What is "Hull Speed" Anyway

[Cadence]

Originally Posted by Cadence

? I had one. A nice little boat 6.4 might even be stretching it?

PS, who cares. She was fun.

[mitiempo]

Originally Posted by Kenomac

There seem to be many owners of displacement boats who don’t understand that the ocean and seas have currents which add and subtract boat speed. We travel through these currents and eddies all the time, and when I see the speedo go up over hull speed.... I know how to interpret the data. It doesn’t mean I have a boat which can magically go 2-4 knots above hull speed... Oh boy, I now have a racing boat!

The speedo will not be influenced by current - it reads speed over the surface, whether that surface is traveling with you or against you at whatever speed.

Gps speed over the bottom will show the effect of current.

[Kenomac]

Originally Posted by mitiempo

The speedo will not be influenced by current - it reads speed over the surface, whether that surface is traveling with you or against you at whatever speed.

Gps speed over the bottom will show the effect of current.

Yes, I know this. It’s some of the other folks who seem to be having a hard time with the concept.

Speedos also require proper calibration, whereas a gps relies on satellites.

[Joli]

Originally Posted by Polux

It is disencouraging to post on forums!

When it has clearly established that there was not not a single hull speed for the same LWL, but a fixed one for boats with a D/L of 240 and over and another one, a variable one, for boats with a D/L lesser than 240, one that for the same LWL is increasing with the lightness of the sailboat, your post implies that nothing of this is a sound reality.

What you say is only true for heavy boats where the hull speed depends only on the LWL. For lighter boats the hull speed for the same LWL increases as decreases D/L. It is not only dependent on the LWL but on LWL and D/L.

So, it is not mostly about LWL but mostly about hull speed that varies for the same LWL with the D/L for most modern boats.

Regarding most cruisers to be happy sailing at about 70% of the hull speed, that is not true. That can only be true for the owner of an heavy sailboat.

Again it has to do with the easiness a boat reach hull speed and that was to do with boat sail power (stiffness) and with D/L. A heavy boat needs much more wind to reach hull speed than a light boat.

On a 40ft boat the traditional hull speed is about 8k, 70% of it is 5.6k. If upwind a performance cruiser can do that with about 6.5/7 kt winds and even if one can sail with that weak wind, or even less, certainly the owner of that boat would not be "happy" with that speed and would sail normally way faster than that.

I'm pretty sure I've asked you before if you have raced and was never given an answer. Race course numbers tell the tale and teaches you how to look at the real average speed of a boat. And pressing a powered up racing shell is not indicative of what the average sailor sees for daily runs or even what the average racer sees for daily runs with a racer cruiser.

Let's look at a couple numbers.

The ARC "Cruising Class" winner this years was an HR 37 with a 34 foot waterline. Theoretical hull speed is 7.81 knots, they won the class averaging 5.32 knots or a bit under 70% of hull speed.

The ARC "Racing Class" winner this year was a Dufour 45e with a 40 foot waterline. Theoretical hull speed is 8.47 knots, they won the class averaging 6.02 knots or a bit over 70% of hull speed.

I think the ARC record held by a Rambler 88, a no holds barred racing maxi is 16.8 knots, the theoretical speed of that boat is 12.6 knots. But who would want to cruise an empty carbon fiber black hole with a crew of 20?

Our heavy old boat designed in the 60's and built in the 70's has a phrf rating or -21. Hull speed is around 9.5 but we've pressed the boat into the high teens when racing. We plan on an average of 7 knots for cruising, it's no faster or slower than modern 60 footers we've sailed against (empty carbon racing shells excluded).

[StuM]

Originally Posted by Cadence

It's just a hypothetical mean no. (1.34} that seems to have worked for years on displacement hulls as a guess. So it seems you nailed it. I'm surprised no one has argued fresh or salt water.

As I understand it, the 1.34 comes from the formula for wave propagation in water - no boat involved. The physics of pushing an object through the water and consequently creating waves is why it is generally applicable to a displacement hull.

Variations in the design of the displacement hull will affect that theoretical value for any number of reasons.


See for instance: Understanding and Utilizing the Secrets of Waves - BoatSafe.com
"We can also determine the speed or period of a wave mathematically by multiplying 1.34 times the square root of the wave's length. [This number originates from many years of scientific research on wave speed.] "

or

Crunching Numbers: Hull Speed & Boat Length - boats.com
To understand why this is and where this mysterious multiplier of 1.34 comes from, you first need to understand that the term “displacement hull” refers to a hull that travels through the water rather than on top of it. Because such a hull displaces significant amounts of water as it moves along, it inevitably creates two series of waves in so doing--one at the bow and another at the stern. These waves are governed by a law of natural physics, which states that the speed of a series of waves in knots equals 1.34 times the square root of their wavelength, which is the distance in feet between the wave crests (WS = 1.34 x √WL).

[Don C L]

Just for fun I went looking for that 1.34. Wikipedia gave me:
The speed of a deep-water wave may also be approximated by:

{\displaystyle C={\sqrt {{gL}/{2\pi }}}} C = \sqrt{{gL}/{2\pi}}
where g is the acceleration due to gravity, 9.8 meters (32 feet) per second squared. Because g and π (3.14) are constants, the equation can be reduced to:

{\displaystyle C=1.251{\sqrt {L}}} C = 1.251\sqrt{L}
when C is measured in meters per second and L in meters. Note that in both formulas the wave speed is proportional to the square root of the wavelength.

So when using meters and meters/second, the constant is 1.251 times the square root of the waterline. length.

So my 22.5' waterline is 6.858 meters and if I did my math right that came out to 11.7939301371 km/hour which is 6.368212822510335 knots

Now with the other formula I get 1.34 x square root of 22.5 ft = 6.35617809694 knots

So this clearly explains why all the European boats are faster. They are all measuring with the metric system.

[Pete7]

Originally Posted by Don C L

So this clearly explains why all the European boats are faster. They are all measuring with the metric system.

The higher average speed for European boats is because a large majority are sailed by the French. Sailing in France isn't a hobby or sport its a religion! bit like guns are in America

Pete

[Polux]

Originally Posted by roland stockham

While this is a fun discussion in the real world of cruising people carry heavy ground tackle, a month or more of stores and everything from a sewing machine to maybe scuba tanks. In the racing world they cut their toothbrushes in half to save weight. Cruising boat are not lightweights once they set off. On traditional hull this makes negligible difference. The more you move toward lightweight race bread hull-forms that rely on planing for speed the more of an issue it becomes.

There are several types of cruisers some just travel light and keep weight to a minimum, others don't resist to carry all the stuff with them.

That has nothing to do with cruising but with life style, people are different on their needs and in what regards the way they like to live and that includes to have fun sailing, or less fun and more stuff.

Yes if you want to sail fast you should maintain your boat as light as possible but a loaded boat is also a boat with a higher AVS and with a bigger inverted stability, so even the ones that like to carry all the stuff on the boat and need a lot of stuff to live the way they like should not exaggerate and not go over the max load considered by the designer, even if they don't care about performance. It is a safety issue.

Anyway a light boat with a load will be always faster than a heavy boat with the same load, provided that it is inside the Max load the boat can carry.

[Alan Mighty]

Originally Posted by Don C L

Just for fun I went looking for that 1.34. Wikipedia gave me:
The speed of a deep-water wave may also be approximated by:

{\displaystyle C={\sqrt {{gL}/{2\pi }}}} C = \sqrt{{gL}/{2\pi}}
where g is the acceleration due to gravity, 9.8 meters (32 feet) per second squared. Because g and π (3.14) are constants, the equation can be reduced to:

{\displaystyle C=1.251{\sqrt {L}}} C = 1.251\sqrt{L}
when C is measured in meters per second and L in meters. Note that in both formulas the wave speed is proportional to the square root of the wavelength.

So when using meters and meters/second, the constant is 1.251 times the square root of the waterline. length.

So my 22.5' waterline is 6.858 meters and if I did my math right that came out to 11.7939301371 km/hour which is 6.368212822510335 knots

Now with the other formula I get 1.34 x square root of 22.5 ft = 6.35617809694 knots

So this clearly explains why all the European boats are faster. They are all measuring with the metric system.

I call garbage on Don C L's post. Fake news.

GIGO = garbage in, garbage out.

Why garbage? It's garbage to calculate a "hull speed" in 12 or more significant figures (e.g. "6.368212822510335 knots") when Don C L uses only 2 significant figures for g and 3 significant figures for pi.

And it's likely garbage to regard g as a constant.

The nominal average g for the planet is 9.80665 m/s^2.

But at the Equator, g is only around 9.78033 m/s^2.

So if "hull speed" were a real thing and if you could calculate "hull speed" to 12 or more significant figures, you'd need (1) an accurate figure of your local g; and (2) a value of pi with an appropriate number of significant figures.

[rgleason]

Polux, Thank you for your clarifying post.

[Polux]

Originally Posted by Joli

I'm pretty sure I've asked you before if you have raced and was never given an answer. Race course numbers tell the tale and teaches you how to look at the real average speed of a boat. And pressing a powered up racing shell is not indicative of what the average sailor sees for daily runs or even what the average racer sees for daily runs with a racer cruiser.

Let's look at a couple numbers.

The ARC "Cruising Class" winner this years was an HR 37 with a 34 foot waterline. Theoretical hull speed is 7.81 knots, they won the class averaging 5.32 knots or a bit under 70% of hull speed.

The ARC "Racing Class" winner this year was a Dufour 45e with a 40 foot waterline. Theoretical hull speed is 8.47 knots, they won the class averaging 6.02 knots or a bit over 70% of hull speed.

I think the ARC record held by a Rambler 88, a no holds barred racing maxi is 16.8 knots, the theoretical speed of that boat is 12.6 knots. But who would want to cruise an empty carbon fiber black hole with a crew of 20?

Our heavy old boat designed in the 60's and built in the 70's has a phrf rating or -21. Hull speed is around 9.5 but we've pressed the boat into the high teens when racing. We plan on an average of 7 knots for cruising, it's no faster or slower than modern 60 footers we've sailed against (empty carbon racing shells excluded).

I have raced a couple of times but I do cruise on a fast boat and I am used to catch and overtake much bigger boats. In fact I have fun with that, keeps me entertained.

I sail a lot of time and I use the engine very little so if I made a global season average speed it would be probably about 6 kt but that is just because I am sailing doing 3 to 5 kt when you would be almost dead on the water (with winds between 3 to 6K) and therefore using the engine.

Regarding ARC you are confusing speed to speed to an handicap. The boats that won the cruising or racing divisions have done so regarding an handicap, there was for sure much faster boats with the same size.

I would look at the speed you mention and the 70% later but I know sailors (a couple) that while cruising have done the atlantic crossing on a much smaller boat averaging a bit over 7kt (33/34ft boat).

Regarding the average speeds you mention you forgot two things: you are considering an average speed over a direct line while they, particularly this year, had to move a lot to have wind and made much more miles than the ones they would do on a direct course.

If you consider all the miles they have done the speed would be considerably higher.

Also, regarding the one that is a relatively fast boat, the Dufour 45, and its average speed you have to consider that when you are motoring (below 6 or 7 kt wind) they are still sailing, or almost without wind, dead on the water waiting for wind to came (on the racing division the engine cannot be used).

Off course, that will count a lot in what regards lowering the average speed.

And besides all that what you said was this: "At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons)."

You are not talking about average speeds over an Atlantic crossing but about to be happy to sail over 70% of hull speed. Nobody with a performance cruiser will be happy with that not even the ones that have mass production modern boats, that are able to do better than that with not too much wind.

[Don C L]

Originally Posted by Alan Mighty

I call garbage on Don C L's post. Fake news.

GIGO = garbage in, garbage out.

Why garbage? It's garbage to calculate a "hull speed" in 12 or more significant figures (e.g. "6.368212822510335 knots") when Don C L uses only 2 significant figures for g and 3 significant figures for pi.

And it's likely garbage to regard g as a constant.

The nominal average g for the planet is 9.80665 m/s^2.

But at the Equator, g is only around 9.78033 m/s^2.

So if "hull speed" were a real thing and if you could calculate "hull speed" to 12 or more significant figures, you'd need (1) an accurate figure of your local g; and (2) a value of pi with an appropriate number of significant figures.

My God Man! You are right! I humbly apologize. I'll get on those calculations again right away.

[JPA Cate]

Round up the numbers, Don, not the vessel.

Ann

[Don C L]

Originally Posted by JPA Cate

Round up the numbers, Don, not the vessel.

Ann

Thanks Ann! My boat just gained a hundredth of a knot! Now I can finally take on that pesky Swan and win next time!!

[Alan Mighty]

Originally Posted by Alan Mighty

And it's likely garbage to regard g as a constant.

The nominal average g for the planet is 9.80665 m/s^2.

But at the Equator, g is only around 9.78033 m/s^2.

So if "hull speed" were a real thing and if you could calculate "hull speed" to 12 or more significant figures, you'd need (1) an accurate figure of your local g; and (2) a value of pi with an appropriate number of significant figures.

About 0.5% of the value of your local g comes from latitude.

So about 0.5% of variation in "hull speed" likely comes from different latitude.

Sources of latitude-based local g values include:

https://www.sensorsone.com/local-gravity-calculator/

and

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