Confused by my propeller ......

[JKW Polar]

Hello,

I bought a new Polar Kraft 160 SE this spring.

It is powered by a 40HP Merc 4 Stroke (EFI). The boat came from the dealer prop'ed with a 10 1/8 D x 15P Black Max. The boat runs fairly well, I just have some issues with the RPM getting up to 6000 RPM and wanting to go higher on smooth water, the problem does not happen on rougher water. I do a lot of river fishing so the water is generally fairly calm.

I started looking into the propeller options to see if I should use a different size to keep the RPM down. That is where the confusion sets in ......

Polar Kraft has a series of "Performance Bulletins" on their site for different boats and engines.

http://www.polarkraftboats.com/images/PLK-OutlanderV160SE-F40LA.pdf

This report is for my boat with a Yamaha rather than Merc engine, but I consider the engines to be indentical in terms of power and weight for the purposes of selecting a propeller.

The specs provided about weight etc are as follows;

Weight : 1750 lbs (boat, fuel and two passengers)
Length : 16' 3"
Hull : V configuration

Engine : 40HP Merc vs 40HP Yamaha

They report shows the boat prop'ed with a 10 5/8D x 12P for the testing with good results.

I ran the numbers in the Mercury Propeller Selector and the same size is recommended. The same tool tells me that the 10 1/8 x 15P is out of expected range for the boat.

A friend of mine had a spare 10 5/8D x 12P Merc prop so I tried it out. Wow, not what I expected.... The boat top speed has dropped to about 18mph from about 28 mph and must be throttled way back or the RPM jumps up and beyone 6000.

So, what gives? I am new to this game and am very confused now.

The engine is mounted one hole up from the bottom, which is different from the report but would it make that much difference? The ventilation plate is level with the bottom of the center keel. In all cases, the propller is not ventilating as far as I can tell.

How can my performance with the same equipment be so much different than the report? Maybe I don't actually have a 40HP, they ran out so they put a 40HP sticker on a 60? Not that lucky eh!

Please any insight is appreciated as I am in the learning process here.

Ultimately I want to know the following;

Is the prop I have now is the best one for my application?

Will going up from 15P to 16 / 17P help keep the RPM away from 6000 when on the river?

Why in the world does my boat behave so much differently than the bulletin or what the mercury prop selector suggests?

Thanks,
Jeremy

 

[Tig]

Re: Confused by my propeller ......

15P does seem like a lot of pitch for a 40hp 1750# V hull boat. On the other hand 28mph does not sound fast enough for 6000RPM with a 15P prop. You should be high 30s. Something doesn't add up. Sounds like your speedo is down about 10mph or your tach is whack. You really need GPS speeds for a sanity check right now.

 

[JKW Polar]

Re: Confused by my propeller ......

I have verified the speeds with my GPS.

I have not verified the Tach, though as stupid as it may sound the tach "feels" right given the sound of the engine etc.

 

[Tig]

Re: Confused by my propeller ......

Run your RPM, pitch and speed through the BAM prop calcualtor and you'll see what I mean. Assuming 10% slip I get 4365RPM.
If your speed and RPM as measured are both correct then you have 35% slip. Is this a vented prop by any chance?

 

[JKW Polar]

Re: Confused by my propeller ......

By Vented, you mean ventilation? Is there an easy way to tell? I can lower the engine one more set of holes, would that help? This is where I have no experience in telling if the prop is venting. It makes sense from what I have read so far that it may be the explanation.

Would the mounting holes in the transom be at a set height from the factory or drilled by the marina when they install the engine? This would give me a hint about if the mounting position performance bulletin was comparable.

 

[Silvertip]

Re: Confused by my propeller ......

You are ignoring a very important number when making Yamaha/Merc rpm/speed/prop comparisons. That number is the lower unit gear ratio. If both the Merc and Yammy have 2.0 gear sets, then it becomes a matter of propeller "design" that can have a significant difference in performance on any given boat. Then the difference in boats can elevate the performance of one prop and degrade the performance of the other and vice versa. The only comparisons you can make is on your own boat, same prop manufacturer and same water/weather conditions. Besides, something is wrong with your numbers. My prop calculator shows 34% slip at 6000 rpm and 28 mph. 28 MPH sounds about right for the boat/engine combo. But something is wrong with the rpm/slip numbers. Better verify the tach and verify that you actually have a 15P prop. A two stroke 40 HP Merc runs a 1.83:1 gear set and the common prop for that combo is a 13 P so 15P sounds in the ball park for a 40 4-stroke with a 2.0 gear set. Check the engine mounting height. If too high the prop may indeed be ventilating at higher speeds which does mess with slip numbers. If that's the case, lowering the engine height would drop rpm and perhaps even raise the speed a tad.

 

[JKW Polar]

Re: Confused by my propeller ......

Thanks for the input. The gear ratio between the Yamaha and the Merc are the same, 2:1. Even the weight is identical for the two engines. I have removed the prop and checked the markings on the hub to verify that it is a 15P.

That leaves verifying the RPM and the most likely culprit at this point being ventilation. I have some work to do.....

Thanks

 

[Tig]

Re: Confused by my propeller ......

Vented refers to a prop design feature. I checked the black max series of props and I'm pretty sure they are not vented, so forget that thought.
Check the stickies at the top of the Prop forum. They will give you lots to think about and also guide you in gathering information that the forum experts will need to assist you.

 

[JKW Polar]

Re: Confused by my propeller ......

Ok, here is an update I would like to run by everyone. Following the Tach question I was in the boat taking a look at the guage to find the proper wire to measure the signal at. I don't have a strobe tach at my disposal to check the RPM at the moment so I was going to count the pulses from the alternator at the guage using my DMM frequency counting function. This is when I started looking at the switch on the tach that selects the number of pulses.

It is a Faria combination guage with 7000RPM tach, Fuel and trim. The switch is set to position 4. When looking at all the documentation on the Faria site, position 4 on the 7000 RPM tach is for a 10 pole alternator. The 2009 40hp 4 stroke Merc uses a 12 pole alternator (can anyone confirm this?). This setting would put my RPM reading off by 20% (6 pulses / 5 pulses) to the high side. i.e. Gauge displays 5800, actual RPM is 4833.

With that in mind

Actual RPM = 4833
GPS Speed = 28 MPH
Pitch = 15
Gear Ratio = 2.0

SLIP = 18

Still an academic exercise till I measure the RPM directly and contact Faria about this exact guage to get the proper setting.

If this is the problem I am not gonna be too happy with the Marina where I bought it ............

 

[Tig]

Re: Confused by my propeller ......

Good work Polar. Things are starting to make sense. Sorry, can't help you with the alternator pole question, but I think you are on the right track.
Sounds like the marina that set up your motor might owe you a prop swap.

 

[JKW Polar]

Re: Confused by my propeller ......

I just want to close out this post.

I went to the dealer this week and after going through what I though was the problem they connected the laptop to the Engine with the Mercury software. Sure enough, the engine had never been above 5000 RPM. They checked the tack and confirmed that is was set incorrectly. So, they set it and put a 12 pitch prop on the boat.

It is a completely different boat now than it was with the 15P. Jumps up on plane very quickly and the top speed has gone up by 3 MPH. It also powers through a turn rather than just boggind down halfway through .........

So happy ........

Thanks for all of your insight, if you had not pushed the RPM as being suspect I would have gone for a while before figuring this out.

JW