Astern Maneuvering for Single screw boat.

Dear All
I have completed the design of a boat of 9m length. It is fitted with a single screw propeller which is left rotating.

The problem faced is that while the boat moves towards starboard, it moves in a straight line for about 15-20 m and turns to STBD. While Port side it turns properly.

Propeller Parameters is as follows:-
Skew- 30 Deg.
Rake-0
Number of blades -4
Dia- 812 mm
Pitch - 673.1
Expanded area ratio- 0.6

Vessel Particulars are as follows:-
Length OA :- 9 MTR
Breadth :- 3.5 mtr
Depth :- 1.8mtr
Draft :- 0.945 mtr
Maximum draft displacement :-11.102t
Engine :- 220 bhp @ 2600 RPM
VCG :-1.272m

The problem might be because of the Propeller walk, but I would like to increase the response of the vessel towards the starboard side. The master on sea trial refused to take over the vessel because of this issue.

Awaiting a solution from experts in the forum.

 

You describe typical behavior of single screw boats. A bow thruster would help, or a more experienced master.

 

Hello Sarath,
Could you possibly post a couple of photos of the boat (ideally while underway and when she is out of the water) please?
I think you are saying that she will turn to port in an acceptable fashion, but when you try to turn to starboard, she carries on in a straight line for a while before turning?
Does she do this at slow revs as well as higher revs?

 

The relationship between the hull, prop and rudder is the issue, most likely, but you say nothing about the rudder. What is its area, dimensions, distance from prop, balance, section, profile, proximity to hull? Outflow from a prop can't be symmetrical unless using a contraprop. Therefore interaction between flow and the rudder are also not symmetrical.

This is a classic problem in aircraft design, where the Messerschmitt 109 could only be flown in a straight line with a heavy boot on one rudder pedal. It's the first I've heard of this in a boat though.

Is the rudder axis offset to one side to allow prop shaft removal without also removing the rudder? If so, it might be an unfortunate hydrodynamic interaction between prop outflow and the adjacent hull where the flow is also offset in the other direction, away from the rudder axis. That would give excellent helm one way, and almost nothing the other way. Which you describe.

I don't know what your resources are. Can you switch to an opposite hand prop, test it in "reverse", see if the helm deficiency also changes hands.

Rudder axis can't be changed now. If there's space to enlarge the rudder, can you increase area forward of the axis, increasing balance ratio and moving more rudder into prop stream?

John

 

It's called propwalk.
Learn to live with it.
That is, use it to your advantage when backing.

 

60' is a long way to go before any starboard helm response. I'm not remotely as experienced as you are, Bluebell. I think I'd prang my boat.

 

I agree DogCavalry, but there is a lot of information missing from the OP.
As a hired driver, one just has to drive what they got under the circumstances of the day.
Also, 60' is his or her perception and report of what's happening.
Video of the experience would be most helpful.

 

Yes. Perception and reality may be different. But if the delay really was 60', I'd let you drive. I'm just not up to that at my level.

 

91SARATH, that is a pretty big prop with a fair amount of skew and not much submergence for a 9m vessel. If this a backing only issue, I would expect a flow over the rudder issue. As this is backing...are we turning the head to stbd (stbd rudder), or the stern (port rudder)?

 

Look at the thread title...

 

D'oh! (Homer slaps forehead)

 

And here's me dragging out my copy of CA Marchaj's Aerohydrodynamics of Sail.

 

Yes, the problem persists in the starboard turning while going astern. Astern turning to the portside is satisfactory. Moreover, while moving ahead the vessel response to both port and starboard is satisfactory.

The only problem is towards astern stbd turning.

 

Hi DogCavalry,
Thanks for the response!!

Rudder Properties are as follows.

Section shape- Single plate rudder.
Balance ratio - 0.3
please find the attached pic for the rudder.

Cb (vessel full load) - 0.47 and 0.38 at lightship.
Rudder Porp clearance - 280 mm, from the fwd end of rudder to prop leading edge
Prop Hull clearance - 154 mm from the prop tip to the hull.

Rudder and propeller are in same line and there is no offset between them.

 

The issue is only while the vessel goes in astern condition. That too in starboard turn only. Turning in fwd direction to both sides are good enough.
Please find the attached image of rudder and propeller.

I too think the propeller is big and have a bigger amount of skew. Does this aggravate the issue of prop walk???