Pitch number
The second number - in this case 15 - refers to the propelor pitch and is in Inches. It means that for each rotation of the propelor it will theoreticaly advance 15 inches. You can calculate theoreticly the speed by multiolying the pitch - in this case 15 - by the propelor RPM and devide by 72,000 to get the nautical miles. For example: At 2,500 engine RPM, with gear reduction rate of- for example -2.63, the propelor rotates at 950 RPM, time 15 inch,,times 60 (minutes in hour), devide by 72,000 (inch per nautical mile)= gives Theoretical speed of11.8 Knots. But there are other factors which makes this theoretic calculation very unaccurate and incomplete.
