I don't know the size of my prop......

[wil7483]

I can't find the size of my prop stamped anywhere on it. I was out today for the 1st time this year with the boat and need to know what size prop I will need. I ran the boat at WOT and was able to get to 32 mph on the GPS at 3300 rpm. Anyone know what the normal rpm range for this engine is at WOT? Any idea what size prop I should be using to get in that range?

1983 SB Chevy 305, Alpha One out-drive, 4 Barrel Quadrajet carb.

 

[Maclin]

Re: I don't know the size of my prop......

You really have to have the outdrive ratio in order to come up with a good guess on the prop pitch from rpm and mph.

That engine's range is 4400-4800 I believe. You may have some more sorting out on your combo...If your tach is correct then the engine is not reaching top rpm. And 32mph is not very fast for that engine unless it is in a 26 foot cruiser or something.

 

[wil7483]

Re: I don't know the size of my prop......

It is in a 24ft Cuddy Cabin. The outdrive is 1:50.

 

[wil7483]

Re: I don't know the size of my prop......

This is the boat it is in.

 

[wil7483]

Re: I don't know the size of my prop......

Any ideas?

 

[codertimt]

Re: I don't know the size of my prop......

Did you pull the prop and check on the inner hub for a stamping?

The calculator I use says you have a 19" prop given the numbers you provided. I really don't have the experience to judge what kind of performance such a boat should be capable of...someone else might can give you a better idea in that regards. From looking at NADA data though it looks like it's a shade under 4000 lbs, so it's no lightweight. The basic list of things to look at goes something like this...

• Check to make sure you boat hasn't gained alot of weight from wet foam/wood. The proper way is to get to the foam and take a sample of it, but it seems like you could weigh the boat and get a general idea if it's in the ballpark of weight still...
• Make sure the engine is making the power it should. At the very least, you could run a compression test to see if all cylinders are pulling their own weight.
• Verify the RPMs are correct with another tach.
• If all of that checks out, then you're just way overpropped. You need to drop down to a 15" or even 13" pitch to get your RPMs up where they should be. You're probably not going to gain speed, but your holeshot should be much improved and your engine will thank you in the long run properly propped...

Hope that helps some. I'm not an expert, so those more knowledgeable, feel free to correct or add to any of my comments...

 

[Silvertip]

Re: I don't know the size of my prop......

Don't even think about chaning props until you get the power issue sorted out. That engine is not making full power or the boat is seriously overweight. Make sure the throttle is fully open and that you are trimming correctly. The prop part number is normally stamped on the barrel of the prop or on the hub. You need to remove the nut and washer to see the number if its on the hub.

 

[wil7483]

Re: I don't know the size of my prop......

The boat weighs 3900 lbs empty. With gas, equipment, and two people on board it weighs in the neighborhood of 4800 lbs easily. The boat has no flotation foam in it so that isn't an issue. The floor is new so no saturated wood. The engine isn't getting to max rpms because the prop isn't allowing it too. That's my question, maybe I phrased it wrong. What size prop should I be using. I am not really interested in speed, I just don't want the engine working extra hard because of the wrong prop. I use the boat to fish so speed isn't a concern of mine. It's a big boat and it really could use a larger engine but for now I have to settle for what I have and make it work as efficiently as possible.

 

[MikDee]

Re: I don't know the size of my prop......

Well yrs ago I had this 79' 24' SeaRay Cuddy with a 350/260hp Mercruiser about the same weight as your boat, with thru hull exhaust, synthetic oil, a slick bottom, and the best I could get out of it fully trimmed out, with a 17" Alum prop & 4600rpm was 40mph!

So, I'd say with that 305 maybe approx. 228hp you probably have a 19" pitch prop, and would need to drop down to a 15" Alum. prop to gain back about 800rpm, and put you in your proper rpm range. You might get away with a 15" Stainless prop, you would probably have to try it to see if you could get the right rpm.

 

[wil7483]

Re: I don't know the size of my prop......

Here are the numbers off of the prop....48-16314-17. So I am assuming that it is a 17 pitch prop. Do you think I should still drop down to a 15 pitch or maybe even a 14?

 

[MikDee]

Re: I don't know the size of my prop......

Here are the numbers off of the prop....48-16314-17. So I am assuming that it is a 17 pitch prop. Do you think I should still drop down to a 15 pitch or maybe even a 14?

That being the case, I agree with codertimt, and suggest a 13" pitch prop, rather then a 15" prop, and I bet your motor will rise into the propper rpm range, and your speed may come up to 35mph, once you take the load off.

 

[wil7483]

Re: I don't know the size of my prop......

I guess I do have some more engine work to do. I went back out today with a 15p prop on and actually had worst numbers than with the 17p prop. Today I was only able to get 3k rpm out of her and 27 mph. I was going to repower her this winter with a Rapido-Marine 350 engine but I guess I'll actually do it earlier now so that i can actually salvage some of this season.

 

[MikDee]

Re: I don't know the size of my prop......

You don't want to repower now, the seasons just starting! I'd check fuel system completely back to the antisiphon valve, & the fuel tank vent, it can't be putting out full HP! I'd say you should be able to do 40 at least, and 4600rpm with the 17" pitch prop, or at least that with the 15" pitch.