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

[Dean Smith]

Originally Posted by MikeJohns

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]

Originally Posted by MikeJohns

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...

[MikeJohns]

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]

Originally Posted by MikeJohns

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]

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]

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]

Originally Posted by Dean Smith

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.

Originally Posted by MikeJohns

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.

[pdwiley]

Originally Posted by MikeJohns

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]

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]

Originally Posted by Dean Smith

......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]

Originally Posted by pdwiley

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]

Originally Posted by tazmann

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