I have had a bit of a disaster this easter weekend with my nsm4 in terms of the rear built in buoyancy tank (3389). 
A safety boat crew member put their knee through the plywood tank, when I had other more pressing issues on at the time!
I am looking at taking out the remainder of the tank and fitting bags as an alternative, as suggested by other Merlin club sailors of the 150lb variety (bags, not advisors!).
Has anyone got any knowledge of any problems that I may encounter?
What is the normal process for securing these items to the hull and what size of bags are normally used? On the crewsaver site, the sizes are in metric in terms of dimmensions only?

Major problem - the tank is actually very structural!

It replaces (And is probably more effective than) a large knee that normally runs down each side and then joins onto the back of the centreboard case.

You can take the tank out, but you will need to put this knee in else the boat will lose a lot of it's stiffness (you can see where it should go as there will be two small knees under the side deck exactly where the larger full width one would go on a non tank boat). Putting the knee in is probably going to work out more difficult/expensive than the relatively simple repair to the plywood which doesn't have to be all that pretty as the boat is painted inside anyway.

Kev
I would suggest you claim on your insurance and have your boat repaired by someome like Laury Smart for example. Not only will it cost you more to modify, but the second hand value will drop also.
In short use your insurance and have it repaired and continue to have fun sailing.

Kev
Another thought, talk to your club or the club you were sailing at. This incident may well be covered on THEIR insurance.

Where are you based Kev?

Hi chaps and thanks for the responses. 
Chris, I'm based very close to and sail at Blackpool and Fleetwood Yact Club?

I remember White Whale when she sailed at Banbury until recently and would agree with Chris and others that you should be looking to get the rear tank repaired properly rather than removed.  It is important structurally in view of the low transom.

Suggest that you should be claiming off the club's insurance policy which they must have to cover the rescue boats (and the actions of those in them) then get it professionally repaired.

If it's the boat I think it is, 'Black is Back' give Charlie Hadfield a call. He had her designed originally and will know if the tank was structural or not. His number should still be in the book as a class member, currently owns 3558.

I remember she used to be a pig to drain out and fixing the rudder on without putting your knee through the ply was equally as frustrating.

Kev,

Bad luck!  That boat seems to attract clumsy rescue boats.  She's also had considerably worse crashes!

I've left you a voicemail on your mobile, give me a call.

Regards

Jon

Lawrie Smart's a bit of a haul, i'd suggest ringing Phil Scott 07973 720 732 he'll sort you out.

Based just north of birmingham.

lots of good advice from you all out there, Cheers Kev.............

Another thought initially was to remove the plywood top of the tank, but keep the rib that supports it to maintain the rigidity of the hull. After a capsize, the boat is very difficult to empty due to the step at the rear tank, which takes ages to drain. Hence the suggestion initially was to beef up the thickness of this rib and cut two drainage slots into it for efficient water dispersal. Any thoughts again would be greatly appreciated? Kev............

Kev,

This is an old merlin chesnut not restricted to that design.  Basically whan you get the hang of it you'll stop capsizing in lighter conditions.  That boat drains actually drains better because once most of the water is out, the transom flaps are higher than usual and water stops sloshing back in.

It is however a real pain in the a**e to drain the boat when she's left in the boat park so you end up leaving here flat, rather than perched up so that the water can drain from the bailers, but then water sits around the bailers and the CB case.

You could get some 1/2 - 3/4 inch tubes fitted from the front of the tank to the transom either side of the keel when you get the tank repaired.  I still have the carbon tubes and bungs I acquired for the job but couldn't be bothered because I would have had to take the top of the tank off.  Your friendly rescue boat crew has done that for you!  Back is Black (the boat mentioned above) had this modification and it was a good one but it won't help the boat drain after a capsize.

Jon

Removing the plywood top won't work either - it makes up the box section that provides the rigidity.

Hi
I took the rear tank out of a Smokers Satisfaction some time ago. True it was a real pain making a new stiffener which I made in two pieces with a scarf joint over the Hog. I was fairly lucky in that the tank was fitted after the decks and removing the tank left the inner gunwhales intact including inlays right up to the transom. I followed this boat with a Summer Wine with rear tank, incidently the rear tank does make it a comfortable boat to sail. This one had inspection hatches in the tops which is another way of repairing the damage and makes ideal storage for those energy bars and drinks when back to back racing.
Hope this is of some help.

Again, thanks for all the advice.
I got my mate to take a look at the damage, who is a carpenter who is currently renovating all the period wood work in my house. 
He tells me that either option in terms of sorting the boat are fairly straightforward. 
Anyway, I will have to make a decision this next few days as to whch route I am going to take! (eek)
Kev

Chris - technically it was replacing it afterwards that put me off!