Prop pitch question

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FrankR
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Location: Florida

Re: Prop pitch question

Postby FrankR » Mon Mar 26, 2018 4:56 pm

Or an optical tach (requires running with hood off for test)
https://www.amazon.com/AGPtek%C2%AE-Pro ... tachometer

fleetwin
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Location: Rhode Island

Re: Prop pitch question

Postby fleetwin » Mon Mar 26, 2018 5:14 pm

NJ-boatbuilder57 wrote:Thanks, guys! Good info all around.

Pardon my ignorance, but it sounds like a tach is pretty important to getting everything dialed in. But as far as I know, my engine has no provisions for a tach. What's a fellow to do?

Absolutely correct on both counts. So, you will need some sort of shop tach. The problem is finding one compatible with a two stroke and making sure it is set up properly...Inaccurate tachometer readings just waste your time and cause even more confusion...

outbdnut2
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Re: Prop pitch question

Postby outbdnut2 » Mon Mar 26, 2018 5:38 pm

The Tiny Tach has a wire that you wrap around the outside of one spark plug wire to sense RPM. There are others out there that have a sensor that clips around the outside of a spark plug wire - all are easy to use. A tach is the best way, but I don't generally use one - I've gotten a bit of a "feel" for proper performance and engine sound. You can buy a TinyTach online at E-BAy , Amazon, and other places. for $45 to $50. They are also good for setting idle RPM, and can be used on virtually any gasoline engine. You just have to, as was said in the previous post, set a tach up for 2 or 4 stroke and the number of cylinders to get a correct reading.
Dave

gdcarpenter
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Location: Raleigh, NC

Re: Prop pitch question

Postby gdcarpenter » Mon Mar 26, 2018 8:34 pm

Hi, looks like a GlenL 'ZIP'. My GlenL 'ZIP' runs 29-30 MPH @ 5,600 RPM with a 1959 35 HP outboard.
(In terms of full disclosure it's a Mercury Mark35A)

My best results have come from a 10" diameter 10" pitch prop giving me the above top end results, using a tach for RPM and a GPS for speed. A 12" diameter 11" pitch prop was too much prop for my boat, which weighs in a little under 800 lb. fully load for a days outing with 6 gal. of gas.

A tach is rally important to dial in your prop, so at WOT you are at or near max. RPM.

FrankR
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Location: Florida

Re: Prop pitch question

Postby FrankR » Mon Mar 26, 2018 9:29 pm

gd, by any chance are we comparing apples with oranges now? How do the gear ratios compare between the Merc and OMC? And was your Merc a two-blade?

NJ-boatbuilder57
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Location: New Jersey

Re: Prop pitch question

Postby NJ-boatbuilder57 » Tue Mar 27, 2018 7:07 am

Hi gdcarpenter: it's a ski-tow, and I'm on that 'other' forum, too. I posted pages & pages of build photos....until photobucket sank.

I guess the mission is to dial it in with as many props as are needed to find the sweet spot. That could get expensive! :)

Tachs: are the $10 "ebay tachs" as good as the $50 tiny-tach? They all look to be just about the same....

Dave Bernard
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Re: Prop pitch question

Postby Dave Bernard » Tue Mar 27, 2018 10:39 am

I may have a merc-o-tronic shop tach for testing it hooks around the spark plug wire. works on any motor that has plug wires.

gdcarpenter
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Location: Raleigh, NC

Re: Prop pitch question

Postby gdcarpenter » Wed Mar 28, 2018 7:22 am

The Merc is Pinion 14 : 23 Propeller for the gear ratio.
The 10" diameter 10" pitch prop is a three blade brass prop.
The 12" diameter 11" pitch prop was a two blade aluminum prop.

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wbeaton
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Location: Hamilton, ON
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Re: Prop pitch question

Postby wbeaton » Wed Mar 28, 2018 10:54 pm

FrankR wrote:gd, by any chance are we comparing apples with oranges now? How do the gear ratios compare between the Merc and OMC? And was your Merc a two-blade?


It is apples to oranges. From my experience, if a 40 ci Mercury runs best with a 10P prop then a 35 hp Big Twin/RD will likely need a 13P prop on the same boat. That is anecdotal based on motor/prop testing on Feather Crafts, which is also an apples to oranges comparison. :)

Also, I have used an automotive timing light with a built in tach when prop testing. It’s one of three tachs I use.

OLCAH
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Re: Prop pitch question

Postby OLCAH » Sat Mar 31, 2018 4:43 pm

Brings us back to NJ-Boatbuilder57's question:

"Tachs: are the $10 "ebay tachs" as good as the $50 tiny-tach? They all look to be just about the same...."

Does anyone have experience with both?


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