diff --git a/include/prototypes.h b/include/prototypes.h
index d31ed61d..5ea7a47a 100644
--- a/include/prototypes.h
+++ b/include/prototypes.h
@@ -399,7 +399,7 @@ extern int check_trade_ok(struct trdstr *);
 /* coastal.c */
 extern void set_coastal(struct sctstr *, int, int);
 /* control.c */
-extern int security_strength(struct sctstr *, int *);
+extern double security_strength(struct sctstr *, int *);
 extern int military_control(struct sctstr *);
 extern int abandon_askyn(struct sctstr *, i_type, int, struct ulist *);
 extern int would_abandon(struct sctstr *, i_type, int, struct ulist *);
diff --git a/src/lib/commands/conv.c b/src/lib/commands/conv.c
index 76a1523a..c6175110 100644
--- a/src/lib/commands/conv.c
+++ b/src/lib/commands/conv.c
@@ -48,7 +48,8 @@ conv(void)
     struct sctstr sect;
     struct nstr_sect nstr;
     int uwtoconvert, newuw, totaluw, uw;
-    int maxpop, civ, mil, nsec, adj_mob, mob;
+    int maxpop, civ, seceff, adj_mob, mob;
+    double secstr;
     double security_extra = 1.0;
 
     if (!snxtsct(&nstr, player->argp[1]))
@@ -66,11 +67,11 @@ conv(void)
 	natp = getnatp(sect.sct_own);
 	maxpop = max_pop(natp->nat_level[NAT_RLEV], &sect);
 	civ = sect.sct_item[I_CIVIL];
-	mil = security_strength(&sect, &nsec);
+	secstr = security_strength(&sect, &seceff);
 	/*
 	 * Must have military control to convert captured civs.
 	 */
-	if (mil * 10 < civ)
+	if (secstr * 10 < civ)
 	    continue;
 	newuw = civ;
 	if (newuw > uwtoconvert)
@@ -88,7 +89,7 @@ conv(void)
 	mob = sect.sct_mobil * 5;
 
 	/* security troops make conversion more effective */
-	security_extra = 1.0 + nsec / 10.0;
+	security_extra = 1.0 + seceff / 1000.0;
 	adj_mob = ldround(((double)mob * security_extra), 1);
 
 	if (adj_mob < newuw)
diff --git a/src/lib/commands/shoo.c b/src/lib/commands/shoo.c
index 45f9e329..28821a6a 100644
--- a/src/lib/commands/shoo.c
+++ b/src/lib/commands/shoo.c
@@ -28,6 +28,7 @@
  *
  *  Known contributors to this file:
  *     Dave Pare, 1986
+ *     Markus Armbruster, 2004-2016
  */
 
 #include <config.h>
@@ -43,13 +44,12 @@ shoo(void)
 {
     struct sctstr sect;
     struct nstr_sect nstr;
-    int nshot;
-    double m;
+    int seceff, nshot;
+    double secstr, m;
     i_type item;
     struct ichrstr *ip;
     int targets;
     char *p;
-    int mil, nsec;
     char prompt[128];
     char buf[1024];
 
@@ -66,21 +66,20 @@ shoo(void)
     while (nxtsct(&nstr, &sect)) {
 	if (!player->owner)
 	    continue;
-	mil = security_strength(&sect, &nsec);
-	if (sect.sct_item[item] == 0 || sect.sct_item[I_CIVIL] > mil * 10)
+	secstr = security_strength(&sect, &seceff);
+	if (sect.sct_item[item] == 0 || sect.sct_item[I_CIVIL] > secstr * 10)
 	    continue;
 	nshot = sect.sct_item[item] > targets ? targets : sect.sct_item[item];
 	if (nshot > sect.sct_mobil * 5)
 	    nshot = sect.sct_mobil * 5;
 	m = nshot / 5.0;
 	/*
-	 * Each security unit lowers the cost of
-	 * shooting a person by 10%. However, you
-	 * can't go lower than 50% of normal cost
+	 * Security units reduce mobility cost of shooting people by
+	 * 10% per 100% unit efficiency, up to a 50% reduction.
 	 */
-	if (nsec > 5)
-	    nsec = 5;
-	m *= 1.0 - nsec * 0.1;
+	if (seceff > 500)
+	    seceff = 500;
+	m *= 1.0 - seceff / 1000.0;
 	if (nshot <= 0)
 	    continue;
 	if (m < 0)
diff --git a/src/lib/subs/control.c b/src/lib/subs/control.c
index 3fc5b1f9..a2df3a3f 100644
--- a/src/lib/subs/control.c
+++ b/src/lib/subs/control.c
@@ -42,28 +42,29 @@
 
 /*
  * Return strength of security detail in @sp.
- * Store number of land units with security capability in @nsecurity.
+ * Store sum of efficiency of land units with security capability in
+ * @seceffp.
  */
-int
-security_strength(struct sctstr *sp, int *nsecurity)
+double
+security_strength(struct sctstr *sp, int *seceffp)
 {
-    int strength;
-    int nsec;
+    double strength;
+    int seceff;
     struct nstr_item ni;
     struct lndstr land;
 
     strength = sp->sct_item[I_MILIT];
-    nsec = 0;
+    seceff = 0;
     snxtitem_xy(&ni, EF_LAND, sp->sct_x, sp->sct_y);
     while (nxtitem(&ni, &land)) {
 	strength += land.lnd_item[I_MILIT];
 	if (lchr[land.lnd_type].l_flags & L_SECURITY) {
-	    strength += land.lnd_item[I_MILIT];
-	    nsec++;
+	    strength += land.lnd_item[I_MILIT] * land.lnd_effic / 100.0;
+	    seceff += land.lnd_effic;
 	}
     }
 
-    *nsecurity = nsec;
+    *seceffp = seceff;
     return strength;
 }