HWMT ?

[graham]

Im sure I should know this but what height is HWMT ?

Should I send my YMticket back in disgrace? /forums/images/graemlins/wink.gif

 

[Twister_Ken]

High Water Medium Tide, innit.

 

[michael_w]

High Water Mean Tide. ie: The average of all High Waters.

 

[graham]

I have worded the question badly.I know the Letters stand for High Water Mean Tide but when calculating your safe air draught clearance how do you calculate what HWMTis in meters above chart datum?

 

[Guest]

[ QUOTE ]
how do you calculate what HWMTis in meters above chart datum?

[/ QUOTE ]
Surely it shouldn't matter how high above chart datum it is. What matters is the distance under the cables to the water surface at any given state of tide. If you're thinking of squeezing under high tension power cables with such narrow margins that you have to do it when the tide is at chart datum then you're a brave man.

But if you do want to calculate distance above chart datum for some reason, then approximation is got by looking at the height at MHWS and MHWN, taking the average of the two, and then adding on the height of the cables above HWMT.

 

[tillergirl]

Init the mean between the Mean HW Springs and the Mean HW Neaps - and that figure would be the height above chart datum

 

[Woody]

Heights are usually measured on charts (bridges etc) above MHWS. I agree with SimonCr and wonder what the relevance is to yachts. Where is this? /forums/images/graemlins/confused.gif

 

[graham]

That makes sense ,averaging between Neaps and Springs.

The reference to CD doesnt imply I would wait a few years for it to occur especia;lly as most rivers around here dry out at half tide /forums/images/graemlins/grin.gif

The height of tide is given or calculated above cd,it would make more sense to have everything on the same datum.Ie for example if the height of tide at 1700 on a certain day was 6.8 meters height of cable was 16.8 meters above CD You would have 10 ms from the water to the cable.

Next question ,how many of us know our airdraught ? /forums/images/graemlins/blush.gif

PS Its the Rumney River near Cardiff.

 

[Guest]

I think they use HW (whether MHWS or HWMT) rather than chart datum to measure air draft not only for power cables but also bridges etc. to allow for numpties who forget about tide heights. The idea being that the height mentioned on the notice is the least clearance that you're likely to find.

Then again, if the datum is HWMT then you have to allow a bit if you're passing at high water springs. Also with high tension power cables I would make an allowance for any extra droop in the cables, the fact that the guy measuring may just have argued with his wife, that the wind or low atmospheric pressure may have blown in a higher tide than expected, the fact that my VHF aerial may be longer than I rememberered, or all of those factors combined together...

 

[Guest]

Here\'s a pic

I took the day before yesterday of a ship squeezing under a bridge on the Kiel Canal. Clearance height was 40m, and the antennae over his bridge juuuuuuust got under. A bit further along were power cables with the same height

 

[Sailfree]

I believe all this recently changed and without looking it up I can't remember which way!!

I believe it is as follows:-

Charts used to have Air Draught given calculated from HAT.

New charts I believe are now calculated from HWMT giving a slightly larger clearance air draught.

So the first thing you need to know is which way was the chart you are using measured. Then do your sums.

Finally if you need to do the sums its because you think it might be close then do the sums thoroughly as when you do approach there never appears to be enough room when you look up!!

I have never passed under HT cables but I assume there must be some places where this occurs what clearance do you allow to avoid the chance of the Electricary shorting to earth through some very moist air and your mast??

 

[Andrew_Fanner]

>>>
Charts used to have Air Draught given calculated from HAT.

New charts I believe are now calculated from HWMT giving a slightly larger clearance air draught.
>>>

Thus allowing HMG to claim that they have "raised the clearance of many bridges"

 

[graham]

Seems a bit unwise to have changed to HWMT .There will frequently be less clearance than the chart or sign says. Around here it could be a lot less clearance on a big spring tide.

Thanks for all the replies. My brain cells have been reactivated now. PS Anyone remember the RYA recommended safety clearance for power lines,1.5 meters I think but dont quote me.. /forums/images/graemlins/grin.gif

 

[Skyva_2]

I thought it had changed from MHWS to HAT to be consistent with LAT, ie the safety margin has improved?

Would be glad to be positive about that!

 

[Daedelus]

Clearance heights used to be quoted as at MHWS, but it has just been changed to HAT. This is not as useful as it might be (although it should give even better clearance tolerances) as Reeds only publish HATs for standard ports and that in the updates to the latest almanac. The only charts I've seen HATs on are the RYA training ones, but in time it will all change.

 

[douglas_family]

i think this is probably insufficient clearence

http://uk.youtube.com/watch?v=p47L3kWCsx0

 

[Cotillion]

It's High Water Mean TEMPERATURE.

Things could get very hot if you've more than 14 metres aloft.

 

[ccscott49]

Why dont they give the clearance as high water spring tide, then you will know the minimum clearance? the you can calculate from there.

 

[stephenh]

Now I am confused..

I thought that air draught calcs on charts used to be based on MHWS and then recently ( in the last 5 yrs or so ) changed to HAT ?

Have they changed it again ??

 

[deep denial]

[ QUOTE ]
Now I am confused..

I thought that air draught calcs on charts used to be based on MHWS and then recently ( in the last 5 yrs or so ) changed to HAT ?
No, you're correct, it's now HAT
Have they changed it again ??

[/ QUOTE ]