Following Seas Dangers?

[cfarrar]

I'm confident that lifting the centerboard / lifting keel helps downwind stability (unless maybe your lifting keel has huge amount of ballast). I know this is true in dinghies and centerboard racing sloops, where reducing the underwater lateral resistance and moving it aft reduces the tendency to broach. I've never sailed an Ovni, but Jimmy Cornell and others have written that this works especially well in those boats, since they're relatively flat bottomed.

Actually, in a strong breeze this also works upwind. Raising the centerboard reduces helm and effectively depowers the rig, since less of the lateral pressure on the sails is translated into heeling moment by the board. In high performance dinghies as you become overpowered upwind you raise the centerboard instead of reefing. (The reduced drag helps when you foot off to build VMG, but that's besides the point. Raising the board helps the boat sail better in these conditions, even if you're not racing).

[Andrew Troup]

Originally Posted by goboatingnow

What I meant was that the very few people I know with retractable keels ( or centreboards etc) never used them in the manner described. , ie lift them in a big downwind storm. I mean what happens if you broach or suddenly need your keel ?

Dave

Two questions: two answers

First: The impulses initiating a broach can arise from the sails or the hull.

The first group is round-ups due to the turning effect of the sail area, if the boat heels or rolls heavily. Heeling is best minimised by keeping sail area small and low, and is eliminated altogether if the course can be held close to dead downwind.

Rolling can be difficult to avoid with some traditional hull forms driven close to or past hull speed, but modern underbodies which develop sufficient lift in the forefoot region generally steady up the faster they run, as long as the speed is due to wind speed, rather than sail area. It can get tricky if there's not enough wind for the size of the seas.

Broaching or turning effects of water on the hull are also largely avoided if the course can be held close to dead downwind, and the best way to help this is to move the centre of lateral resistance as far aft as possible. Swinging the keel aft can be a big help.

If the rudder is large enough, the best control may be achieved with the keel fully retracted, leaving the rudder as the only appendage underwater.
In directional control terms, the rudder then tends to act like the tail feathers on an arrow. Their purpose is to prevent the arrow "broaching", despite the fact that arrows do not have a helmsman. Nor keels.

Arrows are not a particularly close analogy, though: a better one is surf boats (the sort full of bronzed Aussies in speedos and coloured skullcaps, in ads for chewing gum or suntan lotion)

The mission statement for these boats can be reduced to two words: Don't Broach.

The only underwater appendage is the steering oar. A keel would be a Bad Idea.
　
Secondly, to the question "What happens if you need your keel"

The purposes of a keel are fourfold: to carry sail, to prevent leeway, to provide a pivot point for manoeuvring, and to pull the boat back upright if knocked down.

The first two are not relevant when running off under minimal sail. The third is undesirable: you do NOT want the hull pivoting about any point forward of the aft quarters.

The fourth is a tradeoff. Contrary to intuition, raising the keel will generally reduce the risk of knockdown by wave action.

I say this for a number of reasons which I'm happy to go into on request, but I sense some readers' eyes glazing.

This tradeoff requires a skipper to to make a classic and difficult judgement call: is it better to take steps to avoid a knockdown, even if this means something which makes recovery from a knockdown more problematic?

There is a way to have your cake and eat it, but it's hardly ever done: Ideally a swing keel for a sailboat built for serious survival situations would have a way of quickly extending the keel if needed for re-righting moment, regardless of the attitude of the vessel.

I personally think the best way is stored hydraulic oil under pressure in an accumulator.
Such a keel could be deployed in a matter of seconds even with the boat inverted, and this could reliably be arranged to happen automatically without relying on electrics or electronics.

[msponer]

Originally Posted by Andrew Troup

I say this for a number of reasons which I'm happy to go into on request, but I sense some readers' eyes glazing.

My eyes are not glazing! I'm interested in this since our new boat has a hydraulic lift keel. I have almost zero experience with this boat and have been curious if we'd ever experiment with lifting the keel in any situation that is not wanting to reduce the draft while making landfall.

Originally Posted by Andrew Troup

The fourth is a tradeoff. Contrary to intuition, raising the keel will generally reduce the risk of knockdown by wave action.

Would you mind explaining this?

My undergraduate degree is in physics, which I am only mentioning since that means I have a surface familiarity with the types of thought a domain expert would use to figure this out, but none of the details.

Anyways, my non-domain-expert intuition is that raising the ballast will decrease the roll inertia and make it much easier to roll the boat. So I am guessing that part of your thought is that raising the keel decreases other inputs -- reduces 'tripping' if the boat is hit by a wave sideways, or etc?

Is there any math or other thought that makes this idea come up the clear winner?

[msponer]

Originally Posted by Andrew Troup

Once you get significant heel, the heel angle becomes the primary variable for balance.

Yes. What 'significant heel' is for a particular boat, when heel becomes the primary variable for balance, is, to me, a big part of a boats personality. Boats where this effect starts to take over at common angles of heel are really hard for me to settle into a groove downwind, since common waves (a few times an hour, or even one every few hours) will throw them out of their groove and into a jibe or tack. It's frustrating to sail boats like this far, since I find myself having to make a lot of performance (and sometimes comfort) sacrafices just to accomodate this effect, when I know that on other boats this would not even be an issue (because the graph of 'heel vs. turning moment induced by hull' doesn't get steep until much higher, and less common, heel angles). I think some boats even have a bad dynamic pattern they go through with this, where they heel 'too much', start turning into the wind, and bleed off so much speed that the rudder loses control -- I think the shape of that graph is important, not just the magnitude: that if it's linear and gradual you don't get this little phrase the boat finds easy to just repeat.

Originally Posted by Andrew Troup

My vector diagrams suggest that this effect is due to parasitic drag on the topmast and top rigging. This acts directly downwind, and if the windspeed increases past a (very high) threshold, the long lever arm due to heel combined with the direction can cause sudden-onset major lee helm when close reaching. This tipping point happens if the moment becomes sufficient to overcome the usually predominant weather helm, which arises due to the sail area acting out to leeward.

When you have such large forces, it doesn't take much swing in a direction vector for lots of weather helm to suddenly become lots of lee helm.

That's interesting -- thank you. I'll have to pay attention with that model in mind to see it. I think some of these things are hard to see if you don't have a mental model that is looking for the right details.

Originally Posted by Andrew Troup

You mentioned there are exceptional circumstances. For me, one of those rare exceptions happens when there's enough wind to be able to sail hard on the wind nicely balanced with just the jib. Then the feeling goes away.
Except there's an even more rare and exceptional possibility, which most people will probably never encounter.

Yes, I've also experienced that, it has been rare on the boats I've been on. But fun, to have perfect finger tip balance on the helm with just the jib.

[charliehows]

reef down the first time the thought that reefing might be necessary occurs. By the time you've thought about it for a bit, it could be too late. call me a pussy if you like, but pussys like to keep dry...

[Andrew Troup]

Originally Posted by msponer

My eyes are not glazing! I'm interested in this since our new boat has a hydraulic lift keel. I have almost zero experience with this boat and have been curious if we'd ever experiment with lifting the keel in any situation that is not wanting to reduce the draft while making landfall.



Would you mind explaining this?

My undergraduate degree is in physics, which I am only mentioning since that means I have a surface familiarity with the types of thought a domain expert would use to figure this out, but none of the details.

Anyways, my non-domain-expert intuition is that raising the ballast will decrease the roll inertia and make it much easier to roll the boat. So I am guessing that part of your thought is that raising the keel decreases other inputs -- reduces 'tripping' if the boat is hit by a wave sideways, or etc?

Is there any math or other thought that makes this idea come up the clear winner?

Yay ! Someone who won't go "meh...", or "whatever... !" , the moment things get difficult !

( I suppose I should read you your rights ... you are allowed to switch allegiance to the "eyes glazing over" camp after the first thousand words!)

Without knowing more about your boat I'd have to guess there will be some ballast in the keel, and some internal ballast in the boat (either 'formal' dedicated ballast, or informal, such as a chain locker or machinery space situated low and amidships)

When you raise such a keel, the overall centre of mass does indeed rise somewhat (realistically we're probably talking less than 150mm, possibly much less).
However the radius of gyration of the whole system (and hence the rotational mass moment of inertia, or "roll inertia") does not change much.

One way of looking at this: because the mast is so much further from the CoM, and its mass is so spread out, it makes a considerable contribution to the combined gyRadius.

Moving the CoM of the hull and keel slightly closer to the CoM of the rig is not going to exert much influence on the combined gyRadius, and hence the "roll inertia"

Whereas losing the rig has a huge influence, and it's sometimes a major problem after a dismasting in high seas: even though the CoM is (slightly) lower with no rig, the "roll inertia" is so much reduced that the boat can have a perceptibly increased risk of wave action capsize.

In one other respect, it can be a good thing that the CoM is raised with the keel.

I say this because the most inexorable mechanism for a wave-action capsize is generally held to go something like this (although I've fleshed it out with some detail to do with orbital action, which I haven't quantified so it may be controversial):

Assume the boat is beam-on.

Assume further that the wave we're focussing on is at the point of reaching a certain critical steepness, which will result in the crest (consisting of aerated water) becoming detached from the body of the wave. This body of water will continue to act like a semi-solid ramp, advancing at up to forty knots in highly exceptional circumstances, but not taking the water with it (in fact the opposite is true as the boat starts up the face - see below).

The crest will avalanche under the influence of gravity down this ramp which is in turn moving towards the boat, consequently it may reach truly dangerous speed by the time it impacts the topsides.

(Consolation: This mechanism almost NEVER happens in deep water: what most people call "breakers" are actually spilling the majority of water harmlessly down the back, if you look closely)

I'll focus first on the approach of the boat up the flank, before the crest topples forward.

As the hull is lifted up the solid flank of the approaching wave, it has a tendency to conform to the 'new normal' waterplane, which progressively tilts increasingly steeply, causing the boat to roll the windward topsides up out of the water.
If you have a narrow waterline beam and a deep ballast keel (particularly a thin strut and a heavy bulb) this won't happen to any great degree, and in this case raising the keel might be a bad idea.

However with a typical swing keel, which has a reasonable amount of planform area but often is only ballasted towards the tip, leaving it down might do more harm than good in this phase, even with a relatively narrow waterline beam (rare in modern designs).

I say this because there's something else happening here which is insidious and not widely recognised:
(I'll assume anyone who doesn't know about the orbital motion of a notional water particle will look this up on the internet)

Viewed side on to the direction of advance of the wavetrain, individual particles of water are moving contrary to the direction of advance, during the bottom semicircle of their circular orbit.

The radius of the orbit is increased in direct proportion to wave height. If the period is short in relation to height (eg, steep waves on the point of breaking), this speed can be substantial for high or very high waves.

This means the surface water is trying to carry the boat sideways towards and then up the face of the wave by means of the underbody, including (if it's down) the keel. The conservation of linear momentum means the boat's mass resists this lateral acceleration. Because the combined centre of mass is well above the centre of lateral resistance (more so with a lifting keel fully down), this resistance creates a roll moment, again rolling the windward topsides out of the water.

This I think is a bit of a worry, because if you track the change in direction of the water particle vector, as the boat travels up the wave and the keel swings towards the horizontal, the vector is simultaneously swinging towards the vertical (travelling vertically up, carrying the mass of the boat with it against gravity and inertia).

If I'm right about this, the force, at least in theory, could 'follow' the keel around, still tending to increase the roll. If I'm not, most of my explanation (and my conclusions) will, I believe, still apply.

Remember that a typical modern hull form, with plenty of "form stability" (wide waterline beam) is already likely to be at a considerable roll angle as it tries to conform to the local waterplane, which could be approaching 60 degrees to the horizontal in extreme cases.

The problem with all this is that the crest of the wave is about to impact the hull, and we very much don't want to be presenting a broad expanse of topsides, and a high gunwhale, to that crest.

It's the impact of the detached crest against this area (remember, that crest is travelling ABOVE the local waterplane, which is what floats the boat) which is generally believed to be the main motive force for a knockdown.

So rolling away from the oncoming wave creates several problems: it presents more area of hull to the crest, it presents it higher off the water, and it presents it more perpendicularly (angle of incidence closer to 90 deg)

So we have this very large force vector acting on the exposed topsides, acting substantially parallel to the local waterplane, but raised above it by about half the height of the gunwhale on that side.

We now have to think about the reaction force to this imposed, external force vector.

There are two: one is due to F=m.a, or the conservation of linear momentum.
So, (counterintuitively), we actually want the Centre of Mass as high as possible, ideally approaching the midheight of the exposed topsides, so it lies on or close to the force vector.

I think this addresses the reasonable question you raised, which reflects good common sense and conventional wisdom.
After all, we grow up thinking of ballasted keels as conferring security from capsize: that's their primary purpose, certainly in classical recreational sailing mythology. (Which is a fancy way of saying, that's what we tell our womenfolk!)

The other reaction force might be thought of as the "tripping" lever arm, which you referred to. It's obvious that the deeper and larger the keel, the longer this lever arm, and consequently the greater the tendency of the boat to be knocked down. (Dinghy sailors who have to cross a bar regularly will be familiar with this effect). The problem is that the keel is down in green water, which furthermore is unhelpfully travelling in the opposite direction to the toppling crest.

I've only been in this situation once in an offshore sailboat, in seas of serious proportions, so I can't pretend to have plenty of experience.
We had our keel swung way aft, and almost retracted. The rudder was large and deep. Hence the centre of lateral resistance was only moderately deep, but well aft. We were doing maybe fourteen knots at the time of impact, so we had hydrodynamic lift up the wazoo. We were beam reaching whenever we could, (and at the time of our knockdown) because that took us in the direction of our only safe exit.

When I replay the memory tape, I have a sense of the boat turning her bow slightly away from the impact, so that although we were knocked down with the mast well below the horizontal, we surfed down the wave in that attitude without tripping*. We didn't have enough keel down to trip us, and I think it was our residual forward speed, skewed in a more favourable, down-wave direction by the reaction vector aft, which prevented us tripping over our leeward gunwhale. (The problem here is that the boat is thrown down into the trough, where the 'local waterplane' is relatively level, so the lee gunwhale becomes a real tripping danger)

*Our direction of travel during this surf was a compromise between following the mast and following the bow, but mainly the former. I think the lee bow back to the master section must have been providing lots of hydrodynamic lift, otherwise we might well have done a diagonal pitchpole.

[Andrew Troup]

Originally Posted by msponer

....I think some boats even have a bad dynamic pattern they go through with this, where they heel 'too much', start turning into the wind, and bleed off so much speed that the rudder loses control -- I think the shape of that graph is important, not just the magnitude: that if it's linear and gradual you don't get this little phrase the boat finds easy to just repeat.
.

The problem is worst with boats with maximum beam carried well aft. As well as turning the underwater shape into a wedge, presented at an angle to the waterflow, they have a horrible habit of lifting the rudder out of the water* at extreme heel angles, levering it up with their fat rump. I wonder if that's partly what you've noticed...

*or at least close enough to the surface that they start to 'ventilate', meaning they stall

Originally Posted by msponer

....That's interesting -- thank you. I'll have to pay attention with that model in mind to see it. I think some of these things are hard to see if you don't have a mental model that is looking for the right details.

Did you catch the diagrams I posted at:

http://www.cruisersforum.com/forums/...ml#post1074305

?

In post 39, earlier in that same thread, I also posted a view of the underwater shape of a modern hull heeled to 40 degrees (illustrating the wedge effect I mentioned above, exaggerating that nasty syndrome you referred to as "turning moment induced by hull")

The direction of forward progress can be inferred from a line through the keel and rudder.

[Jimbo485]

Going downwind in heavy weather (APPARENT wind well behind the beam)...

1. No main. It is fully battened and can be reefed while running if we are too slow or too lazy to prepare in advance. We do not round up in heavy weather in order to reef it. The jibs are there to keep the bows held downwind with the sterns dragging along behind. Just like an arrow with the weight at the front. Or a front wheel drive car with the motive force at the front.

2. Usually windward daggerboard down, leeward daggerboard up. One is down to help the directional stability, while the other is up to avoid tripping and flipping. Applies only to cats. In less than heavy weather, it is the opposite or they are both up.

3. Speed. Once the wind gets up to consistently above 30 or 35 knots, we often go bare poles. The boat speed is still somewhere between 4 and 9 knots, depending on the degree of surfing and the autopilot can control it fine. The next step is a drogue and we haven't had to do that yet on this boat, so I won't comment. That same low that capsized the yacht recently between Tonga and NZ found us hurtling east from Savusavu to the Lau Group with bare poles. Still too fast. 10 miles off our destination, we rounded up and simply lay ahull until the sun popped up so we could get through the pass. Speed should be enough to maintain decent steerage, but not so much that it excessively buries the nose in the troughs and risks a pitchpole or broach.

4. This is a cruisers forum, not a racers forum. Our autopilot handles it all, even up to 17.5 knots surfing with the spinnaker up. (yeah, that was too much ) We don't hand steer unless the autopilot breaks down and we have have exhausted all possibilities of repair. The sail set up, balance and daggerboard positions are planned to look after the autopilot and hence make our lives easier, not to win races.


But of course, every boat is different and each of us also have different priorities in terms of risk aversion or adrenalin chasing. So YMMV. The only way to know is to get out there, start somewhat conservatively and then push the envelope a little more each time until you and/or your boat doesn't feel comfortable or you start breaking things.

[goboatingnow]

The last few posts here have been very interesting and have me digging out my Marchaj .


Dave

[Strait Shooter]

Great, now I have to google "Marchaj" too.

Thanks all!

[Mr B]

My Cats 34 foot long and 14 feet wide, Not being experienced with centre boards it was trial and error,

Being a motor boat driver, I can use the drive leg to counter bad seas, too fast or too slow, Thats not a problem,

My first effort was too slow and both Centreboards fully down,

which caused the following sea to broach me hard and I almost went over sideways, The boat was at 70 degrees. The lee side centre board Dug in and stopped me from sliding down the wave, Lesson learned,

I then sailed with the windward board fully down and the Lee side fully up, I had no further problems,

In the big waves, I ran before it with the Autopilot working full time, and the drive leg down, That worked as a drogue or brake, so the the bows were always heading down wind,
I used the Genoa only for steerage and to maintain my speed, The main never went up in bad weather,

Guessing how much Genoa to have up, totally depends on the conditions your in at the time, more or less, depending on wave size,

To much Genoa up and your going to fast and its hard to control the boat, too slow and the following sea will fill the cockpit,coming over the transom, I did it twice,

But I was sliding backwards down the third of four very close together Pacific rollers, and the waves were going in two directions at the same time,

[belizesailor]

Originally Posted by Jimbo485

...

4. This is a cruisers forum, not a racers forum. Our autopilot handles it all, even up to 17.5 knots surfing with the spinnaker up. (yeah, that was too much ) We don't hand steer unless the autopilot breaks down and we have have exhausted all possibilities of repair. The sail set up, balance and daggerboard positions are planned to look after the autopilot and hence make our lives easier, not to win races.
...

Modern autopilots are pretty amazing, but be careful, I once did a very spectacular accidental jibe in moderately heavy conditions because I trusted my autopilot just a little too much.

I was running wing-on-wing, reefed jib and main, downwind in the Windward Passage. True wind about 35 knots. Seas about 8-12 with the 12 footers rolling in a at irregular intervals a bit off the starboard quarter. Auto was steering, and it was doing a great job, but I was sitting at the helm the whole time. I debated with myself about stepping forward to tweak a line, and then decided to do it. Right at the moment when I was just too far away from the helm to react in time, a larger wave rolled in from a slightly different angle. Auto of course can't see these coming...as a human helmsman can. It quickly shifted the attitude of the boat and over came the main. Fortunately the preventer did its job and no serious damage was done, but the preventer did make the ultimate sacrifice and parts of it flew everywhere.

Lesson learned: keep auto closely supervised in heavy conditions.

[Andrew Troup]

Yeah, et tu, Jimbo ?

(OOPS - on edit, it was belizesailor, quoting Jimbo)

You reminded me of an inadvertent gybe when some friends were commissioning a hydraulic autopilot.

It doesn't really relate to the topic, but I'll beg forgiveness for telling it anyway:

I was in the aft cabin calling out rudder angles when I noticed that it was staying one side of midway, and sensed that subtle hesitancy in the boat's body language, like a horse fixing to refuse a jump.

I got my upper torso out the aft hatch just in time to grab the mainsheet tail and furiously shorten it, rivalling an America's cup trimmer in a gybing duel, even as the boom was starting to hurtle across. The tail was not around a winch or near a spare cleat, and I was under no misconceptions that I could hand-hold the sheet, but I knew the friction of it running back out through the blocks would help cushion the shock and possibly save the rig. I was also afraid that the slack in the multi-part falls of the tackle might catch around something, like the binnacle (or my neck!) if it wasn't shortened in at least partially.

(My friends were at the transom, where the hydraulic pump, powered by a towed propellor on a line, and the windvane, were both situated, and we had stupidly all omitted to rig a preventer. It was a heavily rigged 40'er, full main, reasonable sea breeze)

I let go the sheet the moment it came tight again, but was nevertheless rewarded with a skiff of melted plastic from the outer cover of the braided sheet on my palm as it started to run out, gravity causing the rope to run across my skin for a few instants before I could snatch it away -- and run to the sink to plunge it in water - no serious burn resulted.

I know from painful experience that melted dacron can be a bit like napalm!

The friction did make a difference and there was no damage to the rig. But it was probably pretty stupid of me to stick my head out the hatch in the first place. Just because something worked, does not necessarily make it a good idea !

[Rakuflames]

Originally Posted by Andrew Troup

Yeah, et tu, Jimbo ?

(OOPS - on edit, it was belizesailor, quoting Jimbo)

You reminded me of an inadvertent gybe when some friends were commissioning a hydraulic autopilot.

It doesn't really relate to the topic, but I'll beg forgiveness for telling it anyway:

I was in the aft cabin calling out rudder angles when I noticed that it was staying one side of midway, and sensed that subtle hesitancy in the boat's body language, like a horse fixing to refuse a jump.

I got my upper torso out the aft hatch just in time to grab the mainsheet tail and furiously shorten it, rivalling an America's cup trimmer in a gybing duel, even as the boom was starting to hurtle across. The tail was not around a winch or near a spare cleat, and I was under no misconceptions that I could hand-hold the sheet, but I knew the friction of it running back out through the blocks would help cushion the shock and possibly save the rig. I was also afraid that the slack in the multi-part falls of the tackle might catch around something, like the binnacle (or my neck!) if it wasn't shortened in at least partially.

(My friends were at the transom, where the hydraulic pump, powered by a towed propellor on a line, and the windvane, were both situated, and we had stupidly all omitted to rig a preventer. It was a heavily rigged 40'er, full main, reasonable sea breeze)

I let go the sheet the moment it came tight again, but was nevertheless rewarded with a skiff of melted plastic from the outer cover of the braided sheet on my palm as it started to run out, gravity causing the rope to run across my skin for a few instants before I could snatch it away -- and run to the sink to plunge it in water - no serious burn resulted.

I know from painful experience that melted dacron can be a bit like napalm!

The friction did make a difference and there was no damage to the rig. But it was probably pretty stupid of me to stick my head out the hatch in the first place. Just because something worked, does not necessarily make it a good idea !

I try to convince people to wear gloves, but it rarely works.

[Capt.Fred]

Someone isn't paying attention