Removing Rocker at the Stern

Discussion in 'Wooden Boat Building and Restoration' started by CraigC, Oct 10, 2017.

  1. CraigC
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    CraigC Junior Member

    My current boat is a wooden flat bottom with rocker both at bow and stern. The design of this boat (20'), uses a 9.9 hp OB with a top speed of 7 kts. By removing some of the stern rocker, flattening out the bottom, will this allow a slight increase in speed? I was told that I could install a 20 hp with maybe a max increase of 12 kts. but will be pushing a huge wall of water. I am not wanting to create a planing hull but simply increase my cruise around the 10-12 kts range without having to go WOT. Hopefully my question is not too disjointed. If any clarification is needed please let me know. Thanks for the replies and help.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    A picture of it might be helpful. The chances are you could do something along those lines, but you will find it slams badly in any kind of chop, at the speeds you desire.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Welcome the the forum.

    If your flat bottom as rocker aft, it was intended to travel at displacement speeds, which is a function of LWL, so speeds will be limited. If you want to go faster, as you've surmised, the aft sections need to straighten out, but then you're making a boat capable of full plane mode. There's really no in between, though some underpowered full plane mode craft can be called semi-displacement, they're really full plane, just lack power. At 7 knots you're already moving past displacement mode speeds (1.65 S/L ratio on an 18' LWL). At 12 knots you'll be asking this boat to move at over 2.8 S/L which is full plane mode speeds. Lastly, without some images of the boat, this is all pretty speculative.
     
  4. CraigC
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    CraigC Junior Member

    Let me give a try to posting a drawing or picture. Thank you.
     

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  5. PAR
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    PAR Yacht Designer/Builder

    Yeah, the RedWing isn't going to get much faster, without rebuilding the entire back half of the boat's bottom. Of course, this will trim her bow down quite a bit and it'll still be what she is. You might want to learn to love 7 knots and search for a different boat, more in keeping with your goals.
     
  6. CraigC
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    CraigC Junior Member

    PAR, thanks for your reply. I think we may just replace the 9.9 with a 15hp. This would eliminate having to continually run full throttle on 9.9hp to achieve the 7 knots. As I said, we are not necessarily looking to plane but have a little extra "umph" available in case of an emergency. I appreciate your impute.
     
  7. SamSam
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    SamSam Senior Member

  8. DCockey
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    DCockey Senior Member

    I've seen reports of wedges at the transom or some "hook" in the bottom reducing resistance near and slightly above "hull speed" but don't recall the specifics.

    You could experiment. Make some temporary wedges, perhaps using tapered cedar shingles, and glue them in place. Try the boat and see if the wedges make a difference. Mostly like difference, if there is one, is in amount out power to get to 7 knots. Any increase in top speed will probably be small.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Again at 7 knots, you're already pushing well past its normal operational envelop, asking for more is just turning water into a froth, behind the prop. Wedges on that hull may help trim at speeds approaching 6 and above knots, but nothing will help get her to a near 3.0 S/L ratio, which is the OP's desire. The wedges will help flatten the hole he's dragging, but he's still dragging that hole. You should be able to do hull speed (about 5.7 knots) with less than 10 HP, more likely about 6 HP. So the remaining 3.9 HP you're using at WOT is managing a whole extra knot. Put differently, 60% of the power you have available gets you to hull speed, while the remaining 40% is what it takes to gain an extra knot. This isn't a proportional tradeoff and you'll use progressively more power to gain ever decreasing speed gains. Simply put a 15 HP outboard will likely get you to hull speed at 1/2 throttle, probably less, so now you're lugging the engine at low RPM, for the same speeds. I estimate you'll just see 9 knots with the 15 HP at WOT and double your fuel costs, assuming you can get her to trim out and remain laterally stable.
     
  10. CraigC
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    CraigC Junior Member

    SAMSAM, thanks for pointing out that PM article.
    I would not want to modifiy my current Redwing at this point. Since I have an unused set of plans for the Redwing 18 and thought that I may build another one and adjust measurements during the frame build so as to reduce the rocker to a more flat bottom at the stern. There is much to think about and consider. I really appreciate everyone's replies.
    PAR, your point is well taken regarding additional fuel consumption and performance with the 15 hp. Would running the 9.9 at a full throttle wear out the motor faster or am I just being concerned over a non issue?
     
  11. DCockey
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    DCockey Senior Member

    You could send a note to the Karl Stambaugh, the Redwing designer, and ask his opinion of your proposed change. Stambaugh is a naval architect who works for the US Coast Guard. Contact information is available at his website cmdboats.com He is a co-author of a paper which includes evaluation of transom wedges on the resistance of USCG patrol vessels. http://www.sname.org/HigherLogic/Sy...tFileKey=904fe327-2c75-8df6-7326-d3c605f8a3a1 See pages 10-12. 7 knots in a Redwing 18 corresponds to a Froude Number of 0.5, which corresponds to 20 knots in the patrol craft discussed in the paper.

    Reducing rocker in the aft portion of the Redwing 18 will cause it to float with both the stem and transom submerged more. Take too much rocker out and the boat will have significantly higher resistance at 7 knots.
     
  12. CraigC
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    CraigC Junior Member

    Thanks DCockey, I'll reach out to Karl.
     
  13. Milehog
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    Milehog Clever Quip

    That flat hull bottom will be quite sporty at planing speeds.
    Your wanting to turn a mule into a thoroughbred, not gonna happen.
    Contacting Karl is a good idea.
     
  14. CraigC
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    CraigC Junior Member

    Well I think I get the picture and will put this idea to rest. It was a good idea just the wrong hull. Thanks again everyone for helping out.
    I'll close this thread out as soon as I can figure how that's done.
     

  15. SamSam
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    SamSam Senior Member

    If you build another one with reduced rocker, you might also want to widen the beam at the transom.
     
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