My House Rules for General Quarters

• Laying Smoke. In WWI and earlier eras, I disallow the use of smoke laying altogether. It's unclear just when or how much smokelaying was used in WWI, but it seems that the British didn't start practicing with it until the last couple years of the war, and I have yet to find an account of deliberate smoke-laying in action during the war. With all the coal smoke around, it's understandable that most naval officers wouldn't think request more smoke.
• Coal Smoke. The GQ rules have some rules about measuring angles and distances from ships to determine what is obscured by the coal smoke from each ship's stacks. I ignore these rules. At the scale of my games (about 3" per mile on the table), my 1/2" wide and 1 to 1-3/4" long bases cover plenty of sea to represent smoke obscuration. So instead, any shot which crosses any part of an intervening ship's base suffers a +1 straddle modifier for each such ship base it crosses.
• Torpedoes. I've completely replaced the torpedo rules with my own.
• Non-linear speed reduction. I saw these optional rules about speed reduction on David Manley's General Quarters web site, and incorporated my interpretation of it into my own SDSes. Basically, ships do not lose speed in a linear fashon when receiving damage (this is supported by historical accounts and numerous laws of physics). Assuming each hull box crossed off represents about a 20% reduction of the ship's effective horsepower (which could be lost boilers, lost engines, increased draught from flooding, etc.), then the speed reduction should follow a curve that starts shallow and gets steeper with mounting damage.
• Large Caliber Secondary Artillery. I got this idea from this web page by Paul French, and extended it (in modified form) to all pre-dreadnought era ships with liberally mixed calibers of guns aboard. Basically, ships with small main and secondary batteries of very similar calibers are allowed to combine the attack factors for these two batteries, as long as they use the straddle values for the smaller gun. I feel this is a nice, abstract way of representing the sort of "hail of fire" ships of the pre-dreadnought period expected to generate. If anything, this isn't enough of a penalty for using both sets of guns on the same target (period artillery spotters complained that they couldn't differentiate the shell splashes of heavy and semi-heavy artillery fire, making range adjustments difficult), but I figure old ships already labor under enough disadvantages against "modern" WWI ships that it doesn't matter too much.
• Tertiary Batteries. I liked the idea on this web page about incorporating tertiary batteries into the GQ SDSes, so I incorporated it into my own SDSes.
• Limited ranges of minor batteries. I haven't tried this rule out yet, but I'm considering it: only main batteries and semi-heavy secondary batteries may fire out to the blue range bands (ranges 0, 1 and 2) on my range finders. This is to represent the limited elevation of most secondary and tertiary battery guns until after WWI, especially armaments mounted in casemates. In general, ships before the dreadnought were designed with a small battery of big guns to "soften up" an enemy at long range, and large batteries of smaller QF guns to produce a "hail of fire" at decisive engagement ranges. After Tsushima, when it was realized that modern heavy artillery firing at long ranges could be decisive (and smaller QF guns could no longer do sufficient damage to heavily armored battleships), main batteries increased in size, elevation, and long range accuracy, but smaller batteries were still designed mostly for the close-range "hail of fire" approach, because their intended targets (light torpedo craft) were fast and maneuverable enough to make the big guns ineffective against them.
• Evasive action. All ships capable of moving 9", regardless of class or size, can perform evasive action. It seems that faster cruisers also used "straddle chasing" as a technique for avoiding damage during WWI, and it only makes sense that light cruisers acting as destroyer leaders should be able to evade along with their flotillas.
• Movement Speeds. I adjusted movement speeds back to the "real" speeds of the ships represented. In the standard GQ rules, ships' speeds are reduced by half, presumably because at a ground scale of 8" per mile, ships steam out of a 10' x 10' playing area too easily and battles end too quickly as a result. With 1:6000 scale miniatures at "centimeter scale" (one inch in the rules is one centimeter on the table), it is easy to have sweeping battles covering vast expanses of sea and scrolling terrain on a reasonably sized table (8' x 4' or 8' x 5'), making "real" speeds useful again.
• Plunging Fire. I liked David Manley's point about range effects on this web page, but I didn't want to complicate the game further with extra tables, so I settled on a modifier to achieve a less accurate effect along similar lines: all shots at extreme range (range bands 0, 1 and 2, the blue range bands on my gunnery range finders) give a -1 to the blue (hull) die. This helps represent increased damage to the watertight integrity of a ship from plunging fire, due to increased penetration of the thin deck armor.