Planing hull - sail vs power

Discussion in 'Boat Design' started by Will Fraser, Sep 18, 2018.

  1. Will Fraser
    Joined: Feb 2014
    Posts: 168
    Likes: 16, Points: 18, Legacy Rep: 11
    Location: South Africa

    Will Fraser Senior Member

    All else being the same, is there a difference in the ideal aft buttocks of a sailing dinghy vs a powered skiff? More specifically, can a sailing dinghy get away with a bit more aft rocker given the fact that its thrust line's vertical displacement already provides some bow-down trim?

    As a hypothetical case I am thinking about a light skiff, say
    Lp = 13ft,
    Bp = 4.5ft
    Displ = 200kg
    V >= 12kts
    FnL >= 1
    FnV >= 2.6
     
  2. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Thats a good observation.

    The V, hard chine transom on a sailing dinghy is very common, and when sailing, the idea is to keep it out of the water.

    The BIG difference, is the hull surface for the main part of the underbody. Dinghys will usually have a convex surface to maximise slower sailing performance.

    The power hull, benefits from a straight run from the turn of the bow.

    One "combination" design I have considered is illustrated below.

    Trim.png
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,585
    Likes: 508, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    So "ideal" for what speeds ?
     
  4. Will Fraser
    Joined: Feb 2014
    Posts: 168
    Likes: 16, Points: 18, Legacy Rep: 11
    Location: South Africa

    Will Fraser Senior Member

    12kts or faster.
    I appreciate the requirement for sailing dinghies to also cater for low speed design, but for this hypothetical case it is not important.
    It can be assumed that an equal amount of propulsive thrust is available and sufficient to achieve the required speed. The sail might just as well be an air-propeller.

    I can rephrase the question as follows: if the thrust-line can be moved at will to provide optimum trim (for least drag) for each hull, one with straight buttocks and one with a bit of rocker, which hull would have the least drag at planing speeds?
     
  5. Will Fraser
    Joined: Feb 2014
    Posts: 168
    Likes: 16, Points: 18, Legacy Rep: 11
    Location: South Africa

    Will Fraser Senior Member

    Another question that may or may not be related: is there a difference in buttock design for a powerboat with transom mounted motor vs a motor mounted further forward in a well?
    Aside from any trim implications, does the longitudinal location of the prop pressure differential have any influence on the local hull shape with regards to efficiency?
     
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,585
    Likes: 508, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I doubt it will make much difference at 12 knots, except the rocker might create more porpoising. Maybe you will get more speed downwind, as the bows on the rockered boat will rise more easily to a wave, rather than plough, washing off speed. Once the boat is really up and running, beyond any "hump", you'd think the rockered bottom would have the advantage of a higher trim angle, and less wetted surface area.
     
    fallguy likes this.
  7. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    The rocker at the bottom has a much more devastating effect on power. It forms severe drag by nature of the curved surface. That's why you never, ever see a planing boat with rocker.
     
  8. alan craig
    Joined: Jul 2012
    Posts: 252
    Likes: 38, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    Modern planing sailing dinghies all seem to be designed as pure planing craft, I guess you just sail them on their side to reduce wetted area when conditions don't allow planing.
     
  9. fallguy
    Joined: Dec 2016
    Posts: 3,091
    Likes: 359, Points: 83, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    How about a semi planing hull?
     
  10. Doug Lord
    Joined: May 2009
    Posts: 16,651
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==================
    "never ,ever" is a bit much. The Windmill, designed by Clark Mills, is a great planing hull design and has just the right amount of rocker.
    Guillaume Verdier is one of the leading multihull designers and considers that the hulls of Gitana 17 are planing hulls. And, as such, make a huge difference in the ability of the boat to takeoff and fly.

    Windmill-planes in 8 kts of wind:
    Windmill illustration.jpg



    Gitana 17 ama with a bit of rocker.

    Gitana 17-probably yann riou though uncredited from stingray -sa.jpg
     
    Last edited: Sep 18, 2018
  11. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Totally different to a powered craft. For a start, the bottoms of those hulls are flat, with some rounded edges. next, the length.width ratio is hugely different to small hull that has to get enough surface to plane
    Finally - its FOILING craft. It doesnt Plane, it Wings it.

    The effect of Rocker on Power craft is so basic, I am surprised you even try to dis it.
    "For maximum speed, a boat bottom should be as flat as possible in a fore-aft direction (longitudinally) for approximately the last five feet (1.5 m) (Figure 7-18). .... Any hook, rocker, or surface roughness on the bottom, particularly in the all-important center-aft portion (critical bottom area) (Figure 7-21), will have a negative effect on speed, often costing several miles per hour on a fast boat." Hull Shapes Review http://www.killcaremarina.com.au/commonly-asked/63-hull-shapes-review

    "Although any hull will plane if enough power is provided and enough speed is attained, a hull designed for operation in the planing realm is sometimes distinguished by a flat run aft. In other words, in side view, the bottom is more or less a straight line towards the stern" Planing (boat) - Wikipedia https://en.wikipedia.org/wiki/Planing_(boat)

    "Planing a sailing dinghy was first popularised by Uffa Fox in Britain. In 1928 Fox introduced planing to the racing world in his International 14 dinghy, Avenger"
    Check out the complete lack of rocker.

    INt14.jpg 220px-Contender_sailing_dinghy.jpg
     
  12. Doug Lord
    Joined: May 2009
    Posts: 16,651
    Likes: 322, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    I didn't even consider "rocker on powered craft" so I couldn't have dissed it.
    On Gitana 17: while it flies, the planing hulls come into their own in the transition between displacement sailing and foiling-that is, leading up to takeoff. And also in rougher water where the hulls make intermittent contact with the water.
    A flat run aft is planing hull 101-but it's a lot different than saying: "Thats why you never,ever see a planing boat with rocker".
     
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,585
    Likes: 508, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Pretty sure you will find planing boats with rocker, but it is a matter of degree, of course. The original Cigarette racing boats did have rocker, the rationale being they were less likely to "stuff" (drive the bows into the back of a wave), a sometimes deadly situation. The rocker made them less stiff in pitch, and allowed the bow to rise, rather than punch through, at high speeds. This typically happened when the boat landed on the front of a wave, after being airborne. Tabs could be used to limit the porpoising caused by the rocker. But obviously not applicable to this thread ! I would say it might be helpful to have a little rocker, but not much, in this little boat.
     
  14. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Yes, it is a matter of degree. The point I was making is that any curve in the buttocks detracts from optimum planing ability, and is a design decision.

    This is in response to the OPs actual request "one with straight buttocks and one with a bit of rocker, which hull would have the least drag at planing speeds?"

    "The tests conducted into warped hull forms ........ conclusions given by Savitsky and Brown suggest that the drag of a warped hull is markedly increased compared to a hull with parallel buttocks,"
    https://pdfs.semanticscholar.org/684c/1eb356659eea427d9dc1bad8b254095aaa17.pdf
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,585
    Likes: 508, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Over the years, I had two 14 foot alloy boats, supposedly the same boat brand and name. but one had rocker, the other a slight hook. How that happened, I don't know, must have been rough-house building practices, but it wasn't from rough use dishing or deforming the bottom. The one that had rocker was drier and handled better, a little bit of rocker doesn't hurt.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.