Keel dynamics question

[peteandthira]

Dear experienced yotties

I was reading an old thread on SB about different keels and their pros and cons. I have been wondering about my own fin keel (lead) and have this question.

I assume the heavy keel has a certain righting moment whilst stationary. I also assume that whilst making way, this righting moment is greatly increased by aquadynamics, ie water flow past it. If you are beating, and heeling significantly, I notice a tendency to heel much more rapidly if I allow the boat to turn into wind, but stand up more solidly if rudder is applied to counter the natural tendency of my boat to turn into strong winds. Is that increased resistance to heeling caused by the keel "flying" through the water, eg "biting" into the sea to apply more of a righting moment?

This leads me to suppose that the boat becomes more stable the more speed you keep on, which when combined with a reduction in sail (to decrease heel) adds up to a whole lot better performance.

Having re-read my post I am aware that this wasn't a "question" really, just inviting opinion.

I welcome any comments on the above from the learned many on here!

Pops

 

[DJE]

Something doesn't seem right there. The keel is gerating lift to windward in order to reduce leeway. This force reacts with the side force on the sails to produce a couple which is trying to increase the angle of heel. Any side force on the keel which reduces heeling will increase leeway. Applying rudder to oppose weather helm will increase the lift generated by the keel and increase the heeling force.
When reaching or beating my boat comes more upright as she rounds up because the sails start to luff. It is only on a very broad reach or a run -where rounding up wouild put you across the wind- that heel will increase leading to a broach.

 

[andy_wilson]

Aquadynamics

When heeled and close hauled, the low pressure side of the keel is the upper surface. The dynamic effect is therefore to tip the boat further as there is a vertical component to the lift on the upper face of the keel.

Your boat stiffens up due to the shap of the hull. If it was designed well, as it heels, the curve of the topsides is such that heeling physically lifts the keel, which gravity of course resists. Furthermore, an angle of heel occurs where the boat sits in a groove, resisting further tipping by the fact that the keel would have to be lifted significantly further for a given increment of heel.

 

[bbg]

I know what you mean about heeling more rapidly as the boat turns into the wind - but that has nothing to do with lift or gravity, and everything to do with centrifugal force. Look at a picture of a destroyer doing a tight, high-speed turn, or, on a calm day, you can do it yourself. Motor along at cruising speed and try to turn the boat in its own length. Watch what happens to the mast. Tell everyone to hold on first.

 

[Lakesailor]

I'll be interested to see where this thread goes. I have the same responses as you. As wind speed increases in a puff when close-hauled the boat heels more. If I let it move towards rounding up (and usually the wind has shifted in my case) the heel increases dramatically. If I fight the rounding up effect by heaving up the tiller, (to maintain my course) the boat stiffens up considerably and heel doesn't increase much.
I'm assuming that it's more to do with the wind speed increase and possible shift that is creating these effects.

I'll be interested to se the reasons people will put forward.

 

[peteandthira]

bbg

I'm happy with what you say, and have done just what you suggest. It is clear that whilst motoring on a flat sea with no wind, I can get quite a heel on by throwing in full rudder at speed. However, it doesn't do that if I turn at a slow RATE of turn.

I am talking about slow rates of turn, ie by allowing a tendency to luff up simply by not opposing it with rudder. Rates of turn gained here are not the same high rate of turn that I get under motor and lots of sudden rudder.

I am also thinking that hull shape can cause this turn into wind by the wetted shape changing as the boat heels. But it still doesn't quite explain our apparent ability to counter the increasing heel by trying to hold the boat off the wind. There is definitely some "assistance" going on somewhere if I fight the weather helm.

Still very interested, especially as Lakesailor seems to have the same characteristic. I am reluctant to call it a "problem" since it seems to be quite a safety thing if left alone.

Pops

 

[tross]

I'll put this one up for people to have a shot at /forums/images/graemlins/smirk.gif

As you turn into the wind you are pivoting around the keel but you boat momentum wants to carry on the prevoius heading, this causes a greater angle of attack of the keel relative to the water flow which, creates greater lift hence, more heeling ?

 

[peteandthira]

tross

Yes, can see that. Presumably any opposite rudder at this point will tend to "dig" the keel in again, helping bring the boat a little more upright, which is about where I had the original query.

I'm rather hoping we might "catch" an aquadynamicist here soon with this one!

Pops

 

[peteandthira]

Re: Aquadynamics

andy

err, sounds good! Here's another thought. If I stood on the harbour wall and hauled on a line from the top of the mast, I would probably be able to get the boat on its side given a force of, say, "x" lbs. My feeling is that it would require about 10 times that force to get the boat on its side if it were doing about 6kts through the water.

Is that possibly true, and if so, where is that extra stability coming from?

 

[fireball]

Re: Aquadynamics

Can't see the argument on that one - not for your standard keel ...

remove the sails from the equation.... so - 6knots under motor ... now you will only have the aquadynamics to think about ...

 

[DJE]

It\'s Called Hydrodynamics.

Easy enough test your theory by placing three of four heavy crew on the rail and measuring the angle of heel when motoring and when stationary. I would bet that there would be no difference.

 

[Evadne]

I thought this at first as well and, although I haven't done the maths to work out the forces involved I feel that it is unlikely, as this effect is noticable at speeds as low as 4 knots, which is little below walking pace. This suggests it is more due to the wind than the boat. More likely is a combination of effects, (a) turning the boat to point higher presents more sail area across the wind, giving a greater heeling moment and (b) pointing higher will, momentarily, increase the boat's velocity into the wind, thus increasing the apparent wind speed, hence wind on the sails, thus more heeling.

 

[Evadne]

Sorry Phil, I didn't see you'd suggested this already, I didn't read all the responses properly.
Dave

 

[jimbaerselman]

There's a sequence of things going on in a gust induced heel angle:

1. Heel angle causes boat to yaw (turn a little around it's own axis)

2. The angle of yaw, as it increases, creates an angle of attack at the keel (leeway).

3. The angle of attack causes to boat to accelerate at right angles to its previous direction (trace a curve through the water)

4. Since the boat's C of G is above the position at which the force is applied, the boat will heel more (inertia coupling if you like. Same as a car rolling as it goes round a bend)

If you hold the boat straight by applying rudder, a different sequence occurs:

1. Heel angle causes a small yaw

2. Helmsman reacts by applying counter rudder

3. If the boat travels straight, there is no net sideways force from under water. Yes, there's a big rudder angle (to counter the turn from the hull shape) pushing the boat to windward, and there will be a small keel angle to cancel that lateral force. The two may produce a small vertical heel moment, since the rudder is usually higher than the keel. This effect would slightly reduce heel.

4. With no net sideways force below water, there is no inertia coupling to increase the heel angle.

Does that theory co-incide with your observations?

 

[oldsaltoz]

G,day All,

The comments so far are very interesting, however it seems no one has considered the effect of the rudder.

Sailing close hauled you have some weather helm (if not your trim is out). at this point the trailing edge of the rudder is leeward so is in effect holding the stern to wind. Push the tiller away and three (not one) forces come into play; The rudder bites and pulls the stern to leeward, but you have also reduced the forces generated by weather helm, also note that your head sail will will lose drive as the wind moves aft, it will change shape, however because the main has a boom the loss of drive is less so the sail plan is now out of balance and the drive is more aft. All three forces are in effect when the sails are still at best trim, but the resistance has changes dramatically when the tiller is pushed away, would this not cause the boat to increase speed and so cause more heel?

Avagoodweekend......

 

[peteandthira]

Jim

That sounds quite convincing and goes a long way to explaining what I think I am seeing in practice. I see you were, like me, ex-RAF aircrew. I have tried applying aerodynamics to all this, but it fails somewhere. Probably because water doesn't quite act the same as air when a foil shape is "flown" through it.

I am still of the opinion that turning the boat away from the wind causes an angle of attack underwater that is tending to drive the keel further underwater, thus righting the boat, but probably only as long as that opposite rudder is applied.

Furrowed brows here!

 

[peteandthira]

Brian

Thank you for your input, it sounds very reasonable. Are you suggesting that the foresail loses drive as the boat turns out of the wind because it is now sheeted in too much and is stalling? Is it not possible in "real" boats to balance the sails so that she will maintain her heading even close hauled, therefore leaving the rudder out of the equation?

Incidentally I can't do that on my AWB, she will always need some pressure on the helm even if I have no main up at all. As I sail further off the wind she will eventually balance up reasonably well. I am aware that this might well be my lack of precise trimming rather than the design of the boat.

 

[peteandthira]

David
quote : (a) turning the boat to point higher presents more sail area across the wind, giving a greater heeling moment and (b) pointing higher will, momentarily, increase the boat's velocity into the wind, thus increasing the apparent wind speed, hence wind on the sails, thus more heeling. "

Thank you for this one. I have a slight issue here, or maybe misunderstanding some basics. (a)Turning the boat into wind must surely REDUCE the sail area presented to the airflow?
...and (b) the increase in apparent wind is only because you are turning more INTO it, rather than the boat accelerating?

Sorry if I have misunderstood your point..

Pops

 

[Krusty]

I'm sure Jimb... has the best answer yet. Most of this 'heeling-while-rounding-up' is plain dynamics, on sea or land, and the only 'aqua-' bit that applies is the additional effect of change of underwater hull shape as heel increases: that can be almost neutral in a well-designed sea-kindly boat, or quite extreme if the designer has accomodation higher in his priorites.
But there is another factor; the aerodynamics. The speed of the 'apparent wind' increases as the yacht rounds up towards the true wind, and since the forces on sails are proportional to the square of the apparent windspeed the heeling force can increase dramatically -- unless the sails were already right on the point of luffing. That is rarely the case, since most sailors habitually over-sheet a little(some quite a lot!) so that the luff is not shivering to every transient variation of wind direction and helm control. And quite right too, since sailing right on the edge is for keen racing men, and the inevitable sail flutter seriously shortens their life.
A sharp helmsman can deliberately 'feather ' a well-balanced and responsive yachts with good sails, correctly trimmed, through a gust with the luffs of mainsail and headsails shivering, to prevent over-pressing and round-up. It is a very useful skill, especially in crowded waters where an uncontrolled round-up may be disastrous, and well worth practising.

 

[andy_wilson]

Did the keel bit lower down.

If you hold your course you are spilling a lot of wind, the sails become inefficient foils. It all gets very noisy, tremendous helm loads, but not much more heel.

If you luff up you turn the sails towards the wind thus accellerating the apparent wind, the angular change AS you turn accellerates the wind over the sails, and these combine to increase the suction on the leeward side of the sails, thus increasing heel significantly, though temorarily.

Add to this the effect of centrifugal force on the parts of the boat (hull, decks, mast, rigging) above the point of lateral resistance, all of which will throw this weight outwards as you turn.

Finally, dependent upon the violence of the manouver, there may also be an element of tripping over the keel. If yo manage to disturb the water flow over the keel so much, you will incur a massive increase in lift from the low pressure side of the keel. thus making the heel even worse, but this may be temporary if the keel subsequently stalls.