Tuesday, August 30, 2011

Steps, and second steering stations on smaller boats, nope, don't like them


All too often advances in technology or performance comes with a price, and a step hull is no exception. Step hulls are much different from the deep V hulls most are familiar with, and come with their own special jargon that contains words such as chine walking, snapping, rolling, hooking, step tripping, porpoising, turn blow outs, and many others, none of which have successful boating connotations. The point is that all boat hulls are compromises, and step hull vessels have more of them than most, particularly for the less than experienced boater.




From a design viewpoint, a well implemented step hull can improve the hull efficiency by up to about 15%, but like all things in life, the laws of physics extracts a price for this. The principle of reducing the wetted surface of the hull using steps, gives the hull markedly different handling characteristics, especially when contrasted with most boaters personal experience with traditional deep V design hulls. The area behind each step may need to be be ventilated with air, and having a wave, or turning the boat in such a way that the ventilation slots get closed by water can cause bad things to happen such as abrupt slowdowns, the back end of the vessel breaking loose, or worse. Improperly trimming, and or letting off on the throttles during a turn can also have severe, and deleterious effects. Design of a stepped hull is extremely complex, particularly if more than one step is incorporated into a hull, and small errors can have dramatic impacts on handling. Designing these hulls so they operate consistently over a broad range of speeds takes experience, good computer modeling software, and a lot of time.

But what I am most disturbed about is the fact that these designs are being blithely sold to the boating public, for recreational boating and fishing. The brochures wax poetically about the speed, and performance characteristics, while burying the handling details in the fine print. I liken it to giving a 16 year old kid a 253mph Bugatti Veyron, and telling him to go have fun. It has the same steering wheel and pedals, as the Crown Vic, but without a lot of experience, and training, the car will eat you alive.


I know from personal experience that in too many cases, step hull buyers are given little if any instruction by the dealers and private sellers, and many, (but not all) owners manuals, if they exist at all, are woefully deficient in clearly explaining the handling characteristics of these hulls. It's not that the hulls are bad, they work exactly as they were designed to, it's just that they are very different from a deep V hull in their handling, and need to be treated that way. These improvements in performance come with requiring even more vigilance, and understanding about how these boat hulls really work. The step hull driver should always be prepared for the unexpected to happen, and should clearly understand exactly how their boat works. All too often the owners don't have a good understanding of why these hulls are truly different, and this is when bad things can, and have tragically happened. As for me, despite the performance gains, I still prefer the more traditional, predictable, safer, and more docile deep V hull. I don't need any additional boating adventures in my life. I have enough already, despite any savings, or performance gains there might be. But in a race, if I was a trained and experienced professional, you betcha, but you "still half to know the territory". (Stolen from Professor Harold Hill.)


You can find an excellent paper by Kobus Potgieter on how a step hull really work here.

Professional engineer Jim Russell's website Aeromarine Research website has a wealth of information on step hull designs.

Now on to second steering stations on smaller boats. This another thing I am suspicious about, and I generally dislike. In the fervor to see the large fish, or the weed lines, I have the sense that other important things are often being overlooked, like the vessels inherent stability. In the robot business we always worried about the moment on a robots arm. This was a simple formula which goes, moment = length of the robots arm times the weight of the load. A simple example of this is to pick up a concrete block, and let it hang from your arm, see that's not to bad is it? Now holding the same block, lift you arm so the block is now being held out at arms length. If you can do it, it will be excruciating. Welcome to the world of moment on a very simple scale.
















The second station your looking at is mounted on a 29' boat. I am guessing the structure has to weigh at least 250 lbs. There is room for two passengers on top, and lets say these are skinny guys that weigh about 150lbs each. We now have a total load of 550lbs, that is now about 7 feet or more above the vessels center of gravity. Now list the boat about 20 degrees to starboard. That equates to about 3' so the moment is now 3 * 550lbs = 1650lbs. Now tip the boat 40 degrees, (6') and the moment is now a whopping 3300lbs, over a ton and a half of weight, and we haven't even considered the momentum involved yet. You should not be surprised if the vessel could not recover, and the second station, with passengers get to meet the seawater up close, and personal.


Only a few manufacturers actually offer second stations as a factory option on smaller boats, and the hope is that the designers have factored the upward shift in the vessel center of gravity into their designs, and I know that several of the ones that do provide the option did consider this loading in their design. The boat you are looking at never had a factory option for this. It's a custom built package, and I seriously doubt that anyone thought much about the impact on the vessel. 


There are many more sophisticated ways to calculate moments, their impact on a vessels center of gravity, and this was a simple way of demonstrating how quickly the numbers can add up with an increasing heeling angle. If you want to do this to your boat, discuss it with the manufacturer, and get an opinion from a naval architect, especially if you're adding it to a stepped hull vessel.  All boats typically work well on a nice day running on flat water, it's when you're caught by surprise, that you have to worry about how well your vessel will really perform. 

4 comments:

  1. Interesting post and perspective. Very educational. Thanks for sharing

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  2. good article, bill. for original text used in Potgieter article, check Russell's original article at aeromarineresearch.com

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  3. Thanks for the comment. I spoke to Jim Russell during the construction of the piece. He has written a lot of definitive articles on the subject. Which article are you suggesting? Tnx Bill

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  4. Bill - was just noticing that the "excellent" article by Kobus was actually taken from one of Russell's articles on step design.

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