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If the towee cooperates (TowedShipsCooperate), it will not break the tow.

Alternative Towing: Towing depends on the tow strength of the tower, and the tow resistance of the tow target. The tow succeeds if the tower's strength equals or exceeds the tow resistance. If the towee has a larger tow resistance, he breaks the tow.

(4.0j) In case both ships have equal tow strength, the ship with the higher experience wins. If the experience level is the same, the tow breaks.

Tow strength is determined as follows:

Tow_strength =
   Engine_contribution + Movement_contribution
                    ...if ship has fuel
   0                ...if ship does not have fuel

Engine_contribution =
   Engine_tech^2 * Eff_engines * Warp_factor * TowStrengthEngineScale

Movement_contribution =
   Movement_distance * TowStrengthDistanceScale
   ...where
      Movement_distance = Min(Waypoint_distance,
                              Max_dist,
                              Max_allowed_by_fuel)

• Engine_tech is the tech-level of the engine (i.e. 10 for Transwarp Drives)
• Eff_engines is the number of engines the ship has, times two if the ship has gravitonic engines, plus an additional 2 if the ship has a Level 2 Tractor beam.
• Movement_distance is an estimate how far the ship will move regularily this turn.
  • Waypoint_distance is the distance to the waypoint. For ships that hyperjump, this value is 350, independant from actual waypoint.
  • Max_dist is the maximum distance allowed by the ship's speed, as explained in the movement formulas section, e.g. 81 for a regular ship at warp 9.
  • Max_allowed_by_fuel is the maximum distance the ship can travel before running out of fuel. For the tower, the computation includes the towee's mass.

Tow resistance is computed using the same formula, but using only the towee's mass in computing the fuel/distance limit.

These rules can be summed up as follows:

• good engines win over bad engines
• ships with many engines win over ships with few engines
• fast ships win over slow ships
• gravitonic engines win over non-gravitonic engines

Note that up to version 4.0i, towing behaved slightly different. First, Level2Tow always was an absolute win. Ships with Level2Tow could always tow ships without, the function did not explicitly affect tow strength. Second, in case of equal tow strength, the tow would have succeeded; experience was not considered.

HOST-compatible Towing: HOST-compatible towing is based on an analysis of the behaviour of HOST 3.22.009. The rules can be summarized as follows:

• A tow breaks if all of the following conditions are fulfilled:
  • The towed ship's warp speed is greater or equal to the towing ship's warp speed. If the towing ship has gravitonic engines, its warp speed is doubled; this does not apply to the towed ship.
  • The towed ship has a waypoint further than warp-squared lightyears away (i.e. more than one turn of travel most of the time).
  • The towed ship has at least 25 kt fuel.
  • The towed ship can tow.
• When two ships try to tow each other (mutual towing), the low-Id ship wins most of the time. Exceptions are:
  • If both ships have the same warp speed, and both are set to travel more than one turn (warp-squared lightyears), they break apart and each moves towards its waypoint.
  • If the high-Id ship meets the criteria for breaking the tow (see above), it will tow the low-Id ship.
  • As for the simple towing case, gravitonic engines for the lower-ID ship double its effective warp speed. This effect does not hold for the higher-ID ship.