I have a sea swirl spyder sport 178, its a 18ft boat with a GM350. dry weight is about 1560 lbs
I have a 3 blade 14.25x19 and had a 4 blade 14x19 (damaged)
after running some online calculators it just give me some generic options in 4 blade: 14x17, 14x19, 14x21.
I want to go back to a 4 blade, I just don't like how the 3 blade preforms (maybe its under pitched)

my old 4 blade got on plane quicker, on plane at a lower speed and had more top end. a couple shops have told me to stay at 14x19, and a few other say I'm under propped and should be at a 14x21 or 14x23

not sure if it makes a difference, but this is a fresh water lake boat

What RPM and speed are you running now? Im thinking 3 blade 14x21 . An 18ft 1600lb boat with a 350 should haul ass.

Chris

I try to cruise in the 4k area but iirc red line is 5,500-6,000. it will keep getting faster till I run out of RPM.

I haven't hit it with the GPS but the vacuum speedo reads between 55-60mph

BTW I plan on sticking with a AL prop, cheaper and more forgiving to the lower unit if you hit something with it.

Well this weekend I tried a 4 blade 14.25x20. the boat is much faster and the RPM are now sitting at 4,800. If I put to to WOT and give is some time I can get to 5,500.

Long story short, that new prop is at the bottom of the lake.... The shop that put it on didn't put the back thrust washer on, I put the boat in to reverse the rpm jumped up and should move anymore. raised the drive and there was no prop back there. the hub and everything else was back there. they are replacing the prop free of charge.

the local shop has a used Stainless steel 3 blade 14.25x21 for $50, I'm gonna grab that and try it out next weekend and see how the boat preforms.