Hi Alan,
generally as starting point the draft could be fixed around 50% of LWL.
It depend later where the LCB will fall also in terms of percentage.
Is not only the Rocker curve that can produce a change on LCB position. You can work also on the summit of the curve of the Water Plan.
An example with the drawing :
This is a paramount parameter when designing. The CF may be largely affected by that choice.
I have not checked the actual design of this discussion to see if the CF is within 1% max from 0° to 25° of tilt.
In order to appreciate the consequence of Water Plan outline shift , here below are the results.
The shift toward the bow of 1.1cm induced a shift of the LCB of 1% to the right.
This demonstrate that a small modification of a curve can produce important effects.
The Blue line is the original design and the red is the modified one. The significant changes are visible with the Curve of Areas.
Also the Prismatic Coeficient is increased a bit.
The water Plan is calculated for 1520cm². This value suggest that a floating variation o 1mm is equivalent to 152g or cm3.
This drawing refers to the Center of Flottation - CF
The CF is verified at 0° tilt and usually at 25° tilt and always in front.
The results shall not differs more then 1% in order to consider as a balanced hull .
In this case, the difference is only 0.5%, therefore the hull is a well balanced design.
I do not need to go further, but remain open to remarks / suggestions…
ClaudioD
PS: I went on Ebay and found some Corel Draw 5 software available !
Claudio, earlier using cut-out of the Curve of Area (COA) and using pendulum method you showed us how you find the LCB, how is the CF found ? for both level & tilted LWL.
The tracing is carried out for the tilted condition since the 0° is already the water plan.
I fix, for commodity, the needle to the crossing of shadow 7 as in the drawing detail.
Cheers
ClaudioD
I’m becoming predictable in my old age it’s very interesting subject for those interested in doing their own boat design, so I"ll keep ball rolling …here’s another
So we have:
LCB longitudinal centre of buoyancy, the fixed point where maximum underwater body volume/water displacement takes place.
LCF is the geometric longitudinal centre of flotation of the water plan area. It is the axis of rotation of the hull that changes by weighting/movement.
What is importance of these two points to each other ? and should they be in same place or +/- to each other to have stable sailing boat ?
Hi Claudio
Back in Post#8 you show a graph plotting LCB against Fn. I’ve never seen that before and I’ve read everything I can. Can you link me with any more info on that? How it was developed, that sort of thing?
Hi Don,
reported in many books about Yacht Design, one off is just on my desk is : “Principle of Yacht Design” of Lars Larsson & Rolf Elatson.
Gutelle is also presenting this subject, like Lodigiani in Italy, etc.
In 1995 a lot of testing were carried out at the University of Delft in Holland.
This article about NewZealand/Delft testing in French with the same graph: http://jels.americacup.pagesperso-orange.fr/delft.htm
ClaudioD
Hi Alan,
a couple of years ago it happen to read an article written by Eric Spongberg : http://www.sponbergyachtdesign.com/Articles.htm and many years before with the Model Yacht Design book of Lewis.
In summary, the sailing stability of a model to maintain the route depends on the variation of the CF with heeling.
Practically when the boat heels, the form of the hull change. It is up to the designer to develop a hull that is changing form as less as possible as such to offer the minimum drag when heeling while keeping the route.
On real boat with "unbalanced design’ it is somewhat easy for the skipper sensitivity to play with the rudder and negotiate the waves.
RC models cannot.
I remember once crossing the Channel how much was difficult to maintain the route with a dual keel boat in a formed sea after a gale. I remember similar crossing with my Rival38, where under the same conditions; the boat required almost no rudder modifications as it was so stable.
When the Radio Control was not yet in use and models were sailing with the Vane, was most important the Hull Balance, so under the same culture, I try to get always a good balance boat, simply because produce less drag , etc., etc.
Ciao
ClaudioD
I’ve just downloaded on internet a version of corel drawX5 (full version cracked!) and before to share on the forum the way to get it and how to install it (so simple) I would like to check if it works with one of your drawing file.
Then could you please load on a post or send it by email a corel draw file of one of your plan?
Hi
the CDR files cannot be loaded directly therefore I converted into a Zip file.
Please let me know if OK, otherwise send your Email via PM.
Cheers
ClaudioD
Hi all, I succesfully open the Claudio’s files in corel draw X5 dowloaded with a torrent application, but it seems we don’t have all the elements corel draw X5 is in fact the version 12 or 13 of corel draw (Claudio has the 5)
step by step procedure to get a full life free version of corel draw X5
This link is currently blocked. The software, regardless of how old, is still copyrighted. It is not legal to download/use “cracked” software because of copyrights. If you like the software, respect the copyright and purchase it.
Just to reinforce what Dick said, buying a used copy of Corel Draw off eBay or some other source is legal; downloading a cracked version is a violation of U.S. law. Linking to pirated software places the existence of this site in jeopardy, which is why we have taken down the link. Please don’t do it.
Claudio,
Quick question if you have a moment. I have some plans for a full size boat that I was thinking of scaling down for R/C, but I wondered if you have run across this in your research. When scaling, is the displacement a linear function, or is it cubed root. For example a boat with 10000 lb displacement, when scaling to 1/5th scale, would that be 2000 lb on the model or would it be 21.5 lbs. I like digging into the math of a thing before I jump all over it, plus I need to know what I’m looking at for transporting a model to the water. Thanks for any help you can give.
it’s very interesting subject for those interested in doing their own boat design, so I"ll keep ball rolling …