Hull speed

[Rick Willoughby]

Discussion thread for Hull speed (http://www.boatdesign.net/wiki/Hull speed). If you would like to add a comment, click the New Reply button

[TerryKing]

Thanks for this Wiki subject.. it was exactly what I was looking for, as I make the transition from V8 planing boats to Old Age (Scratch That! :P ) and larger old boats that need re-engine.

[masalai]

I have been digging around, & downloaded a spreadsheet which was very interesting but does not match actuality in a Robin Chamberlin design which interested me greatly: achieving 16knots max using 2 x 50hp Kubuto engines and delivering a cruise of 12knots at 1 litre/nautical mile from a cat with 10m loa hulls.

I am seeking analysis to ascertain the potential for a 12m lwl with 600mm or 700mm waterline hull beam (depending on engine manufacturer) and displacement of 6500 - 7000kg.

or, having built a 1:10 scale model, how do I evaluate this and scale up to full size data?

[AmnonMikeCohen]

:idea: By eliminating on a displacement hull, its Bow wave, its traditional Skin Friction, and its Drag resistances, and also eliminating the wave-making on planning hulls; a hull equipped with the invented Superior Power FINS is not limited to the original resistances traditionally occurring as a result of hull's shape, while plowing in water.
The displacement hulls can now surf forward 20% faster and the planing hulls can lift the watercraft to fly 7%+ faster on top of its keel!

[charmc]

OK, so we had a commercial break. Back to topic discussion.

I can't find the right references, but I know that the hull speed formula for extremely narrow hulls (very high L/B ratio) is very different. The problem is I can't remember the point at which the 1.34 constant gets tossed. :confused: This reference gets at part of the subject: "There is a sense in which multihulls are always superior to monohulls from the point of view of wave reduction, and in particular wave resistance reduction. After all, since wave resistance varies as beam squared, the total wave resistance of two separate half-beam hulls is half of that of one fullbeam hull." (OPTIMUM HULL SPACING OF A FAMILY OF MULTIHULLS, Tuck & Lazauskas, University of Adelaide, 1998)

The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now.

[masalai]

Thanks Charlie, :D
These commercials are popping up everywhere, and as per post #4 not really wanted...:P
Waterline beam:length ratio is close to 1:17 and closely aligned with Michelet I guess, I have left refinements in abeyance as I am still figuring out which way to go (I need to have a sail or go mad???).

So will I have strictly Iron sails on the live-aboard and carry a couple of 14 beach cats for (not tender) tenders/sail fun. or

Put a mast up and fly a variation of the "hitchhiker rig" as per John Hitch designs....

The two concepts are divergent as the underwater profile will be significantly different. That is why my designing has gone quiet. The cogs turn slowly as age marches to a quieter tune

[AmnonMikeCohen]

:!: My statement(s) are based on R&D which is not yet within the public domain yet, and only few professionals have looked at it - so please accept the limitations of my claims, and just take this as news and not as empty arguments or untrue claims.
The new technology, is not related to how narrow the hull is, it is to do with the introduction of controlled beneficial water lubrication under the hull, created by a new invention of Fins to be added to a hull.

The only known technology, which is not the same, but is also beneficial to a limited extant, is the resulted positive effect of Bilge-Keels, but these are limited add-on appendages, and must be specially designed for each hull and power combination, yet are known to increase performance.

Ami

OK, so we had a commercial break. Back to topic discussion.

I can't find the right references, but I know that the hull speed formula for extremely narrow hulls (very high L/B ratio) is very different. The problem is I can't remember the point at which the 1.34 constant gets tossed. :confused: This reference gets at part of the subject: "There is a sense in which multihulls are always superior to monohulls from the point of view of wave reduction, and in particular wave resistance reduction. After all, since wave resistance varies as beam squared, the total wave resistance of two separate half-beam hulls is half of that of one fullbeam hull." (OPTIMUM HULL SPACING OF A FAMILY OF MULTIHULLS, Tuck & Lazauskas, University of Adelaide, 1998)

The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now.

[masalai]

Please Amnon, post the R&D or do something else apart from cluttering dialogue... with nothing more than diatribe

[Rick Willoughby]

...
The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now.

Charlie
Hull speed is BY DEFINITION set by the waterline length of the hull. The formula is given in post 130 on this thread:
http://boatdesign.net/forums/showthread.php?t=18851&page=9

Thin hulls can readily exceed their hull speed. Wider hulls require much more power to exceed their hull speed.

Rick W.

[masalai]

Thanks Rick, will go have a look... That is not the one I was thinking of - get confused with the names -

[lilies34]

Charlie
Hull speed is BY DEFINITION set by the waterline length of the hull. The formula is given in post 130 on this thread:
http://boatdesign.net/forums/showthread.php?t=18851&page=9

Thin hulls can readily exceed their hull speed. Wider hulls require much more power to exceed their hull speed.

Rick W.


thanks for sharing the link! finally found the formula! yey!

______________________
CRM software (http://www.crm-software-guide.com)

[charmc]

Charlie
Hull speed is BY DEFINITION set by the waterline length of the hull. Rick W.

OK, Rick, I was guilty of loose language. You're right, of course, hull speed, in knots, is defined as 1.34 x the square root of LWL in feet (A rare case in which the formula in English units is simpler and more elegant than in metric).

What I was getting at is the fact that very narrow hulls can exceed this number without either the requirement of getting on plane or of consuming huge amounts of power and generating huge waves. Malcolm Tennant said something similar in his oft-quoted article on power catamaran design, "the hull speed of the displacement catamaran is not restricted by the familiar 1.34 times the square root of the waterline length ..."

[charmc]

I probably should have said that the practical maximum displacement speed of very narrow hulls is much higher than for "traditional" L/B range monohulls.

[Rick Willoughby]

.......

What I was getting at is the fact that very narrow hulls can exceed this number without either the requirement of getting on plane or of consuming huge amounts of power and generating huge waves. ........ [/FONT][/I]

Charlie
I was just being pedantic. And I also agree the imperial formula you have is very simple and the one I remember. I even convert LWL to feet so I can do the sum.

Many people do not realise that the formula is derived from a property of a gravitational wave. They think the 1.34 is a constant derived from best fit.

Rick W.

[terhohalme]

Why not do metric like:

V = 2.43 * (LWL)^0.5 (speed in knots)

This is quite as simple!

Though the hull speed is just a reference or a value of comparison, not a barrier of any hull form.

[Rick Willoughby]

Why not do metric like:

V = 2.43 * (LWL)^0.5 (speed in knots)

This is quite as simple!

Though the hull speed is just a reference or a value of comparison, not a barrier of any hull form.

My first boat book predated metric conversion in Australia and the 1.34 has just stuck in my mind. I regularly do meter to feet conversion so the 3.28 also comes easily to mind.

Rick W.

[Kerry Thomas]

The Froud number was for the average L/B ratio of merchant ships in his time. Very wide or narrow hulls have a different constant.

Also we can get higher power these days with the same displacement.
A 50m tug hull with 80 ton bollard pull would have been impossible then.

Many displacement vessels now have enough power to exceed hull speed, usually with the stern wave above the funnel:-) I hate to think how much fuel they are wasting to get that extra few knots.

[Rick Willoughby]

The Froud number was for the average L/B ratio of merchant ships in his time. Very wide or narrow hulls have a different constant. ...........

Kerry
"Hull Speed" is a defined quantity that is only related to the LWL and the gravitational constant. The value shown in the equation is NOT A CONSTANT related to L/B but a physical property of gravitation waves.

All hulls have a "Hull Speed" by definition. It is the speed where the boat speed matches the phase velocity of a wave having the same wavelength as the hull waterline length. The phase velocity for deep water is:
v = (g * Wavelength / 2 / pi) ^ 0.5

In applying this formula you have to use consistent units so for metric, wavelength is in metres and g is 9.8. Velocity will be given in m/s. For imperial it is feet, 31.8 and speed is ft/s. If you want to get speed in knots you need to know 6080ft/nm or 1852m/nm and 3600s/hr.

I simply find it easier to remember 1.34.

Rick W.

[Kerry Thomas]

Exactly.


Quote; I can't find the right references, but I know that the hull speed formula for extremely narrow hulls (very high L/B ratio) is very different. The problem is I can't remember the point at which the 1.34 constant gets tossed. :confused: This reference gets at part of the subject: "There is a sense in which multihulls are always superior to monohulls from the point of view of wave reduction, and in particular wave resistance reduction. After all, since wave resistance varies as beam squared, the total wave resistance of two separate half-beam hulls is half of that of one fullbeam hull." (OPTIMUM HULL SPACING OF A FAMILY OF MULTIHULLS, Tuck & Lazauskas, University of Adelaide, 1998)

The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now.[/QUOTE]

[Rick Willoughby]

...
The point is that the hull speed of very narrow hulls is much higher than for hulls with L/B of 3 or less. "Narrow" and "high ratio", of course are subjective terms. They are defined, I just can't find the reference now....


For any given LWL the hull speed in deep water DOES NOT CHANGE irrespective of the hull shape. Hull speed is not a speed limit. Hull speed is simply the phase velocity of a wave having wavelength the same as the LWL of the hull. Absolutely NOTHING to do with L/B ratio.

Rick W.

[masalai]

Ok, now I am being thoroughly dumb/ignorant/confused. (Not unusual as I come to grips with failing memory & precise definitions)... Is there a way to evaluate the potential velocity of a long skinny hull (half a catamaran) at various Kw of power delivered through a reasonably effective choice of 2 different screws (optimised to work fully submerged - one without, and the other, with designed "cavitating blades", supported by exhaust injection)

I have moved my request to here, I feel this form is accidently the best I have produced? http://www.boatdesign.net/forums/showthread.php?t=20633&page=6 at post 81 :D Is it worthy of further modelling to make and put to a 1/10 test? What suggestions?

[Kerry Thomas]

I do not recall saying hull speed is a speed limit. Quite the opposite.

I am trying to track down the paper to cite.
This was a study on why multihulls and very narrow vessels appear to dis regard the Froud number.
The conclusion was, hull speed, defined as the point where wave making resistance is such that power required to increase speed slightly increases exponentially. This is what we want to know in the real world!
Not only beam, but angle of entry and displacement also should be factored in. Giving us a different constant and factors.

[Rick Willoughby]

....
The conclusion was, hull speed, defined as the point where wave making resistance is such that power required to increase speed slightly increases exponentially. This is what we want to know in the real world!
Not only beam, but angle of entry and displacement also should be factored in. Giving us a different constant and factors.

It is not reasonable to redefine Froude's work. Hull speed, as he coined the term, is determined by LWL and the gravitational constant. It is based solely on the phase velocity of gravitational waves in deep water - end of story. NOTHING to do with L/B of a hull.

Hull speed, as Froude observed it, has more relevance for wide hulls than for slender hulls. The velocity corresponding with hull speed results in rapidly rising wave drag with wide hulls.

One observation I have made with very light displacement hulls is that the hull speed corresponds with the design speed when the hull is designed for minimum drag. As the displacement increases, the hull speed is increasingly higher than the design speed for the minium drag hull.

I have attached drag curves for two hulls with the same length therefore the same hull speed. One displaces 10t while the other displaces 300kg and is optimised for 8.1kts - the hull speed. You can see the heavy displacement boat drag is dominated by the wave drag throughout the speed range while the light displacement boat drag is dominated by viscous drag. Even so the optimum length for the light displacement boat results in the design speed and hull speed being almost the same. The hull speed is just where the wave drag is starting to kick in despite this component of drag remaining very small.

Point is that hull speed still has significance for very light displacement hulls as it provides the length that results in the minium overall drag for that speed. This is significant for craft like rowing sculls.

The curve for the light displacement boat also shows that there is no point where the wave drag is increasing exponentially. Your redifined term has no meaning for slender hulls. Wave drag remains a tiny component of the total drag.

What I want to know is the total drag on the hull. To determine this I just load the hull coordinates into Michlet and it gives me the answer over my selected speed range. Nothing too complicated there. No need to be concerned with exponentially rising wave drag. It may or may not be the case.

Rick W.

[charmc]

Also we can get higher power these days with the same displacement. A 50m tug hull with 80 ton bollard pull would have been impossible then.

Many displacement vessels now have enough power to exceed hull speed, usually with the stern wave above the funnel:-) I hate to think how much fuel they are wasting to get that extra few knots.

"Thin hulls can readily exceed their hull speed. Wider hulls require much more power to exceed their hull speed." Rick W

True. Tugs are an example. This neat video of the annual tugboat race in NY harbor shows the concept: massive power = massive wakes and a (very)few more knots.

http://www.splashvision.com/Video/10964_2007-nyc-tugboat-races-II.html