Making a rowing 8 hull plane - hypothetically

Discussion in 'Boat Design' started by fernandodlc, May 22, 2015.

  1. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    Hi people.

    This is just a hypothetical as I'm trying to understand the conceptual limitations of the various situations a boat can be in.

    1.
    Lets say you had some kind of high speed catamaran with a magic power source that didn't need prop positioning.
    Now let's say you suspended a rowing 8 between the hulls and you had some magic way of lowering the hull at speed.
    What would stop a relatively flat bottomed 8 from becoming a planing hull that could take over as the main hull?

    2.
    Rowing 8s can get up to pretty high speeds in displacement mode.
    What is stopping you having some kind of transitional aide that would help lift the hull out of the water to plane?

    Fundamentally what I am getting at here is that what seems to make a good displacement hull can have a flat bottom that sits parallel to the waterline. What makes a good planing hull is a flat bottom that can rise and skim across the water. If you have a hull that can reach high speeds in displacement mode (like a rowing 8) and it is light enough, what is stopping you transitioning it to a planing hull with enough power?

    Does the kind of weight you need to plane prevent you from making the most of the design in displacement mode?
     
  2. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    The rowing 8s I've seen are closer to semi-circular than flat in section below the waterline? Why do you think a flat bottom is good for displacement hulls?

    You could pretty much make anything plane if you could get it fast enough, but only in theory. A round bottomed rowing 8 isn't realistically going to be able to plane.

    A flat bottomed one would have a better chance, but I doubt it could be done while rowing. I'd guess the rhythmic nature of its power would mean even if a stroke could put it on the plane it would come off again between strokes.

    I assume if rowers could take advantage of semi-planing hulls they'd already be commonplace.

    A flat bottomed rowing 8 would need to reach incredibly high speeds to be able to generate enough lift to support a decent size cat out of the water - also unrealistic.
     
  3. tom28571
    Joined: Dec 2001
    Posts: 2,473
    Likes: 115, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Oh wow. There is an extremely long and contentious thread on planing in the archives that touches on every aspect of the subject while never reaching any satisfactory conclusions..

    In your example just know that in order to plane, a hull must generate lift. In order to generate lift the hull bottom must move forward at some angle of attack or incidence to the water. How that might be satisfactorily achieved in an eight oared shell is a mystery.
     
  4. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    So I'm right in thinking that it is just the lack of hydrodynamic lift that is the issue?

    Conceptually then should there be any reason why a rectangular cross sectioned rowing type hull would be unable to transition to the plane with enough power and little enough weight?

    Maybe you would have a boat that was say 25ft long and a foot wide that was impractical for pretty much everything. You would however have a boat was able to go 15ish knots in displacement mode and plane at say 20ish knots with one lonely person in it?

    15-20 knots aerodynamics start coming into play, maybe you could have some wings and have a WIG supported craft to reduce the requirement for hydrodynamic lift but with enough speed at displacement to achieve a partially hydrodynamic "take-off"?
     
  5. SaltOntheBrain
    Joined: Feb 2007
    Posts: 123
    Likes: 7, Points: 18, Legacy Rep: 87
    Location: crosbyton, TX

    SaltOntheBrain Senior Member

    Like this?
     

    Attached Files:

  6. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    Aha, I think you are hitting the core of where my questions lie. How much does that angle of attack affect the efficiency in displacement mode? I guess as soon as the LWL changes significantly drag will shoot up?

    I guess I was wondering whether (using magic thrust) it would be possible to ever reach a speed where the same hull would work in displacement and in planing mode.

    Some of the very narrow long hulls seem to be able to achieve 20 odd knots and have pretty flat (if narrow) bottoms. Wouldn't said hulls also be able to skim across the surface at that speed too?
     
  7. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    Not far off lol
     
  8. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,559
    Likes: 502, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I think the design differences of powered catamarans answers most of your questions. Except for issues of wave interference between hulls, and the fact that joining two hulls together makes an inherently more stable arrangement than one slim hull. But the dynamics otherwise are comparable.
     
  9. Rastapop
    Joined: Mar 2014
    Posts: 278
    Likes: 5, Points: 18, Legacy Rep: 46
    Location: Australia

    Rastapop Naval Architect

    Inability to plane is by definition a problem of insufficient hydrodynamic lift.

    Virtually any conceivable object is able to generate some lift while moving in a fluid (at the correct angle), but the issue is whether or not has enough speed to be able to generate enough lift to support its weight.

    I cannot imagine a shell that one person could propel at 15 knots, let alone 20?

    Water gives 1000 times more lift than air, so better to put your wings in it rather than above it.

    Certainly, with magic thrust. The faster you go the more lift you generate.
     
  10. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    Well it was just a thought experiment.

    True but in water foils have so many disadvantages. The holy grail is a boat that goes well under displacement and planes well. It occurred to me that a long thin hull is basically ski shaped. If you could somehow get around the issue of that angle of attack issue using some kind of supplementary lift you might be able to do it. Maybe some outriggers with hulls that are designed to plane that you could lower to assist. That would of course mean you need some kind of crane to force them down and lift the rest of the boat up.
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,559
    Likes: 502, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    As I mentioned earlier, there are significant clues for you in the varying designs of power catamarans. You can get quite high speeds in displacement cat designs that work efficiently at low speeds as well, but there is no dependence on, or use of, significant dynamic lift to bodily lift the thing higher in the water. This results is a boat that, at speed, suffers from high wetted area that does not reduce with speed. Planing cats are typically wider hulled and fuller in the ends with submerged transoms, to get enough surface area and angle of attack for planing action to actually lift in the water. To this end, chines, chine flats, strakes and planks are designed to assist that, and also limit pitching, which is a potential problem with any slender hull. But they do not operate efficiently at off-plane speeds. The true displacement cat is probably the most versatile of these hulls, being efficient over a wide range of speed, but eventually losing out at the top end to boats that can reduce wetted drag.
     
  12. kerosene
    Joined: Jul 2006
    Posts: 1,047
    Likes: 62, Points: 48, Legacy Rep: 358
    Location: finland

    kerosene Senior Member

    foils would probably work the best.
    anyway, borrow or read uffa fox's seamanlike sense in powercraft
    he made 2 "best boat"s a displacement rowing single and a square sectioned planing version. with a really strong rower the planing version could just match the displacement version but didn't really beat it. now 8 guys have more power so in theory a craft might be possible (wider than a rowing 8).
     
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,559
    Likes: 502, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Test the resistance of a loaded rowing eight by towing it, then test the resistance of a candidate planing eight by again towing that, the curves of resistance Vs speed for each, then compared. That of course does not factor in the dynamics of rowers and rower's blades in the water, but will give an indication of how far apart they are in performance at various speeds..
     

  14. fernandodlc
    Joined: Jun 2014
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: United Kingdom

    fernandodlc Junior Member

    I love the http://www.seaski.com.au/ concept idea but it looks pig ugly. I am trying to figure out if you could achieve similar results with a mono/tri and a displacement hull in place of the submerging skis. It appears that one of the basic mechanics of the sea ski is to use displacement (in this case, submerged) "hulls" that cut through the water efficiently and captured air pressure to gain the necessary lift.

    Couldn't you leverage the relatively high speed of a rowing 8 type shape to provide enough forward speed so some kind of assistance could lift the hull into a planing configuration? Foils might do this. If you are going at a decent clip you wouldn't need massive foils. Especially if the objective was to plane on the hull rather than be supported entirely by foils. The problem with foils though is they significantly increase draft, they are fragile and although I've never used them except on an outboard I can imagine they get plenty of rubbish caught on them too. Besides if you are going to use foils you might as well go the whole hog and just make a displacement hull with foils
     
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.