What is "Hull Speed" Anyway

[Dockhead]

Another thread on here got derailed by a discussion about hull speed. One member could not believe that displacement hull monohulls can exceed "hull speed" without planing, and attributed all claims of the contrary to either favorable currents or lying.

So I thought it would be useful to discuss the nuts and bolts of "hull speed" to clear up any misconceptions.

We have some fantastically knowledgeable people on here, and I hope some of them will chime in so all of us can learn something.

I am not a NA and only know the basics. What I do know is that "hull speed" is not a term which is even used by professionals -- it's an amateur's oversimplification of a more complex thing. The pros, as far as I know, talk about "speed/length ratio" and "Froude numbers".

The (over)simplified version of the story is that when a displacement hull reaches a speed (in knots) of approximately 1.34x the square root of its waterline length (in feet) (that's called the "speed/length ratio"), it starts to climb its own bow wave, and can't go any faster without the application of a huge amount of power.

But in reality, it's not quite like that First of all, there is no "sound barrier" which you can sail right up to but then can't break through. There is a curve representing power vs speed which steepens sometime before "hull speed" and continues steepening for some time after, so depending on how much power you have compared to the total resistance you have (skin friction plus wave-making resistance), you might not actually be able to get up to "hull speed", or you might be able to get some distance beyond it.

Secondly, heaviness of the boat expressed as D/L, and different aspects of the hull form, including fineness of the hull, rocker, and width of aft sections, can dramatically affect the place where the curve steepens, and also the steepness of the curve. In fact, 1.34x the square root of LWL is just a rule of thumb invented in olden days of heavy boats -- 1.5x is probably closer to where you feel this steepening on lighter monohulls, and catamarans with their very fine hulls may not experience it at all. Modern "wedgies" with wide and buoyant aft sections and fine entries may also feel "hull speed" very little, because the buoyancy aft keeps them from squatting.

But in any case, the effects of this change in wave-making resistance are felt over a range of speed, so in any case "hull speed" is not a single hard number, even for one given boat.

On my own boat, with 47' waterline, bulb keel, and pretty light at 188 D/L, I feel these effects as follows:

* One knot below theoretical "hull speed", or about 8.3 knots (speed/length ratio of 1.2), is the maximum speed I can maintain for a day at a time even in ideal conditions. The power required to exceed 8.3 knots obviously increases and requires perfect wind and sail trim -- something I can't keep up for days at a time.

* Two knots below theoretical "hull speed" or 7.3 knots (S/L of 1.06) is the fastest speed for economical motoring on my boat. Fuel consumption goes up a lot above that speed. Not coincidentally, I think, that's the maximum speed at which my boat does not produce a noticeable bow wave. Actually, for really effortless motoring I need to slow down to about 6.5 knots (S/L of 0.95).

* A bit less than one knot above theoretical "hull speed", or 10 knots, is the maximum sustained speed I can achieve without surfing. Actually 10 knots for my 47 foot waterline is a speed/length ratio of 1.46, which is still within the definition of "hull speed" according to the way some use it -- 1.2 to 1.51 rather than the artificially fixed number of 1.34.


Some general reading materials:

https://en.wikipedia.org/wiki/Hull_s...d-length_ratio


http://www.wavetrain.net/boats-a-gea...nger-is-faster

https://www.setsail.com/optimum-spee...-length-ratio/



Discuss!

[boatman61]

My personal confirmation of the realities of 'hull speed' effect on different types of boats were on a Hurley 22 when running before a F7 into La Coruna with full hank on No1 jib the bow sank down to where it was barely 6in from being swamped.. it was quite exciting releasing and tensioning the jib to keep her afloat.. turning head to wind was not an option.. neither was letting go off the tiller to go fwd and change sail..
These are heavy semi full keel boats.. often described as 'proper little ships'
My other experience was running bare poles N of Bermuda before a NW gale.. averaged 9kts and surfed around 14kts.. this boat was a HC37 with lead fin.. she definitely demonstrated more 'lift'..
Sorry its not full of techy stuff..
All I can say is.. heavy displacement will start to submarine over hull speed and lighter displacements (fin) will generate lift at the bow as the stern begins to squat..
Okay.. now you have something to get your teeth into folks..
Should keep you all occupied over the hols..
Enjoy

[Kenomac]

Now the entire premise seems to have changed from Dockheads's posts on the previous thread, where outlandish claims of averaging 1.5 knots above hull speed against a .5 knot current for his displacement cruising Moody 54 were claimed over a 30 mile distance whilst kicking back and listening to "Mahler." I don't plan on re-writing all of my posts, and NO... nobody was calling anyone a liar on that thread. Simple disagreement and requesting proof doesn't constitute calling someone a "liar."

[El Pinguino]

My boat has a theoretical hull speed of 8 knots ( using 1.34 as a constant ). On a few occasions in flat water I have had her up to 10 knots for a short period of time on a beam reach ( and once up to 8 knots downhill under bare poles also in flat(ish) water..... ) but that is never going to be sustainable in a seaway. I do know that downhill in flatish water ( sub 1 metre sea) under just the jib ( normally with between 25/40 over the deck ) when she gets above 7 she is on the limit.... so in general terms for me the rule works.... but she can exceed theoretical hull speed..

This is worth a read .. your first link I think

https://en.wikipedia.org/wiki/Hull_s...d-length_ratio


'Hull design implications[edit]
Wave making resistance depends dramatically on the general proportions and shape of the hull: many modern displacement designs can easily exceed their 'hull speed' without planing.

These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels.

Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.

Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.

Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, once above hull speed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases. For example, world-class racing kayaks can exceed hull speed by more than 100%,[1] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes.

Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.'

So what is hull speed for 'big' ships? Back in the dream time - when 'big' for most of us was 500 feet - US companies had plenty of cargo ships capable of over 20 knots as they had big steam plants, fine hull forms, and fuel costs didn't seem to be an issue. My first ship was capable of 18+ knots but in the interests of making a quid for the shareholders we ran at 50 tons of HFO a day which gave about 15 knots... economic speed.... You see there is a reason even today why most ships poke along at 15 knots or less....

Then, in the early 60's , Esso discovered that by fitting bulbous bows to existing ships that had 'traditional' bows they could get an extra knot for the same fuel consumption. Within a very short period of time all but their very oldest ships had bulbous bows and the rest of the shipping world soon followed.

I'll let you do your own research on how that works but hull speed is dependent on hull form... and whatever constant you choose would appear to be somewhat arbitary.

[Dockhead]

Originally Posted by El Pinguino

. .. .

So what is hull speed for 'big' ships? Back in the dream time - when 'big' for most of us was 500 feet - US companies had plenty of cargo ships capable of over 20 knots as they had big steam plants, fine hull forms, and fuel costs didn't seem to be an issue. My first ship was capable of 18+ knots but in the interests of making a quid for the shareholders we ran at 50 tons of HFO a day which gave about 15 knots... economic speed.... You see there is a reason even today why most ships poke along at 15 knots or less....

Then, in the early 60's , Esso discovered that by fitting bulbous bows to existing ships that had 'traditional' bows they could get an extra knot for the same fuel consumption. Within a very short period of time all but their very oldest ships had bulbous bows and the rest of the shipping world soon followed.

I'll let you do your own research on how that works but hull speed is dependent on hull form... and whatever constant you choose would appear to be somewhat arbitary.

Yes, Dashew talks about this also -- what is an economic motoring speed.

It's far below "hull speed". And this also shows that this effect -- disproportionate increase in wave-making resistance -- starts far earlier than "hull speed". Dashew says that 0.9 S/L is about right for economical cruising -- so that's about 2/3 of "hull speed". That corresponds exactly to what you are saying, Ping.

[Dockhead]

Originally Posted by Kenomac

. .. outlandish claims of averaging 1.5 knots above hull speed against a .5 knot current for his displacement cruising . .

Have you noticed that poster after poster after poster, have stated similar "outlandish claims"? Most recently Phil (Boatman 61) who averaged 9 knots surfing over some distance, in a 22 foot boat?

Do you really not start to doubt your preconceptions about the impregnability of the "hull speed" barrier?

What Phil describes, and others described in the other thread, is SURFING. Something which I think you haven't experienced, based on your identification of this with planing, which is completely different.

Well, open your mind, Ken, instead of heaping disdain. It's a real thing which many of us have experienced. Sometimes you can keep it up only for a moment at a time, but when conditions are just right, it can go on for hours. It doesn't happen that often because the sea state has to be just so, but it's very cool when it does. Lighter boats do it much more easily than heavier ones. You can only do it more or less dead downwind.

[boatman61]

Originally Posted by Dockhead

Have you noticed that poster after poster after poster, have stated similar "outlandish claims"? Most recently Phil (Boatman 61) who averaged 9 knots surfing over some distance, in a 22 foot boat?

Do you really not start to doubt your preconceptions about the impregnability of the "hull speed" barrier?

What Phil describes, and others described in the other thread, is SURFING. Something which I think you haven't experienced, based on your identification of this with planing, which is completely different.

Well, open your mind, Ken, instead of heaping disdain. It's a real thing which many of us have experienced. Sometimes you can keep it up only for a moment at a time, but when conditions are just right, it can go on for hours. It doesn't happen that often because the sea state has to be just so, but it's very cool when it does. Lighter boats do it much more easily than heavier ones. You can only do it more or less dead downwind.

Read again Dockhead.. you will see that I was talking about a Hunter Cherubini 37c.. aka HC37.. N of Bermuda.. the H22 was the Biscay and La Coruna..
Part of the problem is folks often do not absorb the full post.. just grab the bit they find objectionable..
Something I have also been guilty off on the odd occasion.. as the song goes..
"Slow down.. you move to fast..!!!"

[Dockhead]

Originally Posted by boatman61

Read again Dockhead.. you will see that I was talking about a Hunter Cherubini 37c.. aka HC37.. N of Bermuda.. the H22 was the Biscay and La Coruna..
Part of the problem is folks often do not absorb the full post.. just grab the bit they find objectionable..
Something I have also been guilty off on the odd occasion.. as the song goes..
"Slow down.. you move to fast..!!!"

Ah, OK.

But a Hunter Cherubini has a 30' waterline, so the point is the same -- 9 knots, your AVERAGE speed, is two knots above "hull speed", so you were surfin', baby . . . .

And 14 knots is DOUBLE hull speed. I've never been that fast myself on a monohull (beach cat is a different story . . .)

[boatman61]

PS: have not a clue what I was doing on the H22.. she did not have a speed log and my hh gps was below..
I do know she was almost keeping pace with the waves which was when I would have to release the jib sheet to keep her from going under.. the release allowed the bow to lift as the wave passed under the stern and moved forward.

[boatman61]

Originally Posted by Dockhead

Ah, OK.

But a Hunter Cherubini has a 30' waterline, so the point is the same -- 9 knots, your AVERAGE speed, is two knots above "hull speed", so you were surfin', baby . . . .

And 14 knots is DOUBLE hull speed. I've never been that fast myself on a monohull (beach cat is a different story . . .)

What can I say.. its the reading that showed a couple of times as I shot along the face of waves coming up behind me.. and these were long Atlantic seas.. not short Channel and N Sea waves.. 14kts was the highest I saw and around 9 was the most frequent..
When your steering solo and concentrating on whats ahead.. to the side and coming astern staring at ones instruments is not a priority..
Another outlandish claim.. A Catalac9 can do 18kts.. this has photografic evidence somewhere on CF.. and I was at the helm..

[Kenomac]

Originally Posted by Dockhead

Have you noticed that poster after poster after poster, have stated similar "outlandish claims"? Most recently Phil (Boatman 61) who averaged 9 knots surfing over some distance, in a 22 foot boat?

Do you really not start to doubt your preconceptions about the impregnability of the "hull speed" barrier?

What Phil describes, and others described in the other thread, is SURFING. Something which I think you haven't experienced, based on your identification of this with planing, which is completely different.

Well, open your mind, Ken, instead of heaping disdain. It's a real thing which many of us have experienced. Sometimes you can keep it up only for a moment at a time, but when conditions are just right, it can go on for hours. It doesn't happen that often because the sea state has to be just so, but it's very cool when it does. Lighter boats do it much more easily than heavier ones. You can only do it more or less dead downwind.

Your Moody 54 isn't a 100ft Maxi like Comanche... please get over it. It's a very nice 54 displacement cruising boat. Carbon sails do not turn a 54' non-planing displacement cruising yacht weighing in at a hefty 45,000 pounds into a 100ft Maxi yacht capable of long stretches of sailing well above hull speed.

I'll continue the discussion, if and when some proof (as in the form of a video) can be presented of YOUR BOAT (not someone else's boat) attaining a relaxed constant speed of 1.5-2 knots above hull speed. Until then, I reserve the right to cast doubt on your claims. Surfing down a single wave.... doesn't count, we've done that too. Big Deal. I also doubt the claims on your Beowolf video of their boat claiming speeds of 27 knots whilst having a relaxing breakfast... which are the same speeds attained by the America’s cup foiling catamarans. Sure don’t look the same to me, but I’m also sure anything is possible on the internet.

Meanwhile, I’m also still waiting for you to point out the similarities between your Moody 54 and the 100ft Maxi Comanche, which enable your boat to attain “Warp Speed” over great distances against tide and current... my “9 year old” eyes can’t find any.

I’m sure I’m not alone in my thoughts regarding your claims.

[danielamartindm]

I suspect that "hull speed" is an oversimplified term thrown around to describe the diminishing returns in increased speed through the water that we see with increasingly larger engines or sail power in a given displacement hull. I would bet that this relationship has already been quantified, and the final equation is complex. It always seemed to me that with a displacement hull, no matter how much horse/sail power (and extra weight) you bring to bear, at some point there is a theoretically infinite-sized bow wave waiting to form to thwart any further increases in speed; but I'm just a ham and egger.

[boatman61]

Originally Posted by danielamartindm

I suspect that "hull speed" is an oversimplified term thrown around to describe the diminishing returns in increased speed through the water that we see with increasingly larger engines or sail power in a given displacement hull. I suspect that this relationship has already been quantified, and the final equation is complex. It always seemed to me that with a displacement hull, no matter how much horse/sail power (and extra weight) you bring to bear, at some point there is a theoretically infinite-sized bow wave waiting to form to thwart any further increases in speed; but I'm just a ham and egger.

Beats me mate.. I'm just beans on toast..

[Dockhead]

Originally Posted by boatman61

PS: have not a clue what I was doing on the H22.. she did not have a speed log and my hh gps was below..
I do know she was almost keeping pace with the waves which was when I would have to release the jib sheet to keep her from going under.. the release allowed the bow to lift as the wave passed under the stern and moved forward.

Another thing to say about surfing --

It doesn't work either when the waves are too BIG. The longer the wavelength, the faster they go, and if the difference in speed to you is too much, you can't catch them.

I would be interested to hear an explanation from someone who understands this. But I think the principle of surfing a displacement yacht is that the waves push you forward down the face of them more than they push you back off the backs of them, the closer your speed is to theirs.

Here's Dashew surfing Beowulf at 27 knots (!)

https://www.youtube.com/watch?v=J4zg5vD_jvI

.

"Hull speed" of Beowulf is only 12 knots. But Beowulf is very much a displacement hull much like ours. She is just light (it's easier to make a boat light, when the boat is longer -- the denominator of D/L goes up with the cube of the waterline length), and narrow, both of which reduce the "hull speed" effect.



Beowulf's hull form.

[Dockhead]

Originally Posted by Kenomac

Your Moody 54 isn't a 100ft Maxi like Comanche... please get over it. It's a very nice 54 displacement cruising boat. Carbon sails do not turn a 54' non-planing displacement cruising yacht weighing in at a hefty 45,000 pounds into a 100ft Maxi yacht capable of long stretches of sailing well above hull speed.

I'll continue the discussion, if and when some proof (as in the form of a video) can be presented of YOUR BOAT (not someone else's boat) attaining a relaxed constant speed of 1.5-2 knots above hull speed. Until then, I reserve the right to cast doubt on your claims. Surfing down a single wave.... doesn't count, we've done that too. Big Deal. I also doubt the claims on your Beowolf video of their boat claiming speeds of 27 knots whilst having a relaxing breakfast... which are the same speeds attained by the America’s cup foiling catamarans. Sure don’t look the same to me, but I’m also sure anything is possible on the internet.

Meanwhile, I’m also still waiting for you to point out the similarities between your Moody 54 and the 100ft Maxi Comanche, which enable your boat to attain “Warp Speed” over great distances against tide and current... my “9 year old” eyes can’t find any.

I’m sure I’m not alone in my thoughts regarding your claims.

What do 100 foot Maxis have to do with anything? As was discussed earlier, Commanche can sail at 40 knots. Nothing to do with anything we've been discussing. That's what we call a "red herring".

So do I understand you also doubt the truth of Dashew's video? What -- you think Dashew fiddled with his speed log in order to impress people with impossible speeds he wasn't actually achieving?

Well, Dashew certainly doesn't need to impress people with false speed claims, nor do any of the rest of us.

Beowulf achieves speeds well above "hull speed" for just the reasons we've been discussing. You've been provided with tons of objective information, to allow you to understand this. Beowulf has a normal displacement hull quite like mine except without the keel bulb. But she is narrow. And she is very light (D/L of about 60 -- like TJ's boat "Rocket Science").

As boats get lighter and narrower, they make less of a bow wave. That means less energy is transferred to the water by wave-making, and there is less of a bow wave to climb. That means the acceleration of resistance which is happening between 1.2 and 1.5 S/L is less steep, so takes less power to overcome.

That's why Beowulf is able to make 350 mile days for days on end -- although that's an average of 14.5 knots, 2.5 knots above Beowulf's theoretical "hull speed" -- and why Beowulf is able to surf for longer periods at more than double hull speed, as we saw in the video.

That's why Jim Cate can "frequently hit 12 or 13 knots, and occasionally over 15 knots" in his light 46 foot boat, and why Phil (Boatman61) could average 9 knots in a storm in his Hunter Cherubini, and why others reported similar experiences.

OK, those are the "outlandish" facts. You can believe that we're all just lying to impress you (!), or not, as you like.