Skool me on prop slip

[Mototrig]

Never really paid attention to it until now.

Been trying out different props lately. The prop that came with the boat last year was a custom deal and it decided to go on a permanent vacation to the bottom of the river. It was a 14 x 24. Fully loaded it got us outta the hole ok and cruised well.

With that prop the boat ran 54 mph @ 5000 rpm and 26% prop slip.

I ordered a Solas 15⅝x23 and the boat ran 52 @ 4100 with 8% slip. Bogged it down hard. It was a dog outta the hole and wouldn't rev out but was a great cruiser.

I have a few more I'm trying out this Saturday.

My main question is what slip should I be targeting? Is 26% too high? 8% seems too low. I've always heard you should prop for RPM not speed. What say the brain trust?

I'm running a SBC 5.7 325ish HP 20.5 Elimiminator cruiser.

 

[evantwheeler]

Do you know the gear ratio in your lower unit? Is it a Bravo1?

I have a 22’ ultra closed bow with 6.2 MPI SBC and bravo 1 with 1.65 gears. A 24p bravo 1 is an awesome prop for me. Bought used here for around $400. I suspect it would be a good prop for your boat, or at least a good starting point depending on what you have in your lower unit for gearing.

 

[lbhsbz]

I don’t put too much merit in prop slip numbers….just find one that works best.

I have a 24P right now that runs about 2-3% slip when I’m light. I got 2 different 26P versions of that same prop that ran about 15% and 19% slip.

There are far to many other factors than the pitch number that will affect how a prop works…

 

[Mototrig]

So I envy you Bravo guys.....I'm running the Volvo Penta 280 deal. It's a good unit just not many props available for it.

My gear ratio is 1.61

 

[Trash]

Don't chase slip numbers. Prop the boat where it will just be shy of max HP rpm with a heavy load. There are always trade offs. One prop may be the best for max speed, but suck out of the hole. Others may have great midrange and holeshot but not hit the max speed seen with other props. Unfortunately there is simply a lot of trial and error.

I will suggest you start gathering data during your prop quest. Very few people do this. When you test a prop, document the rpm and stabilized GPS speed at 200 or 250 rpm increments from idle to WOT. The slip will vary throughout the rpm band, but don't chase this number. Just observe. Put this data in a spread sheet. Do the same for all the props you try. You can add comments as to how the prop 'feels' out of the hole, in turns, it's tolerance for trim etc.

Overtime you will see what your boat/motor combo likes.

 

[LargeOrangeFont]

So I envy you Bravo guys.....I'm running the Volvo Penta 280 deal. It's a good unit just not many props available for it.

My gear ratio is 1.61

Did you GPS your speed? Do what @ Trash just said. Document your cruise speed vs RPM as well as top end, take off and midrange acceleration tests.

When I initially did prop testing going from a 22 to a 24 pitch dropped my 50 MPH cruising RPM by 500! Easier on the engine, and better with fuel. No downsides at all for me on that particular change.

Do you know the gear ratio in your lower unit? Is it a Bravo1?

I have a 22’ ultra closed bow with 6.2 MPI SBC and bravo 1 with 1.65 gears. A 24p bravo 1 is an awesome prop for me. Bought used here for around $400. I suspect it would be a good prop for your boat, or at least a good starting point depending on what you have in your lower unit for gearing.

Also try a Rev4 23 pitch if you come across one. That is what I run now. The B1 24 still works well, I like the Rev 4 a little better, the boat rides better with it and cruises at a lower RPM, also attains the same top speed at a lower RPM.

 

[Drew]

Prop slip is tricky. Need to know what to fix and how to fix it. That being said I don’t know that information . I have a set of labbed 30p that at 3000rpm I’m doing 50mph . With my stock merc 28p 4 blades at 3000rpm still doing 50mph . I run the 30p because it gives me better top speed less slip at wot. Just don’t get why my situation makes no sense. I must have less slip on the 28p I guess? I don’t know. Good luck

 

[boatdoc55]

Put a 15" diameter Lefthand prop on it with a max RPM of 5200 and be done with it. I worked on and propped a million of those boats when in the OC and that is what worked best. Don't worry about prop slip, you don't have a race boat, you have a very nice family boat. I would also keep it made out of aluminum as those are WAY cheaper to repair and a SS isn't going to do anything for you in that boat besides look cool. I don't know how many times I have gone over this prop stuff with people over the years but your boat wasn't built for speed. It's a family daycruiser and that's why Eliminator built it.

When you find the one that works buy another for the spare. Remember to prop it with a normal everyday load. You'd be surprised how much loads very day to day.

 

[Ricks raft]

So I envy you Bravo guys.....I'm running the Volvo Penta 280 deal. It's a good unit just not many props available for it.

My gear ratio is 1.61

Hill propellers can make whatever you want . I had a 290, supercharged 350, hill’s elephant ear, tuned, worked best for me.

 

[Mototrig]

@boatdoc55 I should have specified, I'm not trying to hot rod my family boat. Most of the time the boat is fully loaded cruising at 3500 rpm, 40ish mph. I was just geeking out on the numbers since I'd never tested props before.

Next Saturday im testing a 15x21 and 15x23

Did you GPS your speed? Do what @ Trash just said. Document your cruise speed vs RPM as well as top end, take off and midrange acceleration tests.

When I initially did prop testing going from a 22 to a 24 pitch dropped my 50 MPH cruising RPM by 500! Easier on the engine, and better with fuel. No downsides at all for me on that particular change.



Also try a Rev4 23 pitch if you come across one. That is what I run now. The B1 24 still works well, I like the Rev 4 a little better, the boat rides better with it and cruises at a lower RPM, also attains the same top speed at a lower RPM.

I did some research on the Rev4 looks like it's only for a Merc drive. Also I did in fact GPS my cruising speed, with the 3 props I've tried I get anywhere between 33-41 mph at 3500. I'm good with anything between there.

 

[Mototrig]

Hill propellers can make whatever you want . I had a 290, supercharged 350, hill’s elephant ear, tuned, worked best for me.

Good to know. I had a custom elephant ear SS prop on there. No manufacturer name, wonder if it was a Hill prop?

 

[Crazyhippy]

Some light reading...

Bblades - Props 101 - Learn The Basics About Performance Propellers

Maximizing the performance of your boat is not just getting the right motor for your boat; your prop plays a huge role in its performance.

bblades.com

And a slip claculator

BBLADES - Prop Slip Calculator

Let play hypothetical for a minute. We are going to use a 24" pitch prop. We don't care about diameter, cup, blade count, blade style etc (for now).
Ideally for every rotation of the propshaft, the boat will move forward 24". If you know RPM, and gear ratio of the drive, you can calculate your zero slip speed.
RPM / gear ratio x pitch = inches/minute
inches/minute x 60 / 12 / 5280 = mph

Again this is your speed without slip!

5000rpm / 1.5 ratio x 24 pitch = 80,000inches/minute = 75.7mph

Now if you are only getting 70mph @ 5,000rpm, you have slip! 70/75.7 = 92.4% efficiency, or 7.6% slip.

Be aware the tach is not always perfectly accurate, and Prop pitch almost never is unless it has been measured, tweaked, etc.

Slip is fun to play with, but don't spend too much time chasing it if you aren't racing.

 

[Mototrig]

Some light reading...

Bblades - Props 101 - Learn The Basics About Performance Propellers

Maximizing the performance of your boat is not just getting the right motor for your boat; your prop plays a huge role in its performance.

bblades.com

And a slip claculator

BBLADES - Prop Slip Calculator

Let play hypothetical for a minute. We are going to use a 24" pitch prop. We don't care about diameter, cup, blade count, blade style etc (for now).
Ideally for every rotation of the propshaft, the boat will move forward 24". If you know RPM, and gear ratio of the drive, you can calculate your zero slip speed.
RPM / gear ratio x pitch = inches/minute
inches/minute x 60 / 12 / 5280 = mph

Again this is your speed without slip!

5000rpm / 1.5 ratio x 24 pitch = 80,000inches/minute = 75.7mph

Now if you are only getting 70mph @ 5,000rpm, you have slip! 70/75.7 = 92.4% efficiency, or 7.6% slip.

Be aware the tach is not always perfectly accurate, and Prop pitch almost never is unless it has been measured, tweaked, etc.

Slip is fun to play with, but don't spend too much time chasing it if you aren't racing.

I was using the BBlades calculator when I got the 8% and 26% slips. Good easy calc to use. Good read too

 

[Flying_Lavey]

So I envy you Bravo guys.....I'm running the Volvo Penta 280 deal. It's a good unit just not many props available for it.

My gear ratio is 1.61

@boatdoc55 I should have specified, I'm not trying to hot rod my family boat. Most of the time the boat is fully loaded cruising at 3500 rpm, 40ish mph. I was just geeking out on the numbers since I'd never tested props before.

Next Saturday im testing a 15x21 and 15x23


I did some research on the Rev4 looks like it's only for a Merc drive. Also I did in fact GPS my cruising speed, with the 3 props I've tried I get anywhere between 33-41 mph at 3500. I'm good with anything between there.

Im 90% sure Merc or maybe Sierra makes shaft inserts to put a Merc prop on a OMC drive and vice versa. I know they make all the different hub kits to put the different brand props on the different outboards.

 

[Mototrig]

Ths

Im 90% sure Merc or maybe Sierra makes shaft inserts to put a Merc prop on a OMC drive and vice versa. I know they make all the different hub kits to put the different brand props on the different outboards.

That would be great. Thanks for the heads up!!!

 

[Flying_Lavey]

Ths

That would be great. Thanks for the heads up!!!

Looks like Merc makes one maybe....

Flo-Torq II Hub Kits - Quicksilver Products

Quicksilver Propellers can improve improve performance of any Boat. Use the following Flo‑Torq II chart for proper hardware selection. Industry-leading Flo-Torq II hub system allows you to use market leading Quicksilver Propellers to increase the performance of almost any marine outboard or...

www.quicksilver-products.com

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[mash on it]

Looks like Merc makes one maybe....

Flo-Torq II Hub Kits - Quicksilver Products

Quicksilver Propellers can improve improve performance of any Boat. Use the following Flo‑Torq II chart for proper hardware selection. Industry-leading Flo-Torq II hub system allows you to use market leading Quicksilver Propellers to increase the performance of almost any marine outboard or...

www.quicksilver-products.com

View attachment 1041662

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The problem is, the OP has a VOLVO 280 DRIVE.

Not an OMC.

Details, people details.

Dan'l

 

[SummitKarl]

So I envy you Bravo guys.....I'm running the Volvo Penta 280 deal. It's a good unit just not many props available for it.

My gear ratio is 1.61

I just gave away one of the 14x24s, that's the one you want for a small block, I forget the name of the place right now in pheonix but I ordered a merc conversion output shaft for the 280/290 and now run a bravo 26p with a big block on a 280, it was a very simple change out and worth every penny

 

[Mototrig]

Looks like Merc makes one maybe....

Flo-Torq II Hub Kits - Quicksilver Products

Quicksilver Propellers can improve improve performance of any Boat. Use the following Flo‑Torq II chart for proper hardware selection. Industry-leading Flo-Torq II hub system allows you to use market leading Quicksilver Propellers to increase the performance of almost any marine outboard or...

www.quicksilver-products.com

View attachment 1041662

Sent from my SM-G781V using Tapatalk

This would fit O

The problem is, the OP has a VOLVO 280 DRIVE.

Not an OMC.

Details, people details.

Dan'l

I wasn't going to say anything as I wasn't sure if there was a crossover between OMC and VP. There is in fact an adapter on that Quicksilver website but it is for a 19 spline 290 drive. I have a 17 spline 280.

 

[Mototrig]

I just gave away one of the 14x24s, that's the one you want for a small block, I forget the name of the place right now in pheonix but I ordered a merc conversion output shaft for the 280/290 and now run a bravo 26p with a big block on a 280, it was a very simple change out and worth every penny

If you happen to remember the name of the place in Phoenix I'd love to hear about it. And I know exactly where my 14x24 prop is I just don't have the scuba gear to go get it

 

[SummitKarl]

If you happen to remember the name of the place in Phoenix I'd love to hear about it. And I know exactly where my 14x24 prop is I just don't have the scuba gear to go get it

I will post a thread and see what the brain trust comes up with

 

[SummitKarl]

If you happen to remember the name of the place in Phoenix I'd love to hear about it. And I know exactly where my 14x24 prop is I just don't have the scuba gear to go get it

see my thread in the lounge, RDP brain trust got it... Bergeron engineering 480-834-1531 just prepare for sticker shock, I think it was like $500..but worth every penny to be able to use a bravo prop, stock 290 motor up to 26' vee use a bravo 4 blade 14x22, big block use 4 blade 14x26

 

[Mototrig]

see my thread in the lounge, RDP brain trust got it... Bergeron engineering 480-834-1531 just prepare for sticker shock, I think it was like $500..but worth every penny to be able to use a bravo prop, stock 290 motor up to 26' vee use a bravo 4 blade 14x22, big block use 4 blade 14x26

Perfect!!!!! I'll give them a call in the morning. Thank you!!!!!