Asymmetrical and Symmetrical hull form in powerboat

Hey,

Regarding the symmetry of the hull in real life powerboats (leisure boat or workboat). What are really the rule of thumbs (if there are any) in the hull design regarding these two kind of shapes when we are above 1 Fn.

Offshore racing catamarans pretty much always use this kind of hull shape and I think the reason for this is that the hull interaction is minimized? But then again if you look up some tests made for asymmetrical and for symmetrical hull it seems like the interference drag is greatest below 1 Fn. But then of course also the distance between the hulls play major role in this because the greater the spacing the less there is interference drag. But I also assume that the general wave making drag exluding interference drag is greater for asymmetrical hull shape than for symmetrical one.

If you look at the picture I sent here, this is 3m wide catamaran and pretty long (13-14m) for it's beam so I think the interference drag would be "too" high if it would be symmetrical so that is why the designer choose to go asymmetrical?

So my question is: Is there general rule of thumbs when asymmetrical hull shape is preferred and if there is, why? Is it generally true that with certain length / Beam ratio there is no benefit for making asymmetrical hull shape? If asymmetrical is greater for bigger Fn numbers why is that, since some data show asymmetrical shape having advance in lower Fn numbers? Maybe my expectations for rules of thumb are too high, since there are so many variables that influence this subject?

 

I concur wholeheartedly.

 

Comparing a race catamaran to a workboat, is like comparing a Lamborghini to a Mack truck. They have some similarities, but are very different. You should look at boats that have the same purpose, area of operation, target speed, etc. for a more reasonable example.

 

I've designed several catamarans with asymmetric hulls, and they weren't racing boats but simple purse seine fishing vessels.
There are many parameters in boat design that can influence the type of hull chosen; speed isn't the only factor. It's not enough to just look at your neighbor's boat and copy it; the goal is to optimize the SOR of the project and achieve the best boat the client/builder can construct with their resources. (You only need to design a boat to learn this.)

 

The cat in my avatar photo has asymmetric hulls - yes, the hulls would be more efficient if they were symmetrical, but there were other factors that influenced the choice of going with asymmetric hulls.
Especially as these hulls are already pretty efficient - 15 m LOA, 4.9 m beam, original power was 2 x 70 hp O/B motors, and she did 16 knots top speed with these engines and 10 people on board.

The hulls are aluminium - and we saved a LOT of welding length of hull panels by building the inboard sides of the hulls from simple flat plates joined together.
The outboard sides have two chines and a keel bar, and the chines below the waterline were double continuous welded.
The Builders had calculated how much it cost them to weld every inch (or cm) of seam and while I cannot remember the exact figure they came up with (this was 25 years ago), it was pretty high relatively.
So a significant chunk of $$ was saved during the construction by simply making the hulls assymmetric, and reducing the total weld length as much as possible, while keeping two chines on the outboard side (rather than having just one chine, which might well have resulted in increased drag while underway).

 

Could you share why did you make those specific boats asymmetric?

And yes I understand what you mean.

 

Thanks for the reply!

That is interesting! I was actually thinking that at some point that could there me much saving for money if the building style would be as you have explained!

 

We used one of the monohull molds that the shipyard had. A "normal" hull was built, split along the longitudinal plane, and each of the half-hulls was completed with a vertical longitudinal bulkhead, which could have been any other shape, but for simplicity, a vertical "bulkhead" was chosen.