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  1. #1
    Join Date
    Jul 2003
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    91

    Default If LWL is so important how come some boats are faster than others... read on!

    Ok so the basic principle is hull speed is around 1.37 x sqrt LWL (ft)

    eg 36ft LWL = 1.37 x 6 = 8.2 knots (sounds about right for a 40-45ft boat (which would have a 36ft LWL).

    Now I know we can have planing racy boats such as Ker, Mills, J/111 etc.... but...

    If the LWL is the key performance indication, which is to do with displacement wave forms on the hull, then why would one boat be faster than another?

    Ok, so SA: DISP ratio comes into it, so a heavy boat with a small rig will be smaller than a light boat with lots of sail..... IN LIGHT WINDS.....

    But once "powered up", lets say 15-20kts wind, surely virtually any two boats with the same LWL *should * perform the same?

    So why is it that a (say) Warrior 40, is slower than (say) a Starlight 39, which is also slower than (say) an Elan 40?

    I have a few ideas, but seeing this is the Racing, Tech, and DESIGN forum.... lets open the discussion?!

    Thanks!

  2. #2
    Join Date
    Jun 2009
    Posts
    12,531

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    There's a lot of twaddle talked about this "hull speed" thing! People seem to think it's like the sound barrier for aircraft, but it really isn't - see the article on Wikipedia for example : https://en.wikipedia.org/wiki/Hull_speed. When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.

  3. #3
    Join Date
    May 2007
    Posts
    38,921

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    Quote Originally Posted by maby View Post
    There's a lot of twaddle talked about this "hull speed" thing! People seem to think it's like the sound barrier for aircraft, but it really isn't - see the article on Wikipedia for example : https://en.wikipedia.org/wiki/Hull_speed. When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.
    Very true.
    It's useful as a rough indication of how fast a boat will go with not much power, but that's about it.
    It's more relevant to older styles of boat where the waterplane is narrow at the back. Also the heavier the boat, the more it will stick at 'hull speed'.

  4. #4
    Join Date
    Dec 2014
    Location
    bristol & st mawes
    Posts
    612

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    Leaving aside the hull speed issue, there are issues of power and of hull and foil lift vs drag. So its a question of faster - when and under what circumstances? My last bilgie for example was as fast downwind as a Sigma 33 but upwind sailed a different course and so was slower overall.

    I have heard it said that a length to beam ratio of 14to 1 as in a multi means the hull speed doesnt apply any longer but I doubt it is as black and white as that

    What racer types usually mean when talking faster or slower is faster or slower under one set of handicap formulae. And thats what the designer goes for . . Can he find a way of going faster without affectibg the TCF?
    Last edited by drakes drum; 11-09-15 at 16:21.

  5. #5
    Join Date
    Jan 2006
    Location
    Behind your curtains.
    Posts
    8,295

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    The biggest factor is wetted surface area which translates as drag. A boat with loads of area under the water is going to be harder to push along than one with less and so will on average be slower.

    Secondly hull shape is important. A hull shape that will not plane will be harder to push than one that skims along the top of the water.

    Hull speed is only a theoretical figure and relates to an exponential amount of force that is required to effectively push a boat over and beyond its own bow wave and simply put the easier it is to push a particular shape of boat the less force is required and therefore the more likely it is to exceed its maximum theoretical speed.

  6. #6
    Join Date
    Aug 2002
    Location
    Western Scotland
    Posts
    1,916

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    Quote Originally Posted by maby View Post
    When the boat reaches Hull Speed, keep pushing harder and it will carry on going faster.
    That is only true up to a point. Consider a tug with an incredible amount of power per tonne or unit of length. Once it reaches its displacement limit, using full power (as lots here do when not towing but returning to their berth) it will go only very slightly faster. All the extra energy goes into making large waves to damage the shoreline and potentially other nearby boats.
    Ken.

  7. #7
    Join Date
    Dec 2012
    Location
    Fortrose
    Posts
    1,586

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    How does the original formula equate with regard to an Americas'Cup foiler, where once lifted, the LWL is around about a metre? :-)
    Folkboat 'Stakkr' - Inverness - http://islandrov.wix.com/island-rov-web

  8. #8
    Join Date
    Nov 2007
    Posts
    32,180

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    Quote Originally Posted by jamie N View Post
    How does the original formula equate with regard to an Americas'Cup foiler, where once lifted, the LWL is around about a metre? :-)
    It doesn't. It is not really a "formula" but a quick and dirty way of estimating the speed at which a displacement boat starts to exceed its wave length and the amount of power required (whether from sail or motor) to increase speed rises rapidly. Planing and foiling boats do not have this constraint because they sit on the water rather than in it so do not make waves.

    Most displacement boats operate well within maximum displacement speed and as has already been noted, power/weight ratio, drag and hull shape influence the speed in any particular set of conditions resulting in some boats being faster than others.

  9. #9
    Join Date
    Mar 2005
    Location
    East Sussex.
    Posts
    21,506

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    The other issue is that boat builders quote static waterline length. On some modern boats that is pretty much the same as hull length. On my old cruiser the hull is 45 feet, LOA is 46 ft 5 inches, but waterline length is quoted at 37 ft 6 inches! Once I am up to sailing at 6 or 7 knots the stern has become immersed and the waterline has grown to about 43 feet! So what is my hull speed? I am regularly sailing at over 7 knots in any decent wind and have seen 10 knots on occasion though I don't usually push it that hard..
    Working on immortality - One day at a time.

  10. #10
    Join Date
    Nov 2007
    Posts
    32,180

    Default Re: If LWL is so important how come some boats are faster than others... read on!

    10 knots is not unusual for a 43' LWL. The "formula" gives 9.2. so you should easily be able to get continuous speed in the high 7's. low 8's and surges to higher speed. The extra 5' of heeled LWL increases the potential by .6 knot, which is why long overhangs were popular when rating rules penalised static LWL. This is where the Power/weight, or as it is normally expressed SA/Disp for a sailing boat comes in. Lots of sail and a breeze on the limit can generate enough power to overcome the wave making constraint. Often stains the trousers as well!
    Last edited by Tranona; 13-09-15 at 15:01.

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