This description is taken from the Harpoon4 Quick start booklet. It describes the basis of the game, and also details differences between the Third Edition, published in 1987, and the new Fourth Edition.
Harpoon4 as available boxed or as separate components from Clash of Arms.

The game uses three different Turn lengths. Intermediate Turns are 30 minutes long, and speed play when units are not expected to see each other. Three-minute Tactical Turns are used for most combat. If high-speed units (planes or missiles) are involved, the players can use 30-second Engagement Turns to resolve the movement and high-speed reactions of navel weapons. For all turn types, phases are executed by players simultaneously.

No combat is allowed in Intermediate Turns. Players plot, move, then resolve detection. If units have moved within detection range of each other, the players shift to Tactical Turns, starting either from their positions at the end of the turn, or backing up and using the positions at the sears of the turn.

In Tactical Turns, plotting is followed by movement, then planned fire, detection, and reaction fire phases. Reaction fire allows players to fire at targets detected that turn, with reduced effectiveness. Movement must be plotted. Fire is not declared until after movement is completed. Long-range missile launches and torpedo combo can be resolved using Tactical Turns.

If ships or aircraft are attacked by missiles, guns, or other aircraft, the players must use 30-second Engagement Turns. In each Engagement Turn the players can move, fire, resolve air attacks on ships, resolve detection, then move, fire, and resolve air attacks again. Most units can fire only in the First Fire Phase, but aircraft and some advanced ship-based weapons can fire in the second phase as well.

Ships and planes are classed by size: Large, Medium, Small, Very Small (VSmall), and Stealthy. They also have a radar signature which may be the same size or smaller than their actual size.

After an order to turn is given, ships must advance several hundred yards along their present course, and then can turn up to 45ø. There is a slight speed loss associated with each turn, but it is usually compensated by acceleration provided by the ship's engines. If several turns are made, the accumulated speed loss will slow the ship, which is assessed immediately.

Planes have a Defensive Air-to-Air rating, which is a numeric ranking of their maneuverability. All but the largest planes can turn up to 180ø at the start of a turn, then move in a straight line. Altitudes are divided into bands: Low (which includes a special Very Low band), Medium, High, and Very High, but players should also keep track of their attitude in meters. Aircraft endurance is modeled, and depends on the load carried, altitude, and throttle setting.

Submarine depths are divided into bands: Shallow (which includes a special Periscope/Snorkeling band), Intermediate, Deep, and Very Deep. The Intermediate and Deep bands are also subdivided into five zones, Intermediate I - V and Deep I - V. Depth change depends on speed.

Radar detection depends on the Signature of the contact and the range of the radar. For any radar, contacts with larger Signatures can be seen farther away. The chance of radar detection is the same for all radars, but depends on the type of turn the players are using. Detection is automatic in an Intermediate Turn. There is an 90% rl~ance of detection in a Tactical and 60% in an Engagement Turn.

ESM uses the same detection chances as radar, and will detect any radar within their range. ESM receivers arc ranked by generation, with newer receivers able to see farther than older ones.

Passive sonar detection depends on the noise rating of the potential contact and the passive range of the deducing sonar, as well as the water conditions. Players check the range to the potential contact against the range of the sonar, which allows them to see what the chance of detection is. There are many conditions which will increase or decrease the range of passive sonar detection. Active sonar detection is similar except that there are fewer modifiers and it is not affected by the contact's noise level.

Visual detection is based on the visibility conditions, with optional variations rolled by the players when they attempt to detect something. Infrared thermal imagers improve sighting ability, and are ranked by generation. Lasers are covered in the rules, but their use in naval applications is limited to range finders, designators, and one special system used on Russian Bear F Mod 4 aircraft.

Weapons characteristics are described in Chapter 5. Each of the many guidance systems in use is covered, along with how it affects a weapon's behavior after launch. Weapons with special characteristics, like the ALARM missile, are described in this chapter.

An important feature of guided weapons is their "generation". This describes their technological level, with more advanced weapons able to do more. When matched with a target's electronic defenses, the generation shows the weapon's chance of hitting. If you are familiar with naval systems and modern weapons, this is a good chapter to read first.

Players must detect a target before firing, unless they have a fire control solution, which can be developed over time from a series of passive bearings and othce data Fire control solutions are ranked as BOL (Bcaring-OnlyLaunch), Poor, Fair, and Good. The better the solution, the better the chance of a hit.

SAM fire and long-range AAM fire uses Engagement Turns. Each missile is ranked by generation and it also has an Air-to-Air (ATA) rating, which is compared to the plane's ATA rating to find out the chance to hit. This is modified by comparing the missile's generation to any ECM carried by the plane.

Close-in air-to-air Admit is abstracted, with any plane within 5 nm of another able to declare a dogfight Faster planes can outrun or evade a dogfight. Cannon can only be used in a dogfight, and AAMs may be used as well, although older ones are penalized. Each Engagement Tum (30 seconds) every plane in a dogfight rolls to gain position against its opponents, comparing its maneuverability against theirs. If it succeeds, it then takes a shot.

Air attacks on ships can be by missile, or with unguided ordnance. Missiles which reach a ship or planes which overfly their target make attacks, which are resolved in a special "resolution phase" after the ship's defensive weapons have fired.

Surface guns fire in either the Near or Far range bands, each half of the gun's range. One roll is made for each attack. Damage is fixed and is affected by range. The chance to hit is also affected by the range band. Most ships do not have armor, but those that do will be able to ignore some critical hits. A table shows what types and sizes of weapons are defeated by each level of armor protection. Gunnery attacks against ships and aircraft are made during the fire phases of an Engagement Turn.

Torpedo fire is resolved like missile fire, with the generation of the torpedo's seeker compared to any acoustic countermeasures the target carries. Evasive maneuvers by the target will also affect the torpedo's chance to hit. Torpedo movement is usually resolved with Tactical Turns, although Engagement Turns could be used in close situations.

Ships can attack submerged subs with depth charges, ahead-ehrown weapons, torpedoes, or ASW standoff weapons. Depth charges use patterns and require that the attacking ship see the charges to a particul~r depth Mortars only require that the target be in range and within the weapon's arc of fire. Ship-launched torpedoce behave like sul~launched weapons, although they are normally shorter-ranged. Stand off weapons carry a torpedo, and the player must roll to see where the weapon eneers the welter relative to the aum pomp.

All planes hit by a weapon in the game are considered either killed or damaged so severely that they must abort their mission. Optional rules allow players to track aircraft maintenance and repair damaged planes.

Attacks and damage are resolved in the phase in which thy occur. This means AAM, SAM and torpedo attacks are resolved in the movement phase. Gun, depth charge, and ahead-thrown weapons attacks are resolved in the fire phase they happen.

Damage is measured in points. All weapons inflict the same amount of damage whenever they hit. Weapons that hit the ship have their points totaled, then divided by the amount of damage remaining. This ratio is compared with a die roll to find out how many critical hits the ship has sustained. These may be weapons or sensors knocked out, fires or flooding. It will affect the ship's combat performance in a significant way, and may impose more damage, which will have to be resolved in the next turn.

Each critical is rolled on a table indexed to the target type or in some cases, weapon type. A few criticals can be corrected during the garlic, (fire, flooding), but most will remain in effect for the entire game. Rules are provided for limited repairs after or between battles.