Stability at sea

[John Shasteen]

For those of you who wonder whether your boat is a good boat to venture well offshore (comfortable and stable in a seaway), here's a link to the SailingUSA calculator for the Angle of Vanishing Stability - doesn't take much to figure out what that is.
http://www.sailingusa.info/cal__avs.htm

[John Shasteen]

I'm surprised that all you cruiser wanta-be's (cruisers probably already know), wouldn't be interested in how stable your boat will be in a blow. I knew the AVS of Paloma before I bought her years ago and know it's accurate, because she's been knocked down to port lights fully emersed and popped right back up and quickly nosed up into the wind.
Check your boat out and see how it measures up to known/proven off-shore statistics

[PCarrico]

For all the offshore race boats, there are measurments that give each boat the moments of inherent stability.
There is an inclination method of hanging jerry jigs of water on the ends of the spinnaker poles rigged perpendicular to the centerline with a huge water level on the stern to measure degrees in tenths.
There were minimum moments of stability for racing. I think the minimum angle was about 115 degrees, but with the new IRC rule, we're seeing boats that are much stiffer now with measurements of 130 and higher. All this stability turns into boatspeed, especially on a reach or upwind.
There are two general types of stability
1. Form Stability, such as a beamy boat or a catamaran, then there's keel stability where the lower the center of gravity, and the deeper the keel, the more inherently stable the boat becomes.
The boats I worry about the most are the shallow draft cruisers that rely on beam for stability. These are great for cruising the Bahamas or Keys, but broach one of those babies with too much sail up, and they might not come back up.
I forget how many times I've been on a broaching offshore sailboat, but we force boats to broach in our training.
This is especially fun with green crews.
We'll be sailing along with the spinnaker up, and I'll whisper to the crew holding the boom vang control.
Let's say it's blowing 20 knots, the spinnaker's up, boat stable sailing at about 165 degrees true.
I'll say something like, lets come up a bit to go behind that anchored merchant ship.
Then I'll ask the crew on the afterguy to ease the pole forward to about a foot off the headstay, then I'll ask the mainsheet trimmer to start trimming on the sheet.
It's a great look on the helmsman/helmswoman's face when they start to realise that the rudder's starting to loose it's grip, and then the big eyes on all the crew when the boat winds up into a broach.
Once under control, I'll then tell the crew about how these 3' waves can be 30 feet in the ocean in Gulfstream storms, and by the time we finish our summer, we should be comfortable sailing in the ocean with the spinnaker up in 30 knots of wind.
Still, we all take baby steps generating confidence and skill. No matter what the conditions, it's very important to know your boat's capabilities and limitations. Most cruisers will have lots of loose equipment on board as it's more a home than a machine for pushing to the limit.
One a race boat, everything is stowed low and secure for not the "if" but "when" the boat broaches.
Good luck, and be careful.

[Art Dufresne]

IMHO one big issue with the stability calculations and tests is that they ignore two important dynamics...waves and wind.

Since most broaches and capsizes are likely to occur when the wind and waves are significant you need to consider the boat profile, rigging, etc

A beamy boat turned on it's side presents a large area for wind and waves to exert pressure that is acting against the righting moment. In dinghies you see this where some dinghies are prone to turtling rather than just laying over, some the bow likes to spin up toward the wind while others just sit perpendicular to the wind.

So I suggest that you do the calcs but look at hull form and rig as well and make a mental adjustment to the numbers

[John Shasteen]

The Angle of Vanishing Stabilty takes that into consideration by comparing the beam to the keel depth and weight. Most of the boats with high AVS also tend to be the boats that most people consider as bluewater vessels. Many years ago, I bought Paloma, a Bristol 29.9, which I knew by reputation was a bluewater boat long before I knew about AVS - and true to reputation, she has endured two Force 10 gales at sea.

[PCarrico]

The sailingusa.info link is dead.

[Mike Brown]

Yep the link mentioned didn't seem to want to cooperate so I googled and found this site. Check it out.

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

[PCarrico]

I recognize that list of boats mainly because of the data for the Navy 44.
I put the data for the Navy 44 into the Harken load program about 15 years ago.
Harken stopped using that program about 10 years ago, and I've been wondering ever since what happened to that program. It's possible that someone else gathered rating information, but since most of those boats aren't racing boats, that possibility might be less susceptible.
I suppose that data was then utilized for the sailcalc program.
What the Harken program did was enable you to calculate sheet load to help determine what block would be suitable for your boat.
You could have two 44 footers, but with the displacement ratio different (ie; one boat stiffer than the other) that the lighter boat could go with the smaller jib lead blocks for example.

[Mike Brown]

It looks like it's been fairly well kept up to date. I couldn't find my new Benny 43 on it but there are a number of recent models listed.

[John Shasteen]

Well for some odd reason, the Sailing USA link is working again: http://www.sailingusa.info/cal__avs.htm Now you can see if it's safe to leave the dock in your boat.

[Mike Brown]

Well it looks like we tip easily but apparently we should pop right back up. Reassuring!? Actually all indications are we have a light displacement racer/cruiser. Not what Beneteau bills the 43 as but that suits me just fine. Just can't wait to sail her come spring time.

[John Shasteen]

Paloma's AVS is in the low 150's and we've been knocked down to port lights and handrails in the water with the sails filled with water. She quickly rounded back up into the wind - with water going everywhere as the sails emptied and leaving everything in the cabin in a mess..