# Force Exerted By Towed Object On Tug At Different Speed

Discussion in 'Hydrodynamics and Aerodynamics' started by rspawarr, Oct 10, 2013.

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### rspawarrNew Member

i need to know when tug start pulling any stationary ship, it will have certain amount of force on its tow hook, but when ship start moving that means when speed increase force coming on tow hook will increase or reduce.
please note that i am not asking static bollard pull of the tug.
for eg. we got 50 t bp tug when i started pulling 150 m tanker, tow hook will experience certain amount of force say 10T when towage speed increase then continous force exerted at tow hook will be reduced or increaes? why? how?
please explain in detail.

some people says ship will get some inertia so force will decrease.
some says with speed resistance of towed vessel increases hence force will increase.

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### Mr EfficiencySenior Member

Probably depends on what speed your tug is optimized for.

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### philSweetSenior Member

Welcome to the forum rspawarr.

The answer depends on several factors. The most important being the thrust that the tug is generating at any given speed. The towline force will equal the tug's thrust minus the total drag of the tug minus the rate of momentum change of the tug.

If it is a diesel tug and the throttle is left unchanged (constant rpm's), the thrust will decrease as speed increases. The total drag of the tug will increase as the tug's speed increases. The Tug's rate of change in momentum is determined by the acceleration of the tug and it's mass.

Ftow(V, rpm) = Fthrust(V,rpm) - Dtug(V) - Mtug*A(V,rpm).

read (V,rpm) as "as a function of velocity and rpm". F is thrust, M is mass, A is acceleration.

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### rxcompositeSenior Member

Standard Naval Architecture formula calculates the EHP or the estimated horsepower to propel the boat. This called the towrope horsepower, or the raw power required, without mechanical losses and reduction of power due to propeller efficiency. In other words, it is a bare horsepower needed if you would pull a dead boat/ship. Naturally, if you want to pull it faster, you need to increase your power.

Acceleration is a different thing. It is the time measured to tow a boat up to a speed desired or up to the point where the pulling ship has reached its maximum horsepower rating that it cannot go faster.

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a prop cant produce the same force when moving as stationary due to current inflow
so pure and simple..
bollard pull in still water is the max you will ever see
edit ( via the engines)

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### CDKretired engineer

The maximum force on the tow hook depends on the mass of the towing vehicle and the distance covered before the cable is taut. All kinetic energy (speed times mass) is absorbed at that moment and will cause forces far exceeding bollard pull.

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too true
those clowns that towed (tried to)the Kulluk up in Alaska needed some common sense like that

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### Murat124Junior Member

Physically, it will be necessary to obtain necessary tension on towage line, to get vessel to move ( this tension =vessel's hull resistance + Tugboat's resistance = Bollard Pull)

it will be necessary to tighten towage line up to total resistance of the towage system ( tug+ ship+weather+other effects) to obtain required tension on towage line
Tugboat makes a work for tensioning towage line before the move, A portion of this work will become a kinetic energy of towage system and tension will be reduced on towage line. Result of this system gains a fixed velocity ( theorotically acceleration = 0 like limit speed of a rain drop) if you want to increase the systems velocity up you need to give a little bit force to the system or in other words increase the tension of towage line up.

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### Murat124Junior Member

It is very strange that standart Naval Architecture comments about this question thougt as a different other thing from Newton Mechanics
There is no acceleration concept in Naval Architecture.
Why can not Naval Architects give comments with actual Physic Rules to any problem??

10. ### El_GueroPrevious Member

First, there is acceleration in Naval Engineering. Engineers may discuss it differently than you are used to.

Second, if they gave simple answers, there would not be any need for Naval Architects .....

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If naval architects understood acceleration boats would have been built with X bows since the Ark

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### Murat124Junior Member

I will assign numbers to my reply as you did it
First I am a Naval Architect and Marine Engineer too. I tried to emphasize that Naval Arhcitecture education becomes formal increasingly, Students do not learn how to think
about a problem, and conditioned to give some comments about problems with some solid arguments.

Second Naval Architecture is closely related with towage- towage calculations and towage simulations.

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### jonrSenior Member

> some people says ship will get some inertia so force will decrease.

As far as I know, the static friction involved in moving a boat in water is approximately zero. So ignoring the slack line case, force will only increase as the speed of the towed ship increases.

Pulling it off a reef would be different; once it got moving, force would decrease.

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