Marine News from the Great Lakes

Do I Have The Correct Propeller?

Published: Monday, December 23, 2019
By: Professionals of Henry H. Smith Co.

We all dream of the perfect boat and propeller combination. It would make our boats jump out of the water with ease, speed across the lake faster than anyone else, and our fuel gauges would always read full!

Unfortunately, the realities of physics gets involved. We could consider having a couple of different propellers that have performance characteristics biased toward specific performance objectives and conditions. Barring that, we must search for a realistic, optimal balance providing good all-around performance.

The question of the correct prop typically starts with a boat performance concern. “My boat is using more fuel than I expected”, “It is slower than I believe it should be”, “When I have friends aboard, it doesn’t get out of the hole as quickly”, or “It takes too much effort to stay on plane” – these are all concerns that will have you questioning your propeller.

Before we focus exclusively on the propeller, there are some other important factors to consider relative to boat performance: the hull, extra/added weight, and the engine. First, hull condition is particularly important if kept in the water as algae will create drag and resistance. Next, boat performance is greatly affected by weight from additional people, coolers, and accumulated objects that are normally hidden from sight. Finally, confirm the engine is maintained and running well, so that the propeller performance can be evaluated.

To determine the best propeller for your boat, not only do we consider boat, engine, and transmission information, but the current, actual baseline performance will need to be quantified in order to understand the improvements that are achievable. This performance data should include RPM and boat speed wide-open throttle (WOT). Time to get on plane, cruising RPM, temperature and conditions when the data was taken are also helpful to record. Current propeller design and condition have to be documented to make sure we understand the basis for any adjustments. Your favorite marine service or prop specialist can provide you with a form to ensure all of the necessary data is recorded.

Wide-open throttle (WOT) RPM at the target is most important in optimizing if all-around balanced performance is the objective. If a boat runs out of horsepower at the same RPM as the engine’s target WOT RPM, then the other performance characteristics should fall into place. The boat’s cruising RPM should be comfortably within 70%-80% of the WOT. The boat should then get on plane, with adequate RPM to have the engine properly in the horsepower curve. This is the optimal balance between acceleration and top speed, providing overall good performance.

Performance gaps can be addressed with the assistance of a propulsion professional. Propellers have many design characteristics that all play a role in how a propeller will perform on your specific boat and your specific conditions. These include the material (aluminum, stainless steel, bronze or NiBrAl), diameter (impacting blade area and clearances), pitch (straight, progressive or variable, controlling the progression through the water), rake (changing the attitude of the blades relative to the shaft axis),  cupping (impacting the water flow off the trailing blade edge) and venting (helps increase low speed RPM).

With complete and accurate data, your boat can be matched with the right propeller to ensure your boating experience and performance can be the best one possible.

Founded in Detroit by Henry H. and William H. Smith, the company continues to employ 3rd and 4th generation members of the family, carrying on a legacy established in 1900 - to be your Marine Drive Specialists.

This article first appeared in the Year End Issue (Nov/Dec) 2019 of Great Lakes Scuttlebutt magazine.

tags: Propellers, Service & Repair

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