Stepped hull

Hi!
I am searching for literature of stepped bottom planing hulls.I want to know where is the optimal position of the LCG on the steeped hull with one step, and what happens with the two step hull.
Regards to all!

 

Thread: Stepped hulls
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Old 05-01-2005, 08:27 PM
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Step Design in Powerboats
[...from Tunnel Boat Performance News - Mar. 26, 2004]

"We have had many questions regarding the design and implementation of steps in performance hulls. Here is a “short answer” - step-by-step.

- The design of effectively performing steps is VERY difficult - and will most always achieve a 'benefit' (more than the losses) at ONLY one planing velocity. A step design is usually only good for a single angle of attack with a single dynamic center of gravity (CofG). That is why it is so complicated to find a step design that can "help" the performance throughout the speed range of a performance boat.

For example, the design and manufacturing tolerances are far more critical in stepped hulls than in non-stepped hulls. A slight change in plane angles, particularly the angles of the aftermost plane, has a marked effect on the running of the boat. Changing the after step by only one-eighth of an inch can change the boat performance from one that runs smoothly to a porpoising hull. Even the change in weight of passengers, or fuel weight can be enough to throw off the CofG so that the step design no longer works as intended.

- It gets worse, too! When a stepped hull turns, the wetted surface of the stepped portion can change, which changes the center of pressure of the lifting surface, which changes the dynamic CofG of the hull - scuppered again! The results can vary from loss of lift to "step-tripping" to the ultimate "hook" accident.

- The theory...On a planing hull, the highest-pressure water is just aft of the leading edge, so we want to take advantage of as much of that pressure (lift) as possible without the drag penalty of the low-pressure water farther aft. The efficiency of a planing surface is a strong function of its aspect ratio, (the relation of the width to length). The most efficient planing hull is one that is very wide, but very short. (The aspect ratio of an efficient prop-driven airplane is perhaps 8:1, while the aspect ratio of a non-stepped planing hull is on the order of 0.5:1)

Now, the design of a simple non-stepped hull, we must select a built-in angle of attack and the center of gravity. We can control the angle of attack by specifying keel camber, deadrise, and chine warp (all of which may vary over the hull length). Stepped hull design includes all these considerations, but now we ALSO have to balance the angle of attack of each step section with the distance between the step and the transom or between multiple steps!

At speeds that are different than the speed that the step is designed for, the steps are often entirely immersed, so each step actually adds drag to the hull.

- Introducing "air bubbles" to the pad surfaces? Good grief...why would we do that? A myth about stepped hulls is that the introduction of air into the water that flows under the hull reduces the viscous drag and makes a stepped hull go faster than a non-stepped hull. But in reality, running on air bubbles doesn't reduce the frictional resistance at all. The hull lifts on the water, not the bubbles. So bubbles or "two-phase flow" (water and air) will actually increase the drag.

- And finally, there is the question of "multiple steps". There are two problems with multiple steps:
1) If the steps are located too close to each other, the water attaching to the second step is "contaminated" by the aerated low-density water from the first step (as per my explanation above), so the aft step does not produce the high lift forces desired.
2) Where do we locate the center of weight (CofG) so that the weight is balanced across the steps? Remember, the running trim of your boat changes dramatically as you go from zero to full speed, and this makes a huge difference in the lift-force distribution on your steps. It takes only a small change in the relative locations of the dynamic CofG (and the center of pressure) to change your boat from a stable, efficient boat to one that porpoises at several velocities.

Steps (particularly a single-step) have a significant benefit, though - that is they can run at optimum trim across a velocity range. So, a single step forward of the LCG, often work well.

- Summary - Steps is really complicated design issue. Many often don't really work as intended. When we read about, or experience boats that behave really nasty with steps, we can appreciate how they got there - just not properly designed at all! But when they are designed for a very specific purpose, they can be really effective."

[...from Tunnel Boat Performance News - Mar. 26, 2004]
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Thank you for your help.
I would like to know, if there is any design, or some recomandations of how to design a step, in what posotopn and with what angles.For example, if I know the position of LCG and the trim angle on certain speed, where should I put the step infront of LCG, how it should be designed, what dimension, what angles.How has to be dte surface after the step, should it have also spry rails, etc.
Regards!

 

Try with this....

 

Thank you for your disponibility Mr Otto!
Best regards!

 

You are welcome !

 

designing stepped hulls

I tend to agree with all the above comments. What can be summed up from all that has been said is that in order to design a good set of steps that work, a proper engineering approach needs to be followed and don't expect to design a good stepped hull the first time around.

There are three ways you can set about this:

1. pay an expert to help you and learn something from him the first time around. Expensive and doubtful how much anybody will teach you.

2. Start by doing your own calculations to get a feel for how the different parameters effect behaviour and performance. There is enough research on planing hulls that will allow you to do some calculations on stepped hulls. However collecting all the literature and then programming a method to do stepped hull calculations will be time consuming. If you dont want to program something yourself, you can buy a commercial package such as AUTOWING which to my knowledge is the only software for planing hulls that has routines for calculating stepped hulls. Which ever way you go it will be mathematically intensive.

3. last but probably best is to build a model of your hull and test it. You can tow the boat alongside another boat using a boom and observe its bevaviour in calm water, waves, in turns etc. By trial and error and tweeking your hull you will eventually get something that works for that condition.

 

thanks for posting the Savitsky paper on whisker spray. I was looking for that one

 

You are welcome too!!

 

Its not that hard

Put the steps just forward of the LCG, and things work well,
especially if the steps are large enough.

Look at good recent stepped powerboats by Fountain, Donzi,
Howard, etc, and you'll see that's what works: the steps are
just in front of the CG.

Ignore Outerlimits: they are fast only because they are built of
Carbon. The design itself (steps all over the place) is widely
reported as being prone to porpoising.

I talked to the guy who designed the Howard 28 (howardcustomboats.com).
He built a prototype boat and rigged it, weighed it to accurrately determine the LCG, and put the step right there.

Remember the lift is at the forward end of the immersed prism, so the lift is forward of the step.

The result is a single step boat that never porposies, handles great, and is widely recognized as the fastest of its size for a given amount of power.

It is simple!

I have an early Fountain stepped hull (built in 2000, but one of his earliest designs) and it porpoises at certain speeds. This is because the step is too small, so it does not maintain the constant angle of attack.

More recent Fountains made the step about three times as large, and moved the step forward a bit more to be certain its in front of the LCG, and porpoising went away.

 

Hi, in the forum there are some interesting threads and links about the item. I remember one:http://www.boatdesign.net/forums/boat-design/stepped-hull-resistance-35492.html
may be helpfull in someway to go deeper in the matter.
Regards
K.

 

It's the most understandable comment I've read so far. Why Everything is relative. Even if we have the best plan a bad setup can change all

 

Theres lots of speculation and people saying this and saying that and going round and round but is there any one here on site on this subject that has hands on parctical stand behind at the steering wheel exsperiance ?????

 

Correctly and self

 

disillusioned !!

<off topic attack against another member removed>

In reality the 1920s and 1930- and 40s more development took place than is seen in our times .