Short Wave Sportfishing wrote in message
...


I assume that these are high revolution diesels much like the one in
my truck. How do boats like that Topaz I'm looking at generate their
speed - gearbox?

That and prop pitch and diameter. My Navigator engines (Volvo) are rated at
2800 RPM with a recommended cruising RPM or 2600 RPM. I can't remember the
transmission gear ratio - I think it is 2:1, but I am not sure. The props
are 30" dia. by 30 pitch.

The Egg with the 3126 Cats was also rated at about 2800 RPM WOT. Again, I
don't remember the gear box ratio, and the props were something like 25"
dia. by 28 pitch.

Many of the serious sports-fish types have "trolling valves" installed on
the transmissions that allow a certain amount of hydraulic slip that permits
the boat to troll at a lower speed. Otherwise, the only way to troll below
about 6 knots was to put one transmission in neutral and work with one
engine and even then, on some of the really high-powered boats, you can't go
slow enough.


Oh, and while I'm on the subject, how does a engine synchro work? I
have an idea, but I'm not sure.

On both the Navigator and the Egg, there is a mechanical tachometer take-off
on the main crankshaft pulley on the front of the engines. A cable, similar
to an old-fashioned speedometer cable, transfers the rotational
speed of the tach to a hydraulic controller that is connected to the
throttle of the "slave" engine.

In operation, turning the synchro on disconnects the manual throttle control
of the slave engine and the hydraulic controller takes over and adjusts the
slave engine throttle to match the master engine RPM.
Both engines are now controlled by adjusting the master engine throttle
only.

I was amazed at how well they work. Although you can "tune" by ear, on
bigger boats it is sometimes hard to detect a 20 or 30 RPM difference. When
I first got the Navigator, I used to control both throttles manually at
first and thought I was doing a great job in keeping them in sync. I
noticed one day that the starboard engine was running about 5 degrees hotter
than the port engine after a long run. The temps were still well within
normal, but, being a nerd, I started worrying about why. A friend was with
me who is far more experienced than I and noticed I didn't have the syncs
on. At his suggestion I engaged them and within 5 minutes both engine temps
were identical.

Does that help?

Eisboch


Later,

Tom
-----------
"Angling may be said to be so
like the mathematics that it
can never be fully learnt..."

Izaak Walton "The Compleat Angler", 1653