Economic benefits of parallel middlebody

I can understand that there are potential economic benefits for hulls to have extensive parallel middlebodies. For example, making several identical frames off-site might reduce manufacturing and other errors.

My question is whether there is a benefit if the ship is very large and all components are made and assembled on-site.

At the other end of the scale, is there any real economic or construction advantage to having parallel middlebody on a 18m carbon-fibre or fibreglass rowing shell?

I appreciate that there are hydrodynamic disadvantages, and possible advantages in layout of components and arrangements, but they are separate issues. (Unless I am being naive in separating them out).

Leo.

 

Leo,

I do know that large cargo vessels and tankers are routinely built with parallel mid bodies. There is a considerable savings to the shipyard when they can build identical modules and assemble them end to end. At the speed length ratios these vessels operate I don't believe there are any hydrodynamic disadvantages. I welcome others around here who may have more expertise in this area than I do to chime in to correct me if I am wrong.

 

As I recall long ships don't operate the same way as small pleasure boats. The small boats operate within the wave created with the bow of the vessel and the designer's job is to place the vessel in the center of the wave wheras the stern is located on the face of the 1st following wave. As vessels get longer there comes a time when the hull speed is too high to meet the design requirements regarding speed, displacement, fuel burn and power required. So vessels that long or longer operate with their stern on the face of the second or third wave face behind the bow. Great Lake Ore Boats could operate on waves even further back and of course each wave gets smaller and there is less advantage to "catch the wave" and do a bit of surfing. Surfing is what all good displacement hulls aspire to do. Slab sided ships have great advantages while broadside to a dock and the bow wave can bounce off the flat side nicely and then the stationary sea and then the side again ect ect. Any displacement vessel operating on the 1st returning wave should not have a straight side and if they do the water line midships would be below the static level at or near hull speed. My 30' Willard has sides that are curved it's entire length and the boat is (to a small degree) difficult to deal with at dockside. There are many advantages to a slab sided vessel beyond construction. Well I've had fun w this and now I hope someone that really knows what ther'e talking about will jump in here and clear the air.

Easy Rider

 

Thanks, that's another advantage I hadn't considered.

Leo.

 

Way back when in my shipyard days we built the ships using the module method. For a long time my specific job was assisting the fitters. The riggers would make the gross movements getting the mods roughly into place and then we would make the fine movements dialing the mods into place. The fitters would scribe and fit then tack.

On series built boats I don't think there is quite as much savings in labor between a slab sided vessel and a more curvaceous one as one might intuitively think. Dealing with the mods as units takes away from the benefits of being able to quickly hang plate on the frames. Much of the stuff now is NC cut etc. and I think the differences between curvaceous and straight would be even closer.

I was a kid then and obviously wasn't privy to the numbers used for bidding. But I think the time benefits of module construction may negate the advantages of simpler design based on the premise of laying a keel, building and erecting frames, trueing everything and then hanging plate. At a certain level you can usually break stuff like this down to a $ per # basis. I imagine dead straight and flat is cheaper but I bet you can get some curves without paying a large premium by the time the whole boat has been built and launched. I'd be real interested to find out the opinions of guys with a lot of experience, there's a few of them around here.

 

For cruise ships and such, one advantage of straight sides would be easier fit-out of the passenger rooms.

On the cruise ships I've seen, the plan at the waterline is much more 'curvaceous' than above the waterline, where the slab sides predominate.

Cruise ships operate at slower speeds than the ocean liners of the first half of the 20th century, and the 'Blue Ribband' for the fastest transatlantic passage was a coveted trophy.

 

Leo, I assume you have read The Speed and Power of Ships.

The advantages of PMB depend on the speed length ratio. In simple explaination, PMB adds displacement at no cost to form drag, only friction drag. On the other hand, there is less proportional form drag the longer the entry and exit are. This leads to a tradeoff between PMB, displacement, speed and power. So for two ships of the same length, say a VLCC and a CVN, the VLCC has 80% PMB, 200K DWT and 20knts, while the CVN has 20% PMB, 75K DWT and 30+ knts.

FWIW, there is whole industry supported by the plugging and un-plugging of bulkers to meet market forces. However, the complications in service systems prevent most other vessel types from ever being modified (Been there, done that, have the emotional scars). It is fraught with difficulties, as the USCG will attest.

As for initial construction, if PMB suits my needs, I would put it in a 12' dory.

 

This is a good post as it actually points out where variable PMB economics comes in. As stated, ships are built in modules, mostly 50-80% outfitted then joined together. In the vast majority of cases of bulker and inter-nodal ships, you can have a bow module(s) and a stern module(s) which contains the prime mover, then select a given number of cargo volume modules. Of course, semi-customazation goes on as ships are still built one at a time, but the cost savings in basic design, engineering, and CNC are significant if there is finished basic design to work from.

 

On a small scale, the parallel sided module system has been applied to homebuilt craft, for the purpose of transport as much as for scaling: http://www.craftacraft.com/tims_the_infinite_modular_sharpie . I wouldn't want to try one in rough water though....

 

Rob Denney has developed a simple method of making folding hulls. He uses foam and glass flat panels to start with. The foam is placed where he wants flat sections to separate the glass layers and there is no foam where the panel will be curved.

The ideal hull for this has parallel sides. The whole hull can be made from one main piece with two extra pieces to form the bow and stern being glassed to the main piece once it is folded.

I did analysis on various options for a slender hull and found it cost about 10% extra power.

With large ships the maximum beam is a critical feature for loading and unloading operation. Shiploaders and shipunloaders have to be able to place cargo to the extremes of the hold or even deck. The further the machines have to reach for any given load the larger they will be. It means the wharf structure they are mounted on also has to get larger. The bigger rail-mounted loading machines are up around 2000t so the wharf structure that permits them to travel full length to load a ship is an impressive structure in its own right. Deep offshore berths will permit a loaded draft of 20m and them there will will be a wave and surge allowance designed for a 200yr ARI that is 20m above the low water mark. So these structures stand maybe 40m off the sea floor and have a 2000t machine trundling along them; continually accelerating and braking.

Point of this is that changes to the shape of ships has a big impact on the substantial infrastructure that serves them. You may be aware that the Port of Melbourne has recently being through some challenging times to dredge Port Philip to enable economic size vessels enter - so draft is also an issue.

Rick W

 

What does PMB stand for?

Rowing shell design has significant tradeoffs which I only slightly understand. The people rowing constitute the vast majority of the mass. They oscillate fore and aft which causes pitching and some amount of energy loss through wave generation. The distribution of displacement needs to be balanced between minimizing pitching with more displacment at the ends, and optimum distribution if the mass was fixed. Presumably there are also practical limits on how narrow the hull can be at the first and last rowers and still accomodate them.

On the other hand even a very small decrease in overall resistance and corresponding increase in average speed would be worthwhile. A difference of a tenth of a precent could be winning margin in a race.

 

Parallel Mid(dle) Body, a length of the hull in the middle of the vessel that has constant section shape so the waterlines, buttocks, etc are just parallel lines. Rowing shells have considerable PMB.