Physics/Nautical

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.

Thanks

Mike

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  1. The size of the waves makes a big difference. In a following sea, large waves as they pass the stern lift the stern, then they approach midships, and the stern rises out of the water, rudders and screws. The ship is vernable to the force of the water midships twisting the ship sideways as the rudders are lifted from the water. Once the twisting motion occurs, the ship can roll or be swampled. This of course is dangerous if the wavelength of the waves is nearly equal to ship length (crest midships, trough bow and stern).
    Now heading into seas, or my experience putting the sea two or three points off the bow, allows you to adjust apparent wave wavelength relative to ship to avoid the cork twisting effect, and since the bow of the ship normally has a more vertical line, it can act as a rudder when water lowers. I only have deepwater experience in heavy seas, no tidal effect. But I can imagine a tidal current with the waves would give one more bow steering effect in heavy seas, and allow you to maintain headway. The bows of the ships I had were much more heavily structured in the bow area compared to the stern. In pounding seas, that is a consideration.
    Again, in my own experience, choosing the course depends on how heavy the seas (amplitude), wavelength compared to ship, wind (especially if it is not with the waves, but a cross wind), desired headway direction, and underwater hull shape. Lord help you make the decision in a flat profile bottom vessel, and that may be what you are asking on the 10m.

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    bobpursley

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