Tuesday, April 5, 2016


These stability curves are nice ones, from the new Halberg Rassy 44. They are righting moment curves that are obtained multiplying the boat mass in kg by the length of the arm (GZ) in m. The arm is the a horizontal one that you obtain with heeling, between the vertical CG (center of gravity) line and the vertical line of CB,(center of buoyancy) that is the center of the the underwater volume of the boat. The picture below makes shows that arm increasing and decreasing.

The CG is where the mass of the boat pulls down, the CB is the point where the buoyancy push up. Stability is increased when the CG is lowered (with more draft or a more efficient keel) and when the boat beam is increased (the distance between the two vertical line becomes bigger). The righting moment is positive till the AVS point (point of no return) and negative after that.

A good stability curve is one that has a much more positive righting moment than negative righting moment and you can compare (and measure) that by the the amount of surface between the positive area of the curve, first part (up) till the AVS point (the heel where the boat inverts itself, not returning anymore to the upright position) and the negative area under the second part of the curve, after the AVS point. They are marked on the drawing as upright stability and inverted stability.

A good stability curve is also one with a good AVS and not less important, a good or very goor righting moment at 90º heel.

The minimum acceptable AVS has been maintained constant on the RCD but the way stability curves are considered have increased in fact boat stability. The minimum AVS on Category A depends on the boat mass. Lighter boats need a bigger AVS to be approved, heavier and normally bigger boats can be approved with a smaller one, but never inferior to 100º and that only for a boat over 15 000kg, assuming that it can pass STIX demands. 

That diference regards the smaller stability a lighter boat will have towards a significant heavier boat will be translated by a bigger possibility of a capsize to occur on the lighter one, all other things considered equal. On the below image we can see stability RM curves of boats with very different mass and since the energy needed to capsize a sailboat is proportional to the area under the positive parts of the RM curve, we can see that the diference between those sailboats regarding the energy needed to be capsized is huge.

The formula for the minimum AVS on the RCD is this: 130º- m/500, being m the mass of a boat in kg. For instance a light category A boat like the Pogo 30, that weights only 2800kg, will have to have a minimum AVS of 124.4º. Well, that was during the first years of RCD, since then, without modifying the values the stability needs were greatly increased.

At the beginning the considered value of AVS regarded a lightship Stability curve, that you can see at red on the first image. Some years ago they demanded the AVS to be considered was the one of the boat in maximum load condition and things become more exigent. You can see that the AVS in lightship condition on the HR stability curves is about 125º but the AVS on max load condition is only about 120º. 

Now the demands become bigger and retroactively regarding all boats on the market from the end of 2016, the minimum AVS will be considered regarding a new stability curve that will give a significant lower AVS, a stability curve with the boat on the worst possible configuration, meaning tanks emptied, all the weight of possible boat extras over the waterline considered,  none below,  all allowed crew over the deck and none inside.

It seems not to make a big diference but it does, since extras, like radar on the mast or furling main, empty tanks and the weight of the crew over the deck can rise significantly the CG and diminish the AVS. I have heard about a 40ft aluminum voyage centerboarder (and not one with a low AVS regarding other centerboarders) having to add around 450kg of ballast to be able to comply with the new minimum AVS and also, in what regards STIX, having to reduce the mast and the sail area.

The STIX (that for Category A has to be over 32) is obtained by a complex formula and is the other relevant parameter, in what regards stability conditions, for a boat  to be approved.


  1. Great and interesting post Paolo!

    Interesting that after several years where the trend had been hull form stability at the cost of ballast stability, they are implementing new rules. Of course some brands like Hallberg-Rassy and X-Yachts have not been following that norm. After all the best way to reduce the negative impact on stability by gear, equipment, crew etc. is by having heavy and deep keels.

    Also of note is the recently announced Dehler 34. With both increased hull form stability and a really heavy and deep keel itcertainly is a statement in what regards stiffness. The righting moment curve of the standard version shows that it is actually stiffer than the Hanse 345 and the Hallberg-Rassy 342 counterparts. A big and positive surprise, for me at least. It should be an ideal cruiser for the exposed Atlantic coast. Particularly considering the wealth of useful upgrades. And depending on the coamings and the sheltered feel (or lack thereof) of the cockpit, it could really be the ideal Bluewater cruiser.

    1. Yes, I agree with you, the Dehler 34 will have a much better final stability than an Oceanis 35 and probably an identical overall stability (the Oceanis is beamier).

      The Dehler is more beamier than what it looks like (3.60m) and that is good since beam is a very nice feature on small cruisers to increase stability and offshore potential. Beam increases a lot overall stability.

      What many beamy boats don't have is a correspondent good final stability that can only be brought by a deep keel and a considerable B/D ratio.

      When you join on a a boat, like the Pogo 10.50 (and probably on the new Pogo 36), a huge beam with a huge draft (swing keel) with a relatively big B/D ratio, you have the best of the two worlds in what respect stability: big hull form stability and low CG, due to that we can call improperly ballast stability and this is probably the best receipt to increase the offshore potential of a small cruiser, I mean increase these two factors (within reasonable limits since both have disadvantages too).