Forums » Ask a Question

Use this forum to post your questions to our experts – you need to be logged on to do it (you can register here ), then scroll down to the bottom of this page and click the blue Post Thread words

If you can help answer the question, feel free to post a reply – you need to be logged on to do it (you can register here ), then hit the reply button on the thread.

 

To go back to the experts page click here>>

AuthorMessage
sleddy123
sleddy123

Posted: Mon Apr 20, 2015 5:31pm

Post Subject: Reversing my boat

hi there - I hope someone can help me with my latest narrow boat neurosis. When I bought my boat some four years ago I seem to remember being told that when reversing the flow of water or thrust should emerge on the port side of the boat - when I look at this now this appears to emerge on both sides. Is this a gearbox issue or propeller matter or am I worrying unnecessarily ? I should add that the boat does reverse OK. If anyone has an opinion on this I'd be really grateful to hear it. Thanking you in anticipation. Adrian

Tony-B
Tony-B

Posted: Mon Apr 20, 2015 7:31pm

Post Subject: Reversing my boat

What an almost load of old whatsits you were told. Take a look at the shape of the inside of the swim either side of the stern gland. You will see the sides of the swim come together on a narrow upright flat section where the stern tube goes through the hull to form almost a point. This is the mirror image of the outside of the swim with the propeller a few inches behind that flat upright section. On some boats the two sides of the swim might actually form a point. Now imagine the prop in reverse directing a column of water back against that point. The water has no option that to split evenly along either side. It will flow along the side of the swim, under the flat swim (Uxter) plate and become visible in the area where the swim meets the hull side proper. Apart from one thing you simply must get an almost even split of water to each side of the boat. The above assumes that you are in mid channel but if you are on the side of the channel the canal bed may obstruct the water flow on the bank side so it looks as if more water is flowing from the other side. However if you are reversing the water flow will probably be stronger than the effect of the rudder so it is likely that the prop thrust will force the swim away from the bank so water can flow from both sides. The reason I said "almost" is because the propeller will impart a rotary motion to the water column and this may tend to direct a little extra flow to one side. However I have not noticed this to any significant extent. What I think your informant was confused over is what is known as prop walk. The bottom of the propeller is in denser water than the blades at the top and this means that the rotating prop will tend to force the back of the boat to one side. In ahead where the column of water is flowing over the rudder the rudder forces will all but hide this effect but in reverse the back of the boat will tend to move one way,especially if you make a sudden and significant increase of revs when moving slowly. Which way the stern tend to move in reverse depends on your prop rotation and that, in turn, depends upon your engine and gearbox but if you look at the rotation of the shaft in reverse you can soon work out which way the lower blade will push the back of the boat. Prop walk is nice in theory and in deep water but the effect of the side of the channel and the flow around the swim tends to mask it more often than not. I hope this helps explain what to expect.

sleddy123
sleddy123

Posted: Mon Apr 20, 2015 11:45pm

Post Subject: Reversing my boat

Tony - thanks for taking the time and trouble to post such a lucid and interesting response to my query. Cheers, Adrian

Most Read

Newsletter Sign Up

Canal Boat monthly newsletter
Sign up to receive our regular email newsletter

Our Privacy Policy

Like us on Facebook



Follow us on Twitter

Cache: Disabled for this object  Total Queries: 43.  Total Objects: 162.  Total Unserialized: 3. Total Runtime: 0.55 seconds.