Why is the island called "Ship-Trap Island"? What mean, tricky thing does Zaroff do to ensure ships sink so that he can round up the survivors?

At the beginning of the story, Whitney and Rainsford are standing on the deck of a ship.  It is nighttime.  Whitney points out into the darkness and tells Rainsford that there is indeed an island in the darkness.  Whitney further explains that sailors have a "curious dread" of the island, and the island also happens to be named "Ship-Trap Island."  

"The old charts call it Ship-Trap Island,"' Whitney replied." A suggestive name, isn't it? Sailors have a curious dread of the place. I don't know why."

After falling overboard, Rainsford is forced to swim toward the island.  On the island, Rainsford meets Zaroff, and Zaroff explains why the island has been appropriately named.  
Zaroff's island has electricity on it.  Additionally, Zaroff has placed electrical channel markers out in the island's waters.  The reality is that the channel markers do not actually indicate a real channel.  Zaroff has placed the markers to guide ships into the shallow waters around the island.  Ships mistakenly think there is a channel and run themselves aground.  

"Watch! Out there!" exclaimed the general, pointing into the night. Rainsford's eyes saw only blackness, and then, as the general pressed a button, far out to sea Rainsford saw the flash of lights.
The general chuckled. "They indicate a channel," he said, "where there's none; giant rocks with razor edges crouch like a sea monster with wide-open jaws. They can crush a ship as easily as I crush this nut." He dropped a walnut on the hardwood floor and brought his heel grinding down on it. "Oh, yes," he said, casually, as if in answer to a question, "I have electricity. We try to be civilized here."

The ships are trapped or destroyed, and the sailors are forced onto the island. They are lured to Zaroff's estate and then captured by Zaroff and Ivan.