CD 28 Hull Speed

[darmoose]

Why..Andy

You sly old dog, you. As silly and as embarrassing as you said you thought this discussion about locking or freeing ones propeller is, you've been out conducting "secret tests". I am just beside myself with admiration!!

You are absolutely correct, a video would be very telling, and Mainsail did one and posted it on this board on 4/17/09.

Keep up the good work!
Darrell

[SurryMark]

CD27 Tula showed 6.6 to 6.8 knots on the GPS yesterday, in a supposedly slack current window ten minutes before and after low tide. About 10-12 knots of wind. Small curling waves, very few small whitecaps. Main and slightly larger than 100% jib. A few points off of close beating on two different tacks, towing a toy scow I made for my grandson. Three adults and 2-year-old on board. What does this mean, in terms of hull speed for a 20' waterline, which should be 5.99 kts. Or should I be looking at the GPS? (Garmin 4208)

[The Patriot]

... 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 ...

This is a nice logical test, but why bother with a GPS? GPS speed is a derived speed based on consecutive position measurements which themselves have inherent errors. Any paddle wheel speedo that reads in hundredths would probably do the job, and the speedo does not of course have to be calibrated to any great degree of accuracy. All it has to do is spin more quickly or spin more slowly, depending on how fast the boat is moving.

[Neil Gordon]

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

The link includes the following: "Hold on! Exceeding max hull speed is possible with outside assistance."

Perhaps it's not so surprising then, that my 9,000 lb Cape Dory 28 can exceed hull speed on the downhill part of a wave. (Does anyone out there have a Cape Dory that exceeds hull speed going UP the wave?

[Neil Gordon]

As a sun man, I can't imagine kissing a moon woman, ...

As a sailor, I simply ignore my own sign and kiss sun women during the day and moon women at night.

[WaywardWind]

"The link includes the following: "Hold on! Exceeding max hull speed is possible with outside assistance." "

.....................................................................................

The Hand of God (also known as "outside assistance") pushes thousands of Hobie cats across the country any nice Sunday afternoon to exceed max hull speed, and do so by VERY wide margins. The silly writer of that term simply is not capable of sticking his head out the window of any yacht club to watch the boats go by.

Please note the stupid, stupid, stupid term "max hull speed". Stupid, stupid, stupid is the term for most every last sailboat designed in the last 90 years GOES FASTER than that term allows as possible.

Let me say this one last time. THERE IS NO LAW OF PHYSICS TO SUPPORT THE SILLY CONCEPT OF "HULL SPEED". NONE. NOT ONE. NADA. ZIP. SQUAT. DIDDLY.

[Oswego John]

Well, I'm glad that's finally settled.

I'm not taking sides in this thing so don't anyone start to read anything into the following that isn't there.

You guys got me thinking, (and I get headaches when I do that.) I have often wondered what would be the result if I was sailing under full sail in a stiff breeze and going at (do I dare mention) max hull speed. At that moment in life, I fire up the iron genny at full RPM.

So what happens? Do I go faster? Do I remain (you should pardon the expression) at the same max hull speed? Does my stern sink lower into the wake and my stem rises upward while trying to climb over the increasingly higher bow wave being made?

What's the real skinny? Where is the #@&% Tylenol?

O J :

[WaywardWind]

You'd go faster, and by some predictable amount.

In fact, square rigged sailboats used to do exactly as you say, except they put up more sail (rather then turn on an engine they didn't have). It turns out, quite a number of those old square rigged boats (with way too much sail up) kept increasing their speed until they sunk lower and lower in the water until they actually sailed deep enough into a wave to be flooded over the sides and sunk. A number of square rigger boat were lost this way (according to Royce, who as a young man, interviewed some of those old timer square rigger sailors who survived "sailing under" even if the boats did not). Some of those square riggers were known to average upwards of 18 to 20 knots for some periods of time (l180 to 223 feet or greater waterline length (most were less than 100' lwl, with a few longer), if we are to believe they were not capable of sailing beyond "max hull speed").

[John Vigor]

"The link includes the following: "Hold on! Exceeding max hull speed is possible with outside assistance." "

.....................................................................................

The Hand of God (also known as "outside assistance") pushes thousands of Hobie cats across the country any nice Sunday afternoon to exceed max hull speed, and do so by VERY wide margins. The silly writer of that term simply is not capable of sticking his head out the window of any yacht club to watch the boats go by.

Please note the stupid, stupid, stupid term "max hull speed". Stupid, stupid, stupid is the term for most every last sailboat designed in the last 90 years GOES FASTER than that term allows as possible.

Let me say this one last time. THERE IS NO LAW OF PHYSICS TO SUPPORT THE SILLY CONCEPT OF "HULL SPEED". NONE. NOT ONE. NADA. ZIP. SQUAT. DIDDLY.

Whoa there, Windy One!

Those hundreds of thousands of Hobie Cats you keep on going on about are not heavy-displacement boats like Cape Dories, which are subject to the theory of maximum hull speed -- namely, that the speed of a wave in deep water is 1.34 times the square root of its length between crests. That's a law of physics.

Nobody is saying that light-displacement craft and multihulls are subject to the law of physics that restricts Cape Dories to the speed of the wave their hulls dig in the water. Nobody is saying that all boats are subject to the hull-speed law.

You'll notice Bayliners rushing around pulling half the ocean behind them and using fuel at a ratio of gallons to the mile because they are so-called semi-displacement hulls, which can exceed the maximum hull speed because they derive some lift from their flatter underwater sections -- but it's at the cost of huge horsepower increase and fuel consumption.

But true heavy-displacement hulls like Cape Dories are subject to the hull-speed law, and the only time they can exceed it -- and then by tiny amounts -- is when they are grossly overpressed under sail or when they are being thrown forward by breaking waves.

Hobies don't make a wave to fall into, and planing sailboat hulls get dynamic lift from their flat underwater sections that enables them to climb up and over the front of their bow wave, and plane away at high speed.

As for OJ's query, there is no certainty that help from the engine will make you go any faster if you're already doing hull speed under sail. Your propeller is geared to produce the number of revs to achieve that speed, and it would simply be keeping up with the water flowing past. It wouldn't be adding any push.

Cheers,

John V.

[Neil Gordon]

As for OJ's query, there is no certainty that help from the engine will make you go any faster if you're already doing hull speed under sail. Your propeller is geared to produce the number of revs to achieve that speed, and it would simply be keeping up with the water flowing past. It wouldn't be adding any push.

Keeping up with the water flowing past is what the fabled spinning prop does (or tries to do). So revving the motor to about "break even" RPM would most likely slow you down.

[WaywardWind]

John, stick to writing. You're good at that.

The rebuttal to the 19th century placebo "hull speed" was FULLY explained in terms a more or less average sixteen year old kid could understand early in this thread. There is no value in going over it again. Those who find emotional value in astrology will always seek comfort in the alignment of Jupiter with Mars.

[Neil Gordon]

Those who find emotional value in astrology will always seek comfort in the alignment of Jupiter with Mars.

I don't know about Jupitar and Mars, but the Moon and Venus were in near perfect vertical alignment on Friday morning. Is that significant or is it more telling if it leans one way or the other? (The Moon was on top, if that makes a difference.)

[WaywardWind]

I am going to new drop a slow-breaking, hanging curve ball into the mix, and then whack it out of the park into the upper deck.

Ask a hundred thousand people -- including most people on the Cape Dory owners site, which some claim is the arbiter of all things scientific -- if it's a proven fact Earth goes around the sun, and nearly all will look at you like you have three heads. Push those hundred thou people for an answer and, again, nearly all will tell you, Of course!

Nearly all? Nearly? Who the heck wouldn't agree?

Well, no physicist would agree. Nor would anyone else with an even rudimentary knowledge of the Theory of Relativity.

Huh?

Yeah, huh. Plain and simple, if two or more objects SEEM to be moving RELATIVE TO EACH OTHER one simply has no possible way to know which is moving and which is not. Not if the speed of light is constant (and it is).

That is why one can not accurately navigate in a fog using just a compass and a knot log.

Go ahead, tell me the Theory of Relativity doesn't apply to sailboats (on the water), and "hull speed" is a Law of Physics (capital letters).

[darmoose]

"The link includes the following: "Hold on! Exceeding max hull speed is possible with outside assistance." "

.....................................................................................

The Hand of God (also known as "outside assistance") pushes thousands of Hobie cats across the country any nice Sunday afternoon to exceed max hull speed, and do so by VERY wide margins. The silly writer of that term simply is not capable of sticking his head out the window of any yacht club to watch the boats go by.

Please note the stupid, stupid, stupid term "max hull speed". Stupid, stupid, stupid is the term for most every last sailboat designed in the last 90 years GOES FASTER than that term allows as possible.

Let me say this one last time. THERE IS NO LAW OF PHYSICS TO SUPPORT THE SILLY CONCEPT OF "HULL SPEED". NONE. NOT ONE. NADA. ZIP. SQUAT. DIDDLY.

Dear Windy,

Take a chill pill. People on this board are not impressed by repetition nor yelling nor invoking God.

Seems to me John makes a very solid argument regarding heavy displacement hulls vs lighter, or planing hulls like your Hobie cats.

We all know that in regard to O.J.s query, if you are sailing at maximum speed for your boat, turning on the engine will do nothing for you, until your propeller surpasses the freewheeling RPMs relative to your boats speed(assuming no drivetrain friction), at which time propeller drag becomes thrust. The problem,however, is that most of our boats engines and propellers are configured to max out at "hull speed", and so will not surpass the RPMs necessary to exceed "hull speed" Any RPM less than what I've described has no effect, so if O.J. is sailing at hull speed, he will continue to do so.

And by the way, for Neils benifit, one cannot achieve "hull speed" with a locked propeller.

As to your physics of the planets question, if the Sun is moving around the Earth, does its orbit emcompass all the other planets, or does it go between earth and all the rest, or just exactly which planets does it go between. I think our celestial observations would be quite diferrent if the "possibility you suggest were indeed even possible. The Earth does indeed go around the Sun, whether you physicists think so, or not.

Where did you say you got that physics degree?


Darrell

[darmoose]

I am going to new drop a slow-breaking, hanging curve ball into the mix, and then whack it out of the park into the upper deck.

Ask a hundred thousand people -- including most people on the Cape Dory owners site, which some claim is the arbiter of all things scientific -- if it's a proven fact Earth goes around the sun, and nearly all will look at you like you have three heads. Push those hundred thou people for an answer and, again, nearly all will tell you, Of course!

Nearly all? Nearly? Who the heck wouldn't agree?

Well, no physicist would agree. Nor would anyone else with an even rudimentary knowledge of the Theory of Relativity.

Huh?

Yeah, huh. Plain and simple, if two or more objects SEEM to be moving RELATIVE TO EACH OTHER one simply has no possible way to know which is moving and which is not. Not if the speed of light is constant (and it is).

That is why one can not accurately navigate in a fog using just a compass and a knot log.

Go ahead, tell me the Theory of Relativity doesn't apply to sailboats (on the water), and "hull speed" is a Law of Physics (capital letters).

Windy,

REALLY? ....out of the park?? Please see above.

Darrell