Wednesday, March 19, 2014


This is the first post of a series about stability. I will be using some of the information provided on the old thread, improved and better organized trying to provide a complete view about the subject.

 Let’s start to look at some misconceptions starting by the notion that the “capsize ratio” can give a good idea of a boat stability; that the stability of a boat can be judged looking only at the B/D ratio; that the AVS alone is per se a good measure of a boat overall stability; that a more seaworthy boat is necessarily one with a very small inverted stability, or no inverted stability at all. 

First let me tell that this is a complicated matter and that no simplifications are of much use here but if conveniently explained it is not difficult to grasp or understand the big picture and when understood it will provide a much better global understanding on boat stability and on boat design making easier to understand contemporary boat design tendencies and even to chose a better and more adapted boat to one’s needs, or to stay away from the ones that are not suited.

Many years ago the technical committee of the Cruising Club of America created a very simple formula that was suppose to determine if a boat had blue water capability and for doing that they centered the criteria on boat stability. The capsize ratio formula compares beam with displacement since supposedly excess beam contributes to capsize and heavy displacement supposedly reduces capsize vulnerability. The formula is just the maximum beam divided by the cube root of the displacement in cubic feet: 
Capsize Ratio = Beam/Displacement.333
I don’t know if this was of some use long ago in some particular types of boats but today it is just an odd mixture of misconceptions without any use regarding evaluating boat stability, much less to evaluate a boat ability for offshore work.

The first oddity is the misconception that an increased beam makes a boat easier to capsize. That is just the opposite, beam makes a boat more difficult to capsize (and that’s why multihulls need more energy to be capsized than a monohull). The second one is that weight per se is always a favorable factor in what regards resisting a capsize and the third one is a huge omission on that formula: the position of the boat center of gravity that has a major influence in what regards stability. The CG on a boat with the same weight can vary hugely with the B/D ratio of the boat, with the type and efficiency of the keel design and with the draft of the keel that is normally related with the position where most of the ballast is located (lower or higher depending on draft).

The ballast displacement ratio is the percentage of weight of ballast regarding the total weight of the boat in light condition. It is an important factor to stability because the ballast is put has down as possible on a sailboat and therefore on the same boat more ballast will contribute to lower the center of gravity (CG). The lowering of the center of gravity is one of the factors that contributes to more stability (more righting moment) being the other two: beam and weight.
But as I had said, this is true only in the same boat or in a boat with similar hull characteristics with the same beam, the same type of keel, the same draft. Only with all those conditions reunited the increase in B/D ratio is meaningful indicating that the boat with a superior ratio will be a more stiff and powerful boat (more stable), and if both have the same weight, that it will have an overall bigger positive stability and probably a smaller inverted one (this one relates not only with the CG position but also with the cabin design).

Regarding different boats the B/D ratio can be very misleading in what regards the boat CG and the effect the same has on stability. Let’s have as example two 40ft performance cruisers one with some years already, the 1996 Jim Taylor  Sabre 402 and the 2011 Finot/Conq Pogo 12.50: The Sabre has a considerable bigger B/D ratio (39%) than the Pogo (34%).
The ones that think that B/D per se is an indication of a superior stability would say that the Sabre has a superior stability, especially a superior reserve stability and a better AVS. Well, no, the Sabre 402 has an AVS of 114º while the Pogo 12.50 has one of 124º. That is due to mostly with different keels: An almost non bulbed keel with less draft on the Sabre (1.89M) and a swing keel with most of the ballast on the bottom and bigger draft (3.00M) on the Pogo 12.50. That gives to the Pogo a lower CG not withstanding the inferior B/D ratio. 

The misconception that beam is greatly prejudicial to the boat AVS (that comes from that capsizing ratio conception) is also clearly proved false on this comparison. If that was the case the huge difference of beam between the Pogo (4.50m) and the Sabre (4.06) would not have allowed that big difference on the AVS favorable to the Pogo, even considering its lowest CG.
  (AVS stand for angle of vanishing stability and means the point where positive stability vanishes and after that becomes negative, meaning the angle where the boat will become inverted and when inverted, the angle that it has to reach to re-right itself. Reserve stability or final stability is the boat positive stability that is not used for sailing, regarding high angles of heel, but that is fundamental for the boat seaworthiness and to bring it back from a 90º capsize.)

Regarding the same model with different keels with different drafts, the practice in European brands is to give to all boats the same overall stability increasing the ballast on the keels with smaller draft to compensate and give the same CG to the boat.  The same stability does not mean that the boat sail as well. A smaller draft keel (with more ballast) increases the overall weight of the boat, increases drag (because it has to be thicker and therefore with a lower aspect ratio) and because being less deep works less well as a foil to keep the boat on track close upwind.

 For the ones that think that all this is not very important just remember that the difference in speed between a fixed propeller and a feathered one is between 0.5 and 1k and that a difference of 5º upwind will translate in a very considerable distance lost.
A good example of a boat with a varied offer in what regards keels is the Jeanneau SO 349  with at least 3 keels available, fixed swallow draft keel, a standard keel and a swing keel with a variable draft, a very deep one when deployed (and a very low one when up). All the keels will give to the 349 about the same stability and AVS, the same ability to carry sail but the fixed low draft keel will add 300kg to the boat weight and will have all the negative effects already described. They tried to compensate with wings on the low draft keel and it will work better but not with less drag and not as well as a bigger foil.

 Regarding the comparison between the standard keel and the swing keel we have already talked about that (previous posts). It seems that the swing keel has a slightly lower CG and that means that it is lighter but the weight of the interior lifting mechanism probably will make that difference negligible.  I heard that the Polars would give the swing keel a better performance and I believe so, at least upwind but the testers just messed up with the comparative test and it is hard to tell. Anyway, at least it has a comparable performance regarding the standard keel. 

Two misleading points remained to be addressed: that the AVS alone is per se a good measure of a boat overall stability and that a more seaworthy boat is necessarily one with a very small inverted stability or no inverted stability. I will join two others: That a boat with a better ratio between positive and negative stability is always the one with better overall stability and that the boat that have a reputation of being more seaworthy (stability) is not always so when compared with a cheaper main market mass production boat.

Regarding AVS it is pretty simple: Imagine a nice round wood cylinder on the water. Perfect AVS (180º) and however it would not cease to capsize, I mean being its positive stability as big as it’s negative (considering any arbitrary side as the “deck”) the cylinder will roll at will.
Let’s consider now a board floor from your boat on the water. The board will have a huge stability and it will be needed really a big wave (in proportion to its size) to have it capsized but once capsized it will remain inverted for as much time as it was on the “right” side. The board compared to the cylinder will have a huge positive stability but also a negative stability as big as the positive and a 90º AVS.

Off course if someone was given the choice to be out on the sea balancing himself over a cylinder or over a board with adequate floatability the choice would be evident. In fact the second craft is called a raft and was used on multiply circumstances as a water craft. The cylinder will have a much better AVS (180º) but it is obvious that the raft would be a much safer vessel and therefore the AVS alone is per se not a good measure of boat overall stability and a more seaworthy boat is not necessarily the one with a very small inverted stability or no inverted stability at all.

From here and regarding the design to get a seaworthy sailboat the ways have someone parted in two different directions: the one that consider that having the biggest positive stability is the way to go, and you have the multihull family and the monohull family that consider that a boat should have as much positive stability as possible and as little negative stability as possible, meaning that the ratio between the positive and negative stability should be as high as possible. Such a boat will need a much bigger wave to be capsized comparing with the one needed to put it back on its feet. Later we will see that even this is not so linear and true as it appears to be.

Regarding multihulls stability is an easy story: The bigger the ratio between length and beam the better, even if at some point the risk of pitchpolling has to be considered too and of course, the CG should be as low as possible.
Regarding monohulls it is not as easy and many variables enter in play. Sure, the ratio between positive and negative stability is much more important than the AVS per se, providing this one is an acceptable one, but not an absolute. I mean that not always the boat with a bigger ratio is the more seaworthy boat, or the boat with a better reserve stability. We will see later what an acceptable AVS is and why but for now let’s just focus on that ratio between positive and negative stability and for that nothing better than a practical example:

Let's assume two boats with the same length, same weight, same B/D ratio same type of keel, same draft, one narrow other beamier. Let's assume that the proportion between the negative and positive stability is similar. That should not be very far since they will have a similar CG. Let's assume that proportion is 3 to 1. The beamier one will have more positive and negative stability, the narrow one less positive stability and less negative stability in about the same proportion. Let’s say that difference in stability (negative and positive) is of 50%. Let's assume that the energy needed to capsize the narrower boat is the one of a breaking wave with 2m. That means that the boat would right itself up with a breaking wave of 0.66m, or with a normal non breaking bigger wave. The wave energy needed to capsize the beamier boat will be bigger. It will correspond to the one of a wave 50% bigger, that means a 3m breaking wave and it will also need the energy of a bigger wave to right itself up, in this case a 1m breaking wave, or a correspondent bigger non breaking wave able to roll the boat till its AVS point.

What is the boat in you would want to be, the one that needs a 3m breaking wave to be capsized and 1m wave to be re-righted or on one that will need a 2m wave to be capsized and a 0.66m wave to be re-righted?

In the end there are boats better or worse designed in what regards final (or reserve) stability and one very important factor is the force the boat is making to re-right itself (proportional to its weight) after a 90º knock out, but beamier or narrow boats are not by itself more or less seaworthy even if in what regards smaller boats beam can be very important to give it an overall better positive stability. We will see this with more detail ahead.

 The last point in this list of misgiving concepts, the one that a boat that have a reputation of being more seaworthy (stability) is always so when compared with a main market mass production boat, will be treated in another post through a practical example.



  1. I find this very interesting paulo - I'm buying an RM 890 (I know you like this boat) & sailed one for 6 days last week. I then had to decide 'which version' - twin keels single rudder, or single keel twin rudder. there are advantages and disadvantages to both.... however I've gone for the single keel twin rudder because the CG is 400mm lower and the draft is 400mm deeper (1.9m as compared with 1.5). these are figures from the architect. my feeling is that as I sail with just my wife most of the time, increased stability and stiffness is important - also the 'bite' of the leeward rudder in strong gusts. am I right here do you think?

    of course moving around marinas and avoiding ferries and warships will require a little more practice as propwash will be very limited in its effect, but a small price to pay I think. we don't want to dry out in creeks as being 1.5m up in the air is pretty awkward + the fact that we won't be having a holding tank.

    I like to think that this blog of yours is infinitely better that the average forum, where people just trade hard held and incorrect views.


  2. Hi Dave!
    I believe that in that particular type of hull twin rudders make a significant difference in what regards bite in strong winds and it will give a bit better overall performance. If you don't need the sallower draft there is no reason to chose the twin keel version.
    Regarding maneuverability on the marinas it is not so much a question of propwash since modern boats with sail drive and the rudder as aft as possible have already very little, it is a question of a significantly bigger turning circle that will make it more difficult to handle the boat in tight marinas. That is due not to propwash but to the position of the rudders.
    In what concerns not having a holding tank meaning a black water tank, I don't understand. I thought they were mandatory and certainly they are necessary while cruising most first world countries since there is legislation that makes them indispensable while cruising. That is also a question of civility and good manners: will you shit on anchorages were other people are swimming?