Extending My Catamaran Sugar Scoops

[YPSILANTI]

I am extending the sugar scoops on my 53 ft. Royal Cape Catamaran. See attached pics. If one followed the existing line of the inside of the hull, the sugar scoop extension would narrow in slightly. However, in my instance, we've diverted slightly such that the sugar scoop widens on the inside by a marginal amount of approximately 5 to 10 cm. ( 2 to 4 inches max) I wonder if the slight widening of the aft of the vessel below the waterline will create any undue drag or is it negligible and thus of no significance. Id immensely appreciate the knowledge of experienced fiberglass specialists rather than speculation and opinion from non specialists. Thank you kindly.

[psk125]

Judging from the pictures it seems a bit late to be asking that question now. There are probably naval architects who would be happy to answer you. They may only charge SAR 2000/hr for a non-speculative answer.

[YPSILANTI]

You are correct, the job is basically done. However, it can still be changed, its only the inner skin that would need a change. There are many experienced, hands on boat builders and fiberglass experienced folk whose opinion Id value and respect. Their knowledge is invaluable. I didn't mean to offend.

[smj]

Purely speculation, but the Royal Cape isn’t known to have great performance so my “guess” would be the added waterline May even out with the slight widening giving maybe close to the same performance as before the modification?

[Matt Johnson]

Judging by the image, there doesn't seem to be a need to add that flare in the hull. If you're looking for swim platform space, why not just have the deck area overhang and not push the hull out too?

[catalystcat]

I hate to give the awful, "it depends" answer, but, it depends. Given the width of the hulls on your boat is larger than the scoop widths and given that the hull spacing is something like 12', the impact of 6" (3 on each side) should not be significant. However, it would be interesting to see how the bulge impacts fluid flow at and near that location. I would suspect that, if there were to be any real impact it would come more from that kind of fluid dynamics than it would from the slight flow rate change going from 12' to a 11.5'. You might be surprised to find out what it does. I know that little things like winglets on jets and rear cowlings on trucks can have a handful of percent impact on drag from turbulent-like effects. Sometimes, a little change that increases the laminar flow part can really have a good impact.
Once it is in the water, you might try to see if you can see what kind of eddys, turbulence, or other flow changes arise. That's really a better way than to try to model fluid flow at an interface. That kind of modeling is notoriously difficult and sensitive to assumptions around speed, surface dynamics, and more. I doubt any current model would give a highly accurate prediction of what that bulge would do. That's one reason wind tunnels are still so common.
To help you see what is happening, you might try a little dye in the water or some ash to help you see what the water is doing.
If I had to guess, I'd guess the effect is small, but that, at some speeds, it might be noticeable and not necessarily negative.