DF90 rpms and prop questions

[Shores]

hey guys, new to me boat. 20' nautic star deck boat, It is 2100lbs. Normally a 115hp 2-stroke but this was rigged with a 05 Johnson 90hp 4-stroke. A DF90 of same year if what I have read and seen so far. RPM range of 4500-5500rpm. It's turning a 17p SS Powertech right now. 33mph gps at 6000rpms. Full tank fuel(30gals) and 2 adults into the wind. It was not ideal for a top end run but it's what I had to work with. It does this pretty easy. It's planing out pretty quick, IMO. I was thinking this boat was going to be a bit sluggish with a 90hp on it. I've had several F150's in the past on 21' deck boats and it felt similar. Just didn't have the longer legs up top. I'm think the target rpm with moderate load should be 5500rpm? That tells me a 19p would pick up some speed and and still have some hole shot?

or do these df90's like 6000 rpms and I should leave her alone?

[Shores]

26% slip is a ton.

[Shores]

er, I had the wrong ratio. It a 2.59 and slip is 12%, which is fine.

[Moonlighter]

And, your target rpm is 6000, not 5500 !

[Shores]

That's where I'm having an issue. My owners manual clearly states 4500-5500 for this engine.

[Murray]

I have a 20009 DF90A, specs. are 5500-6300 RPM’s.

[Shores]

I think 09-up are the larger blocks and have the higher rpm range. Mine is the 1950cc.

[Murray]

I think 09-up are the larger blocks and have the higher rpm range. Mine is the 1950cc.

Interesting mine is 1502cc The DF70 and DF80 in 2009 are 5000-6000RPM. If your owners manual wants you to stay in the 4500-5500 then you probably should. If it is a Suzuki with a Johnson badge then it should be able to handle 6000 RPM. With my engine the fun really begins at 5500 RPMs....

[Shores]

ok, I had that backwards. The pre-09 had the big pistons. It is a 119cu. Same as the 115hp.

This 3 blade SS Powertech is spun rather easily to 6k. Might be worth trying a 19p. This prop does have 3 vent holes open.

[Murray]

I’m assuming you have read the sticky re motor height. Sounds like your happy with the performance and 6000 RPM is a respectable number considering the conditions you had to work with. I’d suggest you get ideal conditions and see what you can out of the engine and then re-evaluate. Sound like you aren’t too far off for optimum performance.

[Shores]

I understand height but I have not checked it while running. Just seems like it can pull more pitch. I'd be very happy to pick up 5mph. Or even more so to get a solid 30mph cruise speed. Right now 26mph gps is about it for a cruise speed. I boat in the intercoastal and low 20's will get you overun by other boats pretty often. 30mph will not.

[Murray]

My thinking is get the height correct, it will cost you nothing. Prop. selection is a whole other field which I know very little so I won’t coment. I understand you want just a little more but that’s a large boat and it is only a 90HP. It’s an 05, is the maintenance up to date? Any history to the engine, Grandkids could have run the crap out of it. (previous owner of course)

[D'oh999]

I understand height but I have not checked it while running. Just seems like it can pull more pitch. I'd be very happy to pick up 5mph. Or even more so to get a solid 30mph cruise speed. Right now 26mph gps is about it for a cruise speed. I boat in the intercoastal and low 20's will get you overun by other boats pretty often. 30mph will not.

A 2100 lb, 20' Deep Vee Boat with a 90hp, and you want 30 mph at cruise? with 2 people and 300 lbs of fuel.

Good Luck.

Just as a spot check, I run a 17 140A, on a 20' 10 degree 1300 lb boat. 1 person and 20 gallons, 47 MPH @ 6100 RPM with a 23" pitch, Cruise 30 mph @ 4200 RPM.

[Moonlighter]

Yes.

Think your expectations for a 30mph cruise speed (economical cruise being around 4500 rpm for these engines) are way unrealistic.

But do let us know what you find.

[Shores]

I may be unrealistic but she's at 25-26mph gps @4500rpms right now. It's a V but not deep. It's about an 11 degree deadrise at the transom.