The Earth Moved

[system 4-50]

Several months ago I installed an 18mm bulkhead 4ft wide near the middle of my boat to be a wall of my bathroom.

I fixed it firmly to the 25mm ply floor with 6 metal angle brackets and less firmly to the ceiling with just 3 of the same brackets.

I marked a point at the top of the arc at which I calculated/measured that the centrally mounted Morco flue should be and marked the ceiling at this point as well.

I then went away and did more pressing things like boxing in the gunnels to prevent the buckets of condensation running off them.

The other day I noticed in passing that the ceiling brackets were all twisted, and the centre line marks on the bulkhead and ceiling no longer lined up.

I monitored this over a few days and this is what I came up with.

The sun comes up in the morning (you may have noticed this) and shines straight on one side of the boat. Feeling the boat, the cabin sides feel cool (but the big bus windows feel hot) and the ceiling feels hot. The day is cold but clear. The bulkhead mark is 6mm to the left of the ceiling mark, towards the sun.

By late afternoon the bulkhead mark is 2mm to the right of the ceiling mark, also towards the sun.

 

Is this normal? 8mm movement seems a lot to me, and its only May!

[Ex- Member]

Several months ago I installed an 18mm bulkhead 4ft wide near the middle of my boat to be a wall of my bathroom.

I fixed it firmly to the 25mm ply floor with 6 metal angle brackets and less firmly to the ceiling with just 3 of the same brackets.

I marked a point at the top of the arc at which I calculated/measured that the centrally mounted Morco flue should be and marked the ceiling at this point as well.

I then went away and did more pressing things like boxing in the gunnels to prevent the buckets of condensation running off them.

The other day I noticed in passing that the ceiling brackets were all twisted, and the centre line marks on the bulkhead and ceiling no longer lined up.

I monitored this over a few days and this is what I came up with.

The sun comes up in the morning (you may have noticed this) and shines straight on one side of the boat. Feeling the boat, the cabin sides feel cool (but the big bus windows feel hot) and the ceiling feels hot. The day is cold but clear. The bulkhead mark is 6mm to the left of the ceiling mark, towards the sun.

By late afternoon the bulkhead mark is 2mm to the right of the ceiling mark, also towards the sun.

 

Is this normal? 8mm movement seems a lot to me, and its only May!

 

That is quite a lot of movement, plywood is usually very dry and quite rigid so I would suggest it's expansion and contraction of the structure the ply is fitted to. I've always used stud walling on the boat and cut the 9mm panelling ply slightly short, it's all covered with Oak moulding so you don't see the gaps. A stud wall will cope better with any movement in comparison to plywood fitted tight.

Edited May 21, 2012 by Julynian

[Chalky]

It seems an awful lot. Is the boat sitting in the water or on a hard standing?

 

Out bulkheads are 18mm ply attached on three sides and I've never known them to move.

[nicknorman]

Surely this is expansion of the steel, nothing really to do with the wood. One of the arts of boat building (and I am very definitely no expert!) is the marrying up of a wooden interior structure that expands/contracts only very slowy if at all (with changes in humidity), with a large steel structure that expands/contracts a lot and rapidly when exposed to the sun (especially with a dark paint scheme). If you just attach the wood rigidly to the steel at many points, something has to give in this war and it will be either the wood or the fitting, but probably never the steel structure. I believe the trick is to allow for this movement by not rigidly attaching eg 1 sheet of ply in too many different places, rather than trying to lock everything together and hope the wood stops the steel from moving (no chance!).

[system 4-50]

Its on the water and its painted dark green.

[Ex- Member]

Its on the water and its painted dark green.

 

Is the roof dark Green, you'll be attracting a lot of heat if so and could explain lots of movement.

 

 

 

[Masquerade]

If my memory serves me right, a 60 foot steel boat would expand in length approximately 1/4" over a change of 20 degrees C. However, who knows what stresses have been welded into the boat and which way it would move? Douglas fir would be about half this figure across the grain.

 

I would imagine the boat is twisting in the sun more than expanding in it's length. You'd be amazed what stresses can be put into steel sheets welded together.

[system 4-50]

If my memory serves me right, a 60 foot steel boat would expand in length approximately 1/4" over a change of 20 degrees C. However, who knows what stresses have been welded into the boat and which way it would move? Douglas fir would be about half this figure across the grain.

 

I would imagine the boat is twisting in the sun more than expanding in it's length. You'd be amazed what stresses can be put into steel sheets welded together.

 

We're talking about expanding in its height/width here...

[nicknorman]

If my memory serves me right, a 60 foot steel boat would expand in length approximately 1/4" over a change of 20 degrees C.

 

With dark paint and May sun, probably much more than 20 deg C differential - sunny side too hot to touch say 60 deg, shady side in this cold airmass say 15 - all in the upper part of course, meanwhile the submerged bit remains at canal temperature, probably less than 10 at the moment. It is a miracle that NBs don't twist themselves to destruction after a season!

[bowten]

If you mark more than 2 points you could use it as sun dial.Expansion of the shell can be quite a bit but not this much,look for other reasons.

[bizzard]

Many people will probably have noticed that an accurately fitted internal transverse door in an all steel shell can jamb in the winter and open and close quite freely during the summer.

[Masquerade]

We're talking about expanding in its height/width here...

 

Exactly, hardly noticable over 7ft. Most likely the heat is exaggerating the stress in the shell when it was welded up and twisting in the heat. Stick a thermometer on the sunny side, then the shaded side, you'll get the picture then. 20 degrees ambient will be a lot higher as a surface temp.

 

If it helps I fix my bulkheads to the floor and lock the sides and uppers with the materials I used as my coverings, ie t&g for the roof and above the gunwhales and ply below. You can use temporary blocks to lock it in place until ready to cover the walls and ceiling.

 

As has been stated it can be a bit of a faff to get the equilibrium between the different materials right. It's not unusual to encounter boats with doors that jam in the winter due to the moisture expanding the wood and cold contracting the steel only for the same door to swing open and fail to latch with too much clearance in the height of summer. If you look closely at a lot of boats, you can work out which ones were fitted out in winter and which in summer!

[Ex- Member]

Many people will probably have noticed that an accurately fitted internal transverse door in an all steel shell can jamb in the winter and open and close quite freely during the summer.

 

I haven't had that problem although I haven't fitted all doors yet. The one door I have fitted is the engine room door and did so to a very tight tolerance just a 1mm gap all around. This door has never jammed. I also fitted a mortice lock and the lock has never gone stiff or failed to open or close. This is set into a steel bulkhead on a 10/6 w/b however I did use 25mm hardwood ply which also had some 10mm Oak panelling added around the boarder and Redwood bead to seal the ply edges. The steel bulkhead is constructed with 1.1/4 inch tube frame and 4mm steel and clearly has little movement, or movement that affects the door. The boat is still in Red primer too so does get very hot in the summer.

 

The rest of the doors I'll make in 25mm hardwood Ply again as it will stay flat and not warp especially if you add some hardwood trim, and it won't expand and contract either, all door frames are Oak as well so I'm hoping to avoid sticky door situations if future.

 

 

 

 

[Boaty Jo]

Don't attach a bulkhead to the ceiling (or side wall). Attach to the floor and let the bulkhead sit between 2 battons so there is room for expansion of the hull.

[keble]

Many people will probably have noticed that an accurately fitted internal transverse door in an all steel shell can jamb in the winter and open and close quite freely during the summer.

Yup - I'm one of them!