Prop testing issues- 21'pontoon prop selection

dstnlandry

Cadet
Joined
Apr 1, 2018
Messages
7
Running a 21'4" suncrusier pontoon with 2006 optimax 90hp (2.33 gear ratio). Estimated weight is 2100lbs based on the current 2019 models.

Origial prop is stainless powertech 13pitch 13.5 diamater. Runs 21.3mph at 6500rpm

Tested today with a aluminum stock mercury prop 15" pitch. Diamater was close to 13.5 from comparing the two but i couldnt decifer anymore off the prop serial number 77343a45 p15. Ran 21.7mph at 6400rpm

According the merc this motor should run around 5500rpm. Seems to me that i am way under proped at 13 and15" but all the prop calcualtors say i should be running closer to 30mph with a 15"

Speed was measured with gps. Im wondering of my tach is off, only basis for that is i assmume the motor would have been blown by the previous owner if it was ran this high for 300hrs.

planning to buy a aluminum prop for top speed. leaning towards testing a 17"p 13.5d 4 blade. Any recommendations? Should i get more pitch? Stick with 3 blade?
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Check the proper setting on the tach. My brother ran his new boat for years with the wrong setting from the Ranger factory. It was reading ~600 rpm high
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
Head scratcher there. I would have expected a very noticeable drop in rpm going from the 13" to the 15" (5-600 rpm?). Were the tests run in similar conditions with a similar load?

Our fairly light Honda 90 powered 24' boat (also a Suncruiser if that matters) runs 6200 @ 24mph w/13"p and 5500 @ 26mpg w/15" pitch. 15p prop is the primary prop. 13p provides all kinds of kick when you punch it, but the engine is sounding pretty busy turning that fast. 15p seems much more civilized at WOT, and we aren't pulling often enough to really justify the 13p.

Incorrect tach would have read incorrectly on both props. Would not account for the fact there is no difference in rpm's between them. My thoughts anyway.... -Al
 

Starcraft5834

Lieutenant Commander
Joined
Jun 2, 2013
Messages
1,677
Michigan Match makes a 16p 4 blade.. thinking 30mph out of a 90hp 21 foot toon a bit to optimistic. ive got new 20ft toon with 115 Merc 4 stroke, can hit 30 lightly loaded.. based on your numbers, sounds like you need more prop
 
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