Prop question

[Coonneck]

Ok here goes another prop question. I have just purchased a 27FC 1984 with a 4D61. The wheel I have on it now is a 17 in and cruises at 6.8-7 at 3100-3200 RPM at 180-190 degrees. In reading at many comments across the web it appappears as though I am turning way to many RPM’s to cruise at 7kts. Any suggestions on prop size and pitch would be appreciated.

[DCatSea]

Prop questions always generate lots of responses, so here goes again:

I've just switched out the OEM 15x11 on our 4D61 84FC for an acquired 16x14, just to see what happens. There are numerous opinions on this all over the interweb, so here's my current input.
With the original she cruises at 6.5-7 kts at 2700-2800 rpm, temps and oil pressure well within boundaries, and as with the 4D61 a decibel level that is OK but not great. Engine sounds well at all rpms up to 3500+, but I rarely put her up there.
I switched because I wanted lower decibel levels at cruise, and figured, in my unscientific, "lets see what happens" manner, to make the switch and see if there are (any) positive changes in normal running RPM, and more especially backing into my slip. We are going in on Monday and moving straight to the slip; I am told that slip mates have been selling tickets to see how I manage with the new screw. (Note: I'm happy to revert to the 15x11 if it all goes weird on me).
Good luck with your 27FC.

[ssrig]

Do you know what your pitch is on that prop?

I recently bought a new prop and went to a web site for prop sizing,
https://vicprop.com/displacement_size_new.php/?m=1

I did go oversized on mine and as a result I don’t achieve wide open throttle rpm but I do cruise at low rpms keeping the sound down, you can have your 17” re pitched to bring the rpms down and get a bit more speed, I have the option to do that with mine or even have it trimmed down in diameter.

I have a A25 with a 55hp VP and if I remember right I went with a 18”x 15.

[Ambler27FC]

These boats have a number of different engines/transmissions installed, so for an apples-to-apples comparison you'd only compare to a boat with the same engine and transmission.

My old engine went up to about 3600 RPM and I would hit hull speed around 2800 RPMs. My new engine goes up to 2800 RPM and I hit hull speed around 2100 RPM. In both cases the propeller was spinning at the exact same speed because the transmissions have different gear ratios.

At 2100 rpm, my boat speed will vary from 5.6 to 9.0 knots because of local currents - which I assume your watching. And I think a clean hull can add .1 - .3 knots...

Hope this helps.

[Ambler27FC]

Revising my statement above: you can compare to any engine if you know the transmission ration, which lets you calculate prop speed. I have a 2.0 gear ratio, so at 2100 rpm for hull speed my (18x12) prop speed is ~1050 RPM.

[Coonneck]

The prop I have now is stamped 17 in but no pitch designation. I have a spare with no markings that I will do some measurements on soon. I’m getting a trailer ready for this boat now before hurricane season get into full swing so I’ll be able to switch props and do fine tuning without paying a lift fee over and over.