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Old 20-03-2013, 04:39   #1
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Technical question about Ship Handling

Hi
I’ve just joined this forum - and I am not much of a sailor (some experience in an Albion 78, that is all!)

The main reason I am here is to ask a question to more knowledgeable people. I'm developing a simulation of a ship’s hull travelling through water as part of a project. One thing that I have noted is that as the ship’s speed increases, its pivot point moves towards the bow, away from its centre of gravity. Does this mean that a ship will have a tendency to turn into any oncoming current, if the helm is locked amidships? For example, suppose there is a current from 10.00 o'clock off the bow. If the pivot point was far foward in the ship, it would mean that there is a greater area of hull after this point exposed to the force of the current, which would tend to swivel the ship around to face the current, if not corrected by the helm. I may have this wrong, because the dynamics involved are very complicated, but this is my understanding.

If there is anyone here with any practical ship handling knowledge or experience, it would be nice to hear confirmation or refutation of this.
Cheers
Jason
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Old 20-03-2013, 05:20   #2
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Re: Technical question about Ship Handling

A steady current itself doesn't effect the ships helm or course, both are moving in the same water and the ship doesn't "see" the current

Only when you hit the boundary line between two differently moving flows such as when you go through a strong eddy line, or if the current is different at the lower portion of the hull from the surface does the vessel get knocked of course.

Wind does effect it however in the way you describe, depending on where the bulk of the windage is in relation to the underwater dynamic center of lateral resistance (pivot point). This is most noticeable when you go astern. the pivot point moves aft and the windage blows the bow off downwind so you end up reversing into the wind on most yachts and some ships.

The same effect is a part of the reason why yachts develop weather helm (want to turn into the wind) as they move faster, but this is also caused by the hull shape changing and the force from the sails moving off to the leeward side as the boat heels over.

Sounds like an interesting project, all the best with it.
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Old 20-03-2013, 05:44   #3
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Re: Technical question about Ship Handling

Greetings and welcome aboard the CF, Jason.
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Old 20-03-2013, 06:44   #4
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Re: Technical question about Ship Handling

Many thanks for the prompt reply!

And also thank you for the warm welcome!
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Old 21-03-2013, 21:58   #5
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Re: Technical question about Ship Handling

When I took my 500 ton mates test , there was a book recommended that went into great explanation of big ship handling. I would recommend calling one of the schools that teaches USCG courses, and ask about the book. I hope it helps you. ____Grant.
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Old 22-03-2013, 01:15   #6
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Re: Technical question about Ship Handling

Thanks.

Actually I wanted to hear first hand what people's experience has been handling ships, as I spend too much time in books. Still, it would be worth a look in this one you mention. I'll check it out.

Snow petrel has more less answered my question, though. Since the ship drifts with the current, it wouldn't affect the helm or heading (although drift will affect the course).

However I would have thought that for example, in an estuary where the is a strong current say of 4 kts, a ship coming about is going to be affected, for example if turning into the current because the current at some point will tend to start pushing the stern around.
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Old 22-03-2013, 04:06   #7
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Having been helmsman and Officer of the watch (not at the same time) on many ships up to 40,000 tonnes or so, I have never noticed any effect like this in a smoothly flowing current. As far as I know it's eddys or current sheer that effect our heading.
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Old 22-03-2013, 05:34   #8
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Re: Technical question about Ship Handling

Thanks once again, Snowpetrel - I think that wraps it up!

This seems to be a great forum, by the way. Glad I found it.
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