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Author Topic:   Guardian 17 Low Top Speed

koaman posted 05-27-2009 06:10 PM ET (US)
I have a 1988 GUARDIAN 17 with a Yamaha 90 two-cycle with a black stainless steel 13-1/4 x 16 propeller [turning via 2:1 gears]. I only get 32-MPH and 5,100-RPM at wide open throttle. I know the Guardian is a bit heavier than a Montauk but I think I should be getting more top end. Don't know if I can turn a 13-1/4 x 17 if I am only getting 5,100-RPM. Any ideas or suggestions? I am going to raise the motor one hole to see if that helps. It is in the lowest position and even with the bottom. Thanks
jimh posted 05-28-2009 12:48 AM ET (US)
The propeller calculator
http://continuouswave.com/cgi-bin/propcalc.pl
RPM = 5100
RATIO = 2
PITCH = 16
MPH = 32
SLIP computes to 18.8-percent. That is a bit high. Generally when the propeller is well matched to the boat you can get the SLIP down to 10-percent.
If you could get the Yamaha 90 to turn A 15-inch pitch propeller at 5,500-RPM and get 10-percent SLIP, the boat speed would be around 35-MPH.
koaman posted 05-28-2009 01:06 AM ET (US)
Thanks for that Jim, I raised the motor one hole today as it was in the first hole. I am hoping that will make some difference since the cavatation plate was dragging when on plane. The propeller calculator is very helpful. Aloha
koaman posted 06-07-2009 02:19 AM ET (US)
I noticed my gps was set to knots and not mph so recalculated and I'm getting about 38mph @ 5100 rpm and 4.69% slip. I guess thats not too bad. I still may get the Stiletto 15in to see how it compares.
Phil T posted 06-07-2009 07:18 AM ET (US)
Koa -
I would speculate (based on my experience) raising the engine one hole, the engine WOT rpm's will be 150 higher.
To ascertain the optimum mounting height, run a level or long dowl out from the keel and measure the distance (above or below the anti-cavitation plate) above or below the level or dowel. Having the plate above the keel by 1 to 1 1/2 inches is recommended.
Is the Yamaha 90 on your Guardian the classic 2-stroke or a 4 stroke? If the 2-stroke, the WOT range is 4500-5500. A prudent rigging of the engine would allow 5300-5500 rpm's at WOT with a very light load and a single operator.

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Author	Topic: Guardian 17 Low Top Speed
koaman	posted 05-27-2009 06:10 PM ET (US) I have a 1988 GUARDIAN 17 with a Yamaha 90 two-cycle with a black stainless steel 13-1/4 x 16 propeller [turning via 2:1 gears]. I only get 32-MPH and 5,100-RPM at wide open throttle. I know the Guardian is a bit heavier than a Montauk but I think I should be getting more top end. Don't know if I can turn a 13-1/4 x 17 if I am only getting 5,100-RPM. Any ideas or suggestions? I am going to raise the motor one hole to see if that helps. It is in the lowest position and even with the bottom. Thanks
jimh	posted 05-28-2009 12:48 AM ET (US) The propeller calculator http://continuouswave.com/cgi-bin/propcalc.pl RPM = 5100 RATIO = 2 PITCH = 16 MPH = 32 SLIP computes to 18.8-percent. That is a bit high. Generally when the propeller is well matched to the boat you can get the SLIP down to 10-percent. If you could get the Yamaha 90 to turn A 15-inch pitch propeller at 5,500-RPM and get 10-percent SLIP, the boat speed would be around 35-MPH.
koaman	posted 05-28-2009 01:06 AM ET (US) Thanks for that Jim, I raised the motor one hole today as it was in the first hole. I am hoping that will make some difference since the cavatation plate was dragging when on plane. The propeller calculator is very helpful. Aloha
koaman	posted 06-07-2009 02:19 AM ET (US) I noticed my gps was set to knots and not mph so recalculated and I'm getting about 38mph @ 5100 rpm and 4.69% slip. I guess thats not too bad. I still may get the Stiletto 15in to see how it compares.
Phil T	posted 06-07-2009 07:18 AM ET (US) Koa - I would speculate (based on my experience) raising the engine one hole, the engine WOT rpm's will be 150 higher. To ascertain the optimum mounting height, run a level or long dowl out from the keel and measure the distance (above or below the anti-cavitation plate) above or below the level or dowel. Having the plate above the keel by 1 to 1 1/2 inches is recommended. Is the Yamaha 90 on your Guardian the classic 2-stroke or a 4 stroke? If the 2-stroke, the WOT range is 4500-5500. A prudent rigging of the engine would allow 5300-5500 rpm's at WOT with a very light load and a single operator.