I am contemplating a high jump into water.

Close to where I live is a cenote. From ground level to the water below is 18 meters. The depth of the cenote is 160 meters. So hopefully I won’t hit bottom. My calculus days have been forgotten. I would like some answers, so I have fewer unknowns before attempting this action. I am 65 years old, a good swimmer and weigh 120 Kilograms. If some of you math/physics folks could help with these questions, I would appreciate it.

  1. How is the best way to enter the water? I am not planning on a swan dive. Going to enter feet first. I have done some jumps, and when I entered with feet flat it stung. So, I am thinking to point my toes downward. Or, would it be better to wear shoes?

1a. Should my arms be straight up over my head? I am thinking that a smooth entry would be better, right?

  1. How many seconds will I be flying? Before entering the water?

2a. Speed upon impact?

  1. What would be the maximum depth of my penetration in the water?

  2. Is there other information I should know?

For math folks and others, this question.

  1. Is this a really dumb thing to attempt?

Thank you for your time and your calculations.

Your speed will be about 18.8 m/s (or 42 mph, or 68 km/h). It will take just under 2 seconds to hit the water. The maximum depth will vary based on how you hit, but I’m sure someone else will come along with some estimates.

As for if it’s a dumb idea, I have to say yes. I have a friend who broke his tailbone on a 20 meter jump - he hit the water a little too much on his butt. He was in the hospital for a week and physical therapy for long after that. If you insist, make sure you have a good way to get out of the hole. If your exit plan consists of climbing a rope or ladder, are you going to be able to do that if you injure yourself on the drop?

t= sqrt(2s/g) , where s is the height above the water surface you will be starting from, g is acceleration due to gravity (Assuming you have no velocity at start and neglecting air resistance)

t = sqrt (18/9.8) = 1.4 s

v=sqrt(2gs) = 18.8 m/s or 68 kmph

more here http://boards.straightdope.com/sdmb/archive/index.php/t-123440.html

I am not a high-dive expert, but I am a mechanical engineer with a good theoretical (and visceral) understanding of fluid mechanics. My answers:

  1. Best entry is (IMHO) feet first, with toes pointed down. I would avoid wearing shoes, as they may add surface area that causes a “ruddering” effect that will force your feet and legs out from under you as they enter the water. You could go head-first, but you’d want to arms/hands in front to prevent a direct impact between your skull and the water. I wouldn’t recommend head-first entry unless you’ve had a lot of practice doing so from lower jump points and worked your way up to your 18-meter jump.

1.a. Assuming a feet-first entry, I seem to recall some Boy Scout training that recommended covering your testicles with at least one hand to mitigate the potential for injury upon impact with the water. If not that, then I’d suggest both hands up overhead, assuming you aren’t twirling them madly to maintain a vertical orientation as you plummet through the air.

  1. Expect a 1.9-second freefall.

2.a. Speed at impact, 42 MPH, assuming no aerodynamic drag. For a feet-first orientation at this speed, there really won’t be much aerodynamic drag compared to your weight, so this is pretty accurate (as is the freefall time in 2.a).

  1. For a given speed, hydrodynamic drag is about 800 times aerodynamic drag (owing to water’s density being 800 times that of air). Terminal velocity in air for feet-first orientation is roughly 160 MPH, although this assumes an 80-kg human being. So in air at 80 MPH, there’s (42/160)[sup]2[/sup] * 80 kg = 5.5 kg of drag. So if you move through water at 42 MPH, you’ll experience a drag of 4400 kg, or 9700 pounds. Don’t freak out yet, things aren’t as bad as that raw number suggests. For starters, this wouldn’t all be imparted at your toes and feet; it would be distributed over any portion of your body that is tilted even slightly downward. Your toes and feet will probably experience more than 120 kg of drag as soon as they hit the water though, which means you’ll immediately begin decelerating; you’ll be going a lot less than 42 MPH as the rest of your body is submerged. Moreover, your feet/toes will initially displace a lot of water sideways, so it won’t be there to exert drag force on the rest of your body right away. So you’ll never experience 9700 pounds of decelerative force. It’s pretty difficult to estimate your depth of penetration below the surface, especially since you could do pretty much anything once you’re underwater (splay your legs for braking, rudder yourself into a horizontal trajectory, etc). I would guess you’re not going to penetrate more than 15-20 feet unless you willfully maintain a vertical orientation and keep your feet/legs together and your arms straight up. Acapulco cliff divers fall 35 meters into water that’s no more than 3 meters deep, but I’m guessing they’re pretty deliberate about braking immediately after they enter the water.

  2. It’s not something I would try, at least not without a number of practice jumps from progressively higher and higher platforms. The gutter of a two-story house is about 20 feet up, which is as high as I’d feel comfortable jumping without trying anything lower first. Olympic platform diving is from 33 feet (10 m); 59 feet (18 m) is a damn long way up.

Little correction - you had the formula right, but neglected the “2” when you plugged in the numbers. The time is 1.9 seconds.

On the Westwater section of the Colorado river, there’s a spot where you can jump off a cliff into the river, there’s various ledges, so you can pick how high you want to go. A guy jumped from the very top, about the height you’re contemplating. Problem is that he Burst his scrotum.

chacoguy wins the thread, but I seem to recall reading somewhere that there is also some risk of water being forced in through the back door at an uncomfortable speed and volume.

Why?

That took balls.

My brother took a dive off a bridge, as a teenager, and nearly broke his jaw,because his mouth was slightly open when he hit the water!

Think harder, please!

I’ve gone off 10 meter platforms in pools, and done cliff jumps in the 15-18 meter range. Granted that was a while back. I also used to be a competitive diver so I’m used to being in the air and controlling my body falling towards water.

It’s a dangerous jump as others have mentioned. If you mess up, even slightly, bad things can happen. Entering the water as close to vertical is important, toes pointed is good, I’d fold your arms across your chest - hands at shoulders. I don’t think shoes would matter much for the entry but they can make your launch much more comfortable and secure. You really don’t want to slip before you leap.

I’d definitely try it at a lower height to work out the kinks before you try this one. Even finding a pool with a 5, 7.5, and 10 meter platform will give you the feel of things.

Finally, 18 meters is a long way up. It’s pretty scary that far off the water.

Is this on your “bucket list”? As others have said, build up to it. No substitute for skill level.

BTW I am 62 and have done this with no problems, but Results may vary.

What is considered a normal comfortable speed and volume? I know mine is 0 and 0, rounded up, but I’m not sure if that’s typical,

Minor disagreement from a above post, wear shoes. I have not jumped for that height, but have done lesser stuff that footwear was highly recommended. If you are male be aware of the testies (balls) as they can take a very hard smack and you will not be happy.

Breath out before the jump and in on the way down- too long to breath in before.

Get a person to help you who has done this before.

Get a person to help you who has done this before.

Get a person to help you who has done this before.

What is really important is the last 3.

I’d ask how confident you are in your 18m height measurement. If that’s 100% accurate you’re looking at one set of risks. If that’s an estimate that turns out to be 3 or 4 meters short, you’re looking at a significant increase in risk.

Compression shorts will help protect your “boys.” That doesn’t mean they’re safe, but it will help.

When I was 16 I jumped off the 32’ high dive at a local lake. The water under the diving
raft was about 35’. I touched the icky bottom.

Be sure you can hold your breath long enough.

It takes lots of practice to keep your body straight and rigid. If you can record your lower level attemps, you will see where you bend or twist.

You want tight-fitting, stout speedos, such as worn by water polo players. Two pairs wouldn’t hurt. Definitely not board shorts or loose trunks.

How to have your friends post here.

Among other things, you are too fat to do this safely. It won’t make you drop faster but you are going to hit harder and if you have to ask all these questions, it means you are not prepared.

Post back if you survive.

By the way, wrt to Q2: You will not be flying. You will be plummeting like a sack of lard, and most likely lose your orientation along the way. I’m betting this will kill you.

I did a similar jump into Horseshoe Lake in Jasper, Alberta, Canada. It is pretty high too. Estimates vary and I’ve never gone to the effort to confirm it, but it’s at least 60’, perhaps as much as 80’.

I did this leap before age 30. Many people do it. I wore sneakers, and did a pin drop, hitting the water arrow straight. My form could not have been better.

I ended up with bruises in my armpits and up the side of my body. On one of my jumps, I hit the water with my mouth slightly open as someone described above. In my case, I bit my tongue badly.

My friend who also did the jump damaged his knee and ended up requiring surgery. My brother did it and was looking down “checking his form for straightness” when he hit the water and he ended up with minor whiplash that took a couple of days to get over.

Many others do this leap, and on any nice summer day, one can go and watch them. Of course, I don’t know what the outcome or long term upshot is for those people.

I do know that I would never do it again. I’m 41 now.

Oh, this. You will NOT fly! I jumped off the garage roof with a dish towel for a cape when I was six. I was smart about it, (for a 6 year old) I place a pillow about 10 feet from the garage wall. I was flabbergasted when I landed 6" from the garage. I did NOT fly. I was lucky enough to merely get a spanking and not a broken neck!