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The Norfolk Broads Forum / Broads Boat Owners Q & A / Air Draft
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Posted By Discussion Topic: Air Draft

Similar Threads That Might Help :
Boat air drafts (heights)| Boat air drafts (heights)| Air Draft and Bridges| air draft | air draft |

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Casper
Apr-07-2006 @ 7:09 PM                           Permalink
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Is there an easy and reliable way to determine your airdraft. Casper is a Princess 25ft, the published air drafts vary a bit. I suppose that as she has got older she has also become heavier and thus deeper.

I guess that feeling your way under a bridge is not easily recommended - it could prove to be expensive.

Airdraft is something I haven't had to worry too much about before.

Alan & Gill

AndyH
Apr-07-2006 @ 8:51 PM                           Permalink
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The way i did this was to moor up at a jetty. Measure from the top of the jetty to the top of canopy or whatever, and then measure from the top of the jetty to the water - add the two together. If it's difficult to read across from the top of the boat to the tape measure you just need to get someone to hold a something (say a mop handle) horizontally from that point!

Torty
Apr-07-2006 @ 10:08 PM                           Permalink
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Plan B

Sit in middle of river under low bridge with rising tide. Keep an eye on distance between high point of boat and underside of bridge. When distance less than one inch, move slowly forward taking note of reading on conveniently placed height gauge alongside bridge.

Phil

Speleologist
Apr-07-2006 @ 10:42 PM                           Permalink
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I like the sound of plan B. Would cause such wonderful chaos. (And it might be interesting when the height gauges get renewed)  Smile

Robin
www.robin.me.uk

BarnacleBill
Apr-07-2006 @ 11:38 PM                           Permalink
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Plan C:

Drill hole of known diameter in bottom of boat.  Using square edged orifice equation, calculate rate of inflow of water into boat.  From resultant rate of increase in displacement, determine rate of sinking.  Note time taken for boat to become completely submerged.  Simply multiply the rate of sinking by the time to sink - hey presto!  The air draft is now known.

Hope this helps...

This message was edited by BarnacleBill on Apr-7-06 @ 10:42 PM

Paul
Apr-07-2006 @ 12:59 AM                           Permalink
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Now that laser levels are very cheap, place one on the highest point of your boat, tilt so that the bubble is level, and very carefully, (watching out for anyone's eyes), point the laser at a stationary object as near as possible to the boat, (boatshed wall, post etc.), then get someone to mark where the dot is (being careful not to look down the beam), then measure from the water surface to the dot...

billmaxted
Apr-08-2006 @ 7:47 AM                           Permalink
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If you are doing all this clever stuff, do remember to work out whether your fuel,domestic & blackwater tanks are full or empty if low on fuel and water and just pumped out you will ride a lot higher  Scared

"We seem to have Two dogs on board!
Lord she's not having her pups already!
Not unless they are being born three foot long!!!"
Bill...

seajay
Apr-08-2006 @ 10:19 AM                           Permalink
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I had the same problem.

I measured the height with a spirit level held onto the boat hook with elastic bands and used this to find the highest point of the boat and then keeping it level unwound a weighted lenth of string till it touched the water and measured that.

As  Bill says with fuel and water tanks full and pump out tanks empty and all crew aboard this gives you the air draught for a level span bridge,Potter Heigham is a  different matter,leave that to the bridge pilot.

It's worth measuring your air-draught as our boat's is given as between 6'8" and 7'0" with roof back and screen down in different documents. I measured it in Feb.with the roof up(too cold to slide roof back)and found that height is 7'5"!

  Hopefully though one day we will have 6'9" or more clearance that Ex Pilot says  we need to go through P.H. bridge.


Regards-Steve

This message was edited by seajay on Apr-8-06 @ 9:25 AM

Casper
Apr-08-2006 @ 10:40 AM                           Permalink
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Guy's

Thanks for the ideas, plan B sounds like it could provide a good degree of amusment to the onlookers, especially when Casper becomes jammed under and blocks the river. Perhaps ex pilot could arrange this at PH. Plan C is a lot of work, I would need to remove all the soft stuff to stop them getting wet and service the engine after.

So it looks like a variation of the boathook, laser, spirit level, I suppose I could use Gill as the bobweight to keep the line tight but then I would need to allow for the change in displacement on Casper as Gill entered the water and Archimedes comes into play on her. Some correction would also be needed for the resulting changes in heel.

More ideas very welcome.

Alan & Gill

Casper
Apr-17-2006 @ 9:05 PM                           Permalink
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Used the laser level method and worked out the air draft with empty then full water tanks - dead easy and then checked going through Ludham bridge, the measurement gave us 7ft8inches with half fuel and full water so we expected a clearance of 10inches. We had about a foot clearance so now we know.

Alan & Gill


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