Adventures with Stability.

Thanks for the tip.

Regarding waves. Must say that breakers do concentrate ones mind. Guess on your travels you've seen them. We took a very large one travelling eastwards from the Azores a few years ago. It was a tad 'heart warming' and we were very lucky we didnt loose the few things we had left strapped on deck after seeing the sea getting up. Ripped out stanchions from cans tied to guardwires are gonna be a problem!
 
AndrewB said:
Of particular concern are amateur builders of small steel yachts who adapt the designs to give them better headroom. At one time this was all too common. I remember once getting onto a yacht supposedly of the same design as mine, which tipped almost like a dinghy when one stood on the side. The owner quickly gave up on plans of ocean sailing.

My own yacht had exceptionally deep bilges and it was actually an asset to stability to have them completely filled with supplies for long passages.
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At the Salon Nautique I asked about the possibility of a hard dodger for an OVNI. I was told that it would first have to be homologated for stability purposes.
 
I suppose they will stow the washing before they set of.
Pictures of older heavier boats, A bit cluttered but probably not much difference in stability.
I do wonder a bit about the exposure of the auxiliary engine hanging of the back of the second one.

Rough eyeball guess ok. Not sure what makes but guess longer keel.

If concerned a boats stability may be a bit under the cut.
Try timing the roll, Slack the lines in calm dock. give it a shove. and time the period of the roll with a stop watch.

Faster more stable
Slower less stable.

Really slow. Might be a problem.
 
I find that a useful way to think about stability is to remember that when a boat is rolled over (“a B2 knockdown”, to use the old terminology) she is going to be dismasted if she has any sail up, and maybe even if she has not.

A dismasting (I’ve only had a small one in sheltered waters) can spoil your entire day.
 
As raised on the GGR thread, there's a world of difference between 'Static Stability' and 'Dynamic Stability'. Is one any useful kind of guide to the other?
 
Before I make my comment, I will qualify... it is good and sensible for theskipper to ensure a yacht is fit for purpose. Which means for me that I would expect a boat that I would use to make a major offshore voyage to have a decent level of stability, be well designed to ensure that getting pooped didn’t downflood the boat, that the rudder wouldn’t be prone to catastrophic failure... etc.

But outside of that, I think this obsession with stability is a bit misfounded.. any boat will be inverted by a breaking wave on the beam that is equal to its beam. No matter the top hamper or lack thereof.

Now, I agree, it’s unseamanlike for many reasons to have a load of gear strapped to the rails or WHY... but you are far more likely to loose that gear i think than be inverted because of it... so when we look at the things we should be doing to avoid being inverted... ensuring the boat has a high avs is probably well down the list...and the addition of stuff is probably not going to a significant factor in a roll.. the real advantage of a high avs is the impact on inverted stability and the boats ability to self right quickly when it’s been rolled by a wave which may be unusual.

So, We need to be thinking more about boat handling, weather, and heavy weather strategies to avoid being rolled, than ultimate avs..

We need to be concerned with stuff on the deck because we are simply more likely to lose that stuff overboard.
 
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capnsensible said:
That is not my boat.......
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Oh I didn't mean to imply it was yours, more asking if winches are needed to sail downwind because with a solar panel sat on top winching will be interesting.

They need a nice big arch to mount solar, dinghy windy thing and fenders, its the way forward :encouragement:
 
Pete7 said:
Oh I didn't mean to imply it was yours, more asking if winches are needed to sail downwind because with a solar panel sat on top winching will be interesting.

They need a nice big arch to mount solar, dinghy windy thing and fenders, its the way forward :encouragement:
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Looks like the panels are moveable.....well ones propped up with a stick, anyway!

I kinda had in mind the Arch as a Big Thing in the stability of standard boats. It most certainly makes a big difference if you are trying to get a commercial stability category.

Here is a typical form:

https://www.rya.org.uk/sitecollecti...eb Documents/MCA Documents/MCA SS1 Sail 9.pdf

Obviously if you are not commercial, you can do what you like, but I wonder how many people tell their Insurers about such modifications? A stainless frame is heavy and when loaded with all the usual stuff, very heavy. Its bound to have some effect. But you do see many of them about in the cruising hotspots.

Plus jerrycans all over the place!
 
Indeed:




"Further Requirements

If a vessel under 15m (50ft) is fitted with more than one of the following:

• Roller Furling Headsail
• In-Mast or Behind-Mast Roller Furling Mainsail
• A Radar Antenna mounted higher than 30% of the length of the vessel above the waterline

then the Code of Practice requires that ... the centre of gravity of a vessel should be established by an inclination experiment and, in addition, a curve of static stability (GZ Curve) for the loaded departure, 100% consumables, should be produced."
 
capnsensible said:
Looks like the panels are moveable.....well ones propped up with a stick, anyway!

I kinda had in mind the Arch as a Big Thing in the stability of standard boats. It most certainly makes a big difference if you are trying to get a commercial stability category.

Here is a typical form:

https://www.rya.org.uk/sitecollecti...eb Documents/MCA Documents/MCA SS1 Sail 9.pdf

Obviously if you are not commercial, you can do what you like, but I wonder how many people tell their Insurers about such modifications? A stainless frame is heavy and when loaded with all the usual stuff, very heavy. Its bound to have some effect. But you do see many of them about in the cruising hotspots.

Plus jerrycans all over the place!
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A stainless frame need not be very heavy. I'm sure there are some excessively heavy ones about, but it's not necessary.
The weight is not hugely above the roll centre of the yacht.
It's weight high up which matters.
If you've ever sailed the same design of dinghy with both carbon and ali masts you will know this!
Situation where the boat is turned by a wave and the top of the mast is keen to carry on for example.

Fuel cans on deck probably have little effect within reason. Loading the boat may increase form stability at small angle of heel. In a knockdown situation, the cans will be buoyant, if only marginally. Betwen those extremes, do the math as they say.
Loose deck cargo is obviously a bad thing (including the 'self loading' variety).
 
Ive assisted at a couple of yacht stability tests. Shifting weights about. Normally conducted in the travel hoist dock to get flat water, its very difficult alongside.

Its surprising the effect the weights made, but regret I cant remember the numbers. But Im fairly sure all the excesses that are routine on long distance cruising yachts is gonna have a major effect on the angle of vanishing stability. And a big wave taking a rib and its davits off is gonna leave 'undesirable holes'. Have had some experience of the damage breaking waves can do. Its not my favourite thing.....

One number I do remember is that on my last delivery, I had 20 x 25l cans of diesel extra. I was able to stow those in the cockpit lockers (Bav 42) but they do weigh a lot.
 
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Remembered a Commercial anecdote.

Some friends I knew in Gib were trying to get a new Beneteau 50something rated Cat 0 for transats. They spent a lot of money including fitting two of the correct sized cannister liferafts mounted on deck.

Because of the liferafts, the yacht failed its stability test and they were refused a Cat 0 certificate.
 
I have a fairly heavy stainless steel cradle for two such life rafts on the the foredeck of my ex Cat 0 boat, just where I don’t want them, because it’s the obvious place for the dinghy, and she was built with what I consider a much better set up, with two self draining cockpit lockers for valise life rafts with vertical doors.
 
doug748 said:
If a vessel under 15m (50ft) is fitted with more than one of the following:

• Roller Furling Headsail
• In-Mast or Behind-Mast Roller Furling Mainsail
• A Radar Antenna mounted higher than 30% of the length of the vessel above the waterline

then the Code of Practice requires that ... the centre of gravity of a vessel should be established by an inclination experiment and, in addition, a curve of static stability (GZ Curve) for the loaded departure, 100% consumables, should be produced."
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I'm finding it difficult interpreting 'requirements' such as that above in respect of my 27' refurb project, while accepting the information is probably best practice as determined by an august group of expert designers. I try to consider the effects of changes I'm making and come to a 'qualitative' and subjective judgement. That ain't easy, especially as there are no Stability Curves or similar calculations available AFAIK.

The nominal displacement is 6500lb; the ballast 3250lb. That offers a B/D Ration of ~50%, which suggests a favourable situation in respect of knockdowns and AVS. Given the modest beam and immodest aspirations, I suspect I'll encounter breaking seas big enough and often enough to give me nightmares.

I don't intend to fit radar or in-mast furling, but do expect to have two roller-furling headsails. The ( weight of ) engine has been changed since Alan Hill designed the thing. I expect to stow the chain rode down in the hollow keel, under the engine and below the waterline, and not in a bow locker. I'd like to be able to have a stab at calculating how such changes are likely to affect the static stability, but don't know how.

Does anyone know of an affordable programme or method?
 
zoidberg said:
.......

Does anyone know of an affordable programme or method?
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That extract was taken from here:

http://www.radford-yacht.com/stablty1.html

There is a method for calculating AVS, half way down the page but it does need access to some tricky measurements so you may be no nearer. Note the warning that even designers find this work difficult.

Given your starting point and wary approach, it's difficult to see you going too far off the scale.
 
That's helpful, 'doug748', although I'll have to make some awkward measurements and a few careful presumptions. Intrapolating the table in Radford, I'm aiming at an Angle better than 145° for my <9m boat. I shan't be able to make any Inclining Measurements for quite some time, to establish a VCG, so I'll have a stab at the Formula.

For the avoidance of any doubt, there is not the slightest intention of 'Coding'. It's my own little pink bod I'm concerned about - not clients.

There are other concerns. Calculations on downflooding angles are all very well, but the more dynamic situations are worrying.... such as a breaking sea filling the cockpit with 2 cubic metres of seawater, for example, which is likely to take quite some time to drain away. That can, on some small boats, depress the floor of the cockpit sufficiently that water flows back UP the cockpit drains. There is an argument for part-filling the floor of the cockpit 'bathtub' securely with high-volume, low density objects such as a liferaft or a deflated, folded dinghy. Thus most seawater which lands in this area is quickly shed back over the side as the vessel pitches and rolls.
 
In terms of stability one needs take account of the dynamic effect of breaking waves. The breaking top is actually falling down the face of the wave, much like an avalanche, Because it is overtaking the water underneath it can induce roll into a vessel on the surface.... especially if the vessel is sideways to the wave and has a keel that prevents it sliding down the face of the wave...
 
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