Clr--the Computer Verses The Balancing Pin

Just curious- It's raining and cool-- the new boat building roof is not completed so the boat has to remain tarped over. Lazy and tired--worked too hard yesterday dismanteling a CJ7 for it's fiberglass body, (another project). So being a little on the A.D.D. type A personality side i came up with a personal challenge. I carefully scaled out the underwater profile of my build, including half the rudder area, cut and pasted it on stiff boxboard and located the CLR by the pin balance method. My helping designer is away on a 4 week holiday in Europe(Germany) and my challenge is to see if I can match upon his return his computer calculations both in the horizontal and vertical location of the CLR. What chance do i have ?
P.S. for general interest I will post both method locations upon his return .

A yacht is not defined by the vessel but by the care and love of her owner

 

Kinda depends on how he is working out CLR on the computer - ie if it is simply the centre of the projected, upright, and underwater lateral area: then any variation between computer result and 'pin balance' result will probably be due to weight variation in the cardboard used (negligible?), and also any differences between the numerical model on the computer and your cut out shape (also negligible due to your superb lofting and cutting out skills...).

However if he is using some kind of (numerical) force balance to work out the CLR then the difference could be substantial. This is due to A: the assumptions made in the force balance equations, and B: Centre of lift is not usually coincident with centre of area. I think there is some discussion on (edit) locating CLR for different hull forms in Principles of yacht design (larrsson).

The difference between the centre of projected area and the actual CLR depends a lot on the nature of the lifting surfaces of the underwater body, (eg fin/bulb and rudder, long keel, wineglasss sections etc etc). Generally the actual CLR is forward of the geometric centre of area.
Its an interesting problem to try to solve...

 

Hi Viking

Knowing nothing about boat design, I would have thought the centreline of weight balance would not be important in a boat. Rather the centreline of floatation.

If it makes you feel any better, I took a week off work and the weather here is nasty.

 

I was just pondering the challenge to see if the old timers (pin method) were actually within striking distance of the new fan dangled computer programs. Considering of course they mostly worked with full and long keels. However even the best designers say CE/CLR/LEAD is not an exact science and one can expect alot of fine tuning to get it semi perfect. Thus the high cost of developing prototypes for production runs. For the one off- and home builder it's always a bit of alchemy. I did get my vertical location (45 to 50 %) of hull emersion as recommended but the horizontal should be interesting. I am not working with anything high teck, Long modified foil(streatched by a parallel mid section)/cutout/skeg combo, which basically is a modern version of the full keel so I could be close. Should be interesting . Poida --it's the elusive magic of boat design, as pointed out by RThompson above whose controlling factors are certainly beyond my design capabilities but every once and awhile old time basics still shine thru --Geo

P.S. We had several visitors to our B&B (www.anchorsgatebb.com) from Tasmania, over the past couple of years. Always wanted to visit the other side.

 

viking......

This designer says the whole exercise is pointless.....

The boat will never float exactly at DWL, and sailing trim and heel will move the CLR to places you cannot imagine. Locating a mythical point on a 3D surface via 2D thousands of an inch computer calculations is a waste of bits and bytes......

 

TAD -- You quazi lost this old man at the end of pointless. (I was just boored and wanted to get on with the build if the posting responses were in my favour )I certainly agree on the DWL as I'm not going into detail calculations of "moments" that would drive me completely nuts but I am doing so with the heavy items-Engine-fuel tank-water tank and so on and from there it's gut feeling. From years of hauling in tons of fish aboard smaller vessels it's become second nature, that plus a bit a trim ballast .My last two builds I did the same proceedure and added not one pound of trim -Tis the luck of the Irish I say. Ok from there on are you saying forget the computer or forget the balancing pin method or forget it all and wait until Gerhard returns to update the hydrostatics to include the new keel/skeg/rudder addition. (please don't tell me i have to wait) Here's what i got-my CE is 14in aft of the centerline of the mast which is located at DWL stn. #4. If I extend the CE line down it is very close to 13% of the DWL forward of the CLR (lead)as located by balancing the two dimensional underwater profile(very, very accurately scaled) on the head of a pin by a pin head --Tnx. geo.

 

Either determining it by the balance point of a cardboard cut-out or by a computer program is good enough to use for estimating "balance" of the boat when sailing. The rules about lead are emperical rules which generally work well enough, but are not directly based on correct physics.

"CLR" is a mis-nomer when applied to the center of the immersed portion of the side-view profile. That point is generally not the center of lateral resistance in terms of the point through which a side force applied would cause the boat to move laterally without yawing.

 

I think (I'm sure ) I've written this before but I can't recall where.....

Make an accurate scale model of your hull with appendages. Place it in the bay, make sure it's floating at your estimated waterline and trimmed correctly. Then give it a gentle push away from you....it should travel is something close to a straight line if the two sides are close in shape and surface roughness.

Now with some small weights heel your model 10 degrees (as close as you can) and give it another push. It will probably circle one way or the other, rarely will it still travel straight...though it is possible, usually with a very narrow and fine form. Make a note of which way it circles and estimate the rate of turn (diameter of circle). Try it three times and average the results.

Now try the same thing at 20 and 30 degrees heel. Many hulls will just keep turning the same way in tighter circles, some will turn different directions at each of the three heel angles, and some will switch from one direction at 10 to the other direction at 20 and 30.......

Every sailing yacht designer that I know of has a collection of hull forms (sections which can be scaled) for which he (or she) knows the directional characteristics and how much lead (fudge factor) is appropriate. This is the best one can do, figure it out via experience. This is a trade secret and the designers stock in trade....all I have is my experience to sell....so in the books we put "try this...it might work".

As your keel is long and your skeg is large, and your hull is nothing radical in shape, and your rig is completely conventional...you are pretty safe with the head of pin or edge of ruler guess....but that's all it is.....computer calculations notwithstanding.....

Also I can't for the life of me understand what finding the vertical center of the underwater profile might tell one?

 

DCockey & Tad thanks - the project can proceed which is great because glassing, in an unheated section of the shop could get cold if delayed much longer. TAD that vertical location of the CLR I thought was good because it was recommended by Larsson, why? beats me, I thought it was just a fluke it was located at that point vertically in my case. I certainly didn't design it to be so. Somewhere according to my short notes I read he recommends the location of the CLR be approx at 25% of the keels cord horizontally and 45% of the emersion vertically. On second sober thought he might have been referring to high aspect foil keels and not low aspect long keels. I'm just happy to get a rough location to get the % lead between the CE and CLR, hopefully to create a little weatherhelm in moderate pre-reefing conditionds. If not it's no big deal for me to re locate the mast one way or the other for a little fine tuning(prototype developement in a big tank)One of the benifits of owning a fairley well equipped (now hobby) shop. Payback time Thanks again---Geo.

 

Viking, I imagine Tasmania has many similarities to Nova Scotia – a bit wild, wet, and woolly (and maybe 20 years back in time?). Nah, I joke –Tasmania is very modern; there is wireless radio.

With respect to what Tad was saying about building an understanding of the heeled ‘balance’ of your designs – I had never thought about doing it that way, such an obvious and practical method (thanks!)… Now I wonder –someone must have done some studies on balance using with a systematic series (like the Delft series)? - there are whole sets of ready-made systematically varied models just waiting to be played with. Sounds like a good student project…

Finding the vertical centre of the profile isn’t much help, but I think the vertical centre of lift is (if one is trying to resolve the force balance of the sailing yacht). That is to say a heeling force couple is created by the centre of lift for the sails and the opposing centre of lift for the underwater body, thus the vertical distance between them is important.

Viking - a well equipped shop of your own? mmm one day... I lobbied for one but my dear wife insists the kitchen lounge and dining room are, and always will be, 'family' rooms... (its like she is suggesting boatbuilding is not a family sport)

(edit) yes I think you are right with respect to Larrsson talking about high aspect ratio keels. The book does go on to discuss how it changes with different underwater arrangements (although I dont have the book with me now). I think on long keels/wineglass sections the CLR moves forward and up, as the hull itself acts as a lifting surface. 25% of chord aft of the leading edge of a foil is in the ballpark for centre of lift.

 

Thanks , I am going to use this one on my current model, should be interesting

CLR (not vertical centre of underwater profile) would be important for calculating the location of the sails.

I have seen a sailboat with fore and aft sails pulled in tight, on a beam reach, that just drifted sideways slowly.

This is a handy way to 'stop' in the water, if you are wanting a stable observation platform, or to attend to something without having to put the anchor down.

Not all boats will do this , but it used to be a 'desirable feature' in some designers opinion.

 

Finding the center of lift of a foil is easy and follows a simple rule of thumb. Finding the center of lift of the canoe body cannot be done from a profile view. In fact it can only be done reliably (in my experience) in a test tank. I gather some claim it can be be with new CFD programs but these computations are always confirmed with towed models in real water.

Heeling arm length is useful for comparing Delibaugh or Wind Heeling angles. But this is a comparison tool, useful only for comparing like with like, it is inexact at best.....Just using the 40% of draft estimate is perfectly adequate, calculating areas to 3 decimal points and believing that's somehow better is fooling yourself. Your heeling estimate will change a lot more with the height of VCG than with minute changes in the heeling arm length.

Only for a foil, not the canoe body. Depending on immersed hull shape the center of of the hull and keel might be between station 2 and 3 or 3.5.....the distance forward depends on how sharp and deep the forefoot is......I think L&E show a forefoot cut away to nothing......which is not your typical cruising boat.

 

Wow! from balancing a piece of cardboard on the head of a pin to some of this engineering is like going from a soap box racer to the space shuttle. Having said that there are tid bits between the lines that help in some understanding of the great science and engineering involved in todays modern sailing vessels. To be sure thats not my goal just want to get this conversion project into the water with reasonable handling characteristics, once there my handling skills kick in and other than building the craft it's where i feel most at home. Good stuff keep the post going as it seems others with more qualifications than I are learning also---Geo.

A yacht is not defined by the vessel but by the care and love of her owner--

 

Did a little more study on this CE--CLR topic and was a little confused by -If a vessels CE leads it's CLR it will have Lee helm and if the CLR leads the CE it will have weather helm. I re read several time and at first thought it was a misprint as I was under the impression the design criteria was to set up the CE to lead the CLR by some percentage of the DWL to hopefully create a little weather helm not Lee helm. It seems the reference books i have read omitted a little piece of info--I.E. as designed under so called static conditions a sailing craft is set up with the CE leading the CLR to in fact create Lee Helm. The reason being that under dynamic conditions the CLR moves forward hopefully enough to create sufficient weather helm to counteract the designed Lee helm plus a little extra, resulting in the desired amount of weather helm for good handling. I was also surprised to read that famous designers struggle with this and have made some big bo bo's on famous production boats that needed majour modifications to overcome the in accuracies of their computer programs.( I have names, makes and their related CR/CLR Weather/Lee helm problems and modifications but thats not the point I'm trying to make. The point is the Balancing pin is no more of a crap shoot than some of the computer calculations relating to these elusive polar opposed dancing twins. TAD I now understand the model testing needed as detailed in your post and even at that the model better be accurate and under well controled testing conditions, As for us poor smucks that can't afford such, I'm afraid it's old school trial and error (move the mast a little)(add or take off a bow sprit)(rake the mast) (pour TWO drinks for King Neptune)

A yacht is not defined by the vessel but by the care and love of her owner--

 

Sailboats are not two dimensional. An obvious statement but that fact means the physics are not properly described by just the center of the sail area with the sails on the centerplane, usually called the CE, and the center of of the immersed side view area, usually called the CLR. When the boat is sailing the sails are not on the centerplane, and have swung forward. The forces generated by the sails in general don't act through the "CE", and the hydrodynamic forces don't act throught the "CLR". This topic has been well discussed in previous threads.