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December 9

Lifejackets and buoyancy center

I remember that when I used a lifejacket, it was harder for me to swim because the center of buoyancy at the neck level constantly pushed me in a direction perpendicular to swimming direction, acting as an opposite force. My gut feeling is that the lifejacket should be more like a belt so that the center of buoyancy would be placed lower, allowing more flexibility for swimming. Is is actually a legit concern physics-wise? Thanks. 212.180.235.46 (talk) 18:58, 9 December 2020 (UTC)[reply]

The way I learned how to use a life jacket, (or more precisely, a "personal floatation device"), swimming isn't really part of the purpose.

The goal of the PFD is to keep the survivor at or above water level until rescue arrives.

In almost every single use-case, the survivor should be upright and perpendicular to the water, as illustrated in the US Coast Guard PFD information webpage. In fact, especially in cold water, the guidance is specifically not to swim: "...don't swim unless you can reach a nearby boat, fellow survivor, or floating object. Even good swimmers drown while swimming in cold water. Swimming lowers your body temperature."

...In most of the realistic but hypothetical survival situations on the water, if you could actually reach that nearby boat by swimming, you probably wouldn't have needed the PFD in the first place.

Nimur (talk) 19:06, 9 December 2020 (UTC)[reply]

Some people would panic and drown without the vest, they need it even for a short swim to shore in warm water. Sagittarian Milky Way (talk) 22:20, 9 December 2020 (UTC)[reply]

I don't think your center of buoyancy could be at a neck level. As the linked article defines it, the center of buoyancy is a centroid of the displaced water. So it is a geometric center of the part of body below the surface, whilst the neck should be at the surface level to keep the head above water.... --CiaPan (talk) 21:03, 9 December 2020 (UTC)[reply]

Most corpses are seen floating face down thanks to buoyancy at belt level, see [1]. 84.209.119.241 (talk) 21:52, 9 December 2020 (UTC)[reply]

I don't remember where, but I've definitely read somewhere that early lifejackets were dangerous if the wearer fell unconscious, precisely for this reason. Without a definite effort to keep the head above water, the wearer tended to turn face down. --174.95.161.129 (talk) 23:03, 9 December 2020 (UTC)[reply]

In UK terminology, a lifejacket keeps you afloat when you're unconscious, whereas a buoyancy aid helps you to float but allows you to swim. Therefore, in situations where you're likely to have to swim after falling in (i.e. kayaking or windsurfing), use a buoyancy aid; if you're likely to be in the water for a while (if you abandon a ship or aircraft for instance) use a lifejacket. Ditto if you can't swim. BTW, I'm a kayak coach. On some American films I've seen things that look like a combination of both, so it may be a bit different on the wrong side of the pond. See also Buoyancy Aids & Lifejackets Alansplodge (talk) 00:20, 10 December 2020 (UTC)[reply]

To be more precise: a lifejacket is designed to keep your face out of the water if unconscious. (Briefly mentioned here). Iapetus (talk) 10:57, 10 December 2020 (UTC)[reply]

Quite right, I didn't word my reply very well. Generally a lifejacket supports you in a head-back posture, which is rubbish for swimming except for a leisurely backstroke. Alansplodge (talk) 21:01, 10 December 2020 (UTC)[reply]

The stabilising jackets used by scuba divers are so bad as life jackets that some of them will actively force you into a face-down position at the surface. It's just about impossible to swim any other style than on-the-back, legs only in any of them and this is certainly what I'd try with any other form of bouyancy aid or life jacket. 2A01:E34:EF5E:4640:8848:C97D:6ACB:2E9C (talk) 16:15, 11 December 2020 (UTC)[reply]

The various styles of kayaking buoyancy aids that I have used all allow for a respectable breaststroke as well as backstoke. The requirement to be able to swim strongly in whitewater is rather important to avoid being smashed into rocks and other unpleasantness. Alansplodge (talk) 12:35, 12 December 2020 (UTC)[reply]

Inflatable armbands or "water wings" allow more flexibility for swimming, especially for children under supervision. 84.209.119.241 (talk) 16:24, 10 December 2020 (UTC)[reply]

Why does Richard Bowie Spikes and Jan Ernst Matzeliger have the same image?

Why does Richard Bowie Spikes and Jan Ernst Matzeliger have the same image — Preceding unsigned comment added by 2601:644:4080:9A70:E503:9DF2:FFC6:D638 (talk) 23:57, 9 December 2020 (UTC)[reply]

They don't. Jan Ernst Matzeliger has an image, but Richard Spikes does not. Also what does this have to do with the science reference desk? --OuroborosCobra (talk) 00:44, 10 December 2020 (UTC)[reply]

Are you by any chance referring to a photo or text shown to the right of a Google search? Google's Knowledge Graph uses a wide variety of sources. There may be a text paragraph ending with "Wikipedia" to indicate that particular text was copied from Wikipedia. An image and other text before or after the Wikipedia excerpt may be from sources completely unrelated to Wikipedia. We have no control over how Google presents our information, but Google's Knowledge Graph has a "Feedback" link where anyone can mark a field as wrong. The same feedback facility is also provided on Bing and some other search engines..--Shantavira|^{feed me} 10:16, 10 December 2020 (UTC)[reply]

Yes I strongly suspect the OP is referring to the Google Knowledge Graph which does show the same image for both. The ultimate answer to this is that you'll have to ask Google's engineers why. However it's perhaps not particularly surprisingly since a number of sources seem to use the image we have for Jan Ernst Matzelinger or similar images for Richard Spikes e.g. [2] [3]. The image in our article for Jan Ernst Matzelinger seems to have originated from Encyclopaedia Britannica for our article. Hopefully they are right about the identification, it seems that image has been used for stamps and stuff [4]. Other images for the two are this for Richard Spikes [5] and this for Jan Earnst Matzelinger [6]. Nil Einne (talk) 12:04, 10 December 2020 (UTC)[reply]

Neither of the latter two photos looks to me like they could be of the same person as in the EB photo. --Lambiam 23:03, 10 December 2020 (UTC)[reply]