Change prop, what to expect

Chopperbill

Petty Officer 1st Class
Joined
Mar 26, 2014
Messages
389
signature boat is an inflatable, motor is 20 hp EFI 4 stroke
With two aboard I am running 23 mph @ 6200 rpm. Max is 6300 rpm. Planes fast and smooth
Will have 4 aboard at times and have a 8" prop I can install in place of the 10" as I'm sure the added weight will make it difficult to plane. If I leave the 8" prop running solo or with just 2 aboard how much speed can I expect to lose? Guessing I will have to back out of the throttle to keep from over revving the motor.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Prop calculator says 20 mph at 6300,assuming 11% slip same as the 10".
The 20 on a light reasonably fast hull should manage around 30 or more.
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
I'm not sure I would change props - if you are hitting the max rpm then you should be fine with a full load.... if you said you were having a slow plane time now I would say change it, but you said that was good. just my thoughts
 

Chopperbill

Petty Officer 1st Class
Joined
Mar 26, 2014
Messages
389
Prop calculator says 20 mph at 6300,assuming 11% slip same as the 10".
The 20 on a light reasonably fast hull should manage around 30 or more.
Don't think my inflatable hull would be considered a fast hull. Tell you the truth not sure I would want to over 25 mph in my rubber boat, but in my old age I'm a lot more cautious then I use to be. LOL
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
If you're happy do nothing. If you aren't and inquisitive, change it. If you like the results keep it. If not change back. On using the shallower pitch and being lightly loaded, you don't have to run over the max rpm, just cut it back. Since you aren't all that interested in speed in your particular type boat, then changing might be a good idea.

Mark
 

Chopperbill

Petty Officer 1st Class
Joined
Mar 26, 2014
Messages
389
I summer at 5200 feet and my dad bought me a 8" prop because it was difficult to plane there but never got a chance to use it. I winter at 400 feet and the 10" planes great with my wife and me. But, will probably have more passengers so I was going to put on the 8" to see what it would do. As soon as it warms back up I'll just try all different things
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Make a wot test with 4 and check max rpm achieved, if in middle of min-max revs factory stated fopr that engine should be fine, if near lugging the engine change for the less pitch prop. Inflate well sib's tubes and air deck or expect a poor water performance.

Happy Boating
 
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