Bow extensions

Discussion in 'Hydrodynamics and Aerodynamics' started by DennisRB, Jan 14, 2015.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Could you elaborate more on this Eric?

    We've done extensive research on bow shapes over the years, I'd be interested to hear what they did and found.

    Was it like the bows of the old Chinese junks, where they have an open bow section, i.e full of holes, to allow water to go in and out? Those work extremely well.
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Wharram cats have their centre of pitch midships, one reason they hobby horse so much

    I agree you would need to make radical hull changes to do much if anything. But I doubt if DennisRB wants to cut the bottom of his boat off at the WL and make new hulls, but really thats the only answer to his problem

    And as I say in my article, bulb bows tend to slam and catch anchor warps

    RW
     
  3. Richard Woods
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    Richard Woods Woods Designs

    To Adhoc, like Kamiloa had, and I guess lightships anti-roll ballast tanks

    RW
     
  4. DennisRB
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    DennisRB Senior Member

    Thanks Doug. Yes it is a very nice looking and fast cruiser. Over all we are very happy.

    We still have our house in the suburbs, but it will be rented out to friends and we will keep one room. The boat is moored in the Brisbane River downdown closer to work, so we live aboard more than in the house. But its times like this 34-40 deg C (100F) humid weather that its nice to be able to go home and turn the air con on! When I have to work nightshift on my roster I also go home as its hard to sleep in the heat during the day.

    We are experienced at offshore cruising in monos and plan to do some real long term cruising again after the re fit in the next couple of years.
     
  5. DennisRB
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    DennisRB Senior Member

    Thanks for the advice! I have a lot of time before the re fit, so I have time to attempt to learn the software. Learning this has always been something have wanted to do. I am fairly good with computers. What free software do you recommend?
     
  6. DennisRB
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    DennisRB Senior Member

    Thanks Richard. Attached is 226-B. (I have not found 226-A yet) I did not think rocker was excessive compared to other cats I have seen out of the water. It seems a lot less than production boats, but more than boats like Orams (which have much less draft since they are so light, and almost no rocker forward with a very deep forefoot). The pic where it is out of the water has exaggerated rocker due to the wide angle lens of the go pro.

    You mention a deep forefoot helps going to windward. My hydrostatic calcs would have experimented with deepening the forefoot and extending the bow together. I understand that this will change the handling/sea keeping characteristics and be slower to tack. But as you say the rocker is excessive as it is, I don't think this will result in too many issues.

    The boat floats with the transoms level with the water. Filling in the transom will probably not work, going by my experience of how much water actually washes over in practice. Its probably a few bucket fulls at worst underway. Could I be underestimating the effect of this?

    Looks like I must be getting my head around how changes will effect things as I did postulate that I move the CoB forward, the pitching may be felt more by the crew even if the boat pitches less. I hoped that the increased length and buoyancy forward would however bring and over all comfort increase.
     

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  7. DennisRB
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    DennisRB Senior Member

    No I don't want to cut the boat up :p Which I why I would not just start changing things without the calcs first. The calcs will probably show exactly what you say, then I will have learned how to calculate things myself and knowledge is good right?

    Seems the center of pitch is very far aft on this boat. The bulb which I have access to the mold is not really a bulb in the traditional sense. Imagine a bulb, but its faired into the stem (nothing for warps to catch on). Imagine the front is sharpish in shape not bluff, and the bottom is also Veed so not to slam. Catana still use this today! http://www.catana.com/index.php?option=com_content&view=article&id=62&Itemid=103&lang=en

    The devices priority is to add buoyancy and was very successful on the 40 foot tri it was fitted too. Bullfrog / Verbatim is probably Australia's most Iconic tri so its results are out there for all to see. Its single main hull is of similar proportions and displacement to one hull of my boat.

    I am not arguing for its use based just on that, but clarifying what I meant by bulb originally and I still think its not the best idea.
     
  8. DennisRB
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    DennisRB Senior Member

    I planned to do some rough testing by measuring the draft increase at the bows by getting friends to stand on each bow and noting the how much weight it takes to sink the bows a certain amount. But I would need to go to a marina to do this where its totally calm. I realize since a boat is a 3D shape the results of this may not be accurate when applied in the opposite direction. Eg 100kg may sink the bows 15mm, but 100kg of buoyancy may not raise it 15mm. I thought it would give some sort of idea though.
     
  9. DennisRB
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    DennisRB Senior Member

    BTW I feel grateful that so many experts have replied to this thread. Thanks BD.net!
     
  10. Richard Woods
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    The Crowther tri with bulbs I was thinking of was the Buccaneer 40 (Sonrisa?) in the mid 1970's. Oldsailor may remember

    RW
     
  11. DennisRB
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    DennisRB Senior Member

  12. DennisRB
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    DennisRB Senior Member

    Out of the water pics. Rocker has been reduced by the extension. This was the 3rd one the original owner (and builder) made. Only took 3 days. He made 3 of ever increasing size so he could push the boat harder and harder under extreme racing conditions to ward of bow burying and speaks very highly of the results. This boat was the fastest boat around in those days!
     

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  13. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi John,
    I don't have a copy of Coles' book in my library, so I am writing from memory. As Richard reminded me, the boat Coles was on was too short for the race across the Atlantic, West to East that year (I think this was in the '50s or 60s). And I remember at the time I read it that it was an intriguing solution to an unusual problem. That boat was wood, Wobegone Daze is fiberglass. At any rate, the added length, with attendant buoyancy mostly above the waterline, qualified the boat for the race. The shape was not so much to extend the length on the waterline (although there was a tiny amount added), nor to change trim, but just to change the length of the boat with a faired-in extension on the bow. There was a photograph of the boat in the book, and you could see the gunwale on deck of the original bow which consequently was left in place.

    In the case of Wobegone Daze, the owner had approached me about adding a bulbous bow to the boat in order to change trim. I advised him that adding a bulbous bow likely would not add enough buoyancy. There is also a brief story in Tony Marchaj's book "Aero-Hydrodynamics of Sailing" about a bulbous bow on a sailboat, and the fact that while the boat may have sailed better, it did not turn too well (See 1st ed., page 210). I also advised my client that a bulbous bow is most effective when the boat is moving level (no pitching) in one orientation (upright), and that a sailboat is always pitching and is never upright except when at anchor, moored, or motoring. The solution, I advised, was to change the shape of the bow. Plumb stems were all the rage, so we made the raked-stem boat a plumb-stem boat. The result was that the owner was very pleased with the look and handling of the boat--basically, he got a new modern hull for the price of the new add-on bow.

    I enjoyed your recent articles in PBB on aluminum construction, by the way. Thanks!

    Eric
     
  14. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Dennis,
    I have absolutely no experience with free hull design software as I don't use it. I have been using Pro-Surf from New Wave Systems in Rhode Island for about 25-30 years. The developer is a very good friend of mine.

    Eric
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks for that Eric.

    This does appear consistent with the findings of the Wolfson Unit who conducted some research for the UK’s MCA in the late 90s on multihull stability. Although the conclusions were inconclusive, merely because the fact that there was insufficient data to suggest whether it was simply the raised freeboard or the actual flare above the waterline. But either way, this feature did show an improvement in stability and pitch-poling than those without.

    We did a lot of research with BBs and various shape of BBs too, on cats. The only reason we did so was to appease our ‘masters’ and the salesman that kept complaining we didn’t design with any, unlike our competitors at the time. The research via tank testing very clearly shows, it makes zero difference and as you note too, it adds very little buoyancy. If you look at the simple mechanics behind the factors that affect pitching, the equations of motion very quickly indicate that to make a significant deference the BB needs to be very very large indeed.

    Also noted here and here.

    Yup, that’s pretty much what we did too. It does increase damping slightly so there is some beneficial effects of a plumb bow to that of a conventional bow shape.

    Thanks for the kind words on my articles. Hope you got something out of them…?
     
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