Theoretical hull speed

[Bladerunner]

Can anybody let me know the formula for calculating theoretical hull speed ? /forums/images/graemlins/confused.gif

Specifically, I have an Achilles 24 (triple keel, outboard in a well) and am wondering how it might do against a Hunter 26 (bilge keel, inboard with folding prop) owned by a very competitive member of my club.

How nice it would be to cross the finish line before him in our first club race in a couple of weeks. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/laugh.gif

 

[ashanta]

Having owned a Hunter26 TWIN FIN as they marketed the boat I doubt you would win if the sails are good and the owner can race. Hey! but who knows?
The calculation is based on the displacement of the vessel and it's length. The vessel sit's in trough of water in which all displacement vessels sit and it will not go any faster through the water. You will no doubt be provided the formula from our learned friends but remember the Hunter 26 was derived from a very competive hull which I believe was the Sonata?

Regards.

Peter.

 

[Joe_Cole]

Sounds like there's a grudge match in the making!

Try this: http://www.nwmarinedesign.com/slscript.htm

Put your outboard in the locker if it drags when in the well.

 

[William_H]

Theoretical hull speed is a description of the rate at which the hull speed increases with force applied to make that speed. It describes that speed where the force required to increase speed becomes disproportionate to the speed increase acheived. It is not a brick wall where no further speed can nbe avheived it is just much more difficult. however as pointed out that dificulty is greater with more weight and easier with narrower boat. (As in catamarans) And nif as in a speed boat the hull can be made to plane then the difficulty is vastly reduced. the rule of thumb is that the hull speed is the square root of the water line length in feet. This formular is more usefull in indicating how an increase in water line length will give more hull speed than in actually7 giving a hull speed.
Now in your quest to beat the black knight with the Hunter 26 we presume that your race will include up wind and down wind legs. (An olympic course) Your boat is not necessaruily beaten on water line length (hull speed) You need to be able to get the best speed by getting the best drive downwind by use of spinnacker or at least poled out jib. Get the fore and aft trim of the boat correct not too much weight in the cockpit.
Going up wind you need to point to the wind as close as possible. Remember that when near hull speed or over it it takes a lot more drive to get more speed. That drive couold be sacrificed for pointing closer to the wind. So if you have a log look for around 5.5 or 6 knots up wind not less as you will be going too slow but not more as you will be sacrificing pointing. If you can calculate Velocity Made Good so much the better.
Of course the bottom must be perfectly clean and the O/B stowed a2way and the well covered over. You must work the winds and tides and have a crew giving you smooth glitch free tacking.
So it all points to why racing is a sport related not just to technical excellenece of the boat but in crew and skipper skills.
Don,t despair my 21 ft trailer sailer has consistently beaten a fleet of 22ft 24 ft 26ft and even a 28ft fin keeled boats this season (25 wins usually by 8 minutes in 2.5 hr race) by light weight, large sail area and good crew work. You can do it too. Just get in some practice. ( this was my 21 st season) it is fun to put yourself against another boat. regards olewill

 

[simonfraser]

William-H,
max velocity is aprox square root of length in feet, does this work for cats too ?
thanks

 

[jerryat]

Surely you mean the square root of the water line length X the speed/length ratio??
/forums/images/graemlins/confused.gif

 

[gandy]

Rather than just comparing hull speed, why not look at some of the published handicaps to see how other clubs have compared these boats? See http://www.fyca.org.uk/ for example.

Tony S

 

[reeac]

To put it succinctly, the hull speed expressed in knots is 1.34 times the square root of the waterline length in feet. eg. LWL 25 ft., hull speed 6.7 knots. Applies to displacement hulls only.

 

[boatmike]

Yes as a general rule but it's not a scientifically derived formula and the hull type and displacement will alter the multiplier considerably. It's only a
very general guide

 

[snowleopard]

the multiplier can vary depending on hull form, particularly whether the shape of the ends effectively lengthens the waterline. classic racing yachts had long low overhangs which were immersed by bow and stern waves so making the dynamic waterline longer than the static measurement.

as the hull form gets narrower the wave-making reduces until beyond about 8:1 length:beam there is insufficient wave making to create a true hull speed limit.

 

[Lakesailor]

Have fun with this Hull Speed Calculator
Lots of people will come up with reasons why it's not accurate - wetted area and things like that, but hey - it's not rocket science.