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Author Topic: Here's How To Select A Prop  (Read 744 times)

December 28, 2016, 10:36:52 PM
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boatnamesue

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Here's How To Select A Prop
« on: December 28, 2016, 10:36:52 PM »
Over the past few weeks we've had a number of threads asking how to select the correct prop, or how to determine if the prop you currently have on is correct.  Hopefully this thread answers all, if not most.

First, before you go purchasing a new prop (they ain't cheap), there are a few variables that affect rpms other than the prop.  Of the controllable variables, engine height can make the biggest difference.  The engine AV plate should be between 1"-2" above the bottom level of hull (excluding powerboats, commercial boats, etc.).  If your engine's max rpm at WOT is under spec, and the engine height is under 1"-1.5", raise the engine.  If your engine's max rpm at WOT is over spec, and the engine's height is above 1"-1.5", lower engine.  Check your engine height before spending the money on a prop.

Determine your max rpms at WOT (wide open throttle).  This test should be done with a full fuel tank, one operator and no passengers, and all non essential items off the boat.  Run the boat on a calm day, preferably no head wind.  At WOT, trim up engine to the point just before the bow begins to porpoise.  You want to perform this test with as little drag as possible.  Read your tach.

Now it's time to read your service/owner manual.  In the specification section will be listed the spec rpm range at WOT.  The numbers will be something like 5000-6000, or 4500-5500.  Every engine manufacturer has their own pre-determined rpm range.  Follow their range, not what you heard the range should be.  Operating the engine out of WOT rpm spec will eventually cause a pre-detonation issue, which will then eventually cause bigger problems.

What is your current props pitch?  Look at your prop and find a sequence of numbers like this: 13.25 X 17   The first number is the diameter, the second number is the pitch.  Diameter isn't something you'll have to mess with.  The bigger the engine, the bigger the diameter.  Every prop manufacturer produces a prop for a specific engine manufacturer, with limited diameter choices per pitch, if any choice.  Don't be concerned with the diameter.

Now that you have done all of the above it's time to determine the correct pitch prop to use.  And here is when it becomes a little bit of guessing game.  As an unwritten rule of thumb, every inch of pitch equals 150-200 rpms.  If you decrease pitch, you increase rpms.  And vice versa.  For example, lets say you have a 17" pitch prop.  And with this prop you're turning 7000 rpms at WOT.  Too much, it's under-propped.  Let's assume your manual lists the max rpm at WOT to be 5000-6000 rpms.  So you need to drop 1000 rpms to be at the top end of the range, which is ideally where you want to be.  If you went with a 21" pitch, increasing the pitch by 4", you can approximate the max rpms you'll get to be at the top end of spec rpm range (4" x 200 rpms = 800 rpms).  Question is, do you go with the 23" or the 21".  That's where it becomes a guessing game.  However, if you locate a reputable prop shop in your area and tell them what rpms your currently getting with your prop, and tell them where the rpms need to be, they will give you their best advice.  After purchasing the prop and you determine you need to go up a pitch or down a pitch, most shops will accomodate.  After all, a new SS prop ain't cheap.  You need to be sure you're buying the correct one. 
---------------
Jason
1976 AS 170
1998 S115TLRW

December 29, 2016, 05:15:51 PM
Reply #1

wingtime

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Re: Here's How To Select A Prop
« Reply #1 on: December 29, 2016, 05:15:51 PM »
Lowering the engine to help lower the WOT rpms of an under propped motor is a bad idea. Yes it may lower your rpms it is doing so by increasing the drag and decreasing the efficiency of the boat. This results in increased load on the motor and a higher fuel burn. The best method is test run the boat to make sure the AV plate is riding correctly while a plane. You can do this by having a buddy watch the ride height while running. Or better yet record a video using a GoPro. Once you have the correct engine height then you can dial in the prop.
1998 Explorer w/ Etec 250


1987 170 w/ Evinrude 90

December 29, 2016, 07:37:01 PM
Reply #2

boatnamesue

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Re: Here's How To Select A Prop
« Reply #2 on: December 29, 2016, 07:37:01 PM »
Lowering the engine to help lower the WOT rpms of an under propped motor is a bad idea.

How do you know you have an under propped boat until you rule out the controllable factors that could be causing high rpms??  You can't label the boat as being under propped until you have addressed these factors, engine height being foremost.  Soo, IF the engine is above the widely acceptable engine height range of 1"-2" above the bottom level of hull, adjust this first.  Lower the engine into the acceptable range, water test.  If the reduction in rpms you'll get still does not get into the rpm spec range, THEN you can label the boat as under propped. 

What you're suggesting, respectful, would be like labeling a spark plug as bad without even looking at the plugs, and also confirming proper gap.  I've owned a number of engines made by different manufacturers.  All list the correct engine height in their owners manual to be 1"-2" above the bottom level of hull.  So you don't want to skip over this and make the presumption the problem of high or low rpms is definitively the propeller.  You gotta look at the entire picture.
---------------
Jason
1976 AS 170
1998 S115TLRW

January 01, 2017, 07:13:15 PM
Reply #3

wingtime

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Re: Here's How To Select A Prop
« Reply #3 on: January 01, 2017, 07:13:15 PM »
The engine manufactures suggestion of the AV plate being 1-2" above the bottom of the hull is just a starting point suggestion it is not a number that is set in stone.  Factors such as hull design, motor setback, hull steps, weight, end use of the boat (bay boat vs offshore etc) are also factors.  Also things such as cooling water pick up, handling and blowout while cornering etc are also things to be considered into the motor mounting height. The best thing so go out and test the boat and SEE where the plate is riding. See what happens in turns, see how the hole shot is etc etc.  If the plate is riding in the right place you do NOT lower it to attain a lower WOT RPM for your prop.

Here are two videos of the AV ride height on a recent Etec repower I did on my Explorer. You can clearly see the plate is riding too low in the first video.  The second video is after raising the motor up one hole.  This did not result in much of a WOT RPM change but it did net a gain in speed.  Keep in mind this hull has a step so the water you are seeing is coming up from the hull about two feet ahead of the motor.





In the end I ended up testing three different propellers on this setup to get it dialed in.  Just buying a prop and adjusting the motor height down if your RPMS are too high just to save the $$$ spent on the prop is a bad idea and is NOT the excepted practice of dialing in a engine prop combination.  You should borrow several props and go out and test the boat with each one in several different loaded conditions. Reputable prop shops will loan you or let you exchange props until you have the correct one you need. Also a props pitch can be adjusted, have cup added or taken out, etc to get things dialed in perfectly.  Propping a boat is more of an art than it is a science.
1998 Explorer w/ Etec 250


1987 170 w/ Evinrude 90

 

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