Sailboat Length and Hull Speed (Longer = Faster)

In this video, you'll learn what the relation is between the hull lenght and hull speed, and also how to calculate it yourself. You'll also learn how sailboats are able to exceed maximum hull speed, aka 'sail their own wave'.

[MonolithproductionsT]

Fun fact: during the 1960s, the ACC regulated class by length at waterline. In order to cheat the system, boat designers would give their sailboats long stern overhangs, which reduced the length at waterline. But when they started sailing, the heel would cause that overhang to dip into the water, and caused the length at waterline to increase, thus increasing hull speed. It's because of this that long stern overhangs seem to be the "classic" 1960s design

[macrubit]

The formula results from the propagation speed of a wave with the same wavelength as the boat''s length (The boat will make a disturbance on the water surface that is roughly of the same size as its own length) in deep water c = sqrt(g L/(2*pi)). The factor 1.34 comes from the magnitude of gravitational acceleration and appropriate unit conversion factors. For example, 1 foot/1 m = 0.3048, g=9.81 m/s^2 and (1 knot) / (1 m/s) = 0.514444.

[mboyer]

Great video, thank you! I sailed for 30 years before I learned how a sail really works. I had just bought a Supercat 17 and I bought a catamaran racing book. The book explained how the sail is a vertical wing creating lift just like an airplane wing. So the sailboat is moving forward due to the lift generated on the downwind side! The boat is getting sucked forward!! How crazy is that??

[istra]

This formula is only rough guide to old designs of displacement sailing Newer sail boat designs ( flatter bottom ) are created so that sailing boat going upwind is behaving like displacement boat ( and that formula can apply ), BUT going downwind they can easily TRIPLE the speed of this
That is the reason why designers MUST supply you with speed diagram of different points of sailing ( polar diagram).

Of course as Michael has previously commented this formula can't be applied to all multi hulls ( catamarans, trimarans, proas) and especially nowadays new foiling designs....
Thanks for posting....

[svamour]

I enjoyed the Calculus, Geometry and Physics lecture. I have found most of the information on the internet is like getting a weather forecast. Everyone repeats what they have heard; no one verifies the facts or even knows what the facts are.
Allow me to ask you a simple physics question; take two round objects the same length and size, both have the same nose cone similar to a jet aircraft.
Object A; starts the gradual curve to the end (tail/stern) point at 50% of the length.
Object B; starts the gradual curve to the end (tail/stern) point at 75% of the length.
Simple Question: which object is faster given everything else is constant; Object A or Object B.
Last question what is making Object A or B slower given everything else is the same (constant)?

[nochnkorn]

You do not need any surfing conditions to exceed hull speed. You just need to get more force into the boat. The problem with sailboats is that more force means more rolling over to the lee side of the boat and more slipping of the wind over your sails. A heavier keel could do, a ballast system and ballast at luv could do, even a more rectangular shape of the hull could do.
But .. a heavier keel means more depth and more friction, also does a more rectangular shape in terms of friction. The ballast system is a good way but expensive.
If your mast, sails and ropes can withstand the force you can exceed the hullspeed of your boat by setting more sails with winds from aft.
The hullspeed is just a point were the force to move your boat and the shape (mostly length) of your hull is at the optimum.

[ximono]

It's not true that the hull speed is the maximum speed of the sailboat. It's a theoretical "limit", an _optimal_ top speed, not a physical glass ceiling. It _is_ possible to go faster, though it will probably be uncomfortable. Something I just learned. Otherwise, great video 🙂

Edit: Ah, you said so later in the video.

[ivanostellato]

the keel as i said should be at the nose or a rod forward with a small rudder 20 ft infront of boat makes virtual keel allows into the wind and beyond sailing .. yes sailing counter wind

[ivanostellato]

having a sail 20 feet trailing will also allow for more speed and higher pointing angle

[joedonnelly]

Did a great job of keeping it not too technical but highly informative

[Labrador_Productions]

I think the formula is a bit off. It says that the theoretical max speed of a 100 ft boat is 13.4 kts but modern 100 ft racing boats can do close to 40. I'm not sure what causes the discrepancy but I think it might be that the boat starts to plane on the water like a powerboat. I think the practical reason that longer boats are faster is because the boat has more time to push the water out of the way and thus less drag. i.e. if the widest part of the boat is the midpoint, then on a 100 ft boat, it has a 50 ft range to push the water to the side whereas a 50 ft boat would only have 25 ft. Anyway, I thought it was a good video. :)

[rudynunweiler]

My question is about that formula. It seems that two boats of equal length at waterline with completely different beam width at waterline will have the same top speed? I feel the beam width would cause some drag and is causing more displaced water slowing the wider boat. Is there something I am missing?

[aprobroofficial]

its easier to break hull speed than you think, i sailed across lake ontario in a j35 and our max speed was 14knts

[ivanostellato]

you can make a flexible hu;;; that is long and curved and then point into the wind more .. the hull flex and vibe does what? virtual hull resistance factor

[monoman]

could you do a short video with some of your infgraphics in various bits of sailing you have covered in dutch as i am trying to learn some dutch. i need simple ie sail = etc..thanks.

[markandoyo]

Longer boats makes sails faster (check)

Kindly check from the other findings that the extensive research on the hands of Hydrodynamics, ballasting, sail match can be counted regardless of short vessels with bigger sails, the trimmed of the vessels and the counteractive its defined waterline base into the complete stability platforms and the complete Physics were the another accountable fast sailboats

although the shorter sailboats with Keel oriented can withstand longer in the battles of seaworthiness routes and they can keep at the most turbulent sea currents but they cannot take any fast sails as due to the geometrical round bottoms

as though another feasible facts that the oversized sails much needed flat bottom boats and the required longer lengths can be a true factual basis but having additionally either to mount with Centerboards, Daggerboards and the ballast displacements are the significant hydrodynamic basis and the correct applications in Science in terms of arms race and space edge Sportscraft but limits its trans oceangoing seaworthiness.

[ivanostellato]

as we sail the apparent lateral resistance changes ... it moves forward at speed ...

[dirtcoffeethethird]

I've got a cutter boat from 1922, no manual about the speeds sadly 🙊

[ivanostellato]

i never thought of thys b4 either well not the nose pieces the apparent clr at speed deltasz

[emmengel]

With the boat plays no part in whole speed?