My Coefficient Forms?

[Laranjo123]

Sirs, is it ok if my cb = 0.3739 cp=0.5366 cm=0.6968 cwp=0.6966 is that ok i think my cb cp and cm are ok but i have doubt about on Cwp?....It's a motoryacht......thx in advance ^__^

[Olav]

Originally Posted by Laranjo123

[...] It's a motoryacht. [...]

What type of hull? Round bilge, hard chine? Deadrise?

What size? Length, beam, draught? Displacement?

What speed/Froude number? Planing, semi-planing, displacement mode?

Where do these numbers come from? Actual data from your linesplan or estimated values from empirical formulae or any "recommendations" from a book?

CB, CP and CM fit together (since CP = CB / CM), but are quite meaningless without any further information about the vessel.

[philSweet]

Like olav suggested- these parameters don't stand by themselves; but given a linesplan and a statement of requirements, they are useful tools in determining trends in performance. They have proven useful in reducing the amount of data needed to make some informed decisions, but you still need the original data to reduce and similar craft of known performance to compare with.

[Laranjo123]

LOA-45.8 m
LWL-42 m
B-9
D-5.25
d-2.43
Displ Wsw 360 tons
Vol. Displ - 351 Cubic m
Speed 13

Thus the Particulars came from the average values of the parentships. the coefficients came from books some are empirical and some are approximate.. It's a round bilge....and i don't have lines plan yet.

[rxcomposite]

With the above particulars your S/L ratio is 1.1 or Fn number 0.33. Typical of High speed liner/Ferry.

The following should guide you;

Cb= 0.6
Cx=0.97
Cp=0.62
Cwp= 0.71
Entrance angle= 6 degrees
Displacement to length ratio= 80 to 100
LCB= 1.5 to 2% aft of Midsection

No parralel mid body, Bulbous bow might be beneficial, straight or slightly hollow (S) forward section.

[Laranjo123]

so my Coefficients wasn't right?

rxcomposite sir do you have reference on what are the corresponding coefficients for Motor Yachts?...

is this right? i just recalculated the coefficients cb=0.36 cp=0.55 cm=0.66 cwl=0.71 ?..or still not?

[Olav]

Originally Posted by Laranjo123

so my Coefficients wasn't right?

No, at least CB doesn't fit together with the particulars you stated (I can't comment on the others due to lack of knowledge about other particulars, but I assume they are also not quite correct, but probably in the right ballpark though).

Assuming your displacement is right (again: Where does it come from? Weight study, even a preliminary one? Taken from a similar vessel?), your CB is

CB = V / (L * B * T) = 351 m³ / (42 m * 9 m * 2.43 m) = 0.3821

Why don't you just go and make a lines plan based on the main dimensions of your project (L, B, T, deplacement etc.) and take the "real" coefficients from that? With nowaday's hull design software this is done in the twinkling of an eye (compared to drawing by hand) and you get all hydrostatic data in real time when refining the shape.

Optimum parameters can be obtained from common literature depending on hull shape, Fn etc. In another thread you wrote about Michael G. Parsons' paper on parametric design that you have, another good source would be "Ship Design for Efficiency and Economy" by H. Schneekluth and V. Bertram, for example.

So:

- Set up a SOR for your project
- Collect data, drawings and everything from similar vessels
- Make a preliminary weight study and determine your main particulars
- Make a lines plan
- Optimize the lines plan using optimum coefficients from the literature
- Perform a resistance estimate and propulsion calculation
- Evaluate the design and go further loops within the design spiral if necessary

Edit:

Originally Posted by Laranjo123

is this right? i just recalculated the coefficients cb=0.36 cp=0.55 cm=0.66 cwl=0.71 ?..or still not?

Again, this makes me wondering about what kind of studies you do... No offence meant, I'm just wondering... Really.

[rxcomposite]

Those coefficient came from Principles of Naval Architecture, Volume 2- Powering. On the same volume, there is also a range of prismatic coefficient and D/L ratio. Ad Hoc has posted it somewhere in this forum if you don't have the book.

Those are just guidelines to give you a head start in design using a parent hull. But if you look closely at the empirical data, the coefficients are very close together as you go to higher froude numbers, hence you must have something to start with.

Say for a motor yacht, you must determine your SOR first. You gave a speed requirement. Is it top speed or crusing? At FN 0.33 that is bordering on the next group where the coefficients changes and no "S" curve on the hull. Design around cruising speed where it is most economical to operate. Choose a hull form then tailor your parent hull to fit the coefficient. Make a spreadsheet so you can optimize your (least) resistance based on the hull form, that is the underwater hull form. Whatever shape you design the above water line has very little effect on your study as its effect is minimal. Just air drag and aesthetics which is way down the design spiral.

Olav gave a methodlogy for you to follow so you won't get confused. It is a good advice. If you are familiar with the design spiral, follow that also.

[Laranjo123]

but my prof didn't gave us the SOR. He said we should depend on our parentship? And my prof told us not to us blocking. Because the time he dne his thesis he didn't used blocking just formulas.

[rxcomposite]

Well, Maybe it is time you prove your professor wrong (or maybe he is correct). If he said use what he has instructed you to use, what is the point of making the thesis? How will you learn?

First make a block drawing using the the B/T and the L/B ratio. Choose a hull type. Since this is a semi displacement craft, you narrow down your choice to SSPA or NPL type hull. The shape section calls for V shape aft, turning to U. The SSPA hull hull seems too bulky forward. Chooses the NPL hull. You need to modify the NPL type hull though. Don't worry about displacement at this stage because you will be working with proportions, not dimensions. Once you get your coefficient right, scale your drawing to get the displacement.

Run some hull resistance test and tweak your hull form to get the lowest resistance while keeping well within the published boundaries of the NPL data. At this point, you need to to watch closely the Atx/Cx (transom immersion to midship section ratio) It is discussed in the PNA and used in Savitsky/Mercier speed prediction. Or if you are familiar with Michlet, try using the program. Note that the parent NPL hull has a deeply immersed transom.

You should be able to get near optimum hull form. Do a second set of calculation for the wind drag. Add that to the hull resistance and you should get a close estimate of EHP.

I think at this point, you will be able to see if your prof came close to optimizing or you did a better job. Good luck.

[Laranjo123]

thank you sir your advices really made my day. Thanks really. And to be honest i don't know the SSPA and NPL. i really want to finish my thesis. Though it will take time before i realize that studying hard is the best key. Thanks to all who gave their advices. haha RXcomposite was my favorite of them all..

[rxcomposite]

Practice, practice. It is just an organized stream of thoughts and you learn much from the forum.

I find the hull coefficients in 12 minutes and the hull optimization (mono, cats) in another 10 mins. Using Excell of course and the program I wrote. If I want to do more analysis, I use Michlet for different hull forms and Mercier/Savitsky for the Atx/Cx ratios. No need to reinvent the wheel. Other great minds have done it.

If you want to draw hull lines quickly, use Hullform. It is downloadable for the trial version. For serious work, try Freeship. Other programs are available but will cost a lot of money.

[Laranjo123]

i have maxsurf v11. is that ok?...