Cappy,
I don't know if there is any "slack" sea water in coastal areas. This fall I'm going into Lake Washington (behind Seattle) where they have a measured mile marked off alongside a floating bridge. I want to make a rpm/speed chart over the measured mile. I'll use the measured mile for 2000rpm to 2500rpm. Then I'll make some other recordings re other speeds .. including top speed.
Part of me wonders why because I'll be applying the results in tidal waters ????
Yes crab/lobster floats can be bothersome.
One of the questions I have to ask when at work often is: what is your slack water speed? I often ask, because in the process of proceeding up or down a channel, it is necessary to discuss overtaking (or to be overtaken) of another vessel. I know (for instance) that my tug (and barge) make 9.4 knots when loaded, and 10.4 knots when empty. So, if I ask another vessel what speed he makes in slack water, I can readily determine if it is even possible for me to overtake, or make it in time for a next bend in the channel, or to allow for other traffic. But way too often, the response I get is: "Well, I'm doing 8.5 with the tide" That wasn't the question. Because, I know we have fair at the present time, and the channel has varying current (based upon stricture, and confines of the channel) that will change over the mile or two it will take me to overtake him. I have to know what the slack water speed is, to make informed decisions of both vessels situation. This is particularly appropriate when two vessels are meeting at the confluence of two separate rivers, with differing currents effecting each.
This brings up an interesting topic. The measured mile is only effective if you run it one way, then turn around and run it the other way. This way you have one leg with foul current, the other leg with fair. (assuming this was not at slack water.
It is not important to know the speed over the bottom, unless you have erased the current effect. However, it IS important to know your speed through the water, when applying set and drift in voyage planning.
The other way to know current (other than by visually seeing it ((Then estimating) on buoys and fixed structure) is to use a current book, and extrapolate the velocity at the time you are running.
On the East coast we have an excellent reference book that pretty much has all the current velocity at each hour of the day already printed and ready to use. (Eldridge Tide and Current Book) They are worth their weight in GOLD. In addition to about 100 pages of extremely helpful seamanship hints (customized to the local waters) they have tide and current predictions from Maine to Miami. All in easily usable format for the boater. We even use them in the coastwise tug industry.