Cp - Prismatic Coefficient Fore & Aft

[rxcomposite]

Leo,

I'm looking at my book where there is a plot of wavemaking resistance. After the humps and hollows, the resistance decreases but the graph stops there.

Why does it decrease? is it because at high froude numbers, there is a lift generated or is it just the way the formula predicts the resistance?

Thanks,
Rx

[Tim B]

Rx, that's WAVE resistance, you've still got skin friction to add to that as per ITTC-57/58 or Schoenherr.

Of course, skin friction is subject to the old 1/2*rho*V^2 bit, and V^2 goes up pretty quick.
Tim B.

[Leo Lazauskas]

Originally Posted by rxcomposite

Leo,

I'm looking at my book where there is a plot of wavemaking resistance. After the humps and hollows, the resistance decreases but the graph stops there.

Why does it decrease? is it because at high froude numbers, there is a lift generated or is it just the way the formula predicts the resistance?

Thanks,
Rx

The decrease in wave resistance at very high Froude numbers is not because of lift, the same occurs for fixed attitude vessels.

Regards,
Leo.

[rxcomposite]

Tim B,

Thanks. I'm doing that. I'm actually using a very sophisticated program to analyze the resistances. Like most program, you have a very powerful tool to do the number crunching but you dont see the formula.

Canoes are easy to analyze as their S/L ratio is in the range of about 0.7. What i am investigating are S/L ratio greater than 2.0.

Most published design guide stops at S/L of 2.0. The wave resistance graph implies that eventually it will diminish to zero, while the frictional resistance suggest that there is a point where it will be very stiff, almost vertical.

What am i looking for? the correct hull shape that will give the most efficiency at S/L of greater than 2. At this point, semi displacement hull "destroyer" type lord it over. What is the next hull type? Planing hull? Wiggly shaped hulls? or SWATH type hulls?

Michlet program does a fine job but i,m not a mathematician so i would say i'm fiddling.

I dont have enough information/dat yet to make a convincing decision.

Cheers

[Leo Lazauskas]

Originally Posted by rxcomposite

Tim B,

Thanks. I'm doing that. I'm actually using a very sophisticated program to analyze the resistances. Like most program, you have a very powerful tool to do the number crunching but you dont see the formula.

The main difficulty (for non-mathematicians) is that the equations for wave resistance are not simple functions like the ITTC line. The method I use in Michlet is due to J.H. Michell who originally expressed the wave resistance as a quintuple integral. For some mathematically-defined hulls the number of integrals can be reduced: for example, the wave resistance of thin parabolic struts can be expressed in terms of Bessel functions. Unfortunately it is not easy to see what is going on with even those simple cases.

If you want the mathematics behind the methods in Michlet see the early reports at:
www.cyberiad.net\wakepredict.htm

A very useful concept to aid the understanding of (thin-ship) wave resistance is the free wave spectrum (FWS). The graphs of the FWS that you can produce with Michlet will show you at what wave propagation angles most energy is being shed. That can give you clues on how to shape the hull so as to reduce the largest peaks. (Well it does if you know what you are doing!) Reducing the peaks will not necessarily produce an optimum hull (from the point of view of low wave-making) but it's not a bad strategy. Another strategy is to just let the optimisation program grind out something for you.

You should also keep in mind that Michlet can only guide you towards a reasonably efficient hull. I think that the best way to use Michlet is to let the optimisation program find a simple mathematical hull of the desired displacement and subject to any other constraints you have in mind. Once you have a reasonable looking hull, you can fiddle around with transom sterns, add flare, or make any other changes that lead to a practical hull.

Then repeat the process again and again.

Good luck!
Leo.

[rxcomposite]

Thanks Leo,

I have downloaded most of the papers in the site. I am studying it. I am very much engrossed on the pressure signatures on planing hulls. When i first heard of surface pressure prediction, it was for the stealth bomber.

I am following your advice to the letter. I find that michlet will help me accomplish what i am doing.

I switch to Michlet as i find it easier to punch in the parameters than let Godzilla do the grinding. Godz takes hours to find optimum hull even for a simple kayak. I,m using a 1.7Ghz PC.

I,m making progress now. have finished transom hull evaluations.

thanks again,
Rx