Stability curve

Sailvayu · Oct 11, 2014

You may be able to find it by searching boat reviews as I have seen this published for some boats in the review. There is a way to figure righting moment with weights but it has been years since I did any of that have forgotten much of the details. I do not know of any real way to calculate the full curve without a lines drawing or at least the offsets. I could be wrong like I say the last time I messed with that was in the 80s

Jackdaw · Oct 11, 2014

Normally for boat this number is calculated by the designer. I can guarantee MacGregor never did this for any of their boats.

But for a trailer sailor this is very simple.

Keep your spreaders out of the water and you be fine.

Get them wet and your troubles begin.

Honestly, most trailerable boats have an AVS (angle of vanishing stability) of about 110-115 degrees. That's the mast top being in the water. Avoid that.

finding41 · Oct 11, 2014

Diver. Once your rudder is out of the water your boat will head up. I know I can't get water in the cockpit or knock over one of my Mac 26D's.
My Siren 17... I can get water in the cockpit while still sailing strait. But the knock over... I'll let you know next year.

topcat0399 · Oct 11, 2014

I have had the side decks in the water more than once but it takes damn careful steering to get her over that far and not round up. I think its exciting; the Admiral not so much.

If we ever get some decent sails I will probably push this boat a lot harder....

Jackdaw · Oct 12, 2014

Stability indexes are used to reflect a boats resistance to capsize due to a breaking wave or to a lesser degrees, an overloaded sail plan taken squarely on the beam.

Any boat will round up with enough wind while going to windward. First she'll put her rail in the water, which looks exciting but is very slow.

Bill1565 · Oct 12, 2014

Why do you need the stability curve. Your boat will round up when over powered, not capsize.

topcat0399 · Oct 12, 2014

Diver960 said:
That's correct, Jack. I've deduced that "the minimum recommended LPS...(AVS)...for traditional designs under 40' can be approximated by the following simple formula: 160 degrees minus the LWL in feet."
My issue is that barring exceptional circumstances, I don't ever want to get to that point, so if I can figure out what my angle of maximum stability is, or get somewhere close, I can use my instruments during sailing as a preventative measure and enjoy myself a whole lot more while pushing my boat closer to its limit without fear of being knocked down. Again, I realize this is only one preventative measure, and the variables are many, but I'll figure it out eventually and add it to my 'toolbox'.
What I do know is that "as poorly understood as it is, stability represents one of the only measures of how well a boat will stand up to extreme conditions". (Leonard, The Voyagers Handbook, 2nd edition, p. 62). Since we are working our way towards a voyaging retirement, we are doing our best to get prepared for any contingency. Figuring things out on a small vessel, should assist us in transferring our knowledge and skill sets to our retirement vessel.

You should push the boat trying to find its limit at least a few times in my opinion. It will demonstrate to you the abilities of the boat and give you confidence in your equipment and yourself. I have TRIED to put this boats spreaders in the water and couldn't get her done - a round up happens.
I've been out in 25 knots with full baggy main and 130 hank on genny. Way too much sail but I want the experience. The admiral is such a good sport.

The only way you would ever capsize that boat is you being out in huge breaking waves and taking a bad one on the beam I'm thinking. That issue is more about the crew than the boat.

Sailvayu · Oct 12, 2014

Stability and a stability curve are based on hull form and weights. It has nothing to do with sail area or waves. Stability curves are done on most new designs as it will effect sail carrying, safety and comfort. It is sort of like calculating a pendulum and its momentum and how far it will travel. Every boat has a resistance to heeling based on hull form as well as weight placement. Weights are calculated and a figure called the metracentric height or GM is calculated. This is the height of a static equilibrium of the weights. This combined with hull form resistance (as in wider beam more heeling resistance) is used to plot a curve of resistance or the stability curve. This is done on power and sail boats. Comfort becomes a factor because if a boat is too stiff it will have an uncomfortable motion being very abrupt in its side to side motion. Designers strive to find a middle ground that will be stiff but allow a smooth (and more comfortable) roll from side to side. Of course sail carrying is also factored in on sailboats. As a boat rolls to its side the hull form adds resistance as the weights or GM move to counter this. Hence low ballast is harder to overcome than a high weight like the house structure. AS more of the freeboard is submerged its bouncy is greater and more resistance to heeling. At some point when the hull side is fully submerged this resistance drops off dramatically and the boat turns over. On mono hulls the heavy ballast is now up high and with luck will turn the boat back over. Hope that helps some understand not sure I did a great job of explaining.

Doc_holiday · Oct 13, 2014

I can't imagine a Mac going over without a rouge wave 6' over the rails just flipping it. Caught in the surf or something.

I'm pretty sure anyone in the cockpit is in the drink at around 70 degrees anyway.

That's all I got, no scientific evidence though. I do agree that at 60 degrees you won't have a rudder in the water and the boat will round up.

walt · Oct 14, 2014

If you could compare those curves between boats (which would be difficult), it would tell you some things about the way the boat will sail. Its not just the point way at the end of the curve that is interesting.

Remember that the righting moment (what is plotted on the Y axis) results from lifting a weight at the end of a moment arm that always originates at the boats center of buoyancy. Since its lifting against gravity that results in a force, it is only vertical movement of the weight that matters. Horizontal movement of the center of gravity doesn’t contribute to righting moment. And the moment arm depends on how the boats center of buoyancy moves from its initial at rest spot at the middle of the boat.

You will see internet post that a water ballast Mac is initially tender but stiffens up. This is not due to the water ballast moving above the water level - it’s actually due to the hull shape. As the boat heels, depending on hull shape, the boats center of buoyancy will move to the outside and create the moment arm. How fast it moves depends on hull shape. A "hard bilge" has the center of buoyancy move quick and a soft bilge has the center of buoyancy move slower. The old Macs tended to have a softer bilge (more V or rounded). The X is more of a planning hull so is squarer on the bottom.

I suspect that the curve for an X model vs. a D/S model would rise faster at very low heel angles (like stepping on the boat at a dock) and feel more stable because the center of buoyancy would move to the outside quicker on the planning (hard bilge) type hull.

The high location water ballast is also very good at having the moment arm move the ballast vertical at low heel angles– almost all of the movement is lifting against gravity. This should show up as a relatively fast rise in the stability curve at lower heel angles. This also says you want to sail a water ballast boat at lower heel angles as this is where you get the best bang for the buck with ballast weight.

As you get to heel angles on the high side of where you normally sail, how deep the ballast starts to really affect the curve. At low heel angles, the shallow water ballast really works well as the center of gravity movement is almost all vertical - against gravity. But as the heel angle gets high for the water ballast boat and it’s rotating about the center of buoyancy, the center of gravity starts to move more sideways - less up and down. Sideways movement does not lift against gravity. So in this region of the curve, you would see a deep keel boat like the older 25 keep on increasing its righting moment (and get stiffer) but the water ballast boat will have its righting moment level off - and not be so stiff. This is a tradeoff you make with the water ballast boat. In higher winds, the deeper ballast boat is less work to sail as it hits some angle and gets stiffer. The water ballast boat doesn’t stiffen up like this - but you can compensate by working the sail more - and I think this is also why a traveler is so nice to have with water ballast.

At very high heel angles - past where you would want to sail - the freeboard actually starts to influence the stability curve. At very high heel angles, the boats center of buoyancy actually starts to move from the bottom of the boat up the side of the boat. The higher the freeboard, the more the center of buoyancy can move up the side of the boat and create a longer moment arm lifting the boats center of gravity. Longer moment arm = higher righting moment.

So at very high heel angle, the deep ballast boat (like the 25) would have a higher righting moment than either the 26 S/D or the X and this should also show up on the curve. But because of the X higher freeboard, it would still likely have a higher righting moment value than the D/S (maybe.. the V hull shape of the D/S probably results in the center of gravity being a little deeper than the X - also increasing the lenght of the moment arm).

walt · Oct 15, 2014

What did you think about the Catalina 27 you rented compared to your boat? (hope I got that right). That boat has a fin keel and the ballast alone is about the weight of my entire boat and trailer..

I think I would find it a luxury ride.. but the old water ballast is more practical for where I live