How To Calculate Tension Of The Towline

[SeaManiac]

Dear boat enthusiast,
great to be this forum member, i've been working in tug and barge operator for almost three years..
this year was the worst one...since one of our tug had unable to control her barge (towed) caused her towline was parted.
i wonder you guys could help me to find out the simple way to estimate the tendion works on the towline, so i could briefly explain the Big boss to settle the matter...
tug spec.:
LOA = 30 mtr
beam = 8 mtr
draft = 4 mtr (max)
speed = max 10 knots (free running)
BHP = 3200

barge spec:
length = 115 fts
beam = 26 mtr
draft = 1.0 mtr (light)
no prop. with spoons type bow.

appreciate if you guys cud help me to work with this matter...big cheer

[Kay9]

SWL on the tow line should be at least 1.5 times the gross wieght of the towed vessel. Scope of the towline should be at least 6 times the length of the towed vessel. I have no idea how you would caculate the actual strain on the towline without a device on your tow winch showing the bollard pull. But if you parted a towline you can probably bet your towline SWL had been severly reduced due to age or your towline's SWL was WAY to small to start with. If you are in heavy seas and you see the catnary of your towline comming taught then you need to pay out more scope, to reduce the stress on the towline.

Some info that would be helpfull here would be the size of your towline and its make, (IE Manilla, nylon etc) and the wieght of the barge you are towing. Also the conditions you found yourself when your towline parted.

[Rick Willoughby]

If you give some more detail like the prop diameter and the maximum prop rpm I can estimate the bollard pull from your 3200HP engine. I expect it would be up around 25 to 30T.

The bollard pull can be exceeded due to differential inertia between the vessels. Like Kay9 implied, towing in a seaway can cause shock loading if the tow line is not of adequate length and weight to absorb and cushion the shock.

Rick W.

[Kay9]

Rick I would love the formula for that calc. If you dont mind sir. Thanks

[Rick Willoughby]

Originally Posted by Kay9

Rick I would love the formula for that calc. If you dont mind sir. Thanks

I have my own software for it but JavaProp produces very similar results:
http://colaco.freeshell.org/mhepperl.../jp_applet.htm
It has limitation on the foil selection. The more information you have about the prop then the closer you can estimate. If I knew the maximum rpm then I can cross check the speed with the lightly loaded case. The Re# of the foils are a bit low for most of the foils offered but the coeeficients do not change much once Re# gets above 300,000.

The theoretical thrust limit is:
F = rho^(1/3) * pi^(1/3 * [1.5 * Power * Prop Diameter) ^ (2/3)
This provides a first order check. Of course you need to use compatible units (simple with MKS). This has no allowance for the viscous drag of the water on the blades but this is not a particularly high number for a well designed tug. Tip losses can be reduced using a Kort nozzle.

The figure I gave was based on a single 3m prop. If it has two engines and two 3m props then it could be more like 35T. The more prop area you have, the higher the static thrust for your power input.

Rick W.

[Kay9]

Thank you Rick.