Boat Speed

[BlewBayouToo]

What is the average speed that everybody's boat does? I ask because while we are heading out the channel everone passes us no matter the boat size. We are doing about 4.3 - 4.4 knots at 2400-2500 RPM. I have done the searches about propeller size and it seems as if the stock is a 15/9 and some people change to a 15/10 or even a 16/10. I have read on one post that some one said that a 16" was too large. I went and checked what the surveyor marked for propeller size which I verified at the time of purchase. We have a 19/9 !!! Could this be what is causing what seems to be such a slow speed. We have no problem reaching 3000+ RPM but recommended is in the 2500 range. Any comments or ideas would be appreciated.

[RV61]

I found with my 86 when I first bought it that the tachometer was reading to high and I could actually bury the needle past 4000 RPMs on the gauge and experienced the same as you are now at 2500 rpm she would do a little less than 5 knots. So I decided to crank the throttle all the way and then back off about 25% and found on a flat lake or channel with out current the knot meter reads 6.5 knots and  full out approx 7.4 knots however my GPS reads slightly slower. There are some threads on how to callobrate the tach I just have low on my to do list right now. One thing I also remember noticing is the engine ran the best at about on my tach at 3200 to 3300 with out being technical I believe the tach is off by 1000 RPM

[BlewBayouToo]

That is kind of what I was suspecting. When I push the throttle all the way forward I can get about 4000 RPM or so out of it. This weekend I am taking a industrial type tachometer with me to verify the RPM's. I just found it kind of hard to belive that it could be off by 1000 RPM. Thanks for the insight. Any thoughts on that propeller size?

[Stu Jackson]

Good idea to check with a tach, but remember that the RPM come off the alternator, and that the tach should have settings on the back.  Check the "Knowledgebase" or use a search on this board for tachs, Teleflex and alternators.  If you have the stock OEM alternator, it is unusual for the RPM to be off that much.  The only way to check max RPM is in neutral, you just won't get high that in gear.  If you've changed the alternator, it means it has a different number of poles.  Either way, you should be able to adjust.  Or, do what so many have done:  knowing what the max is, just proportion it to 3,000.

Also check on your prop size.  a 19 just won't fit, unless it's about three feet behind the rudder!  There needs to be adequate clearance between the hull and the tip of the prop.  All well documented in earlier references.

[BlewBayouToo]

Well I went back and looked at the pictures the surveyor took of the markings on the propeller. Plain as day on the propeller 16x9. In his wording description in the survey, what I had originally quoted he had 19x9. I guess I should have checked the pictures first.

I am not sure if the alternator is OEM or a upgrade it could have been changed before I got her.

So are you saying that on the back of the tach is a way to calibrate it?

[Stu Jackson]

Yes

[Craig Illman]

Terry - You need the S024.pdf and the S024cor.pdf from the Teleflex website or send me an email and I'll forward them to you.

Craig

[sail4dale]

I went thru a quandry on the boat speed vs. RPM and finally got a mechanical tach and calibrated my knot meter. 

I have a 15 x 10 Flexofold prop (replaced my "draging" 3 blade fixed) and found my tack was off by a factor of 1.25.  When it read 1000 RPM I was actually doing 1250 RPM at the engine.  It was a straight line function throughtout the speed range.   I now cruise at about 1700 -1900 max.on the tach which is actullly 2125 -2375 at the engine and I'll cruise at about 6.8 - 7 knots.  No black smoke.

[BlewBayouToo]

Thanks for all the help yall. Craig sent me the pdf files that explain how to adjust the calibration on my tach.