Tide measurements and will my boat fit?

[OopsieDaisy]

Hi. I’m a complete newbie to all of this, so I apologise in advance! I barely know what I’m doing but I’m trying to learn.
I have a small sailing boat with max draft of just shy of 1.7 (lifting keel)… so does this mean, if a low tide is 1.8 for example that my boat would be okay? Although I can lift it higher, I’m just trying to grasp an understanding of what the low and high tide measurements actually mean for me coming and going from the marina - I’m very confused!

thank you in advance

 

[MoodySabre]

Yes it does mean that - in theory. The actual tidal height may differ due to atmospheric pressure, wind etc. It's only a prediction. Test it out on a rising tide and don't push your luck on a dropping tide.

 

[john_morris_uk]

The tidal height from the tide tables etc is the height of tide above chart datum.

So if the the tide is 1.8 metres and the charted depth is 1 metre then you’d have 2.8 metres depth at that spot.

Tidal predictions can vary slightly with air pressure and wind, but file that away for now and just leave a bigger margin for error in your calculations.

What you mustn’t ignore is the effect of swell and waves if you’re trying to enter a shallow harbour.

If you’re asking whether your 1.7 metre draught boat will float in 1.8 metres of water then the answer is yes of course. However any waves or wash and you’ll be bumping the keel on the bottom.

 

[LadyInBed]

A couple of links for you
Advanced navigation courses - coastal and offshore Ch7 deals with tide predictions
Tidal Height related to Pressure : Monty Mariner

 

[oldharry]

Dont forget too, that even LW springs very rarely goes down to Chart Datum. Tide tables show not only depth at HW, but depth at LW, So the predicted LW level for the day should be added to the depth shown on the chart The actual level varies depending on wind, and atmospheric pressure, just to make life more interesting, though! So for your boat The tide table shows LW at 1.8m, and the chart shows a depth of say 1m so there will actually be 2.8m predicted depth. So there should be 1.1m of depth under your keel.

But you say you can raise the keel higher: Its not a good idea to leave the boat with the drop keel down, as it can cause significant damage to the the boat if it runs aground, particualrly if there is a current putting sideways strain on it. I've seen 9mm plate steel bent like cardboard in storm conditions on quite a small boat!