Myth: Hull Speed

[Tropic Cat]

Originally Posted by gramos

.... there must be a seperate set of formulae for assessing a boats hull speed when it has 2x hulls linked by a bridgedeck?.....

Yes, you are right. The displacement hull speed formula has been modified for multihulls. I've generally seen it written as hull speed= 1.74 x (sq rt of LWL), however, I believe it's either Crowther or Chris White who thinks the multiplier should be 1.89. In either case it's modifying the traditional displacement hull formula to compensate for narrower hulls which as Gord pointed out may be an obsolete way of calculating hull speed.

I think I'd have to see more complete data from you mono guys before I chuck the hull speed formula completely or declare all mono's semi displacement hulls as this is a term reserved for much narrower hulls and certainly can not be applied to full keel boats. Yet we've seen examples of exceeded theoretical hull speeds from a bunch of guys regardless of hull design.

I'm sure your boat's going 12 knots, I have no reason to disbelieve you, however, if you reversed course, would this still be true? SOG is what we are taking about here, when we should be talking speed thru water. I've seen zero provisions made for currents or tidal flows in any of your examples.

In any case, as Joli has pointed out, cruising boats aren't racing boats and most cruisers happily putter along at 4 knots.(not our catamarans of course )

[Joli]

Well, here is what a S/L ratio of 1.8 to 2.2 looks like.

YouTube - Wally 101 Indio volando en popa

Here is what a S/L ratio of 3 looks like. (ie 600 mile day, 25 knot average)

YouTube - No Hiding Place (29/10/08)

Can modern mono's sail beyond a S/L ratio of 1? Yes, easily but the real question is do we want to? If you look at rally results or cruise logs you will find very few cruising boats sailing faster then a S/L ratio of .85.

Want a faster passage? Buy waterline.

[neelie]

Originally Posted by GordMay

Quoting Wiki is for amateurs.

.

Probably.

But its still a substantial repository of FREE of cost information and not to be discounted purely out of intellectual snobbery.

Like everything on the internet, it has to be taken with a pinch of salt. Use the Discussion and history tabs for each article to get a feel for its value. There is sometimes lots of very interesting information embedded there.

[barnakiel]

Originally Posted by GordMay

Wikipedia, is the free encyclopedia that anyone can edit.

Anybody can post on Cruisers Forum too. Does it make quoting from Cruisers Forum amateurish?

Articles in Wikipedia may be well written and insightful, and may provide useful links, but they are not embedded in the world of scholarly discourse.

Not being embedded in the world of scholarly discourse does not imply being amateurish either.

Without knowing who wrote the article, it is more difficult to judge whether the author's writing is worthy of consideration, or to critique his or her motivations or qualifications.

Why judge the author by their name, not by the quality of their work? And please do not forget about the fact that the editors are there and that their work is parallel to Cruisers Forum moderators.

Without a known author, Wikipedia articles cannot be considered authoritative.

You derive authority from having a known father. I will derive authority from correctness and coherence of the information presented in an article.

If the definition of 'hull speed' in Wiki is bad, then anybody can correct it (you too). If it is good, then it does not need to be authoritative.

Would you say that a incorrect article (e.g. an outdated one) from an authoritative source is better than a correct one from an unauthoritative one?

Religious treatises address one’s faith, not intellect.

Wikipedia has been created by people who have faith, AND intellect. It is not as authoritative as religious treaties but it is a way more certain source of information.

Cheers ;-)
barnie

[barnakiel]

BTW

Quoting from YouTube, now this is for real amateurs ...

LOL, wildly ;-)

hugs to everybody and gd nite mates,
barnie

[Chief Engineer]

If one looks at the early speedboats they were long and narrow....My Grandfather had more than a passing aquaintance with Christoper C. Smith....he was one of the guys who used to work on his engines.

[daddle]

This is a goofy bunch of sailors. I sometimes wonder just who here has actually been sailing. Hull speed is an arbitrary point on the speed/power CURVE. Add a little power and you go a little faster. No need to plane or surf. The wave simply gets longer than the waterline. Planing or surfing happens, generally, at much higher speeds. My 7 knot hull speed displacement boat will easily go 8 or 9 just off the wind without any wave push. It start to feel like surfing at 11. If hull speed is any guide it is that a properly sailed boat should go upwind at about that speed in sweet conditions. Complicating the situation is the effective waterline length is anything but the measured length. Boats designed for the old CCA rule had short waterlines at at the dock that magically got really long at speed. Presto. Cheater boat.

[GordMay]

The term “amateur” can have a pejorative ring, but in science, it retains the meaning of its French root "amour"; for amateurs do science without remuneration or reward, for the "love" of it.

The journal Science itself was begun by a famous amateur scientist and inventor. Expelled from school at the age of seven for being "retarded," Thomas Edison was taught at home by his mother. His life changed forever when he found an old copy of Experimental Researches in Electricity by Michael Faraday, himself a talented amateur, and promptly built every project in the book. The rest is history (& myth).

Clyde Tombaugh's discovery of Pluto (whatever it is) ranks among the best known amateur achievements.

A paper on massive storms on Saturn that appeared in Science (1992) was coauthored by Donald Parker, who discovered the storms, and who is famous for his detailed planetary images. Although astronomy is his passion, Parker earns a living as an anesthesiologist.

I submit, that most of us, aboard the CF, are amateurs, as far as most of our discussions go. I know, I certainly am.

[dcstrng]

Originally Posted by GordMay

I submit, that most of us, aboard the CF, are amateurs, as far as most of our discussions go. I know, I certainly am.

Well said, bravely offered…

[Tropic Cat]

Originally Posted by daddle

This is a goofy bunch of sailors. I sometimes wonder just who here has actually been sailing. Hull speed is an arbitrary point on the speed/power CURVE. ......

Of course hull speed is an arbitrary point on the speed power curve. Who's said it's anything else?

(apologies for this graph, it was the only one I could find quickly)

The trouble with your post is that any graph plotted with speed vs horse power in displacement hull vessels, shows that at hull speed the graph goes vertical. It's not a 'little' amount of power required to increase the speed of the vessel, it's a whole lot of horse power for an infinitesimal speed gain.

Unless, of course, the vessel begins to plane. In order for a vessel to plane we have to begin discussing hull loading factors which are completely irrelevant in a sailing forum. Cruising boats with tons of lead hanging from their keel do not plane, they dig a hole when exceeding hull speed.

Your post is wonderfully written and just as wonderfully misleading. Actually it sounds like it was written by a power boater.

Just my opinion

[Joli]

Well, as they say "a picture is worth a thousand words".

Do you disagree with the videos showing the S/L ratio that the boats are sailing at?

Originally Posted by barnakiel

BTW

Quoting from YouTube, now this is for real amateurs ...

LOL, wildly ;-)

hugs to everybody and gd nite mates,
barnie

[denverd0n]

Originally Posted by Randyonr3

A modern sailboat is not planing when heeled at 12 degrees....

Says who? It is by no means impossible for a sailboat to be in a state of planing or semi-planing just because it is heeled over.

Hull speed is hull speed. Boats do not, and CAN NOT, exceed the physical constraints of hull speed without achieving some degree of planing.

Don't believe me, though. Take an introductory fluid dynamics course. This is all basic physics.

[denverd0n]

Originally Posted by Oyvind

Hull speed, per the popular formula, is not a set in stone figure...

Actually, hull speed is, in fact, a set in stone figure.

What is NOT set in stone is the notion that boats with a "displacement" hull cannot exceed their hull speed. In fact almost all boats can exceed hull speed under the correct circumstances--no matter what their hull design--and many boats that appear to have a displacement hull will routinely exceed hull speed.

This seems to me to be where the confusion is coming from. That is, people have read that hull speed is the "maximum" speed that a sailboat can achieve and so they imagine that when they exceed that there must be something wrong with the concept of hull speed. No, hull speed is a well understood, thoroughly tested, and completely documented physical phenomenon. It is simply that the people who have, for years, been saying that a sailboat's maximum speed is its hull speed have been wrong all along. Hull speed never really was an absolute maximum, and with modern hull design it is barely even a constraint anymore.

[denverd0n]

Originally Posted by GordMay

It looks like the issue here is one of semantics, not one of naval architecture, engineering, or science.

PRECISELY!

[s/v Jedi]

Racers plane. We have 7 metric tons of lead under the keel but we can plane. We can start into plane when heeled, but we heel much less once planing. A monohull with a hull-speed of 7 knots isn't just pushing that a bit when doing 15-18 knots... it is planing at that point or the measured speed is SOG instead of SOW.
Basically, everyone in this thread is right but we're all talking about a different boat.

A couple more: cats don't plane because they do not have the flat underwater surface area for that. The formula you use is only valid for the type of hull used for creating that formula. That means a medium to heavy displacement boat without flat underwater sections for the formula in the Wiki. If you have those flat sections (aft) you can plane if you add enough power to propulsion, period. A light displacement boat will need less power than a medium displacement boat.

And for those who think only hulls designed for planing can plane:

and even this will plane:


cheers,
Nick.