Godzilla, Form Drag and Hull Half Angles Issues

TJ Cameron · Apr 9, 2012

(Post Deleted)

Leo Lazauskas · Apr 7, 2012

TJ Cameron said: ↑

I am using Godzilla to find optimized hull forms for an A-class development rule sailing catamaran.

A-class Box Rule:
5.5 m max. length
75 kg min boat mass
2.3 m max. width
13.93 m2 max. sail area

85 kg assumed crew weight
160 kg assumed total mass
Optimized for 6 m/s speed

These boats are balanced on one hull both upwind and downwind. That makes mono-hull calcs. a better fit than catamaran calcs. There is no need for hull form stability so semi-circular elliptical sections are optimum. Above 6 m/s the boats are partially lifted out of the water by curved daggerboards set at between 1 and 2 degrees angle of attack.

I have modified Corey’s concrete canoe mlt file (series 7) and the single crew skull mlt example (series 9) for 5.5 m length and 160 kg mass and made multiple optimization runs.

The series 7 file optimizes the design space for lowest total resistance with elliptical cross sections, elliptical bow and stern waterplanes and very close to rectangular bow and stern sideview profiles (ie. little to no rocker). Achieving adequate rocker for maneuverability by forcing elliptical side profiles results in about a 2 percent resistance increase. The drag increase is minimized somewhat by using cross sections which are fuller (more rectangular) than pure ellipses. So far, so good.

My problem comes in when I try to account for form drag caused by the bow and stern half angles. The Godzilla optimized elliptical waterplanes create half angles of 90 degrees at the bow and stern. Successful A-cats typically have bow half angles between 5 and 7 degrees.

Am I missing something?

Leo posted the following:
“For rowing singles and doubles, Scragg and Nelson found a
form factor that could be applied to the (empirical)
1957 ITTC skin-friction formula. Their factor is:

1+k_f = 1 + 0.0044 *(a_in + a_out)

where a_in and a_out are the entrance and exit
half angles in degrees.

I have also used Prohaska plots to convert their experimental
data so a factor can be used in conjunction with Grigson's
skin-friction formula. In that case:

1+k_f = 1.0289 + 0.0015*(a_in + a_out)

These factors are only available in my personal version of
Godzilla, but you can add the form drag after finding several
candidate hulls to see if it changes their ranking. I'm guessing it
won't gain you much more that 1%.”

I am not sure how to use the equations above. If I use 90 degree half angles for the elliptical waterplanes, the right hand factor goes to 0.79 in the top equation. Does that mean my total resistance increases by 79 percent? The form drag for blunt ellipses versus 7 degree half angles intuitively would seem to be considerably greater than the 1 percent speculated above by Leo.

I think that if I modify the elliptical waterplanes to have 7 degree half angles in Freeship, the prismatic coefficient would be substantially reduced with the result of a large increase in resistance. If I can’t seamlessly calculate the effects of half angle form drag for each optimized design using Godzilla and Michlet, then the programs have much less utility. I have no way to effectively evaluate the performance of different half angles. This is the only design problem I have left before finalizing my design.

Does anyone have any suggestions?

I notice that Rick's pedal boats V11, V14 and V15 have half angles I estimate to be between 7 and 10 degrees.

I want to thank Leo for making available such a great program and Rick for his tips for using Michlet without which I never would have gotten Godzilla to run.

TJ
Click to expand...

I suggest that you constrain the shape parameters so as not to allow
elliptical waterplanes. They are not realistic for this type of vessel.

Good luck!
Leo.

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Godzilla, Form Drag and Hull Half Angles Issues

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