Always wondered what exactly is meant by 'Maximum Hull speed'?

[markymac1961]

I recently watched a hi end, very professional on line video sail test and review of the brand new Jeanneau Sun Odyssey 350. (Interestingly... turns out Jeanneau have 'stolen' our beloved B&R backstayless rig for their shiny new pride and joy.)

So here's this overly well dressed Pommie dude sailing around a bay somewhere talking to camera, and on hitting 7.5 knots SOG, commented that this would be "close to Maximum Hull Speed" for this model.
I recently delivered my 1999 build Hunter 340 from Mackay to Airlie Beach (Whitsundays QLD) with 20-25 kts up the jacksie, reaching happily along with 2 reefs and a full headsail (100-120 AWA.)
My mate and I were delighted to be averaging 7-9 knots SOG for a few hours with this sail plan.... and SV 'Chummy' was absolutely loving it. She handled like a sweet thang and never once even got close to feeling out of control, unwieldy, or overpowered.
At one stage she hit 11.3kts down a wave.... YeeeeHaaaaah!!! Still not a handful at all. Still rock solid and stable.
My question is, given she's a similar size boat and configuration to the brand spanking new 'JSO 350', was I unwittingly in constant excess of the 'Maximum Hull Speed' ... and therefore exposed to potentially having my beloved boat suddenly explode into a million GRP fragments, and me and my mate paddling cockpit cushions to shore?
Love some thoughts and clarification Guys n Gals. Just curious before I potentially blow up!
Happy Sailin' from Mark Mac - Airlie Beach QLD, AUSTRAYA
(Please note: In vid attached below, you may notice tacho reading at 1000rpm. The needle mysteriously sits there with the motor switched off! With plenty of wind... we certainly had no need for the Donk!)

 

[kappykaplan]

My question is, given she's a similar size boat and configuration to the brand spanking new 'JSO 350', was I unwittingly in constant excess of the 'Maximum Hull Speed' ... and therefore exposed to potentially having my beloved boat suddenly explode into a million GRP fragments, and me and my mate paddling cockpit cushions to shore?

I'm sure this is a "tongue-in-cheek" question! My 1983 Pearson 37 routinely exceeds hull speed when the wind pipes up - hasn't exploded yet.

 

[kappykaplan]

And here's all you wanted to know about hull speed, courtesy of the Copilot AI:

Hull speed is the theoretical maximum speed a displacement hull can efficiently travel, determined primarily by its waterline length.
Definition and Concept
Hull speed, also called displacement speed, is the speed at which the wavelength of a vessel's bow wave equals the waterline length of the boat. As a displacement hull moves through water, it generates waves; when the wavelength of the bow wave matches the waterline length, the boat experiences a sharp increase in wave-making resistance, making it inefficient to go faster without planing or additional power Wikipedia+1. This phenomenon is often associated with the "displacement trap," though modern hull designs can exceed it yachtingmonthly.com.
Calculation
The traditional formula for hull speed in knots is:
Hull speed (knots) = 1.34 × √LWL (ft)
Where LWL is the waterline length in feet. For example, a sailboat with a 36-foot waterline has a hull speed of:
1.34 × √36 = 1.34 × 6 = 8.04 knots hullmetric.com+1.
In metric units, the formula can be expressed as: V (knots) = 2.43 × √LWL (m). The constant varies slightly depending on units and sources, typically ranging from 1.34 to 1.51 in imperial units Wikipedia.
Froude Number and Speed-Length Ratio
Hull speed corresponds to a Froude number (Fn) of approximately 0.4, where the bow and stern waves lock together, creating maximum wave resistance hullmetric.com. The speed-length ratio (SLR) is the ratio of boat speed to the square root of waterline length, and hull speed typically occurs at SLR ≈ 1.34 for displacement hulls funcfish.com. Exceeding hull speed requires exponentially more power due to increased wave drag.
Modern Considerations
While hull speed is a useful guideline, many modern hulls can exceed it without planing. Long, narrow, or wave-piercing hulls, as well as multihulls like catamarans and trimarans, can surpass traditional hull speed limits due to reduced wave-making resistance yachtingmonthly.com+1. Semi-displacement hulls can partially plane, allowing speeds beyond the theoretical hull speed, while true planing hulls have no theoretical speed limit once they lift onto the water surface hullmetric.com.
Practical Implications
Understanding hull speed helps sailors and boaters estimate cruising speeds, fuel consumption, and passage times. For example, a 40-foot cruiser with a 35-foot waterline has a hull speed of approximately 7.9 knots, which informs planning for offshore passages hullmetric.com. It also helps in comparing hull designs and understanding why longer waterlines generally allow higher efficient speeds.

 

[sailme88]

Who hasn't exceeded the "potential" maximum hull speed.. the formula number of 1.34 has been around for a long time. There once was a time that speed was determined by counting knots in a line over a given time period. That said, things, many things have changed. Glass hulls vs ship lap planking, lighter yet stronger materials, full aft beam canting keels, bulb and wings and on and on..So, I guess my point is that it might be time to reassess that 1.34 number..

 

[markymac1961]

And here's all you wanted to know about hull speed, courtesy of the Copilot AI:

Hull speed is the theoretical maximum speed a displacement hull can efficiently travel, determined primarily by its waterline length.
Definition and Concept
Hull speed, also called displacement speed, is the speed at which the wavelength of a vessel's bow wave equals the waterline length of the boat. As a displacement hull moves through water, it generates waves; when the wavelength of the bow wave matches the waterline length, the boat experiences a sharp increase in wave-making resistance, making it inefficient to go faster without planing or additional power Wikipedia+1. This phenomenon is often associated with the "displacement trap," though modern hull designs can exceed it yachtingmonthly.com.
Calculation
The traditional formula for hull speed in knots is:
Hull speed (knots) = 1.34 × √LWL (ft)
Where LWL is the waterline length in feet. For example, a sailboat with a 36-foot waterline has a hull speed of:
1.34 × √36 = 1.34 × 6 = 8.04 knots hullmetric.com+1.
In metric units, the formula can be expressed as: V (knots) = 2.43 × √LWL (m). The constant varies slightly depending on units and sources, typically ranging from 1.34 to 1.51 in imperial units Wikipedia.
Froude Number and Speed-Length Ratio
Hull speed corresponds to a Froude number (Fn) of approximately 0.4, where the bow and stern waves lock together, creating maximum wave resistance hullmetric.com. The speed-length ratio (SLR) is the ratio of boat speed to the square root of waterline length, and hull speed typically occurs at SLR ≈ 1.34 for displacement hulls funcfish.com. Exceeding hull speed requires exponentially more power due to increased wave drag.
Modern Considerations
While hull speed is a useful guideline, many modern hulls can exceed it without planing. Long, narrow, or wave-piercing hulls, as well as multihulls like catamarans and trimarans, can surpass traditional hull speed limits due to reduced wave-making resistance yachtingmonthly.com+1. Semi-displacement hulls can partially plane, allowing speeds beyond the theoretical hull speed, while true planing hulls have no theoretical speed limit once they lift onto the water surface hullmetric.com.
Practical Implications
Understanding hull speed helps sailors and boaters estimate cruising speeds, fuel consumption, and passage times. For example, a 40-foot cruiser with a 35-foot waterline has a hull speed of approximately 7.9 knots, which informs planning for offshore passages hullmetric.com. It also helps in comparing hull designs and understanding why longer waterlines generally allow higher efficient speeds.

Well very interesting and thanks for replying. What AI doesn't figure in... is how absolutely exhilarating it feels to be flying outside the 'Displacement Trap'

 

[kappykaplan]

Well very interesting and thanks for replying. What AI doesn't figure in... is how absolutely exhilarating it feels to be flying outside the 'Displacement Trap'

Absolutely!

 

[markymac1961]

I'm sure this is a "tongue-in-cheek" question! My 1983 Pearson 37 routinely exceeds hull speed when the wind pipes up - hasn't exploded yet.

... and don't you just love those moments Cap'n. Safe sailing Bud.

 

[markymac1961]

Who hasn't exceeded the "potential" maximum hull speed.. the formula number of 1.34 has been around for a long time. There once was a time that speed was determined by counting knots in a line over a given time period. That said, things, many things have changed. Glass hulls vs ship lap planking, lighter yet stronger materials, full aft beam canting keels, bulb and wings and on and on..So, I guess my point is that it might be time to reassess that 1.34 number..

Great summation mate, but I've never been good at maths. I think its high time 'Maximum Hull Speed' is redefined as the exact number of seconds to go before you're about to poop your pants

 

[Davidasailor26]

My question is, given she's a similar size boat and configuration to the brand spanking new 'JSO 350', was I unwittingly in constant excess of the 'Maximum Hull Speed' ... and therefore exposed to potentially having my beloved boat suddenly explode into a million GRP fragments, and me and my mate paddling cockpit cushions to shore?

Nothing about hull speed means the boat can’t go faster. Doing that just takes a lot more energy than staying below hull speed. To get to hull speed you basically need to start lifting the hull up, climbing up the bow wave. Then you need to keep supplying energy to hold the boat up there. Imagine trying to stand under the bow of your boat and holding it up a couple feet. Then imagine how much easier it would be to just use a dock line and pull it forward through the water. Both are technically possible, with enough energy, but just going forward within the bow wave is a lot easier than climbing up it.

Going hull speed, and even exceeding it, is achievable with pretty much all recreational sailboats, especially in the high wind reaches you described. The advantage of higher performance designs is that they can do it at lower wind speeds and wider wind ranges.

 

[markymac1961]

Nothing about hull speed means the boat can’t go faster. Doing that just takes a lot more energy than staying below hull speed. To get to hull speed you basically need to start lifting the hull up, climbing up the bow wave. Then you need to keep supplying energy to hold the boat up there. Imagine trying to stand under the bow of your boat and holding it up a couple feet. Then imagine how much easier it would be to just use a dock line and pull it forward through the water. Both are technically possible, with enough energy, but just going forward within the bow wave is a lot easier than climbing up it.

Going hull speed, and even exceeding it, is achievable with pretty much all recreational sailboats, especially in the high wind reaches you described. The advantage of higher performance designs is that they can do it at lower wind speeds and wider wind ranges.

Great explanation cheers mate. Now I get it thanks.

 

[Will Gilmore]

"Maximum Hull Speed" is a misnomer; it's "Hull Speed". Don't get confused by the word "speed". It's just a number like saying "Length Waterline"; it's a measurement of a property of your boat's hull. It is related to the efficiency of your movement through the water and has very little to do with how fast you can go.

Since the AI summary used the term "wave resistance" instead of "gravity", note that wave resistance is the driving of a boat up the bow wave against gravity. When surfing down a wave, that wave resistance disappears while your boat is on the downward slope of the ocean wave. No upward climb, no hull speed effect.

-Will