Broaching tendency - correction possible?

Discussion in 'Boat Design' started by skypoke, Sep 16, 2010.

  1. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    The boat is a 24.5' X 9' Privateer Renegade. It is a well known and regarded large skiff in production for many years. It has a shallow V hull, a small keel running the length of it, maybe 1" in height. Transom mounted 200 hp outboard. It is a great boat with one bad little habit....following seas keep you on your toes cause she will broach if allowed. Usual technique involves loading her aft, raising the bow to minimize these tendencies. No trim tabs though these will be added but no use downsea.

    I'd like to correct this tendency. It's repower time and I was wondering what, if any, help moving the engine back with a flotation bracket might be. Looks like most brackets basically extend the hull aft with a step up from bottom at transom.

    I suspect removing the "keel" would help but would rather not get into surgery of this order plus the boat would no longer be rated for light ice breaking (no kidding). Do you think increasing the lever arm by setting the engine back might help or aggravate?

    Operating conditions are offshore Gulf of Mexico to 50 miles.

    I'd appreciate your thoughts.

    Chuck
     
  2. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    If this is the Privateer built in NC that I am familiar with, it is stern light and bow heavy. Not a good condition on any boat and bad in following waves. It had scuppers added at the forward end of the cockpit to allow draining at the dock. Prior to that we would load several big water cans aft to stop rain from flooding the cockpit. The boat has very little on the aft bottom to prevent slewing around if a wave takes it on the quarter. A quick hand on the wheel was needed to prevent the broaching you mention. I would give it a lot of outboard thrust in the proper direction to keep it on course. A large skeg would help if you can stand the added draft.
     
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  3. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    Tom,

    That's the one. This is a center console though, so no weight of a cabin forward. It drains aft with no problem but like your experience it wants more weight in rear. That's one reason I was considering the bracket....move the weight back a bit. I don't have a problem catching a developing sheer but I'm not the only one operating the boat. I'd like to fix it.

    Interesting idea on enlarging skeg. Not draft limited so a bit of tig work could accomplish this.

    It seems that the shape of the stern has a lot to do with keeping her straight in a following sea. Adding a bracket might allow a change to a more advantageous configuration?

    Chuck
     
  4. Petros
    Joined: Oct 2007
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    Petros Senior Member

    A simple skeg is the easiest fix I should think. This is a common problems with sea kayaks, and a skeg makes the difference. You might also put two skegs, one on each side of the engine location, so you always have at least one in the water.
     
  5. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    The simplest would be to add to the skeg that exists on the outboard. Extending the height of the skeg a bit and extending it aft would be possible, could probably double the surface area without overloading the steering.

    As far as mounting a skeg on the bottom of hull, somewhat limited due to need to trailer the boat. Probably a max height of 4" or so but could be pretty much any length.



    Is there any general rule that governs skeg shapes?
     
  6. Bruce46
    Joined: Jul 2006
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    Location: Stuart, Fla.

    Bruce46 Junior Member

    Chuck
    When I Googled your boat I noticed that it was available with a stern bracket as an option. Shifting the weight of the engine back about two feet would shift the center of gravity aft, which should help additionaly movinging the engine to the bracket allows a cleaner flow of water to the prop and allows you to raise the engine for better top end speed. On my race boats I mounted a roughly 3"x 6" deep fin on the transom, which made a big difference in keeping the boat from slidding on the turns.

    While the origin of the problem lies with the design of the hull, I believe that gluing things on or cutting things off could be hazardious. I would also strongly advise against making any changes to the lower unit of the outboard. Good luck and let us know how it works out.
    Bruce
     
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  7. tom28571
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    Location: Oriental, NC

    tom28571 Senior Member

    I don't think adding anything to the engine lower unit would have any appreciable effect. I would not do that anyway. I'd vote for the bracket even though I am not a big fan of them in a lot of cases. Adding the pod/bracket would also allow extending the length of the skeg to, or near to, the transom. I don't remember the aft shape of eh topsides as this was someone else's boat. Ideally chine beam should decrease toward the stern up to 15% less than max beam to help with downwind steering but changing that is also beyond a reasonable change to an existing boat.
     
  8. mcollins07
    Joined: Jan 2006
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    mcollins07 Senior Member

    rules of thumb

    Tom,
    Thanks for this rule of thumb. I don't think I've heard this stated like this before. Is it for power boats, sailboat, any boat? Any other guidelines for applying it?

    ~ Michael
     
  9. srimes
    Joined: Sep 2008
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    Location: Oregon

    srimes Senior Member

    x2. Also, what are the common ways of doing this without adding rocker on v-bottoms? I thought how you did it was pretty slick.
     
  10. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    The chine beam does decrease significantly fore to aft. It's easily 15% less beam from the forward hull swell to transom.

    I contacted another owner of the same boat and he tells me the problem was largely eliminated when hydraulic steering was installed, this happens to be a new addition to my boat but haven't had a chance to run her with it yet. So, I'm hopeful.

    Taking a closer look at the bottom after getting boat on a trailer, I see the "keel" does not extend as far aft as I thought. I'm going to experiment with an aluminum fin aft, maybe 2' long, 6" tall for starters. This can be accommodated by raising trailer bunks a bit. Easy to try out using some 3M VHB tape mounting along centerline. Test and trim with a sabre saw as necessary.

    Thanks for all the great advice. I'd like to point out that this boat has many absolutely outstanding sea handling qualities, I'm confident this undesirable quirk can be corrected.

    Chuck
     
  11. tom28571
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    tom28571 Senior Member

    If the keel or skeg extends too close to the propeller,there will be problems with ventilation of the prop in turns.
     
  12. skypoke
    Joined: Dec 2002
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    skypoke Junior Member

    Tom,

    Duly noted. I've decided to approach this systematically. Will rig new engine, try boat as is with hydraulic steering. Next will try adding skeg if needed. If problem persists will opt for the engine bracket.

    Apex, strong words there. Let's see your perfect boat.

    Chuck
     
  13. HughGWrecktion
    Joined: Sep 2010
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    HughGWrecktion Junior Member

    How about a bigger engine to move weight to the rear? :D
     
  14. EgliVincent
    Joined: Sep 2010
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    EgliVincent Junior Member

    Hi Chuck,
    Do you still have your powercat you built or did you move on.
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I have seen outboard powered flattish-bottomed planing hulls that had broaching tendencies modified with skegs on both sides, maybe 3" or so deep, over the last 3 feet of the hull approx., halfway between the centreline and the chine. That avoids interfering with the water flow into the prop that may occur by deepening the central keel. Moving fixed weights aft may not be that helpful, as the "lever arm" working to broach your boat will be lengthened by it. Dragging the centre of lateral resistance back by increasing the underwater profile aft will dampen broaching.
     
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