View Full Version : Stability test

Brian Wood

11-29-2003, 05:21 PM

I wish to perform a stability test on my hull. Anyone know of a good discription or where i can view pictures of said test being performed on a sailing hull? The simplest, the better.

Hull is 21' loa, 17" lwl, 7" beam and 5' 5" beam at wl. Shallow v bottom, full keel w/cutaway forefoot. draft 18" (hull) 48" to bottom of keel.

Boat will be crusing in puget sound region.

thanks,

Brian

1. why not

2. to help to determine sail area

3. becuz I want to know

4. can this be accomplished with a mast, a boom, a known wieght, logging heel angle each time the wieght is moved out along the boom?

The copy I purchased of Skene's Elements of Yacht Design just arrived in todays post.

Thanks everyone for responding.

bw

[ 12-03-2003, 08:55 PM: Message edited by: Brian Wood ]

jwaldin

11-29-2003, 05:45 PM

What hull?

John E Hardiman

11-29-2003, 08:58 PM

When a question like this comes up it begs;

1) What type of boat (open daysailer, costal crusier, etc..)

and

2) Why do you think you need to determine the stability?

George Roberts

11-30-2003, 01:09 AM

The Coast Guard has a 2 part stability test for boats. It is required for boats under 20'. You can look at their web site and find it.

Lucky Luke

11-30-2003, 03:27 AM

...stability test required for boats UNDER 20' ????

does that go down to windsurfers? smile.gif , or do you mean OVER 20'?

Anyway, mmd's question remains: why a stability test for this boat?????

Ian McColgin

12-01-2003, 07:31 AM

You may well want to do the CG tests for insurance and registration purposes, but the must useful is also the most obvious.

You could do small angles by weighting the boom and doing the appropriate leverage-arm calculations but that only gets you boom into the water. I guess you could peak the boom up on the topping lift a bit as you go.

At angles approaching 45 degrees, it might be easier to simply hang a weight from the mast. Tilt toward the dock so's you can control how far it goes and add or subtract weight.

Normally, the maximum righting moment will be when the gunnel kisses the water, and whe'll loose righting moment after that.

You want to look at whether she experiences swamping when the cockpit begins to flood and whether she experiences any significant down-flooding through hatches or cockpit lockers.

I had a nice little 22' cruising sloop I tested this way (back in mid-60's). Sank her at the dock when the starboard cockpit locker flopped open. Gave me one thing to improve.

If you've an inboard you may not want to carry this test to the ultimate. Same if the cabin furniture can't be easily cleaned.

If you've an outboard and much other on&off gear, try putting similar weights in place to duplicate.

Try the test with friends doing the tipping to see what your crew weight does.

I tested my little sloop all the way to inverted stability. It was the old Electra, like the more popular Ensign hull but masthead rig, longer cabin and small cockpit. She was fairly stabile +/- 5 degrees of inverted. In seas that could roll her, she'd get tossed enough to roll right back. We turned the test and subsequent clean up into a party. To my cowardly shame, I did not perform the test with myself in the cabin.

You don't really need this information for determining sail area of the basic rig, but it will give you an excellent insight as to where the "Whoopie!" passes to an expression that won't get past Scot's filtres.

Have fun.

Brings back memories of the old "Glasgow Roll Test". :D

John E Hardiman

12-01-2003, 01:53 PM

Here is ASTM F1321-92 (http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/F1321.htm?E+mystore) Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity of a Vessel. Some racing classes/organizations require a stability test where, in addition to haveing the sails up and wet, a weight is attached to the hounds and the masthead is put in the water. The boat is expected to recover.

But Brian, do not confuse the ability to carry sail with what is measured by a single stability test.

Depending on the type of test, a stability test measures either initial stability or righting moment at a given angle phi(not pi, can't do greek letters here). They are different things. Initial stability is the ability to resist a dynamicly applied moment and is dependent on the waterplane inertia at the given displacement and heel angle. Righting moment is the force generated by the hull at a given displacement and heel angle attempting to restore "up" to vertical. A continious plot of righting moment to angle of heel gives the righting curve which when intergrated over the heel angle 0 to phi gives the righting energy; i.e. the energy needed to heel the boat over until it reaches heel angle phi.

The ability to carry sail, at a given heel angle phi, is more dependent on the remaining area under the righting curve (i.e. the available righting energy after the vessel has been heeled over to phi) and to some extent the initial stability at that angle of heel. For a small open boat, this is so dependent on crew weight, their actions, and downflooding as to render a stability curve superfluous. For other craft, it is a careful tradeoff between angle of heel at a given windspeed, and the probability of a gust of sufficient duration to capsize the vessel(i.e. applies more energy to the vessel than there is available energy left in the righting curve) . For a detailed procedural method of how the USCG evaluates this; see 46CFR171.055 (you'll need to see the figures, not just the text; use the GPO CFR website).

Now that we understand that the ability to carry sail is dependent upon righting energy; we can now reduce the ability to carry sail to three things; the heeling moment, the area under the righting curve, and the zero righting moment and downflooding angles.

The heeling moment is just the sail area * the wind pressure* cos phi* a fudge factor for margin (due to the effects of sail lift attempting to overturn the vessel even at large angles of heel). As everyone knows, more sail can be carried in light winds than in heavy weather. Therefor reefed configuration and spar and rig selection is intimately tied to the ability to carry sail. It is a function of the intended use and sailing area of the boat. How designers decide how much sail area to use for a given rig and hull is as individual as the designers and the area. But in the final analysis, the rig should be small enough to keep the boat from capsizing in a gust during worse conditions than what it was intended for. A good rule of thumb is to look at a well respected design of similar configuration a just follow the example.

Now we need to maximize righting moment in order carry the most sail area. In your case Brian, the hull is already designed/built(?). Which gives us very little to do other than calculate it (easier) or measure it (harder). Additionally, in general practice, the maximum value of the righting moment curve should be also be near the center of the area under the curve so as to give "good" behaviour to the vessel.

As for the angle at which righting moment goes negative and the downflooding angle, both should be maximized. The USGC desires a minimum angle of downflooding at 60 degrees, and a minimum angle of zero righting moment of 70 degrees for protected waters and 90 degrees for exposed waters. Some rules for round-the-world races require even higher values for both zero righting moment and downflooding to reflect what happens in the real world.

Finally, a capsize or swamping rarely happens in mirror flat water. There are several Express 27's and other boats at the bottom of SF bay that would have survived the knockdown, had not a wave washed into the fore/mainhatch that was open/unsecured. One as I recall, was driven under while running downwind (not a capsize!)with the forehatch open and never came up. I know of a 40' Choy Lee that ended up with 3' of water in her after a pooping. Only reserve buoyancy and adaquate righting moment saved her.

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