If your boat has a draft of three feet, the lowest part of it, probably the skeg, will touch bottom in water that's 2'11'' deep. If the mean low tide at a given spot is 3.0 feet and you cross on an average low tide, you won't touch bottom. If you cross on a -0.2 low tide, things will scrape and bump and possibly bend.
But there's more. If there are waves rocking your boat up and down, the water on the shoal will go from deeper to shallower with every cycle of peak to trough. On the bottom of the trough, you may strike bottom, even though you could cross the bar in calm seas. You have to use your judgment and perhaps wait for the tide to flow back in before you cross that particular spot. You refer to the tide tables to see when the water will be rising, and "guesstimate" about when you can cross by looking at the time of high tide and the sea conditions.
By the same token, perhaps there's only a foot of water over a bar at a creek mouth at mean low tide, and that's two feet less than you need to get in. But wait until the peak of a +3.0 tide, and there's no problem. That is, there's no problem unless you want to leave that creek at low tide. You're locked in until the next high tide, so keep that in mind any time you go exploring in the shallows.
The tide tables also list nearby locations with time corrections for the nearest main reporting station. They list the latitude and longitude of the location so that you can confirm with your GPS, and the difference in time from the nearest station. The difference may be anywhere from a few minutes to several hours, and is not the same for high and low tides. You subtract minus differences and add positive differences.
For example, suppose you want to figure out the tide for a fishing spot that's 15 miles up your favorite bay from the mouth, and the tides are only reported in the tables for the mouth. Say the report indicates a noon high at the bay mouth, and a +2.0 hours time difference for your fishing location. You would figure it this way:
12:00 + 2 = 2:00 P.M., the time of high tide at your spot
There are very slight variations in tide height at various locations on a given tidal period. In most of the southeastern United States, this difference is insignificant, but in the northeast it can be considerable. The tide tables list this variation as well, and it should be considered if you're making an iffy passage across a bar.
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.