Prop Guidance for Mercruiser

1) Performance issue you are trying to correct.

- Not sure yet. I'd like some opinions whether the boat is capable of higher cruising speeds / top end and MPG

2) Current prop manufacturer, model, aluminum or stainless as a minimum.

- Boat currently has SS Merc 3 blade 13 3/4" x 21P (48-73982-A4) on the drive
- Included with the boat was (1) Alum Merc 14 x 19P prop and (1) Michigan 14 x 19 SS prop
- All 3 props are in good shape, but I've only tested the 21P prop that's on the drive

3) Current prop diameter and pitch (required).

- 13 3/4 x 21P

4) Wide open throttle RPM and speed with an average load (very helpful)

-4200 @ 45 mph

5) Engine/drive make, model, year, and HP

-Mercruiser 5.7 260HP / Alpha One, 1987, 260HP (not original motor, but OEM replacement)

6) Boat make model, year, length and weight

-1987 Chris Craft 230 Limited / 22' length / 2800 dry (3800 with gas and family aboard?)

I purchased this boat from my wife's grandfather last year. I had transom redone, etc, etc....lots of work (still in progress but mechanically good to go, just touching up on some cosmetic issues now)

My wife's uncle that helped maintain the boat said they tried several different props on this boat and 45mph is top speed at 4200rpms. Cruising speeds around 3400-3600rpms in the 35-38mph range. I know they tried the 19p Michigan prop but obviously had better results with the 21p, since that's the prop that was on the drive when I bought it.

Merc prop selector suggests 16 or 17 pitch or Rev 4 15 pitch. This is main reason I'm posting this thread for advice since they are suggesting a much lower pitch. I'd like to add that they only have the 1.47 Gear Ratio option, not 1.5 which is what I have. Also would like to know what the max RPM should be for this motor?

Re: Prop Guidance for Mercruiser

I'm thinking that you could go to a 19" pitch, based on the rpm's . . . I do not think you will see much more speed out of it though . . Maybe 1-2 mph, that's about it. Go with the Michigan Wheel 19 SS that you have and see what you get for #'s

I'm surprised that the Merc Prop selector app came up with 16-17" pitch . . . seems a bit low.

Merc Rev 4's are pricey and even used ones go quick on eBay . . . I'd try a 17" Rev 4, if you were going that route, 15" may be too little, and it is not the kind of pitch that will go quick on eBay

Re: Prop Guidance for Mercruiser

Actually, the 1st results on Merc Prop Selector were from my boat weight, length, etc. When I re-entered it using my current prop, rpm, and top speed, it suggests 17-19P, so that seems more accurate. I had planned on trying the 19P next time out, hopefully this weekend, but just wanted some opinions on whether or not I would benefit from a different prop. A little research and I learned that the RPM range is 4200-4600, so I'm on the lower end of that range.

Top speed is great, but I'm more concerned about my speed at 3200-3500 RPM. If I can pick up 3-4 MPH at that range AND increase my MPG's, then that would be ideal. And worth the cost of a new prop IMO.

Re: Prop Guidance for Mercruiser

mguidry629 said:

Top speed is great, but I'm more concerned about my speed at 3200-3500 RPM. If I can pick up 3-4 MPH at that range AND increase my MPG's, then that would be ideal. And worth the cost of a new prop IMO.

Click to expand...

Wait. Do you realize, that if you lower your pitch, that you will LOWER your speed at 3200-3500 RPM?

Re: Prop Guidance for Mercruiser

BobGinCO said:

Wait. Do you realize, that if you lower your pitch, that you will LOWER your speed at 3200-3500 RPM?

Click to expand...

Maybe not. He's running at cruise with around a 35% slip; if he can reduce that slip number he MIGHT actually see better speeds with a lower pitched prop.

To the OP: At WOT your prop slip is on the high side of "normal" at 20%. That's fine, but the high slip number at cruise speeds tells me you MAY pick up some speed (and efficiency) with a prop change. An extra blade MAY help here.

I throw all the "MIGHT"s and "MAY"s in there because the only way to know for sure is to test.

Local prop shops often have a test or trade-in program. That's one option. Another is to buy/sell on ebay. Neither option is free, but if you can pick up some efficiency at cruise it will pay for itself. You are 100% correct about that one.

One final thing; Mercury isn't the only one making props to fit your Alpha drive. There are other brands/options that MAY (here we go again) produce better results. For example, the best performing prop we found on our 20' bowrider (5.0mpi/Alpha) was a Turbo 2+2TH. Again, don't think you have to stick to Mercury products because you don't....and you may not even want to.

Re: Prop Guidance for Mercruiser

Thanks for the insight...how do you calculate slip? I guess I can google it and find out....I think I'll try the 19P I have and post up the results and then go to a prop shop and see if I can test a 4 blade. I'm not partial to Merc props, but the Merc Prop Selector is, so that's what I was going off of.