round bilge hull bottom should be easier to clean as well, when compared to a clinker construction (?)
     
    Last edited: Sep 22, 2020
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    If was sufficiently young and enthusiastic ( I am neither) to contemplate such an excursion, I think I would scour the publications of those that were successful in the past, for clues as to what the most important factors were, in making the trip more of an adventure, and less of an ordeal. It is a rare person that undertakes such a trip, for the vast majority, the novelty would wear off far short of landfall. I don't think it would be too difficult to get self-righting nailed down, but I guess you want that to be achieved with the least effect on stability, that it is less likely to need righting in the first place. I don't know how many rollovers could be endured and the rower and integrity of the boat and its contents remain preserved. I think it would be better to be kept to a rare event, by having a boat less prone to suffer it. This is really a very complicated exercise in design, ergonomics, human physiology etc, And don't forget the Vitamin C !
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    @Eric Sponberg is undoubtedly the expert with the most amount of experience of trans-Pacific rowing boats on this Forum - please do read through his thoughts about the design of Jacob's boat (they are very detailed) - and make a lot of time available for reading Jacob's blog (have a good supply of tea or beer handy :) )

    I think I have tagged Eric above - but he is currently sailing around Australia, so he might not find this too quickly.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Yes, Eric's thread should be compulsory reading for the OP, as would other books and publications about what is an extremely specialised kind of craft. You don't want to be finding unanticipated shortcomings that might have been avoidable.
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I recall a fella that completed one of these marathon rows, saying he was still rocking around about three weeks after coming ashore, you almost become a part of the sea, and the memory is long retained. I guess it is rather different than a similar size sailboat that has the steadying effect of the sails.
     
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  15. Tom Robinson
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    Tom Robinson Junior Member

    Thanks guys.
    This is a long held ambition of mine. I have I have spent many hours pouring over photographs and plans of ocean rowing boats, as well as rowing a couple. I have thought long and hard about the ideal boat. My vessel will be quite similar dimensionally to most ORB’s, however she will have flatter midship sections for greater initial stability, which will help with rowing a clean stroke. I would like to keep cabin height to an absolute minimum as I want to row across, not sail. A lower cabin will also mean a lower centre of gravity. Ultimately my vessel will be traditional in nature and appearance.

    RE the drag of the laps, I am yet to find information on the relationship between wetted surface area and drag, of a vessel at very low speeds (max 2 knots). I can’t see the laps adding much resistance. This being said I am not fixed on the idea of glued lap.
     
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