Massive Stem bars ..........Why [Archive]

LyndonJ

03-18-2010, 06:33 PM

Looking at class rules for a 60 foot motorsailer

The bow is a raked relatively sharp with the plates coming together onto a solid round bar.

The rule asks for a stem bar of something like 10mm by 100mm but the well supported and restrained bow plate is already so strong that the stem bar adds nothing structurally just keeps the shape during construction. There's no analysis I can do which shows even the remotest amount of significant stress in that member.

The class construction surveyor isn't happy leaving out the stem bar
but it seems superfluous as I already have a massively greater (100 X) structural result with the acute plate angle.

Is the stem bar aimed more at rounded bows which are relatively flat?

I was wondering what experience other designers had here ?

tazmann

03-18-2010, 08:20 PM

I am not an expert on the subject but my take on it, without a stem bar would be strong enough if you never hit anything but if you figure your displcement at full speed hitting somthing solid, you would be glad you had a stem bar
tom

marshmat

03-18-2010, 10:22 PM

My (admittedly limited) understanding is that such "overbuilt" stem bars are there mainly for impact resistance, as "normal" operating loads ought not to put such huge stresses on this part of the boat.

Consider the case where, at 10 knots, you hit a 40' container sitting an inch above water (as they occasionally do). If you just have the bow plating up there, with no stem bar, the bow plates are likely to buckle outward- the stem bar takes up some of this load and serves to keep everything roughly where it ought to be during the collision. So instead of a cascading structural failure that tears plating from the forward bulkhead, you end up with a nasty but more manageable hull breach.

LyndonJ

03-19-2010, 04:13 AM

Thanks
The plates are 6mm and the round is 25mm solid bar, I think on the event of a collision it will ride up regardless. Anyway if you have to have a collision bukhead it's all very redundant.

I think it's more a case of ship structure ending up being in small vessel rules. I know ABS ORY is now obsolete but they didn't even have a stem bar requirement.
Then if you want to consider collision even without the stem bar its about 5 times as strong for collision and considerably harder than layered wood or foam core.
So I really think its a case of apply directly to the rule society and show them te calcs and ask for an omission. I was really wondering if anyone else had been there.

gonzo

03-19-2010, 06:13 AM

Why do you want to eliminate the bar, is it the wieght? If you are using it to keep the shape during construction, it can't be any extra labor to keep it in place.

waikikin

03-19-2010, 07:09 AM

LyndonJ, the only advise I can give is to leave it there- it may be stuctrually redundant but if it aids set up in holding the round & bobstay attachments or forestay attahment are incorperated the surveyor has a valid point(his job is to verify the boat is built as drawn), also importantly(very) building boats is a business & getting on with it gets the job done & paid for. All the best from Jeff.

Wynand N

03-19-2010, 07:46 AM

why not get the designer's view on this?

LyndonJ

03-19-2010, 08:48 AM

jehardiman

03-19-2010, 11:19 AM

Yep the designer thinks it's total overkill too! but if you want class approval then you do it.

Just seems some things are so heavy that it's dumb, I'm a recent grad engineer and I've just spent too much time analysing this every which way and I just cannot see that it's required, I can see it's needed on a flattish stem but an acute angled bow is so strong anyway.

Never mind I can take all my calcs to one of the senior engineers and get him to sign it off then they will be happy. It's all about having someone who can carry the blame if it goes pear shape :-)

It saves on weight and allows the welders to get at the inside of the stem, you can offest the stem bar too, then it wouldn't help with collision or attachments.

Makes me wonder just how much framing is actually superfluous on heavily skinned steel boats.

Thanks for the replies

There is actualy very little superfluous structure rerquired by the rules. Almost every thing required is there because it provides maximum reasonable assurance that in a damaged condition the loss of structural intigerity is limited to the initial damage site. As you are a recent grad, and say thay you have analyzed the structure, I have to ask if you analyzied the structure in ALL the damaged conditions?

As you gain work experience, it will quickly become apparent that the limiting factor in steel design is not strength, but stiffness and keeping structural geometry. The reduncency of ship structure is not for strength, but to prevent loss of sectional properties which leads to cascade failures. As tazmann and Marshmat pointed out, damage forward can be quick and large. If your vessel is designed for 10 knots, did you design the forward compartment for an internal pressure of a minimum of 406 psf (3 psi) with the stem removed? I've seen photos of bows opened up like a flower with the shell plating ripped away from the collision bulkhead, or the collision bulkhead stove in due to ram pressure. Will your bow support all the loads with all structure from the LWL down back to the colision bulkhead gone? Damage can cripple major framing structure and force the loads in the secondary and tertiary structure. There must be sufficient structural material with adequate geometry available to take up the primary loads.

The reason for the heavy stem bar is to keep the deck and bottom plating attached and acting together in the event of collision or allision. It's size is not predicated on strength, but on the historical probability of surviving the impact, damaged, but still functioning as a strictural load path. A fair amount of the rules are predicated on "we've never seen a part of X by Y dimensions fail, so the minimum sizes for certification is X by Y".

And then, for laughs and giggles, are you RTing your stem to ensure you are actually getting the assumed strength? Or have you done an analysis for the case of a weld root flaw if there is no NDT? What is the maximum expected pit size expected in the seam weld between drydockings? There is a plethora of other considerations besides strength for the structural requirement of a 10mm x 100mm stem bar.

Edit to add, remember, the rules are not about optimum structure. They are about providing reasonable assurance to the insurer or owner that the vessel will perform adaquetely throughout it's useful life.

LyndonJ

03-19-2010, 07:10 PM

JEhardiman
Thanks, I'm still frustrated though

There are lloyds approved foam cored FRP boats sitting here right now that have nothing similar re-inforcoing the stem, just thicker laminates on the hull itself. Way weaker than the steel structure.
Having seen photos of the damage resulting form collision in this sort of construction and it's extensive but even then it all survives.

A steel boat with bow plating at a relatively acute angle already has a very large structural capacity and in this design the longitudinals run into the equivalent of horizontal floors every 2 feet or so up the stem, this same frrame -floor type construct carries the massive stem bar as well. Those brackets tie the hull plates together and stiffen the stem anyway. If the first frame at the waterline were a full bulkhead appropriately stiffened (with a massive stem bar :-)) you could apparently call the bow a 'false bow' and do what you like.

If it were ice rated the plate would be 8mm thick on the bow .

As the stem is well raked I think that you could run the vessel onto a low rock all day and not even dent the stem, if it were a sharp rock and ripped into the plate then it's going to do it regardless.

What is interesting is that the different classes orgs all seem to differ on whether or not it's required and what size it should be.

I think under ISO and it can be left out completely too.

It's frustrating when you can't see behind the blind standards and what the intent is. And Istill think a lot of these are generalisations that will catch problems with other bow types. Like a big lazy U shaped bow.

There's so much framing tieing the hull deck and all together anyway.

Welds they only check 5% or so the overhead butt welds seem to be their favorite. But on vessels this size weld failure is apparently unknown.

Now Lloyds will accept specific calcs and so long as the eng is appropriately cross insured. So yep it's about liability

Still leaves me thinking some overkill is often present in smaller vessels. I can definately see the requirement in a ship, but in a boat?

Thanks

waikikin

03-20-2010, 03:03 AM

LyndonJ, 100 x 10 mm is hardly massive on a 60 footer, I can understand your frustration but as I see it the main issue raised is ease of welding access(a very important consideration esp' to the guy cramped in the bow doing it)- in that case you could & have suggested terminate the bow in another fashion- maybe another flat set in front at 90 degrees to the 100x10 & attach the pointed part of the bow as a non structural addition to the outside in the possibility of a range of materials(steel, staino, composite, timber?), a vessel my dad built had similar to this.
All the best from Jeff.

LyndonJ

03-20-2010, 06:45 PM

Thanks Jeff

baeckmo

03-20-2010, 07:06 PM

The stiffness against local damage in case of hitting a solid object is what counts, like noted by Jeff, Jehardiman, and Matt above. Twin 6 mm plates meeting a 25 dia bar are easily damaged; there will be sharp buckling with plates ripping and/or welds opening!!!! Believe us, you don't want that when you realise that you can't steer away from that floating container you just spotted......! Man, you are talking about a 60-footer, there are forces.

LyndonJ

03-23-2010, 05:46 PM

Yes but I just look at the foam FRP composite vessels also built to classification rules and they don't have anything like the strength, so why for one construction already stronger do you make it yet stronger again ?

The trad stem bar runs right through and has the plate butting at some angle into the side, if the plates meet on a solid round instead then the stem bar has to be added as a separate item . In transverse frames we were always told that floors are better than massive Central keel bars and to put more floors in and reduce the CVK size. Shouldn't this apply to the bow too?

LyndonJ

08-01-2010, 11:40 PM

I've been intending to come back to this finally I remembered:

The stem bar was not required it was a typo and the error grew. There was a bar called a stem bar in the specifications which was not really a stem bar just a shape defining curved bar. The plan approval process just beefed it up to the required dimensions for a stem bar.

It was actually a plate stem ( no stem bar required ) The plate has to be a minimum thickness and it has to be 'shape supported' with horizontal webs every 1.8m or so max. I thought it was daft to have two 6mm bow plates meeting at a moderately acute angle and to have to add another bar.

Just thought that might be of interest to designers in metal. Lloyds register were very helpful in the end. It's all in their SSC, plate stems or bar stems and a table for plate thickness for a plating only stem . Funny that it's not in GL <24m, just need to know I guess or stick to LLoyds.

baeckmo

08-02-2010, 05:06 AM

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).

LyndonJ

08-02-2010, 06:39 AM

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. ?

pdwiley

08-02-2010, 07:25 AM

MikeJohns

08-04-2010, 07:08 AM

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.

pdwiley

08-04-2010, 11:07 PM

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

MikeJohns

08-06-2010, 12:21 AM

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.

Ad Hoc

08-06-2010, 03:56 AM

If the bottom of the boat were an acute V you wouldn't bisect it with a central vertical keel bar,...

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.

MikeJohns

08-06-2010, 05:52 AM

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.

tazmann

08-07-2010, 09:27 AM

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

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

LyndonJ

08-07-2010, 10:24 PM

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

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 ?

pdwiley

08-08-2010, 12:41 AM

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

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

MikeJohns

05-16-2011, 01:37 AM

........
I am having a prob coming to grips with STEM pic, usually there is a big alloy flat bar right there, and they do not dent like that, but then that is a very small pic

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.

Dean Smith

05-16-2011, 01:51 AM

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

MikeJohns

05-16-2011, 05:11 AM

......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.

Dean Smith

05-16-2011, 05:25 AM

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

MikeJohns

05-16-2011, 07:37 AM

............
But you are ignoring sailing yachts, simply put a 400 tonne disp. needs a stem bar as does any sailing yacht
please show me a ship , icebreaker whatever, sans bar.........

Ships may have a relatively small solid round bar bent to the profile shape. It's called a stem bar but it's not a structural member . Many don't have a stem bar at all. Some have a centre line web. Some just have extra floors and Breast hooks further up. There's lots of potential designs. You can make it as strong as you want and carry any load you want. Whether it's breaking ice or carrying a sailing rig fore-stay load. There's never only one way to achieve strength.

Dean Smith

05-16-2011, 06:45 PM

does this make you salivate, it should this is real boatbuilding--at its very best and most skilful
Yes there is your stembar
built by Bos and Carr NZ

MikeJohns

05-16-2011, 07:49 PM

Well to recap.

If you have a stem bar as the primary structural member it's quite large and heavy in a steel boat. It's sized as though the plating has no structural input. It's one way of framing but it's not the only way.

The bar is not mandatory and can be safely omitted or reduced considerably in size providing the plate in the bow and stem are of sufficient thickness and is adequately shape supported. That thickness is often already inherent in a steel sailboat and if not is easily achievable unless relatively thin plate is being developed for a complex bow shape.

The bar is favored in fabrication as a strongback to carry and position the frames prior to plating and as such it does aid construction and define the limits of the bow plates.

But if the bar is present it does not need to match the dimensions given in the class rules for a stem bar and can be as small as you like if you specify that you are using an all plate stem. That's the crux otherwise the plan approval process will mistakenly reject your stem bar size as inadequate if you label it a stem bar in the materials list. If you label it a stem softening bar or a profile definition bar there's no problem.

A round bar used as a nose softening at the stem can suffice as both a construction aid and local collision reinforcing. I think you'll find it's a lot stronger than you imagine.

It is a very common misconception and leads to the use of a lot of unnecessary material in small boats. I think a lot of this confusion arises because plan approval see's 'stem bar' specified as say 25mm round and say "no that has to be 150 by 20" !

In the pic: That's an acute enough angle to make a very simple and strong plate stem. The webs are what give it strength to stop buckling, It can be lighter and stiffer than using a big heavy bar and it can have just as good a collision strength if you want.

If you still need convincing I can calculate the strength for you. Or take two plates of 1m by 300 by 6mm use a 25mm round bar at the apex and weld them at 45 degrees, add a triangle web at each end and one in the middle to keep the shape then try and dent it with your sledge hammer ! And that's only a small part of the strength since the stem is also backed up by framing and adjacent plate as well, so the real strength is closer to tack welding the base of the triangle to a base plate. You could run over it all day with a 20 tonne excavator and it wouldn't budge a mm.

Dean Smith

05-16-2011, 07:50 PM

and for a boat that will lock a lot and be in heavy river traffic, what else would you do?
The topside plate is 8mm al al. I have never seen a barge or river ship without a bar and they date from turn of 20th century til now

Ad Hoc

05-16-2011, 08:29 PM

and for a boat that will lock a lot

Lock a lot, don't know what that means....can you explain. New terminology for me.

what else would you do?

Totally depends upon the SOR and hence where it is opertaing and how. If collisions are likely and frequent, you ascertain what type of collision. A collision between a submerged container, is different to that of a quayside. Thus, where is your risk coming from and then design to suit.

But you also need to weigh up the balance between designing for a nominal "collision" and a full on "crash case". It has major weight and hence cost implications.

The topside plate is 8mm al al.

Topside increase in plate thickness is a throw back to "old days" and wooden boats. Although on container ships/tankers etc this is common owing to the higher stresses that occur round the deck-side shell connections.

Only if you have large fenders and/or rubbing streaks, otherwise an increase in plate thickness as "top side" is unnecessary.

Dean Smith

05-16-2011, 11:21 PM

Lock, Sluice slisen call it what you will , a place where ships go to go to next river level

Dean Smith

05-16-2011, 11:28 PM

Mike, my whole life I have been constructing in metal.
I am not convinced and round bar is a messy thing to use, Welding into a flat bar stem from inside is hard near bow unless you are soft nose, but welding into a round bar a big one, pray tell me how are you going to get the gun nozzle in there
NZ is not staffed by just NZ,ers, the guy we used was from Croatia , from the times they were heavily into shipping
You have overlooked the massive loads carried on megayacht forestays although I did mention it
The bow structure you see in the rhino was approved in UK, they did not alter my dwgs and I am content to carry on that way
I can then out a single jack under any point if I want to life there. Now tell me you can do that with 2 plates meeting at 10 degrees
good day to you:)

Ad Hoc

05-16-2011, 11:35 PM

Lock, Sluice slisen call it what you will , a place where ships go to go to next river level

Ahh..i wasn't sure if this was a verb or adjective or a noun.

So you mean technically "Lock through". To take a boat through a lock.

If that is all your concerned with, with regards to collisions, then there is nothing to worry about. The speed at which you shall be traversing going into and through a lock is slow, very slow. Therefore the impact shall be minor. Unless your scantlings are so light, in which case a minor jolt against the jetty would cause damage. As the impact is roughly the same.

Dean Smith

05-17-2011, 04:49 AM

Ahh..i wasn't sure if this was a verb or adjective or a noun.

So you mean technically "Lock through". To take a boat through a lock.

If that is all your concerned with, with regards to collisions, then there is nothing to worry about. The speed at which you shall be traversing going into and through a lock is slow, very slow. Therefore the impact shall be minor. Unless your scantlings are so light, in which case a minor jolt against the jetty would cause damage. As the impact is roughly the same.

<removed>
I have been at sea in tugs sailed and built boats all my life
Please try to to not be superior and I am sure we can communicate, and believe it or not just maybe we can learn off each other
<removed> no I do not mean Lock through, You in UK may call it this but the rest of the world does not

Ad Hoc

05-17-2011, 05:49 AM

I’m sorry but you seem to be reading a totally different post to that which I have added.

<removed>
Please try to to not be superior …

There is nothing superior in my postings, only your interpretation and approach in reading them. Since to reply in such a manner suggests you feel you know more than anyone else, ergo, why should you be questioned, as the implication is you know more than everyone else?...otherwise, I simply haven’t the foggiest what you’re on about.

Why……<removed>……and superior John

And you seem to be calling me John…for such a personal salutation do I know you??.

no I do not mean Lock through, You in UK may call it this but the rest of the world does not

Then please explain what it is you mean. Unlike you seem to be, I am not a mind reader, I can only reply to a post that is written not sent via ESP.

Boat Design Net Moderator

05-17-2011, 06:00 AM

Please, let's try to keep this discussion polite to one another so it can remain productive and focused on the topic at hand.

MikeJohns

05-17-2011, 09:06 AM

Some stem bars running from the stem head to the keel base are often so overbuilt it appears an obsession with the designer.

I was at the boatyard this afternoon to get some measurements for alterations I'm designing on a sailboat, I found one frustrated shipwright !

This stem bar was frankly bordering on a FOS of around 25.

The 8mm plate with a couple of transverse webs would have been far more than adequate on it's own. It's a good example !

Dean
I've never seen any problem with welders with round bar. In all my steel design work I always check access for a Mig gun. You wouldn't be very popular if you didn't consider the welders.

powerabout

05-17-2011, 09:40 AM

Dean Smith

05-17-2011, 04:49 PM

ok Mike
those pics, all I see is a mess, that tube ? what is it doing? the plate is miles fromit in a place that often strikes bricks first. I am quite sure neither you or I designed that
i do not think I overkill . I asked Dubois and Frers if I could do my own structure . they were ok with that IF I DID NOToverbuild
In a 17m sailing yacht I may use 130x16 stem ,cut from 5083 which is the cl bar and then climbs to up to 300 by way of the mast step Often in a modern yacht with U sections you can plate OVER the bar in other words you will have your bottom plate run maybe 500 wide each side
the aim is to maintain continuity of the bar and step .I was fortunate to build under Lloyds for an owner who was a very eminent engineer and founder of a firm that employed 700 design engineers. I learnt so much. It was funny to see him cover reams of A4 with blue ink arguments and fire them off to London. His arguments were always accepted. He was older, passed away 2 ,3 years back. He was also a very good sailer. Rambling and remembering

MikeJohns

05-17-2011, 07:01 PM

Dean Smith

05-17-2011, 07:18 PM

often people aren't aware that specific design is an option. If you think some structure is redundant then a NA/ME who does structural analysis can give you and the class society a workable alternative.

Currently they are more interested in seeing what's called a Finite Element analysis which gives a very clear indication of whether an alternative is strong enough or not. I do this commonly and it gives me a lot of work. There's nothing like design to first principles to teach anyone just what structures are redundant.

perhaps you may put up some screenshots of your work, say a complete small boat say under 80 feet construction dwg Mike
Is is astonishing that say Huisman has not changed since day one and people commission the worlds great offices to design ships such as Athena
Ted Hood Jnr once came to my yard and told me that Huisman was his yard of choice , expensive but
The Dutch , the Germans stay with traditional boat building with modern tools CAD for one, but they rarely deviate from the trad methods of construction
We are not talking ships that indeed have changed, alot. But vessels such as frigates seem to have retained

Ad Hoc

05-17-2011, 07:26 PM

Currently they are more interested in seeing what's called a Finite Element analysis which gives a very clear indication of whether an alternative is strong enough or not.

Indeed, very sad.

DNV no longer accept basin trials for cranes/davits structure with an overload test. Must be FEA now. Like you gives me more work, but it is a nuclear warhead to crack a nut!

To stem or not to stem...that is the question!
Whether 'tis nobler in the mind to suffer the failures and success of outrageous fortune... :p

MikeJohns

05-17-2011, 07:54 PM

Dean
Here's the pointy end of a 36 tonne 50 foot (edit sorry 60 foot) sailboat I designed in steel for a client .
I can provide plans with or without that stem bar both class approved to any class you want. If it's omitted the web between the longs is deeper . It may also need more of those intermediate floors, I'd have to check. If you can keep the shape in the plate you make just as strong a structure, this is the basis of monocoque and the weight saving can be significant.

I've never seen a stem bar as the type you envisage in a ship, if you have any books on NA have a look at the framing options for the bow stem structure in shipbuilding. Or I could scan some and post, later since I have to work.

Dean Smith

05-17-2011, 08:03 PM

Dean
Here's the pointy end of a 36 tonne 50 foot sailboat I designed in steel for a client .
I can provide plans with or without that stem bar both class approved to any class you want. If it's omitted the web between the longs is deeper . It may also need more of those intermediate floors, I'd have to check. If you can keep the shape in the plate you make just as strong a structure, this is the basis of monocoque and the weight saving can be significant.

I've never seen a stem bar as a solid bar in a ship, if you have any books on NA have a look at the framing options for the bow stem structure in shipbuilding. Or I could scan some and post, later since I have to work.

work;) oh well please just cup the bow abt 3m back and r4 it so I can see what you are doing , when you have time
thanks. yes I have all manner bibles here
Ad Hoc, forgive me if I do not answer your posts, I am 65 I do not like being treated as a child. there is no need to talk down. I think it is unintentional on your part. But there you go
Enjoy your day, be happy, smile and the world smiles with you:) Watch how Mike responds

MikeJohns

05-17-2011, 08:23 PM

Here's a scan from Ship construction by Eyres

The 'stem bar' is just a round softening bar that leads into the rolled plate upper stem, the whole fore end gets it's strength from the plate being restrained so it doesn't buckle. Just like the hull girder, you can sit it on a strongback which is fully structural or you can simply use the hull itself as the girder which is the only really valid option as the vessel gets larger.

Dean Smith

05-17-2011, 08:55 PM

well the structure looks ok, I spose the chain pl could pick up the first Panting , seeing as she is not so big, but I thought you were considering the welder? where is his/her egress? I see a nightmare working in there, I am sorry

A good idea is a 8wall pvc pipe as a ch lkr
thanks for showing

Ad Hoc

05-17-2011, 08:59 PM

Ad Hoc, forgive me if I do not answer your posts, I am 65 I do not like being treated as a child.

Well, I’m sorry you feel that way. There is no air of superiority nor arrogance in my postings, I’m just stating facts, which you are free to counter accordingly. My posts are to be read in a neutral manner, however, if you are reading more into them than its actually stated, I’m sorry but I have no control over your interpretations. If you feel chagrined by any of my posts, please highlight them and I’m happy to explain further. I only respond to what has been written.

Here's a scan from Ship construction by Eyres

Mike, you beat me to it. I was just about to post the very same image from the same book!!

Having said all that, for the type of boats we design, they are high-speed. Thus the fore end shape of the bow and forefoot has a fine angle of entrance. Therefore to ensure a good full penetration weld, which can only be obtained from one side, we add a stem bar. This is purely for fabrication purposes, none other. It allows a decent edge prep and full penentration into the stem, since it is just "half" a welded joint.

57263 57264

pdwiley

05-17-2011, 10:42 PM

Dean
I've never seen a stem bar as a solid bar in a ship, if you have any books on NA have a look at the framing options for the bow stem structure in shipbuilding. Or I could scan some and post, later since I have to work.

The 'Aurora Australis' has no stem bar. I know this for a fact as I've crawled around inside the bow spaces more times than I like to think about.

My Colvin Witch has a 100x6 stem bar and if I were to build her again I'd replace it with a 20mm solid round bar set half into the plating. I can't see how anyone can claim that it's easier to weld inside a nasty narrow triangular space where the bow plating meets a stem bar rather than welding a half-inset round bar with much better access.

That's a pure amateur's POV of course.

PDW

MikeJohns

05-19-2011, 06:33 PM

Yes Peter
An intelligent pure amateur has an advantage over the compromised position many part professionals find themselves in. Sometimes it's a case of the emperors clothes.

This thread is a classic illustration of the confusion that can arise.

The trouble with simplified construction rules written by people who know that there are alternatives, for people who don't. It's simply not clearly explained that there are optional approaches.

I believe D Gerr's “Elements of boat strength” is very poor in this regard, and that book desperately requires a clear concise explanation of valid options with steel boats in particular. Designers using that book think the stem bar is mandatory and it’s scantlings are given as though it’s the main structural member. Consequently it’s huge, heavy (and often entirely unnecessary). It’s no wonder small steel boats end up too heavy with that sort of design approach.

Many steel boat designers fall into this trap because in the face of a confusing issue they prefer to err on the side of overkill. Also as Lyndon posted, plan approval to scantling rule design is not about analysis but simply a look up table approval process.
If you call a construction bar a stem bar they will demand a stem bars scantlings. Then the whole merry go round becomes a sort of urban myth, it’s required because its always there because the class societies require it so it has to be specified…..

Small boat yards and builders like Dean who have limited experience then start to believe it's actually a requirement in all steel vessels and are then completely and utterly incredulous at the suggestion that in most cases you can simply leave it out with some very small modification.

Ship builders wouldn’t blink an eye since they gave up using solid stems years ago.

MikeJohns

05-19-2011, 11:29 PM

......I thought you were considering the welder? where is his/her egress? I see a nightmare working in there, I am sorry

A good idea is a 8wall pvc pipe as a ch lkr
thanks for showing

I think your scale might be out, this is a ship fore end in this case and there's no problem for the welders access at all.

tazmann

05-20-2011, 10:20 AM

The 'Aurora Australis' has no stem bar. I know this for a fact as I've crawled around inside the bow spaces more times than I like to think about.

My Colvin Witch has a 100x6 stem bar and if I were to build her again I'd replace it with a 20mm solid round bar set half into the plating. I can't see how anyone can claim that it's easier to weld inside a nasty narrow triangular space where the bow plating meets a stem bar rather than welding a half-inset round bar with much better access.

That's a pure amateur's POV of course.

PDW

The 100x6 stem bar does not sound too massive for the size you're building.
depending on design but with the flat bar style it would be a little easier to connect chine and stringer bars to and keep plumb at the same time.
Tom

pdwiley

05-22-2011, 03:00 AM

The 100x6 stem bar does not sound too massive for the size you're building.
depending on design but with the flat bar style it would be a little easier to connect chine and stringer bars to and keep plumb at the same time.
Tom

Exactly right and that's why I left it there. Next time however I think I'd use 20mm solid round and tack some flat bar to the outer side as a stiffener while pulling in the deck edge pipe, chine bar & longs. The inside welding would be a lot nicer to do.

I'd want *something* there because I know my layout skills aren't up to getting just the plate to meet fair. I used one of Tom's tricks to keep alignment right down the CL by stringing a tight wire exactly over the CL for the full length & dropping plumb bobs down. Working inside a big shed makes a lot of things simple compared to doing it outdoors.

PDW