Electric Ship Propulsion - Motor Characteristics

[johnnyw]

I have a theoretical question that myself and some colleagues are debating which I'm hoping someone can shed some insight on.

Given a ship driven by a propellor via an electric motor and motor drive is operated at rated current and voltage which produces rated ship speed. What is the affect of increasing the hull length (increased drag and ship weight) while utilizing the same propulsion system? For example, can the motor still be operated at it's rated conditions and the ship speed is reduced, or will the motor be torque (current) limited before rated motor speed is reached due to the increase in load (at which point the motor & drive would not be fully loaded) - again resulting in less than rated ship speed. The propellor in our debate is enclosed and not open to the sea water at the edges if that makes difference (i.e., more like a pump in a pipe than a simple fan/open type propellor).

Any opinions?

 

[IRstuff]

Forward motion is dictated by the balance of forces between drag and motive force.

TTFN

 

[johnnyw]

I realize ship speed will be reduced due to the increased load (drag). However, can the motive force remain at the same level, or will it decrease due to motor and drive limitations (i.e., torque limitation).

 

[itsmoked]

Sorry but as ships get longer they go FASTER. You see that funny bulb all ships now sport at the front? That is there purely to make the ship act longer.

Furthermore; A propellor system is really just an unenclosed pump. The end result is thrust. So the Prime mover-generator-motor-propellor don't notice the ship really. Hull dynamics + thrust = speed. What ever hull you have, the thrust is still the same. (within limits of course)

Again there is no effect on the motor or "azipod" because it only cares about the fluid it is in not the ship it's under.

 

[Barry1961]

I would look at it this way.

If the drive is held stationary it will have a certain set of characteristics. These will be determined by the shape of the tube, density, temperature, rpm and shape of propeller. As you allow the drive to move forward the flow of the water through the drive will change. How much it changes and to what effect will be determined by several factors including those above.

You may see a dramatic change with a very small change in speed if that change causes cavitations in the wrong place.

Barry1961

 

[Warpspeed]

Have to concur with itsmoked.

I know of two cases where a cargo hull was split and extended to add an extra hold, and later sea trials actually produced a higher top speed. It may not always happen that way, but don't just assume a heavier ship with greater displacement will always go slower.

 

[itsmoked]

Hull speed in knots = 1.3 x sqrroot(waterline length in feet)

 

[blackwed]

<<You see that funny bulb all ships now sport at the front? That is there purely to make the ship act longer.>>

It makes a nice spot for a sonar array, too.

DB

 

[SteveKW]

I have seen many shows on ships that note how much faster and less power it takes to move the ship as it gets bigger!
And the bulb on the front can add one more thing for a ship...bouyancy for the bow.

 

[itsmoked]

Both true too. Though most tankers and container ships fore go the 2 million sonar options.

 

[aimtobeengineer]

Actually the funny bulb in the front is known as the "bulbous bow", and has the function of reducing friction / drag