Prop Pitch vs Speed

[Mumbles]

I took my freshly rebuilt ’65 Johnson 18 horse motor to the lake today and swapped props while I was there. The 9 X 10 pushed the boat at 25.4 mph but when I put the 9 1/4 X 11 prop on, the speed dropped one mph to 24.4 mph. The 11" prop has wider blades on it and I’m wondering if that is the reason or maybe the motor doesn’t have enough power to turn it. I’m not sure what RPM the motor was turning as I keep forgetting to bring the mini tach Wedgie gave me.

The ten inch is on the left.

[johnyrude200]

You’ll have to use the tach to figure that one out. Just recently played with props on a 30hp etec…went up 1 pitch higher, and gained 2 mph, but the RPM’s were 200 below the minimum, so switch to the lower prop for longevity of the motor (lost 2 mph, but worth it to see the engine last longer).

[chinewalker]

More pitch does NOT always result in more speed. Your motor has a "happy spot" where it puts out its rated HP at a certain RPM. Typically on those old Johnrude 18s, that’s around 4500 RPM. If your motor were spinning 5000 RPM with the 9-inch prop, then putting the 10 on it might give you more speed as the higher pitch would bring the RPM down closer to the 4500 benchmark. However, if your motor were already pulling 4500 with the 9, putting the 10 on it would bring the RPM below that benchmark, thus not allowing it to reach its rated RPM/HP.

Think of it this way, you’re towing a trailer with a truck with a 6-speed transmission. You’re cruising along at 55mph in 5th gear on flat ground, akin to your 18hp revving at 4500 with prop A. You approach an uphill grade and suddenly your truck isn’t cruising along so smoothly anymore, so you downshift to 4th. This is akin to going to a power prop to tow a skier, using a prop with lower pitch. You have more load (uphill/skier) so you need to adjust the gear ratio/prop pitch to keep the motor happy. Same truck, but take away the trailer and you can now cruise along on flat ground in 6th gear, as you’ve taken away the load, akin to going UP in pitch. Hope that makes sense…

Bottom line is you need to borrow that tach and get a handle on where your RPM are. The 10 might only work well on a really light boat with minimal loading. You really want to avoid loading up the engine. Letting it rev a couple hundred over is better for it than lugging it a few hundred under.

[pappy]

Try and find a two blade to play with. Less blade area means less parasitic drag. You were correct in your thinking on those two propellers.
The "disc area ratio" of a propeller (how much of a circle is occupied by blade) can make a big difference in the way a boat performs. We don’t have many choices in props for these engines but there are some out there. Can always add a bit of cup to the smaller prop as well.
Still think you have speed left in that one.
My 15’6" fiberglass Gheenoe (with the 18) ran a solid 27mph with a trolling motor battery up under the front seat and some water in the live well.
Your boat has to be lighter and should respond to a good prop.

[Mumbles]

I’ll bring the tach next time so I know better what’s going on here. If I look around, I’m positive I have an 11" prop somewhere with small blades on it. As for the boat, the hull probably has a lot of drag as the hull is beat up and there’s lots of sealer on the loose rivets. The new paint is what’s holding it together! 😀

[pappy]

Wick a little medium thickness super glue into any leaky rivet and that issue goes away immediately. Hobby Shops have it or order online. Make sure everything is nice and dry of course

[fleetwin]

This is the engine you put the 25hp powerhead on, correct? If so, those engines kinda need higher RPM versus the 18s to develop power. Follow Pappy’s propping advice to optimize performance.

[Mumbles]

No, it’s an 18 horse powerhead from a ’72 18203 Evinrude motor. The original block was toast so I put the ’65 Johnson crank and pistons in the Rude block as I had the early rings on hand. The later two rings seem to be quite pricy when you can locate them.

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