Ballast/draught question

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bozlite

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I wonder if someone would be able to help me with a quick question.

We've currently got our eye on a dutch steel cruiser. Brand new engine, and, on a quick inspection at least, in fairly good condition. However, it has an air draught of 8' 2" - and we need to get it under an 8' 1" bridge to get it home!

The boat is 10m x 3.3m. I've no idea what capacity the current tanks are - or how full they were when the boat was measured.

Is there any way of (roughly) calculating how much additional weight we'd have to add to sink it by, say, 3cm?
 
Something less than 990Kg. If I can remember my school physics and arithmetic the calc goes like this. Assuming the boat is a rectangular box you would need to displace an extra 1000 x 330 x 3 cc =990000 cc of water. That volume of pure water weighs 990kg (for the pedants has a mass of 990kg). The boat isn't a box and it's not floating in pure water but it's a start. If you can put in 300L of water into your tanks that's 300kg and 200l of diesel would add 170kg (850 grams/liter) more and you'd be about half way there. Is there anything you can take off the top of the boat?
HTH
A
 
Last edited:
albineer said:
Something less than 990Kg. If I can remember my school physics and arithmetic the calc goes like this. Assuming the boat is a rectangular box you would need to displace an extra 1000 x 330 x 3 cc =990000 cc of water. That volume of pure water weighs 990kg (for the pedants has a mass of 990kg). The boat isn't a box and it's not floating in pure water but it's a start. If you can put in 300L of water into your tanks that's 300kg and 200l of diesel would add 170kg (850 grams/liter) more and you'd be about half way there. Is there anything you can take off the top of the boat?
HTH
A
Click to expand...

Thanks so much - that's actually quite good news. Seems doable.
 
sarabande said:
remember the 'squatting effect" of a boat moving at speed - especially in shallow water.


http://www.vht-online.de/PDF/Practical_Advice/SQUAT-uk.pdf

A quick calculation shows that at 27 knots, you should be able to reduce the airdraft by approx 3 inches. More than enough.....
Click to expand...
And if you needed 4" one hell of a dent at 27 knots !!!

Likelihood is an advertised bridge height will account for highest expected water level, so may be enough without worrying - drive to the bridge with a tape measure within a day or so of the expected trip.
 
If you do need to add weight 200 litre plastic drums are around £15 each and water can be pumped in / out effortlessly.

When you've finished sell the drums on again.

If you need a permanent solution then it's broken paving slabs in the bottom of the boat. Much harder work !

Henry :)
 
sarabande said:
remember the 'squatting effect" of a boat moving at speed - especially in shallow water.


http://www.vht-online.de/PDF/Practical_Advice/SQUAT-uk.pdf

A quick calculation shows that at 27 knots, you should be able to reduce the airdraft by approx 3 inches. More than enough.....
Click to expand...

Actually, now I've had a look at that page in a bit of detail, it seems that even a very low speed (say 3 knots) can make the boat squat by 4 cm in a very narrow, very shallow channel (eg a canal). I've often heard owners of deep-draughted ex-'working boats' on the canal say that they draw 36" when stopped and 39" at tickover - an exaggeration, possibly, but it now sort of makes sense.
 
bozlite said:
I wonder if someone would be able to help me with a quick question.

We've currently got our eye on a dutch steel cruiser. Brand new engine, and, on a quick inspection at least, in fairly good condition. However, it has an air draught of 8' 2" - and we need to get it under an 8' 1" bridge to get it home!

The boat is 10m x 3.3m. I've no idea what capacity the current tanks are - or how full they were when the boat was measured.

Is there any way of (roughly) calculating how much additional weight we'd have to add to sink it by, say, 3cm?
Click to expand...

Really? You want to take a 10m boat under a bridge with 5mm clearance?
 
Nick_H said:
Really? You want to take a 10m boat under a bridge with 5mm clearance?
Click to expand...

You're quite right, of course. That'd be foolhardy (if potentially amusing for onlookers). I'm just getting my head round some possibilities. I'll be doing a spot of bridge measuring this weekend (I doubt they're as low as the published clearance figures), and then find out about the tanks, and then actually measure the boat myself.
 
my little joke above about 20 fat blokes above does actually have some basis. Some friends we travelled in convoy with down the French canals had a similar issue to yours. On one occasion he stripped off everything he could on his Trader 42 and the highest point on the boat were the morse throttle controls which he calculated would only clear the bridge if lying flat at WOT. No he didn't take it at 25 knots! With engines off and as many passers by as he could muster on board, he then towed the boat through on a long rope from the canal bank and just scraped through (literally).
 
Is the air draught of 8' 2" to the top of the upper helm window frame or the top of the coachhouse roof? If its to the top of the window frame can the window be folded down? A lot of Dutch steel cruisers I see have foldable windows on top.
 
Whopper said:
Is the air draught of 8' 2" to the top of the upper helm window frame or the top of the coachhouse roof? If its to the top of the window frame can the window be folded down? A lot of Dutch steel cruisers I see have foldable windows on top.
Click to expand...

It's 8' 2" with the screens folded down.
 
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