Thread: 4 blade prop
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Old 09-23-2020, 08:07 AM   #22
rslifkin
Guru
 
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,557
In general, more blade area (from more diameter and/or more blades) will improve hole shot as well as low RPM thrust. More blade area will also reduce WOT RPM with no other changes. Pitch will determine RPM and also speed. But sometimes adding blade area and reducing pitch reduces prop slip enough that you don't lose any speed at a given RPM.

Also, if efficiency is the goal, your most efficient slow cruise will likely be in the 6.5 - 7 kt range, as you'll be pushing much less water than at 8+ kts.

For my boat (38 feet, 33.5 foot waterline, twin 454s) I typically consider slow cruise as up to 1300 RPM (around 6.5 - 6.7 kts in flat water) unless I'm pushing a good headwind where I may bump it up to 1400 - 1500. Fast cruise for me is typically 3300 (17 kts) unless I have a reason to run a little slower. 3400 is generally considered max continuous for 454s. WOT speed this season was about 25 kts at 4200 RPM (a bit slow for this boat), so something is causing me a little extra drag (but nothing major). I remember years ago seeing this boat hit 27 kts at WOT (which should give about 4300 RPM with these props).

I've got 2.57:1 reduction transmisssions behind my Mercruiser 454s and 22x25 3 blades (turns 4200 - 4300 at WOT). Original props were 22x26, but it was a little overpropped with those (just barely hit 4000 RPM), so the 25s were swapped on. I'm currently hunting for a set of 22x25 4 blades, as it could use a little more blade area. If I don't come up with any, I'll probably take the other 3 blades (the original 22x26) and have the de-pitched to 25, as they have a bit more blade area than the currently installed props.

EDIT: Weight and hull design wise, the boats are probably similar. Mine should be about 26k lbs with full tanks and a normal load of stuff. It's a low deadrise (10*) modified V hull, pretty much looks like an old Hatteras with a shorter keel. From a picture I found, your hull looks pretty similar.
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