Power for planing vs displacement operation

Discussion in 'Powerboats' started by Tom_McGuinness, Jan 29, 2004.

  1. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Hi Folks,

    I've got what should be a simple question for you powerboat guru's.

    As a planing-type powerboat hull accelerates from standstill to planing operation, it "appears" that less power may be required to operate at minimum planing speed than before getting fully onto the step. It also "appears" that once on step, the boat can remain on plane at a somewhat lower speed than it took to get onto step in the first place. I recognize that this may be an illusion since the tendency for some of us is to gun the engine and get onto step quickly, after which throttling back is required to remain at desired cruise speed.

    Is there in fact a hysteresis effect around the planing transition speed, or does the required power vs speed curve rise continuously from zero?

    My reason for asking this pertains to human-powered boat applications where available power is, shall we say, limited. ;)

    Thank you for your help.

    Regards,

    Tom McGuinness
    Pacific Tailboats
     
  2. Tom Lathrop
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    Tom Lathrop Junior Member

    I'll bump this back up to the top since no one has ventured an opinion. I really don't know the answer as to whether there is a theoretical hysteresis effect. I suspect that that there is. I assume that your phrase "on step" refers to the initiation of planing and not to any step in the hull. It's a confusing term anyway and should not be used. Certainly, on many boats, especially heavy ones, it takes more power to initiate planing than to maintain it. If the boat is well designed and very light though, and the throttle is advanced very slowly, no such effect is detectable. On slowing down, I find that the boat holds trim angle to a lower speed than when speeding up. Don't know how universal this is since the effect is different on all the planing boats that I've experienced and I've not paid close attention.

    Not a very good answer but maybe someone else can break in.
     
  3. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Clarification

    Tom,

    Yes, by "on step", I mean initiation of planing mode....not a physical step on the bottom of the hull, as I've seen on some cigarette boats. It's a commonly used term by laymen. Sorry for any confusion caused.

    My main interest is with regard to human-powered applications which are severely power limited. However, if it can be established that a hysteresis effect exists on larger boats, the same can be implied for smaller boats. If such an effect exists, there might be a speed range where human powered boats can plane for extended periods.

    Fluid systems do have the ability to exhibit such bimodal behavior, such as in the vicinity of stall speed on an airplane's wing. Once the flow detaches, you need to overcorrect a bit to reestablish attached airflow. I attribute this to the formation of and subsequent extinguishing of vortex flow...vortices being a stable flow regime, as is irrotational flow. Two stable flow patterns separated by an activation energy barrier.

    Thank you for your help.

    Kind regards,

    Tom
     
  4. captword
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    captword Junior Member

    this may be a stupid question,but what about variable pitch propelers? we use to have river raft races here. i have seen some local home antrepenuers that would set up a kinda like skulling style canoe with a ten speed geer system to run the propeller. the theory being the skulling style would move easier and one could use the gears to ease the transition from dead still to cruise, in these races they would almost always try to gear to high and kill there energy for the rest of the race. it would seem to me that it might be posssible to deploy a variable pitch prop in work through the process easier. Now the in the first place i will state that i have no experience in var pitch props. i do have experiencein boat building and repair. you may have already have tried it, but i thoughtit worth passing on for your thoughts on the process.
    Captword
     
  5. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Hull design

    Hi captword,

    The majority of human-powered boats (hpb's) are displacement hulls. But there's also a few hydrofoils. I've not seen any planing hpb's.

    Going long and thin to attain speed is the usual route for speed, but at the expense of maneuverability. A planing hull might give us speed, without unduly sacrificing maneuverability...just as it does with powerboats.

    At this point, I'm just exploring the displacement-planing transition to see if there's a potential operating point for a properly designed hull. Just pedal like heck to get onto plane, and then cut back for cruise on plane. I suspect this wheel has already been invented...many times over, but maybe not. Sometimes the obvious has been overlooked. And sometimes it's been overlooked for good reason. ;)

    Best regards,

    Tom
     
  6. Chris Krumm
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    Chris Krumm Junior Member

    Tom-

    If your questions are directed toward low/human powered craft, perhaps MIT's human powered hydrofoil Daedulus might be worth looking at:

    http://lancet.mit.edu/decavitator/VideoClips.html

    Hit 18.5 knots in 1991 using an air prop. They found the reduction in appendage drag and getting rid of the issue of changing prop depth was worth the trade off, but noted the increased crosswind disturbance was a problem.

    Foil-borne craft design is complex, but I wonder if the much higher lift/drag ratios you can get with full lifting foils vs a planing hull bottom are the reason experiments with high-speed, human powered craft have focused on foils rather than planing hulls.
     
  7. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Hi Chris,

    Many thanks for the lead. I'm aware of MIT's Decavitator. Hydrofoils are hampered by a number of practicality issues...weeds, submerged obstructions, and payload for example. Probably for same reasons hydrofoils have never "taken off", as it were, for powerboats.

    A planing hull avoids these problems.

    Much work on hpb's has been aimed at maximizing straight-ahead speed. However, this route quickly leads to unacceptable sacrifices in maneuverability and practicality. The only parameters I want to maximize are fun, versatility and utility.....not straight-ahead speed.

    Best regards,

    Tom
     
  8. CDBarry
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    CDBarry Senior Member

    I worked on an interesting approach to the 20 knot prize, but it works well at lower speeds too: hybrid planing hydrofoils - lots of differents choices depending on the mission.
     
  9. Tom Lathrop
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    Tom Lathrop Junior Member

    There is an earlier thread on the forum that discusses hull shape for operation in the transition range:

    http://boatdesign.net/forums/showthread.php?t=2545

    It's about using a high L/B displacement hull for buoyant support beneath an upper hull of planing shape. Maybe not exactly what you need but food for thought. It is one method of gaining lateral stability while also gaining some of the advantages of a long and slim hull.
     
  10. Tom_McGuinness
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    Tom_McGuinness Junior Member

    Tom,

    Thank you for the link to the previous thread. Some interesting reading there.

    Food for thought.

    Best regards,

    Tom
     

  11. Tom_McGuinness
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    Tom_McGuinness Junior Member

    CD,

    Interesting. I'm still trying to visualize the hybrid hydrofoils to determine whether they would address concerns for weeds and submerged obstructions. I'm trying to avoid any design that is not self-clearing of weeds.

    Best regards,

    Tom
     
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