Sub: Add throttle channel gain scalar parameter

ArduSub/Parameters.cpp

@@ -517,6 +517,13 @@ const AP_Param::Info Sub::var_info[] = {
 	// @Range: 30 400
 	GSCALAR(lights_step, "JS_LIGHTS_STEP", 100),
 
+	// @Param: JS_THR_GAIN
+	// @DisplayName: Throttle gain scalar
+	// @Description: Scalar for gain on the throttle channel
+	// @User: Standard
+	// @Range: 0.5 4.0
+	GSCALAR(throttle_gain, "JS_THR_GAIN", 1.0f),
+
 	// @Group: BTN0_
 	// @Path: ../libraries/AP_JSButton/AP_JSButton.cpp
 	GGROUP(jbtn_0,                   "BTN0_", JSButton),

ArduSub/Parameters.h

@@ -206,7 +206,7 @@ public:
         k_param_disarm_delay,
 		k_param_terrain_follow,
         k_param_rc_feel_rp,
-
+		k_param_throttle_gain,
 
 		// Acro Mode parameters
         k_param_acro_yaw_p = 220, // Used in all modes for get_pilot_desired_yaw_rate
@@ -345,6 +345,7 @@ public:
 	AP_Float				maxGain;
 	AP_Float				minGain;
 	AP_Int8					numGainSettings;
+	AP_Float				throttle_gain;
 
 	AP_Int16					cam_tilt_step;
 	AP_Int16					lights_step;

ArduSub/joystick.cpp

@@ -46,7 +46,7 @@ void Sub::transform_manual_control_to_rc_override(int16_t x, int16_t y, int16_t
 	uint32_t tnow_ms = millis();
 
 	float rpyScale = 0.4*gain; // Scale -1000-1000 to -400-400 with gain
-	float throttleScale = 0.8*gain; // Scale 0-1000 to 0-800 with gain
+	float throttleScale = 0.8*gain*g.throttle_gain; // Scale 0-1000 to 0-800 times gain
 	int16_t rpyCenter = 1500;
 	int16_t throttleBase = 1500-500*throttleScale;