Prop Selection - Bayliner 175

[gsbarry]

New to me 2015 Bayliner 175 w/ Merc 3.0 Alpha 1 Gen 2. Prior owner put completely the wrong prop on there, boat barely gets out of the hole at WOT and only 2 passengers/no gear. The prior owner put an outboard prop on there that’s the wrong everything, flow through exhaust hub, 17 pitch, 13.75 diameter. I think he just got the cheapest one he could find.

Anyways, doing some reading it looks like the stock prop for this boat is 21 pitch, 14.25 diameter, 3 blade aluminum. I’ve also read that some folks go down to 19 pitch for better whole shot and towing sacrificing some top end of course. I’ve also read that a stainless prop won’t provide much benefit on the 3.0 motor due to the lower hp.

I’m leaning towards 19 pitch as well, plan to do more towing/skiing, but I’d like to hear y’all’s thought on specific prop brands/models? Ideally I’d like to keep the part cost under $400.
This mercruiser prop seems like it would fit the bill. But I’m not sure if there are better aftermarket brands/models in my price range. Also not too sure if 4 blade makes any real difference vs 3 blade in this application.

 

[Pmt133]

I ran a 19p on my 3.0 in my 19 footer and it did alright. That being said I'd take what it came with and everything else with a grain of salt... the 3.0l isn't a powerhouse by any means but it'll get the job done... now mind you this is a much heavier and larger vessel than yours. Time to plane with 2 people was... well... bad. Even with a 17p it was not really any better and that particular prop put me into the rev limiter... about 300 rpm over the reccomended max rpm range. But once it got up on plane it still cruised at 26 mph and would wide open around 33 with a load of people. This was with the 19p. Knock 2mph off for the 17p.

I would start out with as much information you can find on the wrong prop first. Including wide open throttle RPMs and speeds so at least there is some data to go off of. Otherwise buying anything would be a shot in the dark to an extent.

I'd also reccomend looking around Facebook marketplace and the likes... typically can find good used take off props for cheap on there. That's how I've gotten a few emergency spares over the years.

 

[jimmbo]

If the boat is a Dog with a 17, it will be even slower, maybe not even able to plane with a 19.
The 3 liter was geared quite low and usually came with 21 -23 inch pitch Prop.
You are propped very low, and normally that engine would be severely over revving with a 17 inch.
What is the Gear Ratio? It can be found on the Mercrusier Wrap/Decal on the Upper Gear Housing. Maybe it is a swap/replacement from a higher Horsepower Engine.
However, before spending any Money, I suggest a Compression Test be done

 

[gsbarry]

If the boat is a Dog with a 17, it will be even slower, maybe not even able to plane with a 19.
The 3 liter was geared quite low and usually came with 21 -23 inch pitch Prop.
You are propped very low, and normally that engine would be severely over revving with a 17 inch.
What is the Gear Ratio? It can be found on the Mercrusier Wrap/Decal on the Upper Gear Housing. Maybe it is a swap/replacement from a higher Horsepower Engine.
However, before spending any Money, I suggest a Compression Test be done

I performed a compression test, and it seemed good to me. Results were: 171, 182, 181, 171 psi on cylinders 1-4 respectively. It did seem to me to be over-revving at WOT, pushing 5300 if i recall correctly (I didn't run it for very long at WOT considering how high it was). I think the manual calls for 4800 at WOT. I also started a separate thread on the timing. The base and initial timing are testing right on spec at 2*ATDC w/ jumper wire and 15* BTDC at idle w/o jumper wire, however the total timing advance at 3000+ rpm was only 22* vs the 26* called for in the manual. So likely also leaving some power on the table there. I plan to replace the ignition module in the distributor to see if that improves the timing specs. The drive ratio marked on the outdrive sticker is 2.00. Does that seem correct for the 3.0L on 2000 lb. 17"6" boat? My feel for this is that there are several things wrong with the prop that's on there which is p/n 3413-138-17: diameter too small, exhaust flow through hub, and pitch is too low. The way it feels is like trying to get a car up to highway speed stuck in 1st gear (which has tons of low end torque, but your speed is going to be very limited). So... any suggestions on prop brands? What do y'all think of Solas in general? Or stick with Mercruiser? Others?

 

[alldodge]

I performed a compression test, and it seemed good to me. Results were: 171, 182, 181, 171 psi on cylinders 1-4 respectively

Your gauge is not worth an crap or you have no idea how to do a compression test. Your readings should not be above 150 and that's for a brand new motor

 

[jimmbo]

Most V6 Outboard Props are the same as I/O Props
If it is a dog and you are turning 5300, which above the RPM Range for that Engine, it is going to have even slower acceleration with greater pitch.
171, 182, 181,171 for compression Numbers? That engine has about 8.5:1 for a Compression Ratio, there is no way the numbers can be that high. Even if it were 10:1 the numbers can't be over 150

Compression Ratio to PSI Calculator

The compression ratio to PSI calculator is here to help you calculate the compression ratio and pressure in psi.

www.omnicalculator.com

I would also verify that the Tach is set correctly

 

[gsbarry]

Your gauge is not worth an crap or you have no idea how to do a compression test. Your readings should not be above 150 and that's for a brand new motor

It's certainly possible my gauge is not accurate. My procedure for performing the compression test (cold motor) was to disconnect the distributor wire, remove 1 plug at a time, thread in the hose/gauge into the cylinder, then crank the starter for ~5 seconds until compression stops rising on the gauge, record result and move to next cylinder. My understanding w/ compression tests is that it's less about the absolute value but rather that each cylinder reads within ~10% of each other.

 

[gsbarry]

Most V6 Outboard Props are the same as I/O Props
If it is a dog and you are turning 5300, which above the RPM Range for that Engine, it is going to have even slower acceleration with greater pitch.
171, 182, 181,171 for compression Numbers? That engine has about 8.5:1 for a Compression Ratio, there is no way the numbers can be that high. Even if it were 10:1 the numbers can't be over 150

Compression Ratio to PSI Calculator

The compression ratio to PSI calculator is here to help you calculate the compression ratio and pressure in psi.

www.omnicalculator.com

I would also verify that the Tach is set correctly

When I checked the timing, the tach on the instrument cluster matched the tach on my timing light, so I would think they are both accurate. As far as compression ratio, I could be wrong, but would that not be for each compression stroke? So, while cranking the engine for ~5 seconds we're getting several compression strokes, therefore a higher overall pressure reading?

 

[jimmbo]

There is a Check Valve in the Compression Gauge to allow the gauge to eventually measure the Max Pressure. If that valve wasn't there the Gauge would drop as the Piston descended. When the Gauge is connected the actual Compression Ratio drops a bit as the Gauge adds Volume to the Combustion Chamber, so the Check Valve helps there too by keeping the Gauge Volume at higher Pressure when the cylinder pressure drops. Eventually the Pressure Maxes out and is retained by the Gauge. On a 4 stroke that can take more than 10 revolutions(5 compression strokes)

The 10% rule is a used mainly cause it points out a Cylinder that is not going to carry its load as well as the rest. But if all cylinders are even, but low, that doesn't mean the Engine is fine, it is likely worn out.

 

[gsbarry]

There is a Check Valve in the Compression Gauge to allow the gauge to eventually measure the Max Pressure. If that valve wasn't there the Gauge would drop as the Piston descended. When the Gauge is connected the actual Compression Ratio drops a bit as the Gauge adds Volume to the Combustion Chamber, so the Check Valve helps there too by keeping the Gauge Volume at higher Pressure when the cylinder pressure drops. Eventually the Pressure Maxes out and is retained by the Gauge. On a 4 stroke that can take more than 10 revolutions(5 compression strokes)

The 10% rule is a used mainly cause it points out a Cylinder that is not going to carry its load as well as the rest. But if all cylinders are even, but low, that doesn't mean the Engine is fine, it is likely worn out.

Makes sense, Jimmbo. The results here are high and consistent readings (in the 170-180 range).

 

[Scott Danforth]

FWIW, the largest prop I could ever run on a 3.0 was a 19P on a 1600# 17' long boat. and that was with only 2 people in the boat. most of the time it was 17P for people hauling or 15P for water sports and trolling.

not sure there is enough motor to spin a 21p prop unless the hull is a stingray (really light hulls). your boat is a bit heavier at just under 2000 dry weight. so you are looking at about 2300# with fuel and gear, add 185# per person, so with two people you are at just under 2700#

the 17p should be right. however if you plan to ski behind the boat, you may need to get a 15p

 

[Lpgc]

A dynamic compression test will usually read higher than static compression because the compression heats up the air in the cylinder.

If you could turn the engine slowly enough to allow the heat of compression to dissipate a 10:1 engine would read max 150psi and probably lower if the inlet valve didn't close right at bdc or exhaust valve didn't close right at tdc. Even at cranking speed there isn't enough time for heat of compression to dissipate, so it's still trapping the same volume of air, still compressing it into a smaller space, plus the heat of compression increases pressure further.

 

[gsbarry]

Another thing about this boat that seemed to be perhaps less than ideal was the weight distribution. By that I mean when I was trying to get out of the hole, with no passengers up front (just 2 of us in the center seats), and really no gear (other than basic safety stuff), no cooler, but full tank of gas, fully trimmed down, the bow to me seemed excessively high, at least higher than what I’m used to on other bowriders. I’ve read about trim tabs, and maybe that’s an option but I don’t love the idea of drilling a bunch more potential leak points in the transom. So, other than putting passengers and gear up front, are there any other ways y’all can think of to lower the bow when trying to get out of the hole?

 

[Scott Danforth]

. So, other than putting passengers and gear up front, are there any other ways y’all can think of to lower the bow when trying to get out of the hole?

Drop down in prop pitch
Add more motor
Put a sand bag up front
Add trim tabs

 

[Stinnett21]

Another thing about this boat that seemed to be perhaps less than ideal was the weight distribution. By that I mean when I was trying to get out of the hole, with no passengers up front (just 2 of us in the center seats), and really no gear (other than basic safety stuff), no cooler, but full tank of gas, fully trimmed down, the bow to me seemed excessively high, at least higher than what I’m used to on other bowriders. I’ve read about trim tabs, and maybe that’s an option but I don’t love the idea of drilling a bunch more potential leak points in the transom. So, other than putting passengers and gear up front, are there any other ways y’all can think of to lower the bow when trying to get out of the hole?

Might be waterlogged. Soaked foam below deck carrying hundreds of pounds of water.

 

[gsbarry]

Doing more reading on trim tabs, I'm not as leery as I was earlier about it. That's probably the route I'll go given the results many others have had. I guess I'll find out if it's water logged when i drill the pilot holes for the trim tabs!

 

[Stinnett21]

New to me 2015 Bayliner 175 w/ Merc 3.0 Alpha 1 Gen 2. Prior owner put completely the wrong prop on there, boat barely gets out of the hole at WOT and only 2 passengers/no gear. The prior owner put an outboard prop on there that’s the wrong everything, flow through exhaust hub, 17 pitch, 13.75 diameter. I think he just got the cheapest one he could find.

Anyways, doing some reading it looks like the stock prop for this boat is 21 pitch, 14.25 diameter, 3 blade aluminum. I’ve also read that some folks go down to 19 pitch for better whole shot and towing sacrificing some top end of course. I’ve also read that a stainless prop won’t provide much benefit on the 3.0 motor due to the lower hp.

I’m leaning towards 19 pitch as well, plan to do more towing/skiing, but I’d like to hear y’all’s thought on specific prop brands/models? Ideally I’d like to keep the part cost under $400.
This mercruiser prop seems like it would fit the bill. But I’m not sure if there are better aftermarket brands/models in my price range. Also not too sure if 4 blade makes any real difference vs 3 blade in this application.

Need clarification. When you say it has a wrong prop and that it is a "flow through exhaust hub", 95% of props on the water today are flow through exhaust. Just wondering if you mean over the hub exhaust as in you currently have a 4 1/4" hub diameter prop installed on a drive that asks for a 4 3/4"?

 

[jlh3rd]

Every compression test I've ever done is with all plugs out and WOT while cranking. That's what I learned 50+ years ago when I started messing with engines. There was no internet so this knowledge came from quality, reliable
service manuals and other publications.
didn't see that mentioned. Maybe it doesn't matter, maybe it does.
Ass-u-m(e)-ing the motor is healthy as is the tach, you are 500 rpms high @WOT. Regardless of gears, weight, boat type, it's too high.
The general consensus on paper is that for every 1" increase in pitch...or diameter....or additional blade, you decrease rpm @WOT by 200rpms.
Throw in cupping, or different blade designs from different manufacturers and numbers will vary.

After reading all of this, it just seems you might have more going on than a wrong pitch.

I don't understand the advice about going to a lower pitch, you are already 500 rpms too high.

Water logged stringers is spot on. Many other threads on here from experts that mention this as the first thing to trouble shoot with an older boat.


The prop hub/gear case diameter comment is spot on also. Furthermore, there is a plastic ring that snaps on the gearcase between the gearcase and prop on my 2021 Merc. outboard. Maybe you are supposed to have that also.
Propping gets expensive when just guessing.

 

[gsbarry]

Need clarification. When you say it has a wrong prop and that it is a "flow through exhaust hub", 95% of props on the water today are flow through exhaust. Just wondering if you mean over the hub exhaust as in you currently have a 4 1/4" hub diameter prop installed on a drive that asks for a 4 3/4"?

This is the prop model that is currently on there:

Solas Amita 4 Blade 13.75 X 17P Propeller 3413-138-17​

The Solas website doesn't seem to indicate the proper gearcase diameter fitment:


I see some 3rd party websites indicating this prop fitment for 4 1/4" gearcases, others indicating 3 1/4" gearcases, all seem to be outboards, so it may be the hub diameter is 1/2" too small for the alpha 1 gearcase. I'll double check the hub diameter when I get home tonight. Based on my initial testing, the combination of the outer prop diameter being 1/2" smaller than factory spec and the 17" pitch, 17" pitch appears to be too low due to WOT rpms too high (~5300), and top speed too low. I'm also experiencing poor attitude in the hole (bow too high), which I'm thinking about adding trim tabs to address. The Bennett SLT-10" appear to be a reasonable choice for this application (17'5" 2000# boat). The electric/hydraulic versions seem overkill for this application to me given the much higher cost, but happy to hear opinions. It's possible that with the addition of the trim tabs, and the correct diameter prop, a 17" pitch would perform well. Perhaps the 19" will be ideal. I'm not adverse to having a couple different props on hand for use in different purposes. I'm also chasing down a timing advance issue where the 3000+ rpm advance is 4* lower than it should be (new ignition module on the way), so that may also be a contributing factor. Also, not to be forgotten is the relatively lower power output of the 3.0L to begin with, meaning the combination of less than ideal setup/tuning becomes apparent very quickly (i.e. everything needs to be tuned well for this motor to perform adequately).

I acknowledge that water logged stringers is certainly a possibility, however the boat is only 10 years old, has always been trailered, and by all appearances the hull is in pristine condition. My amateur knock testing of the hull passes (no soft spongy dead/heavy sounding areas), although I freely admit that many novices can't discern a water logged hull this way.

 

[gsbarry]

Every compression test I've ever done is with all plugs out and WOT while cranking. That's what I learned 50+ years ago when I started messing with engines. There was no internet so this knowledge came from quality, reliable
service manuals and other publications.
didn't see that mentioned. Maybe it doesn't matter, maybe it does.
Ass-u-m(e)-ing the motor is healthy as is the tach, you are 500 rpms high @WOT. Regardless of gears, weight, boat type, it's too high.
The general consensus on paper is that for every 1" increase in pitch...or diameter....or additional blade, you decrease rpm @WOT by 200rpms.
Throw in cupping, or different blade designs from different manufacturers and numbers will vary.

After reading all of this, it just seems you might have more going on than a wrong pitch.

I don't understand the advice about going to a lower pitch, you are already 500 rpms too high.

Water logged stringers is spot on. Many other threads on here from experts that mention this as the first thing to trouble shoot with an older boat.


The prop hub/gear case diameter comment is spot on also. Furthermore, there is a plastic ring that snaps on the gearcase between the gearcase and prop on my 2021 Merc. outboard. Maybe you are supposed to have that also.
Propping gets expensive when just guessing.

Your point on the compression test is interesting. When I performed my test the throttle plate was in neutral/idle position and the other non-testing cylinder plugs were still installed. You have me curious if this makes a material difference, so I'll re-run my compression test with those 2 setup changes (throttle plate open and all plugs out at same time).