Prop Selection

[carpedium]

Current rig has a 115hp 4 stroke. At WOT the tach shows around 5200 RPM when properly trimmed, with 25 gallons of fuel gear and two people. The manual says that the motor should operate between 5500 and 6000 RPM at WOT.

There is currently a 3 blade 17" prop on the motor. It ventilates easily on turns right now, but other than the low RPMs it performs ok. From my research it looks like a 4 blade prop may be the way to go. Should I be looking at a 15" four blade? Any recommendations for a fishmaster 196?

 

Edit: The existing prop is a mercury black max. 

Sent from my SCH-I545 using Lake Ontario United mobile app

Edited June 17, 2015 by carpedium

[ray koziatek]

any motor trimmed for top speed will cavatate or vent as you say on a turn,just gotta drop it down a tad and it will grip..yea 800 rpm is a bit much for loaded boat probly workin a tad to get outa the hole,also,,I too have a 115 4 st. and had same issues with 4 people full tank and diving gear,dropped to a 15 in stainless tho alum would work fine and it helped a lot..i have a 19.6ft key largo,,, even with full load it still wont hit top rpm drop 200 lbs and im good to go..the 115 4 stroke is not as strong as i would like, even alone it just don't jump outa the water  ,once on plane then all is good..Bottom line we need at least 150 hp..but thats cost prohabative for me and i really enjoy 7mpg when i take trips out the back islands..So yea the 15 i would recomend ....

[ray koziatek]

i used 3 blade

[carpedium]

any motor trimmed for top speed will cavatate or vent as you say on a turn,just gotta drop it down a tad and it will grip..yea 800 rpm is a bit much for loaded boat probly workin a tad to get outa the hole,also,,I too have a 115 4 st. and had same issues with 4 people full tank and diving gear,dropped to a 15 in stainless tho alum would work fine and it helped a lot..i have a 19.6ft key largo,,, even with full load it still wont hit top rpm drop 200 lbs and im good to go..the 115 4 stroke is not as strong as i would like, even alone it just don't jump outa the water  ,once on plane then all is good..Bottom line we need at least 150 hp..but thats cost prohabative for me and i really enjoy 7mpg when i take trips out the back islands..So yea the 15 i would recomend ....

 

 

Thanks for the input Ray! 

[carpedium]

This is the existing prop:

 

http://www.amazon.com/Blkmax-13-25X17Rh-Mercury-Mercruiser-48-77344A45/dp/B0038ZZTH8

 

 

i used 3 blade

 

 

Ray, how much does your rig weigh?

[carpedium]

 

 

 

       3-blades provide good "hole shot" and top-speed performance.

Compared to 3 blades, 4-blade boat props provide better "hole shot" performance with less steering torque and less vibration at high speeds.

When switching from 3 to 4-blades, generally a 1" pitch decrease is required. So, a 21" pitch 3-blade is replaced with a 20" pitch 4-blade. Pitch is increased when switching from 4 to 3-blades.

 

The above is from iBoats.com. Can anyone verify that these statements are true?

[final flight]

Just went through this , 115 merc four stroke with a 4 blade 15" vs a3 blade 17" the 17" killed the hole shot and cavitated at w.o.t. , the 15" has a great hole shot but allows too many RPM at w.o.t. My decision was to stay with the 15 and run the boat at 5400-5800 rpm instead of w.o.t.

[carpedium]

Just went through this , 115 merc four stroke with a 4 blade 15" vs a3 blade 17" the 17" killed the hole shot and cavitated at w.o.t. , the 15" has a great hole shot but allows too many RPM at w.o.t. My decision was to stay with the 15 and run the boat at 5400-5800 rpm instead of w.o.t.

 

Thanks for the input. 

 

What was your WOT? What is your rig's weight?

[final flight]

21' Arima hard top 6400 Rpm at WOT no idea what she weighs

[carpedium]

Fishmaster - 1430lbs dry

Arima - 2150lbs dry

Key Largo 1950lbs dry

 

This suggests that a 15 would probably rev even higher. All based on if the tachs are correct I guess. 

[G-Daddy]

Michigan Wheel used to have a prop selector tool on their website.  I used that last time I changed outboard props, but that was years ago.

[ray koziatek]

mine is a older motor 2004 max rpm is 5500..also 40 gal tank and 2 batteries  mercury but really a yamaha motor made for merc.......plus a t top probly another 150 lbs ..4200 rpm is my "happy spot" 24mph  

[Shellback]

I've got a Mercury Quicksilver prop 13.75x15 3 blade aluminum. It's as new, only had it on a motor once. It's a 4" flo torque hub model. I'd let it go for $100 plus shipping. PM me an email addy if you want some pictures.

[carpedium]

I've got a Mercury Quicksilver prop 13.75x15 3 blade aluminum. It's as new, only had it on a motor once. It's a 4" flo torque hub model. I'd let it go for $100 plus shipping. PM me an email addy if you want some pictures.

 

PMed but never heard back from you. Found this prop on amazon for $27 plus shipping, picked up a hub kit - I'll report back on how it works. 

 

http://www.amazon.com/gp/product/B00EVINAJA/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=AMLHJ402PTWDD