Wave heights: what's a lot?

[fisherman]

So the 47 ft swell I was in is phenomenal, but it was flat glassy calm. You can't quantify it with any useful accuracy. We all know that 1 metre seas with the wrong period can be blody uncomfortable.

 

[Talbot]

Going downwind on saturday/sunday looks fine, but an upwind leg would be a bit testing.

The first forecast is for Sandown, which is sheltered fro westerlies if I recall correctly, yet is forecasting 2m swell from a westerly wind.

I would much prefer a forecast for Ventnor as that is more likely to provide info on conditions at St Cathrines.

I would definitely time any trip there to make sure that wind and tide were in sync rather than opposition.

 

[AntarcticPilot]

Reminds me of between the landing stages in the Mersey. Brick walls either side a little bit of traffic strong currents a bit of wind all the things you need to make waves close and vertical with big holes behind them...

I would see how much of the wind comes through to make the swells, the forecasts at the moment are not exactly precise...

Yes, and Hong Kong Harbour seems to suffer from the same thing - hard shore-lines, with lots of traffic, and a big fetch to the south. The Star ferries always seem to wallow like pigs!

BTW, I have relatives in HK - I haven't sailed there!

 

[AntarcticPilot]

So the 47 ft swell I was in is phenomenal, but it was flat glassy calm. You can't quantify it with any useful accuracy. We all know that 1 metre seas with the wrong period can be blody uncomfortable.

That's an interesting case, because pretty much all the wave energy would have been at one frequency. You were presumably measuring peak-to-trough; in this case, I think the SWH figure (which is what the Met Office figures are based on) would be significantly less.

You could argue that given the long wavelength of such swells, the LOCAL waveheight (i.e. over a distance of a few boat lengths) is practically speaking zero

Incidentally, 47' is a very precise figure. How did you measure it? I can see that under the circumstances of a single long-period swell with no short wavelengths it would be feasible to do a good measurement, but I'm curious how you did it.

 

[fisherman]

Incidentally, 47' is a very precise figure. How did you measure it? I can see that under the circumstances of a single long-period swell with no short wavelengths it would be feasible to do a good measurement, but I'm curious how you did it.

As you say, local WHT zero, boat was very stable, several (digital) readings off the echo sounder, max was not quite 8fm, I swapped the reading to feet. Only problem to me in this case is if the gear comes fast something will break, usually the rope.

 

[KellysEye]

>Isn't there something in the heavy weather books that gives a wave height to LOA percentage (35% comes to mind), beyond which you might come a cropper?

It's generally acepted that if the wave hight is the same or more than your beam you could roll, so never beam reach in those conditions. If the wave height is the length of your boat or more you could pitchpole, so don't run downwind in those conditions. Those were the rules we followed.