Big prop low rpm or small prop high rpm

[bep078]

I was just wondering. Take the following scenario.

A boat running at 6000 rpm (peak power) with a 1.75 ratio and 28" prop
A boat running at 6000 rpm (peak power) with a 1.62 ratio and 26" prop

Boat and spec are identical for both runs.

According to a slip calculator, keeping slip constant, speed is nearly identical for both runs.

But in the real world, what is preferred? One must assume that it has nothing to do with the motor as the rpm is kept the same for both runs so same power for both runs.

One can say that a smaller prop will have less slip at slower speeds, making acceleration better. However more pitch seems to lift the bow more. It also depends on weight I presume.

What are your thoughts?

[staylor]

Assuming everything else is to be constant- like prop model, prop diameter and boat loading- then the difference between gear ratios is so small that there wold be very little difference in performance. If you start to play with prop diameter at the pitches listed- and prop type, then there may be some improvement with one combination or the other, depending on boat size and weight. In general, as the boat and boat weight get smaller for a given horsepower, then the numerically lower gear ratio- the 1.62- will have more performance potential. Back in the 1950s we used to run beat-up kneel down race boats just for fun with various combinations of lower units for the old 20 cid Mercs. You could run a Mk20 Merc with either the stock shifting gearcase- numerically high ratio, or swap on a low profile non-shifting Merc KG-7 lower with a lower ratio, or if you could afford one- swap on a 1:1 ratio Merc Quicksilver racing lower. The 1:1 ratio with its tuny props was the fastest by far, but was very slow to accelerate compared to the higher ratio cases with their numerically higher ratios.
Doug

[staylor]

Spelling challenged this morning- I meant "tiny" props. A typical shifting Mk 20 case ran a prop in the 9.5 inch diameter range, the low ratio KG-7 lower ran props in the 7.5 in dia range, and the 1:1 ratio racing lowers ran props around 6.75 inches diameter. Years back I tried a comparison between an old 1951-52 Merc KG-4 (15 cid, non-shifting case, more or less a 10-12 hp rating) against an early 1980s Evinrude 15 with it's stock case running much larger dia props than the ancient Merc. On the same 16 ft aluminum fishing boat, both engines had nearly the same top end- even though the old Merc had what was most likely less hp, but planing time with the old Merc with it's small prop was around triple the time for the larger props on the Evinrude.
Doug

[Hoss Marine Propellers]

Changing gear ratios is just like changing pitch of prop.
In your example... it ends up with basically the same result.

When you get into really high pitch props, it becomes a game of "slippage"..... You are better off running a higher gear ratio with a slightly lower pitch prop..... you end up turning the prop more efficiently at a higher prop rpm vs. a big pitch prop at lower prop rpm.

Engine torque starts playing an important role with gear ratios as well.

In Staylor's post above, there are too many variables with gearcase design, prop design (ie. dia. & pitch & blade area) to have the same comparison to your original question of gear ratio & pitch. It is amazing to try different combinations and see the BIG differences you end up with!

[PropWash]

I was just wondering. Take the following scenario.

A boat running at 6000 rpm (peak power) with a 1.75 ratio and 28" prop
A boat running at 6000 rpm (peak power) with a 1.62 ratio and 26" prop

Boat and spec are identical for both runs.


What are your thoughts?

I tested a ski racing combo and tried this exact thing last weekend, on two SportMasters.

With boat only the 28/1.75 combo had lower top speed and hit the limiter easier. Towing a skier it was also slower, with a few more rpm in the top end. It accelerated a little bit better (I think - could not measure accurately).

The 26/1.62 was the fastest combination, with and without a skier. It had slightly more effective pitch (i.e. a bit less slip than the 28). I have had the same results comparing 24/1.62 and 26/1.75 with a 300x.

I think somewhere in the 24-28 inch range with 14-14.5" diameter is a good place to be if you can spin the prop fast enough.

At the end of the day the differences are going to be really small and you need to test the combos. Even two apparently identical 26 props might give slightly different results. I was looking for less than 1 mph between setups.

As a side note, a nose coned Torque Master I had was significantly faster.