Technical » Stem Shape

Release Date: 11/20/2010

Stem Shape

Several boats that have been built recently or are being built to Bernd Kohler's designs show very blunt stems, with sharp angles where the front joins the sides. This is an undesirable practice that will result in turbulence and thus drag.


I have looked at Bernd's drawings of the KD860 and find only a brief hint of what to do at the bow of the KD860. A diagram mentions a false bow but doesnt give any details. I think the bows are blunt at the front edge of the plywood planks and stem panel and the builders are just stopping there. Here are pictures of some of the resulting boats.




This is a computer generated graphic that shows

the open, unfinished bow on the right and closed

off bow on the left with square edges. Perhaps

the finished boat will not be squared off,

but it illustrates the matter.








Derek Kelsall's drawings for the KD920K shows a foam former ahead of the side panels (epoxy/glass/foam panels) with this former rounded off with the diameter appearing to be the width of the plank joint at the front at station 0. That makes sense.


The problem with the blunt bows is that they will add drag to the hulls. The two hard edges appearing on Bernds designs are wrong, in my opinion. They will generate turbulence and thus drag.


A really sharp bow apparent in the photo below is also a turbulence generator for a sailboat because a sailboat operates at a slip angle through the water. This too causes drag.



Rounded is best, and I think there is a proper dimension to this based on the length of the boat and the design boat speed. My recollection from long ago is this relates to the Froude number which takes waterline length and design boat speed into consideration.


I have written to Bernd for clarification. He replied "I know the theory but never use it after I am always looking for a better way to design a hull. Now we found that rounding off the bow is sufficient. No matter how you do it nothing changes, not even the bow wave. Here in France I see a lot of multi hulls as you can Imagine. I talked to a lot of designers about hull shapes etc. The bow was never a concern to anyone. Of course you make them not to blunt that is all. (Emphasis added.) Mr. [Edmund]Bruce in England made a lot of hull tests in his water channel and using the Froude numbers but his results were not conclusive".


I am also thinking of writing to other professional catamaran designers, such as Shuttleworth, Morelli & Melvin and Kurt Hughes to see if they have design standards for this.


Interested builders might look at pictures of sailing catamarans in magazines. Occasionally there is a picture that shows bow shape and the effect on turbulence at the entry waterline. Meanwhile, round them off!



Calvin H. Markwood

Engineering Analyst

Multihull Dynamics, Inc