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Author Topic:   13 performance
Jiles posted 03-25-2003 01:50 AM ET (US)   Profile for Jiles  
I have a 1985 13 footer with a 40 Johnson. I have a depthfinder that has a speedometer and I have installed a tachometer. At W.O.T. my r.p.m. is 4200 and the speed is 32 m.p.h. My motor is trimmed out correctly and the total weight of myself and gear is app. 300pounds. I have no reason to believe the tachometer is inaccurate because the motor idles at about 800 to 1000 r.p.m. and the motor seems to be in a strain at w.o.t.. The motor starts instantly and runs perfect with a light tan color to the plugs. My prop is stainless 11 3/4 x 17. What size prop should I get to raise my r.p.m. to about 5,000 and what will this change do for my top speed? ---THANKS---
Bigshot posted 03-25-2003 08:55 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Something may be wrong with the engine my friend. Try a 15" and see, you need to get that engine closer to the 5500 mark with just you in it.
LiquidDiet posted 03-25-2003 10:47 AM ET (US)     Profile for LiquidDiet  Send Email to LiquidDiet     
I agree with Bigshot, something may be wrong with the motor. I ran the exact setup a 40hp Johnson (1989) on a 13' Sport swinging a 11 3/4" by 17" for ten years and acheived constant 40mph with 1 person on calm water.
msullivan006 posted 03-26-2003 01:47 PM ET (US)     Profile for msullivan006  Send Email to msullivan006     
Isn't it likely that part of the problem is the different "effective" pitch of his stainless prop versus stock uncupped aluminum? As I understand it, the SS is similar to adding close to 2" to the pitch, giving him an effective 19"? Isn't a cupped blade a similar 2" effect also? On my 13 with J40, its current cupped 17" prop is clearly slower at hole shot than the old uncupped 17, and cupped blade seems to drive bow down more as well (on same pin setting).
jimh posted 03-26-2003 11:06 PM ET (US)     Profile for jimh  Send Email to jimh     
It is my experience that the speed data provided by a paddlewheel type transducer (used with most depthfinders) requires careful calibration in order to get any decent accuracy.

I would not rely on the indicated speed from such a device, particularly for evaulating the propeller and engine performance. Borrow a decent GPS with WAAS enhancement to provide better speed indication.

By the way, after several days of fiddling with my paddlewheel calibration, I did get to a setting where at most speeds the GPS and the paddlewheel were in agreement within plus or minus one mile per hour. Prior to calibration they were far apart.

See my earlier comments about transducer calibration and some of the effects of location and boundary water.

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