correct prop/pitch for 20' w/ 175 opti

[whitly]

OK, so clearly I don't know sh_t about props and pitch and RPMs! My new Mercury service guys, who I love, ask me what kind of RPMs I'm turning w/ my 175 Optimax. Answer: "Um, I dunno, but I'll get back to you." Well , now I know. But before I go into it and before I tell you how I managed to get talked into the wrong prop, let me ask this question:
What do you all think would be the best prop/pitch for a 2008 Optimax 175 25" shaft mounted on a 20' Seafari? And what should I be seeing for RPMs and speed?

[eggsuckindog]

17" Stilletto - should pull 5400/5600 and with the cabin maybe 46, with a cruise at 3500 of about 25/26

these are trying to adjust for my CC w/200 and a T Top vs cabin - I'm swinging a 19 with the extra HP 5600 with a cruise at 3400/3600 of 28/29 - WOT 52 @ 5600

I was just out yesterday and marveled at how the boat just danced over the chop in the bay, it just rides so much higher on the chines than it ever did before - trim adjustment is just flick here and there and it responds.

[DonV]

OK...OK.....I don't get too much into this prop thing, however who cares what it runs at WOT?? How many times you gonna run wide open??? To me it's what prop works best with the LEAST effort from your engine at the normal speed you run, using the normal loading of the boat, the safety of the boat, etc. You do not want the boat to struggle to run your NORMAL cruising speed. You want the engine to use the LEAST effort to cruise.....at let's say 4000 rpm @ 30 mph. Your bank account will thank you!

[eggsuckindog]

First of all he asked


The WOT RPM does allow to figure that, or you could get a 13" prop and the motor wouldn't work hard at all - just wouldn't go very fast at cruise. Actually a motor over propped will cruise at a faster mph just like overdrive in a car but hole shot would suffer. So it is a balancing act since we don't have transmissions and RPM at WOT is the best way to determine it.

I am a good example - my cruise when the motor would only turm 5300/5400 WOT was /26/27/28 @ 3400/3600 - by raising the motor and now getting 5600 WOT my cruise increased to 28/29/30 at the same RPM depending on water conditions. I also found it stays on plane and sounds much better at 3000/3200 than it did before - which is good because 29/30 is pretty quick for cruise and I find myself pulling back to 3000/25 much more which it didn't really want to do before. BTW the 52 is close to 4/5 mph at WOT higher than before - which is fun when I want to feed it and according Ridge Runner floats like a butterfly

Thats why I suggested you try my prop, its a bit big but should give most everything as far as feel except WOT RPM - the feel factor was very noticable even before I raised it.

[RidgeRunner]

Sting like a bee there ESD.
I think a large diameter big blade 15P like the Mirage would do it for the Seafari with the 2.5liter 175 Opti. Maybe a 17 Stiletto or Tempest. Post them numbers Whitly.
I got an education on props. I could turn a 19p Quicksilver to 5800 RPM on my boat or a 17P Mirage at 5800. BUT a 17p Quicksilver was the fastest WOT overrevving to 6100. Blade design and diameter make a huge difference. Ken at PropGods is one way to go about testing before you buy. Wealth of knowledge there..

[eggsuckindog]

RR when I came on here the Mirage was the go to prop and with the larger blade area may still be for 23's. I also had a Quicksilver when I got the boat - looks like a Vengence, kinda rounded blades, reminded me of MM ears. I looked and looked for a used Mirage or even one to try and never did. So possibly the 20 hull works as good or better with the smaller blade area of the Stilletto and 23's may work better with the Mirage - would love to do a test but the hull seems to like that design.

An aquaintence had this 19 Stilletto and in checking with Ken@Prop Gods it was similar to a Mirage/Tempest as to design - rake, progressive pitch, cup, stuff I really don't fully understand except rake and cup. The one I had I couldn't trim it at all before it blew out so knew I needed something else.

My experience was the same as yours - I had a 19 to 5700@48 mph - so bought that 19 Stilletto and it would only turn 5300/5400@47/48. The boat handled a ton better at cruise and was much much dryer, just rode higher, trimmed fine so I was much happier except the WOT RPM was low and low speed planeing so thought about a 17 too.

Raised the motor 1 hole and it made a huge difference, I'm still amazed, numbers up everywhere, and even low speed planeing its much much better which is important to me - so overall set up is as important as which prop I guess. I am at 2 1/2 inchs above the hull bottom on the Cav plate and it still will not break loose.

This is much more complicated than most understand, including me, thats why I refer most people to Ken with his knowledge of different hulls. With the Mirage knowledge from here and Ken telling me it was similar I lucked out on that Stilletto. - blind squirrel syndrome

I want to try the Stilletto on Don V's boat someday, a bit big pitch wise but a try anyway