Hi every body,
I was reading severals books on Naval Architecture and I was surprised to observe that graphics depicting the relation between the Prismatic Coeficient vs the Speed-length-ratio are different from one book to another.
There is anyone that possess a trust-worthy graphic ?
Hi every body,
Any ideas yet ?
As I understand it, prismatic coeficient is related to the underwater hull shape. It is different for each hull form for hulls of equal length. It is more complicated for a sailboat as underwater hull shape changes with heel angle.
If so then a graph comparing PC to the speed/length would be unique for each hull design. This could explain why the graphs you are looking at are so different.
I’ve been away from this stuff for along time, so I cannot help with useful references.
I’m not sure that there is an answer to this one Claudio. CP is really just an indicator of fullness in the hull, and the spread of that fullness fore and aft and vertically. I’m not sure that this can be matched against speed without taking in a whole lot of other variables too. Obviously fullness is a factor in drag.
But you know all of this already. Maybe I just haven’t understood your question.
i too have been playing with this. the best way i can describe it. it too show you 2 boats. lets take a ts2, very wide stern. this boat should have a high prismatic coefficent. now we look at a nimbus. a narrow stern boat. this should have a lower cp. I take these boats because they are on the far end of the spectrum. both both are good. but at different wind ranges. what i can gather is the higher the cp. the faster you are in high winds. the cp basicly meens the hull if full. that is the entire hull. I could be wrong. and most times I am. this is just want i have gatherd from 15 years of designing. I am just putting names to what i have taught myself.
I don’t think you can infer performance based on CP in this way. CP applies to the hull in displacement mode. The TS2 (wide, flat aft) is faster in stronger winds as its shape promotes planing better than the narrower hull.
However the other half of the argument, that the narrow hull is faster in light air (both in displacement mode - no planing) sounds valid.
Is there a correlation between CP and wetted surface? Narrow boat has less wetted surface and therefore better light air performance?
Cougar,I think you are way off here.
Most of the wide beam IOMs I have studied show a low Pc and narrow beam ones a high Pc.
Reason…all IOMs displace 4000cm3 right? so a wide beam boat with a typically high midship section area must have a lower Pc than the boat with a smaller midsection area.
The more I draw boats and think about them the less notice I take of Pc,s
It really is a pretty meaningless ratio when its all boiled down.
Though I always make a check of it to ensure the value is in the "normal " range.
The various full size graphs I have seen showing Pc/speed don’t seem to apply to model yachts as far as I can tell.
My latest designs seem to have a Pc of around 0.53 if that is off any use to you.
reading books, it is said that without considering an important degree of hell, during running, the relative Fineness or Fullness of the hull?s ends has an important effect on the wave-making resistance.
It is convenient to use the PC to define therefore the fullnees or not of the hull?s ends.
The prismatic Coeficient is given by :
PC = V/(Am x lwl)
where V is the Volume of the imersed body
Am is the Main section area
Lwl is the water line length.
It is also reconisesd that for each Speed-length ratio = V/√Lwl, there is an optimum Prismatic Coeficient (PC).
Full ends will produce relatively high PC and slim hull entrances will exibit low PC.
It is also implicit that the optimum PC for that Speed correspond to the lowest Resistance of that Hull at that Speed.
It is also described that at Speed V/√Lwl < 0.70, the optimum PC remains close to 0.53 / 0.54, while for V/√Lwl > 1.0 , the optimised PC would increases around 0.56 / 0.57 and more.
Here below, two diagrams : one relating to the Speed (V/√Lwl) against the Resistance for various PC.; the other diagram instead, is the collection of various graphs extracted from different books concernig the relation : PC vs V/√Lwl.
It is interesting to see the contradictions existing for a given V/√Lwl.
This is why I search for a trustable graph PC vs V/√Lwl.
you could be right when i compare the 2 different sizes. wide vs narrow. what i was trying to do was to bring people into the conversation. whe i first started to design. i drew by hand, and worked with an existing design then built it. then drew my improvements. now I draw my own. i got into discussion. and people were thoughing words at me like cp. foreward boayancy vs displacement.
i would assume that the wide beam boats like the ts2 and v3 would have a high cp. because simply they do well in the high winds. that are around the south. the narrow boats that do well. would have a lower cp, do well in the low end spectum. the boats i do now ( iom) have a cp around 55-56
Claudio, use the Delft series values. The results from the delft research are based on tow tank test. Speaking as a physicist, it?s the only model that looks realistic to me.
Have you read ?principles of yacht design? by Larsson & Eliasson?
no, I have severals books of Marchaj, Badeer, Gutelle, Douglas-Phillips Birt, Bethwaite, etc. but not that one.
What it says about ?
‘principles of yacht design’ by Larsson & Eliasson is one of the best Texts around,you should find a copy and read it.It really is the best book on yacht design I have read.
The Delft series is most likely the best one to use to start with.I used to follow the values given by the Delft tow tests as the bible,but as time as gone on I now have my own thoughts
i agree with the above book. principal of yacth design. although i warn you . you have to read it atleast 3 times. before you understand it. it gives you almost everything you need. and it is well worth the pick up
I will search the book via Amazom.com
Actually I have two class M on my drawing table , one for ligth winds with a PC of 0.53 and the other for the breeze with a PC of 0.57.
Rigth or wrong ?
I personally would drop the high winds cp to .55
I wonder if there is big difference than between 0.53 and 0.55. Extrapolating I would cut in the middle and drawing a single hull with 0.54 and hoping to be an All-round.
?principles of yacht design? gives a graph showing the amount of drag caused by a non optimum cp.
"Principles of yacht design"Also shows a graph showing the increase of drag for non optimum LCB.
Interestingly the LCB of most sucessfull model racing yachts is further back…comments??
Using the graphs from the Delft series( Pc,LCB etc) would be a safe way to design your boat…But I don’t think it is optimum for model racing yachts.
i think the disconnect, is that are models are at ‘plaining mode’ speeds, but still in displacement mode.
I agree Dan,thats why the Pc graphs from the delft series don’t apply to us either…