Hull speed... - theory or fact?

I have heard of the hull speed theory(math concept) but without consideration of wind speed & sail area or h.p. is it accurate?(displacment changes with conditions)

 

If you are talking a true displacement hull...no remotely reasonable amount of power will get it past 1.3-1.5 times the sqrt of the waterline length. This is related to the length/speed/height of waves and isn't really negotiable. You can surf the front of a big wave and get above hull speed but that the the power of the wave and gravity. Once the boat is riding on the bow wave and the stern wave with the trough in the middle that is pretty much as fast as it will go. Any faster and the stern wave moves aft and the boat's stern drops into the trough...the bow wave becomes an insurmountable obstacle. Various hull shapes will tend to go a bit faster or slower but again...if it is a TRUE displacement (not semi-planing or planing) hull then the limit is reached somewhere between 1.3 and 1.5 depending on the shape of the after quarters of the boat.

Steve

 

follow up

please help me understand,...hull speed theory is based on LWL, but sails lift....so LWL changes with sail area and wind?

ps ... my boat has a canoe stern

 

Sail lift is hardly relevant here.. but LWL will change somewhat depending of the hull form, speed and heeling.

 

"If you are talking a true displacement hull...no remotely reasonable amount of power will get it past 1.3-1.5 times the sqrt of the waterline length. This is related to the length/speed/height of waves and isn't really negotiable."

True enough for heavy fat boats of 3-1 LWL /BWL..,

,But once you get past 6-1 , the waves you create may be small enough to NOT tip the boat trapping you attempting to climb the bow wave , as the stern sinks ever lower .

Most cats and tris are displacement boats , yet have no problem double at "hull speed."

BEAM is one key to fast, light weight is the other .

FF

 

the change in LWL will only affect in tenths of a knot...not really a factor unless you are racing for big money over a long course.

I assumed a monohull with a relatively traditional design seeing as no qualifying additions were added to the question.

 

As people have already pointed out hull speed is fact. If you check passage times for any cruising monohull in good tradewind conditions you will typically find average speed lies just below the hull speed.

There is more to it that just physically "climbing out of the trough".
Boat wake represents energy lost as the boat moves through the water - bigger wake, more power required.
Both bow and stern generate a wave as the boat moves through the water. At most speeds these wave cancel each other out somewhat, and so the total driving energy is less than the sum of the two waves. At "hull speed" the bow and stern waves exactly add together, so it gives biggest wake, requiring most energy to drive forward. As you approach this speed the slope of a power/speed graph gets progressively steeper.

In a sail boat the amount of power input is limited by the stability (which dictates sail drive), and a typical monohull cannot practically have sufficient stability to provide power to pass the steepest section of the power curve.
A displacement motor boat could have sufficient engine, but it turns out that if you have that by the time you have enough power to get through the hump you have enough power to plane, and you can go fast while ignoring hull speed.

Multihulls tend to have low displacement and high stability, so they can carry more sail while requiring less driving power at a given speed, so most sailing multihulls, (and ULDB monohulls) can sail faster than hull speed.

 

There's a good thread on this subject, worth checking out:
http://www.boatdesign.net/forums/boat-design/hull-speed-1220.html

Hope it helps