Finding the right propeller for your outboard can significantly impact your boat’s performance and your boating enjoyment. If your propeller is too big or too small (in pitch and diameter) you’ll be frustrated by either sluggish acceleration or low top-end speed. The wrong size propeller can also cause wear and tear on your engine.
Sea trialing different props is hands down the best way to determine the right prop for your boat. On our latest sea-trial for a new Honda BF200 fit-out, we took a range on stainless steel props out on the water. I love this part of my job - not only because I got to enjoy an epic day on the Hauraki Gulf in glassy calm conditions- but mostly because it’s an opportunity to learn and refine the boat’s performance.
Our test findings
The first prop we tested was the new Lexor 15.5 x 17 pitch. This is a big bad boy prop with heaps of cupping and plenty of blade surface, suited to gear ratios of 2:1. With a light boat load it performed excellent, reaching 5700 RPM and with three people on board reached 37.8 knots. The boat felt lively and accelerated hard in every rev range.
We then tried a smaller Titan prop 14.75 x 17, this has a high rake for extra bow lift and plenty of cupping, which gave us 5900 RPM and 42 knots. Obviously with the extra bow lift there was less drag from the hull, resulting a higher top end speed. The big difference however was the acceleration through the rev range - the boat just didn’t perform as well, as it also lost its stern lift.
Throughout the day we tried smaller diameter props which gave good speed but poor boat handling. The huge amounts of torque from the engine made the prop slip and cavitate (bubbles of steam generated by the prop rotating quickly causing low pressure on the blades) under heavy acceleration.
From 3500 RPM and above it goes to show that a big diameter blade performance doesn’t always give you the best top end speed but the performance gains are in the boat handling. There’s always a compromise between speed and boat handling.
The new BF200 proved itself over the sea-trial using only 25L of fuel for 2.5hrs of motoring. Not bad considering 90% of it was above 5000 RPM.
With the correct prop we saw the tacho hit the magic 5800 RPM and 39.9 knots and at 21 knots it’s only using 21lph.
Unsure if your propeller is the best for your engine?
The best place to start is to look at your outboard manufacturer’s recommendation, as they’ll recommend a propeller that keeps your boat in the optimal RPM range. You can check this by running your engine at wide open throttle and recording the RPM on your tachometer. If the RPM is lower or higher than the manufacturer’s specifications, it’s likely your propeller isn’t right.
Another consideration is how you use your boat the majority of the time. Whether you’re mostly cruising around islands, doing a bit of sea-biscuiting or loading your boat up for fishing and diving missions, you’ll have different needs in terms of speed and acceleration.
Therefore, we recommend testing a range of props with the gear and people you typically take on the water. That way you can be sure your boat will perform to your needs the majority of the time.
Talk to the team at GT Marine about your prop set-up today.