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650 cfm vs. 750 cfm

J

jumpin

Member
I have a pair of 1987 big block Crusaders. I am trying to determine if the previous owner has the boat, a 1987 Chris Craft 426 Catalina propped correctly. I have installed new distributors, new racors and new spark plugs and through investigating have determined that previous owner bought remanufactured and converted 650 cfm carbs. My props and bottom are clean and when at full throttle I get 3850 out of the port engine and 3900 from the starboard. I have 2.9 to 1 transmission and am turning 24" diameter by 22" pitch props. Engine are timed to 10 degrees at idle and at 3000 rpm's advance is 24 degrees. Do you guys think the 100 cfm difference is costing me at least the 400 to 500 rpm's that I should be getting at WOT? Thanks
 
I doubt it. Assuming all is well in the power department (and you've done well checking things out there) then other factors need to be addressed: Clean bottom and prop blades? Props true (not bent)? Boat overloaded with "goodies"? Last on the list would be removing some pitch from your props.

Question: Does the boat labor to attain and maintain a plane?

Jeff
 
Fastjeff, just looked at survey and it says the props are 24" diameter by 25" pitch. Does that sound like alot of pitch even with the reduction gear I have? Also, what do you mean by goodies?
 
the carbs' size is more than adequate.

That is a lot of hull so what i would call 'normal' growth over the season will impact you a lot more than most of us - more surface area = more friction.

"Goodies" - typically items on board that are not really needed on a regular basis, excluding safety equipment; functional equivalent of ballast.
 
My dad had a 1990 38 eggharbor that was 15 ft wide. He had 454's with 2.5 reduction and turned a 24X21 prop. His boat would turn 4200 rpm in fresh water. Without doing the math it seems the lower gear ratio should allow you to turn 4 more inches of pitch.
 
When doing the math using pitch, RPM's, gear Ratio anf GPS speed the slip is 35.53 percent. Does this seem excessive? Formula- 25" pitch-2.9 reduction-3800 rpm and 20 mph GPS speed. Could this slip factor be telling me something that I am not aware of. Props are clean and I have no vibration that I can feel from the bridge. Do you think it could be that my throttle linkage is not opening the carbs up all the way? Not likely I thought, because I didn't think they BOTH would be not operating properly.
 
Think about it. Imagine 2 guys on a tandem bicycle. Your in great shape and your partner is like me, winded after 2 minutes on the bike. Now you are pedaling your legs and trying to pull my legs which are dragging you down. Same thing with your engines. If one isn't turning up, youir asking the other to turn 4200 plus pull the other engine up to 4200 rpm. It won't happen. To make a short story long one engine not turning up will cause them both to drag down. Easy to check. HAve you checked any Chris Craft websites, like the commander club? They may have some info for you. Also, I think we have that boat at out club. In another month or 2 it will be out of the water. Remind me with a private message and I can tell you what he has for props.
 
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