Massive Stem bars ..........Why

Thanks for coming back on this LJ! We actually see a few different approaches when studying different materials for equal application. Some of the rules we have had to use for alu (which is "my material") produce a far stronger structure, than required for a similar GRP part. I have a feeling that we see a reflection of a "metals for commercial use" approach, resulting in more focus on overall redundancy where rules are applied to steel (and to some extent alu as well).

 

It's all in the name. If it had been labeled as a plate stem and I'd actually known that's what you call a stem without a stem bar then I wouldn't have wasted a lot of time running around saying why does this need to be so huge.
The answers I got are like the answers posted above....it needs to be that size because the class society determined that's what's needed !

In reality all non-commercial metal boats could dispense with the stem bar, the plating is always thick enough to give the strength at the stem.

You just have to know if it doesn't have a stem bar then it relies on it's plating and its called a plate stem ! No-one ever told me that at uni.

Why couldn't the rules say something easy like... with a stem bar as the primary member its size is.....or without a stem bar the plating shall be this thick

I wonder how many small metal boats get heavy unnecessary stem bars just because the rule writers overlooked the fact that we didn't know smaller boats didn't actually need them. ?

 

Yeah thanks for pointing that out. The boat I'm building has a 100x6 flat bar stem with 4mm hull plate joining it. I'm really looking forward to doing the inside welds....

PDW

 

Stem bars are a pain, particularly in a curved stem. On a bigger boat if they are the principal structural element they are massive indeed.

But they can be left out, providing usually as a rule of thumb that the bottom forefoot plate thickness is carried up into the stem plating. If you have a large radius rolled plate stem which meets the thickness criteria then there is a centerline web frame as well as horizontal webs.

I've often seen a heavy stem bar along with bow plating plenty heavy enough to eliminate the stem bar. Apparently it should be made clearer in the class rules.

I think it's a problem for people applying the scantlings in Gerr's Elements of boat strength too. They may end up with a very heavy completely unnecessary part that really does nothing but just adds a lot of weight.

Some people take too much structure out and others put too much in.

If in doubt any of these sorts of structures can be omitted if analysis shows that what's left is strong enough. Our own Australian commercial standard USL has always allowed stem bars to be replaced by heavier plate. On small boats say up to 45 feet the plating is nearly always thick enough to omit it.

In pdWiley's case above he probably would have needed to go up to 5mm and add a few small horizontal webs. It's much easier to fabricate. Paint and maintain without the acute half stem angles.

 

Good reason to leave it in, then. A single piece of 100x6 flat bar weighs a lot less than increasing the plate thickness to 5mm unless it's for quite a small area of hull.

Yes it's going to be a PITA to weld in there but I'll manage it. In theory (and I emphasise the theory bit...) I could weld vertical-up from the outside only using E4111 with a small root gap and get 100% penetration but I might also get a lot more distortion than I'd like. Might be getting near time to switch over to the MIG welder.

Almost finished making the flanges & tank tops.

PDW

 

The Dutch even in the 1930's often opted for just a plate stem but they often used 8mm plate on 45-50 foot boats carrying on a foot or two back from the stem. More recently Lloyds certainly by the early 80's allowed the stem bar to be dropped as unnecessary, so it's hardly a new method.

As Lyndon said earlier there are foam cored GRP boats with much weaker stems in collision than a pair of acute bow plates welded together offer. You just need some webs to stop them buckling outward in collision. In collision I think the material is actually better being in the plating by a millimeter or two rather than in a heavy bar the whole structure is lighter but it's just as strong.

If the bottom of the boat were an acute V you wouldn't bisect it with a central vertical keel bar, you'd make the plate maybe a little thicker and put floors in and call the whole thing a keel. The acute angle on many sailboats is just a continuation of that same 'keel'

A lot of steel boat designers seem a bit hung up on stem bars. They will even create a hard to plate stem so as to fit a straight bar when it should have been curved.

A solid rod is a good stem piece to control the shape , it rounds the stem too and is not hard to bend into curve.

 

This is precisely what we do on all our high-speed boats. We have a thick stem bar, bisecting an acute V angle. We do this principally to aid in construction. The stem bar ensures a good quality full-pen weld between the two plates, if the detailing and weld prop details are done correctly. Also during construction the stem bar helps in setting up the frames.

 

Yes I should have said you don't need to rather than you wouldn't
I was talking of that steel motor-sailor in the opening post.

Stems are worth talking about.

With alloy a bar offers good redundancy as well as reducing the stress running through the stem welds.

It sounds like you use the edge of the bar as a weld backing plate with all the edges being bonded with the weld. Then does it only get welded from the outside in one pass, any internal fillets as well ? (You need to write a book. )


In the picture attached of the 60 footer, the stem is just a flat plate with full corner welds. to the adjacent plating.
The flat leading edge can be used to define the frame shape, it curves over the horizontal stem webs which in turn have the longitudinals running into them.
It's all terrifically strong even before the plate goes on and the shape is well defined with a good edge to work to.
The leading edge plate is bent around fairly easily and can be formed in situ.

Otherwise most 60 foot steel displacment boats would need quite a thick deep stem bar that would take a lot of fabricating if it's curved.

 

Hello PD
Gapping the plate away from the stem is an interesting idea, The only thing I could think of that would be a con to the idea is the inside apearance ? If your sides panels land on the sides of the stem and you keep the outside edge inline with the outside corner of the stem bar then the inside corner of the side panels are set back a ways. Nice open V allmost the same as a lap weld . Welding the inside tight V is not that dificult, If you are going to weld uphand in there be real carefull not to under cut on the side panels.
Tom

 

Actually it shouldn't make any difference in weight, it's basically moving the same amount material from the bar into the adjacent stem plate , you just have to restrain the plates from buckling for whatever distance gives the same Z (section modulus).
Your prob would be getting the thin plate fair at the join. But on my example there was 6mm plate on both the bottom and topsides anyway and that let us drop a huge very heavy bar that had to be cut out or press curved.

Whn you weld an acute plate angle inside don't yo get a pocket of ionised condictive gas and the arc wanders all over the place ?

 

Inside appearance doesn't matter as it's up in the chain locker area. Who cares what it looks like as long as it's sound, there's no distortion and you haven't created any stress risers?

I've found that with 5mm plate if I grind it to a single V prep I can burn through the root using a 2.5mm E4111 rod and get 100% penetration. I use a double V prep on 6mm plate and am getting 100% penetration (had to cut a couple of welds open when I stuffed something up so I got to have a look). You need to be careful not to weld too long a bead in one go in the 3mm or 4mm plate and to grind out the bead where you stopped to get a decent start again but it does work. Then in places where it's difficult to get at the seam from the other side at least you've got confidence that the weld is as sound as it's possible to be. If the seam is accessible a light grind and a capping run will give you a nice weld.

I expect that I'll switch to the MIG for the internal weld because it's a lot easier to get 0.8mm or 1.2mm wire into the tight gap than a 2.5mm flux coated rod. I hate doing vertical up welds where I don't have a good view of the weld pool. It's not undercutting, I have the technique to prevent that, it's trying to keep the bead reasonably flat and not get too much metal in the centre. I've gone through a lot of grinding disks and points in a die grinder making sure the welds are dressed nicely.

As for arc wander, good point, dunno. I've not experienced this much with AC stick welding, DC it's more of a problem. If I use a MIG with flux cored wire and a decent extractor fan, probably be OK.

I'm not a professional welder, just did some courses 25+ years ago so while my theory is pretty sound if well out of date, my technique sometimes sucks badly. That's why I have 5 angle grinders and a die grinder

It is a lot easier to set everything up with a stem bar WRT alignment. I follow the designer's plans and as it calls for a 100x6 flat bar, that's what I have used. Left to my own devices I think I'd have used say 19mm or 25mm solid round bar giving the benefit of alignment AND easy welds. Maybe next time if there is one....

PDW

 

In many vessels particulalry large ones there is no stem bar as such, the alternative is what's called a plate stem which is shape supported by webs and may require a thicker plate.

On many smaller metal boats the plating comes together at an acute enough angle to omit the stem bar completely and with no increase in plate thickness either.
The bar is a good aid to construction and then it's often oversized becasue the designer makes the mistake of thinking it's structural and sizes it accordingly. Look up plate stems in whatever class scantling rules you use.

It's a common mistake.

 

well I will not change I would still have the bar to come into a cl bar and bresthooks too So IF that boat had had a bar she would have a scratch
Also in large sailing yacht can you see a stay attached to a 75mm thick chain plate with no stem bar I very much doubt it
Plus where the stem stops and THEN SHE hit then one would really be in the mire In all the structures I submitted to NZ plan approval I could never omit the bar

 

It's a very common area of confusion.
Many designers and builders have never been told there was an option. But all class societies allow for a 'plate' stem.

Small stem bars are sensible but designers will often add a full heavy structural stem bar where it's simply not required even by class rules. That's one reason metal boats can be so heavy when following simple design rules like for example Dave Gerrs scantlings which I don't think even acknowledges that the stem bar is often not even required. You can end up adding hundreds of kilo's of unnecessary weight right where you don't want it.

For example Two 6mm plates coming together on an acute angle with some horizontal webs to keep them in shape and a maybe a smaller 'construction bar' represent a very robust and strong stem for collisions with floating objects, and it's already orders of magnitude tougher than any composite bow structure.

 

Mike
where you have a very steep deadrise and two heavy plates coming together, maybe, but deadrise soon flattens. Take your V take a hammer to it, you can easily deform such. Take your bar, it takes abt 30 tonne to shift a 150 x 16 bar in a press between 600 centres
You see this is the diff between you theory wallahs and us hands on lackies
Heavy? as in say up to 80 feet and a bar 200x16, ?
the benefits are obvious
But you are ignoring sailing yachts, simply put a 40o tonne disp. needs a stem bar as does any sailing yacht
please show me a ship , icebreaker whatever, sans bar. You have posted tupperwares ? all to obvious is it not
Besides as you said in first post , bars aid construction