Prop pitch question

[frankr]

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

[fleetwin]

quote NJ-boatbuilder57:

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]

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]

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]

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]

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]

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]

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.

[wbeaton]

quote FrankR:

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.

Wayne
Upper Canada Chapter

uccaomci.com

[olcah]

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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