Following Seas in a Gillnetter

Discussion in 'Stability' started by moose60, Dec 19, 2008.

  1. moose60
    Joined: Dec 2008
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    moose60 Junior Member

    Hello all,

    I have a 36' x 11' gillnet boat that I fish in Southeast Alaska and Puget Sound. Over all it's a good boat. It packs around 12k lbs of salmon max and handles fairly well in most conditions.

    My question regards following seas. When the boat is light, no fish or ice and less than full fuel tanks I have a tough time keeping the thing pointed in the direction that the waves are traveling. Hand steering or autopilot produces the same wild yawing. If I add weight the boat will track straight, but I push more water and slow down. I usually make 6-7kts @ 2 GPH .

    I'd like to keep the boat light and be able to track well. Judging from the helm, the rudder is not well balanced. I have the space to increase the size of the rudder, esp. forward of the pivot. I would also like to change the rudder profile. If you look at my rudder from the stern now, it looks like an " l " . I want to box the rudder so that it looks like " I ". This has helped some other boats in the fleet.

    Anyhow, I've attached a photo of the boat. Any suggestions are appreciated.


    Byron
     

    Attached Files:

  2. PAR
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    PAR Yacht Designer/Builder

    Putting end plates on your rudder will help steering response a little, but the wandering butt syndrome is a function of the hull type and not much can be done without very major surgery.

    This happens because the beam is carried aft to the transom, which offers more hull volume, but also screws up stern too handling unless she's got a healthy load on. With some taper in her stern quarters, this would be much less a problem, though stern waves will still try to manhandle that broad transom.

    The "downeast" fishing boats so loved by the yankees in the northeast, had nicely tapered sterns, but then some wise guy, got the brilliant idea of increasing hull volume aft, so a few more tons of fish could be carried. This happened at the end of the 1950's and it ruined the handling qualities of this hull type. Eventually it was figured out and the hull shapes went back to properly shaped quarters, but it seems not all designers, learn from this historical lesson.
     
  3. tom28571
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    tom28571 Senior Member

    There is too little attention paid to handling downwind in waves. I see many powerboats with chines that run straight aft to the transom at maximum beam. This is guaranteed to make downwind handling more difficult if not downright dangerous. As Paul said, this is usually the result of a desire to get the most volume aft to carry load or support large or multiple engines. A taper in beam of 15 percent will improve things but is not an option to an existing hull. I see boats designed by people who should know better with straight chines. Many of these get away with it because they have huge and powerful outboards that don't so much as solve the problem as trample it to death.

    The basic problem is that the water passage over a rudder is minimal with slow speed and the additional forward motion of water in the waves. A following wave hits the transom at some angle off center and starts to slew the boat around. Unless corrected by effective steering, the boat can broach or even capsize. Even if it doesn't do that, it can be hard work for the helmsman and no fun at best. Outboards are much better in this situation because steered power bursts can correct a course far easier than deflected water from a screw of an inboard.

    One possible solution is a more effective rudder that deflects water better, especially at low relative flow speed. Such a rudder is shown by Dave Gerr in the August-September issue of Professional BoatBuilder. It is the Maclear-Thistle rudder. This rudder has flared trailing edges which generates much more lift at small angles for better control at slow speed. A modification to an existing rudder would not be too difficult. Adding fences will also help, especially at large steering angles. If you don't have access to that issue, send me an Email and I will run a copy and send it to you.
     
  4. rasorinc
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    rasorinc Senior Member

  5. Fanie
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    Fanie Fanie

    Simple question Par,

    How would two keels, one port and one starboard, if added, effect the handling when it's not loaded ?

    Maybe stretch the aft third of the hull.
     
  6. moose60
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    moose60 Junior Member

    Tom,

    Thanks for the info. I believe that I have seen rudders like the one in the article you mentioned. I can probably get a copy at the library. The ones I've seen have small wedges on the trailing edge. A friend recently told me that he was happier with his steering after adding two rolled "ramps" to the rear end of his rudder. These function just like the wedges, but perhaps allow a bit smoother flow/ vector the force more effectively.

    PAR,

    The boat has about a 1' taper from just behind the house to the stern. I have heard folks say that the closer a rudder is to the stern the better a boat like mine will handle following seas. I can't move the rudder because it's very nice to have the wheel and rudder tucked under the hull away from the net. Besides, I'd probably need a longer shaft, and a new strut.

    A year ago I entertained the notion of adding a couple feet to the stern and increasing the taper dramatically. The idea fell by the wayside, partly because any addition would push the rudder farther from the stern.

    so, (in you folks' guesstimation)

    1. What would an addition need to look like in order to show a handling improvement?

    2. Would a 2-3' addition give me more speed for a given power input? Or would the added wetted surface cancel any gains from a longer WL?
     
  7. PAR
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    PAR Yacht Designer/Builder

    Fanie, his boat already has a substantial amount of lateral area, Adding more will just encumber it with more drag. Stretching the aft portions of the hull will not solve the issue, which is the huge hole it's dragging in it's wake, because the beam is carried all the way aft.

    I'm just guessing here, but if the beam of that boat is 11', then the transom beam would likely be on the order of 9' 4". This provides a nice taper to the chines and helps fill in the hole, so the rudder can operate in less disturbed flow. Of course with any well immersed and broad transom, there's going to be some suction aft of the transom, but you can limit this to a fair degree with taper in the plan forms.

    A few different rudder configurations could help her to some degree. Tom has mentioned some and you may want to look up the "kitchen" rudder too.

    It might be possible to put an extension on the back of her, but I think you'll need a good bit more then 3' in length to do any real good. If you're running 6 - 7 knots on an assumed LWL of 33', you're running at 1.15 to 1.2 S/L. If the boat was 3' longer you'd only gain about a quarter of a knot in speed at the same S/L ratio.

    Make the rudder top as close to the hull as you dare, which will serve as an upper fence and weld on a lower fence. Square up the shape of the rudder, so the trailing edge gains a little extra area in the process (the bottom too) and lastly consider adding some wedges.
     
  8. tom28571
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    tom28571 Senior Member

    1. I can't tell, but handling improvement is not guaranteed by lengthening the boat.

    2. Adding a little length won't increase your speed or improve handling very much though and surely not enough to justify the cost. I'd do the rudder thing first.

    The waterline aft does look like the boat is dragging a hole behind the transom. It would take a lot of added length to bring the bottom up enough to get rid of that.

    The rolled edge you mention is the same as the rudder I referred to, not flat angled wedges. this would be an easier mod than anything else I can think of.
     
    Last edited: Dec 20, 2008
  9. Fanie
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    Fanie Fanie

    Sorry if I created the wrong impression.

    The keel is large while the stern area seems flat. In my mind it is going to act like a forklift in following seas with the prop and relative small rudder to do the steering, hence the stern being pushed sideways easily.

    The idea would not be to lengthen the boat, but if a keel is added both sides of the prop, won't that keep the stern from tending to get pushed or throwed sideways so easily ?
     
  10. PAR
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    PAR Yacht Designer/Builder

    If a double ended stern was added, it might help considerably. You'd gain some volume aft to help hold up the stern and the quickly rounded stern sections would help split following seas, rather then resist them. It would place the rudder in better flow, but not in an ideal location for steering leverage (it would be better at the aft end of the hull addition, ditto the running gear).

    Additional skegs wouldn't help Fanie. Following seas will grab the transom, lift it up and toss it to one side or the other, depending on direction from which they came. No amount of additional lateral area (skegs or keels) will prevent this. More lateral area (new skegs) will slow down the movement a little, but not enough to warrant installing them.

    Substantially increasing rudder area probably isn't a good idea. When the rudder is in the prop blast, it has to be relatively small or it can get over powered by the wash. Ditto additional balance in the blade. The rudder appears to have about 15% in it now and I wouldn't increase this at all, as it's just under the amount I'd consider maximum for this arrangement.

    Considering the costs associated with adding a double ended stern compared to increasing rudder efficiency, you're much better off with rudder modifications.

    One other rudder modification that should be considered is a trailing edge flap. I'm not sure what it's called (some one help me out here) but the flap, which typically amounts to about 15 to 20% of the total area is the articulated portion of the rudder, while the rest is free to move with current and prop wash. It dramatically eases the effort needed at the helm and can "steady" her down a good bit. Of course this means a redesigned rudder and related gear, which is more costly then some end plates and other modifications.
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    There are not many (reasonable) things that can be done to correct the broaching tendency in a following sea for such a hull. I'm with Paul and Tom about just trying some redesign/type of rudder to improve its efficiency, with end plates being the cheapest/easiest solution.

    moose60,
    I see you have what I think are stabilizing paravanes installed. In what conditions do you use them?
     
  12. Gilbert
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    Gilbert Senior Member

    Go to metalmarinepilot.com and click on publications, then download a pdf file of the paper titled "Rudders". Then download a form called "Vessel Eval" which, after you fill it out and send it to them, will supply the folks at Metal Marine Pilot with the information they need to give you a free optimizated rudder layout for your boat. The optimized rudder may not solve all your problems but it will certainly improve the steering and give the autopilot a better chance to do what it is supposed to do.
     
  13. moose60
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    moose60 Junior Member

    Guillermo,

    I use the staybees when the weather is crummy. Generally 25kts or more and working the net. We spend alot of time laying in the trough. These make it easier to work on the deck when it is windy. You do have to be careful not to get 'em caught in the net. I almost never use them when running because they cost at least a knot if both are in the water. But they do work quite well when running.

    If I know that it will be nasty I'll drop the poles early but leave the staybees on the boat. The poles can be tough to handle alone when the boat is really bouncing.

    A cousin asked me how I like fishing where I do, I said "it's ok". He had fished the same place for 10+years. He laughed and said, "yeah, sure, it's fine. You just have to keep all the windows closed so that your stuff won't fly out of the boat."
     
  14. Guillermo
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    Guillermo Ingeniero Naval


  15. robherc
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    robherc Designer/Hobbyist

    Hmmm, I'm by NO MEANS an expert here, but it seems to me that you could fairly easily design a "bolt-on" box to bolt to your transom & add some much needed bottom-up, and sides-in taper to your stern. If you made it about 11"x3' at the front, and tapered it to, say, 9'x2.5' that the back, and say 2' long, I'd think you'd gain substantial handling in following seas, and with very little cost in terms of speed (if not a SLIGHT improvement).
    If you filled your box with flotation foam, you'd also be tilting the balance of your boat a bit towards the bow...would raise the stern a little more & provide a little extra buoyancy, but I can't recommend it, not knowing how that'd affect your overall handling. I, personally, would be tempted to make the box waterproof inside & out, then drill a couple 1" holes in the bottom & top, let it fill with water, so it'll improve your handling, without affecting the fore-aft balance of your boat. ...it should also be (fairly) cost-effective to make, if you made it yourself.

    Just my two-cents worth. Like I said, I'm not an expert (yet, at least)
     
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