Boat propellers are a big mystery to a lot of boat owners. But determining what size boat propeller you need to use for your boat and motor combination is not as complicated as you might think. There are just a few simple things you need to know and a few things you might need to find out that you can do yourself.
If you haven’t read our two previous blogs, Looking for a new boat prop? and Looking for a new boat prop? Part two, please do so for the basic information about diameter and pitch.
What you need to know is every engine has an RPM ( revolutions per minute ) range that you need to stay with in for safe engine operation, at full speed or also called wide open throttle ( WOT ) trimmed up if equiped with power trim. All sterndrive engine drive systems have power trim and tilt and most of them have tachometer gauges that show what the RPMS your engine is producing so what you need to know is what diameter and pitch boat prop you have and what your WOT RPM range is for your engine, the range can be found in your engine owners manual or ask your local service dealer that works on your engine brand what your range is.Then take your boat out with just you in it and with a light load, go full speed with it and play with the trim going up as far as you can that produces the fastest speed attainable without trimming up too much then look and see what your RPM’s are. The RPM range is about four to six hundred RPM’s. If the range for your egine is 4400 to 4800 RPMs and you are reading 4700 RPMs then you have the right pitch prop. If you are getting over 4800 RPMs you need a 23 pitchthree blade prop. If you are getting less than 4400 RPMs you need a 19 pitch three blade prop. Your engine’s horsepower is rated maximum horsepower at about 4700 RPMs, that is the RPM number you try to achieve. If your prop is an aluminum 21 pitch three blade propeller and you want a spare get the same size prop. If you want to pull tubers and skiers get a four blade 19 pitch prop for better takeoffs and better pulling performance. A four bladed 19 pitch prop will produce about the same RPMs as a 21 pitch three blade boat propeller. The same goes for stainless steel propellers RPM wise with just a little less RPMs than aluminum propellers, maybe two or three hundred in either the three or four blade because an aluminum 21 pitch boat prop is closer to a 20 under load because the blades will bend a little in aluminum and stainless steel prop blades don’t bend under load.
Next blog will be how to size boat propellers for outboard motors.
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