Hull speed

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Thanks!

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! ) and larger old boats that need re-engine.

 

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?

 

Hull Speed can now go beyond traditional resistances' limitations

: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!

 

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. 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.

 

Thanks Charlie,
These commercials are popping up everywhere, and as per post #4 not really wanted...
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

 

New Technology, does not comply to older ones

:!: 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

 

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

 

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

 

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

______________________
CRM software

 

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 ..."

 

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.

 

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.

 

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.