Relative effect of wind and current on a boat with sails down

[prv]

The 30:1 that we find high is to imagine that you were holding this theoretical 50:50 long keeler (Pete's picture above is very relevant) alongside a pontoon by hand and there was no wind blowing but a 1 knot tidal stream pulling the boat away from you, and then imagine that there was no tidal stream but a 30 knot wind blowing from behind you.

Would you have the same difficulty holding the boat alongside the pontoon in both these situations?

Apparently Tom Cunliffe's Westernman once pulled the cleats out of a pontoon where she was moored across a strong tide; the tale doesn't mention any particular wind being involved. Assuming the same berth was regularly used without problems by more modern boats, it shows how much more a long keel grips the water than a stationary fin.

Pete

 

[ex-Gladys]

I think that's the wrong comparison though. The relative resistance of air and seawater is 850:1 and that is what you would "feel" if you ran through them.

The 30:1 that we find high is to imagine that you were holding this theoretical 50:50 long keeler (Pete's picture above is very relevant) alongside a pontoon by hand and there was no wind blowing but a 1 knot tidal stream pulling the boat away from you, and then imagine that there was no tidal stream but a 30 knot wind blowing from behind you.

Would you have the same difficulty holding the boat alongside the pontoon in both these situations?

I would have thought that you could hold the boat against a 1 knot current but holding it against a 30 knot wind would be totally impossible .... but this is only based on gut feeling rather than any quantifiable experience.

Richard

I doubt you could just hold that by hand in one knot of current. That's several square meters of immersed boat, and a significant force per square meter. I know it's different, but you don't even try to stop a reasonably sized boat without getting a line round a cleat

 

[LadyInBed]

Would you have the same difficulty holding the boat alongside the pontoon in both these situations?

I would have thought that you could hold the boat against a 1 knot current but holding it against a 30 knot wind would be totally impossible .... but this is only based on gut feeling rather than any quantifiable experience.

Richard

I think you are taking your gut feeling from practical experience, where the relative areas exposed to the force are considerably different.

 

[bbg]

I think that's the wrong comparison though. The relative resistance of air and seawater is 850:1 and that is what you would "feel" if you ran through them.

The 30:1 that we find high is to imagine that you were holding this theoretical 50:50 long keeler (Pete's picture above is very relevant) alongside a pontoon by hand and there was no wind blowing but a 1 knot tidal stream pulling the boat away from you, and then imagine that there was no tidal stream but a 30 knot wind blowing from behind you.

Would you have the same difficulty holding the boat alongside the pontoon in both these situations?

I would have thought that you could hold the boat against a 1 knot current but holding it against a 30 knot wind would be totally impossible .... but this is only based on gut feeling rather than any quantifiable experience.

Richard

I don't think you would have any hope of holding that boat, side-on, to a 1 knot current just by hand. At least I wouldn't. Maybe your name is Dr Bruce Banner?

 

[prv]

I think you are taking your gut feeling from practical experience, where the relative areas exposed to the force are considerably different.

+1

Lots of topsides and superstructure and rig, versus a fairly shallow canoe body and a relatively small fin. True of most "modern" monohulls, even more so for a multi.

Pete

 

[Deleted member 36384]

I would not hazard a guess at the maths behind it as hydrodynamics are very complex to model to any degree of accuracy on complex shapes.

I was on a mooring in the Menai Strait, on the Angelsay side, near Beaumaris. It was blowing into the straight a gale and we were sitting quite happily bow to the wind. As soon as the tide turned the yacht rotated and stuck her stern into the wind; Westerly GK29.

 

[markhomer]

You all remembering that current can stratisfy too , you can see this by pouring milk over the side as it sinks you can see it going off in different directions at different heights you can have top layer going 180 degrees to next layer down , i was quiet amazed at how statified it can be first time i did it .

Sometimes i have noticed , our boat can be sitting on mooring with strong tidal," surface " current ( 0n windless day ) let go mooring and boat doesnt move !

So theres a lot going on .

 

[RichardS]

I think you are taking your gut feeling from practical experience, where the relative areas exposed to the force are considerably different.

Yes that's right, which is why my experience is not really relevant and why I'm suprised at the result of the maths when using a 50:50 boat just to simplify things.

However, I do accept the experience of others on here that a long keeler in a 1 kt stream would exert a much bigger force that I have envisaged.

The reason for looking at the calculation was related to my earlier thread linked above, where my boat was straining ahead into a 3 knot counter-current with a modest breeze on the port aft quarter. Unfortunately the maths does not help me understand the previous phenomenon as it appears that the wind required to overcome a 3 knot current would probably be quite significant, even with my cat.

Richard

 

[bbg]

Yes that's right, which is why my experience is not really relevant and why I'm suprised at the result of the maths when using a 50:50 boat just to simplify things.

However, I do accept the experience of others on here that a long keeler in a 1 kt stream would exert a much bigger force that I have envisaged.

The reason for looking at the calculation was related to my earlier thread linked above, where my boat was straining ahead into a 3 knot counter-current with a modest breeze on the port aft quarter. Unfortunately the maths does not help me understand the previous phenomenon as it appears that the wind required to overcome a 3 knot current would probably be quite significant, even with my cat.

Richard

Possible explanations
- windage on the cat is very high compared with the size and shape of the hulls below the water
- would the wind have been enough to blow the cat at 3 knots? That is quite possible.
- how did you determine 3 knots current? From instruments? Any chance of a calibration error?
- as some have already said, there can be a stratification of currents, particularly with an opposing wind. So there might have been 3 knots at the paddlewheel, but somewhat less acting on most of the hull.

 

[Lon nan Gruagach]

Not exactly the typical deep-bodied long-keeler mentioned in the OP

Fair do, but even if that was up to its gunwales it would still be more streamlined across ways underwater than above. But hey, lets get an idea of the boat that is in question, like:

All but the middle 1/4 is not square on to the flow whereas above deck there are hardly any surfaces that arent vertical. As I said, profile and drag coefficient is as important as area.

As for being able to hold a boat against a 1 knot tide, try this, try dragging a boat at 1 knot through still water. Then consider it takes a stonking great horse to pull even a skinny narrow boat.

 

[JumbleDuck]

The math aspect of this question has got to be right up Mr Duck's neck of the river and it would be interesting to hear his take on it.

Someone rubbed my lamp. What do you wish, O Master?

Yes okay, but even there it gets tricky. I'd imagine the simple approximation that the boat collides with air/water molecules at a rate proportional to 'v' (basis of the v^2 relationship) only holds over a specific range of Reynold numbers. At very low Reynolds numbers one will get laminar flow where drag is proportional to v (?), whereas at the extreme upper range it will roughly constant(?). Then there is the problem that the keel will act as a blunt object in certain aspects and a laminar aerofoil in others. Finally there is the problem that an anchored yacht balanced sideways in an opposing current/wind will be dynamically unstable. Happy to be corrected on any and all of this BTW.

Reynolds number is, roughly speaking, an indication of whether viscous forces (low Re) or inertial forces (high Re) predominate. It has a huge effect, but that effect is rolled up in the drag coefficient C_d and the lift coefficient C_l; the drag and lift are given by D = 1/2 C_d rho v² and L = 1/2 C_l rho v². That's because the overall flow pattern is dependent on the Reynolds number and we know that the drag and lift coefficients are the same for the same flow patterns, even when the scale and situations are very different.

So ... relative effects of wind and tide. It depends. I don't have a good table of drag coefficients to hand, so I'll do this for a 1 metre sphere. In sea water, Re ~ 3500, while in air Re ~ 300. That's enough to make a difference in the drag coefficient, but not a huge one: C_d will be about 0.8 in air and about 0.5 in seawater.

The main difference in forces is therefore due to density, which as has been said is about 850 times more for seawater. Combine the two effects and I'd expect the drag on a reasonable streamlined object to be about 500 times in seawater than air.

However, there will be other important effects. Veering about is caused by lift, which for a reasonable aerofoil section, like a keel, can be many times higher than drag, Against water flow past a keel, air flow past the topsides doesn't have much chance. Of course if water is not flowing past the keel the wind can do its stuff, hence veering about at anchor. From experience, though, I know that it takes about 30 kt of wind from the beam before my long keeler (NACA aerofoil keel, though) has any problem holding course at 4 kt through the water. At walking speed through a marina she is rather more sensitive!

Which leads to the observation that where the force is applied matters a lot, which is why my relatively high bows have a big effect when the wind catches them - lots of moment arm around the centre of gravity.

Finally and back to the original model of a boat at right angles to wind across tide, being kept in balance by the two. The simple question is "How fast would you expect her to drift sideways downwind if there was no tide?" because that is how much tide you'd need to hold her in place. In practice, she's need a lot of holing because as you (Dom) says, she'll be horrible unstable and will prefer to align with the flow above and below the waterline.

Hmm. Maybe more smoke than light. Back in my bottle.

Jeannie with the Light Cream Hull

 

[dom]

Someone rubbed my lamp. What do you wish, O Master?

Reynolds number is, roughly speaking, an indication of whether viscous forces (low Re) or inertial forces (high Re) predominate. It has a huge effect, but that effect is rolled up in the drag coefficient C_d and the lift coefficient C_l; the drag and lift are given by D = 1/2 C_d rho v² and L = 1/2 C_l rho v². That's because the overall flow pattern is dependent on the Reynolds number and we know that the drag and lift coefficients are the same for the same flow patterns, even when the scale and situations are very different.

So ... relative effects of wind and tide. It depends. I don't have a good table of drag coefficients to hand, so I'll do this for a 1 metre sphere. In sea water, Re ~ 3500, while in air Re ~ 300. That's enough to make a difference in the drag coefficient, but not a huge one: C_d will be about 0.8 in air and about 0.5 in seawater.

The main difference in forces is therefore due to density, which as has been said is about 850 times more for seawater. Combine the two effects and I'd expect the drag on a reasonable streamlined object to be about 500 times in seawater than air.

However, there will be other important effects. Veering about is caused by lift, which for a reasonable aerofoil section, like a keel, can be many times higher than drag, Against water flow past a keel, air flow past the topsides doesn't have much chance. Of course if water is not flowing past the keel the wind can do its stuff, hence veering about at anchor. From experience, though, I know that it takes about 30 kt of wind from the beam before my long keeler (NACA aerofoil keel, though) has any problem holding course at 4 kt through the water. At walking speed through a marina she is rather more sensitive!

Which leads to the observation that where the force is applied matters a lot, which is why my relatively high bows have a big effect when the wind catches them - lots of moment arm around the centre of gravity.

Finally and back to the original model of a boat at right angles to wind across tide, being kept in balance by the two. The simple question is "How fast would you expect her to drift sideways downwind if there was no tide?" because that is how much tide you'd need to hold her in place. In practice, she's need a lot of holing because as you (Dom) says, she'll be horrible unstable and will prefer to align with the flow above and below the waterline.

Hmm. Maybe more smoke than light. Back in my bottle.

Jeannie with the Light Cream Hull

That’s an excellent beam of light onto this tricky area. It’s a good idea to pull back to the case of a steady flow past a solid sphere as a staging point to understand the principles and techniques required for at least basic dimensional reasoning about the flow of water and air across a hull, its foils and its rigging.

A boat sailing along is one thing, but a vessel yawing about at anchor where several different shapes are at play. Flows are laminar one minute; separated, oscillating and turbulent the next. Meanwhile the anchor rode is swinging around yanking the boat this way and that thereby setting up all sorts of inertial systems. One can only imagine how difficult that wld be to model !!

 

[JumbleDuck]

A boat sailing along is one thing, but a vessel yawing about at anchor where several different shapes are at play. Flows are laminar one minute; separated, oscillating and turbulent the next. Meanwhile the anchor rode is swinging around yanking the boat this way and that thereby setting up all sorts of inertial systems. One can only imagine how difficult that wld be to model !!

You're right about the wild flow variations; remember that those will arise because of changing angle of attack rather than because of changing Reynolds number. And yes, adding the effect of chain, which is itself dynamic, makes it harder still. Years ago I started trying to create a numerical simulation of a glider on a winch launch, which is actually very similar as it involves interactions between the force in the launch cable and the aerodynamic forces on the glider. I gave up after a bit; the glider end was OK but trying to incorporate a reasonable model of half a mile of 6mm steel wire flapping about made it more than I wanted to play with in my spare time, and more than the computer I had was up to. The Akafliegs have probably done it by now.

 

[rogerthebodger]

There has been a lot of study on the forces on bridge pillars in a flow of water which is not the same but similar and needed for the correct design of the bridge structure.

 

[LadyInBed]

Finally and back to the original model of a boat at right angles to wind across tide, being kept in balance by the two. The simple question is "How fast would you expect her to drift sideways downwind if there was no tide?" because that is how much tide you'd need to hold her in place.

Thanks JD, that sums it up nicely for me.

 

[Lon nan Gruagach]

That’s an excellent beam of light onto this tricky area. It’s a good idea to pull back to the case of a steady flow past a solid sphere as a staging point to understand the principles and techniques required for at least basic dimensional reasoning about the flow of water and air across a hull, its foils and its rigging.

A boat sailing along is one thing, but a vessel yawing about at anchor where several different shapes are at play. Flows are laminar one minute; separated, oscillating and turbulent the next. Meanwhile the anchor rode is swinging around yanking the boat this way and that thereby setting up all sorts of inertial systems. One can only imagine how difficult that wld be to model !!

I'm pretty good with some wood and a penknife....