Value of capsize ratios?

mber · #1 (**permalink**) 08-25-2003

I''m looking for a coastal boat 26-29'' long. Stability is one of my more important criteria so I have focused on boats with capsize ratios of less than 2.1 . I realize that the usefullness of this number has been debated but boats with lower capsize ratios generally have higher motion comfort values associated with them - and this is another important boat selection criteria for me. What do you all think of this approach?

Jeff_H · #2 (**permalink**) 08-25-2003

I think that your approach is worse than useless. The capsize ratio includes almost none of the critical determinants of the likelihood of a capsize (such as the vertical center of gravity, vertical and horizontal center of buoyancy, weight and bouyancy distribution, waterline beam and the depth of the canoe body, center of effort, roll moment of interia, etc) By the same token the Motion Comfort Index has almost none of the critical factors related to motoin comfort, (such as the vertical center of gravity, hull form stability, weight and bouyancy distribution, roll and pitch moments of interia,dampening, etc)Both of these formulas were developed when boats were very different than today and were all very similar in hull form and weight distribution. These surrogate formulas allow some comparison of very similar boats but in almost all cases provide no accurate or useful information. In fact the information is so misleading that I consider it dangerous to rely on these information derived from these formulas in any real way.

The example that I give is two identical sailboats, only one has a rig that has a 1000 lb weight at its masthead (and yes I know no one would actually put a 1000 lb weight at the mast head of a boat.) Under the capsize screen the additional 1000 lbs would make this boat seem more stable, but in reality the 1000 lb weight would raise the vertical center of gravity and make the boat with the weight more prone to capsize and more prone to a much wider pitching and rolling angle (albeit at a slower speed) and so less comfortable.

Similarly if that same 1000 lbs were a bulb at the bottom of the keel,the 1000 lb weight would lower the vertical center of gravity and make the boat with the weight more far less prone to capsize and more prone to a much narrower pitching and rolling angle still at a slower speed, and so far more comfortable.

As you can see nowhere in either formula are these factors taken into account but if you strickly followed the formulas the boat with the lead weight at the mast would look like the better boat in terms of seakindliness and stability.

Jeff

mber · #3 (**permalink**) 08-25-2003

Jeff,
Thanks for your comments. I am amazed by all the boat choices out there & am looking for a way to shorten my list of boats to investigate further. I was hoping there would be a reliable tool available to compare the stability and comfort underway of 1 boat relative to another. For example, if I am considering 2 boats of similar hull form, displacement, beam, LWL, etc - how can I get at which one will have the most seakindliness and stability?

doubleplay · #4 (**permalink**) 08-25-2003

Marc,
You said you are looking for a coastal boat(meaning you will sail in protected areas and close to shore)in 26-29 lenght.
I dont think seakindliness and capsize ratios should be on top of the list while looking for a boat that you will be using primarily for coastal cruising.
I would suggest set up a budget and determine your intended use (racing,cruising,gunkholing)and the area that you will keep the boat(shallow,deep)that might help to narrow down your choices.
Respectfully,
Al

Jeff_H · #5 (**permalink**) 08-25-2003

First of all, as Al observed, if you are coastal cruising seakindliness and stability probably should not be your only concerns. Coastal cruising generally rewards boats with well rounded sailing characteristics. The three most meaningful ratios for a coastal cruiser are L/D, SA/D, and Bal/D. The best coastal cruisers have an L/D under 150, and SA/D over 20 (with a 100% jib) and Bal/Disp of .35 or more.

In terms of stability, there are a number of factors that I look at. I start with the comparatively simple ratio of ballast to displacement. But that does not tell the whole story. I also look to see if it is a high density ballast (Cast lead) or a lower density ballast (like lead shot in resin, cast iron, strap steel in concrete or water for that matter) The higher the density the more stability for a given weight. I look to see how deep the boat is. All things being equal the deeper the draft the more stability. I look to see if the keel is encapsulated or bolted on. A bolted keel generally yields a several inch lower VCG for the same exterior profile. I look to see the shape of the ballast. (Is there a bulb? is the ballast stretched along the long bottom of a longish keel?) I look to see where tanks are located and whether there are a lot of lockers high in the hull. I look to see if the boat has a heavy rig (A heavy rig means less stability and larger roll angles). I look to see if a boat has a heavy interior or heavy deck structures or lots of free board or top-hamper which also reduce stability and increase roll angles.

I look at the waterline beam and the distribution of the displacement to see if the boat has a lot of form stability. A little bit more form stability is more acceptable for a coastal cruiser where the angle of positive stability is less on an issue because you are less likely to encounter waves capable of rolling you boat far enough that ultimate stability is an issue.

All of that combined can give you a sense of how stabile a boat is likely to be.

Many of these are the same factors in motion comfort. Does the boat appear to have a low vertical center of gravity? If so it will be less probe to excitation rolling and will potentially have a higher roll moment of inertia. Does it have a tall light rig? This is good because it can help dampen roll. Does it have a fine vee bottomed bow and a long waterline? A fine bow has less impact with each wave and is less prone to hobby horsing. A long waterline dampens pitching. Is there a lot of weight in the ends of the boat? Weight in the ends of the boat tends to slow pitching rates but increase pitching angles which is far less comfortable for most folks. Where are the main functions in the boat in relationship to the roll center of the boat? The head, nav station, and galley should be low and near the center of buoyancy of the boat. Does the boat have a lot of form stability? Form stability gives a boat a quicker motion but generally through a narrower roll angle.

In the end you will probably not buy a boat by the numbers but by a collective sense that this is a good boat for your needs. The numbers help but only so far.

Regards
Jeff

mber · #6 (**permalink**) 08-26-2003

Thanks for the input - it''s been very helpful.