Glen-L.com

Finally got my ZIP out for its' second run, sad I know.
This time had replacement stator and working tach and GPS.
Also had relocated battery from under front deck to very rear at transom with 6 gallon cruiser tank.
Ran great, and read about 4,200 RPM & 28 MPH @ WOT (Wide Open Throttle).
Was able to drop engine from second to lowest hole to get max speed, good planing.
Engine in question 1959 Mercury 35A - 35 HP
Specification for that engine calls for max RPM around 5,200-5,400 RPM @ WOT

Question is, is my tach off or should I be looking to try another prop to increase MPH/RPM @ WOT, and if so what prop - current prop original 2 blade and 13 pitch. Carb butterflies wide open at WOT.

Any feedback welcome and appreciated.

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This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com

28 mph on a older 35hp sounds about right to me on a Zip.
My old 40hp is 30-31 mph WOT with my wife and I on board. 290 pounds combined weight. Im turning 5600 rpm with current prop. 10-3/8x13, 3 blade. I cruise at about 5200 rpm which gives me 26-27 mph

The mph youre getting makes me suspect the tach is off. Was 28 mph with just you on board or with 2?

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My Zip build log...
http://www.vupilot.blogspot.com
http://picasaweb.google.com/vupilot/Chr ... O0x7SvsQE#

"Nothing screams poor craftsmanship like wrinkles in your duct tape."

I get about 28 mph in my Zip at 4200 rpm with my 50 hp Merc 500. I am running an 11" diameter prop 13 pitch. My top rpm is 5200-5300 with a speed of about 35 mph. If you are topping out at 4200 rpm you could be over pitched.

My Merc book shows the Mark 35 using a 10 3/4" prop with 13, 12 and 11" pitch. The speed range at WOT and 5200-5400 rpm at 35-40 for the 13, 30-35 for the 12, and 28-32 for the 11" pitch. I would think the 12" pitch would be a good prop to try. You're already hitting the lower speed range for the 11 and that might not be enough pitch to keep the rpm's in the correct range. How's the compression on the motor? If it is low, that could be a reason for not getting into the rpm range. The above figures are for a gross load of 700-1100 lbs.

Roberta

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Roberta
Built Zip "Oliver IV" and Super Spartan "Jimmy 70"

Thanks,
Compression 120 PSI on all 4 cylinders, 2 on board for run, about 330 pounds all in people wise, 6 gallon cruiser tank, big battery.

If I understand correctly a steeper pitched prop (13) puts more load on engine, ergo lower RPM, and, therefore lower pitch prop would allow RPM to increase, and possibly MPH - at the cost of less 'oomph' out of the hole.

Really not worried about it just curious as to what might help prop wise if I wish to experiment.

_________________
This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com

Actually the lower pitch will give you more oomph out of the hole as the RPMs will come up quicker. Kinda like taking off in 1st instead of 2nd gear. The speed should also increase a little due to the increase in top end RPM to a point.

Roberta

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Roberta
Built Zip "Oliver IV" and Super Spartan "Jimmy 70"

I typically see 30 MPH @ 6,000 RPM with our Zip, with a well used 1956 Mark 55 and a 10"-pitch aluminum prop. Installing a prop with 11" or 13" of pitch drops the engine speed to 4,500 with little improvement in performance. Our tach is a $30.00 el cheapo from the local Pep Boys, and I take its pulse from one side of the alternator, so it's possible that my RPM readings are erroneous. In any event, it's better to be slightly under-propped than over, since engines with needle-bearing bottom ends don't take kindly to lugging. I also have a three-blade, 10"-pitch brass prop that runs very nicely and is great for hole shots with one or two folks on board, although it limits the top end to around 28 MPH.

We did install fixed-jet carbs from a Mark 35 to get rid of the adjustable-needle units on our early Merc, so it's entirely possible that we de-rated the motor in the process. Last time I checked, we had around 115 PSI in all four holes, so I don't think we have a compression problem.

-Mark Shipley

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A boat is just a wooden box with no right angles.