Using Ballast To Get Under Potter Bridge

[ColinL]

Hi, I have an air draft of 6’3, so only go under when’s it’s 6’5 or more, which in last two years has been rare. Is there  any data or formulas that say for example, if I add say 200kg I will be an inch lower in the water , if you know what I mean

[Bernard]

You would need a drawing of your Hull design, 200kg would displace 200 litres of water or 2000 cubic cm, knowing the footprint area of your hull at the waterline you could work it out roughly enough.

[Bikertov]

The Archimedes' principle at work here - https://en.wikipedia.org/wiki/Archimedes'_principle

[Mouldy]

12 minutes ago, Bikertov said:

The Archimedes' principle at work here - https://en.wikipedia.org/wiki/Archimedes'_principle

Richard Digance explains it quite well in verse 4:

https://youtu.be/x4oiX7yrdDA?si=iJ1TD3khr5FMvtF3

 

[MauriceMynah]

There's a flow chart that may help.

Is clearance enough?

No, go into pub and wait.

Yes, go into pub and celebrate. 

[annv]

Hi Why not ask the Pilot at the bridge. John

[pastboating]

Or Martham Boats, I have seen heavy weights on the bank by the bridge they used when clearance was tight.

[Wonderwall]

Or, after my festive indulgence, you could just invite me on your boat 🤭🤭🤭🤭🤭

Or ask Grace on board with her weekly wine supply 😜. 

[Bikertov]

20 minutes ago, Wonderwall said:

Or ask Grace on board with her weekly wine supply 😜. 

Really ?

They are just trying to ballast it down a few inches, not sink the bloomin thing !

[BrundallNavy]

I regularly used water to ballast Nipper down to get under potter. It went from 6-5 ft aircraft to 6-1 ft at the lowest. 

[BroadAmbition]

I would be concerned at letting too much water into the bilge and getting to the point of no return 

Basically we and 'B.A' are stuffed, I doubt I'll ever get her t'other side of that bridge again now and if by some miracle we did, then I'd be mightily concerned about getting her back again.  She was of course designed to pass under PHB back in the days when the Lower Bure was maintained correctly.

I need 6ft 5" then we are fine all day long but could squeeze her through at 6ft4"

Griff

[Bikertov]

Presumably, even if you could in theory add a load of ballast to make your boat sit lower to get under varioua bridges, you would have to take into account your through hull fittings ?

I know I have about 4 fittings not that far above the natural waterline, so would risk a bit of an ingress if I sat too low in the water  

[grendel]

Having been a party to an unintended ballasting event last year, I have to say that 4" of water in the front cabin is no fun thing to clean up after. And that your Hull design may well mean that only one end of the boat sits further in the water.

[Vaughan]

It is unlikely that a boat would sink by ballasting as modern ones have to have all skin fittings 40cm above the waterline, unless they have an accessible sea cock inside near the skin fitting.  Older boats such as Bounty have skin fittings at the waterline but these go straight up to the galley sink or washbasins, so would not take water until the decks were almost awash!

All the same, as modern boats are built with very little access to the bilges, it would be impossible to make sure that all water had been pumped out afterwards and was not hanging around in under-floor compartments that you can't get at.

I think the best way is Richrdsons' method, of putting big plastic oil drums on deck and filling them with river water from a motor pump.

[Bikertov]

1 hour ago, Vaughan said:

It is unlikely that a boat would sink by ballasting as modern ones have to have all skin fittings 40cm above the waterline, unless they have an accessible sea cock inside near the skin fitting.  Older boats such as Bounty have skin fittings at the waterline but these go straight up to the galley sink or washbasins, so would not take water until the decks were almost awash!

Interestingly, on my boat (a 1996 Broom 29), the galley sink and washbasin sink in the heads both drain to skin fittings under the hull, ie below the waterline. I double checked this recently by pouring water down and seeing where it drained from (I'm out the water for the winter)

The shower tray however, being at/below the waterline, drains to an open sump below the floor panels, and from there is pumped out of a fitting above the waterline.

[Smoggy]

I hope they have working seacocks on, and work them from time to time or likely to seize.

I have 4 below hull fittings and close them whenever I leave to boat on it's mooring. 

[Vaughan]

1 hour ago, Bikertov said:

The shower tray however, being at/below the waterline, drains to an open sump below the floor panels, and from there is pumped out of a fitting above the waterline.

This is the eternal problem with shower trays : they are below the waterline!

Which is why traditional hire boats all have a "shower button" which , when pressed, empties the shower tray by way of an electric pump. The good old Jabsco "Water Puppy" was good for the job!

But it will still not sink the boat as the Water Puppy has a vaned impeller which acts, in fact, as a non-return valve. 

[Bikertov]

35 minutes ago, Vaughan said:

This is the eternal problem with shower trays : they are below the waterline!

Which is why traditional hire boats all have a "shower button" which , when pressed, empties the shower tray by way of an electric pump. The good old Jabsco "Water Puppy" was good for the job! 

Mine has a automatic pump with a float switch, like a bilge pump.

[Mouldy]

1 hour ago, Bikertov said:

Mine has an automatic pump with a float switch, like a bilge pump.

We have a similar set up on NL.  The container looks like a Tupperware box, but I know it isn’t, with a float switch and pump mounted inside.  The shower drains into the box and water is then pumped out through a fitting on the starboard side.

Our galley sink and basin in the heads both drain through fittings above the waterline on the port side, marked in the photo.

[Broads01]

So Malcom, with a bit of ballast you'd slip under Potter no problem 😊. 

[dom]

6 hours ago, Bikertov said:

Mine has a automatic pump with a float switch, like a bilge pump.

It's a typical arrangement, but always strikes me as particularly inelegant. You'd think by now someone could have come up with a quiet pump which'll run in free air without damage, which could be connected directly to the tray outlet. Or even a pump which only operates when there's water in its incoming line.

[Mouldy]

1 hour ago, Broads01 said:

So Malcom, with a bit of ballast you'd slip under Potter no problem 😊. 

Simon, we sometimes struggle to get under Ludham!  I think the rudder would be digging in the mud before we had enough ballast to negotiate Potter!  The last time we went under ‘that bridge’ was around 2006, when we hired Grenada Girl from Summercraft.

[kpnut]

7 hours ago, Bikertov said:

Mine has a automatic pump with a float switch, like a bilge pump

At least you don’t have to warn guests that if they’re in the shower too long, they’ll flood the floor as it comes up over the shower tray without them noticing. 
Mine simply pumps out with a jabsco like Vaughan said, after I’ve finished. Makes a racket. I always think everyone nearby will hear it too. 

[Smoggy]

Diaphragm pumps will happily run dry but mine sounds like a randy duck....

[Broads01]

13 hours ago, Mouldy said:

Simon, we sometimes struggle to get under Ludham!  I think the rudder would be digging in the mud before we had enough ballast to negotiate Potter!  The last time we went under ‘that bridge’ was around 2006, when we hired Grenada Girl from Summercraft.

Yes I know, I was only being silly. Joking apart, do you think passage under Ludham for boats with your airdraft and similar (many hundreds of them) is getting harder?