Searay prop question

[sparks4540]

I have a '79 Searay CC. The outdrive is a newer performance pre-alpha. I think the gearing is 1.47? I dropped in a 350 that's making around 330hp. Boat runs pretty well overall. Comes on plane good and cruises around 28mph at 3200rpm. WOT is about 4500 and 39mph. That is with 3 people onboard and some gear. Max rpm on the cam is about 5500. Still have some tuning to do on the quadrajet secondaries and probably could use a little more timing adjustment. It's pretty good on gas as well, but I was only able to get maybe 3 hours of runtime on the new rebuild last year. Prop numbers are 78-78116 A40 and that's all I can make out on it because of some pitting. Research shows me it's a 15 1/4 X 15. Searay listing says it should be a 16 x 19. I can't really see how running a prop like that will get me better performance out of the boat? I am happy with it as is, but I think I should be able to get a little more on top if I simply go with a 15 x 19 prop or a X17. Was hoping for easy plane and top end of about 45 once the motor is dialed in. Trimming nets me a few mph, but that's about it. I would keep the old prop though because there are many times the boat has 3 or 4 people on it and it seems to plane pretty good as is...Any thoughts on that other prop for some more top end and decent holeshot yet. Seems that I am falling pretty short somewhere. thanks,Chris

 

[jestor68]

Re: Searay prop question

If you're only turning 4500 rpm with the current prop, you can't put anything "bigger" on there. If you do, it will drop your WOT rpm out of the recommended range and lug your new motor to an early death.

The guesstimate rpm range for that motor is 4600-5000 rpm, so you're on the lower edge of the range now.
.

How big is the boat?

Your prop number begins with 48; not 78. 48-78116 A40

Here's what the Mercury prop selector says it ought to do: 17 pitch at 4600 rpm @ 45 mph; which represents a 10% slip.

It appears that your "350" should be able to spin that 15" prop to about 5000 rpm. The question is; why isn't it doing it?

 

[sparks4540]

Re: Searay prop question

Thanks Jestor. The boat is 24' and if I remember right, weight was around 4000lbs. Typo by me on the prop pn. and the 48 is right. I can't figure it either why she won't spin any faster. I have some tuning to do yet and maybe it's as simple as a lean condition. I am going to pull a couple plugs and see what that tells me. May just need some more fuel, but I have my doubts being that quadrajets secondaries usually can handle that hp without rejetting. Just not sure. Actually, I checked again on the SeaRay prop chart. For a '79 240CC the prop should be 16x15C at 4200 with a 260 Merc.

 

[jestor68]

Re: Searay prop question

I noticed in the prop section that TP offers a 14.25 X 17 aluminum prop.

That might be worth trying, as the smaller diameter will be easier for the motor to spin up, while the added pitch should get you closer to the speed you're looking for. I expect with that prop you should be able to turn about 4400 rpm; which according to the slip calculator should net you around 43 mph at a 10% slip.

 

[sparks4540]

Re: Searay prop question

That's what I was thinking as well. Motor spins up really nice now, but just tops out at 39-40 and 4400-4600rpm. Since the diameter is an inch smaller than mine now, do you think trying a 14.25X19 would be worth a shot. I would think that might net me around 46-48mph in that RPM range. I am just going to see what happens this spring when I get on the water. I want to see if I can find a dealer around here that I can borrow a prop from for a day and try out..I do know that I pulled about 2 degrees of timing last time out and gained 2 mph and about 200-300 rpm at wot. I can probably squeeze some more of it with a little time on the water. Thanks for the input.