Question about hull speed

[Mhvoiceuk]

I know nothing about boat design so wondered if someone here could satisfy my curiosity about hull speed.

If the standard equation for calculating displacement hull speed is correct (Square route of boat length at waterline x 1.34 =maximum displacement hull speed) then why is it that boats in races like the Vendee Globe often exceed what should be their maximum speed?

Mike Golding's Open 60 class boat is 59.97 feet at the waterline so his maximum hull speed should be 10.37 knots, but he is often traveling at 15 knots. Tidal flow would increase speed as would surfing. Are these the factors that are giving the increased speeds or has that equation become outmoded with modern boat design?

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[kds]

The standard equation gives an appriximate maximum speed for a DISPLACEMENT hull. It relies on the wavelength of bow to stern distance, as this is a function of speed.
Modern lightweight yachts of the type you mention are planing (ie. hull lifting well above normal dispacement waterline, so they make a much less wave pattern)- just like a dinghy or mobo, so they can exceed this value.
Ken

<hr width=100% size=1><A target="_blank" HREF=http://www.canongrange.co.uk>Bed and Breakfast, cathedral Green Wells, Somerset Canon Grange</A>

 

[kds]

The standard equation gives an approximate maximum speed for a DISPLACEMENT hull. It relies on the wavelength of bow to stern distance, as this is a function of speed.
Modern lightweight yachts of the type you mention are planing (ie. hull lifting well above normal dispacement waterline, so they make a much less wave pattern)- just like a dinghy or mobo, so they can exceed this value.
Ken

<hr width=100% size=1><A target="_blank" HREF=http://www.canongrange.co.uk>Bed and Breakfast, cathedral Green Wells, Somerset Canon Grange</A>

 

[William_H]

As I see it what really hasppens is that as the "hull" speed is approached the driving power required to increase the speed increases enormously out of proportion to the speed increase. This increase in power required depends on weight and hull shape so is not so dramatic in catamarans or narrow or light boats and becomes less significant as the boat tends to plane.
So "hull speed" is most usefull in indicating the improvement of speed with hull length but does not give an absolute barrier speed. will

 

[branko]

Hull boat speed is calculated aprox. value which we obtain in Ship building institutes basin trials or by computer programs today. It is dependable off boat water length, downwater form of the hull, and as kds said wave length from bow to stern distance.
Deplacement boat speed is defined in moment when laminar flow on back third of hull is changing into turbulent flow. Boat hull is carrying some quantity of water around it, called boundary water layer. In the beginning of increasing speed boundary leyer have laminar flow and later laminar is transfering to turbulent . If the speed increases even more the turbulent flow becomes all around boat hull and boat begins to surf.
This is in theory very shortly as I remember from my studies.

 

[charles_reed]

The formula you quote is extremely outdated and in any case only a "rule of thumb" approximation.
Marchaj in his 1964 seminal "Sailing Theory and Practice" uses 1.4 x the square root and Olin Stephens by the early 70s was quoting 1.48.
Modern naval architects would essay a figure, especially for such craft as the Open 40s and 60s of nearer 1.7 and such a figure is mentioned by Marchaj

This is the speed at which the particular hull shape will dig itself a "hole" between bow and stern wave which requires a geometric increase in force to increase speed - on occasion they can obtain this increased force and coupled with a reasonably broken seastate can start to surf almost continuously.
I personally have experienced such a situation whilst reaching in a F6 seastate 7 when a 29'10" LWL waterline boat has put between 11.1 - 11.3 miles per hour into 3 consecutive hours
I must say the suggestion that a displacement boat will plane is not supported by any experimental data or by much educated opinion.