Prop recommendations?

[jbutler67]

Prop recommendations?

I have been reading many threads about choosing the right prop. So much to consider!

Before I begin my assessment of different prop pitches and diameters, I am looking for some recommendations to use as starting points. Ultimately, I will test different props -- measure RPMs at WOT.

Boat Info:
1988 20' Four Winns Horizon
5.0L V8 I/O Stern drive
200HP at crank case (175HP at prop shaft???)
Boat weight with engine, full fuel, 3 passengers and accessories - 3,320lbs
Max WOT, per boats specs - 4,200 - 4,600
Limited to max of 14.5" diameter due to clearance from bottom of lower unit

 

[HT32BSX115]

Prop recommendations?

I have been reading many threads about choosing the right prop. So much to consider!

Before I begin my assessment of different prop pitches and diameters, I am looking for some recommendations to use as starting points. Ultimately, I will test different props -- measure RPMs at WOT.

Boat Info:
1988 20' Four Winns Horizon
5.0L V8 I/O Stern drive
200HP at crank case (175HP at prop shaft???)
Boat weight with engine, full fuel, 3 passengers and accessories - 3,320lbs
Max WOT, per boats specs - 4,200 - 4,600
Limited to max of 14.5" diameter due to clearance from bottom of lower unit

Howdy,

It will probably be better to post this in "Prop Questions and Topics"

You'll get more "prop-stuff" there!

Maybe a MOD will move this to the prop area!

You will need to know your WOT (GPS) speed, accurate RPM (known tach) and drive ratio before you consider whether to change what you already have.

Regards,


Rick

 

[jbutler67]

So sorry I posted in the wrong section. I will wait for MOD to move to prop area.

 

[jbutler67]

Although, a quick question?

How does one calculate drive ratio, short of pulling outdrive apart and counting teeth.

I know the OMC part numbers for the forward and pinion gears. But, I can't find any specifications that tell how many teeth on each. Are the number of teeth standard? Or is the only way to know to count?

I am guessing 26 over 14?

 

[southkogs]

Moved the topic for you.

If you can't find the ratio printed anywhere (in a manual, for example), you can rotate the drive manually and count the rotations of the propeller.

 

[jbutler67]

Thanks for moving.

I think I figured it out?

GMC 5.0L
Upper gear case - 21:17
Lower gear case - 14:26

(21/17)*(14/26) = .665
1 rev of the U-joint shaft results in 0.665 revolutions of the propeller shaft

1/.665 = 1.50
1.50 revolutions of the U-joint shaft results in 1 revolution of the propeller shaft

Total Ratio is 0.665 and similarly the Overall ratio is 1.50

Yes?

 

[HT32BSX115]

Thanks for moving.

I think I figured it out?

GMC 5.0L
Upper gear case - 21:17
Lower gear case - 14:26

(21/17)*(14/26) = .665
1 rev of the U-joint shaft results in 0.665 revolutions of the propeller shaft

1/.665 = 1.50
1.50 revolutions of the U-joint shaft results in 1 revolution of the propeller shaft

Total Ratio is 0.665 and similarly the Overall ratio is 1.50

Yes?

Possibly! The main thing to understand is that in the nearly 30 years from new, someone could have changed that drive more than a few times! and just put what ever they had on the back. OR, the drive could have been rebuilt with whatever gears were cheap and available!

You might know what drive ratio *should* be used with your engine but that doesn't guarantee your drive being that ratio!
The easiest and most simple way to determine the ratio is to remove spark plugs, place the drive in gear and have someone turn the engine.

count engine revolutions and prop revolutions and create a fraction. Then use a calculator using the math you indicated above. you can measure fractional (prop) revs by knowing that a 3 blade prop has a blade every 120 degrees. 120 = 1/3 or a turn (0.333)

You can get pretty accurate.

After you do all of this, get out on the water and run the boat at WOT with your current prop and see what you get, Make sure that you have the engine tuned up, the boat is not overloaded, the bottom is not overgrown with moss, you're not dragging the anchor, still hooked up to the trailer etc!!!

Get up on a plane, trim for max speed, (move people around in the boat to) ensure you're level, and note the (GPS) speed and (known tach) RPM.

If the engine is turning at or near the max recommended RPM, you're right where you want to be for best performance. Then go into the prop-slip calculator to verify. you should see slip numbers around 15% or so for most run-A-bout planing type boats.....

We love pictures and videos too!

Cheers,


Rick


Let me also add that if your ratio is indeed 1.5:1 ........3 engine revs will result in exactly 2 prop revs!

 

[jbutler67]

Thanks Rick!

I'll count the engine and prop revolutions (spark plugs out).

Then I'll do the test for RPMs (tach) at WOT, noting speed (GPS).

I appreciate all the great help on this forum!

 

[jbutler67]

Some notes for different props. Unfortunately, tide has influence on river. And I wasn't always alone. So, I tried to note all conditions for each of the 4 props.

Not sure if next test should be 14x21 or 14.25x21.

I'm thinking 14x21 based on results of 14.25x19 - very close to where I want RPMs at WOT. Dropping diameter by 1/4 to offset the loss in RPMs based on 2 increase in pitch.

***14.5 x 19 - 3 blade
3,700 RPM at WOT - 32 MPH into wind and chop with 3 passengers

4100 RPM at WOT - 36 MPH wind at back. With 3 passengers.


****14.5 x 17 - 4 blade
4,600 RPM at WOT - 40 MPH with outgoing tide, no passengers

4,400 RPM at WOT - 32 MPH against outgoing tide, no passengers

Slower moving through water in no wake zones when going against current.


***14.25 x 19 - 3 blade
4,400 RPMs at WOT - 40 MPH moving with outgoing tide, no passengers

4,400 RPM at WOT - 35 MPH moving against outgoing tide, no passengers

 

[HT32BSX115]

****14.5 x 17 - 4 blade
4,600 RPM at WOT - 40 MPH with outgoing tide, no passengers

4,400 RPM at WOT - 32 MPH against outgoing tide, no passengers

Slower moving through water in no wake zones when going against current.


***14.25 x 19 - 3 blade
4,400 RPMs at WOT - 40 MPH moving with outgoing tide, no passengers

4,400 RPM at WOT - 35 MPH moving against outgoing tide, no passengers

If you have verified your tach and speedometer as accurate, 4600 @ 40 MPH using the 14.5 x 17 sounds like your best performance AND with the 4-blade, you'll get the best holeshot :thumb:

 

[jbutler67]

Question about the following...

"way to determine the ratio is to remove spark plugs, place the drive in gear and have someone turn the engine
count engine revolutions and prop revolutions and create a fraction."

How best turn the engine manually?

I read elsewhere to... "Make sure the alternator belt is tight and use a socket on the alternator. WIth the plugs out, the engine should turn easily."

Or, use a breaker bar and socket on the camshaft bolt?

Thanks in advance for entertaining my newbie questions.

 

[HT32BSX115]

With the plugs out, either way works....it should turn fairly easily.. as long as you can tell how many turns the engine makes. and you don't additionally overly tighten the (you meant CRANK-shaft) bolt