CD 28 Hull Speed

[WaywardWind]

Steve, the answer to your question is this:

The "calculated speed" is equal to the propagation rate of an unconstrained water wave of infinite fetch which length is equal to to the waterline length of the boat in question.

That's it, in its entirety. Nothing more.

Well, there WAS more in that it satisfied mid 19th century British Naval brass, who didn't have a clue as to what they had heard.

[John Vigor]

Simple sailors who don't have degrees in science might like this down-to-earth explanation of why heavy-displacement sailboats like Cape Dories don't sail faster than their theoretical hull speed:

http://www.cncphotoalbum.com/technical/ ... lspeed.htm

Cheers,

John V. (BS, Calcutta, failed)

[WaywardWind]

I have a hard time wrapping my arms around the denial of the existence of tens of thousands of 10- and 12- and 15-knot Hobie cats "because they can't go that fast." They DO go that fast and there are thousands and thousands and thousands of them, deep-vee displacement hulls every last one of them.

Every year, on December 16th, the Man Will Never Fly Society meets for dinner in a small town in North Carolina, secure in its "knowledge" that while birds most certainly can fly, man is not capable of it.

[WaywardWind]

Ah, John? You merely restated the same wrong argument once again.

The premise of that simple, but wrong argument is that displacement boats "ARE REQUIRED TO CLIMB THE BOW WAVE". There is no such law in the world of physics. In fact, **ALL** (displacement) boats (even barges) to "some degree" __slice through__ the waves, and the narrower the beam of the boat relative to its length, the greater the "slicing" action. Hobie cats -- with their long narrow hulls -- annihilate "hull speed". Nacra's, with their better hulls, handily beat Hobies.

Nothing special happens to a boat at "hull speed" that hasn't been happening all along. Even the geometric (not exponential, as is usually claimed) effects of "climbing the bow wave" (such that any effects exist at all) ARE ABSOLUTELY ZERO at hull speed (the sine of zero equals zero), and increase very, very, very, very, very, very, very slowly at first, and only get "out of hand" at speeds 10,000 to 100,000 to 1,000,000 times hull speed.

[John Vigor]

Dear Wayward:

Well, never mind about Hobie cats and things.

What does British naval brass think about propellers on sailboats?

Do they lock their props or let them spin free?

Cheers,

John V.

[WaywardWind]

Don't know about props vs British brass, but the U.S. Federal Aviation Administration has oodles of data from today and going back to the days of the CAA to show that twin engine aircraft with a dead engine are considerably better off feathering the dead engine rather than let it freewheel. I have not personally seen a required-to-be-onboard Aircraft Flight Manual in quite some time, but I believe AFM's for multi-engine propeller driven aircraft list both the Glide Ratio's of both freewheeling props (less) and feathered props (greater). I am sure a call to Cessna's PR dept would get you that info in a few minutes. Certainly, all student pilots are taught from the get-go to make even considerable effort to stop a freewheeling prop from a dead engine to increase the glide ratio of the aircraft.

[darmoose]

Does anyone else notice that when someone steps up to claim the virtues of "proplocking", they invariably ignore anything to do with water, and begin to explain airplanes and perhaps helicopters, or appleseeds, or whatever else that never sees a wetted surface!!

Why is that???

Dear Mr Wind

Do they teach in physics to completely ignore the question at hand and talk about airplanes? Are there no "physics" in the water?

I thought you were going to share some wisdom and explain how the Strathclyde Ocean Engineering and M.I.T. people got it all wrong, Cannot you do that?

Darrell

[Steve Darwin]

John,

Thanks for the link. The way boats behave when under tow seems to confirm the wave-resistance theory.

Steve

[WaywardWind]

Darrell, in physics, fluid dynamics is known as fluid dynamics. Physics is strange that way.

Steve, in the world of computers there is an acronym that goes GIGO. The Pop Lit term "hull speed" has not a thing to do with "wave theory". The basic assumptions of what is erroneously known as "hull speed" by Pop Lit writers are obviously incorrect from the get-go (the term "intuitively obvious" comes to mind). There is NO requirement that a boat MUST "climb the bow wave" rather than slice through the wave. There is no "theory", not even a hypothesis. Just a junk explanation to British Naval brass a 160 years ago.

BTW, the constant, 1.340, has such a finality to it that most people assume (erroneously) that the term results from thousands of empirical observations. Such is not true, there were no such "observations" to come up with the (nifty sounding) number of great, two-decimal precision. The constant is **NOT A THING** but the number left over from the calculation of a wave speed for a given X wave length IF THE WATER DEPTH IS SUFFICIENTLY GREAT (i.e., greater than 1/4 the wave length) **AND** UNLIMITED FETCH.

Hard science "hull speed" is not. Soft science it is not. Hokum, hoolium and phoolium it was to hoodwink British Naval brass back from a time before Lincoln was a congressman from Illinois.

[John Vigor]

Dear Wayward:

I read somewhere that British Naval Brass was 80 percent copper and 20 percent zinc. Or was that British Naval Bronze? Never mind, I'm just wondering if the resulting electrolysis could have affected the outcome of the experiments.

John V.

[WaywardWind]

Ya kno, I never done consider that. 20% zinc, ya say? Maybees dat itz? Shore duz sloz downs a boat. Much worserer din Dinty Moore stew, dats fer shure.

[darmoose]

BTW, aeronautical engineers have known for 70 some years that free wheeling props have more drag than stopped props.

Dear MR. Wind,

Same old.... Same old...foolish for us to expect anything different.

Darrell

[WaywardWind]

Darrell, my boy, throw a pinch of salt over your left shoulder, don't leave your batteries on a cement floor, paint no boat green, and start no voyage on a Friday.

Amazon dot com has hundreds of books on astrology for sale.

Astrology or not, Hobies cats by the thousands across the country annihilate "hull speed" every nice Sunday afternoon, and Nacra's go even faster.

[Oswego John]

Darrell, my boy, throw a pinch of salt over your left shoulder, don't leave your batteries on a cement floor, paint no boat green, and start no voyage on a Friday.

I hope that this doesn't come to the point of inferring, or the nay-saying, of the value of knocking on wood.

BTW: Yikes, I may be in a peck of trouble. One of my Tys is painted "Seafoam Green".

As Captain Commanding of my Ty, I strongly enforce the rule of no one opening an umbrella inside the Ty's cuddy cabin.. You gotta be firm but fair.

Aaaarrgh,
O J

[Andy Denmark]

Here's what's NOT foolish ---

Sailing on a comfortable reach with the boat on autopilot put the GPS next to the gearshift and observe what happens when the tranny is put in gear (prop locked) and taken out (prop freewheeling). The consistent difference and direction of the hundredths digit on the GPS tells the story - it winds up, it winds down, and it does so every time. Perhaps some of the "experts" here can try this for themselves and report back. I won't give the secret away as some folks wouldn't believe me anyway. (P.S. A video of this in real life would be telling and a great posting.)

Nowhere in all this so-called scientific conjecture is swept area taken into account. That's what slows aircraft down with the prop freewheeling and it holds true with any fluid (water is still a fluid, I think).

Now I'm going to calculate the "hull speed" of a 20 ft, 2 inch foot length of string. Hmmmmm .... it's the same as my CD-27. Then I might try a python of the same length, a piece of PVC pipe and a 2 X 4. Interesting. Same result.

Guess it doesn't matter all that much ----- and what if it did?
________
Bmw e60