Prismatic Coefficient

My company has survived in the yacht design business for almost 20 years without even knowing about Jay Paris' curve or the half angle formula, so you just might be right...

 

That's not a boat..... that's a ship!
And a load carrier to boot, utterly different design paradigm. If you wanted to look at ships look to the clippers.

ADDED

The old bluff bowed sailing ships were also quite efficient for their speel length ratios which were usually significantly less than 1.

 

Like any rule of thumb it's always where you start your design and then you will model resistance test a few variants of course.............don't you

A huge amount of work was done refining and observing displacement craft in the first half of the 20th century then two things happened, fuel oil became cheap and engines became powerful in the commercial scene and in the sailing boat arena the racing rules started to drive hullshapes and fashion, now those boats have moved out of the displacment craft altogether and the leading edge testing is to optimize light displacment semi-planing performance with completely different pressure and flow fields.

Many power boat designers and Many displacement sailboat designers show a poor understanding of hull-form related to chosen speed length ratio's, in the commercial field where I have spent most of my design life you see the economics daily of a hull designed for SLR of 1 being pushed at 1.4 with a massive diesel and yet the boat's design is for 1/3 the power and 2/3 the speed.

 

I think that's very much the case - probably more so - in the recreational arena too Mike. Witness the madness that has brought about the 'fast trawler'. Enormous diesels fitted to displacement hulls to satisfy a market segment with absolutely no common sense. The Grand Banks 42 comes to mind as an example......
Of course this madness eventually led to the virtual extinction of the true displacement cruiser (with a few notable exceptions) and we now see trawlers (hate that word) "fitted" with planing hulls... still sold as 'long-range cruisers'! As if by virtue of their otherwise traditional styling they are still sensible passagemakers. Few have a range of more than a couple of hundred miles for heavens sake.....

Phew... rant over.....

 

Since virtually every keelboat sails in "displacement mode" upwind maybe you would like to name who these "many..designers" are, and what their poor understanding is.

I know of quite a few designs of heavy steel yachts built by yards like Jongert and Royal Huisman that sail beautifully and were drawn without any regard to Paris Curves or half angle theories.

 

I would not call that a rant Will! That was a nice, honest and polite way to describe the mental illness of a much celebrated market, Trawlers. Or what Joe sixpack buys as a Trawler!

 

Paul
I said Many and that does not mean all by any stretch of the imagination.

You of all people should not need to look far to see examnples of what I'm talking about just go into any yard and look around the boats on the hard.

If you'd like to open a new thread on this it might be informative.

Paris's observations are not a hard and fast rule either, they are more a rule of thumb and they allow a designer to make allowance for underwater variations to a canoe hull at the initail design stage. Naval architecture goes a lot further than this but you need to start somewhere.

I posted that as a helpful guide and was expecting some discussion I wasn't demanding that everyone use it ok?

 

Funny, I never mentioned ALL either. Nice attempt at distraction.

You said MANY designers show a poor understanding of hull form. To me MANY is more than one.

Seems you make these wild statements, yet cannot even name ONE designer that fits your description.

 

Not funny , what distraction? Just look at your logic.

You know of "quite a few designs" wich are well designed and so do I but that's not what I said is it.

I didn't say ALL designers were amiss therefore it comes as no surprise that you know some that are good.

In the past you have talked of "the slow piggies" that can't get out of their own way, you have already identified those boats.

What's actually threatening to you here I wonder, perhaps the fact that medium heavy dispalcment craft could be good low resistance performers if well deisgned ?

Please try and contribute decently.

 

What you said is Originally Posted by MikeJohns
Many displacement sailboat designers show a poor understanding of hull-form related to chosen speed length ratio's,.

I've simply asked you to name these designers so we all know whose work we should avoid using as data points for our own design work.

Really? What boats or designers have I "identified" as having poor understanding of hull-form (sic) related to chosen speed length ratio's (sic)?

I guess you missed the comment in my post about some of the steel yachts produced by Jongert and Royal Huisman, and my impression of their performance.



Maybe you should broaden your horizons and not reference the same book in so many of your arguments. If you are going to do so, maybe you should pick a better book on the subject. All you are doing is showing us all how little you really know about sailboats.

Obviously I bow to your superior knowledge of welding plates onto workboats.

 

Where do you start when people trying to get a grip or a feel for something like prismatic coefficient.

If it is a technical discussion then you should always look for a decent paper but when it's something as fundamental as Cp it is a challenge to find material which popularizesa a subject sufficiently as to give an idea of the way it relates to hull design. John Teale is a Naval architect and his book is a good introduction for newbies as would Brewers primers or sorensons power boat primers, they are all aimed at the neophyte but Teale goes further.

Many other boat designers have other methods of arriving at a hullform which work but it doesn't make the methods mutually exclusive. Prof naval architecture material even uses different terms which only confuse people looking for a basic underestanding. If you would like more material just ask me for it please.

As for your dismissal of books I presume you refer to authors like Marchaj on seaworthiness?
I would note that in the past I have several times offered to give you papers or a technical discussion on several aspects of naval architecture but you have never taken me up on this.

You have a tendency to try and drive a thread that you find threatening into inane arguments and then start name calling. and I will not play this game. I have yet to understand just what put the bee in your bonnet here.

Instead post something helpfull on Cp or discuss your own design process with respect to this for a displacment boat.

 

Mlke: I cheerfully and respectfully defer to your experience and knowledge. I know that a VLCC is not a "boat". A lubberly transgression on my part, however 600 foot submarines are stubbornly referred to by their yankee crew as Boats.

The big cargo carrier is a floating object that displaces water when it is moving. Those big ones operate at very low S/L ratios of course. Even so they may be moving at 25 to 30 Kn. With impact velocities of that magnitude it seems to me that mass acceleration (of water) is more than a casual matter. I will also guess that those blunt nosed things are miserably wet in a seaway. I'll guess some more and opine that the scantlings of such boxy vessels must be signifigantly more robust because of the additional forward bouyancy. Would you comment on some of that please?

 

Ships are long relative to the wave length of the generated waves and they are relatively deep. Block coefficient is more important here, Ships are also designed for a constant Service speed and are easier to optimize. Consequently there is a lot of effort in the design of a carrier as a pressure source and a sink separated by a long parallel mid body, the relative wave generation at bow and stern are chosen carefully along with bulb design (produces a wave train out of phase with the main hull). Then there’s wake….

Optimal block coefficient of a carrier can be quite high although the waterline entry may not be quite as bluff as you first think from looking at the upper stem and it ties in with the bulb-stem fairing, placement and design.Once the block coefficient gets over 0.8 parabolic bow shapes at the waterline entry become sensible and they are very bluff.

Forces are always high on ships bows and scantlings are set by class requirements with commercial shipping.

If you’re interested there’s a lot of material we could dive into here but probably better in a new thread. There are some very good books on modern ship design.
Here’s another graph which you might find interesting. from Ship Design Rawson & Tupper.

 

In simple terms- unlike the previous discussions!

You guys are mean. In simple terms, the PC is how tapered a boat hull is under water. More volume in the ends = a 'blunter' boat, with a higher percentage of its underwater volume in the ends = a high PC. Less volume under water in the ends means less boat area underwater in the ends, ie. more tapered under water. A PC of 1.0 is not tapered in the ends at all- each slice of the loaf is the same area underwater. A PC of .49 is very fine, ie. very tapered - like an ocean kayak. A high PC (.65 to .70) is good if you are going to push the hull very fast. If you are going to push it slowly, by manpower, a low PC is better. Fast and slow are relative to the waterline length, not fast or slow in some absolute sense.

 

You have given a simple description of prismatic coefficient that is easy to understand. However the additional information offered on manpowered craft is not correct. The reason you see low prismatic coefficient on something like an ocean kayak is driven by stability requirement not drag reduction or ease of pushing.

The minimum drag manpowered single person boat for sustainable effort will have a Cp around 0.65 for a male of average fitness. Racing kayaks approach this but are usually a little lower because going higher requires a lot of skill to stay upright. They are difficult to operate because of the low maximum beam but for a given effort they have considerably higher speed than a hull with a Cp of 0.49.

The reason lower Cp is seen on an ocean kayak has nothing to do with ease of "pushing". It is related to staying upright statically rather than with the aid of a paddle.

Rick W