Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!att!cbnews!military From: jrll@Portia.stanford.edu (john ralls) Newsgroups: sci.military Subject: Multiple Screws Message-ID: <6501@cbnews.ATT.COM> Date: 12 May 89 02:47:33 GMT Sender: military@cbnews.ATT.COM Lines: 34 Approved: military@att.att.com From: john ralls >From: royf@roxanne.StPaul.GOV (Roy Forsstrom) >Each shaft/propellor combination is rated at a certain horsepower. Two >shafts with two engines can give you more power with less draft than >with one large prop. Nuclear-powered ships, at 10,000 tons, draw over >30 feet as it is. Each shaft/propellor combination has an engine attached to it which is able to provide a certain amount of horsepower. The naval architect designs the engine, gear, shaft, and prop[ellor to work together to use that power in a way consistent with the mission of the ship. It is true that where draft is of concern, twin screws can reduce the size of the screws, which may, in some hull configurations, reduce draft. But the interest of adding power has more to do with engine size and screw efficiency than draft. The original comment in this thread referred to the Iowa-class BB's. Those ships carry 212,000 hp worth of engine, using old M-type superheating boilers. I think (redundancy issues aside) that it would be challenging to build a reasonable engine which would provide all that power and be capable of the transients involved in shiphandling. On the other hand, the Virginia class CGNs have twin shafts and a total of 70,000 shp. They draw more in the bow (by 7 feet) than in the stern because of the SQS-53 sonar dome. Since the powerr could easily be provided by a single engine (the Ohio class SSBN has similar power on one shaft), the reasons for multiple shafts are obviously not for draft or powerr concerns, but rather for redundancy and maneuverability. There is seldom a single reason for any particular characteristic of a ship. Naval Architecture involves far too many compromises. John