This was discussed in an article which I believe is to be found somewhere on this site. I made a copy of the relevant parts at the time, and namely:
If a following sea is found at the entrance to a harbour then the conditions can get even more lively. Almost by definition as you move towards land then the depth of water is reducing and thus the bottom of the wave is slowing as it drags along the bottom. The wind pushing the wave though is still having its effect and thus the wave tends to grow in height and potentially break. If this occurs where there is a particularly sudden loss of depth (eg at a 'harbour bar' - area of water far shallower than that either side of it) then the change in conditions will be dramatic.
'Crossing a harbour bar' should under no circumstances be attempted without considerable experience and a highly capable vessel. That said there are times when such conditions simply happen upon an unsuspecting vessel entering a harbour after a day out, knowing what to do could make all of the difference. Entering a harbour over a bar with an ebb tide is the worst combination and there needs to a very good reason for not waiting it out or finding another harbour as a refuge. Before making your move over the bar stop and assess the conditions, where are the waves breaking?, do some areas look easier than others? The difference between the approach advocated for the following seas and crossing the bar is the same it is simply the magnitude of the waves and their frequency which makes all of the difference. As the depth of water decreases then so the will slow down and get steeper thus making careful throttle control all the more important. As has been stated earlier it is imperative that the following waves don't flip the RIB. Speed and power are essential elements in the safe passage of a RIB, being able to power away from and through trouble is key.
The necessity to cross a bar is clearly a local factor and you will no doubt know of conditions around your area of operation that merit learning about. It could be a tidal race around a headland or particularly viscious conditions near the entrance to a harbour - either way its worth getting to understand how the conditions vary under different tide & wind conditions, then when you experience them you will be better placed to handle them. Whilst there is no substitute for experience there is a strong argument for understanding how to deal with the conditions you might one day face.
That said, it is important that as a RIB helmsman you exercise restraint and care and make sure you don't put your crew, your boat or the rescue services in a dangerous situation, make sure your vessel is capable, and you and your crew build up your experience over time. The RYA Advanced Powerboat Course tackles rough water handling and there are a number of other specialist courses for RIBs that cover crossing bars and extreme conditions.
Hope that this is useful.
Cheers,
rupert.
