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Not reving high enough |
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87Dominique 
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Joined: August-20-2006 Location: United States Status: Offline Points: 11 |
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 Topic: Not reving high enoughPosted: August-20-2006 at 6:11pm |
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Hi, I have a 1987 Correct Craft Dominique with a PCM 351. It has a 13 by 13 three blade prop but the engine only revs at 3400 RPM when the throttle is completley open and the boat is going about 40 mph, the manual says it should top out at 4400. I thought the secondaries weren't opening but I had my father drive the boat and I could see the linkage under under the secondary diaphragm move. I can't figure out the problem. The boat idles perfect, starts perfect, and accelerates perfect. I was wondering if anyone could offer any suggestions. Thanks
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77stang
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Joined: May-19-2004 Location: United States Status: Offline Points: 342 |
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Posted: August-20-2006 at 6:58pm |
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i dont feel like working the math (i seem to have lost my little spreadsheet that calc's it) but i dont think 3400rpm, 13" pitch, 1:1 drive will equate to 40mph especially when you consider slip. either you tach or your speedo isnt accurate.
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87Dominique
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Joined: August-20-2006 Location: United States Status: Offline Points: 11 |
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Posted: August-20-2006 at 7:46pm |
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3400RPM X 13" Pitch = 44200 inches per minute
44200 x 60= 2652000 inches per hour 2652000/ 63360(inches in a mile) = 41.86 MPH The speedo may be off by a little but i think its pretty close. |
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GottaSki
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Joined: April-21-2005 Location: NE CT Status: Offline Points: 3363 |
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Posted: August-20-2006 at 7:50pm |
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nope, something is off, I'm certain
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole |
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87Dominique
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Posted: August-20-2006 at 7:56pm |
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The speedo could be off but both of them say the same thing. Its unlikely that they're both off. THe Tach may be off but when the engine is idling it's right around 600 i think.
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87Dominique
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Posted: August-20-2006 at 8:14pm |
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I guess the tach could be way off but how fast do these boats usually go with 13 by 13 inch props. I just got the boat last week so i don't know if it ever went any faster. How would i hook up another tach to the engine temporarily just to find out for sure?
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"The worst day out boating is better than the best day working in the office"
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 11119 |
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Posted: August-20-2006 at 10:58pm |
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87 Dominique
Your math is right, but you forgot to take slip into account. You came up with it's theoretical top speed with no slip accounted for. Probably 15 % slip or maybe even 20 on that boat. 15% would then get you about 35 to 36 MPH. 20% would be about 33 MPH 4400 RPM would theoretically be about 54 MPH and with 20% slip you're at about 43MPH which would be about right for that boat/engine combination. keno |
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87Dominique
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Posted: August-21-2006 at 8:41am |
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Thanks I'll try a different Tach and use a GPS for the speed.
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"The worst day out boating is better than the best day working in the office"
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