Is this close??

I have a prop query. Boat is 1973 Olympic, 19', bow rider, with 1973, 120 hp Chrysler outboard. Current prop is 13 1/8 x 17 aluminum three blade. Finally got the engine to run at WOT. As it sits now, WOT gives me 4400 rpm @ 33 mph. Am I close for this engine?? Do I need more RPM?? As expected with an outboard, fuel consumption sucks, even at a moderate cruse. Thanks to all of the old salts out there who share their experience. Been boating 30 years, but just started back after ten years off pouring money down a race car. I think I forgot what I did know!

Re: Is this close??

I had to guess at your gear ratio as 1.73 Motor turns 1.73 to one for the prop.
I don't have those specs for a 73 Chrysler. You can help us by turning the flywheel by hand and counting the revolutions for each one rev of the prop.You may have to pull the plugs for this.There are 3 different systems used, Don't do any trouble shooting without a manual
as damage to the system could result.
Your Revs are low ,should be at least 5250, but your speed is in the ball park for your setup.
You may be over propped Someone more acquainted with Chrysler may be along.
But before jumping to a prop be sure the motor is in excellent tune.
It needs to be timed correctly and then the throttle sync to the triming advance needs to be set.Throttle openimg all the way as well.
Motor height is important too.The antivent plate (just above the prop) should be visable at wot.And you should adjust trim for best speed.

Re: Is this close??

I bought a '71, 85 Chrysler and '71 16' Chrysler Sport Fury Tri-hull and experienced the same poor fuel economy, and for 85 horses, my buddies with Loop charged, 60-65hp OMC engines could stay up with me and run loops around me on fuel economy.....So the next year I bought a heavier, smoother riding boat with a 125 OMC engine. It wasn't looped but it did have exhaust tuning and other things that made the fuel economy very tolerable.

But then some years later I went to Merc, direct charged at first then looped and things never were the same for me again.

The other problem I had with that engine was the OEM prop, white (to match the engine) alum with a reverse lip on the leading edge of the blades. It caused intense cavitation to the point where in no time at all the cavitation had eaten the paint off and started on the alum in an area about the size of a dime just behind the reverse curve.

Mark

Re: Is this close??

Operating range for that engine is actually about 4500-5500 with max horsepower (at the crankshaft, not prop) developed at 5000 or 5250. The exact number isn't critical. The lower unit is 1.73 or 1.76--I forget exactly. You are just below the operating range but really, that speed is about all you can expect even if you prop the engine for higher RPM.

You could try a 15 pitch prop but that may reduce your top speed a bit. The thrust pin, spline drive props are no longer available through manufacturers and you would need to be satisfied with what is available on ebay--yes, there is some choice but the good ones tend to be a bit pricey.


I do have a 15 pitch so send me a private message if you decide you want one.

NOW---If you can find a 90 HP lower unit, the gear ratio is 2 to 1 and you will find that using the same prop, engine RPM will increase as will speed.

If your 120 has the small WB carbs, you can also retrofit with the bigger manifolds and TC carbs off later Chryslers and Force engines. This will give a small boost to horsepower --not a lot, but you may see 1or 2 MPH more