owerless to shake off
her opponent.

Suddenly, and without the slightest warning, a huge, shiny, wet body
shot out of the water almost directly in front of the amazed and
startled boys, and settled back with a mighty splash that sent the
spray flying in a salt-water shower bath over their heads.

"Whatever was it?" gasped Hiram in awed tones.

"A shark," replied Merritt, "and a whopper, too. What are we going to
do, Tubby--keep on or cut loose?"

"Just a little longer," pleaded the other. "He must be tiring by this
time. If we can only wear him out, we can tow him ashore and make a
little money out of him. You know shark skin is valuable."

"I'd rather have a whole skin of my own," quavered Hiram, who had been
considerably alarmed by the momentary glimpse he had had of Tubby's
quarry.

"He's off again!" shouted Merritt, as the sea tiger started straight
ahead once more.

Suddenly the line slackened again.

"Look out!" Tubby had just time to shriek the warning before a mighty
shock threw them all off their feet in a heap on the bottom of the boat.

"Zan-g-g-g!"

The line twanged and snapped under the sudden strain, and a great rush
seaward showed the boys, as soon as they recovered their senses, that
they had lost their shark.

"And a good line," moaned Tubby.

"What are you kicking about?" demanded Merritt. "It's a lucky thing
the beast didn't start some plank of the boat when it charged; but as
far as I can see, the Flying Fish stood the shock all right."

"It felt like an earthquake," murmured Hiram, whose face was white and
eyes frightened.

"Well, I suppose we'd better head for home," said Tubby at length.
"Those bluefish will go fine for supper."

"Spoken like a Tubby," laughed Merritt. "All right, I'll start up.
Hullo--" he looked up with a puzzled face from the reverse lever. "I
can't get her on the forward speed."

"What's the matter?" gasped Hiram.

"I don't know. Something's stuck. Shut off that engine, will you,
Tubby, while I see?"

Tubby promptly shut down the motor, and Merritt struggled with the
refractory lever. It was all in vain, however; he could not get it on
the forward speed.

"I've got to investigate," puffed the perspiring corporal; "something
must be wrong with the reversible propeller."

"Well, whatever you are going to do, hurry up about it," spoke Tubby,
with unwonted sharpness in his tones.

"Why, what's the--" began Merritt.

Tubby checked him with a