Bow extensions

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

  1. DennisRB
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    DennisRB Senior Member

    After having some discussion in this thread http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/bulbous-bow-canoe-51787.htmlabout about adding bulb bows on my boat to correct bow down trim, I would like to start a separate thread on bow extensions. This option appears to give the best physical outcome by way of correcting trim, and more favorable DLR with a light weight extension.

    Here are a couple I can find. This one by Eric Sponburg is a master piece, and it looks to have cost a bomb. On much narrower hulls I think it will not be as difficult as this.

    http://www.sponbergyachtdesign.com/Wobegonbow.htm

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    This on the Kurt Hughes blog and looks to be within my abilities when it comes to construction at least. At first glace it seems very simple. But I am sure the design needs careful attention so that the required buoyancy is added without too much weight and the result is fair.

    I would appreciate any links, pics or general info on the subject.

    http://multihullblog.com/2014/02/how-did-famu-get-longer-amas/

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  2. DennisRB
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    DennisRB Senior Member

    This one was from a poster on the forum, but he admits it was for fashion. I don't think much buoyancy was added in this particular case. But it does look well executed unlike the horrific pic Richard Woods posted, which I cant seem to find.

    http://www.boatdesign.net/forums/multihulls/nacra-5-8-based-foiling-trimaran-50134.html#post683969

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    I have to say that for my application, the reverse bow style appears a good option as todays fashion is clearly the most important consideration. I read it in a glossy sales brochure that all the fast boats have it as it pierces waves :p

    But seriously, I think it will be good as it will be the lightest and probably even easiest way to get the buoyancy where it is needed, down low, not up in the air.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Denis

    AS noted on the other thread. Firstly you need to establish the current situation of your boat. By that I mean a set of hydrostatics for your boat. Once you have this you can very quickly determine how much buoyancy you will need to correct the trim. Since you may find that the amount is rather larger. Until you can gather some hard facts, it will be just supposition.

    Also whilst it does increase the L/D ratio making resistance more favourable, don't forget, it may also effect the seakeeping in following seas if it is purely a BB rather than extending the hull lines as such.
     
  4. Richard Woods
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    Richard Woods Woods Designs

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  5. upchurchmr
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    upchurchmr Senior Member

    Is there no possibility of fixing the trim by moving stored items or equipment?
    Much less cost and time if it is possible.

    If this is still your canoe, can't you just shift the people back a bit?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I was assuming it was a cat of some kind.
     
  7. DennisRB
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    DennisRB Senior Member

    Thanks for all the replies I concur and agree with all you have said.

    I do accept that I can just leave it how it is. The boat still works fine and has good performance. But the trim does have undesirable affect in choppy or lumpy conditions by way of reduced forward bridge clearance, made worse by how fine the bows are (low pitch resistance at the bow). She does not hobby horse as such, as the rear is so much more buoyant that the front, but the bow goes up and down with ease. The motion remains comfortable aft.

    The first step is getting a set of hull lines for my boat a Crowther. Does anyone have lines for a 226-A? I have a set of lines for 226-B, which is not exactly the same, but close. This may serve as a rough estimation of buoyancy needed for feasibility, but not good enough to make CNC stations out of.

    My plan was to create a virtual hull going by those lines in some sort of software (which has not yet been determined, any suggestions?). I would then need to learn how to use the software and adjust CoG so that the virtual ship is floating at the same lines as my boat. Then changes to its volume forward would be made to see the effect on trim. Estimations of the weight of the extension would be made to evaluate the increase in buoyancy VS decrease due to mass. Is this a valid method? Even if I do not end up doing any changes I will have learned something.

    As you can see by the pics the boat needs a major haul out anyway. If she had a nice paint job now I would not consider this, but since I will be painting and fairing anyway, then would be the time to do it.

    These pics are just for amusement, as obviously they contain little to no technical advice on the hull and the shape is even more distorted by the go pro wide angle lens.
     

    Attached Files:

  8. TANSL
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    TANSL Senior Member

    The variation in the trim you will get, increasing the length of the hull towards the bow, is very small since the volume that you can add in that area is minimal. You will change the position of the center of buoyancy but also the center of gravity of the water plane. It will be much more effective repositioning of the weights of equipment or tanks inside the ship.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Dennis, that's a gorgeous boat! Do you live aboard?
     
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  10. Eric Sponberg
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    Eric Sponberg Senior Member

    Hi Dennis, I am glad my efforts on Wobegone Daze gave you some guidance. What you propose, as quoted, is exactly how it is done. It will likely take you longer to do than a naval architect because you'll have to learn the software first--that will take awhile. Not bad if you're not in a hurry. If you want the job done right and well, hire a naval architect.

    On Wobegone Daze we had the original lines from Gary Mull that I could input into the computer for the hydrostatic calculations, and from there I could create my new lines (which at the time I did by hand) but then had the computer hydrostatic model altered so that I could calculate the new flotation with the new bow. Accounting for the extra added weight, I could determine the new center of gravity location, and therefore, the change in buoyancy and trim.

    By the way, if you read Adlard Coles' book "Heavy Weather Sailing," you'll see a story in there about a boat that he added a false bow to in order to sail across the Atlantic. That was the story that gave me inspiration to advise my client on Wobegone Daze to change the shape of the bow. The whole project was extremely successful. That boat, by the way, was sold about 1-2 years ago and now belongs to an instructor at the International Yacht Restoration School in Newport, RI, now her new home port.

    Good luck.

    Eric
     
  11. Richard Woods
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    Richard Woods Woods Designs

    I don't think it is time/cost effective to do. Your boat has a lot of rocker and very fine hulls which is always going to cause pitching problems. That's why Crowther designs were never very successful in the normal choppy UK conditions.

    A deep forefoot helps going to windward. You might do better filling in the hull side where the transom steps are. When you pitch there is no buoyancy aft right now, the sea washes over the steps.

    Right now it appears your pitching centre is well aft so you don't feel it in the cockpit. Add buoyancy forward and your pitching centre may well move forward making the pitching more obvious to the crew

    BTW I sailed across the Atlantic a few years ago on Rush, a 10m Crowther, owned by Alan Morris

    Just read Erics post. I thought the reason for the A Coles bow addition (a "clipper bow" IIRC) was because the Bermuda race had a minimum 40ft LOA and his boat was 38, something like that, so a race rule cheat and nothing to do with seaworthiness. The new bow wasn't even in the water

    Richard Woods
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    Richard, I think you are right. But it was possible to do, and on Wobegone Daze we did change buoyancy forward to get the boat to change trim significantly and sail much better as a result.

    Eric
     
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  13. TANSL
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    TANSL Senior Member

    I totally agree with upchurchmr. I added bulbs in 8 ships and can assure that changes in trim, if not preliminary calculations are made, can be surprising if not ineffective. What is the moment to trim 1cm the boat to the current draft?. How many centimeters do you need to change the trim?
    It's simple, but not so fashion, find cheaper and more practical solutions.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    But not in following seas, quite the opposite in fact. Worse! Thus a holistic approach is required to such thoughts of change.

    Catamarans have their centres of pitch well aft, all of them do. Best place to sit is always as far aft as possible. Adding a bow extension will not alter this. To move the CoP fwd you need tor radically alter the hull shape so it is symmetrical and move all the mass to the centre, i.e. like a canoe. In effect you're changing the radius of gyration to become more favourable.

    Making the hull more slender i.e increasing the L/D ratio helps more so in the trimming, but to increase the L/D on this boat to such an extent may not be possible.

    Still far too much is unknown, factually. Thus getting a set of correct hydrostatics for the hull and establishing where the LCB(LCG) is and the LCF will provide insight whether adding an extension is feasible. In the sense that the size of the extension to correct any trim problems may end up being more than expected. Since the amount of buoyancy in an extension is (generally) not much thus, the trimming moment will also not be much either. It's only effect being its lever.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    The way I read it, Dennis' problem is not so much gyration, but a too-low bow attitude putting the forward portion of the span between the hulls too close to the water. Could be stowaways hiding up forward.
     
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