A question re motor boat directional stability (as a result of keel modifications)

Discussion in 'Boat Design' started by bajansailor, Oct 7, 2012.

  1. bajansailor
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    bajansailor Marine Surveyor

    I was down at the local haul out yard yesterday (there is only one here), and I saw this commercial fishing boat having some modifications carried out to her raked keel - a few photos are posted below.
    She was originally a displacement (almost semi displacement) sportsfishing boat, with twin diesels and shafts and twin rudders. A refit was carried out some years ago to convert her to a single screw commercial fishing vessel, where the deckhouse was shortened, and a large ice box installed in the now much bigger cockpit.

    She currently has a fair bit of rake on her keel. However the Owner was telling me that when they are fishing, they lie beam on to the seas, with their flying fish net deployed from midships, and apparently what happens is that the bow tends to 'blow off', because there is not much keel up forward, and relatively quite a lot aft.

    Hence why they have decided to deepen the keel up forward.
    Approximately half of the new 'add-on' keel has been tacked on to the existing keel so far (you can just see it in the photos).
    The forward section of the keel (still to be added) will follow the same bottom line of the keel (ie straight), hence the keel will progressively get deeper as it goes forward, until the keel reaches the point where the bow curvature starts to curve upwards, and then they will rake it forwards and fair it in to the stem.

    The Builders told me that the reason for doing this is to stop the bow 'blowing off' when they are drifting and fishing. Ideally they want to remain beam on to the seas (and always port side to windward - this is a tradition here).

    I am very dubious about this keel modification - I have a feeling that they will find that the deep forward keel will tend to want to 'take over' re steering the boat, especially if say they are motoring down wind in a significant following sea (which often happens here).
    The current single rudder is relatively small, and I doubt that it will be able to over-ride the effect of this deep keel forward, once the keel has decided what direction the boat will go in as they ride down the face of a wave.

    What does the panel think?

    I am thinking that if they really want to stop the bow 'blowing off' while stopped in the open ocean and fishing, then it might have been better to build a simple retractable dagger board.

    But the guys doing the work are adamant that it will all work out ok.........

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    You need to be very very careful when playing with the directional stability of a boat.

    Your thoughts regarding the result are probably correct. This is a shallow draft -from what looks like in those pix you posted above. Any addition of 'area' up fwd, in this case the keel/stem line dropping, shall alter the directional stability. How much is difficult to say without any hard data on the vessel.

    However, what we can say is that, in general, it may not improve matters. It could reach the stage where too much area fwd has been added that the boat does indeed exhibit very poor turning ability and difficult to remain on course. And your comment re following sea is pretty much on the money. I would be concerned too.

    Tell them to make sure they have plenty of life jackets on board, their life raft is up to date and the EPRIB works properly. In anything other than flat calm, they could be in trouble very quickly.

    But then again, they might, just might, be lucky and hit the "sweet spot". But I doubt it.

    The quickest safest way would be to reduce the amount of "windage" area exposed to a beam wind. Or at least move the location of the exposed surface to the wind. The wheelhouse size and location is what is "driving" the boat when beam-on that causes them to have a wandering bow. Take away the new wheelhouse/cockpit, the problem shall be resolved. The size of length of wheelhouses is often overlooked, but they are where they are and sized for a reason too, not just to keep you dry.
     
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Thank you Ad Hoc for your most informative reply above.

    Re safety equipment, commercial fishing vessels here are required to have lifejackets, but there is no compulsion for a liferaft or an EPIRB, hence I doubt that they have these......

    The vessel does have a fair amount of windage forward, with a high bow and a sweeping sheerline, coupled with a forward wheelhouse. The wheelhouse has already been reduced in size, during the conversion from sportsfisherman to commercial fishing vessel, to allow room in the cockpit for a 2 tonne ice box.

    I hope that they will find the sweet spot with this new extra keel when they take her out on trials after re-launching, but I think the odds of this happening are very low.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    It was more a tongue in cheek riposte ;):p

    They should really do some simple directional stability tests, in calm water, when they have completed the refit, to gauge how much their mod has effected the boat's performance. If only for their own information, i.e. forewarned is forearmed!

    I see in your Bio...another ex-Soton chap :cool:
    I had dinner with the Vice Chancellor on Saturday night. He was "in town" for a meeting.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    A small sail-like arrangement at the rear. Maybe the tail off an old DC-3. :D
     
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Here are a few more photos, including one that shows a bit more of the boat.
    The builders have 'tacked on' another section of keel - the forward edge of this will now be faired into the stem.

    I quite like the idea of a small riding sail at the aft end as suggested above, to help stop the bow from 'blowing off' while beam on to the seas (I also posted this question on the YBW Forum, and this had been suggested there as well) - however I cant see these guys embracing that!
    And I did suggest to them about using a small drogue streamed from the bow, but they didnt like that idea at all.

    I suppose that Fishermen here are generally very conservative and resistant to change (same as the rest of the world), yet these guys are happily and confidently venturing into completely unknown territory with this new modifcation.

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  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    What is it they say about fools rushing in.... :D
     

  8. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Ummm, yes!
    I am getting a bit worried about a typical worst case scenario - eg running home in blustery tradewind conditions where the wave height might be 8 - 10'; they try to steer down the wave and the bow takes over, slewing her around to one side in a broach.
    Good possibility of a knock down occuring?
     
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