Prop help

[Grum Man]

I've got a 40HP Merc 4 stroke on my 1958 Grumman "Runabout". Running with a 3 blade, stainless, 10.5x13 Mercury Vengeance prop I get 6100 RPM and 32 MPH. I installed a Solas 10.5x14 Amita 4 blade aluminum and I get 5100 RPM and 29 MPH (everything the same).
I love the smoothness of the 4 blade, and would ultimately like to end up with a 4 blade stainless. From everything I have learned and read these numbers between the two props shouldn't be so far apart. Anybody have any explanation, or thoughts about an appropriate SS 4 blade prop? Oh BTW, recommended max RPM is 5500 to 6000. Thanks for any responses.

 

[Scott Danforth]

the additional blade adds "pitch" and at 5100 RPM, you are over-propped.

as for a 4 blade stainless, my guess you will be in a 13p or 12p

 

[Grum Man]

Thanks for the info Scott. Solas makes a 4 blade stainless (Rubex) in 12,13, 14 pitch. The diameter is 1" larger than my current props. If I chose the 13 would you think the added diameter and 3 blade to 4 blade conversion could account for more than a 600 RPM drop (the max RPM cushion) I've got to work with?
Dealer in my area doesn't have that prop to test, going to call around and see if I can find one.

 

[Sea Rider]

If OB is already revving slight max from its 6K max wot rpm range as loaded which is nothing, what do you deslike about current 3 blade 13 pitch prop ? Slight over revving increases going out of the hole faster along rpm revving nicely throughout the whole throttle range.

IK wot rpm difference between a 3 blade 13 pitch prop and a 4 blade prop is unbelievable. Have experienced myself + 500 to 1000 wot revs going for one less pitch while playing with 3 blade alum props.powering different boats with fixed loads.

Should have gone the opposite way for a 4 blade 12 pitch prop to maintain theoretical 6K wot revs. Only checkable under a wot water test. The issue is how much true wot rpm the OB will gain going per one less pitch from a 14 to 13 pitch 4 blade prop ?

Happy Boating

 

[jimmbo]

Adding an inch of pitch, an inch of diameter, plus an additional blade, the 1000 drop in rpm doesn’t sound unreasonable. Ditch the extra blade, ditch the diameter, and keep the extra inch of pitch and you will be close to what you want/need

 

[Sea Rider]

What do you plan doing next, go for a 4 blade 12 pitch prop ? A 12 pitch should "theoretically" make OB to rev towards 6K same as it's revving right now with a 3 blade 13 pitch prop as currently loaded. That's if that rule of thumb applies spot on as repeatedly is forum stated.

Happy Boating

 

[WesNewell]

Before you mess with the prop, check your engine height. It will have much more affect on performance if it's not right. Check the Mercury site prop selector and see how you compare with what you have. Not knowing what the specs on your doesn't help making a recommendation.. Raising my engine height 1.5" and going 2 pitches higher increased my top speed by 20%. Assuming your boat dry weight is 1000 lbs with 200 lbs of people/gear and 12 gal of fuel, you should get ~37mph with a 15 pitch ss prop @~5800rpm

 

[WesNewell]

Just found the specs on the 1958 Grumman runabout. If correct it says it only weighs 255 lbs. If that correct, you should get well over 40mph with a 40 hp 4s properly mounted with a 16p ss prop.