Other causes for prop slip?

[CapRedbeard]

2000 Four Winns Horizon 170
Volvo Penta 3.0
SX outdrive 2.18 gear ratio
Manufacturer recommended WOT RPM: 4200-4600rpm

I bought the boat with a beat up 14-1/4” x 23 aluminum prop and was getting about 30% prop slip. I installed a new Solas 14-1/4” x 23 aluminum prop and now I’m down to about 22% slip (5000rpm, 39mph WOT)

What are some other factors that I should look at for my high prop slip and over-revving at WOT? The bottom of the hull has some scratches and dings in the gel coat and fiberglass, could that be causing problems?

Thanks in advance for the help!

 

[89retta]

Are you sure the tach is reading properly? Do you have the throttle wide open or do you still have throttle left ?

 

[CapRedbeard]

Are you sure the tach is reading properly? Do you have the throttle wide open or do you still have throttle left ?

I’m not sure that I’ve ever second guessed the tach, but I haven’t tested it. It reads properly at idle fwiw. Is the easiest method to use a multimeter at the wires on the back of the tachometer?

My prop slip measurements in the OP were at WOT.

 

[Scott06]

I’m not sure that I’ve ever second guessed the tach, but I haven’t tested it. It reads properly at idle fwiw. Is the easiest method to use a multimeter at the wires on the back of the tachometer?

My prop slip measurements in the OP were at WOT.

No test against a shop tach or timing light with tach feature

 

[Texasmark]

Your speed was measured how? Instrument, GPS?
How do you know the tach reads accurate at idle? A 5% error on the instrument or your guessing it's correct at 650 idle RPM, if on the low side would reduce your apparent slip to 17%. A 10% would drop that to 13%. 13% could be expected out of an I/0 depending on numerous variables at WOT.

There are battery operated, hand held, digital tachs available numerous places for less than 50 bucks whereby you put a piece of white tape on something connected to the crankshaft and get a digital pulse count.

So, It may be that you just have instrumentation errors. Changing props, especially manufacturers and types within a manufacturer's offerings can and will change your boats performance which explains the slip differences with the old and new prop.

 

[CapRedbeard]

Your speed was measured how? Instrument, GPS?
How do you know the tach reads accurate at idle? A 5% error on the instrument or your guessing it's correct at 650 idle RPM, if on the low side would reduce your apparent slip to 17%. A 10% would drop that to 13%. 13% could be expected out of an I/0 depending on numerous variables at WOT.

There are battery operated, hand held, digital tachs available numerous places for less than 50 bucks whereby you put a piece of white tape on something connected to the crankshaft and get a digital pulse count.

So, It may be that you just have instrumentation errors. Changing props, especially manufacturers and types within a manufacturer's offerings can and will change your boats performance which explains the slip differences with the old and new prop.

Got it, thanks for the thorough information! Speed was measured with GPS.

If there is a discrepancy when I’m testing my tachometer instrumentation, is there a sender or wiring that could be faulty or would the problem be with my tach on my dash?

 

[Scott Danforth]

Got it, thanks for the thorough information! Speed was measured with GPS.

If there is a discrepancy when I’m testing my tachometer instrumentation, is there a sender or wiring that could be faulty or would the problem be with my tach on my dash?

Dash tachs are notoriously off and their inacuracies are usually not linear. they are nothing more than a digital to analog converter and a volt meter unless its a CAN BUSS gauge reading directly from the engine ECU (which your 2000 3.0 would not have)

just a bit of corrosion on the dip/slide/rotary switch can cause inaccuracies. a bad solder joint on the board can cause inaccuracies and the small trim pot on the board to adjust can cause inaccuracies

other things that can cause high prop slip in addition to improper tachometer reading is improper drive trim and a dirty boat bottom.

 

[poconojoe]

I don't think anyone asked this...
Did you replace the rubber hub too, or just the prop alone?

 

[CapRedbeard]

Dash tachs are notoriously off and their inacuracies are usually not linear. they are nothing more than a digital to analog converter and a volt meter unless its a CAN BUSS gauge reading directly from the engine ECU (which your 2000 3.0 would not have)

just a bit of corrosion on the dip/slide/rotary switch can cause inaccuracies. a bad solder joint on the board can cause inaccuracies and the small trim pot on the board to adjust can cause inaccuracies

other things that can cause high prop slip in addition to improper tachometer reading is improper drive trim and a dirty boat bottom.

Thank you for the explanation. So I’m not wrong in thinking that the scratches and dings in my hull bottom gel coat and fiberglass could be causing some of the problems with my readings also? Would that be due to cavitation caused by the hull or another reason?

 

[CapRedbeard]

I don't think anyone asked this...
Did you replace the rubber hub too, or just the prop alone?

Both the old and the new props had pressed hubs.

 

[poconojoe]

Both the old and the new props had pressed hubs.

I just realized it's a Volvo and I don't know much about those.
I just assumed most props had the replaceable rubber hub for any fluctuations and to have a bit of give to save the prop.

 

[WesNewell]

Most/all? tachs have settings for the engine installed for. If it's not set to the proper setting, then the reading will be off. And I've seen them set wrong from the factory.

 

[89retta]

Try a 4 blade 19 pitch turning point prop.

 

[Texasmark]

Thank you for the explanation. So I’m not wrong in thinking that the scratches and dings in my hull bottom gel coat and fiberglass could be causing some of the problems with my readings also? Would that be due to cavitation caused by the hull or another reason?

They would have to be pretty bad and be in the "new water" area that precedes the prop.....but yeah it could, just like on boats like aluminum whose hull is stamped in two pieces with a "keel" along the bottoms center line to cover up the joint and are an aid in reinforcing it....they cause problems especially in tight turns at higher speeds. A cupped prop helps to overcome that....holds it's RPM even though the new water may be contaminated by bubbles.....BTDT

 

[CapRedbeard]

Hi guys, update on this as I’ve been working through some other stuff with the boat (all fixed) and having some fun this summer on the lakes and river.
After testing with a Hz meter at the spark plug, I discovered somewhere in the 3-5% range of an over-representation on my tachometer vs actual rpm. That means I was getting roughly 4800rpm WOT when measured at the spark plug (5000rpm on the tach).

That gives me about an 18% prop slip, still high. I’d also like to get my rpms down under 4600 if possible. What could I expect to change from switching from my 3 blade 23p aluminum prop to a 4 blade 23p aluminum prop?

 

[PB48052]

I've got a brand new Solas 4 blade 14-1/2 dia. × 21 p, if you're interested. I'm Not a dealer

Hi guys, update on this as I’ve been working through some other stuff with the boat (all fixed) and having some fun this summer on the lakes and river.
After testing with a Hz meter at the spark plug, I discovered somewhere in the 3-5% range of an over-representation on my tachometer vs actual rpm. That means I was getting roughly 4800rpm WOT when measured at the spark plug (5000rpm on the tach).

That gives me about an 18% prop slip, still high. I’d also like to get my rpms down under 4600 if possible. What could I expect to change from switching from my 3 blade 23p aluminum prop to a 4 blade 23p aluminum prop?

It was too steep for my 18' Searay bowrider, only got 3500 rpm. Run only one day in deep blue freshwater

 

[PB48052]

Hi guys, update on this as I’ve been working through some other stuff with the boat (all fixed) and having some fun this summer on the lakes and river.
After testing with a Hz meter at the spark plug, I discovered somewhere in the 3-5% range of an over-representation on my tachometer vs actual rpm. That means I was getting roughly 4800rpm WOT when measured at the spark plug (5000rpm on the tach).

That gives me about an 18% prop slip, still high. I’d also like to get my rpms down under 4600 if possible. What could I expect to change from switching from my 3 blade 23p aluminum prop to a 4 blade 23p aluminum prop?

My Searay has 3.0 mercruiser. Solas in box, new condition

 

[QBhoy]

Try a 4 blade 19 pitch turning point prop.

Think he wants to reduce prop slip . Most effective way to increase prop slip is More blades !

 

[QBhoy]

Was

Hi guys, update on this as I’ve been working through some other stuff with the boat (all fixed) and having some fun this summer on the lakes and river.
After testing with a Hz meter at the spark plug, I discovered somewhere in the 3-5% range of an over-representation on my tachometer vs actual rpm. That means I was getting roughly 4800rpm WOT when measured at the spark plug (5000rpm on the tach).

That gives me about an 18% prop slip, still high. I’d also like to get my rpms down under 4600 if possible. What could I expect to change from switching from my 3 blade 23p aluminum prop to a 4 blade 23p aluminum prop?

id say you’re doing not too badly. You don’t want to be too low in the rpm range on the wee 3.0. Don’t strain it anymore than you need too. Already working hard.
what slip numbers are you aiming for ? Because the truth is that your boat and engine aren’t realistically going to be able to air the hull out enough to see anything much less than you have now. Even less likely with an ally prop and no proper prop to give any lift. With less rpm like you’re suggesting, even less a chance of it. More rpms through the prop equals more lift of the hull (if the prop had the ability to do so)

 

[Texasmark]

My opinion is that 18% slip isn't all that bad if you have a deep V with significant dead rise of the hull at the transom, with an I/O engine, with a lot of weight to be carried....folks, gear and all. If you are carrying a large wake at WOT and best trim you have what I am talking about.