Stepped hull design concept

[batistuvan]

Could anyone me teach me the concept stepped hull boat?
My questions are:
1. What is the best location to put a single step and/or multistep?
2. What is the ideal height of the step in comparison to its length and width?

Thank you for sharing! :)

[Jimboat]

there is a good thread here on step design (http://forums.boatdesign.net/showpost.php?p=47245&postcount=10).

[batistuvan]

Thanks Jim.
An interestid post, but do you know what is the height of the step related to its length/width, I think it also important.

[Jimboat]

batistuvan - Yes, height of step is important and does indeed influence performance. Adjusting location of step and height of step can help control the angle of attack to fairly tight limits, aiding design. There is also a minimum step height required to be effective (dependant on planing surfaces configurations, hull weights/forces, and velocities).

[Captain RAH]

your website is hard to read!

[Jimboat]

Captain RAH - give me a call, and I will try to answer any questions you have.

[batistuvan]

Yes, height of step is important and does indeed influence performance. Adjusting location of step and height of step can help control the angle of attack to fairly tight limits, aiding design. There is also a minimum step height required to be effective (dependant on planing surfaces configurations, hull weights/forces, and velocities).

Do you have some sort of formulae to calculate this?

[Jimboat]

batistuvan - unfortunately there is not one simple formula that will optimize step placement. The influence of step design on overall hull performance is a function of many of the other hull performance parameters. So it is not a simple variable relationship and must be solved with simultaneous equation solutions. It is worthwhile understanding the principles of how the step design can affect performance generally. Solving it numerically is more complex. We use software to determine best solutions. And remember that a step placement can only work optimally at one velocity.

[batistuvan]

Should you put an angle after the step?
I heard somewhere that to decrease the drag, you need to incline the hull, is that right? but how much angle?
Can anybody help?

[fede]

Have been looking for "formulas" about stepped hulls for years, couldn't find anything, Hydrodynamics of high speed marine vehicles by mr Faltinsen gives the best scentific explanation of the issue but it's still not an in depth research on the topic.
Being a ultra fast crft enthusiast I?ve seen anything on race tracks,high steps small steps, 2 steps 5 steps,I?m talking 100 mph boats,all of them seem to work but until somebody does an in deep research in the tank comparing different designs and combinations of steps config/strakes/CGs/LCP/trim/watr conditions etc. none will ever know and that's a tricky and complicated/time consuming and expensive test to run that nobody will ever take into consideration.
I myself considered step design on my ribs but let the ball drop not being 100% shure of what I was doing.
The only way to get it right is to make aproto of the boat you want to make and test and modify and test and modify until the result is achieved.
Just my 2 cents...I'm none around here.

[yipster]

"formulas"
was just reading this formula in "fluid-dynamic drag" by Hoerner but offcourse there is more

[fede]

Cool,found a couple of used drag and lift on amazon...100 and more $ each.

[tspeer]

was just reading this formula in "fluid-dynamic drag" by Hoerner but offcourse there is more

I believe the page is talking about the aerodynamic drag of steps when the airplane is in the air, not the hydrodynamic drag of a stepped hull in the water. It's not going to be of any help in designing a boat.

[fede]

Yes but I?m pretty shure the books talk also about drag reduction in water.

[yipster]

Tom, your right and good setting that straight.

recalled reading drag of surface imperfections and the scan above starts on the page before with

"steps cut into the lower side of floats and flying boats are necesairy to break the suction of the water; and to keep the airplane balanced at angles of attack desirable for take-off. the drag of these steps, plotted in fig 25 can be calculated in a way similar to that of sheet-metal joints"

but its from the chapter "aircraft components - drag of floats and boat hulls" and dont think x800 calculates.

from the chapter "water-borne craft - characteristics of planing craft" some more here but no in detail stepped hull answers.

turning the page i see hydrofoils

[tspeer]

...from the chapter "water-borne craft - characteristics of planing craft" some more here but no in detail stepped hull answers....

Hoerner is most useful for estimating a starting point that will give you ball-park numbers from which you can then start doing more detailed design. For example, Figures 27 and 28 in Chapter 11 show that you're going to need a thrust to weight ratio on the order of 0.22 to 0.27 to get over the hump for a typical narrow (l/b = 7 - 9) stepped hull.

The preceding figures in the chapter have data for flat plates that cover the various drag components and how they vary with geometry. If you were to try to predict the drag for a complete configuration using the earlier material on planing flat plates(or Savitsky, etc.), you should come up with numbers that are reasonably close to those shown in figures 27 and 28. Once you've validated your methodology, you can then strike out towards your desired geometry and have some confidence in the validity of your predictions.

I think the way to approach a stepped hull is to first consider it to be a single hull, cut off at the step. Then you'd consider the stern of the hull as though it were another boat following closely behind the first, taking into consideration the stern wave left by the stepped portion. Finally, you'd add in an allowance for factors like the spray from the step wetting the portion of the hull that would otherwise be dry in between the step and the stern, and constrain both hulls to have the same movement. In this way you could use the methods you currently use for estimating the drag, trim, and dynamic stability of a non-stepped hull to the stepped hull.

[Verytricky]

OK,

I am new at this but...

Stepped hulls primarily are there to reduce the drag of the surface of the wetted area.

Another advantage is that you can run the boat 'flat' and the hull has the correct angle of attack for the boat, without the losses incurred in using trim to raise the front of the 'flat' hull to get the correct angle of attack.

If you are thinking of a single stepped hull, the step should be in the region of the COG. The angle from the transom should be IRO 5 degrees to 3 degrees and the length is more or less determined by YOUR design of the boat, as this travels till it reaches the step.

The height of the step is usually to bring the boat hull at that point back to zero with the transom, or near to it depending on your design, often a few mm lower than the transom 'height'


Hopefully this will help: Feel free to pull apart these basic concepts as I need to learn as well!!

[fede]

One thing I think is always got wrong on stepped hulls is that the step should be positioned keeping in mind where the CG is,this is incorrect, step placement and step height has more to do with pressure distribution of water under the hull,and
water separation/free surface prediction that implies considering trim,immersed depth (draft froud number) at the considered speed.
In fact, there is no such thing as a correct step for all speeds.
In a few words, best step placment is where the least hydrodynamic pressure occurs, that's the point where water will easyly separate from the hull and benefit the best.
As always I might very well be wrong !

[Jimboat]

fede is dead right that there "no such thing as a correct step for all speeds". One of the trickiest challenges (and one of the more dangerous features) of step design, is that the step location is only efficiently effective at one velocity. Some designers try to alleviate this issue by using multiple steps, however this too creates some additional challenges. The most advantageous benefit of the step design is that it allows the designer to 'fix' the angle of attack as desired....proper location and depth of step will do this. The down side of steps is that the mislocation can actually create less efficient lift/drag and also can cause serious "tripping" in turning maneuvers - and there will be mislocation of any step design at non-optimum velocities.