Floating Duck

Sailing into ADVERSE current is FASTER? Explain this to me...

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I've heard and read that apparent wind boats have certain situations where sailing INTO adverse current is actually faster than sailing in slack or positive current...

Can you explain this to me like I'm 5? Is this only when going upwind? Only when going downwind? I assume it doesn't always make sense, so when does it?

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I have experienced this on a sailboard.  It seemed to me that once the speed of the water under the hull reached planning speed the board popped up to a plane and the board went faster.

I would guess the same thing happens with other hull designs that have a different orientation as the speed (and force of the water on the hull) increases.  I doubt this would be true of a displacement monohull.  On the other hand if the increase of the speed of the water going past the hull was able to lengthen the bow wave it might have some effect.

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1 hour ago, Floating Duck said:

I've heard and read that apparent wind boats have certain situations where sailing INTO adverse current is actually faster than sailing in slack or positive current...

Heard from whom and read where?  I'm curious since it isn't anything that I've experienced in the 20 years of sailing in current.

 

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This has been covered in another SA thread but I’ll try to summarize.

All you need is for your down wind velocity made good to be greater than actual wind speed.

It’s a bit counter intuitive but if you break it in pieces it’s easier to digest.

So you want to go straight North but have an adverse current from the North – flowing straight south.

Let’s start with the effect of current on apparent wind. Imagine you are sitting on your boat with an untrimmed sail in a river moving really fast – say 10 knots but NO WIND. To someone standing on shore you are obviously drifting downstream southwards at 10 knots. Your sail does not care what that landlubber thinks. As far as your boat and sail are concerned your boat is still in the water and feels a breeze of 10 knots in the opposite direction the water is moving. So your boat sees south wind of 10 knots.

For traditional sailboats your best bet is drop the anchor and open a beer and wait for more wind. If you try to sail downwind towards the north even on your fastest angle you will be moving slower than 10 knots Velocity Made Good (in the north direction) so you will drift down current (more slowly than if you were not sailing but still down current).

Higher performance boats can sail faster than the wind. For example the AC 72 cats can sail at almost three times the wind speed. Foiling AC boats can sail faster than the wind so much so that the boats speed is the most significant part of the apparent wind – which is the wind the sail “sees” / generates lift from.

The aerodynamics of sails means they generate much more lift or force in the direction you want to go if the wind is flowing over them rather than just pushing on them. That is to say you are better off gybing downwind than just sailing in the direction you want to go. (you already know that but you did say a 5 year old).

Although the distance is longer the extra speed makes up for it. The DMG – distance made good towards the downwind point is shorter than the actual course sailed (think zig zag string and then pull it tight – string/actual course is longer). The VMG - Velocity Made Good – is also slower than the boat’s actual speed.

Now here is the brain twist part...some boats are so efficient that their actual speed is so much higher than the wind speed – that the VMG made good even downwind is greater than the actual true wind velocity. That is the key point – the velocity made good straight downwind is actually faster than the wind speed (reminder boat is not going straight downwind it is zig zagging down wind).

In the simplest case of no wind and just adverse current your boat sees the adverse current as wind. If your VMG down wind is greater than your current you can actually gybe down current and make progress against the current...even without any wind!

Up to a point more wind the faster you go...so more adverse current the faster you sail against the current.

So if the current is strong enough to equate to wind speed needed to foil downwind then you can actually sail up current on a windless day. You get up foiling while pointed across the current accelerate and then turn downwind to your best VMG angle.

If you happen to have any real wind then you can go even faster.

Wiki_sailing_vector_downwind.png

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Found the original thread -

 

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