You're conflating "B:L" and "L:B".Terry Duncan wrote:6 - 1 is not a high B:L ratio, it is roughly that of a WWI dreadnought and nothing like the roughly 10:1 the Japanese needed for their cruisers to
1:6 is a high B:L ratio; 6:1 is not a high L:B ratio.
I stated "high B:L ratio."
No. The primary fun in this exercise is learning what dictates ship HP/speed, understanding how the tradeoffs went historically, and how they might have gone at different scales. An accurate program would steal that fun from me; an inaccurate program based on simplistic rules of thumb would perpetuate bad common knowledge.Terry Duncan wrote:Have you ever tried to use any of the ship design programs out there to create what you are proposing?
I want to understand, for example, why Emma Maersk needs less horsepower than SoDak BB's for near equal speed, despite Emma being ~5x larger.
The biggest thing missing from your analysis is the relationship between hull length and wavemaking resistance; longer hulls have less wave resistance (all else being equal). Again, this depends on speed-length ratio: (S in knots) / Sqrt(waterline length in feet).Terry Duncan wrote:With that length to beam ratio a high speed is very difficult to obtain, which is why the Iowa's had such a long thin bow
As megaship is ~4x longer than Iowa, at 33 knots its proportional wavemaking resistance would be similar to Iowa's at 16.5kn (33 / Sqrt(2)). For illustration of the issues, here's Basic Ship Theory:
[To convert S/L ratio to Froude number, multiply by .298]
As you can see, Iowa's S/L ratio of 1.12 (Fn=.33) puts it at an extremely unfavorable point on the wave drag curve, just right of the resistance "hump" at N=3 (Fn=.31). The Iowa's designers therefore had very good reasons to concentrate on ameliorating wave drag.
By contrast, the 40kn megaship (S/L=.70, Fn = .21) is well left of N=4 on Figure 10.5 above.
Also this diagram from Wikipedia illustrates the basic issue fairly well:
So megaship's main resistance issue is friction resistance, Iowa's is avoiding wavemaking resistance.
Iowa's hull form follows the basic precepts for ameliorating wavemaking resistance.
Friction resistance follows different optimization parameters from wavemaking resistance.
Friction resistance is largely a matter of wetted area, pressure coefficients, and speed.
The pressure coefficient is applied to wetted area to give total friction resistance of a given hull:
[note- I've been using "friction" interchangeably with what the text describes as "skin friction" plus "viscous pressure" resistance. Shorthand terms vary here and in aerodynamics in my experience]
If you "plug and play" with realistic numbers, you'll see there's a tradeoff between total wetted area and friction coefficient. The minimal wetted area enclosing a given volume (displacement) is, of course, a sphere. But a sphere (L:B ratio of one) has a friction coefficient so high that's it's better to have more wetted area (therefore more skin friction drag but less viscous pressure drag).
If wave drag were not an issue in ship design, the optimal L:B ratio (point at which friction drag is minimized) is usually around 5-6, depending on draft and other parameters (especially block coefficient).
Now let's circle back to the point about Speed-Length ratios and wave drag: It should be apparent that the lower the S:L, the closer to the friction-minimizing hull form we should be. Thus we can't look simply at raw speed, we have to look at speed versus size. Thus it makes sense that optimal L:B decreases with increasing size at a given raw speed (i.e. a 33kn BB has optimally lower L:B than a 33kn CA, which has optimally lower L:B than a 33kn DD).
Extending the reasoning of our BB-CA-DD L:B curve to megaship gets you most of the way to understanding why a fast megaBB has higher L:B than a normal BB. Of course it doesn't cover the even faster MegaBB directly, for that we need step further along our understanding of the fundamentals.
...but I've written enough already, willing to answer additional questions though.