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Old 12-04-2013, 11:43 AM   #20
rjtrane
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City: Palmetto Bay
Country: USA
Vessel Name: Sunshine
Vessel Model: Island Pilot DSe 12m Hybrid
Join Date: Apr 2013
Posts: 268
Florida Bay Coasters- Key Largo & Florida Bay

When we first introduced the Florida Bay Coasters in 1988 with the 50' "Florida Bay," we chose a pair of Lehman 120s. These were perfectly sized for the 120,000# Coaster, providing a cruising speed of 8 knots (never did reach predicted 10 knots, though).

When building the 65', 160,000#, "Key Largo," we thought a bit of "extra" HP would be a good thing, so we installed CAT 3208 turbos at 320 HP each. We NEVER used the HP provided for by the turbos - in fact, the most expensive repair was to a turbo due to lack of use. When we did put the hammer down, we dug a hole and blew out a LOT of black smoke. We would have been better advised to use the NA 3208 at 210 HP each.

Over the production life of the Coasters, we installed lightweight Yanmars (still working OK after a single owner of over 20 years), 4 cylinder Fords (rough), 6 cylinder NA Perkins 135 HP (smooth and reliable). As long as we had enough HP to counter head winds (the Coasters have an impressive amount of windage), they were never going to exceed their hull speeds.

Not mentioned in this discussion of over-powering is propping correctly. With a FD hull and a wall at the hull speed, if one over powers, then when running at the real cruising speed, not only is the motor running off its peak performance, but so is the prop. To meet the engine manufacturers requirements, the prop is sized to hold back RPMs at WOT. With a prop sized to an overpowered engine, most likely, the prop will not be as efficient as possible at the cruising speed.

With the advent of modern diesels (read: computer controlled, common-rail or similar electronic injection), it does not hurt the motor to run below optimal RPMs and it will retain fuel efficiency throughout its power curve (except at the lowest and highest RPMs).
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Reuben Trane
"Sunshine" - Island Pilot DSe 12m
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