CD 28 Hull Speed

[bill2]

Methinks you may have had the benefit of going with a nice current . . .

But who am I to say - for all I know you had the rail buried - which lengthened your waterline by a heretofore longer amount - which allowed you to cheat the wind

You might try google for more info - such as below :

"
Are you thinking of displacement hull speed? Displacement hull speed, simply put, is dictated by the waterline length. The longer it is the faster she'll go. When the vessel reaches 'hull speed' it will not go any faster unless a tremendous amount of extra "power" is available to climb the bow wave.

The formula for working out theoretical hull speed is: 1.34 times the square root of the waterline in feet
e.g. The square root of a waterline length of 25 feet = 5 x 1.34 = 6.7 knots. Whereas something with a 100' waterline could reach up to 13.4 knots hull speed (depending on the sail area available), which is why the fast clippers of old were long and lean, with every inch of sail they could possibly hang out.
"

fair winds

[WaywardWind]

"Hull speed" is a not a fact of physics, but rather a 19th century explanation given to British Naval brass as to why doubling the horsepower of a warboat didn't add all that much speed to the boat.

The "explanation", because it was stated by "scientists", took on the appearance of a fact of physics. Yet, it is REALLY nothing more than the speed of a unconstrained water wave of a length the same as the waterline length of the boat in question. The _assumption_, and it is only an assumption, is that a boat "has to climb the bow wave" once it reaches that speed. A couple problems with that. 1.) the boat hull is in no way -required- to "climb the bow wave" (most modern boats don't much, instead kinda plowing through the wave kinda climbing it, in fact Hobie cats -- with their long, narrow hulls -- slice through waves at far above "hull speed", and 2.) the supposed "exponential" increase in power needed "to climb the bow wave" is in fact geometric, meaning very, very little extra power is needed at "hull speed". Geometric, if it were the problem stated and it is mostly not, would not become much of a problem until about 150% of "hull speed". Other issues get in the way far before that. (Geometric becomes truly HUGE at about 4x to 6x "hull speed" IF "hull speed" were a valid concept, which it is not.) FWIW, going 150% of "hull speed" requires 3.37 times as much horsepower WITH NO BOW WAVE AT ALL as going "hull speed", and going twice speed requires 8x the power, for doubling boat speed squares the drag resistance and requires the cube of power, plus a whole bunch of other issues.

Net net, "hull speed" is not science, not a fact of physics, not a limitation it is represented to be. The geometry of a "bow wave" was merely a quickly and scientific sounding explanation given to British Naval brass long, long ago. Most modern sailboats can easily exceed "hull speed". Anyone who says different doesn't have a degree in physics.

[John Vigor]

For those of you who got lost among the exponentials and geometrics of hull speed, let me give you simple expanations from two of North America's best known naval architects:

Dave Gerr, New York, in The Nature of Boats:

"One of the principal factors governing speed for sailboats is length on the waterline. Since the majority of sailboats--especially cruising sailboats--are displacement craft, their top speed is determined as--you guessed it--hull speed in knots (1.34 times the square root of the waterline in feet.)"

Ted Brewer, Canada, in Understanding Boat Design:

"Speed/Length ratio -- Displacement hulls obtain no lift from their speed. Their maximum S/L ratio is 1.34. After that, despite added power, they just dig the stern in and go little, if any, faster. Typical displacement boats are ocean liners, tugs, trawlers, sailboats, canoes and rowboats."

We all understand the a hull can temporarily be thrown forward faster than normal by a breaking wave, but in reasonably still water, if your GPS is showing a speed greater than hull speed, suspect a helpful current -- whether you think there's current or not.

John V.

[Amgine]

I understand the arguments regarding displacement speeds, however we all know that racing boats tend to exceed these rather dramatically. What many people don't realize is that this is nothing new. L.F. Herreshoff suggested 1.36 or even as high as 1.38 for particularly narrow L/B racing hulls. His own Big Ti had extended verifiable speeds even above this multiplier over its racing career, and it's unlikely to be mistaken as a semi-planing modern racing craft.

In several somewhat extreme wind conditions my CD 25D has recorded startling average SOG, including crossing the Georgia Strait E-W in just over an hour, averaging a touch over 7 kt and peak speed of 8.4. There was certainly some current help, but not all the way *across* the strait.

With too much sail for conditions it is clearly possible to exceed the theoretical hull speed over long courses.

[Drew]

"Too much sail" is far more likely to slow you down than add speed (excessive heal). I think the current commentors (pun intended) have it right. GPS is not always pinpoint accurate, either, as the devices don't communicate with satellites continuously - they take a snapshot at short intervals and can lose contact, which will really screw up a reading if it happens while you're tacking the boat.

[Amgine]

Well, I happen to have the data available if you'd like to examine it. Unless you can find some way for the Georgia Strait to have a 2-3 kt current for an hour, I don't think current can explain it. I *can* say that Sand Heads weather station recorded 30+ kt of wind, gusts 40, and I had a double reef and 100% jib, which resulted in 40 degrees of heel, sometimes more, and at least 20 degrees of leeway, until I got out of the channel and was able to knock off the wind a bit, at which point the boat picked up her skirts and flew.

And again, this is not an isolated incident. Please look up some of the course times of Big Ti, and the Bermuda races of the 50s and 60s. You'll find a *lot* of boats somehow manage over long periods of time to outsail their theoretical hull speeds. Maybe it's just magic, if you can't come up with a mechanism.

[bill2]

. . .

But who am I to say - for all I know you had the rail buried - which lengthened your waterline by a heretofore longer amount - which allowed you to cheat the wind

You might try google for more info - such as below :

"
. . .


The formula for working out theoretical hull speed is: 1.34 times the square root of the waterline in feet
e.g. The square root of a waterline length of 25 feet = 5 x 1.34 = 6.7 knots.

. . .

"

fair winds

Your cd 28's lwl of 22' 2 1/2' gives you your hull speed of 6.3 - when sailed " on her feet " . Ahhhh - but you say you had the rub rail buried . . . . hmmmmm

When heeled ( Alberg designed to racing rules then ) your lwl lengthens - for discussion lets say to 25' . Checking the google calculation quoted above gives you a speed of 6.7 without a current - current of course adding or not as the case may be.

Could be more complicated - but maybe we can get by with a simple solution here . . .

enjoy your high speed sails while you can - you may find that like some you enjoy going out when there's a small craft advisory

[darmoose]

For what it's worth... I'm with John V. and Dave Gerr....after all he's the guy who recognized early on, the benefits of allowing your propeller to freewheel if you want to go faster.

Darrell

[Steve Zwicky]

Hi John, I'm interested in the swing arm mount you mentioned. I can't seem to find it on Amazon. Can you help? Thanks, Steve

[Andy Denmark]

More than anything hull speed, as defined by the formula, is a designer's benchmark and has nothing much to do with maximum attainable speed. My understanding from designers is that "hull speed" is a rough means of comparing one hull design to another and is a rough indicator of wave-making resistance. Most modern boats will routinely exceed this number.

In reality "hull speed" should be a unitless quantity but because it has the word "speed" in it some genius decided it must be in knots.

Comments:

(1) Many folks think heeled over with the rail in the water is fast. That's decidely wrong. It just feels that way (to them, anyway). A Cape Dory with 12 - 15 degrees of heel is close to its designed configuration upwind (per Mr. Alberg). The concept that the erstwhile "increased waterline" when heeled makes the boat faster is erroneous.

(2) Except with spinnakers, too much sail area will generally slow you down. I'd rather have a smaller, well shaped and set headsail that a larger one that overpowers the boat

(3) My Moore 24 and Cape Dory 27 have equal LWL's. The Moore can easily double the max speed of the Cape Dory. Of course, the Moore is planing at the 18+ knot speed. What it takes is 20+ kts wind, beam reach, chute. lots of weight on the rail. On rare occasions in flat water and with the right sail combination, we can plane the Moore going upwind at 7+ kts, a speed more often seen with 33 footers. At 2,100 lbs (1,100 in the keel), the Moore is lighter than the CD-27 by a factor of better than 1:3 and that boils down to a significant power-to-weight ratio difference.

FWIW
________
Roll blunts

[Oswego John]

There is another factor to consider when boiling along on a white knuckle passage with the lee rail buried.

Whether this is true or not, many years ago I was told that the greater the angle of heel, the less the amount of lateral resistance given by the keel.

O J

[WaywardWind]

I ask which schools you guys got your BS in Physic from?

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

Damn! Who were those clowns who voted in those Laws of Physics?

[tartansailor]

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

So do mechanical engineers, chemical engineers, and rheologists.

Dick

[bill2]

At the risk of perpetuating another freewheeling-v-fixed controversy I submit the following ( rather lengthy ) quotes as I only read 'em and repeat 'em

From Practical Sailor

The late Carl Alberg designed the 35 as a coastal cruiser. With LOA of 34’9" she has a short waterline of 24’0" and moderate beam of 9’8" good proportions for slipping along in light air. The waterline lengthens when the hull heels, and the boat foots nicely in a breeze.

. . .

For this he drew a 37-foot yawl. It compares to the 35 as does a thoroughbred to a Shetland pony, both from a good stable. The Alberg 37 raced under the CCA rule and the design has long overhangs and a short full keel ending with a raked rudder to reduce the wetted area. Showing the typical Alberg moderation of basic ratios, a slim, slippery hull

From Ted Brewer's Primer on Yacht Design

DISPLACEMENT/LENGTH RATIO: The D/L ratio is a non-dimensional figure derived from the displacement in tons (of 2240 lbs) divided by .01 LWL cubed, or, Dt/(.01 LWL)3. It allows us to compare the displacement of boats of widely different LWLs. Some examples of various D/L ratios follow, but are generalities only as there is often a wide range within each type.

BOAT TYPE D/L RATIO

Light racing multihull 40-50

Ultra light ocean racer 60-100

Very light ocean racer 100-150

Light cruiser/racer 150-200

Light cruising auxiliary 200-250

Average cruising auxiliary 250-300

Heavy cruising auxiliary 300-350

Very heavy cruising auxiliary 350-400

STORM, a wonderful 27' LWL sloop on which I raced with Bill Luders many years ago, had a D/L ratio of 386 so she would be considered very heavy by today's standards. However STORM was 39' LOA and when she heeled to a breeze her long ends would increase her sailing LWL, thus reducing her D/L ratio to a more reasonable figure when we were beating to windward. If she picked up 3 feet of WL her D/L ratio dropped to about 281, a significant change, and one that made her a very competitive racer in the 1960s.

Rodney Johnstone of J Boats

Where does performance come from? Length is most important. Average speed in knots for the typical sailboat is roughly equivalent to the square root of its waterline length. Hence a boat with a 36-foot waterline length should sail at about 6 knots under "cruising canvas"; a performance-cruiser should be able to exceed this pace in all but very light winds.

the Boatbuilding Community

HSPD = Hull Speed (top)
HSPD = SQRT(LWL)*1.34
This is the theoretical hull speed for a displacement hull (like most sail boats). It is a function of the length of the wave created by the boat as it moves through the water. Wave speed is a function of wavelength, longer wavelength is faster. Longer boats make longer waves. Since longer waves are faster boats that make longer waves are faster [Garrett]. The hull speed may not be as precise a figure as the formula leads you to believe (1.34 sounds pretty precise doesn't it?). LWL is not static. As the boat heels it can increase. Older racing boats with long overhangs used this to get some extra un-measured LWL to beat the rating rules of the day. As the boat goes faster the bow and stern are immersed deeper in the wave made by the boat. This also increases waterline. In the 19th century sailing ship speeds were often expressed with a hull speed factor. A ship might have a speed factor of 1.17, meaning it can make a speed of 1.17 x sqrt(lwl) [Chapelle]. There are conditions where you can exceed the theoretical hull speed. Surfing down a wave for example. I have had my boat (LWL=21.6 feet, HSPD=6.25) surfing at 10 knots on waves. When surfing or planing the hull is not in a displacement mode. So hull speed may be a bit of a moving target.

Let the thread freewheel to find a fixed answer !

[mattlydon]

On my CD28, the paddle wheel sticks out the hull on the starboard side just forward of the v-berth bulkhead.

I find that under power, I typically show 5 knots, maybe 5.2 if I go max throttle, but she's really just digging in at that point. So that 5.2 should REALLY be the theoretical hull speed of 6.3.

When sailing in brisk weather, I find I can typically get it to read over 7 on both tacks - it seems to read a bit faster on starboard tack - and last month I hit some squalls on the tappan zee that had me reading 8 knots, with the rail -just- buried, a reef in the main and about 90% on my roller-furled jib.

I haven't really had a chance to calibrate the knotmeter using a GPS, as on the hudson the current is strong and will greatly effect the speed readings, but even allowing for the off-center paddlewheel, a 3 knot difference between WOT and hauling ass on a close reach seems excessive.

I typically don't worry about the overall accuracy of the instruments, as I don't really navigate traditionally much - I use the relative gain/loss to figure out best trim for a given condition.