hull speed, what it means.

[jon hansen]

so i'm reading on a post by benny17441 and he writes that 'hull speed' is an old formula used when boats were wood an mostly similar. and some of the formula is a 'trash' number. he implies that hull speed really does not work for modern boats.

i first thought, nope, benny is wrong, i know better. then i read about the 'froude number'. never heard of this.

so, benny, i'm not trying to pick on you, instead, i want to learn.

i thought hull speed is that speed when the back of the boat falls off the secondary wave created by the bow wave there by making the boat climb up the hill of the bow wave. of course any boat can go faster but it takes way more force as the boat is now climbing a hill. i may be wrong that hull speed still works on modern designs. benny says so. i am starting to question whether hull speed is valid for todays designs as benny quite clearly states.

what is the 'froude number' does it replace the concept of hull speed.

can anybody explain it to me. i want to learn. is benny correct that hull speed no longer applies. i thought it did. what say yawl?

 

[jon hansen]

what i understand about hull speed i read in frank bethwaite's book, high performance sailing. maybe it is no longer valid.

what say you?

 

[Charlie Jones s/v Tehani]

It still most definitely applies It's 1.34 times the square root of the waterline length in feet . The hull speed of my 25 footer, with an 18 foot waterline is roughly 5.5 Knots

It's based on wave length, crest to crest. As you approach hull speed the bow begins to climb the bow wave and the stern out paces the quarter wave, so begins to sink lower. My boat, at that point begins to suck water in from the drains
All in all, running under sail or motor, inshore or off, I figure my average speed made good is about 4.5 knots

 

[Will Gilmore]

So far, this is the simplest explanation of the Froude Number, that I can find.
Froude number | physics
"Froude number (Fr), in hydrology and fluid mechanics, dimensionless quantity used to indicate the influence of gravity on fluid motion. It is generally expressed as Fr = v/(gd)1/2, in which d is depth of flow, g is the gravitational acceleration (equal to the specific weight of the water divided by its density, in fluid mechanics), v is the celerity of a small surface (or gravity) wave, and Fr is the Froude number. When Fr is less than 1, small surface waves can move upstream; when Fr is greater than 1, they will be carried downstream; and when Fr = 1 (said to be the critical Froude number), the velocity of flow is just equal to the velocity of surface waves. The Froude number enters into formulations of the hydraulic jump (rise in water surface elevation) that occurs under certain conditions, and, together with the Reynolds number, it serves to delineate the boundary between laminar and turbulent flow conditions in open channels."

The Wikipedia explanation reads like a bad mystery novel, where element after element is introduced later in the story to explain the inexplicable, leaving the reader feeling like they must have missed half the story not to have heard of the thing before.
Froude number - Wikipedia

Basically, what I got out of it is that the Froude Number, if you can figure out how to use it, takes into account the specific water density so you can calculate a more accurate version of "Hull Speed" in fresh, salt or brackish water. For the purposes of finding a standard by which to measure a given sailing displacement hull's performance, I can't see any advantage.

If you are designing a modern high performance planing hull, it might help to understand the various stages of wave propagation and resistance on the hull's journey to a full plane, but it seems like more detail than the effort would be worth. Of course, once you start designing on a computer, those complex equations become automatic, so why not?

For me, it is nice to know about the Froude Number, thanks Jon, but I don't see it having more useful information than what I'm already familiar with. In saltwater looking for that speed where effort and headway are optimal, the two numbers should give the same result, so I will still use "Hull Speed" as traditionally defined.

-Will (Dragonfly)

 

[dlochner]

Interesting question. After an early morning shallow dive (deep enough to gain some understanding, not so deep that I hit bottom ), I think the Froude number is good for starting arguments in yacht club bars and for licensing exams for Naval Architects, for the rest of us it is an interesting side note.

The classic hull speed formula that we all know and mostly love is based on the formula for deterring the relations between wave speed and frequency, Hull Speed = constant * sqrt of waterline. Waterline is the wavelength. The constant seems to vary a bit, some have said it is 1.34, I learned it as 1.22. I think the difference is whether hull speed is being calculated in miles per hour or knots per hour. Regardless, this is for all practical purposes good enough for most of us.

In the real world in real applications, things are not so simple. The Froude number, Reynolds number, the gravitational constant and the rest of the cast of characters @Will Gilmore talks about try to compensate and account for real world effects. I don't have the time, interest, or math skills to calculate the differences, however, I suspect if we accounted for all the effects on a boat traveling through water the difference between the classic formula and the compensated formula for our boats would be trivial at best. When designing boats measured in tenths of a mile, the numbers will be larger and the differences less trivial. So our naval architect friends need to pay attention to these factors.

The Froude number seems to be an attempt to compensate for the effects of gravity on water flow, which is relevant for running in a current and the Reynolds number seems to be compensating for friction and the disturbance of water flowing over the hull and the transition from laminar flow to turbulent flow. If we plug these ideas into our calculations for hull speed we can more precisely tune the estimate of hull speed and in designing the hull we can optimize the hull for various conditions.

For us small boat sailors, the most relevant characteristic affecting hull speed is hull design, a double ended canoe hull boat is going to have a lower hull speed than a boat with the same waterline that has a big flat butt. The difference will be due to the increased buoyancy in the big butt boat which keep it from squatting in the wave trough as fast as a pointy canoe stern. Other differences include overhang and heeled waterline length.

The waters we sail in will also haven an effect on hull speed. In shallow water there is more drag on the boat and the bow and stern waves will get steeper faster, slowing the boat down. In deeper water the waves are not as steep and thus offer less resistance. Water movement at the surface can slow or speed us along, while water movement a few feet below the surface can be different and contrary to the surface movement. Hydrologists and Naval Architects worry about these things, for the rest of us its nice to know someone is thinking about all this but it doesn't make much real world difference.

Cheers!

 

[Joe]

The "Theoretical Hull Speed" formula was based on a full displacement hull's inability to progress without pushing through the water .. i.e. displacing it.... rather than rising up and planing over it.

It's agreed then, that in the full displacement hull design world... a boat's waterline length will limit its maximum speed .... Theoretically.... because the additional power required to get the displacement hull out of the water, up on plane, so it can lift out of the trough created by its bow wave... is exponentially greater than what's needed to maintain speed at or below the theoretical level.
It is also a truth that there are many sailboat hull designs that are NOT full displacement. It therefore stands to reason that using the theoretical hull speed formula for all sailboats is just not practical. It can't apply evenly to every boat, because there are so many semi displacement and plaining hull designs in the world.

The theoretical hull speed formula, therefore, has limited use today. But, back in the age of steam, it was critical to balance the power and fuel requirements of those heavy, full displacement vessels with cargo capacity and it's revenue potential. So the formula was important in determining the optimal power needed along with the amount of coal, wood or oil that was carried for long trips. Today we might use the formula to help us determine optimal auxiliary motor size.

Yet...see where the exceptions are. What of the power sailors like the Mac x and m? Those boats get up on plane, do they not... so the formula certainly doesn't apply to them. How about the Laser? or any planing dinghy, scow, beach cat... how about the Pogo... or the open 60's......... on and on... But of course, my little tank would probably fit within the formula's scope... except... I've routinely had my boat exceed 6.25 kts... what does that mean? Am I a really fast sailing kind of guy... is the boat bottom slicker than you know what? And that's with old sails... or maybe it's current, or gravity, or heeling over to lengthen the water line........... who cares???? NO MORE theoretical hull speed discussions, please. (wonder why they call it "theoretical?)................ End of Rant. Go have some fun.

 

[shemandr]

He said "Reynolds Number." Is there an emoji for sticking my head in the sand?

 

[sailme88]

The shallow water issue is known as "squat". It affects ships more than sailboats. Operates similar Bernoulli's principle. In brief, a large displacement vessel, traveling at speed in shallow waters will settle more deeply into the water (squat) because of the difference in water speed and compression. This is how the QE2 struck bottom in the Massachuttes Bay some years back..

 

[JohnShannon]

Hull speed for boats is like the sound barrier for airplanes. Once thought unbreakable. In no sense an actual theoretical limit. To break that barrier you need lots of power/weight and a hull shape designed for it.
Many dinghies will plane ( exceed hull speed) they are unencumbered by a heavy lead keel. Some keelboats will plane high SA/D ratios and wide flat aft sections.

 

[CrispyCringle]

The Hull Speed formula and associated Froude number and as valid as ever. Its just that they are not perfect. Fluid dynamics are extremely complex and the formula we use for hull speed is the last line of a long "show your math" word problem. In other words, a very complex formula that is reduced to its simplest terms that we all know and love. It is also based on ideal fluid conditions. That means close to real life, but not quite.

I don't know what frank bethwaite's book is saying. But if its about high performance sailing he is probably talking about planing hulls or boats that are on the transition from displacement to planing, and if that's the case its not really in the relm of classic hull speed.

 

[rgranger]

 

[dlochner]

View attachment 184380

Well, that's a little extreme!

 

[rgranger]

Well I could have found a way to ask.. "what is the best anchor"

 

[dlochner]

I think the Froude number is good for starting arguments in yacht club bars

I was mistaken in this statement. It should have included sailing forums.

 

[rgranger]

I was mistaken in this statement. It should have included sailing forums.

That just made my day

 

[sail sfbay]

.................what is the 'froude number' does it replace the concept of hull speed.............

The Froude number (Fn) was invented by a 19th Century British naval architect called William Froude as a way of measuring and analyzing the ship's resistance. Rather than rely on waterline length to calculate hull speed, the Froude number is a coefficient that can be applied to any size of vessel, depending on hull shape. The Froude number is a speed:length ratio and the formula is V in m/s divided by the square root of (g*L), where g is gravity and L is waterline length in metres.

Jon, like you noted, the Froude number is used to compare the wave making resistance between bodies of various sizes and shapes. The you tube video entitled "Hull speed and Froude number" provides a good discussion about sailboat speed vs. hull shape and Fn.

As the Froude number increases and the boat's hull form moves from displacement to planing and the dominant resistance acting on the hull also changes. A Froude number below 0.4 is the point it becomes very power inefficient if you want to go faster. A Fn between 0.5 and 1, is a semi-displacement hull and at Fn of 1 dynamic forces start lifting the hull and at a Fn 3 you are fully planing.

In a different application, I used the Froude number many times to design equipment gravity drains by limiting the Fn to less than 0.3 to ensure vertical pipes are running full.

 

[DrJudyB]

The old formula, hullspeed=1.34 times the square root of waterline length, was developed by Anthony Deane in 1670 for designing heavily built British Men O'War. In his 1989 book, Propeller Handbook, Dave Gerr, naval architect, updated the formula to reflect the effect of displacement. His version add the variable of displacement. It has proved to be fairly accurate for many Monohull sailboats (with non-wave-piercing hulls) . It has the advantage of being rather easy to calculate from basic boat specifications.

here’s an article about the modern calculations for hull speed I wrote more than 20 years ago. It’s based on Dave Gerr’s work.

How Fast IS a West Wight Potter? A New Look at Hull Speed.

 

[JRacer]

View attachment 184380

 

[jon hansen]

nice video. thanks.
clearly "hull speed" and what it implies is still very relevant as to how a vessel goes through the water. designing vessels that transition to a planing mode more easily is the trick.
of course my 120 year old, first built of wood, Ascow does the planing thing. nothing new there, getting any vessel to transition still takes exponential power. that still takes huge sails and the crew to handle them.
So, i'm happy. learned some things from you wise old men and woman out there.

..... the 'fisherman anchor' is still the very best. all the other discussion is about which is number 2

 

[jssailem]

Very interesting discussion.

'hull speed' is an old formula used when boats were wood an mostly similar...... hull speed really does not work for modern boats.

i first thought, nope, benny is wrong, i know better.

While I enjoy the clarity shared in discussing the math involved identifying the fluid mechanical effects of hulls on the water, as a boat owner it is an engineering exercise in precision. I am comfortable with reasonable accuracy.

Once you calculate (even using the "ancient" method) a theoretical hull speed, the actual (true) speed (as measured by your boat instruments over various conditions) is of greater importance.

Knowing the type of hull you have is important to your seamanship of the boat. Newer hull designs have altered the boat speed by applying the advancements in engineering relative to 1600's. I am certain that when Mr Deane calculated the speed of Her Majesties ships, he could not envision the hydrofoiling catamarans/surfboards of today. Mr Deane's formula still works on my 1974 plastic boat to help identify the displacement hull speed with reasonable accuracy needed to safely sail her.

Not sure you "know better" or worse than "benny". I do think your knowledge is relevant to the needs of sailing your beautiful boat.