Help with prop sizing please

Chuckles88

Seaman
Joined
Aug 28, 2018
Messages
51
Volvo 5.0gxi with a 3 blade stainless 14x19 currently. The engine will hit the governor at 5000-5100 rpm at 47 mph gps checked and I still have apx 1/4 throttle left so I am unable to find my wot. We tow and cruise mostly. I want to take advantage of the remaining power and was thinking of a 4 blade stainless. Do I jump to a 21p? 20p? Or keep it at a 19p with adding another blade? Thanks!
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,583
21-3 blade or 19-4 blade

3-blade keeps boat with similar attitude, 4-blade provides more lift which can pose its own problems or benefits.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Cosergeant,

It's more important to be aware of the min-max safe wot rpm range when motor is working at wot than the speed gain or loss achieved. So if with a 3 blade alum 11 pitch prop and 2 up achieved 5200 wot rpm will need to know the same with 4-5 up to know how much pitch drop the motor needs to work with to rev at least middle to max wot rpm range. Wot rpm is all, speed is secondary and boating with a varying load as in your particular case is a wot rpm killing factor.

With 2 up, and motor reving at just 5200 wot rpm which is the extreme lugging side of the wot range with an 11 pitch, image doing same with 4-5 up and a 15 pitch prop.. Need to decrease pitch, not increase it in order to pull wot revs up.

Happy Boating
 

Chuckles88

Seaman
Joined
Aug 28, 2018
Messages
51
It just seems a little slow getting up with riders behind. Sounds like a 4 blade would help a little
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,583
Increase pitch or adding blades (4-blade) increases drag. More drag is more time to come up
 
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