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Mike,
You are right: this "1/2 of the wave height" rule is not designed for small boats.
It assumes that your height over water (when it is still) is much larger than the wave height.
It cannot be applied to a small yacht when the waves are high.

To see this, try to apply it to the extreme situation when a wave close to you 
simply obscures the horizon.
What you see then as your "horizon" is just the wave crest.

Taking altitudes in these conditions is hardly possible at all, unless you 
climb your mast (I remember reading in some 
yachtsmen memoirs that this was actually done when an observation was badly needed).

Alex.
________________________________________
From: NavList@navlist.net [NavList@navlist.net] on behalf of Mike Freeman [NoReply_Freeman@navlist.net]
Sent: Sunday, December 8, 2019 2:46 AM
To: eremenko@math.purdue.edu
Subject: [NavList] Waves and Dip

"It is normal on the ocean to have a sea running, in the NE trades in mid 
atlantic there will probably be a ground swell of 8 to 10 feet in height. In 
the N Atlantic after heavy weather there could easily be a swell running to 
20 feet or more. If this is the case youll have to estimate the wave height, 
divide by 2 and add the result to your height of eye, youll only see the 
horizon from the top of a wave so that is where youll be when you take your 
sight"

I was surprised to read this.

My imagination tells me when my small yacht is on a crest my horizon is also 
made up of crests so no adjustments required?

I also read some other special instructions for taking sights in large waves 
which I am unable find but were different to either of the above.

I guess different vessels vary?

Look forward to replies

Mike

   

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