Author Topic: Propeller Rotation & Configuration  (Read 3446 times)

0 Members and 1 Guest are viewing this topic.

The Mate

  • Newbie
  • *
  • Posts: 3
Propeller Rotation & Configuration
« on: June 01, 2015, 15:00:08 »
A few days ago I was having a discussion with a fellow member of my local boat club about what’s the best configuration of prop rotation for a twin-screw scale model tug.

I have a lot of ship handling experience from driving fishing boats, twin screw inshore ferry and my day job as Chief Mate of a 102,000 ton LNG Carrier and I wanted to share some of my knowledge to help clear up some of the issues that scale modellers have when deciding what’s the best propeller/motor rotation configuration for your model tug.

I recently took delivery of a new model that hadn’t been on the water and it was down to me to set her up. She’s a one 1/32 scale model tug built in Canada for the great lakes towing company. “Handy One”

I fitted her out with two MFA 919 series 6:1 reduction gearboxes.

Propeller configuration is as follows, 60mm Right hand prop on the starboard motor and a 60mm left hand prop on the port motor. Both motors outward turning.

Now when ship/boat handling the most important thing is knowing the characteristics of your propeller and the effect of transverse thrust and the effect it will have on your boat.

Before I begin I want to outline the setup on outward turning and inward turning propellers.

Outward turning propellers: right hand propeller to starboard and left hand to port.
Inward turning propellers: left hand propeller to starboard and right hand to port

If you imagine a right-handed propeller (outward turning) viewed from astern to drive the boat forward, it must rotate in a clockwise direction.  The advantage of knowing the direction of rotation of a propeller is important when astern power is applied. When a right handed propeller goes astern the propeller is rotated anticlockwise and the helical discharge i.e. the wash from the propeller blade is directed to the starboard side of the boat, this in turn swings the stern to port and pushes the bow round to starboard.

Now if we imagine a left hand propeller  (inward turning) it needs to turn anticlockwise to push the boat forward when viewed from astern. When astern power is applied the propeller turns clockwise to move the boat astern, the helical discharge is directed to the port side of the boat pushing the stern to starboard and the bow to port.

Ok with me so far? Right so what does this have to do with propeller rotation right? As I mentioned I was having a discussion with a mate of mine as to what was the best direction to have your motors rotate.

The answer is outward turning!
The reason I say out ward turning over inward turning is because if you want to turn the boat to starboard in her own length, push the port motor ahead and the starboard motor astern, very easy, and round she swings. Give some starboard helm and she’ll turn even quicker. The advantage of having outward turning props over inward turning is that when going astern on a right hand propeller inward turning it would mean the wash would be directed inboard or the centreline. this can cross over onto the surface of the opposite side rudder and completely mess up the manoeuvre
I know for a fact that there are very few vessels with inward turning propellers used on tugs and anchor handlers because in all essence there an absolute pig to handle.

Another advantage of outward turning props is when coming into berth. Imagine approaching at 45 degrees to the berth, say berthing starboard side alongside, slowly drag your port proper astern and you’ll see that the stern will tuck in nicely and allow you to berth bodily alongside in a controlled manner.


I want to quote a passage from I book that I used when I was studying ship handling in college.

“The Ship Handlers Guide” by Capt. R.W Rowe, FNI

Outward Turning Propellers
In relation to each other when going ahead, the blades of these propellers are outward turning in the upper half of their circle of rotation, when viewed from astern. (See fig 56a) If however the starboard propeller is put astern, to assist for example in turning the ship to starboard, it will now be rotating in the opposite direction. (See fig 56b) This propeller is therefore now behaving in exactly the same way as a right-handed propeller on a single screw ship and part of the helical discharge will be deflected up and onto the starboard quarter. The resultant transverse thrust will cant the bow to starboard, not only assisting the turn, but also working in conjunction with both rudders and propeller torque.

Inward Turning Propellers.
These propellers when viewed from astern are now inward turning in the upper half of their circle of rotation. (See fig 57a) if once again the ship is turning to starboard and the starboard propeller is put astern to assist, it will be rotating in the opposite direction (see fig 57b). This propeller is acting in the same way as a left handed propeller on a single screw ship so, whilst going stern, part of the helical discharge will be deflected up towards the port quarter! The resultant transverse thrust will attempt to cant the bow to port, not only in the opposite direction to the desired turn, but also working against the rudders and propeller torque. The astern wash from the starboard propeller may also seriously deflect the smooth flow of water from the port propeller onto its own rudder.

The effect of inward turning propellers upon a ship can be extremely severe and render it totally unmanageable from a ship handling point of view. In the worst case it has been found necessary, when manoeuvring, to stop one engine completely and work the vessel in the same manner as a single screw ship.

I hope this information is useful to anyone mystified by the direction in which propellers rotate.if any more clarification is needed on this topic please drop me a line.

Thanks for looking

Martin


Never hit the beach with both anchors home!

des

  • Sr. Member
  • ****
  • Posts: 422
Re: Propeller Rotation & Configuration
« Reply #1 on: June 01, 2015, 15:29:07 »
Hi Martin

I worked in a ship building yard in my youth.  We built a number of twin-screw tugs and anchor handlers, and I don't remember any of them swinging inward turning screws.

We did, however, build a number of twin-screw tugs with multiple rudders to assist increase manoeuvrability - this was at a time when Voith-Schneider and ASD drives were still in their infancy.

Des.
« Last Edit: June 02, 2015, 01:56:53 by des »