Friday, March 28, 2014

STABILITY 1: MISLEADING BOATS - Hanse 345 / HR 342

And to finalize  lets see how a boat evaluation based on hear say and "common knowledge" can be misleading. Let's look at two great boats, one with a more conservative design, a Halberg-Rassy 342 and other that is  a main trend mass production 34ft boat, the Hanse 345. Most would say that the HR 342 has an offshore potential while the Hanse 345 is a coastal cruiser, some would even call the Hanse a "marina boat" with lots of interior space, by opposition to the HR, that is made for sailing.

We can see that they are very different boats, the Halberg-Rassy with a hull a bit dated and the Hanse with a modern hull with all beam brought back. When I say that the Halberg Rassy hull is a bit dated it is not because the beam is not all brought back, but because on the modern designs that opted for that solution the hull design has already evolved and the transom is not so narrow as it is on that design. 

The beam is slightly different being the HR a narrower boat with 3.42m, while the Hanse has 3.50m. Looking at the two boats we would say that the difference is much bigger. The Hanse is a bit heavier with 6200kg versus 5300kg on the HR. Regarding B/D ratio the HR has 37% and the Hanse 33%.
 This is also a great example to see how the B/D ratio can be misleading if the keels or drafts are not similar. In this case the draft is close (HR 1.82m, Hanse 1.87m) but not the keels: The Hanse has a modern high efficiency torpedo keel (an iron one) and the HR has an all lead keel with a bulb. Also a good example to see that a typical all lead keel, even with a small bulb is not a match (in what regards efficiency) to a modern steel/iron torpedo keel.
Curiously and also contrary to what most would expect the sail performance of the Hanse, compared with the HR, will improve when the wind gets stronger and it will be better in medium to strong winds since the HR will have to reef sooner. That type of hull will also allow the Hanse a better ride on a beam reach with less heel, more power and most of all with less roll downwind. It will  allow it to carry more sail safely, to be faster and easier to sail, specially in autopilot.
 Some of those things we can tell by the hull shape but others are only disclosed by a stability curve, in this case a righting moment (RM) stability curve.
 If some information that is contained on a RM curve can be accessed sailing the boats (stiffness and power) that regards only the first 30º. All other information will remain hidden and that information is fundamental for accessing the boat reserve and final stability, that are very important to the boat's seaworthiness.
 
Only a comparison of stability curves will indicate the heel angle where the boats will invert themselves. That angle is called Angle of Vanishing Stability (AVS) and indicates the point where the positive stability will disappear being substituted by negative stability (with the boat inverted). It also indicates other important factors, as the force that the boat is making  to right itself up at 90º and over or the max righting moment, or the total amount of energy that would be needed to capsize the boat or to re-right it (correspondent to the area below the positive and negative areas of the RM stability curve) and finally, the ratio between the positive and negative total areas, that indicates the difference of energy (and proportional size of the waves) needed to capsize the boat or re-right it.
Comparing all these factors on the Hanse and HR RM stability curves we will have some big surprises. It is not a surprise for the Hanse to have a better "sailing" stability,  a bit of a surprise for the HR to have a slightly bigger max RM but the real surprise regards the rest of the stability curve and the facts will contradict what most would imagine:
 The Hanse AVS is better as well as its final stability from 95º till the AVS (it means that the boat will be making more force than the HR to right itself up when capsized to 95º and over); the inverted stability of the Hanse is smaller and the ratio between positive and negative stability is better. It means that while the energy needed to capsize both boats is about the same (the area under the positive part of the RM curve) the energy needed to re-right the boats will be smaller on the Hanse than on the HR.

A final note to say that these are two great boats, that the HR 342 AVS ( about 125º) is good in what regards modern designs, that the ratio between its positive and negative stability is a good one and that big difference in power in what regards the first 30º of RM curve has to be balanced by the smaller power the HR needs to go at the same speed than the Hanse, due to its smaller weight and lesser beam (more "finesse"). 
The point here is not the Halberg-Rassy not being a great boat but the Hanse 345 showing that it is as much an offshore boat as the HR 342. Of course this comparison regards only these two boats and is not valid regarding all modern mass production cruisers. When you chose a boat ask for the stability curve. Most builders will not like to give it, others have them available on their sites (a minority).

Some magazines publish them (Yachting Monthly and Yachting World) when they test the boats but even if the data is relevant to what cannot be accessed sailing the boat, no magazine makes a relevant analysis of the positive and negative points they reveal in what regards reserve and final stability. Regarding these points I can tell you that the stability curves from different boats are not all the same, being them beamy or narrow, they can be very different. Now you know what to look for in what regards having more information about a boat besides the one that can be given by a polar speed or by visiting the boat interior on a boat show. ;-)

5 comments:

  1. Hanse are great boats combining combining sufficient sailing(for a cruiser) and competitve price. But isn't 345 a33'?

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  2. Stability and AVS are undoubtedly important but sea kindness is what really matters. Wide transoms and shallow hull make a boat sail faster in normal coastal conditions but uncomfortable to say the least while crossing an ocean. And if the stern gets pushed around by every passing wave they are not so fast either. My 2 cents.

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    1. Probably you ignore that today all offshore racers, including the solo ones, that are designed to be more sea kindly and easy to drive, have these kind of transoms and for a good reason: they are more efficient on offshore conditions. Those boats are designed not only to cross oceans but to circumnavigate passing on high latitudes and some of the worse seas on the planet.

      In fact they are much easier to sail than those old "sea kindly" designs from the 70's and 80's with their rolly motion downwind. Contemporary designers are using those shapes and transoms on cruising boats because after being proven extensively in racing they can improve cruising designs in what regards easy of sailing and overall sailing performance.

      HR is just a more conservative brand and that means that it takes more time to change but I am sure that German Frers, that is a great designer, will convince them to gradually adapt their hull forms to more contemporary and efficient designs. He is just doing that and if you look at the last Halberg-Rassy hull, the 412 you will see that it is a lot more like the Hanse 345 than like the HR 342.

      Take a look:

      http://www.hallberg-rassy.com/fileadmin/images/Dokument/HR412deckplan.pdf

      March 29, 2014 at 7:59 AM

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  3. I just saw this analysis and though I agree with most of it, being AVS a pretty objective seworthiness factor, I must observe that strong stability plays a very relevant role on capsizing in waves, because is the stability (over the tilted plane of a wave) which capsizes the vessel. ("Seaworthiness the forgotten factor" page 148, Marchaj), also one small. About the wide transom being more seaworthy, in fact thats only true if the vessel is able to go faster than the waves (hence the design of Open 60s, volvos, etc...) going slower a wide transom will indeed behave like a beach ball on the waves. Very nice site by the way.

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  4. I don't follow you when you say that a strong stability plays a very relevant role on capsizing in waves. A bigger overall stability means that it is needed more energy, meaning a bigger wave to capsize the boat.

    But that is not the more relevant point regarding a boat with a bigger AVS in what concerns resistance to capsize: A big AVS is necessarily associated with a bigger RM at 90º of heel and that is as important as the AVS itself since it will contribute in a determinant way to re-right the boat QUICKLY when it is capsized by wind/wave to a 90º or even bigger heel angle.

    Most boats that are inverted are so by a set of two waves: the first one capsizes it to 90º and because the boat takes too much time the to return to a moderate sailing heel, the second one catch the boat still deeply heeled and with a remaining small positive stability and then the energy needed to capsize the boat (size of the wave) will be much smaller than the one needed if the boat had already recovered from the first capsize.

    That's why the time the boat will take to recover from a knock down is very important.

    There is a configuration where a bigger AVS and a very low center of gravity can cause problems related to excessive rolling but that will only happen on a narrow hull.

    Today's beamy hulls have a huge initial stability and will prevent that excessive rolling even on racing or very fast boats with almost 50% of ballast deep down on a torpedo at the end of a big foil keel.

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