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Author Topic:   2003 170 MONTAUK 90-HP
Nottoo posted 06-14-2015 11:27 AM ET (US)   Profile for Nottoo   Send Email to Nottoo  
[On a 2003 Boston Whaler 170 MONTAUK], I [want] to change to a stainless propeller from the aluminum prop that came with the 2004 Mercury 90-HP two-cycle outboard engine. The propeller was chewed up pretty badly by the previous owner. The boat does 35-MPH at 5,000-RPM. The maximum engine speed for the 2004 Mercury 90-HP two-cycle outboard engine is 5,500-RPM.

I was looking at the STILETTO ADVANTAGE Q 425. I think I should go with the 17-pitch but I'm not sure. My current aluminum propeller is a 21-pitch.

Tom W Clark posted 06-14-2015 01:12 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Use either the 13-1/4" x 20" Mercury Laser II, part # 48-16990 or the 13-1/4" x 19 Stiletto Advantage, part # D811319

You do not want to use an Advantage Q which is a reduced blade area version of the Advantage.

jimh posted 06-15-2015 11:10 AM ET (US)     Profile for jimh  Send Email to jimh     
When Boston Whaler introduced the 170 MONTAUK in c.2002, the performance with the standard Mercury 90-HP two-cycle-power-stroke outboard engine with an aluminum BlackMax propeller of 12.75-inch diameter and 21-pitch was reported to produce a top speed of 43.7-MPH.

This narrative reports the boat under discussion hits 35-MPH. That is almost 9-MPH less than the test report from Boston Whaler. This discrepancy is probably caused by using a propeller that has been "chewed up pretty badly."

The Mercury 90-HP has a gear reduction of 2.3:1. At a maximum engine speed of 5,500-RPM with 19-pitch, assuming SLIP = 10, we'd see a boat speed of 38.7-MPH. Adding an inch to account for cupped blades, the boat speed would be 40.8-MPH.

A Mercury LASER II 20-pitch probably has cupped blades. Figuring it as 21-pitch, the propeller calculator says 42.8-MPH. That is in good agreement with the report from the factory. The SLIP might be lower in the actual outcome than the SLIP=10 factor I am using as an estimate. If SLIP =8, the prediction is right at 43.7-MPH, the factory target.

http://continuouswave.com/whaler/reference/170Montauk/
http://continuouswave.com/cgi-bin/propcalc.pl

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