Yes - In case anyone is following this, let me explain:

Here in the States we use Mean High Water (MHW) as the datum for bridge clearances. This
means that, on average, half the tides will be higher than MHW, so the bridge will be
lower than expected. To make matters worse, with the common semi-diurnal tide, one tide
(the "higher high") will often be significantly higher than the other, so the discrepancy
can be large.

From chart 13270 of Boston harbor there is a table:

Height referred to datum of soundings (MLLW)
Mean Higher High Water 9.7 feet
Mean High Water 9.3 feet
Mean Low Water 0.3 feet
Extreme Low Water -3.0 feet

Thus, the average "higher high" is 0.4 feet above the datum used for bridge clearance.
However, today the Higher High is at 10.22 feet. almost a foot higher than the datum.
Anyone thinking they have a foot of clearance could be in for a nasty surprise!

I should add that a strong onshore breeze can often add an extra foot to the predicted
tides.

-jeff



"The_navigator©" wrote in message
...
Hmmm. That's a worry. Might mean you'll need tide tables to decide if
you can pass an overhead...

Cheers MC

Jeff Morris wrote:

We don't often use "spring tides" here in the States - people like RB would be saying
they
only sail in Summer.

We just use MHW - that leads to excitement since the bridges will likely be lower than
their reported clearances once a day!

That reminds me - Happy Spring!


"The_navigator©" wrote in message
...

Heights on land and clearances are above MWHS -right?

Cheers MC