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Propeller Modeling
Propeller Modeling
A standard 3-blade fixed propeller such as supplied by Pearson with a stock 424 is not an
expensive piece of equipment, but a folding or feathering propeller is dear. I had already
replaced my stock propeller with a 2-blade folding prop several years ago. This made a
tremendous improvement in the boat's sailing performance. I'd be reluctant to regress to a
fixed propeller again. Unfortunately, my budget was limited, so replacing the propeller yet
again was not in my re-power budget. I expected to reuse the propeller, but I found that I
really didn't understand the relationship between propeller size, gear reduction, and engine
power. Since I chose a larger engine, the old propeller just wasn't large enough to properly load
the engine. The propeller just couldn't transfer the engine power to the water because the
blades were too small. I changed the V-drive gear ration to adjust for much of the difference in
engine speeds (2400 vs 3800), but there wasn't enough bite in the old propeller to transfer that
power to the water.

There are a number of new folding propellers on the market in the past few years that offer
significant performance improvements over older folding propellers such as my old 2-blade
Martec. These include both 3-blade and 4-blade propellers. I checked with a couple of
manufacturers and found that I could purchase a new 3-blade propeller for about the same price
I paid for my old 2-blade unit 10 years ago. The new unit not only offers the smooth
performance of a 3-blade propeller, but also has the blades geared together. This eliminates
the need to rotate the propeller into a specific position to fold efficiently. While it can be
argued that folding propellers suffer in reverse, I've never had difficult stopping or backing. It
has been many years since I used my stock fixed blade propeller so I can't offer a direct
comparison. My research indicates that performance of a new 3-blade folding propeller is as
good or better than a fixed propeller. Performance in reverse is close to a fixed propeller,
bearing in mind that you must apply some shaft speed to open the blades.
There are many trade-offs when selecting a propeller. The link below will provide a propeller
size estimate based on boat size, displacement, engine power and RPM. I eventually selected a
Slipstream 3-blade folding propeller. I liked the stainless steel construction and they had a very
competitive price. The factory calculated 18.5x12 as the correct size. This computation proved
correct, but slightly small. If I had it to do again, I'd probably select a slightly higher pitch,
probably 13 since the full throttle propeller speed is at the high end of the recommended
engine spec.