April 27, 2015

Homework Help: Physics/Nautical

Posted by Mike on Saturday, November 24, 2007 at 1:56am.

Further to my post yesterday re wave speed.

If a fast 25Kn 10m power boat had the choice of making a passage against the wind & waves (head sea) or with the wind & waves following (following sea). Assuming 15Kn wind in open sea conditions & no tide the skipper would choose to make the passage with a following sea. This would give a smoother & faster passage.

Once the tide is brought into the equation the picture changes. Wind & tide together creates a smoother sea state than wind opposing the tide. Apparently a point is reached when it would be preferable for the skipper to make the passage into a head sea providing the tide is flowing in the direction of the wind. Rather than in a following sea in wind against tide conditions.

I understand wave speed is a very complex issue but I have noticed waves generated by 15Kn wind overtake my 8Kn fishing vessel. I assume a wave crest speed of 10Kn which I believe is sufficiently accurate to use in the following examples.

1. A vessel doing 25Kn into head sea impacts the waves at 35Kn.

2. Same vessel doing 25Kn following sea impact speed reduced to 15Kn.

Example 1 wind & tide together makes slightly smoother sea state.

Example 2 wind against tide slightly rougher sea state.

Considering the above I fail to understand how a passage into a head sea would be the preferred option.

One thing I have not included is the rate of tide. Obviously the stronger the tidal stream the more noticable the difference in sea state between wind with & wind opposing the tide.

I have personally made a few passages in a 25Kn 10m vessel and have never encountered a situation when we would have preferred to take on a head sea than run with it.

I do hope I have explained my question clearly.

Can someone explain why make passage into head sea and what rate of tide would be applicable to make this preferred option.



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