Air draught...

[DeeGee]

No, not a posting about a nice big breath of fresh air.

OK, you have done the calculations. The bridge is 17m clearance, top of mast about the same, MHWS = 10.5m, HW today = 7.7m, so that means about 2.8m clearance, or about 9ft... then there is the vhf aerial, but that is only about 1m at best... so we have plenty of room.

Ahhhh, but even with 10m clearance, it APPEARS as you go under to be a close-run thing... how accurate are the chart markings? how accurate is a tidal prediction?

So, here's the biter bit, what sort of safety margins do people use on air-draught ?

 

[Talbot]

depends whether it is a bridge or electricity cables /forums/images/graemlins/shocked.gif

 

[Blueboatman]

Now if you had a crows nest and a tape measure ,there would be no problem !
I have - only once-shot a bridge backwards, stemming the current with the outboard and easing under the bridge oh-so-gently as it was a too close call,on paper. ( Mount Desert Island ,Maine,which we ''circumnavigated'' , can't think why )

 

[DeeGee]

[ QUOTE ]
depends whether it is a bridge or electricity cables /forums/images/graemlins/shocked.gif

[/ QUOTE ]Is that some lack of faith in the marked clearance, or gut feeling for the consequences of error... /forums/images/graemlins/smile.gif

 

[snowleopard]

[ QUOTE ]
depends whether it is a bridge or electricity cables /forums/images/graemlins/shocked.gif

[/ QUOTE ]
Exactly what I was going to say. Probably something to do with sailing ont he Tamar. A bridge tends to stay in the same place from year to year. Electric cables move about, may not have been measured at the lowest point, may have been re-strung since the chart was surveyed etc. Also hitting your VHF antenna of a 500kv line or whatever it is could spoil your whole day.

Isn't it amazing how even if the obstruction is twice the height of the mast you still feel a compulsion to duck! If it's a bridge I know well I generally avoid looking up.

 

[Evadne]

Electric cables also get lower in warm weather. Now I'm sure that the quoted height for them will take this into account, but I'm just glad I don't have to go under any that low in my regular cruising ground.

 

[Talbot]

[ QUOTE ]
[ QUOTE ]
depends whether it is a bridge or electricity cables /forums/images/graemlins/shocked.gif

[/ QUOTE ]Is that some lack of faith in the marked clearance, or gut feeling for the consequences of error... /forums/images/graemlins/smile.gif

[/ QUOTE ]

Risk reduction exercise.

 

[Pie]

A good breeze and well heeled over no problem /forums/images/graemlins/crazy.gif

 

[DJE]

I worked on a new bridge in Hong Kong with 50 metres clearance above high water. It was bashed before it was finished by one of those big derrick barges that they use for transhipping containers. The bridge now has large warning signs saying "41m clearance".

So sometimes there may be a bit more clearance than indicated.

 

[snowleopard]

[ QUOTE ]
A good breeze and well heeled over no problem /forums/images/graemlins/crazy.gif

[/ QUOTE ]
I just hope you don't lose the wind as you pass between the piers of the bridge!!

You wouldn't be the first. I remember a report of a race that used the Royal Sovereign tower as a windward mark. Someone cut it fine and came upright as he bore away leaving his masthead trapped among the girders underneath the helicopter platform.

 

[Stemar]

For power cables, I think I want about twice the distance the current can jump in ideal conditions - I suppose a nice salty fog, or heavy rain. Anyone know what that's likely to be?

I suppose with my little mast, the question's fairly academic. I'll just follow a bigger one through. If he doesn't get an instant perm, I'll be fine :smug:*


*I know it doesn't exist, but it should /forums/images/graemlins/smile.gif

 

[snowleopard]

In dry air, a spark can jump 1cm per 10,000v. I can't say what the figure is for wet air, certainly several times that.

 

[Rowana]

I was once off Copenhagen swinging round the mast in a bosun's chair. If you know that part of the world, you'll knpw that the airport is close by and the approach, depending on the wind of course, is over the sea.

Anyway, there's me up in the air, and this fecking great 747 is coming in to land. Now I KNOW he's going to go over a good couple of hundred feet above my head, but when he's a couple of miles away it somehow feels as if he's going to collect me in the passing ! !


Wonderful sight though ! !

/forums/images/graemlins/shocked.gif /forums/images/graemlins/shocked.gif /forums/images/graemlins/shocked.gif

 

[brianhumber]

Only too true - I agree, I have never had the bottle to take Ronhilda (21m AD)up to Pontrieaux cause I always had kittens with my old UFO which was a lot lower /forums/images/graemlins/confused.gif. Same for the Villiane upstream of LaRocheB and several other places in Britanny

Brian

 

[brianhumber]

In my case sheer fright - have been authorised to switch and work on HV in my past and can still see in my mind the blackened photos of burnt people who were used to scare new trainees like me in avoiding getting anywhere near a situation of getting zapped with 11,000V+.

 

[DeeGee]

[ QUOTE ]
Only too true - I agree, I have never had the bottle to take Ronhilda (21m AD)up to Pontrieaux cause I always had kittens with my old UFO which was a lot lower /forums/images/graemlins/confused.gif. Same for the Villiane upstream of LaRocheB and several other places in Britanny

Brian

[/ QUOTE ]I agree - don't even try !! That is a whole lot of mast. /forums/images/graemlins/crazy.gif

Interesting nearly a quarter of peeps who go for less than 1m /forums/images/graemlins/ooo.gif

 

[capsco]

Brian you seem to be familiar with the area, do you know how far the Villiane is navigable, and to keep on thread what is the air draft restriction to a Motor Boat with 3.2m [air draft]
This query is because a local told me [or I think he did, in French] that it was possible to reach St Malo.