Question for the masses: Stern faces the swell while drifting. Why?

Discussion in 'Hydrodynamics and Aerodynamics' started by djaus, Nov 14, 2013.

  1. djaus
    Joined: Jun 2013
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    djaus Salted Nut!

    I've got a lengthy thread in the wooden boat building section here on Boatdesign.net. The Hartley Flareline power boat.

    My question is: when I go offshore, turn the engine off & drift (& fish) why does the boat always turn the stern to the oncoming swell.

    As per the last trip (& the couple of previous outings) off the central coast of Tasmania I had a Northerly (onshore) breeze & the tide was running out.

    Regardless of wind & tide though the boat always turns the stern to the swell.

    I also leave the outboard leg in the water so I can start it quickly & move if need be. I'm wondering if a sea anchor might rectify this issue & keep the bow to the swell, this obviously depends on the wind too I suppose.
    I'm guessing the dead weight of the outboard leg may cause the boat to turn around even if I stop the motor whilst heading into the swell.

    Plus I prefer not to anchor or use a drogue as I don't know the local waters that well yet. I do know however that it's not that deep, even a kilometre or a half mile offshore it's really no deeper than about 5 - 8 metres so I'm wary of drifting to close to shore & drifting for too long.
     
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    There is no universal rule for that.
    My boat keeps turning around in a quit sea. When the waves get higher she keeps one side towards the swell, usually starboard, aiming for maximal discomfort.
     
  3. Stumble
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    Stumble Senior Member

    Low windage on the transom would be my guess. Mine like to be side to the waves, either side is fine.
     
  4. LP
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    LP Flying Boatman

  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The boat has a lot of drag aft, which the sterndrive increases. The two ways to go are either to increase drag forward (sea anchor) or increase windage aft (some kind of sail). What is the concern about a sea anchor? If you are worried about drifting into the coast, firstly pay attention to your location, and secondly the sea anchor will decrease the speed of your drifting.
     
  6. michael pierzga
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    michael pierzga Senior Member

    There are many mysteries in life. we just dont know why. There are known knowns; there are things we know that we know.
    There are known unknowns; that is to say, there are things that we now know we don't know. But there are also unknown unknowns – there are things we do not know we don't know.

    For instance...whenever i leave the dock on a thousand mile trip...as soon as i clear the breakwater and steer course , the wind is always on my bow, my basebal cap blows overboard, my glasses get covered in salt .......for the next one thousand miles .?

    Why ?
     
  7. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Same reason that there is more uphill than down...........................
     
  8. whitepointer23

    whitepointer23 Previous Member

    lift the leg out of the water and see what happens.
     
  9. mij
    Joined: Nov 2013
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    mij Junior Member

    I wonder if this behaviour is simply due to the weight of your engine relative to the weight of the rest of the boat? As you roll over a wave the rear of the boat will tend to slide down the back of the wave faster than the front this will create rotation that will continue until the next wave arrives. Of course the boat will tend to do the reverse as it goes over the front of the wave, but the period of rotation won't be as long (more downhill than up?). Once the boat is perpendicular to the swell this effect will stop. You should be as to test this by seeing what happens it you have a lighter engine, and perhaps some weight on the bow.

    If this turns out to be the cause you could rectify the problem by loading your catch in the bow, or perhaps installing a weighty figurehead;)
     
  10. midnitmike
    Joined: Apr 2012
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    midnitmike Senior Member

    In my mind it's always been about two forces contending with one another...the current flowing by the underwater portions of the hull and the wind affecting everything above the waterline. If you change the shape of either of these you should be able to alter the way the boat lays to. If you want to weather cock into the wind an aft stay sail should do the trick. Since it's only a few meters deep a small anchor could prove beneficial depending on wind and tide of course. Personally I prefer to lay with the stern into the swell but that's just me.

    MM
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Current or tidal run wouldn't have a lot to do with it, unless it was that fast the "apparent" wind speed and direction altered substantially as a result. I'd try tilting the outboard to see what difference that made. Drifting stern to the swell is not a bad attitude imo, better than side on which will increase rolling.
     
  12. mij
    Joined: Nov 2013
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    mij Junior Member

    I think that this is the important point:

    Have a look at the boat djaus is referring to, it is quite small with a very big motor.
     
  13. djaus
    Joined: Jun 2013
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    djaus Salted Nut!

    All these scenarios make sense. I can understand basically why the boat sits this way. When all said & done I don't mind it, I just worry about a big wave topping the outboard well.
    Having said that the Hartley is designed in such a way that this shouldn't be a worry really. I don't like the idea of a sea anchor as I most likely have to walk on to the bow to set & retrieve it, bit of an annoyance if I only drift for 10 or 20 minutes at a time then want to move.
    If I went further offshore then a sea anchor would be ideal.
    I also agree with the posters who said they prefer the stern to face the swell. It could be worse I guess. It's just the bow is more equipped to handle that 1 in 7 bigger waves & for me, the newbie, it's a little unnerving.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    The (breaking) wave that will endanger your boat when the stern is facing the oncoming swell will be more dangerous beam-on, I don't think you have to worry about it.
     

  15. michael pierzga
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    michael pierzga Senior Member

    The best skiff i ever had used an extra long shaft 50 hp honda the i bought secondhand from a catamaran.

    The skiff looked a little strange, with its tall outboard profile, but it could lay stern too the waves and not take a drop of water over the transom. I wish they marketed an extra long 20 hp motor
     
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