Does anyone know of an accepted method for estimating a towline’s cantenary without all the advanced geomentry and physics of the hyperbolic cosine?
I think that the US Navy Towing Manual has something in it…
This is the double propeller version:
We have been haveing this dis oboard and the easyest way we can up with is you know how much you have out use the smallest scale on radar and figure out the feet then subtract the dif and that’s haw much is under water but there are way to many variables that we see speed weather weight balard pull draft cable size any many others
thank you for the response guys…I am working my way thru the army manual now…I found a homemade drawing and algebraic formula in my “stuff” but the formula doesnot work out…was hoping for a program to download that you plugged in a few variables and the solution mysteriously appeared…might come in handy for those who tow coastwise…with the price of wire and all the fish havens these days the old method of waiting until the wire starts jumping off the caprail won’t do.
Experience it the best way, I have tried all of the formulas for years. Nothing beats flat out experience. After staring out the window, jumping up on the chart table to see and generally being scared for a few years you begin to learn what is too short. Then it is just a matter of more time to figure out when it is to long. Sorry it sounds like a lame answer but it is the best way I have found.
The formula does work…well…kinda works. The formula is fine regarding a “Paper Ship on a Paper Ocean” or to “define” a vessels towing arraingement by a naval architect or a lawyer. This is why it is useless in actual pratice. It leaves out the following information:
- Size and HP of the towing vessel
- Size and “Block coificient” (shape) of the towed vessel
- Speed you wish to tow
- Depth of water in which you are towing on your route
- Towing gear (bridles) on the towed vessel ( single point, bridles, chain, wire, pigtail chain (and length), shockline (and length) etc.
- And last but not least, the weather you are towing in
Why not do what everyone else does… Put a few layers out, bring the boat up to speed. If the wire snaps out of the water, either slow down or put more wire out. Continue this process until you “FEEL” you are in good shape. Adjust your wire length (look at the wire scope going from the Tug into the water and the scope of the wire coming out of the water up to the barge bridles. The more “vertical” the scope, the deeper the cantanary. If, when you pull the wire up, it is shiny, then you know you were dragging it on the bottom. If you feel the barge “surging” then you need more out. When you do it for awhile, you will know what to do…