All,
I've seen the light on the issue of displacement. I was thinking in terms of the weight of the boat equalizing with the water and not the weight of the water being pushed aside (displaced) by the boat. That settles my cognitive disconnect! Thanks, Jim _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Kevin Morton Sent: Thursday, August 14, 2008 5:00 PM To: [email protected] Subject: Re: catalina27-talk: Hull Speed and Displacement(s) Even with this revised hull speed, I can still look with pride at hitting 7.5kts on a broad reach. Not bad for someone that is self taught, and has had the boat out exactly twice at that point, eh? By the way, not all Californians are "svelte", as I can serve as a great example of that. I don't skinny dip, I chunky dunk. lol That aside, I LOVE my 1974 C-27 standard rig, fin keel. I hit that 7.5kts with a 110 Genoa and and a full batten main with no reefs tied in. Kevin M. Morton S/V Serenity #1920 _/)_ --- On Thu, 8/14/08, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: From: [EMAIL PROTECTED] <[EMAIL PROTECTED]> Subject: Re: catalina27-talk: Hull Speed and Displacement(s) To: [email protected] Date: Thursday, August 14, 2008, 2:06 PM George Well done. But just to make it a bit more interesting: At rest (no bobbing please) the weight of the water "displaced" of a ship or hull is EXACTLY equal to the weight of the boat, its outfit (sails, safety equipment, and other normal fittings), and its deadweight (crew, fuel, water (beer), and other items "carried"). Note that merchant ships, most notably bulk carriers and tankers, often have their size quoted in deadweight tons. That's the amount of stuff they can carry, not the total weight of the loaded ship. We all recognize what happens is as more weight is brought aboard - the volume needs to be bigger, so the boat sinks further into the water, displacing (important verb that) more water until equilibrium is reached. The boat's draft (measured from the lower reference - usually the bottom of the keel) also increases. So why the explanation? Well it turns out that displacement (weight) quoted for design purposes is derived from either the expected weight of the boat and stuff, or from the volume displaced by the hull (actually don't know if salt or fresh - ship design standard is salt water at 64lbs/cuft ) when floating at the intended (aka design) waterline. When Frank Butler designed the 27, I'm sure the numbers matched, assuming a given engine, standard outfit (cushions, etc), and crew (probably 4 svelte Californians). With every change in the boat (think taller, heavier mast, different engine, dinette vi ce traditional, etc) the actual weight changed, but the draft quoted stayed the same, 4'. Probably not worth changing the brochures (it takes about 600 pounds to "sink" a C-27 one inch). Since the molds and hull form didn't change, the differences in displacement quoted on the various brochures (6550lbs in 1974 - 6850lbs in 1980) probably do reflect added weight in hull and outfit. Those values represent the designers best guess at operating weight. Singlehanding, it's less, unless you're Mr Childress, whose upgrades necessitated changes to the hull form so the would displace more volume at close to the design waterline. So, what's printed is the design displacement, quoted as weight in pounds. For the original design, that weight matched the volume of (probably salt) water displaced by the hull and appendages (rudder and keel) when immersed up to the design waterline (at which draft = 4'). Looks like the reported displacement was changed in later years to match the weights as the construction and outfit matured. That will match the weight of the displaced water, but in reality the draft will be from 1/2 to 1" deeper than design for the later boats (not to mention even deeper in fresh water, or less in Great Salt Lake). The fact of the matter is that hull speed is a fungible concept. It turns out that effective waterline length, immersed volume, and a host of other parameters change with weight changes and draft changes, all of which affect the amoun t of resistance the hull has to forward motion, and thus "hull speed". Throw in changes due to heel angle, trim, and the dynamic forces on the hull and we might as well settle for the old standby which says that without surfing a displacement vessel is limited to V = 1.5 x SQRT(LWL). That's 6.99 knots for a C-27. Almost never see it, but that's what keeps us trying :>) Peter Z Snagglepuss #2622 -----Original Message----- From: George R. Wiltsie <[EMAIL PROTECTED]> To: [email protected] Sent: Thu, 14 Aug 2008 10:28 am Subject: Re: catalina27-talk: Hull speed Gentlefolk - Perhaps it is best to resort to an old fashioned tried but true approach. . . . . looking it up in the dictionary. . . . . . The Oxford Dictionary states the following: displacement . noun 1 the action or process of displacing. 2 the amount by which a thing is moved from a position. 3 the volume or weight of water displaced by a floating ship, used as a measure of the ship's size. 4 Psychoanalysis the unconscious transfer of an intense emotion from one object to another. Putting aside for the moment whether or not we should be using definition 4 since we all own holes in the water into which we throw money, definition 3 appears the most appropriate. It also establishes that there are two correct answers "volume" and "weight" each of which is independant of the other since they are not the same thing. Nothing weighs a cubic foot, nor does anything fill up a pound. Now that we have explained how an aircraft carrier floats, can we move on to my little phobia, why jetliners that are really nothing more than the equivalent of huge bricks don't fall out of the sky, but instead fly. . . . . . George <http://www.askoxford.com/dictionaries/compact_oed/> ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] <mailto:[EMAIL PROTECTED]> Sent: Thursday, August 14, 2008 8:54 AM Subject: RE: catalina27-talk: Hull speed It looks like people are getting displacement, density and bouyancy mixed up. Any object that has a total density less than water will be bouyant and will float. The weight of the water displaced by the object will equal the weight of the object. Objects that are more dense than water will not float and will not displace as much water as their weight. Looking at it from another direction, if you take a cubic foot of water and a cubic foot of another object (lets say a piece of wood) and weigh each, any object with the same volume that weighs less than the water will float. The hull of our boats create a very large volume (mostly filled with very light air and other things that are less dense than water) compared to the total weight of the boat, so the density is much less than water, and our boats float instead of sink. Fair winds -------------- Original message -------------- From: "Joe McCary" <[EMAIL PROTECTED]> I don't think that is how the displacement measurement works. My understanding is all floating or non floating bodies have displacement. When those bodies are placed in or on the water a certain amount of water is displaced. Example, fill a bucket to the absolute brim and float a piece of wood in the bucket; some of the water spills over the rim of the bucket. The water that over flows is equal to the displacement of the wood now floating. The same is true if the object is a toothpick or an aircraft carrier; the water pushed away by the placement of the object in or on the water is the displacement. The steel aircraft carrier floats because it's hull is deeper that it's displacement. Joe McCary Aeolus II, West River, MD <mailto:[EMAIL PROTECTED]> [EMAIL PROTECTED] Behalf Of Jim Bernstorf I understand the definition of displacement. The logic of that though is that if the 6000lb boat displaces 6000 lbs of water then it theoretically would be sitting with the water at the gunnels of the boat and adding another pound would push the boat under water and sink it _____ It's time to go back to school! 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