Stability of ferries and sailing boats and motor sailers?

[NUTMEG]

Discussing the awful events unfolding in South Korea with the missus I made a discovery.

We were talking about the degree of list the ferry seemed to be unable to recover from. Either because of shifting loads or water ingress she went on to capsize and tragically sink. The figure of 5 degrees has been bandied about the press. I mentioned that our new boat, a CW 28'6" ketch rigged motor sailer would recover from a something like 90 degrees due to the three and a half tons of ballast encapsulated below the water line.

My dear missus was astounded. I went on to explain that sail boats often (?) suffer knock downs that lay them nearly flat in the water and they pop up again thanks to big metal lumps hanging down below. Something those fairly flat bottomed ferries do not have. She honestly thought that from about twenty degrees of heel (list?) a sailing boat would continue on to capsize.

Now I am wondering if I have over egged the pudding a bit. My wife is very nervous of boats and I am desperate to reassure her that the new boat is safe.

Does anyone know what the righting moment (is that the correct phrase) of a CW28 is? How stable is she and from what angle will she 'pop back up' from?

Thanks in advance

 

[Searush]

If properly designed & with all hatches & doors closed she ought to recover from 180deg - ie totally inverted. Part of the ferry problem is that the cars & trucks on the car deck will all slide to the side at quite a low angle of listing, then the problem just keeps getting worse.

 

[NUTMEG]

Thanks Searush. I'll see if I get anymore replies then will bring the assembled wisdom of these forums to the attention of the missus!

 

[Searush]

Give her a masthead halliard on the dockside one day & ask her to pull the boat over.

 

[AngusMcDoon]

Read all about it here...

http://sailskills.co.uk/Stability/sailskills_stability_stability_explained_AVS.html

For most of us here with the type of sailing we do it's just never going to happen.

 

[gregcope]

Give her a masthead halliard on the dockside one day & ask her to pull the boat over.

In Jersey about 5 years ago, the skipper/owner wanted to replace something at the masthead.

Whilst waiting outside the Marina, I noticed that the due to the tide being out, the Wall was about mast hight.

I thought we could just lean her over a bit, and do the work from the wall. Everything lined up. We needed about 20deg of heel to be able to reach the masthead from the wall.

So, we took the main the wall, and a bollard, and winched.

Could we get more than 10deg with the winch?

No.

The righting moment of loads of lead in the keel on a Sailing boat is huge ...

The righting moment of Ferrys is much different.

 

[doug748]

AS Searush has said, the RYA stability index gives all the Fishers (sorry no data for Watsons) an ISO AVS of 180 and a commendable Stix of 33 for the Fisher 34.

See here:

http://www.rya.org.uk/newsevents/news/Pages/stabilitylist.aspx

Click on: View Stability List.

 

[PabloPicasso]

Yes ocean going yachts don't capsize unless the keel falls off. Not impossible. But can capsize if a breaking wave of equal height to the width of the boat hits beam on, or equal height to the length of the boat can pitch pole the whole caboodle. But for most of us being out mid sea in braking waves of 10ft or more is unthinkable. Breaking waves now, not swell height, it is the C shape of the breaking wave that can flip a boat over. Usually taking the mast off in the process. But you'd need force 10 or more for this I would suggest, or some very unusual conditions.

For most of us a prediction of F5 might be a limit, less for many, and a bit more for some. A few May decide to venture out in F7 and above F8 I suggest it would be just daft. But for sailing with the Misses who is nervous about the whole thing a max of F3, with clear sunny skies, and short duration with a lovely restaurant at the end is what's needed. Then the thought of breaking waves rolling the boat from any aspect is just not an issue

 

[NUTMEG]

I have,certainly, no intention of exposing the missus to anything above F3 within the confines of the R Blackwater. I think, as you say, a few nice quiet trips in very settled conditions are the order of the day. Next year we will tackle Biscay in December!

 

[NealB]

Sailing boats (except dinghies, multihulls, and a few other exceptions), rely on ballast to bring them upright once they have heeled (the analogy to the old-fashioned child's doll that keeps getting up when you knock them down, is often used).

Ships, fishing boats, multihulls and rafts rely on form stability. ie they're roughly shoe-box shape. Very stable at first, but once they they go over, they can't right themselves.

 

[JumbleDuck]

We were talking about the degree of list the ferry seemed to be unable to recover from. Either because of shifting loads or water ingress she went on to capsize and tragically sink. The figure of 5 degrees has been bandied about the press.

I'm sure that's far too low.

 

[LittleSister]

If you have a look on YouTube you can see a variety of yacht knockdowns/capsizes - variously, big swells side on; over-canvassed racing yachts getting out of shape; and some deliberately done (with, e.g., crane and strops). You'll see that all need huge power to overturn them, and they bob straight back up.

 

[Searush]

No stabilisers visible on that.

The Korean ferry was carrying vehicles & cargo that could very well have shifted with a smaller list than shown in that vid.

 

[Colvic Watson]

Rest assured that you will never, ever, capsize that beauty. A CW will roll a fair bit in a quartering sea, pitch a bit going into the waves, and won't go close to windward at all but if the angle of heel is more than 15 degrees then you're stopped on a sand bank and starting to dry out. We've never reefed our CW34 in under 25 knots of wind - they're somewhat under-canvassed! In 22 knots constant plus a bit of gusting, all white sail up there was a perceptible heel to the girl and it's fair to say a coffee cup would have needed a non slip mat; but we tramped along at 6 and sometimes 7 knots through the water, just the conditions a CW excels at. We always said no family sailing above a F5 but with a CW we've raised that one or two wind forces, I can't think of a safer yacht for you and SWMBO.

 

[William_H]

Sail boat stability

DSC_1897.jpg (96.1 KB)

Here is a photo of my little boat for OP to show to his wife. The excercise was to measure force required to hold down the mast. In this case 40kg at the hounds. (top of the side stays) This just met the requirments for cat 5 ocean racing tough it is a relatively tender boat having a ballasted centre board 100kg and more lead under the floor.
So certainly OP boat will be far more stable.

Interesting the video on the cruise ship and I am glad I was not aboard. I suppose strictly speaking healing is the angle away from horizontal so quite large. However boats that rely on form stability and most do to a greater or lesser degree, will always heel in a side swell. The face off the wave may be say 20 degrees from horizontal which means that because of form stability the boat will roll that 20 degrees because one side of the hull is lifted up higher than the other. This roll can move cargo of course but can not be considered heel in the sense of the stability failing. Of course the pendulum effect of a lead keel will tend to hold the boat horizontal but mostly from trigonometry this does not have real effect until roll is much more than 30 degrees.
I will certinly be interested as will the world to know why this ferry capsized. olewill

 

[vyv_cox]

Our Colvic Northerner Cecilia draws about 0.7 metres, very round bilged. All ballast is internal lying on the hull, small bilge keels only about 30 cm long, even less as the centre keel. She rolls a lot but seems very stable. Last weekend we brought her from Milford Haven to Port Dinorwic in less than ideal conditions of north/north west moderate winds and spring tides. Off Strumble Head and Bardsey Sound we met quite lively overfalls that chucked sizeable weights of water over the wheelhouse. Rolling was quite considerable, maybe 20 degrees (?) but there was absolutely no fear that we would be capsized. I was quite surprised how quickly we returned to upright after each wave.

 

[Greenheart]

The ferry disaster is appalling of course, but its instability in this matter mustn't be allowed comparison with the behaviour of sailing yachts - nor should such confusion be permitted.

The ghastly Estonia and Herald of Free Enterprise ferry disasters involved bow-door failures which enabled ingress of huge weights of water, which naturally destabilised the vessels...

...and whatever happened to the Sewol, as an ocean-going vessel, it's unlikely that without a critical movement of ballast or cargo, she could have rolled & sunk in calm seas.

Nobody's SWMBO, however much of a novice, ought to confuse such disastrous loss of designed stability, with the ordinary, exaggerated rolling of a hefty long-keel motorsailor.

My SWMBO squealed when we heeled over 25°...but we were in a dinghy, and she had no toestraps or trapeze harness! If she hadn't squealed, she'd have heard my gasps of dread.