Richardw's Narrow Boat Project -BILGE AND BALLAST

[richardw66]

Many of you have been following my 'Plate Thickness' thread and now working from the bottom upwards I wish to discuss the next subject of 'Bilges and Ballast.

Do we really need to have a Bilge under the floor of a Flat Bottomed Boat? What's it for?

Ballast: concrete paving slabs, engineering bricks and old bits of pig iron; what's that all about? Is ballast just there to weigh the boat down and increase the draught or can some other use be extracted out of it?

Of course you can guess by me asking the question that I already have my own answer prepared but I want to hear your suggestions first.

 

[DCockey]

One reason for a bilge is to keep the cabin sole/floor dry if any water gets into the hull from rain or other source.

I mentioned one reason for ballast in your other thread. Depending on where you want to travel in the boat there are maximum "air draft" (height above water) limitations. Working from the top down if standing headroom is desired then this implies a minimum draft requirement, which for a given hull shape and length implies a minimum displacement. Ballast may be needed to achieve that displacement.

 

[richardw66]

One reason for a bilge is to keep the cabin sole/floor dry if any water gets into the hull from rain or other source.

I mentioned one reason for ballast in your other thread. Depending on where you want to travel in the boat there are maximum "air draft" (height above water) limitations. Working from the top down if standing headroom is desired then this implies a minimum draft requirement, which for a given hull shape and length implies a minimum displacement. Ballast may be needed to achieve that displacement.

David,

I read your previous response and was writing a reply as you posted this one.

Thank you for the input. It is totally appreciated. You've said what the ballast does and it is all undeniably correct but how can I put this? I really don't mean to brag and I know I keep going on about seeing myself as fresh pair of eyes bringing in 'new ideas' which are obviously not new to me because I encounter them on a daily basis in my trade but I think would transfer over to boat building

On that note; I'll wait for a few more out of the box thinking ideas of what else ballast can be used for and then I'll post my suggestion tomorrow along with some detailed drawings and then you can all tell me if I'm mad or not.

 

[DCockey]

Batteries could be substituted for ballast if electric propulsion of some form is to be used for which the batteries are needed, though batteries would required more space.

The thickness of the bottom could be increased which would have the advantage of requiring less height for the same weight in concrete or "loose" ballast, and reducing/eliminating any potential internal corrosion problems.

 

[TANSL]

The fixed ballast, in my opinion, should only be used in order to lower the c. of g. ship and make it more stable. From one point of view we can say that the more fixed ballast, lower payload, which is often not interesting to the shipowner. Sometimes the need to place fixed ballast denote an error in the project.
Many boats require varying ballast to reach a depth that allows the propeller to work at the correct depth.They are two completely different issues, fixed ballast and variable ballast.

 

[DCockey]

TANSL, what solution other than fixed ballast would you suggest for a narrow boat which needs sufficient draft to provide head room while meeting air draft (height above water) requirements? Should the scantlings be increased beyond what is otherwise needed just to add mass so that the draft is sufficient? Perhaps water ballast could be used but the water tanks would require significantly more volume than fixed ballast such as concrete, steel bars, pig iron, etc. The additional volume for water ballast could otherwise be used for accommodations.

The boat Richard is designing is a pleasure boat for operation on UK canals, not a cargo vessel. Operation on the UK canals introduce some requirements which are not present for most other vessels. The air draft limitations can require ballast in excess of what would be needed for stability.

 

[richardw66]

The fixed ballast, in my opinion, should only be used in order to lower the c. of g. ship and make it more stable. From one point of view we can say that the more fixed ballast, lower payload, which is often not interesting to the shipowner. Sometimes the need to place fixed ballast denote an error in the project.
Many boats require varying ballast to reach a depth that allows the propeller to work at the correct depth.They are two completely different issues, fixed ballast and variable ballast.

That is very interesting, so varying ballast would also include the fuel and the water tank? I realise a narrow boat is not a ship but the basics of payload will still apply. At this early stage of development with the thick plates I estimate the shell will weigh 10 tonnes and have a draft of 280mm so in order to achieve a draft of 600mm I would need to increase the total weight of the ballast, fit-out and liquid storage by at least another 10 tons. There is a response in the plate thickness thread that suggests a boat should never be over weight so if I reduce the plate thickness I would have to increase the amount of ballast to maintain the same draft depth. I suppose the ballast material can be cheaper than the steel so there is a balance to be arrived at between cost and centre of gravity resulting in better stability.

 

[TANSL]

A pleasure boat not need to carry a heavy load but certainly carries load (passenger, luggage, supplies, etc.). A pleasure boat, or any boat in this world, needs as much space as posible and if spaces are filled with solid ballast it lose space for passenger services. Like you guessed, increase the scantlings may be a solution. A thicker keel could be very convenient. A thicker sides can be very convenient. Reinforcements on the sides, which protect the ship blows with other boats, can be very convenient.
It is difficult to speak of a boat you do not know but, before completing something concrete and metal pieces, there are many solutions that can be applied. I'm sure you know them as well as I do.

 

[richardw66]

A pleasure boat not need to carry a heavy load but certainly carries load (passenger, luggage, supplies, etc.). A pleasure boat, or any boat in this world, needs as much space as posible and if spaces are filled with solid ballast it lose space for passenger services. Like you guessed, increase the scantlings may be a solution. A thicker keel could be very convenient. A thicker sides can be very convenient. Reinforcements on the sides, which protect the ship blows with other boats, can be very convenient.
It is difficult to speak of a boat you do not know but, before completing something concrete and metal pieces, there are many solutions that can be applied. I'm sure you know them as well as I do.

Sorry boys but this reminds me of when I first got into the construction industry and first came across the word 'soffit'. What is 'scantlings'?

 

[TANSL]

Variable ballast can be used, effectively, to offset the weight lost by consumables fluids and also can be used to get a better boat trim.
But the liquid ballast may pose stability problems due to the destabilizing effect of free surfaces in tanks
Scantlings : physical properties of plates and profiles

 

[DCockey]

"Scantlings" are the sizes and materials of the parts of the vessel which contribute to the structure.

 

[PAR]

Other then trimming up the hull, why would you decrease the capacity of a vessel, with these perimeters employing ballast? For ballast to effectively improve the vessel's stability, it'll need to be canted well to one side, which is rather unlikely in this type of craft. I generally agree with TANSL and will add that well designed, baffled tanks can mitigate the free surface effects of their contents.

Back to the ballast issue. The design should be scaled, including it's narrow beam to suit the GA. It doesn't need to be overly heavy, nor ballasted to pull it down to preferred lines. The design should offer preferred lines at it's cruising load displacement, not be forced to them. Some argument could be made for tankage to accommodate varying loads, but a craft like this, even if unusually narrow shouldn't be terribly overwhelmed, from a light to heavy ship loading situation. If it is, someone screwed up in the design process or an unusually high light to heavy displacement is incorporated, which doesn't seem to be the case with this design.

 

[DCockey]

That is very interesting, so varying ballast would also include the fuel and the water tank? I realise a narrow boat is not a ship but the basics of payload will still apply. At this early stage of development with the thick plates I estimate the shell will weigh 10 tonnes and have a draft of 280mm so in order to achieve a draft of 600mm I would need to increase the total weight of the ballast, fit-out and liquid storage by at least another 10 tons. There is a response in the plate thickness thread that suggests a boat should never be over weight so if I reduce the plate thickness I would have to increase the amount of ballast to maintain the same draft depth. I suppose the ballast material can be cheaper than the steel so there is a balance to be arrived at between cost and centre of gravity resulting in better stability.

The mass beyond what is needed for the structure, accomodations, etc can be added in a variety of ways - increased thickness of the metal structure, "loose" weights in the bilge, concrete poured into the bilge, etc. The choice of which to use should be based on impact on the overall design including the effect on (in no particular order) accommodations, layout and positioning of systems, access for general maintenance, maintenance required, longevity and stability.

 

[Ad Hoc]

I haven't followed this thread much, but...This is one I did many years ago.



It changed many times from the GA shown. But in each variant from concept to finish i did a mixture of solid ballast and increased scantlings. The combo depends upon the final weight estimate and of course the hulls buoyancy for the layout/hull.

This one I did was built in aluminium, hence the requirement for ballast fro reasons that you have already noted. The final layout i reduced the solid ballast from 25 tonne down to 5 tonne by using a combo simply for such air-drafts requirements as well as being able to cross the Channel on a calm day to France.

 

[DCockey]

Other then trimming up the hull, why would you decrease the capacity of a vessel, with these perimeters employing ballast?

So that it fits under bridges and through tunnels with standing headroom inside. If the draft is not sufficient then the top of the cabin can not be high enough to stand up inside and still fit under the bridges and through the tunnels.

Very simple but perhaps not obvious to those unfamiliar with UK canals and the boats used on them.

For ballast to effectively improve the vessel's stability, it'll need to be canted well to one side, which is rather unlikely in this type of craft. I generally agree with TANSL and will add that well designed, baffled tanks can mitigate the free surface effects of their contents.

Back to the ballast issue. The design should be scaled, including it's narrow beam to suit the GA. It doesn't need to be overly heavy, nor ballasted to pull it down to preferred lines. The design should offer preferred lines at it's cruising load displacement, not be forced to them. Some argument could be made for tankage to accommodate varying loads, but a craft like this, even if unusually narrow shouldn't be terribly overwhelmed, from a light to heavy ship loading situation. If it is, someone screwed up in the design process or an unusually high light to heavy displacement is incorporated, which doesn't seem to be the case with this design.

Ballast is usually included as an inherent part of recreational narrow boat design to get the needed draft and corresponding displacement, not as an add-on fix.

The first requirement for a narrow boat is that it fit on the canal system. Narrow boats have to be designed to fit inside a "box". The width of the box is just over 7 feet / 2.1 meters. Wider than that and boat won't fit through the locks on canals designed for narrow boats. Overall length of the box depends on the canal systems which the boat needs to fit. Again, too long and the boat won't fit through the locks and may not be able to turn around.

Another key part of the box is the height of the box above the water, or the air-draft. This is determined by the minimum height of fixed bridges and tunnels. Again, too high and the boat won't fit.

There are also draft limitations which determine how far the bottom of the box is under the surface of the water.

These limitations result in most narrow boats have a close to rectangular cross section.