THEORETICAL PERFORMANCE CHART
Last update: 03/27/17

In a perfect world (100% efficiency), calculations derived from this chart would yield true speed.  The laws of physics dictate the answers.  In the real world, prop design and condition, hull design, condition and load and many other factors affect speed.  This is commonly called "slip" or "slip factor".  Although the prop MUST slip to work properly, many factors increase that slip.  I prefer "efficiency loss" to describe the whole system's performance.
Most speedometers are inaccurate (usually fast).  Only radar, GPS or accurate distance over time measurement can yield true speed. The difference in theoretical and "real world" speed is efficiency loss.
If you don't know your gear ratio, check the RPM/Gear Ratio Reference Chart.
The table below is based on the formula: (Tach RPM * Pitch) / (1056 * Gear ratio) = TMS and simplified to one step. Find pitch in first column (cupping may add 1" of effective pitch).  Follow across to number under correct gear ratio.  This number is the constant for this prop/gear ratio combination. Divide actual tach rpm by the constant to get Theoretical Max Speed (TMS).  Divide Real World Speed (RWS) by TMS to get efficiency.
    EXAMPLE: For 25P/1.86 gears, constant is 78.6. If RPM is 6000, then 6000/78.6 = 76.3 TMS. If RWS is 68, then 68/76.3 = .891 or 10.9% efficiency loss.
    I have added a link to an online "Prop Calculator" and provided a downloadable "Prop Workshop" program on the TechLinks page
 

PROPELLER
ENGINE LOWER UNIT GEAR RATIO
Pitch
Revs/Mile
Revs/MPH
2.30:1
2.08:1 
2.00:1
1.87:1
1.86:1
1.81:1
1.75:1
1.62:1
TACH REVS/MPH (CONSTANT) 
17
3727
 62.12
142.9 
129.2 
124.2 
116.2 
115.5 
112.4 
108.7 
100.6
19
3335
55.58
127.8 
115.6 
111.2 
103.9 
103.4 
100.6 
 97.3 
 90.0 
20
3168
52.80
121.4 
109.8 
105.6 
98.7
 98.2 
 95.6 
 92.4 
 85.5 
21
3017
50.28
115.7 
104.6 
100.6 
 94.0 
 93.5 
 91.0 
88.0
 81.5 
22
 2880 
48.00
110.4 
 99.8 
 96.0 
89.8
89.3
 86.9 
84.0
77.8
23
2755
 45.92 
105.6 
 95.5 
 91.8 
85.9
85.4
83.1
 80.4 
74.4
24
2640
44.00
101.2 
 91.5 
 88.0 
 82.3 
81.8
79.6
77.0
71.3
25
2534 
42.23
 97.1 
 87.9 
 84.5 
 79.0 
78.6
 76.5 
73.9
68.4
26
2437
40.62
 93.4 
 84.5
 81.2 
76.0
75.5
73.5
71.1
65.8
27
2347
39.12
 90.0 
 81.3 
 78.2 
73.2
72.8
70.8
68.5
63.4
 28 
2263
37.72
86.8
78.4
75.4
70.5
70.2
68.3
 66.0 
61.1
 29 
2185
36.42
83.8
75.7
72.8
68.1
67.7
 65.9 
63.7
59.0
30
2112
35.20
81.0
73.2
70.4
65.8
65.5
63.7
61.6
57.0
31
2044
34.07
78.4
70.8
68.1
 63.7 
63.4
61.7
59.6
55.2
32
1980
33.00
75.9
68.6
66.0
61.7
61.4
59.7
57.8
53.5

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