Stability Curve diagram

[oldbilbo]

I see these small diagrams in each of the new boat reviews, and wonder if there isn't a straightforward means of producing one for an elderly Cutlass 27?

I would have purchased an elderly PBO or YM review or two from IPC's archive, but there's no way of knowing in advance if that info is included - so I've demurred.

Should anyone have such a copy I could peruse, or can point me to an application that will chew on 'input' and present me with a neat little Stability diagram, I'd be pleased to hear from 'im.... er, her.....

 

[bitbaltic]

I had a chat about that on a recent YM theory course with a bloke with a proper boatbuilding degree. He reckoned that the answer was to tie up alongside then use a cherry picker or whatever to pull her down into the drink via a masthead halyard.

Not an attractive proposition. Although I'd have thought it was a part of prototype design even back in the day; but as no one has these curves for older yachts, perhaps not.

Cheers

 

[TimBennet]

There is software that will do that.

Model the Cutlass 27 in MaxSurf
Do your weights and moments in GCalc.
Run them together in HydroMax.

And it will spit you out a nice little diagram.
But quick . . . easy . . . cheap? Afraid not.

The easiest thing to do is find a boat that's been done and is of 'similar form'. The shape of the curve will be similar, even though the absolute values will not correlate. Can you find one for the Contessa 26?

 

[sarabande]

MaxSurf has a student version, and there is discussion on free software here:-

http://www.boatdesign.net/forums/design-software/basic-software-intact-damage-stability-5692.html


Here's a link to a lot of programs:-
http://yachtdesign.info/Software/

(I haven't checked for free ones)

Err, if it is not urgent I know two students at Plymouth doing Marine Architecture, and they may be able to advise, but I won't be seeing them till after Cmas.

 

[sarabande]

this page has the potential for inputs to produce some data, and also a library of already computed info. Looks a good start.

http://www.image-ination.com/sailcalc.html

No full blown stability curve though.

 

[Roberto]

I see these small diagrams in each of the new boat reviews, and wonder if there isn't a straightforward means of producing one for an elderly Cutlass 27?

I would have purchased an elderly PBO or YM review or two from IPC's archive, but there's no way of knowing in advance if that info is included - so I've demurred.

Should anyone have such a copy I could peruse, or can point me to an application that will chew on 'input' and present me with a neat little Stability diagram, I'd be pleased to hear from 'im.... er, her.....

There are two ways to derive a stability curve:
1. Direct measurement, the most accurate and often the easiest, especially for small boats; especially in classic boats with AVS angle of vanishing stability quite high, you may not be able nor want to determine it;
2. Theoretical computation, that means:
2a) having access to the line plan of the boat to determine the movement of the Centre of Buoyancy when heeled (plus flooding angles etc if needed),
2b) having detailed construction and layout drawings and specifications to determine the position of the Center of Gravity.
By definition, option 2 means having access to the designer and builder data, which is by no means an easy task. Also, not taking into account the various modifications that have been done to the boat during the years

Without these informations, no way any method or software is going to give a sensible result.

If you really want the info, do not dismiss the direct measurement, it's nothing to be afraid of. Mast, shrouds, chainplates etc are all designed to withstand a lot higher loads.

 

[oldfatgit]

So, all you need is a quiet day, a couple of piles to moor fore and aft between, and a nearby point to which to attach the mast head. Then winch her over whilst measuring tension (some kind of measuring device needed here) and recording angle of heel. Do some maths to resolve the righting moment and plot your graph. Do it for port and starboard and finally understand why she is better on one tack than the other. If you decide to go ahead can I come and watch? I'd be fascinated.

 

[Uricanejack]

First question. Why?

Unless you intend to overload the heck out of it.

Sailing vessels have very low centers of gravity. or are very stiff. due to large hunk of lead or iorn hanging underneath.
some angles can be figured out from hull cros section..
Angle of deck edge imersion.
on cross section draw waterline, bysect with center line, draw line though this point up to deck edge and measure.
or Tan angle of Heel=Freeboard/(0.5xbeam)
For angle of down flooding measure to first opening.
The angle of diminishing stability or
The angle of vanishing stability on a sailing vessel are so high you will have long since S@#$ yourself should you evere heel that far.
Most sailing vessels with external keels have vanashing stability angles well beyond 90. in otherwords they are allmost self righting.

 

[Skylark]

If you decide to go ahead can I come and watch? I'd be fascinated.

Can I watch, too, please. Especially when you're measuring the righting forces with heel angles greater than 90 degrees

Remember to allow plenty of water depth for once you've found the AVS.

An interesting practical application of acedemia.....but why

 

[snowleopard]

The diagrams produced for new boats are theoretical, produced by the design software. Beware of believing them beyond about 80° as they assume that when the superstructure is immersed there are no leaks and the hatches are shut!

There are formulae for deriving a theoretical AVS from a few simple dimensions. A friend had it worked out for his classic 60s Admiral's Cup boat. The figure it came up with was 185°, a clever trick! Here is a website that claims to do it.

For angles up to about 40° you can do practical tests involving hanging weights on the boom and measuring the heel.

 

[BruceDanforth]

I see these small diagrams in each of the new boat reviews, and wonder if there isn't a straightforward means of producing one for an elderly Cutlass 27?

I would have purchased an elderly PBO or YM review or two from IPC's archive, but there's no way of knowing in advance if that info is included - so I've demurred.

Should anyone have such a copy I could peruse, or can point me to an application that will chew on 'input' and present me with a neat little Stability diagram, I'd be pleased to hear from 'im.... er, her.....

Get a lot of hefty folks of known weight standing on the pontoon. Transfer them 1 by 1 to the side deck. Measure the clinometer. Stop when people start falling off. Retire to the bar. Do the maths.

Software indeed....

 

[lw395]

There are two ways to derive a stability curve:
1. Direct measurement, the most accurate and often the easiest, especially for small boats; especially in classic boats with AVS angle of vanishing stability quite high, you may not be able nor want to determine it;
2. Theoretical computation, that means:
2a) having access to the line plan of the boat to determine the movement of the Centre of Buoyancy when heeled (plus flooding angles etc if needed),
2b) having detailed construction and layout drawings and specifications to determine the position of the Center of Gravity.
By definition, option 2 means having access to the designer and builder data, which is by no means an easy task. Also, not taking into account the various modifications that have been done to the boat during the years

Without these informations, no way any method or software is going to give a sensible result.

If you know the weight accurately (or can measure the waterline), and have the hull lines, you can do a couple of measurements to deduce the CofG height, then calculate the rest.

When measuring it, you need to be clear whether the heeling force is supposed to be pushing the boat down (like fat people on sidedeck) or across (like the wind) as pushing the boat down will immerse the hull further, hence increasing stability (in general).