Stability curves - how relevent?

[Guest]

Partly as a result of looking for a new boat, and partly after reading a rash of articles in YM and YW, I have been thinking about it a little. I am not entirely sure I agree with the emphasis on stability curves to point of excluding all else. Surely how you handle the boat is the dominating factor?

A few points:

1. Any boat can be rolled if hit beam on by a moderate breaking wave. The determining factor is size, not how high your AVS is, for most boats. Given that, the only important issue is being confident the boat will right itself. Since most boats lose their mast in a roll, very few would not pop up pretty quickly. The worst thing that could happen I imagine would be if you were rolled with some sail up, and did not lose it.

2. You don't get rolled if can keep your bow or stern to the waves at all times, or if you were hit at an angle the boat would tend to align itself fore and aft rather than broach. The reality is that most boats are vulnerable to being broached and rolled if left to their own devices. A way to address this is required - the most obvious of course being a drogue to force the boat to slew when hit.

There is an interesting report prepared some time ago by the US Coast Guard:

http://www.sailrite.com/droguereport.htm

This goes into the mechanics of breaking waves, and when compared to anecdotal evidence elsewhere, is fairly convincing in its conclusions:

1. Any small boat without a drogue is asking for it (this is the short version!). With a drogue, you have a high chance of coming through unscathed - in other words a boat with an AVS of 100deg and a drogue is in far better shape than a boat with 160deg and no drogue - and if you have a drogue out in both cases, the AVS is probably irrelevent.

2. Only stern drogues should used. One from the bow is simply not as good, and exerts much higher forces. They recommend a series drogue as being more reliable (the best being the 100+ parachute type).

3. To use such a drogue requires stern cleats capable of standing at least 5 tonnes each for a 40 foot boat.

4. The boat may be swept from astern, so they recommend that no crew remain in the cockpit, and everyone locks themselves below for the duration. If you think about it, not much is gained keeping watch - you might be better served sitting below watching your instruments and sending regular security messages of your position and lack of steerage.

So survival comes down to strong cleats and washboards. Oh, and if it all does go wrong, enough fuel to get home (see below).

In YW this month was an article about a Swan 65 that was rolled and dismasted. This is a very big boat to get rolled - quite unusual I suspect from the various reports and statistics. I may be wrong, but she appeared to be running without a drogue, and the watch was on deck. As they say, it all happened real fast, and they were lucky not to lose crew. Interesting point to note was that they felt the boat right itself very quickly after the mast snapped.

Any thoughts?

P.S. I am going to buy/make a series drogue

 

[ean_p]

mmmm a few thoughts
1 first requirmennt is to not capsize...
2 second is that boats with masts resist capsize better than those without (inertia)
3 those with higher avs right themselves quicker (all other things been equal ie they need to be disturbed from the inverted point of rest 180 degrees to a lesser extent then craft with a higher avs
4 having been inverted to any extent a primary requirement is to keep water out as this reduces the value of the crafts avs.
so
a boat with good strong rigging and a high avs and small cockpit and strong ports hatch and companionway is the ideal anything else is moving away from the ideal......

 

[Twister_Ken]

And the rudder?

I hate the thought of streaming a drogue astern. That way the transom and the rudder take the force of the sea. How long will the rudder resist? Will water be forced through the exhaust and into the cylinders? How long will the wash boards stand being battered by breaking seas? Will the cockpit drain quickly enough to be empty by the time the next wave crashes aboard - if not you'd better be confident the cockpit lockers are water tight, and that the stern will rise to meet the next wave with a ton of water weighing it down.

The pointy end is the one which should meet oncoming waves, not the blunt end.

 

[Guest]

Re: And the rudder?

All true (also ean_p's note).

The point is that priority number uno is not to get rolled in the first place. I go with their assertion that a long drogue from the stern is the preferred method - I can understand the physics of this. If this is the best method (and I really want to confirm this), then surely one should consider how to achieve it, rather than using an inferior method - particularly if you are starting from the view of getting a new boat built for long distance cruising.

They say this is the preferred method, but can your boat take it? You need strong attachment points, and some confidence that the boat won't fall to bits. I don't have that confidence looking at most of the products on the market.

As you say, things you need are:

1. Massively re-inforced stern cleats
2. A cockpt that when full does not significantly affect the bouyancy aft
3. Hatches that can take a pounding
4. Cockpit lockers that are not open to the inside of the boat, and do not affect the bouyancy.
5. Superstructure & windows that are strong enough or can be reinforced when required.
6. As a point, I would say that an engine exhaust should have a sea-cock
7. A strong and well supported rudder (all metal?). I can see that a transom hung rudder is very vulnerable, but one on a full skeg underwater?

I am warming to the idea that a metal boat (including superstructure) might be best - preferably aluminium to improve performance.

One of the reasons I was re-visiting the stability issue was that one of few such builders is Alubat - the Ovni 435 and 455 fit most of my other criteria, but have an AVS of 115-120 deg in standard configuration due to their internal ballast design (centreboard).

I am going to see Alubat next week, and I intend to ask them about these issues (they are semi-custom and seem happy to make changes if you are willing to pay - the prices are still very reasonable though - an Ovni 435 is 60% of an HR43).

Another interesting point, which was also mentioned in YM, was that the situation could be improved in a centreboard boat if you lifted the keel - this reduced the tendancy to "trip" in a broach, and you can clearly theorise that if you had a stern drogue it would tend to enable the boat to slew more easily when hit by a wave, pivoting on the drogue and rudder/skeg. The bulk of the ballast in an Ovni is in the boat, not the keel - so I would think it would not dramatically affect the AVS if you lifted it.

My rather simplistic view in summary is:

1. The boat should be pulled from the front (windage on mast and bow generally) - a small sail if necessary.

2. It should be restrained at the stern - drogue, skeg, rudder

3. It should be as free as possible to weathercock from the stern - so try and concentrate lateral resistance at the stern - large skeg etc, and minimise keel area forward.

The above is probably the opposite of what you need to heave to, so a variable keel (such as a centreboard) might offer advantages in its flexibility.

regards

 

[halcyon]

Re: And the rudder?

Interesting point, our Halcyon 27 had been modified by the previous owner and done 2 trans-Atlantics, some mods were

lazerrette added, reduces cockpit by 30/40%, and increases bouyancey of stern, and storage.
2 cockpit drains added, 4 inch diameter.
All hatches and cockpit seats have two straps to hold them down.
Hatch has two masive catches that swing down and lock either side of washboards, stops hatch moveing for or aft, and locates washboard inline with hatch cover.

Are we danger of worrying about a minor problem ( on probability of meeting it ) and ignoring problems that you will meet. Looking at our boat know in retrospect, most mods are for day to day sailing / living, and average weather / sea state. What they will meet, not what they may meet.


Brian

 

[kgi]

Re: And the rudder?

the problem with a drogue from the bow of the boat is as the boat is picked up by the wave and thrown backwardsthe rudder is put under intense torsional stress (remember a rudder is built to handle the stresses front to back not back to front) and it will damage/destroy your rudder/steering system

 

[AndrewB]

Beware wishful thinking!

There is a lot of experience behind the belief that a high AVS and small area of negative stability is very desirable in an ocean cruiser.

Sure, plenty of people have done the ARC in low displacement yachts, the Ben/Jen/Bav's. But then the chance of experiencing winds of F9 or more on the trade wind routes are really remarkably low. And boats used to be smaller - you can get away with much more in a larger yacht. But once you have experienced real heavy weather, your ideas about ideal boats change.

When we were ocean cruising I made a point of asking other yachts about heavy weather experience, specially regarding rolling/inversion, and what survival techniques lightly crewed yachts used. I found 12 crews that had experienced some degree of inversion in heavy weather - one believed they had pitchpoled. Not one had lost their mast.

None of them were using sea-anchors, drogues etc etc. You don't the first time you encounter really heavy weather in open sea. The Hiscocks didn't, Adlard Coles didn't, the Pardy's didn't. What they all did the first time, as well as 10 of the 12 crews I spoke to, was heave to, make everthing as secure as possible, go below and rely on the yacht to see them through.

Incidentally, I've never seen the technique of trailing a drogue and leaving the yacht to its own devices advocated. It has to be steered to prevent rounding up.

What do you think about when inverted? The crews I spoke to were unanimous. When is she going to come upright again! Every second lasts an hour, with water gushing through vents, portlights, hatches, washboards - even if they are closed. There's a lot of value in a yacht that recovers quickly.

And it can happen real fast, so the Swan guys weren't necessarily being stupid. I suffered a complete knockdown beyond 90 deg in a quite moderate F6 in mid-Atlantic. Aries coping nicely, boat going well, perhaps slightly too much sail, but I wouldn't have begun to think of trailing a drogue. Just a freak wave out of nowhere.

Oh, and don't count on the engine to get you home! From the people I talked to, if you should invert, its far more likely your engine won't start than that your mast will break. But then, they almost all had stout yachts designed for ocean cruising.

 

[spark]

Stability curves seem to have some relevance, especially when the duration of inversion is considered: the quicker you are upright again the better. However, a high AVS is of little use if your cockpit lockers open into the hull or the washboards are weak/fall out, or the microwave/spare anchor/battery etc. are inadequately restrained and punch a hole in the hull. Safety is about covering as many of the angles as you can.

Probably the best thing you can do is read (and re-read) the latest edition of 'Heavy Weather Sailing' and then apply the measures that make most sense to you and your particular boat.

 

[peterk]

hi,
ANY boat can/will get rolled given the right/wrong conditions.
or - as your cousins so quaintly put it:
'go ass over tea kettle'
(get pitchpoled).
Just hink of TSU HANG's experience.

'Tehani' was rolled on June 27, 2000
at 42 S 100 miles west of Isla Chiloe
at the tail end of my 'storm of a lifetime'.

Horrendous waves, I estimated 13 to 15 meters(T.s mast is 12m)
huge also in the forward - aft extent
breaking all over ,
not just on top
like the fotogenic(computerbuilt)
ones in 'The Perfect Storm' did.

The mast stayed up,
even though
the inner headstay and the
starboard fwd lower shroud broke.
The boat stayed upside-down far too long for
my numbed perception/taste.

when it happened
it was dark and I I had been trying to muster the
energy to crawl on deck to raise the storm trysail again.

if interested try www.juprowa.com/kittel
go to FORUM
scroll down to 'Please, please tell me how...'
for part 1 and 2,
then scroll up again for 5 more shorts at 'Turned Turtle'

My opinion/advice: keep her moving, but not straight downwind


...peter