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  1. #11

    Default Re: Maximum hull speeds ?

    I'm aware of the formula, but I suspect that you are taking the word "maximum" too literally. Earlier, I questioned it when you said that a boat will never exceed its displacement speed. My understanding is that these formulae give you a practical (rather than an absolute) maximum. However the application of more power will give more speed, albeit with increasing inefficiency.

    You then went onto say that it's different when under sail. Granted it is different in that the "balance" of the boat will differ, but in general terms surely 30HP of thrust from the sails is much the same as 30hp from the engine?

    I'm not trying to score points...honest. Just curious about the truth of this.

  2. #12
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    Default Re: Maximum hull speeds ?

    But then there are displacement hulls and then there are displacement hulls!!

    Modern forms tend to have a lwl near equal to loa, whereas more traditional lines have a greater difference, thus the lwl increases as heeling angle increases, giving gains in hull speed. The 1.3 to 1.4 figure is only an approximation: any displacement hull can exceed its theoretical max given the 'right' conditions, it's just a matter of generating sufficient power.

  3. #13
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    Default Re: Maximum hull speeds ?

    hence the distorted I O R hull forms to take max benefit from the rules
    I may be wrong but not always

  4. #14
    Stemar's Avatar
    Stemar is online now Registered User
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    Default Re: Maximum hull speeds ?

    IMHO, the reason a sailing hull tends to bury the bow is because the centre of thrust is above the waterline and the centre of drag is below, so there's a turning force trying to trip the boat over the bow. It's like the heeling moment heeling on a reach. With one of those free-flying kites set high and well forward, the force would still be there, but largely counteracted by the lift from the kite.

    It's possible to exceed hull speed significantly under sail without planing. It's just a matter of power. I've read something about a Warrior 35 caught in a squall doing 15kts, and I'll take a lot of convincing that that hull was planing! (Almost as much as it would take to get me on board if someone offered me a chance to experience it - it must have been a real brown trouser moment! [img]/forums/images/graemlins/tongue.gif[/img])
    Steve
    Never believe anything until it has been officially denied.

  5. #15
    Quiddle's Avatar
    Quiddle is offline Registered User
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    Default Re: Maximum hull speeds ?

    [ QUOTE ]
    unless its a "planing hull" it will never go faster than displacement speed no matter what extra HP is applied , all that will happen is the transom will bury its self

    [/ QUOTE ]

    I don't know the physics but I am sure this is not true. Directing the thrust downwards will prevent the transom burying itself. If I tie my displacement boat to the back of a RN destroyer at full ahead it will exceed its displacement speed. I think Bergmans answer is more feasible.

  6. #16
    trouville is offline
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    Default Re: Maximum hull speeds ?

    J24s in Irland have clamed up to 20knots!! ??

  7. #17
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    Default Re: Maximum hull speeds ?

    the boat is then being towed NOT propelled by the prop or am i the DUMBO [img]/forums/images/graemlins/blush.gif[/img]
    I may be wrong but not always

  8. #18

    Default Re: Maximum hull speeds ?

    Not quite sure why the silly yellow faces.

    If you read the line below the formula you would see that the same formula is also used to calculate wave length, which is is main use.

    U-boat skippers used it to estimate speed of a target by estimating length of bow wave.

    Which is exactly how the formula limits the speed of a vessel.

    In effect the vessel has to climb its own bow wave and you need a lot of power to drive a boat up hill. just think for a moment how driving into a wave when beating will slow down a boat.

    perhaps if you do a little more research you wil find the formula that links boat speed to power and you will see that the curve does not suddenly move to a vertical line but does curve more steeply.

    Have a think about the mechanics of moving a "planing" hull from displacement mode onto the plane, at the same time think about what sort of speed a motor boat needs to be doing to make the most wash.

    Then think why

    All this about ballast and thrust on quarter is moonshine.

    I've sailed a Catalac at 11-12 knots and they sure as hell don't plane at that speed.

    And my 29 ft LWL monohul has topped 10 knots under genoa only and that wasn't planing either.

    Hi & Dri makes a good point about being towed - a lifeboat will get a small sailing boat well over "displacement speed"

    Would be interested to know how to surf in flat water, I always thought you needed waves for that

  9. #19
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    Default Re: Maximum hull speeds ?

    I do know the physics and you are absolutely correct. The calculation of "hull speed" is a question of efficiency. The hull resistance will increase up to a point in a fairly straght line relative to power applied. Then it will still increase in speed but the curve will become very steep. The normal calculation of hull speed is relative to the speed above which a law of diminishing returns will apply.
    It is also rubbish to relate the higher speeds achieved with cats to planing. Most cruising cats will never plane. Their increased speed is due to narrow beam relative to OAL.

  10. #20
    Lakesailor's Avatar
    Lakesailor is online now Registered User
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    Default Re: Maximum hull speeds ?

    There's a guy around who has designed a steamboat and brings it to the Windermere Steamboat Rally. I believe he is from around this area (the North West) It's quite long and slim and will travel a way above it's theoretical hull speed. (Not now of course as Windermere is speed limited).
    I seem to think I was told he is a naval architect.

    I'm sure this it


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