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1:1 tranny rpm=mph |
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delhidream 
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Joined: August-27-2006 Location: United States Status: Offline Points: 28 |
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 Topic: 1:1 tranny rpm=mphPosted: September-21-2006 at 9:18pm |
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Saw 79nautiques comment on rpm equivilant to mph on 1:1 tranny, how accurate is that? Speedo work is a winter project this year. Just curious how close it is and if only at certain rpm ranges.
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Tim D
Grand Poobah 
Joined: August-23-2004 Location: United States Status: Offline Points: 2635 |
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Posted: September-21-2006 at 9:53pm |
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My Mustang, with a 1:1, 12x12 prop would run 20 at 2000, 30 at 3000 etc. But this summer I put a hotter cam in and it runs faster than it used to at the same rpms. I've also got a 12x14 prop which only gives you about 4 mph more on top end.
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Tim D
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Munday
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Joined: August-17-2004 Location: United States Status: Offline Points: 538 |
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Posted: September-21-2006 at 10:24pm |
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I think the 12x14 changed the mph/to rpm not the cam,2,000 rpm is 2,000 rpm no matter how many horses you got.
I've always seen a ratio of 1,000rpm to 10mph in correct crafts too.I preset my speed after getting the skier up with the tach and get close to right very quickly. Good luck Munday |
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Tim D
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Joined: August-23-2004 Location: United States Status: Offline Points: 2635 |
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Posted: September-22-2006 at 8:00am |
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I haven't put the 12x14 on since the new cam.
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Tim D
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Tim D
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Joined: August-23-2004 Location: United States Status: Offline Points: 2635 |
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Posted: September-22-2006 at 8:03am |
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I haven't put the 12x14 on since the new cam. You can believe it or not, I don't care, but after 16 years with the old cam,and then a new one, there's a speed difference. I've driven this boat so much in 16 years, I can tell by the exhaust how fast I'm going.
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Tim D
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David F
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Joined: June-11-2004 Location: United States Status: Offline Points: 1770 |
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Posted: September-22-2006 at 10:00am |
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I think it all depends on the pitch of the prop. 1:1 just means that prop and engine are turning at same rpms. Pitch will determine what speed vs rpms the boat is travelling. Right?
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jimbo
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Joined: September-07-2004 Location: United States Status: Offline Points: 473 |
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Posted: September-22-2006 at 12:29pm |
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Tim, I don't dout you are running faster with a new cam (or getting to that same speed faster
 ). I think what they are saying is even with a different cam, at a given rmp, the prop is still turning at the same rate and the boat is still moving at the same speed.
Now, if you change the prop.... As I recall (which changes darsticaly from time to time) the first number of a prop description is the diameter and the second is the pitch. The pitch is how for the prop moves forward in one revolution. Theoretically, with no slipage, in a vacuum, on a perfect day, on a full moon, when jupiter is algined with the Grand Poohbah.... |
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boat dr
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Joined: June-27-2004 Location: United States Status: Offline Points: 4245 |
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Posted: September-22-2006 at 1:13pm |
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jimbo jimbo jimbo you are wrong,when jupiter is algined with the Grand Poohbah,thats SATURN.everything else i totally agree with. slippage will vary 10 to 15 percent , so with a 13" prop it will pull itself forward 1 foot.this is in a perfect world. so if your tach is spot on and you can rev. your motor to 5280 rpm,s you are real close to 60 mph.the HOLY GRAIL for Knot Correct.......... boat dr
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TRBenj
Grand Poobah
Joined: June-29-2005 Location: NWCT Status: Offline Points: 21123 |
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Posted: September-22-2006 at 1:55pm |
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I have noticed on most inboards (CC in particular) speed vs. RPM is very close with a stock sized prop (3000RPM = 30MPH). This seems to hold true with both the 1:1 (~13x13) and 1.23:1 (~14x16) boats. Go to a smaller pitch propellor and you'll turn more RPM at a given speed, of course.
Strangely enough, I swear I noticed the same thing that Tim D does. After my motor upgrades (with the same prop), I could swear my motor turns less RPM at skiing speed. I wouldnt have thought speed vs. RPM would change depending on the power the motor is making... maybe we're both crazy! 
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Hollywood
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Joined: February-04-2004 Location: Twin Lakes, WI Status: Offline Points: 13512 |
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Posted: September-22-2006 at 2:02pm |
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Well Tim, I do think you're crazy.
You took a boat in "pretty good" shape and look what you've done, turned it into "great"! You remind me of a guy on the chain who wore his top hat driving around his powerboat appropriately named "mad hatter". |
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TRBenj
Grand Poobah
Joined: June-29-2005 Location: NWCT Status: Offline Points: 21123 |
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Posted: September-22-2006 at 2:17pm |
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I appreciate that, the boat was in good shape when I got it- and Im happy with the changes Ive made so far.
Do you know where I can pick up one of those cool hats? Can I borrow yours? |
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Hollywood
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Joined: February-04-2004 Location: Twin Lakes, WI Status: Offline Points: 13512 |
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Posted: September-22-2006 at 2:20pm |
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Tim, send me an email and I'll forward you the link of the hats we use and all that, get you on the next order.
midwestrebel@gmail.com |
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PLBC
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Joined: January-25-2006 Location: United States Status: Offline Points: 568 |
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Posted: September-22-2006 at 3:15pm |
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I think Tim is referring to the mad hatter hat.
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