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
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.
Hello Mike,
I understand that you are wondering about the preference of making a passage into a head sea versus a following sea, especially when tide is taken into account. Let's break it down and try to understand the reasoning behind it.
When a power boat is moving against the wind and waves (head sea), it can experience a rougher and slower passage. The impact speed of the waves on the boat is increased, which can lead to more discomfort for the passengers and potentially reduced speed.
On the other hand, when the boat moves with the wind and waves following (following sea), the passage can be smoother and faster. The impact speed of the waves is reduced, resulting in a more comfortable and efficient ride.
Now, let's consider the impact of tide on these scenarios. When wind and tide are together (flowing in the same direction), it typically creates a smoother sea state. This means that the waves generated by the wind will be less choppy and more uniform, causing less disturbance to the boat.
However, when the wind opposes the tide (flowing in the opposite direction), it can create a rougher sea state. The waves generated by the wind can become choppier and more irregular, increasing the impact on the boat and potentially slowing it down.
Although it may seem counterintuitive, there can be a point at which it becomes preferable to make a passage into a head sea if the tide is flowing in the same direction as the wind. In this case, despite the potential for increased impact speed, the overall sea state can be smoother due to the combined effect of wind and tide.
The rate of tide is indeed an important factor to consider. The stronger the tidal stream, the more noticeable the difference in sea state between wind with and wind opposing the tide. If the tide is relatively weak, the impact of wind opposing the tide may not be significant enough to warrant a change in preferred passage direction.
It's important to note that individual experiences and preferences can vary. While you haven't encountered a situation where a head sea passage was preferred in your personal experience, it doesn't mean that it's never preferred. Different vessels, skippers, conditions, and destinations can influence the decision-making process.
I hope this explanation clarifies the factors behind the preferred passage direction in different wind and tide conditions. If you have any further questions, feel free to ask.