Conference unifix::sailing

    According to Practical Boat Owner magazine here in UK. The maximum hull
    speed of a displacement (non planing) hull is given by the formula
    
    		1.34 x Square root of length at waterline = knots
    
    Regards
    
    John

    
    This formula would give:
    
    20 feet -> 6 knots
    30 feet -> 7.4 knots
    55 feet -> 9.8 knots  expensive to gain knots!
    
    what is the physical theory behind the formula? is it related to wave
    length/ frequency/ wave speed on water?
    do you know for which size range this formula is reliable?
    what about calculation when planing?
    
    thanks.

    Coupla" additional comments:
    
    1.34 is not a magic number.  It is a coefficient, and is subject to
    other factors.  Many texts give the number as a range, perhaps from 1.2
    to 1.45 or so.  Certain design specifics can "fool" the formula.
    
    It is related to the wavelength vs. speed curve of water.  The longer
    the wavelength, the faster the waveform travels, by the same formula.  
    When you are in displacement mode (moving water away from and around the 
    hull rather than planing, which is skimming over it without
    substantially moving it out of the way at all), you are limited by the
    speed of a waveset you create.  This waveset has the leading crest right 
    at the bow and the following crest right at the stern--hence the reason 
    for using LWL in the calculation.  If you pour on more power to go faster,
    depending on your hull type, one of two basic things happens:
    
    1) Displacement hull:  You start trying to move up the back of the wave
    in front, and to outdistance the trailing wave.  But since your power
    is producing that following wave, the faster you try to go, the more
    you have to haul around.  The power to speed curve starts turning
    vertical.  Additionally, most true displacement hulls, especially
    double-enders, will try to bury their sterns.  Water moved aside by the
    bow needs to "come back together" to fill in behind the stern, and in
    effect one gets a real suction there.  I've seen a 30' double ended
    whaleboat-type bring water in over the stern when it was towed at 12
    knots (briefly!).  At 7 knots, everything was hunky-dory (so to speak.)
    
    2)  Planing hull:  This type is designed to handle enough power to get
    past the point of pushing "uphill" on the leading wave, to break the
    hullform resistance and overcome the skin friction, and to accelerate
    well beyond the speed of the wave it produces.  It virtually requires a
    sharp trailing edge to induce clean separation of the water from the
    hull, to avoid the above-mentioned suction problem.As most of you know, 
    it takes far more power to get onto a plane than it does to maintain one
    after getting there.  
    
    The dynamics of the transition state (loosely referred to often as 
    "semi-displacement") are mushy and complex; designers of quite a few 
    large power cruisers (35' and up) have worked to exploit this performance 
    region, with quite varied success.
    
    BTW, that bulb you see (hear about) protruding from the pow of most every 
    freighter, tanker and cruise ship?  It's intended to "fool" the water by 
    setting up a pressure wave which begins the forward wave earlier, thus 
    lengthening the *effective* LWL, which in turn gives a higher speed 
    (tenths of a knot, which is a lot to those guys in terms of fuel savings) 
    for the same power.
            
    John.
    

From:	SALEM::GILMAN       "JUST CALL ME SKIPPY" 27-AUG-1992 17:48:15.40
To:	ULYSSE::JOUSSE
CC:	GILMAN
Subj:	No write

The string your question is as to hull speed is set to no write so I am
replying in mail.

The theory behind hull speed is related to WAVE LENGTH.  i.e. the longer
the boat the longer the wave length the boats passage through the water
creates.  As the wave length exceeds the boats length the boat starts to
CLIMB the bow wave which puts the boat at a slight climbing attitude, 
since the boat has to constantly climb the bow wave with the stern 
settling into a trough it takes much greater power to maintain that 
attitude and speed.  As speed increases even more the bow wave climbs
even high and the  stern settles even deeper thus requiring even MORE power
to increase speed.  A longer boat or ship can go faster that the shorter
boat or ship before she reaches hull speed... thus the longer the displacement
hull the faster the boat or ship can go before passing the hull speed.

A planing hull is another matter entirely because the hull is lifted partially
out of the water.

Jeff

    Arnaud,
    	Sorry, I quoted the formula from the magazine. They do not discuss
    the theory behind it. Planing speed must depend on the hull shape. This
    is a question for the marine architects/boat designers not a mere (ex) VAX
    service engineer! Any takers??
    
    Regards
    
    John