PROPELLER SHOOT OUT (Alloy Boat # 9 Spring 2016)

Barry 2019-03-15

We took eleven different Mercury Marine propellers and tried them all on our McLay 4.8 Fortress powered by a Mercury 75 4S, to see what difference to performance and fuel economy, a few degrees of pitch and inches of diameter made. 

We have been running a McLay 4 .5 Fortress with Merc power for three seasons now and after starting with a Mercury 60 4S, we upgraded to a Mercury 75 4S last summer. It is a rocket ship and also very economical, although until now I didn’t have a handle on just how good it was.

When I go fishing, I fill a 25-litre tote tank and a smaller 5 litre spare and head out for the day. I have never run out and even if I did I always have the electric Minn-Kota outboard to get me home at a slow and leisurely pace. Before the multi-prop test, we ran on Orakei Basin, I had been running a Mercury Spitfire 15 X7, four blade stainless propeller. But was that really the best one for the boat/engine combination?

The boat has a manually adjustable jacking plate on the transom, but since setting it so the cavitation plate of the outboard is 40mm above the keel line, I have never moved it. I know if I raised it and ran a different propeller I would increase speed, but as this is a totally dedicated fishing boat and not a speed machine (some would disagree), top end has never been that important.

Selecting a performance goal is crucial to finding the right prop. For example, one propeller may be better suited for acceleration while another propeller will be best for mid range cruise/fuel economy. The first thing is to decide what you want from your boat and then look for the most suitable propeller for the task. The Mercury 75 runs a Big Tiller, with a steering damper to take some of the torque out of the tiller. I find when the engine is trimmed out a little over ¼, there is no torque feedback through the helm.

However, when I trim it right in, as I sometimes have to do when heading into a head sea, I can feel it in my arm. However, overall it’s not an issue and I love the fact that with the tiller steer I have the rest of the floor space in the boat uncluttered and maximise it as a fishing platform.


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