Which prop?

[Starkenwolf]

So I've got to replace this prop - it's seen more than it's useful life.

The boat's a 26.5 ft Regal Ambassador (cabin cruiser), 5.7 Merc 260 hp, with an alpha I Gen II sterndrive. 1.65:1 gear ratio.

Currently it has prop 1912 17 pitch 3 blade. My marine mechanic fellow suggests either going with the next larger diameter in the same pitch, or the same diameter in a 19 pitch.

Operating rpm's are 4200-4600 and cruise is around 3200. Top end is not my concern. 30 mph is more than fine with me (this is as is with a full fuel tank and 2 men out fishing).

Any advice?

Thanks!!

 

[Silvertip]

Re: Which prop?

You indicate operating rpm is 4200 - 4600 RPM. I take it that's what you determined the engine manufacturers "recommended" operating should be. This is only 1/2 the equation. You need to tell us what you actually "observe" on the tach at WOT. We realize you are not interested in WOT operation but the fact is, that's what you need to do to determine what the best prop is. If the prop is wrong at WOT it is wrong across the entire rpm band. Check the READ THIS FIRST sticky at the top of the page.

 

[Starkenwolf]

Re: Which prop?

Yes, the manufacturer's rec is 4200-4600. WOT gives about 5400 rpm with 35mph when the boat is perfect. Mechanic tells me not to run it over 5000 for any extended period.

These results are with boat setup the way I run it most of the time. Mechanic is all about speed, but as we know, I'm not necessarily. I'd like it to get on plane a little faster and remain on plane at lower rpm's if possible. She's a heavy boat, and almost all the weight is in the rear.

 

[Starkenwolf]

Re: Which prop?

Tested a 15.2" stainless steel prop with a 17 pitch. Worked perfectly WOT at 4900 rpm at 36mph gps. Begs the question, does stainless steel really make that much of a difference? Would an aluminum prop of same yield the same results? Obviously, the alum would be much less expensive.

 

[1980Coronado]

Re: Which prop?

Tested a 15.2" stainless steel prop with a 17 pitch. Worked perfectly WOT at 4900 rpm at 36mph gps. Begs the question, does stainless steel really make that much of a difference? Would an aluminum prop of same yield the same results? Obviously, the alum would be much less expensive.

That's 300 rpm over max rated wot rpm....that's not working perfectly IMO. As far as Aluminum vs Stainless...usually you can reduce the pitch of a stainless prop (by 1") and match the performance of a 1" larger pitch aluminum, because it doesn't flex as much as aluminum.

 

[Silvertip]

Re: Which prop?

So lets analyze this a little bit. In your first post you said the prop was a 1912 17P prop. What is the 1912 in relation to. I take it that is a typo for diameter, but what diameter? If you went from one 17P prop to another 17P prop and dropped 500 rpm and picked up 1 mph I question the original numbers or the validity of the results from the second prop. Two props of the same pitch, one being stainless and the other aluminum does not produce those numbers. If the new prop is heavily cupped and the old one wasn't that would account for some drop - but not 500 rpm. Keep in mind if you are happy with the result then so be it. I'm trying to figure what data is wrong.

 

[Starkenwolf]

Re: Which prop?

That's 300 rpm over max rated wot rpm....that's not working perfectly IMO.

You are correct. However, I was wrong on the mfr recommended operating range. I incorrectly listed it as 4200-4600 when it is in fact 4600-4900, according to marine mechanic who has been helping.

The 1912 is the number stamped on the original prop. It is a 14.5" 17p aluminum.

The current "loaner prop" is a 15.2" 17p SS. I haven't bought the prop yet, but will need to in the very near future. Please give me your honest opinions.

I'm a new boater and this has been one very enlightening couple of months...to say the least. This forum has already proven invaluable.

Thanks!

 

[1980Coronado]

Re: Which prop?

That's 300 rpm over max rated wot rpm....that's not working perfectly IMO.

You are correct. However, I was wrong on the mfr recommended operating range. I incorrectly listed it as 4200-4600 when it is in fact 4600-4900.

The 1912 is the number stamped on the original prop. It is a 14.5" 17p aluminum.

The current "loaner prop" is a 15.2" 17p SS. I haven't bought the prop yet, but will need to in the very near future

Ok...that makes sense then...increased diameter by almost 3/4" (big change) and switched to stainless with the same pitch......you reduced the rpm by 500 and got another mph....sounds good to me.

 

[Starkenwolf]

Re: Which prop?

Where's Harlan? I spent 18 years living in Terre Haute and Indy before I closed my shop and moved to S. Florida.

Thanks again for your help.

 

[1980Coronado]

Re: Which prop?

NE Indiana......Fort Wayne area....I live just off the Amish Turnpike

 

[Starkenwolf]

Re: Which prop?

LOL nice! If you're ever down this way give me a shout - we'll try our luck on some dolphin!!

A'Salt Weapon, monitor VHF channel 68, most weekends out of the St. Lucie inlet!

 

[1980Coronado]

Re: Which prop?

LOL nice! If you're ever down this way give me a shout - we'll try our luck on some dolphin!!

A'Salt Weapon, moitor VHF channel 68, most weekends out of the St. Lucie inlet!

I hope to get back down there sometime next summer....maybe spring break. We stayed in Lauderdale by the sea many times on family vacations. Have been diving with South Florida Dive out of Hillsboro Inlet just south of you many many times. Only been fishing down there once.....kite fishing...caught a nice sail fish....but I love Mahi Mahi...yummy!