3 blade SST vrs High five

[Randy stoner]

I have a 1991 sunbird corsica 175 Cobra with a brand spanking new 140 hp Chevy four cylinder engine. Out drive is total rebuild as well. The prop that originally came with this boat/motor is a stainless 391290 15 x 17. I also have a stainless 48-821154 17 high five. The motor like i said is brand new and has maybe 8 hours running time on it since i was installed last summer. 5 hour break in at the marina who installed it and about 3 hours of my running since. By GPS the boat with several people in it will run 35 mph every time. The big 3 blade prop however will only bring my rpms up to about 4200 yet the high five with 5 blades will run at 4700 rpm. Both speeds by GPS is 35-36 mph. The boat weighs 2000 lbs dry. I have two questions and or observations. Ideally you want the motor to run up to and or near its proper WOT rpm so why is the five blade doing that where as the original prop isnt? Secondly, i have a four blade aluminum 48-83485 c18 prop that I have yet to try. The big three blade weighs far more than the high five and of course the 4 blade alluminum will weigh half of that. Im thinking since the 3 blade is not allowing the motor to run near its proper rpm I should be using the high five even though every where i read, the high five should be too much for that motor. I know some will chime in and say their boat with similar configuration goes a whole lot faster and yes that probably true, how ever no two hulls are the same so im not so much concerned that way but i dont want to harm the motor by lugging it down.

Randy.

P.S. Be safe and thankyou

 

[Texasmark]

My guess is the High 5 is ventilating, causing the problem referred to as "prop slip" which is exactly what you are telling us you are getting...too many blades for the speed travelled...that's the problem with too many blades. Prop slip is bad operating efficiency for one thing. It's good if you get it from "Ports" in the prop (holes drilled under the leading edge of each blade...around 5/16" to maybe 1/2" depending on application. Ports allow the prop to ventilate in the hole shot to get RPMs and Thrust up fast giving you a fast hole shot even though it is inefficient...but not really compared to a long drawn out hole shot. Once up the Ports seal off, the prop slip goes way down and you are off like a bullet.

Weight matters not.
Going to an 18P is the wrong way.
Cut your pitch on a 3 blade prop to achieve near the upper end of your full throttle operating range at WOT.....of which I am unaware or I could suggest my best guess for the right pitch in 3 blades.

Rule of thumb is 150-200 RPM change for every inch (either direction) of pitch change....you will be looking at maybe a 13P of approximately the diameter of the 3 blade...but diameter wiggles a little bit (fractions within 1 inch) with the pitch and design of the prop.

 

[Randy stoner]

I guess that makes sense. Of the three props, the high five will lug the motor down far less than the others allowing it at least to rev closer to proper rpm although not efficient in its performance and lots of slip.? Ill have to find another prop i guess, although my experience with 13 pitch props on three other boats were not very good. (always over reving) What diameter and pitch of a three blade prop would you recommend based upon the specs below?

Specs on the boat are as follows :
16.5 sunbird bowrider
3.0 liter 140 hp vortec i/o motor (brand new)
cobra outdrive (totally rebuilt)
dry weight 2000 lbs.

thanks

Randy

 

[Texasmark]

I guess that makes sense. Of the three props, the high five will lug the motor down far less than the others allowing it at least to rev closer to proper rpm although not efficient in its performance and lots of slip.? Ill have to find another prop i guess, although my experience with 13 pitch props on three other boats were not very good. (always over reving) What diameter and pitch of a three blade prop would you recommend based upon the specs below?

Specs on the boat are as follows :
16.5 sunbird bowrider
3.0 liter 140 hp vortec i/o motor (brand new)
cobra outdrive (totally rebuilt)
dry weight 2000 lbs.

thanks

Randy

Repost under question about I/O prop and include your specs listed. You should get the guys currently running I/Os and can dial you right in.....some really good I/O talent frequent the site. I haven't had one for 40 years and I am not current.

 

[QBhoy]

The high 5 is doing that because it’s very inefficient at the top end with lots of slip. They usually act like less pitch with the rpms. Only good at one thing. Getting someone pulled out the water with a boat that might usually not have the power to do it...without lots less pitch.