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Sail pumping technique

Created by choco > 9 months ago, 30 Aug 2009

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4175 posts

sandman said...

in fresh water, yes.

1 liter of fresh water weighs 1kg. So, lets say you have a boat that weighs 100kg and as you lower it into the water it gradually displaces more and more water until it has pushed 100ltr out of the way. that is the point at which the boat will float.
All that water that the boat pushed out of the way is trying to get back where it was before the boat came along and in turn, pushes the boat upwards.
If you put 10kg more weight into the boat it will sink slightly lower, pushing 10ltr of water out the way until equilibrium is reached and the boat levels again. It is a constant fight for balance.

So as you can see, every kilogram of weight that acts down on the thing pushing water out the way will need to be compensated for by 1 liter of displacement.

As a side point, in your video, your feet and ankles will probably be providing about 3 liters of displacement, helping keep you above water. If you had an even lower volume board, you would have kept sinking until your body and the rig had pushed enough water out the way to help reach equilibrium again.

Thanks for the info Sandman,we sail in "fresh" water and because of the drought out lake has been diconnected from the Murray river for nearly 2 years now in the next few weeks they will be adding 26 gigs of water or 1.7m.
If they don't add the water the ec levels will reach 70,000ec which is about 20,000 more than sea water not sure what the level is now but it must be very high because i seem to be able to use my speed board alot more when the wind is consistant and when the boards dries large salt systals are left on the hull.
Next time i buy a board for light to moderate winds i now have a better understanding of what volume i should get.

sandman

WA

432 posts

sandman

WA, 432 posts

glad to help, just remember, as soon as you start moving forwards, the width of you board starts to to have a huge effect on your ~~boyancy~~ ability to stay above the water.

But that spiel I gave before is true for when you slogging at speeds like that in your video.

evlPanda

NSW

9207 posts

evlPanda

NSW, 9207 posts

I'm in a rowboat on a very small lake. I stop in the middle and throw the anchor out. Does the water level of the lake go up or down?

sandman

WA

432 posts

sandman

WA, 432 posts

water level goes down

EDIT; assuming of course that your anchor hits the bottom.

Windxtasy

WA

4017 posts

Windxtasy

WA, 4017 posts

evlPanda said...

I'm in a rowboat on a very small lake. I stop in the middle and throw the anchor out. Does the water level of the lake go up or down?

Are you a physics teacher?
That is the sort of thing they ask in physics tests.

I think the water level in the lake goes down (infinitessimally), as the weight of the anchor would have added more to to the displacement of the rowboat than the simple volume of the anchor. Once the anchor is dropped, the boat displaces less water and floats higher allowing the water level in a theoretically tiny lake to go down even though the anchor displaces it's own volume of water.

NotWal

QLD

7430 posts

NotWal

QLD, 7430 posts

Boat loses weight of 1 anchor so displaces less. Water level drops by smidge 1.
Water is displaced by anchor which must be denser than water or it wouldn't be an anchor so water level is raised by smidge 2 due to volume of anchor.

Water level drops by smidge 1 - smidge 2.

Windxtasy

WA

4017 posts

Windxtasy

WA, 4017 posts

NotWal said...

Boat loses weight of 1 anchor so displaces less. Water level drops by smidge 1.
Water is displaced by anchor which must be denser than water or it wouldn't be an anchor so water level is raised by smidge 2 due to volume of anchor.

Water level drops by smidge 1 - smidge 2.

That's a much simpler way of putting it.

MikeyS

VIC

1508 posts

MikeyS

VIC, 1508 posts

But is the anchor tied onto a rope which has a density less than fresh water and how much line was paid out? Or did you just chuck the anchor overboard without it being tied to anything?

And by the way, if it's such a small lake, why the hell do you need to anchor there anyway?