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Old 04-21-2020, 09:46 AM   #99
rslifkin
Guru
 
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,554
How big are your trim tabs? Going by the pictures, I'm going to guess they're either 24x9 or 30x9, so maybe a hair small, but nothing terrible. Going to wider span (covers more of the transom) tabs will let you run with them dropped to a smaller angle which gives less drag for a given amount of lift and will save you a little bit of fuel.

For my slightly stern heavy boat with big (48x12) tabs, I usually guesstimate (by time pressing buttons) the tabs to about where they'd need to be while on plane (about 1/3 of the way down or a hair less). Then throttle up and she climbs up on plane nice and smoothly. Once the boat stops accelerating, tweak the tabs to find best speed unless sea conditions require otherwise (trim down further in a head sea, up further in a following sea for ride comfort and handling at the expense of speed). I've never found a reason to drop the tabs all the way.

For an efficient slow speed cruise, you probably don't want to exceed 6.5 kts (estimating based on a 30-ish foot waterline length). With a 33.5 foot waterline length, I consider my slow cruise to be around 6.5 - 6.8 kts. I can push it to a little over 7 easily enough, but fuel burn and wake production definitely start to climb at that point.

Propped to turn 4000 at WOT, assuming you have Quadrajets or other vac secondary 4 barrels on your 350s, most efficient cruise will probably be somewhere in the 2900 - 3200 range. Re-propping to get WOT up to 4200 - 4400 may save you a little fuel up on plane, as it'll be easier to get to a speed where you're fully up on plane without getting into the secondaries.

Based on what you've said, it sounds like your fuel flow meters are either malfunctioning or just way out of calibration.

With small blocks, as a safe estimate based on the numbers you've given, I'd expect (for planning purposes) somewhere around 0.7 - 0.9 nmpg on plane, probably around 1.5 - 1.6 nmpg at low speed, maybe closer to 2 nmpg if you're lucky. With my fuel sucking big blocks in a somewhat bigger, heavier boat, I figure 0.5 - 0.6 nmpg on plane, about 1.2 at 6.5 - 7 kts.
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