I totally agree with your explanation of prop-walk, John. 'Paddlewheel' and differential water density explanations are implausible because the moments are so small. The accuracy of the prop wash on the hull theory is reinforced, in my view, by the way that the degree of prop-walk varies with the underwater profile of the boat. As a generality, long-keeled boats with keel-hung rudders have a very large amount of underwater surface very near the prop and tend to have prodigious prop-walk. Medium keel boats, like Coric have less deadwood, so a bit less prop-walk. Fin and skeg boats have less material near the prop and hence modest prop-walk. Extreme fin keelers with spade rudders (eg Sunshine in our experience) have very little prop-walk. Of course, all of these theories totally fail when you are relying on more or less prop-walk in a difficult berth!
Something to do with Bernoulli's principle.....saw something recently about it which said that in reverse the bow is likely to get kicked off to the side
I think it is very much about the direction of the Propeller. There are left and right-handed propellers. The effect is much more pronounced at high revs and slow speed, which says to me that this is not so much about water flow over the hull as she moves through the water, but the flows across back end of the boat.
I am triggered by your theory on propwalk. It would be good to have some data as of different hull shapes? Never saw a credible explanation so far actually. Maybe in naval literature?
I have had a good look online and in various seamanship manuals. They’re very light on the description of why and much more about how to use the effects.Nick’s comment earlier, looks like a very good description of this and how it various between different hull shapes.
Well done, Elizabeth! Williamson and Anderson turns... reminds me of my initial shiphandling training.
Thanks for this - definitely worth doing
I totally agree with your explanation of prop-walk, John. 'Paddlewheel' and differential water density explanations are implausible because the moments are so small. The accuracy of the prop wash on the hull theory is reinforced, in my view, by the way that the degree of prop-walk varies with the underwater profile of the boat. As a generality, long-keeled boats with keel-hung rudders have a very large amount of underwater surface very near the prop and tend to have prodigious prop-walk. Medium keel boats, like Coric have less deadwood, so a bit less prop-walk. Fin and skeg boats have less material near the prop and hence modest prop-walk. Extreme fin keelers with spade rudders (eg Sunshine in our experience) have very little prop-walk. Of course, all of these theories totally fail when you are relying on more or less prop-walk in a difficult berth!
Thanks for this Nick , and I think your description is much more eloquent and detailed than mine.
Something to do with Bernoulli's principle.....saw something recently about it which said that in reverse the bow is likely to get kicked off to the side
I think it is very much about the direction of the Propeller. There are left and right-handed propellers. The effect is much more pronounced at high revs and slow speed, which says to me that this is not so much about water flow over the hull as she moves through the water, but the flows across back end of the boat.
I am triggered by your theory on propwalk. It would be good to have some data as of different hull shapes? Never saw a credible explanation so far actually. Maybe in naval literature?
I have had a good look online and in various seamanship manuals. They’re very light on the description of why and much more about how to use the effects.Nick’s comment earlier, looks like a very good description of this and how it various between different hull shapes.