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Posted: Tue May 20, 2008 1:08pm

Post Subject: ballast

Hi on recently purchasing a sailaway floored and ballasted by the builder i found it necessary to remove the floor,on removal i found support battens and ballast in the form of brocken concrete slabs.The slab pieces (A SINGLE LAYER )sit very loosely on perforated bitumen felt,i am concerned that this is not adequate ballast.On measuring the gap from the top of the ballast to the underside of the floor which is only 25mm i can not add any extra ballast unless the floor height is increased.Is a single layer of slab an adequate amount of ballast if not i will have to increase the floor height and provide access for extra ballast in the future.What would you consider as a adequate measurement for the maximum internal height between floor and underside of ceiling.The narrowboat is a 57ft cruiser.Many thanks Rod the beard.


Posted: Tue May 20, 2008 1:58pm

Post Subject: ballast

Well Rob, the first thing is that one does not specify ballast by how many layers. I certainly do not like the sound of broken slabs sitting on perforated bitumen felt, but I expect budget had a lot to do with what boat you bought and basically you get what you pay for. Ideally you need to avoid water being trapped between the slabs/felt and the base plate otherwise you may well get rusting. However if the base plate is 10mm thick you will have very many years before such rusting becomes any sort of issue. Ideally your ballast would be placed on thin non-absorbent upstands like floor tile spaces thus allowing any water that gets into the bilge to eventually flow to the lowest point. More importantly it will also allow a little air circulation to dry any moisture out. Ideally the whole of the base plate will be "painted" in some way and if coating will not be attacked by it then a good coat of something like Waxoil before fitting the ballast. However there are all sorts of practices in this area and it would be hard to decide which is correct and which is wrong. How much ballast you will need depends upon the all up weight of the boat when completed and also where abouts that weight is located. You do need to make provision for placing "trimming" ballast once the boat is completed. This is to balance any particularly heavy parts of the fitout that are not balanced by something on the other side of the boat or to get a suitable fore-aft trim. I have heard that boats with 15mm to 20mm base plates do not need any main ballast. Now for the amount ballast. Your first task is to get the uxter (swim) plate at least one inch below the static water level. Within reason the deeper the better because under certain conditions the prop can draw air down and lose efficiency, making a growling noise in the process. Next you need to ensure your air draft is low enough to allow you to cruise all the areas you may want to. Finally you need to ensure that you have not increased the draft to such an extent you keep running aground on shallow canals. Once the main ballast has been placed and the boat completed you need to trim it. I find a slight trim down by the stern usually gives optimum handling. The pivot point moves up or down the boat according to angle of trim. You just have to find a trim that suits you. Then you have to trim for list. Often you need to place some extra ballast opposite the holding tank so the boast lists away from the tank when it is empty and towards it when full. You must also consider the internal headroom before you go raising the floor. By all means raise the floor if you have plenty of headroom, otherwise cost some alternative ballasts. You can buy steel ballast - look in the back of the magazine. If you can find someone demolishing a building with a lift in it I understand the lift balance weights make good ballast. For the final trimming of my boat I bought a quantity of scrap lead, but if you do this you must ensure it is held away from the steel otherwise galvanic action might start to eat the hull away. I know you wanted a simple "you need X layers" answer, but I am afraid there is not one. The weight required could be worked out, but for that you would need to know the weight of the boat and unless the crane driver who lifted it told you I doubt you know that. Tony Brooks

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