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Author Topic: Bulkheads, why not..?  (Read 1931 times)

GG

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Bulkheads, why not..?
« on: November 21, 2019, 05:09:47 pm »

One of the saddest sights you can see in this hobby is someones radio controlled pride and joy sinking, no matter how gracefully, beneath the waves.  Well, assuming it's not a submarine model in which case the saddest sight is the operator wondering why it hasn't resurfaced!


I still find it surprising that so many models do not have the rudimentary safety precaution of the simple transverse bulkhead to divide the hull of the model into watertight compartments. Take a hollow hull, often found in kits as a GRP or plastic moulding, Fig 1. Now if such a hull springs a leak, gets a hole punched in the side (often accompanied with a cry of "where the hell did that come from!") or whatever, then water will flow into the hull and the whole hull can fill with water.  Don't blame water, if it finds an opening it will flow through it, that's its nature, Fig 2.


If our unfortunate modeller is lucky, the inrush of water is modest and there may be time to sail the model back to shore before the internal workings decide they have had enough and cease responding the increasingly desperate assault on transmitters controls.  Or, they could be lucky and another person might be able to make a recovery with their model.  However, I have seen the occasional enthusiastic "helper" cause more damage and even make the model they are trying to save sink even faster..!


Now even a single bulkhead placed somewhere roughly amidships can save the day.  The water will be confined to one half of the hull, Fig 3, and there's a good chance that the model will still float.  Many models with a decent amount of freeboard will have a more than adequate reserve of buoyancy even with half the hull open to the water.  Even if this is not so, a single bulkhead can make the model "sink slower" and give you more time for recovery.


I must admit that the way most of my models are built means that they have multiple bulkheads.  This is even safer as any flooding will be limited to one, or if unlucky, two adjacent compartments in the hull, Fig 4.


The value of these bulkheads was brought home to me many years ago.  A model based on a destroyer was being sailed in a club steering event.  It was quite rough but my model seemed to be coping with the course that had been laid out on the lake, although at times it could be momentarily lost in the waves.  It was only when it was coming in for the final docking maneuver did I see that the rear half of the deck was underwater!  The rear three compartments had been flooded by water coming over the decks and leaking inside the hull.  Luckily the first two compartments were dry and able to keep it afloat.  They also contained the receiver and allowed me to retain control. 


One final point about bulkheads is that they will offer no protection from sinking if holes are cut through them.  If wires have to be routed between compartments I try to run them over bulkheads.  If this is not possible small slots can be made in the top of the bulkhead to accommodate the wires.  If forced to make a hole in a bulkhead, I make it as small as possible and well above the bottom of the bulkhead.


Glynn Guest
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Klunk

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Re: Bulkheads, why not..?
« Reply #1 on: November 22, 2019, 02:58:42 am »

reminds me of Titanic! But seriously what is being said is true!! I saw a Paula 2 sink as water came over the stern while in reverse, go down the rear hatch into the rear compartment, flush fitting hatch and go down in 30 seconds. the only thing that stopped it was the front compartment had been totally sealed when built so made a natural boyancy compartment!
if possible, always put some foam in the boat up front where it help it bob on the surface, foam in any superstructure that can come off easily and attach string between the hull and superstructure so if it floats or sinks, they are in the same area!
PPS this is NOT allowed in Springer Demolition Derby!!!
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GG

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Re: Bulkheads, why not..?
« Reply #2 on: November 22, 2019, 02:37:04 pm »

Good point Klunk, in a "Hollow Hull" (maybe "Coffin Hull" would be a better description..?) at least put some buoyancy material inside it.


Another puzzle is why some designers insist on making bulkheads with big holes through them.  True, in a hull with lots of Frames then such holes are probably needed if you plan to fit anything inside the hull. Although, I'd still like to see a few solid bulkheads.


I'm tempted to think that it may be a hangover from a designers earlier experience building model aircraft fuselages where internal holes are needed for control runs, etc.  Since fuselages are not required to float, except for working models based on flying boats of course, a hollow body is no problem.  This is based on one American magazine which was more used to aircraft plans, who tried to add large holes to the bulkheads in my boat plans until I pointed out the value of solid ones..!


If I see a model boat plan featuring such "holey" bulkheads it suggests the designer has little experience or has a "Titanic" level of optimism....


Glynn Guest
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warspite

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Re: Bulkheads, why not..?
« Reply #3 on: November 22, 2019, 02:58:01 pm »

Though you have to also consider the necessity of holes due to weight constraints, on a 50,000 ton freighter a few holes in the minor bulkheads allows a greater free-board, on a 1 kg model boat however, it needs to save weight where ever the opportunity arises a compartmentalized hull is not always possible.


I haven't fitted bulkheads in Victory due to the weight constraint, as I want to add detail, since not using brass for masts etc. the boat on a crowded waterway is not going to happen as it is susceptible to being damaged by others, plus finding a water where I can test it is proving difficult to find anyway.
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RST

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Re: Bulkheads, why not..?
« Reply #4 on: November 22, 2019, 08:00:26 pm »

Full scale vessels don't really have "lightening holes" in bulkheads.  In the case here. WT bulkheads are very tightly controlled for flooded stability condition. A few years ago now, but you might be surprised just how many few watertight compartments are required for class.
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RST

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Re: Bulkheads, why not..?
« Reply #5 on: November 22, 2019, 08:22:45 pm »

...before you discount Titanic. She was probably one of the "safest" designs in her day, quite revolutionary? The same is true now if you flood too many spaces at once it will always sink!
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nemesis

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Re: Bulkheads, why not..?
« Reply #6 on: November 23, 2019, 03:14:48 pm »

Having been through the constraints of bulkheads on plank on frame and coming to the "freedom" of glassfibre hulls with all the space you now have why put bulkheads in and negate the advantages. I can understand one at the fore castle, just in case. Been building and sailing since the 60s and never sunk a boat, you do build to keep the water out. My thoughts only. nemesis
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NoNuFink

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Re: Bulkheads, why not..?
« Reply #7 on: November 24, 2019, 08:20:57 am »

As a relative newcomer to model boats I have a couple of queries.

1)  If I build a boat hull out of wood, as part of the testing procedure in the bath, should I ballast to the waterline and then fill the hull with water to see if it sinks or merely floats submerged?

2)  If the model is fitted with an automatic bailer or even a pump, aren't bulkheads the exact opposite of what is required?

I seek enlightenment 

NNF
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GG

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Re: Bulkheads, why not..?
« Reply #8 on: November 26, 2019, 01:09:26 pm »


Being an Engineer I'd rather design things to fail-safe and/or give me early warning and time to take action before damage/loss occurs.   


It's hard not to come to the conclusion that bulkheads and internal buoyancy are things that modellers might not appreciate until they wished they had used them.


Glynn Guest


(RST Yes, the Titanic probably was "one of the safest designs of her day" but I was referring to the human decisions and actions that caused the disaster when I mentioned a "Titanic level of optimism- sorry I wasn't 100% clear on that) 
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SailorGreg

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Re: Bulkheads, why not..?
« Reply #9 on: November 26, 2019, 05:12:51 pm »

As a relative newcomer to model boats I have a couple of queries.

1)  If I build a boat hull out of wood, as part of the testing procedure in the bath, should I ballast to the waterline and then fill the hull with water to see if it sinks or merely floats submerged?

2)  If the model is fitted with an automatic bailer or even a pump, aren't bulkheads the exact opposite of what is required?

I seek enlightenment 

NNF

I wouldn't intentionally sink my model.  I'm not sure it would prove anything worthwhile and it gets the inside all wet!  And I suspect that nearly all models ballasted to their scale waterline will sink properly if flooded. The buoyancy in the hull material won't overcome the weight of the motor, battery, speed controller, ballast and so on. (Although if you do the experiment do let us know the outcome.)
If a model has a bilge pump it is usually positioned in a compartment that might be prone to leaks such as where the prop shaft emerges or where a water cooling inlet is positioned. Other sealed compartments are assumed to be watertight.

Hope that helps.

Greg

NoNuFink

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Re: Bulkheads, why not..?
« Reply #10 on: November 27, 2019, 05:31:18 am »

OK, Thanks for that.  My build for this Winter will be a springer style hull built with ply - not very bulkhead friendly.  As a matter of interest I will try the experiment before installing the works. 
I will report back but it won't be soon.

NNF
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tigertiger

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Re: Bulkheads, why not..?
« Reply #11 on: November 27, 2019, 08:29:32 am »

Just a thought for the Springer. Is there room for bulkheads to run fore and aft, different to the normal athwart ships.
Although to be honest, losing a stringer to the deep is not as upsetting as losing a ship model that has taken several years, and several hundred pounds, to build.
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Geoff

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Re: Bulkheads, why not..?
« Reply #12 on: March 09, 2020, 02:20:23 pm »

Fore and aft bulkheads are lethal as they introduce a significant list which puts the deck edge under and causes loss of stability. A good number of real ships were lost this way due to engine room centerline bulkheads. In real ships a list of 10 to 15 degrees is extremely serious and equipment stops working.


Despite early understanding of this phenomenon designers persisted in using centerline bulkheads as they gave great longitudinal strength. Only ships that are sufficiently beamy can take centerline bulkheads but even then there are problems as the flooded compartment is more off the centerline than a narrow ship!


Flooding was a major problem to warships particularly after damage. A torpedo hit would often open a hull for 30 feet in length and 20 feet deep. The explosion also caused significant shock damage with engine mounts and boiler mounts and pump mounts fracturing. Bulkhead distortion was also caused and leaking glands and pipe runs could eventually overwhelm a ships ability to control flooding.


Loss of pumping capacity due to shock damage would often doom a ship.


From a modelling perspective our models are inherently much more stable and can easily be constructed to deal with a certain amount of flooding but realistically its probably going to be a beam collision and roll over that impacts our models or if sailing in bad weather the wind and waves with slow flooding. Actual hull damage is quite rare as all out models are massively strong relative to the original.


In my opinion the best way to protect is beam bulkheads where possible and foam at the bow and stern if possible to preserve buoyancy, however if we have a complex model we tend to forget doing this!


Free surface effect is also very serious in a model (Herald of Free Enterprise) comes to mind so no bulkheads allows water to slosh around soaking the electrics and causing loss of control.


Preserving buoyancy is a very complex subject but real ships have damage control and various internal decks which slow flooding. I read a report on the loss of Prince of Wales and it said it was like the tide coming in slowly as the stern sank lower and lower and they were unable to establish a flooding boundary and hence loss of the ship.


Cheers


Geoff
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