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Author Topic:   Props and RPM
TampaTom posted 08-06-2003 03:18 PM ET (US)   Profile for TampaTom   Send Email to TampaTom  
I've had my 65 "Makotuak" (Mako console/16'Whaler)with a 82 90 Johnson for about 3 years. The compression is and has been at 130 psi on all 4. Until recently I've been able to get to 41 mph on the gps at 5,000 (WOT) throwing a 13 x 19 SS prop. Two years ago, I put in new instruments. The new tach reported same 5,000 at WOT. At the Rendevouz on Shell Key, I tried Bigshot's 13x17 SS prop. Still maxed out at 5,000, speed dropped to 38. One thought is the signal the tach is getting. More likely is that linkage was not set correctly (although both butterflys are flat at WOT and everthing seems to move full range.) I decided to leave it alone. 41 is plenty fast and I'm likely to use the extra RPM if I can get to it (kind of out of the warentte period.)

In the last few weeks the RPM has raised up to 5600, but the speed dropped to 38. I thought the hub may be slipping. Sent oout the prop out to be reconditioned and picked up a reconditiond 13x17 SS ($150!!). It gets me to 37 mph @ 6100 rpm. Now I don't think the hub was slipping on my 19". Is there another point besides the hub that could slip? Why do I get lower speed at higher rpm? The trailer is not attached and the bottom is clean. I will re check my gps to see if it reset from mph to knots. Your ideas.

Bigshot posted 08-06-2003 04:01 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
First not rev more than 5500. 2nd most 17's run 17" props below the 5500 redline so your 19 sounds right at 5k.
acseatsri posted 08-06-2003 04:53 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
The prop could be a 17" pitch and still perform differently, depending on whether it's cupped, double cupped, or has no cupping. Unless you're comparing the same make and model prop, you really don't know.

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