Catamaran drag rise at less than hull speed

[Ad Hoc]

Originally Posted by sundancer

... I think the bottom would have to have an outboard deadrise of 20-30 degrees to bias the flow of displaced water away from the tunnel, possibly minimizing interference drag. But this hull is a little more complicated to build, and probably has more drag at Fn<0.25 because of bigger turning angles - hence the selection of the symmetric hulls.

It is rather difficult to comment when you have alerady come up with the solution/answer. Not sure how you arrived at these conclusions in the absnce of any hard data or the SOR.

If you want objective comments, you need to post the body plan, profile and also more hard data on the boat.

Otherwise all you're asking is for "other people" to agree with your hunches or guesses of what to do.

[Alik]

If boat is in alloy, probably it is too heavy and first thing to do is try reduce weight.

[sundancer]

I didn't consider chine vortex shedding as a possible candidate. Thanks

[sundancer]

Originally Posted by Ad Hoc

It is rather difficult to comment when you have alerady come up with the solution/answer. Not sure how you arrived at these conclusions in the absnce of any hard data or the SOR.

If you want objective comments, you need to post the body plan, profile and also more hard data on the boat.

Otherwise all you're asking is for "other people" to agree with your hunches or guesses of what to do.

Noted: I think all I seeking was a comment, based on any research someone might be aware of, on whether a straight tunnel configuration had any clear benefit over two symmetric hull with regard to the phenomena illustrated in the graph posted earlier.

[Manfred.pech]

Hi Sundancer, you will find a lot of research about Wigley Hulls with Google, mostly academic and very interesting. If you really want to get hulls with optimized hydrodynamics you have to look for the flow of water around your hulls. May be it is helpful for you to get some photos from another boat to analyze the vortices and waves around and under your catamaran. May be I am wrong, but from my experience I would avoid too much gradient of curvature and reduce the wetted surface as much as possible. This can be done by slight hydrodynamic lift from an optimized water flow.

[sundancer]

Originally Posted by Manfred.pech

Hi Sundancer, you will find a lot of research about Wigley Hulls with Google, mostly academic and very interesting. If you really want to get hulls with optimized hydrodynamics you have to look for the flow of water around your hulls. May be it is helpful for you to get some photos from another boat to analyze the vortices and waves around and under your catamaran. May be I am wrong, but from my experience I would avoid too much gradient of curvature and reduce the wetted surface as much as possible. This can be done by slight hydrodynamic lift from an optimized water flow.

When the boat goes back in the water later this coming spring, I plan to do a photo & video survey around and under the boat to check out the various suggestions I've received. I will search Google to see if there is anything about Wigley hulled catamarans, and hull spacing.
Thanks and Tschüß

[sundancer]

Thanks to Manfred.Pech's suggestion to Google Wigley hulls, I came across a paper published in Ocean Engineering by Moraes, et. al. in 2004 (vol 31, pp 2253-2282) To my pleasant surprise, they studied the wave drag of chine hull catamarans as well as the Wigley hulls as a function of their separation distances. With the help of the data in this paper, I was able to establish that Sundancer's drag rise behavior is entirely consistent with this class of catamarans, and not something odd. The details follow. Thanks again!

Sundancer's hulls fall well within the ranges analyzed except for the block coefficient, since Sundancer's deadrise is 0 degrees (flat bottom). Sundancer has following parameters:
L/b = 10.6
b/T ~ 2.5
S/L = 0.32
Entrance/exit = 0.31L
Block coefficient = 0.755

Although the focus of the Moraes paper was on high speeds (Fn > 0.3) they did include data points at Fn = 0.2 & 0.3. Of particular interest was the non-dimensional form of the wave drag, C*, given in figures 31,33 & 35, based on the 'Shipflow' 3D analysis code. Albeit a little hard to read accurately, these figures show ~ 45% drag rise in this neighborhood, with only a slight dependence on hull separation distance (S/L). The rise is very significant above 0.3, with a significant dependence on hull separation between Fn = 0.4 and Fn = 0.6. Since Sundancer's low power limits its speed to ~ 0.33, I concentrated on trying to read the Fn = 0.2 & 0.3 data point values as accurately as possible by enlarging the charts.

Acknowledging that propeller and motor efficiency vary over the speed range, Sundancer's drag trends can still be roughly inferred from the motor input power, After removing known constant overhead power, I divided out the cubic speed dependency on power (quadratic drag from the definition of a fluid drag coefficient). I then superimposed C* values from the Moraes paper rescaled to my 'funny units', and achieved a very reasonable match as shown in the attached chart.

I appreciate all the comments that I've received, and have certainly learned about some improvements that could reduce power consumption at lower speeds if and when another hull is built. However, it is very clear that no great improvements will occur in top speed with < 3kW of motor power. Thanks again to all the contributors for the help.