Prop choice

[Deleted]

1998 sea pro 180cc with evinrude 90hp spl. Current prop 14x17. (Seems a little big to me)
Wot 4500 rpm with 2.5 people (the boy is 8) 27 kts. Anyway that seems way too low for rpm if I remember correctly.
Thinking of dropping down in pitch and maybe size. What do you guys run? I want to go stainless also. Any suggestions would be helpful.

John

[Deleted]

May 5, 2013 19:23:41 GMT -6 Juan said:

You're right... the rpms are too low.. I'd guess you should be running about a 13 x 21 or something close to that to get the rpms up to where they should be... There a many sites on the net that will help you choose the right prop... google search "prop selection"

Wouldn't the 21 pitch bring the rims down? I have been googling but wanted to know what others are using for their set up.

[Juan]

Yep, that's why I said "or something close"... you might have to go with a 19, 17 or even 15...it's hard to say since it depends on many factors that vary from boat to boat and what you want to achieve... but you should try to get the rpms within the factory recommended range...(double check.. but I think is between 5800 and 6000 on most Yammies)

[karlfdburg]

I actually have three props I use with my 2004 90HP Johnson -- two to use and one spare. The main ones I use are a three blade 17-pitch and a four blade 21-pitch. If I'm just going out fishing with the family, I use the 21-pitch blade. If they want to tube, I use the 17-pitch.

I learned more than I ever wanted to know about props last year after hitting some hidden rocks on the James River. As a quick primer, moving to a higher pitch will result in a DECREASE in your wide open throttle (WOT) RPM; going lower in pitch will INCREASE your full-throttle RPM. Although it depends on some other characteristics of the prop, the general rule is that 2 inches of pitch will average 300 to 400 WOT RPM change, i.e. going from 17-pitch to 15-pitch will move WOT RPM from 4,500 to ~4,900 RPM.

Check the manual for your motor to see that the WOT RPM should be. From that, you can figure out what pitch may work best for you.

A couple of other things to think about:

1. Stainless steel sounds great from a performance and strength standpoint, but ... most efficiency isn't gained until exceeding 50 MPH, they are expensive, and repairs are harder and more expensive. If you boat in an area with obstructions or shallow water where you may strike bottom, think hard about that factor.

2. Chosing 3-blades vs. 4-blades has much to do with preferences for lift "out of the hole." 4-blades gives more power out of the hole. There are other factors, but four blades may give you some benefits -- worth a look.

3. Calculations for WOT-to-RPM are slightly different for 4-blade props. In general, subtract 1 inch of pitch for the equivalent RPM of a 3-blade prop, e.g. a 19-inch pitch 3-blade is replaced with an 18-inch pitch 4-blade.

Davie

[brian186dc]

Try Ken at ....www.propgods.com

[BoutTime]

I run a 14 3/4 x 19 three blade stainless on my 190cc powered by a 115 johnson ocean-pro, this model has the bigger V-6 lower unit.
As for figuring out your needed prop size go to the Evinrude website. There is a link that will tell you how to correctly size a prop for your needs. Some info. you will need is boat weight, both dry hull and fully loaded, draft, W.O.T. rpms,(ideally this should be around 5500rpms) and enigne horsepower

[CaptWoody]

Semperfido, you got it backwards on the pitch. Going to a 21 pitch would lower his RPMs, not increase them. He may need to go down in pitch, although there could be other factors such as engine mounting height, engine tilt and trim, prop fouling or boat bottom fouling, etc. I would suggest taking the rig to a prop shop if there is one nearby.

[Deleted]

May 8, 2013 7:33:03 GMT -6 CaptWoody said:

Semperfido, you got it backwards on the pitch. Going to a 21 pitch would lower his RPMs, not increase them. He may need to go down in pitch, although there could be other factors such as engine mounting height, engine tilt and trim, prop fouling or boat bottom fouling, etc. I would suggest taking the rig to a prop shop if there is one nearby.

Thanks guys.
Capt woody
Yeah that's what I thought about the pitch. I figured I needed to go down. Posted at propgods and learned a few things. before I change props I need to raise the motor. AV is buried. Thanks again

[acdaddy0]

Semperfido, I am new to this site but have owned boats for a long time. One example of pitch would be a 70 hp I had on a fish and ski. I had a 17 pitch three blade and a 19 pitch three blade, both ss. The 17 was fast out of the hole and would run up to full rpm. The 19 was slower out of the hole and would not reach full rpm. It actually was slower on top end than the 17. If the motor won't spin the bigger pitch it is slowing you down and puts more strain on your motor. Hope this helps. I am however not a pro on prop diameters.

[acdaddy0]

Forgot to mention. I am currently in a 180cc with a johnson ocean runner 115. She's got an aluminum three blade 13.25 x 17. It runs great on this boat! I can push it up to 5900 rpm trimmed up. I would recommend going to a 15 pitch .