Need some advice on hull action in following seas

Discussion in 'Boat Design' started by 5akman, Sep 1, 2012.

  1. 5akman
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    5akman Junior Member

    I recently purchased a mid '70's 40'x11' fiberglass Snowball gillnetter. The SB hulls have a reputation of being a handful in following seas. The stern is rounded and the bow has a fine entry and needs some more "substance" to it. Prior and current owners of these hulls say it takes constant tending to the helm in anything over about a 3' following sea and can get downright scary! Is there anything that can be done to the boat (short of getting a different boat!) to help? I've got an extensive background in fiberglass work and I'm thinking of trying to increase the width of the bow below the sheer line by a foot or so. Or, would a bulb help at all in this situation?
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    On a boat that size and type, there is little you can do. The only real option open to you, is to be far more vigilant when running in a following sea. Make sure the wave length is not very close to the length of your boat. When the conditions are so, change course. Also the speed of the wave, make sure you're not running at similar speeds so slowing down helps too.

    In other words sharpen your observational skills to recognise when a broach or dangerous behaviour occurs and then change the boats speed/direction to prevent any accident or poor motions that may occur under such conditions.
     
  3. 5akman
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    5akman Junior Member

    one owner says he is usually able to "outrun" the following seas but then wouldn't that put you in a situation to bury the bow into the wave that you're overtaking? Guess I'll just have to deal with it.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The prop and rudder seems well forward of the stern, or is that an optical illusion ? Keeps nets well clear of trouble, but can't be helping the broach ? The worst boats I have been in for this problem had the combination of a deep forefoot with a lot of hollow flare in the bow that gave a 'blunt' waterline higher up on the stem. That creates the 'brakes', the deep foot the fulcrum, not much in the water aft allows the stern to swing and whoooaaaaa.....
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    It all depends upon the boat's speed relative to the speed of the wave, and importantly the wave's wave length to boat length ratio. And to a lesser extent the steepness of the waves.

    But if in doubt, slow down and change course.
     
  6. 5akman
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    5akman Junior Member

    That describes it perfectly! That's why I was wondering if somehow adding some mass/width whatever in the bow would prevent the "submarining". I realize its a long shot but thought it was worth asking about.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm more than happy to defer to the wiser heads around here on this, but it seems to me the only thing in the way of changing the boat that might help is some more lateral plane aft, possibly a rounded fin on each side well aft, say halfway from the centreline to the sides, provided you won't be fouling the nets. Should slow the 'jack-knife' effect, but the steering response will be altered. Altering the topsides forward seems like a big job and a real speculator.
     
  8. 5akman
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    5akman Junior Member

    Are you talking about something vertical under the boat like fins? I was wondering if this hull would benefit from stablizers (it came with them when I bought it) or rolling chocks. While those might ease the roll at slower speeds, I'm not sure what the effect of either of them would be in the following seas situation.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    It is not unknown in the history of aviation for 'afterthought' alterations to have been made to quite famous aircraft to correct yawing tendencies, I guess boats too, are not immune to review in the light of experience. A boat not tracking well in following seas has some parallels to a jack-knifing trailer truck, the retardation when ploughing into the back of big waves is analagous to sudden braking, the stern wanting to overtake the bows corresponds, albeit imperfectly, to a jack-knifing trailer swinging around on a slippery road because the rear tyres have insufficient resistance to sideways motion. Vertical keel surface area aft resists sideway movement at the stern. But fitting off-centre keels would be a big deal as they would need to be secured with thru-hull fasteners to structural members. If you are real keen you could scale model your vessel into a radio-controlled replica and tinker with all sorts of alterations, on the local lake when the breeze suits !
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Directional stability, or lack off, and trying to improve this should be treated with caution. From the pix shown above, there is already a large skeg. So adding more “area” aft will make the boat more difficult to turn. Not ideal when you need to get away from THAT wave which shall swamp your boat.

    The wave crest at the stern that picks up your bow and drives the head down, once initiated, the amount of area required to prevent such a loss of stability is disproportionate for your boat outside this condition.

    Fins etc are only “get out of jail free” cards when broaching is concerned. Thus, treat them as such and not as a cure and do not over size them. But looking at your hull, not really worth it.

    Improving your recognition of the warning signs is your best “cure” as such. In other words, avoid the situation to begin with. All boats are prone to broaching, ostensibly monohulls though, whereas multihulls exhibit different behaviour in such conditions.

    Running in waves of similar speed and length as the boat from astern and the feeling as if the boat is surging/accelerating fwd and with a bow down trim, is a sure sign you’re about to broach.

    There is little you can do; it is a characteristic of your boat. Just recognise this behaviour and know the warning signs when they exist and slow down and change course.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course adding fins will change the turning characteristics, will create drag , might mean more helm is needed to hold course in crosswinds etc., I suppose in an ideal world the best set-up would be retractable skegs that are only deployed when needed. Sounds expensive and difficult to arrange..
     
  12. Frosty

    Frosty Previous Member

    The heavy bridge up forward wont help.

    From the shaft angle it also looks like the engine is way fwd.

    Basically I would get some weight aft. Even the water line shows a heavy bow.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Looks like an uncomfortable helm position right forward too, hard life that fishing caper !
     
  14. viking north
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    viking north VINLAND

    One experiment you might try is to lighten her load forward. Anchors, chain and what else you can remove in prep for a sea trial. Often it is not a case of poor hull design but a case of poor distribution of internal and superstructure weight as we both well know when loading fish. (I was a former fishermans kid on the north atlantic). It doesn't require alot of extra weight forward to create a cranky vessel. There seems to be a very sharp threshold where this occurs.The addition of a bulb is not as far fertched as it seems however as prev. mentioned this should be carried out under modelling conditions before laying out the hard cash only to find out it "worstened" the situation. In any case the fact that you have purchased this old craft indicates operating cash is tight. Usually the case of most independant fishermen :)
    P.S. Just took a closer look at the photos --Frosty has a point you do have alot of superstructure and possibly engine forward. That to accomidate the gear and the fish holes no doubt. Possibly in addition to lightening her forward areas you can get some water ballast aft and pump out as you fill with fish.
     
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  15. tom28571
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    tom28571 Senior Member

    Adding fishtail wedges to the trailing edge of the rudder would allow greater steering control, especially at low speed and when you are on the face of a forward moving wave. Professional Boatbuilder magazine had a good article on that a couple years ago. That, plus more throttle when needed and constant helm attention is called for in following waves. Terrible barnacles, but you can see the idea here on a trawler. http://www.trawlerforum.com/forums/attachment.php?attachmentid=11751&d=1340340448
     
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