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Old 09-30-2010, 10:50 PM   #26
malloyd
 
Join Date: Jun 2006
Default Re: Seaplanes & Submarines

Quote:
Originally Posted by Anaraxes View Post
Yes. I'm not enough of a hydrodynamicist to know, but I'm curious as to whether fully submerged travel ought to be faster or slower, all else being equal. Under water, you've got more area wetted by a more viscous fluid, water instead of air. On the other hand, on the surface there are waves and pitching to deal with, which probably cost you forward progress.
Normally the biggest factor on the surface is the energy you lose generating the wake (or if you prefer to look at it that way, in climbing your own bow wave). For conventional ship designs wave drag is usually quite a few times skin drag at high speeds, and a substantial fraction of your surface area is wetted anyway, so for the same power you would go faster submerged. But there will be exceptions - boats where there is substantial hydrodynamic lift involved (hydrofoils, anything capable of planing), boats that never try to go very fast in the first place (wave drag stays small), boats that are very light compared to their cross sections (producing a big change in wetted area if submerged) designs where there are large additional drag terms (say for water flow through the propulsion system, induced drag from that hydrodynamic lift, or towing something else through the water)....
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