Measuring Inboard Boat Propellers
What is an MRI machine?
When boat propellers come into our shop at Tacoma propeller, we check them for several measurements such as pitch, track, camber, rake, and assess the condition of the metal. Visit our blog about bronze and nibral materials. With inboard propellers over 16″, this type of data is taken on a digital pitchometer we call an MRI. No, no, not a medical MRI machine. This is a propeller “Measurement Recording Instrument.” Our shop uses a Hale MRI machine. Components of the Hale MRI machine include the stand, table, computer, arbors, and measurement arm. It allows professional shops to get digital readings of the pitch at multiple points on each blade, check original design data, and print reports. This becomes especially valuable as propellers get larger in diameter.
How does it change the boat repair process?
By going into more detail on measurements, the propellers can be restored for synchronized propeller loading, minimized noise, and comparison between two propellers is also possible so that exact matches can be compared. Repairs can include restoring boat propellers for operating conditions and even modifications for maximized performance. Read more about the process of marine propeller repair.
Using an MRI machine for inboard measurements gives guidelines for different levels of conditioning. There are different classes of pitch and repair, each with a different tolerance allowance. Class III, II, I, and S Class are the tolerances that may be shown in MRI reports. More radius lines on each blade are observed, with tighter tolerances as the class moves up.
This handy chart from Propellerpages.com shows tolerances in pitch by class:
PITCH | S | I | II | III |
Local pitch | 1.5% | 2% | 3% | — |
Prom. pitch for ratio | 1% | 1.5% | 2% | 5% |
Prom. pitch for blade | 0.75% | 1% | 1.5% | 4% |
Prom. pitch for propeller | 0.5% | 0.75% | 1% | 3% |
A new inboard boat propeller, directly out of the box should meet Class II specifications. Here at Tacoma Propeller and generally within the industry, we will recommend repair for anything not meeting at least Class II specifications. Our NMPA certified shop will repair an inboard to Class I or II specs depending on what the metal will allow.
Class S re-pitch work is extremely precise and time-consuming. In special cases such as high-speed vessels like hydroplane or offshore racers, S Class can be achieved. Rest assured, we want to see you running a well-conditioned propeller. You’ll always receive a printout of your finished MRI reading when your job is complete. We recommend keeping these should you ever find yourself in need of repair or replacement down the line.
Still have a boat propeller repair question? Call 253-272-5065 or email us at sales@tacomapropeller.com
