Safe passage up/down the Bristol Channel

[vodzurk]

Hi All,

Sorry for swamping your forums once more, but I feel that I really should ask this, as we're intending on heading down to Ilfracombe tomorrow (from Portishead)...

A couple of weeks back, we came back up the channel from Swansea, right between the Holms, then once past the sandbars, ducked over to Clevedon to wait for the Portishead lock in.

Link to course we too: http://tinyurl.com/ycourgbr.

What concerned me was the wave height between ~2 miles before to ~5 miles after Flat Holm. I think WindyTV and Portishead Marina website put wave height at somewhere between 0.1-0.3m.

There WAS a SW wind of 8mph... though on the day you'd struggle to even notice. I know SW-erlies are bad news, but is that true when it's barely even noticeable to your face? Around before Barry, the going was pancake flat, and couldn't feel any wind.

Our approach past Barry started at about 1 hour after low water. Until now, I assumed that the peak/trough of the tide is the calmest time.

The wave height was pretty concerning. We managed fine, but an main engine failure at key points could have been quite hairy. I kept our speed a bit faster than the waves, which were going north-ward... so no risk of flooding from the rear. The missues disagrees with me, but I'd swear those waves were between 1-1.5m. Our mobo is only 6.5m. I got wet.

So my questions...
(1) is low water around the holms always hell?
(2) if it gets a bit ropey, are there better ways to make my way north? Like sticking close to the coast past Cardiff?
(3) not quite related, but where would you guys wait if arriving early at Portishead lock? We did Clevedon... but with those waves, it was... a little less than fun hauling the anchor back up (I wore a harness, and hunkered down into the anchor locker for stability).

Also, any other tips are very welcome!

 

[bitbaltic]

The main channel, which is between the Holms, funnels the tide. You can get very substantial tide-driven waves in there even in a flat calm. We had this on the Holms Race a couple of years ago- wind S around 2-6kn, waves up to 1m and surprisingly steep too, but a longer period than wind-driven waves of the same size would be in the inner BC. You can see them at 2.20 on this video (we were sailing less than it looks; sails were filling but boat going nowhere; also the GoPro tends to make the sea look flatter):

https://sailingkarisma.com/2016/09/19/holmward-bound-the-movie/

If you were coming through on a spring tide I think that explains what happened, also be aware on springs there is not really any length of slack water.

This is why, if you put a fresh breeze against this sort of tide, it can get very rough between the islands.

In light airs I would not be too bothered (in a mobo!) but if you want to plot courses around this sort of effect the best advice is to avoid areas where the bathymetry changes rapidly as this is where things get funnelled. You see some similar stuff on the edges of the sand banks eg Welsh Grounds sometimes.

If you stay shallow of course races become a problem, tbh it’s frying pan vs fire in the BC for this sort of thing!

Re. Portishead, if arriving early i’d Just wait in the pool behind the breakwater.....

 

[vodzurk]

The main channel, which is between the Holms, funnels the tide. You can get very substantial tide-driven waves in there even in a flat calm. We had this on the Holms Race a couple of years ago- wind S around 2-6kn, waves up to 1m and surprisingly steep too, but a longer period than wind-driven waves of the same size would be in the inner BC. You can see them at 2.20 on this video (we were sailing less than it looks; sails were filling but boat going nowhere; also the GoPro tends to make the sea look flatter):

https://sailingkarisma.com/2016/09/19/holmward-bound-the-movie/

If you were coming through on a spring tide I think that explains what happened, also be aware on springs there is not really any length of slack water.

This is why, if you put a fresh breeze against this sort of tide, it can get very rough between the islands.

In light airs I would not be too bothered (in a mobo!) but if you want to plot courses around this sort of effect the best advice is to avoid areas where the bathymetry changes rapidly as this is where things get funnelled. You see some similar stuff on the edges of the sand banks eg Welsh Grounds sometimes.

If you stay shallow of course races become a problem, tbh it’s frying pan vs fire in the BC for this sort of thing!

Re. Portishead, if arriving early i’d Just wait in the pool behind the breakwater.....

Cheers for the advice... looks tough to plot a course around it all though... but think i'll avoid going between the Holms if it's less than VISIBLY perfect, and stay closer to whichever coast I'm on.

Nice video by the way .

This doesn't show how bad it got between the Holms, because surprisingly, I was concentrating on the helm... but when I felt we were a safe enough distance through the other side... it still wasn't ideal! ...