How much ballast?

[ronie]

Hi again,

 

We are hoping to ballast the boat this week, and I know that the ballast will need altering when we are in the water, and need trimming, but is there a formula somewhere to work out how much ballast you need?

 

We are planning to use high density concrete blocks because they are cheap and will fit nicely in the spaces in the floor. We will have room on top of them for concrete slabs or metal if needed to trim the boat. We reckon we will get about 4 tonnes in the spaces without adding concrete slabs etc. Is this likely to be a reasonable amount???

 

Thanks

 

Ronie

[Keeping Up]

As the boat is 70ft long, each ton of ballast will lower the boat an extra 1" in the water.

 

The amount of ballast you need will depend on how heavy the shell is in the first place, and how much extra weight you will be adding during the fitout. Plus of course how deep you want it to be after you've finished.

 

For example if the shell weighs 10 tons, and the fitout (including engine) weighs 5 tons, then it would be only 15 inches deep in the water. So to achieve an average of 18 inches you would need 3 tons of ballast - positioned somehow so as to bring the stern down to 21" and the bows therefore to 15" (average of 15 and 21 is 18) remembering that the engine is one of the heaviest parts. On the other hand if the shell weighs 13 tons and the fitout including engine will be 7 tons, then you'd have an average depth of 20" wirthout any ballast at all.

[ditchcrawler]

Hi again,

 

We are hoping to ballast the boat this week, and I know that the ballast will need altering when we are in the water, and need trimming, but is there a formula somewhere to work out how much ballast you need?

 

We are planning to use high density concrete blocks because they are cheap and will fit nicely in the spaces in the floor. We will have room on top of them for concrete slabs or metal if needed to trim the boat. We reckon we will get about 4 tonnes in the spaces without adding concrete slabs etc. Is this likely to be a reasonable amount???

 

Thanks

 

Ronie

I put a couple of ton in our 57 foot boat sitting at about 2ft 4in level in the water. I had to use iron ingots as I needed most of the weight in the front half of the boat.

I knew the weight of the shell and to that I guestamated the weight of thye fit out. half ton engine X sheets 18 mm ply etc. and the guest how much balest was required.

If you know the length of your boat, swims etc and how deep you want to sit, you should be able to estamate the total weight.

[Rick-n-Jo]

Hi again,

 

We are hoping to ballast the boat this week, and I know that the ballast will need altering when we are in the water, and need trimming, but is there a formula somewhere to work out how much ballast you need?

 

We are planning to use high density concrete blocks because they are cheap and will fit nicely in the spaces in the floor. We will have room on top of them for concrete slabs or metal if needed to trim the boat. We reckon we will get about 4 tonnes in the spaces without adding concrete slabs etc. Is this likely to be a reasonable amount???

 

Thanks

 

Ronie

 

First you need to know (or estimate) the weight of your shell, engine and fittings. Thickness of baseplate etc. will obviously make a big difference, but for a typical 10/6/4 shell at 70' 9-10 tons before ballasting and fitting out is somewhere near.

 

A good rule of thumb fo a 70' narrowboat is that it will sink 1" into the water for every ton of weight. So if the final design draught is say 20" (average) you can see that another 10 ton needs adding to give a final displacement of 20 tons. 20-23 is typical for a 70 footer. That 10 tons obviously has to be split between ballast and the weight of all the fit out materials, tanks etc.

 

It's surprising how much more, proportionately, a longer boat needs in ballast compared to shorter ones. This is because there is more full width hull compared to the less bouyant pointy ends.

 

You'll have to do your own estimates. We put about 5 tons of paving slabs in our boat, but we have some particularly heavy items on board. Or you could try your 4 tons and see how she floats, then use the 1" rule to decide how much more to add.

 

Rick

[nb Innisfree]

Hi again,

 

We are hoping to ballast the boat this week, and I know that the ballast will need altering when we are in the water, and need trimming, but is there a formula somewhere to work out how much ballast you need?

 

We are planning to use high density concrete blocks because they are cheap and will fit nicely in the spaces in the floor. We will have room on top of them for concrete slabs or metal if needed to trim the boat. We reckon we will get about 4 tonnes in the spaces without adding concrete slabs etc. Is this likely to be a reasonable amount???

 

Thanks

 

Ronie

 

After much conflicting advice we calculated 2.3t for our 60' narrowbeam at 22" draught, which proved to be spot on. We had more trouble with fore and aft balance, all sorts of advice, again we worked it out ourselves and later (already launched by then!) heard about a ratio of 2 thirds forward 1 third aft, again spot on. this gave a boat trimmed almost level.

[ronie]

Thanks everyone!

 

It sounds like 4 tonnes is a good start, and once the engine and fitout is in will see what else it needs - it will have 2 adults and four kids living on it permanatly with all their associated tat so I guess that will help!!

 

Thanks for the tip about the ratio of fore to aft will bear that in mind, will also be trying to get more in the corridor sides and less in the cabin sides so should be fun - not to mention lifting 288 concrete blocks up into the boat - won't need to go to the gym that day!!!!

 

Ronie

[Martin Nicholas]

The "Barry Wren" method was to fit out completely and then ballast the boat when in the water. You need access to the bilge throughout the length of the boat, but that's no bad thing anyway.

[ChrisPy]

anyone fitting out a new boat (like a sailaway supplied with an Annexe 3 Declaration) becomes the 'boat builder', and may want to consider how they will comply with the RCD stability test requirements.

 

.................. or ignore the RCD and be prepared not to sell the boat for 5 years.