Stepped hulls

What is the advantages and disadvantages by making steps in a hull ?
Is there any papers regarding this ?

Vibtor

 

You do not need any papers on this subject. A WWII 50 ton PT boat had 5 steps installed along the bottom and the speed increased from 40 knots to 55 knots. Lucky guess or damm good designer? I have the booklet stating those speeds. Not much changes , except hype, greed and PR claims to sell boats to the uninformed. Speed, was the only advantege then. Negatives, slower to plane, dropped off plane at a higher speed in a turn, reverse and steering were less responsive at different speeds, boat became more load sensitive. USN dropped idea due to handling complaints, fuel and oil consumption increases, obviously did not match props to new parameters of operation. Never mentioned cost as a factor. War time budget, money no object.

 

Stepping's been around for a while, as Cyclops said. The idea is that the step sucks air under the boat, reducing drag on the hull-water interface.
Key advantages are higher top speed and lower power requirements.
Disadvantages include: difficulty of designing it, less predictable in turns, more expensive, more sensitive to differing loads.
The mathematics involved in stepped-hull design are horrendous. Modern commercial stepped hulls (Regal, Apache, etc.) have thousands of hours of computer simulation and test-hull time behind them. Get it wrong- set the step angle a few degrees too steep, or put the step a few inches farther aft- and the boat will behave in a completely different manner, or might become uncontrollable.
If you're designing your own stepped hull, you should have a good marine CAD/CFD software package (often over $10,000) and be willing to build several outboard-powered test hulls. It's quite possible to look at existing designs and adapt them... but you cannot predict how the stepped hull will act until it is actually out on the water.

 

Marshmat is right about the R & D. Small to large wood racing hydros can tell you a lot on the % from the stern end of different shaped boats. It is usually a totaly emperical project due to the dumb luck needed to help you before you give up butchering the original bottom. You will DEFINATELY weaken the bottom if you do not have a experenced designer. Boats have sunk during this cut and try stuff. Carefull!!!!

 

Plum stepped hulls

One of the most interesting recent articles on stepped hulls was by Eugene Clement in the April/May2004 issue of Professional Boatbuilder. The article is about John Plums stepped hulls; his boats use a sort of aft foil and he found that a step could be quite small and still help a boat achieve a plane earlier than it would otherwise. Also see a letter to the editor regarding this article in February/March 2005 issue of the magazine.
My interest is primarily in seeing if a stepped hull could be successfully adapted to planing sailboats.
Yves Parlier has been experimenting with a "normal" stepped hull on his 60' sailing catamaran where he found that drag could be reduced 80% at speeds over 20 kts. compared to a typical high beam to length ratio displacement multi hull. It's the lower speed on this kind of boat where the step like Parlier's hurts.Some designers in the F14 multihull class and other beachcat types are experimenting with various types of step.
Plums work seems to permit something along the lines of a retractable step with a rear hydrofoil to control pitch attitude.
Interesting article-well worth the read....

 

IT really comes down to getting a film of air under the entire bottom skin. Any and all methods will work. Steps or a load of planing strakes . Whatever you do should be a big help. ----------------A after thought. A flat bottom hull helps greatly at getting on plane quickly. Now you have to trade off some of the V ride and handling for the higher average speed. You can go back to a bow keel and rudder to retain steering and prevent the wind and current skidding. Your R & D. ----------Old bones like smooth V's. Wipper snappers like speed. And so it goes.

 

Always room for more argument. The top speed of a planing boat (all other things being equal) is surface area (friction). Steps are one way of acheiving that, Hickmans Sea Sleds, IVBs or tunnel hulls are some others that attempt to help support the hull on pressurized air under the hull. There is no presurized air under the usual step hull. Some boats introduce pressurized air aft of the step with powered fans, ram air intakes or even exhaust ducts. Some of these ,CAT or controlled air technique designs, use chine skirts to keep the pressurized air from escaping.

One disadvantage of steps is increased longitudinal stability caused by the distribution of lift into two or more spots near the steps instead of just one near the stern. This makes the boat run more level (usually good) but also makes it slam into chop or waves (not good). The latter makes for a very rough ride in waves and is the main reason steps are not used more. Another advantage is that stepped hull don't like to porpoise due to this division of lift. In waves, the air inlets behind steps can get closed off by water and create a vacuum (definitely not good).

The optimum angle of bottom trim for planing is about 5 degrees. On a non step hull the boat must ride bow high to maintain this angle. A stepped hull can introduce a greater angle of attack at the step and stern and maintain the best planing angle while keeping the boat more level.

 

Vibtor. Are we making it difficult enough yet?------------Tom, The Scott Paine 55' British MTB's would go from stop to full speed while remaning perfectly level. Good design always show it's rare head on occasion.

 

stepped hulls/ previous posts

Here is some more info:
http://www.boatdesign.net/forum/showthread.php?t=202

 

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]

 

Why do I feel he is saying the ELCO PTB REALLY did not do what it did? Because the fine art of stepping is, a artform, and not able to be found in a computer program, because it takes too much effort to write the program, if you do not have the experience. That sounds right.

 

Vibtor. I can scan and send you the pages of my booklets that have pictures of the boats with the text on the same page. Would that help?You can also buy the booklet and gain more insight on the boats abilities.I have the companies address.-------------I have just re read the field report again and realize that I am wrong? Yes. The handling WAS improved! And the boat had 6 not the 5, I stated. Page 23. Go for it Vibtor. My age is showing.

 

Jim, your information is certainly well beyond mine. Fortunately, I know enough about steps to keep me from designing or building one.

On the other hand, I had a close friend in the 50's who had a crude 14 foot boat built with 1/4" plywood and a single step about 4 or 5 inches deep about midway. The forward bottom was a V and the aft bottom was just a flat sheet of ply. With a 6hp Wizard, this thing would plane three semi-adults at a speed I thought was in the high teens. It was very lightly built and the hull was a bit wiggly. Of course, I knew practically nothing about hull design and didn't mind if a boat jumped from wave to wave or tried to throw its occupants overboard.

 

I have to disagree with this statement. A bottom surface running over a bubble-water mixture does experience lower drag than when it runs over pure water. I saw it first hand when I did my senior thesis in college. It makes sense too when you consider the vicosity (resistance to shear) is two orders of magnitude higher for water than for air. Any air in the mix will reduce the overall shearing force acting on the bottom (aka wetted surface friction). Whether the air-water mixture reduces the lift produced by the planing surface is another story. I don't see why it necessarily would reduce lift because the air bubbles will be at the same pressure as the surrounding water and they should produce the same upward force on the bottom surface.

 

This thread is really making us all think with all the little grey cells of accumulated knowledge. Engine exhaust into the steps to increase speed. Dirty but faster bottom. There has to be more tricks tried and found to help.