One for the thinkers....

flooding your cockpit is not like adding an eleventh plastic cup with no bottom to your raft... Your ski always have the same volume, flooded or not. It would be more like punching a hole in the bottom of one of the 10 cups you already have : do you think removing the bottom of one of the cup will alter the waterline of your raft ?

Todays project:
Leave your surfski at home
Go down to the sea with full bladder and in my speedo (preferably without)
Baggies or neoprene shorts will give me a false reading
Lie in the water
Fill my lungs with air
Expell all the air from my lungs
Note the difference
Piss while stationary
Piss while swimming forward
Make notes of sensations
Write and post on "one for the thinkers blog"
or maybe just go for a paddle

Rightarmbad wrote:

Its been a while since my high school physics, but If the water in the bucket is at waterline, then the entire craft is displacing a tiny bit less water, and your boat will sit a tiny bit lower in the water. So you will have more wetted surface and go slower. And be a little more stable.

How can it change displacement?
Unless you have a sealing bailer like a V12, the craft should feel no weight from the water in the bucket.
The water is free to move in and out of the bailer.

The boat is not holding the water, merely being surrounded by it.

OK: Even though there is a venturi effect and the water is being sucked out while moving forward, you have in effect a loss of water plane area equivalent to the area of the bucket at the waterline in the bucket, as the water outside and inside are common. So yes, you will sink slightly deeper into the 'Oggin. In effect if you have the plug out you are losing the proportional amount of buoyancy of the waterline in the bucket.

Make sense? Its Damage Stability (Hole in My Ship Sir...) 101.

Question:

If the water in the bucket is free to move as stated earlier in the discussion, will the acceleration and deceleration not cause the centre of gravity to shift and hence cause a rocking motion till it settles at when at a steady pace? Wont this create an effect where there is more resistance/waterline due to the jerking action?

The boat will never settle into a steady pace.
Just listen to your venturies or look at the residual water in the bucket, it is always moving forwards and backwards in relation to the ski with every paddle stroke.

meant that it would become more stable when up to speed, because when you accelerate it would move to the back, and slosh back non-rhythmically and great an uneven glide/displacement by you craft . As soon as you are at a steady pace, there will be more of a rhythm to it and thus the craft should be more stable and be able to "rock" less.

Well, lets rather just go out and paddle

Guys,
After many years studying astrophysics at Macquarie Uni let me put this issue in a more mathematical format.

Question 1 - The law of displacement states that an object will float when it displaces more liquid than the mass of the object.
Therefore - If your bucket is dry then it too is exerting a displacement of water for any volume below the waterline ie creating lift. If you add water to the bucket by either opening the bailer or adding water then this area no longer exerts the lift.
The easy way to prove this is to sit your ski in the water and mark it with a fine felt tip pen then open the bailer and see the change. It will be small as the skis are large but it will change!!

Question 2 - The mass of an object will require force if it is to move. Energy cannot be destroyed only converted or transferred. The energy the body puts into the stroke will be transferred into motion of the water around the blade and forward motion of the ski. The heavier the ski and paddler the greater the force to move the ski forward. Thats why any machine, ski, or bicycle will be made as light as possible to assist acceleration. Even a liquid will require force. That force you see is translated into the motion of the water. Whether it free or not the motion you see in the liquid is still energy that was originated by the paddlers stroke. Therefore if its being wasted on the water its not being transferred into forward motion. ie slowing acceleration.

Someone said before that if you put 20 litres of water into your ski it would slow acceleration. They are correct!! This water is still free to move in the hollow of the ski but is mass that still absorbs energy if you are to get it moving forward.

The only exception to any of this is if you are in a vacuum ie space ( but that clearly isn't the case.)

Regards

Shanan

But in the overall scheme of things, how much does 3-5 litres of water sloshing around in the bucket effect your overall speed?

The whole point of this thread.

Rightarmbad wrote: But in the overall scheme of things, how much does 3-5 litres of water sloshing around in the bucket effect your overall speed?

The whole point of this thread.

Well, yes it must. For the reasons stated:

1. Commensurate loss of buoyancy equivalent to the surface area of the water in the bucket. Therefore you are sitting deeper in the water.
2. 4-5 Litres = 4 to 5 kilos. That could be 40% of the total displacement weight of your watercraft if you have an ultralight.

Would it be measurable? Probably in test conditions in a controlled environment. But given in a race you and everyone else would be in the same boat to overuse a cliche, it would be moot.

You could always race a kayak with a skirt......

Here`s my simple answers and explanation to the original questions:

Yes and Yes.

Imagine pushing and holding a soccer ball under water. Can you feel that upward force? That`s what your empty footwell does to your ski`s waterline.

Water in bucket = boat deeper/heavier = slower than having air in bucket.

Can we end this thread now please, I wanna go paddle!

a Welshman talking of a soccer ball?! I think Bill McLaren turned in his grave

This thread is rubbish and makes me support a benevolent dictator and censorship. Shame on you RAB for starting this under the guise of being a 'thinker'.

Please Rob close this thread!

So consensus is that an empty footwell with open bailers at speed is identical to an empty footwell with a closed bailer?

Also that the 5litres in the bucket is also equivalent to a 5kg heavier ski; I don't buy into that one.

Therefore my V10 with always an inch and a half of water in the bucket unless it is really scooting along is basically a 20kg job;
Don't buy that either.

Thinking expressed in this thread so far doesn't match my reality, so more thinking required.

Matches my reality.
With no water in the ski there would be a difference...the friction caused by the open bailer.Closed bailer ...much less friction.
When I've paddled like a dork and filled my ski with water it does not just sprint off no matter how hard I paddle (granted I am past my prime)
The ski is heavier...like you said ...a 20 kg ski!!!Once empty it respomds again to my efforts with lots more snap.
and thet water sloshing around the bottom of your footwell...it will add weight.
Paddle faster or get better bailers...ony solution cause that water is adding weight
But as all are saying...enough now!
Let's go paddle and trust your sensations on the water

I think Rightarmbad is playing a game to see how long he can string every one along , either that or he really is In another reality , enough I think.