Stepped hull design concept

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.

 

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!!

 

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 !

 

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.