Density of salt water vs fresh water.

[oldgit]

Have plans to try and negotiate a very low bridge up on the river at Tonbridge.Have actually been up with tape measure to measure bridge height 2.6M and then to measure actual air draft on boat err....2.6M ..........it will be tight.
Any thoughts on just how much lower my boat will float in fresh water as opposed to salt.even a few cms will help. /forums/images/graemlins/smile.gif

 

[landaftaf]

well, first I would suggest you fill up the celler with some good wines, and then the bar with loads of g & t. that alone should do the trick

if you still need more sinkage, work on 0.75 x 1025/1000 of the tpc (tonnes per centimeter sinkage), its going to be wrong but not by too much assuming your hull is the normal go fast chine, with a waterplane area increasing at your max draft about 0.75/cm2/cm sinkage /forums/images/graemlins/cool.gif

 

[oldgit]

So basically the faster we go the more likely we are to appear out the other side with a open top sports cruiser /forums/images/graemlins/smile.gif
Actually all I really want is an inch or two.

 

[landaftaf]

if there is enough water then ballast down /forums/images/graemlins/cool.gif

 

[boatone]

Frankly, I dont think the water density will make sod all difference and your inevitable errors in measuring bridge and air draught will probably negate such a calculation.
Fill the tanks, fuel, water, holding and invite lots of peeps on board will probably be the easiest way to get her down a bit.

Many moons ago, I am told kids would wait by Osney Bridge on theThames and offer to jump onto boats that needed an inch or two to get under the bridge.

 

[onenyala]

new draft/old draft = old density/new density

eg salt water draft = 1.5m (old draft)
salt water density = 1.025(old density)
fresh water draft = X (new draft)
fresh water density= 1.000

result : draft in fresh water = 1.5375m

That is assuming the water at the bridge 'fresh' 1.000 sg

 

[oldgit]

Water tanks are full and fuel is also nearly up to the top.Cupboards are full of cheap fizz type lager
So really we need a few friends aboard who have spent two much time in the pie shop........... Any suggestions ......../forums/images/graemlins/grin.gif

 

[Double arvor]

would also check on day as if this rain carrys on could raise water leval

 

[oldgit]

Actually we draw about 1m so reckon about 2.5cms should get me under.

 

[landaftaf]

mmm

you need to know the waterplane area at the current draught plus the waterplane areas at subsequent draughts to work out the tpc , thus sinkage ......in your calculation you are assuming the block coefficient is 1.0 which is unrealisic for anything but a barge, so in my opinion you are misleading ppl into making incorrect decisions /forums/images/graemlins/shocked.gif

but whatever - /forums/images/graemlins/cool.gif

 

[oldgit]

Hope to be going over Aug bank holiday,so will of course be blisteringly hot.
If it rains will nip down to next lock and ahem 'adjust" water level.

 

[Salty John]

"Actually all I really want is an inch or two"

Don't we all?

 

[[2068]]

You're pushing your luck if it's that close. I'm not sure exactly which bridge that is,
but the water levels can very up to a foot or so either way in 24hrs depending on rainfall and what they decide to do with the locks/weirs/sluices etc.

I spent years bimbling around Tonbridge (the first 18, to be precise), but mainly in a Humber inflatable with a Chrysler 15 on the back, so air draft was as lo as the passengers could duck...

dv.

 

[boatone]

[ QUOTE ]
So really we need a few friends aboard who have spent two much time in the pie shop...........

[/ QUOTE ]

Aren't our Scuttlebutt friends a tad partial to the Fray Bentos variety?

 

[alb40]

I had the same dilema with the old boat i had. The mast wouldnt fit under the bridge.

I didnt like the mast, so it did a grand job of removing it for me in double quick time! /forums/images/graemlins/grin.gif

 

[Giblets]

You could always try running under the bridge at high speed and thereby, utilsing the "squat" effect, pull the boat down closer to the river bed. /forums/images/graemlins/grin.gif

 

[Captain Coochie]

Ive heard a few stories of people opening up to get the wheelhouse under a bridge , but then ive also heard that they can and will drop the river levels if you talk to them and tell them what times and dates your going to be needing it .
Must be true , a bloke in a lock told me . /forums/images/graemlins/grin.gif

 

[carlton]

Be careful here - I once clipped the wheelhouse roof going under a bridge after I'd calculated 2" clearance. It was a bank holiday, so BW decided that, as it would be a busy weekend they'd better drop an extra three inches of water into the basin..........without actually telling anyone........

Luckily it only cost me £40 to get it fixed.

 

[woody001]

How about you go on a day of high air pressure??
Would that make much difference?

 

[Andrew_Fanner]

That's Cannon Lane Bridge, last one before Tonbridge Lock and the last bridge you will get under, if you worry about that one then Big Bridge is "no chance". Look for the scrape marks left by All Gold's anchor light last August, had to drop the mast when I found that it was 1/2" too tall after some recent rain.

What's the tallest bit of the boat?

After last year's scrape, with cracked light glass and a bent bracket my father made a facetious remark about leaving the paddles up at Eldridge's Lock for an hour or so I asssume it chucked it down in Kent today as it did in Essex.