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Author Topic:   Correct prop?
Robob2003 posted 07-26-2004 11:02 AM ET (US)   Profile for Robob2003   Send Email to Robob2003  
I have a Sakonnet with a 100HP Johnson turning a 13X17 stainless prop painted black. That's how I bought it.

At WOT with two aboard it turns 40MPH on a two way run at high tide according to GPS. The OMC classic tach just installed goes up to 6000RPM and at WOT the tach needle is off the scale.

I have read others not being able to exceed 5500 or so. Do I have the wrong prop or is it a tach error or should I let it alone? Tach seems accurate at idle apeed (1000RPM)

I rarely run at WOT but it seems to like 4500-5000RPM.

Thanks in advance,

Bob on Tampa Bay

Bigshot posted 07-26-2004 11:25 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Either tach or prop is wrong. 100hp should fetch you about 5300 or so with a 17P SS prop. Try another prop.
Robob2003 posted 07-26-2004 06:56 PM ET (US)     Profile for Robob2003  Send Email to Robob2003     
What diameter adn pitch?

Got any other suggestions?

At $300 a pop, experimenting with props gets expensive:-)

Bob on Tampa Bay

Bigshot posted 07-27-2004 01:13 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Most places let you experiment or exchange so not expensive at all as long as you don't hit anything. It will be roughly a 13x17.
Robob2003 posted 07-27-2004 04:03 PM ET (US)     Profile for Robob2003  Send Email to Robob2003     
Bigshot,

I don't understand how a 13X17 swap out prop is going to make much difference since that's what it has on it now.

It must be the tach.

I'm going to look around and see if somebody can calibrate it.

Thanks,

Bob on Tampa Bay

Bigshot posted 07-27-2004 04:30 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I know that is why I said try another prop, yors might be worn down to a 16" or was repitched, etc. You can buy them all day long on E-Bay for $100 or so used...VERY common prop. Heck you can try my 18" if you come down to the Manatee river area.
Bigshot posted 07-27-2004 04:32 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Better yet, get that prop to general Propeller in Bradenton and have them give it a "once over" and check pitch, etc.....$40-75 depending if they put a new hub in it(can't hurt).
Robob2003 posted 07-28-2004 06:10 PM ET (US)     Profile for Robob2003  Send Email to Robob2003     
Bigshot,

I just got it back from them via Fox Propellers here in St. Pete.

It has the diameter and pitch stamped in it so I think they would bring it back to the original specs.

It had a few small blade nicks in it so I had it fixed.

Thanks for the help,

Bob on tampa Bay

Bigshot posted 07-29-2004 11:38 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Not unless you ask them to check pitch & diameter.
Robob2003 posted 08-01-2004 09:02 AM ET (US)     Profile for Robob2003  Send Email to Robob2003     
Bigshot,

Thanks for the info. I didn't realize that they didn't automatically do that with the balancing:-(

I'll have to call Fox and find out if he can do it. I'm sure he can, they seem to do everyting else.

Thanks again,

Bob on Tampa Bay

Robob2003 posted 08-02-2004 03:40 PM ET (US)     Profile for Robob2003  Send Email to Robob2003     
Bigshot,

What do you think of the composite props like Piranha?

I can buy a universal hub and then change the blades for less than $100 (for three blades)if I get the wrong diameter and pitch.


Bob on Tampa Bay

Nausiat posted 08-06-2004 10:29 AM ET (US)     Profile for Nausiat  Send Email to Nausiat     
Don't know the gear ratio on that motor but I do know you are over reving it. I had a number of people tell me that 19" was right for an 80 Merc on a Nauset. GPS told me I was hitting 41 and tach said 6300. I was 'told' the tach must be wrong. I almost destroyed this 36 year old motor (started a melt down on #3).

I found a prop calculator on the web, punched in my confirmed speed, the pitch, 10% slip and the gear ratio (2.3:1) and guess what, the calculator showed 6300rpm. Trust your tach, work the numbers!! I had to move all the way to a 23" to get into the right band.

http://www.rbbi.com/folders/prop/propcalc.htm

Running the numbers with 10% slip (from calculating 3 Whalers of friends, all blue interior 16's, seems Whalers run 10%-15%) and assuming a 2.1:1 ratio, the calculator says you are getting just under 6K, if it is 2.0:1 then 5650-5700.

You need your gear ratio, looks like most would have been 2:1 however some are heavy geared over the years, certain models had much higher ratios, presumably 'heavy use' cases etc.

Try that calulator once you find out what ratio your lower is, then you'll know. Trust your tach until you can prove it wrong, power heads are very expensive to repair!!

Greg

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