# help with calculating tensile strength for tow line

Discussion in 'Hydrodynamics and Aerodynamics' started by TwistedSister, Jun 6, 2017.

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### TwistedSisterNew Member

anyone have a generic calculation for determining how much force is generated pulling an object through the water -

looking to determine what tensile strength line I would need to pull a 1000lb
boat through the water at 20 knots.

any quick general formulas with just those variables - I know there are more
factors that actually effect it one way or another, just trying to get a ball park number.

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### gonzoSenior Member

It will depend on the shape of the hull.

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### TwistedSisterNew Member

fiberglass deep V type planing hull. anyway to determine approx value without getting too technical?

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### gonzoSenior Member

For example, a SeaRay 160 open bow weighs 1500 lbs empty. Are you towing something around 14 feet or so?

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### TANSLSenior Member

Naturally, and the speed of the towed object with respect to water, and the specific weight of the water, and the area of the maximum cross-section, and the hydrodynamic coefficient of the hull and many other things.
F = k * rho * A * V ^ 2 + Friction resistance + Wave resistance + some other things that I do not remember....

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### TwistedSisterNew Member

I had a feeling it was complicated - was hoping there could be some generalizations to use and not be so specific. It is actually 13' RIB, weight is 950lbs. deep v hull planning hull design, 4.5 feet wide. salt water - looking to get the right size tow line. Thought this would be a little easier but can certainly understand all of the variables involved. I sincerely appreciate you guys trying.

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### gonzoSenior Member

Power= Force x Velocity. If, for example, the RIB goes 20MPH with 30 HP, you can calculate Force by estimating about 75% efficiency.
30 HP = 22kW 20 MPH = 8.9 m/s Then [(22kw)(0.750)]/8.9 = 1.9kN or about 416 lbs. Size the rope for twice the force for safety. You can use the formula by figuring out how much power you are using at 20 MPH. 3/8" braided nylon working load is about 940 lbs

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### TwistedSisterNew Member

I understand - surprised it isn't more than that - even doubled... Sincerely appreciate the help.

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### TANSLSenior Member

You do what you want but I would analyze in detail the formulas and efficiencies that can be applied in these calculations.
Almost with similar scientific rigor I could say that it will be enough to take a fat nylon rope and drag the RIB, without further calculations.

Last edited: Jun 6, 2017

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### BarrySenior Member

As rope is relatively cheap and the difference between say 1/2 inch and 3/8 inch is small it does not make a lot of sense to do a lot of drag calculations to determine a size of rope
In perfect conditions, ie slow acceleration, smooth water, the actual load in the tow line should not exceed the weight of the boat so you could get by with a small diameter rope to suit this
value.
But,
You need to consider a few other things

1) How much does the type of knot reduce the strength of the rope
2) If the rope is not ultravioletly protected, how will the rope degrade in the sun, ie poplyprops etc
3) Does the rope float, so you do not have it get caught in the prop when you are sitting still
4) How easy is it to untie knots, the thinner the line, with max loading CAN make untying knots tougher
5) You need to attach a factor of safety, ie certainly under calm conditions, a thin rope will pull the load, but if the boat surfs in the wake, there may be situations where the boat
will be accelerated if it has "made up some distance" tow boat and then come taut suddenly. The acceleration can triple the load in the line.
6) Consider a line that will stretch a bit so the attachment point in the towed boat and the towing boat loading can be minimized due to shock loading
7) Are there any other times that you might need to use the rope for other uses, tying a long rear anchor etc

We towed a 1000 pound RIB for several years between 12 and 24 knots and used a 1/2 inch line, breaking strength was about 5500 pounds, some stretch, floated. In order to reduce stress in the towing cleats we had a longish bridle that we attached to both rear cleats to make a V looking back, it was maybe 15 feet long ( 30 feet overall), This was one piece with loops backbraided so we would push the loop through the center of the rear cleats then throw the loop over the cleat. Then another 40 feet of line with a loop that rode on this bridal and the towed boat bridle
The Rib had two towing cleats so it got the same treatment, a 10 foot long bridle with stainless snap loops braided in. Having two hook up points on the RIB lessened the hunting that the RIB would do as compared to a single tow point.

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