Propeller Mystery

[Kab Dreamin]

I have a 1997 Mercury 115 hp two stroke on which I have a Mercury 13X19 aluminum prop. The normal operating range according to the owner's manual is 4750-5250 rpms. At 5250 I am running about 33 mph (I have a Alumicraft Trophy 190). Wide open I peak out at about 6200-6250 rpms and 37-38 mph. I thought if I increased the pitch some, I could pick up a few mph and use up some of the 1000 rpms I'm wasting with the current prop so I bought a Michigan Wheel 12.75X21 aluminum prop. I took the boat our for a test run with the new prop and it performed BADLY. It was a pig out of the hole (I thought I might lose a little hole shot with the bigger pitch)and the top end dropped to 24-25 mph at 5250 rpms. The boat seemed to ride higher, too. It didn't seem to be cavitating but that is the only thing I can think of that could have caused that kind of loss in performance. Anybody have any ideas?

[kjgmh]

Get your tach checked, something is not adding up. If you are spinning a 21 at 5250rpm you would be in the upper 30's.

[ChuckN]

Strange.

A 19 pitch prop is about right for that motor. I think your tach is incorrect, and I agree, have it checked.

I tried a 21 pitch on a 115hp motor with a 17' boat that only turned 5,000 RPM's, but 50 mph GPS after it was on plane. It hardly planed up and the motor vibrated terribly.

I believe your Trophy 190 is a 19' boat and a 115hp is on the weaker side. You should not be turning those kind of RPMs with that boat and motor combo.

There is also the chance that the hub is spinning within the 19 pitch prop, but the 21 pitch prop brings my opinion back to the RPM gauge in the boat.

[Kab Dreamin]

I got on the internet and did a little research. The correct tach setting for my motor is 12 poles. Just to show my ignorance, I didn't even know that there were setting adjustments you could make to a tach. Anyway, the tach was set on 4 poles. I switched it to 12 and the problem went away. WOT rpms are now 5500 instead of 6200-6300 and I know run in the 37-38 mph range at 5250 rpms instead of 32-33 mph. Now I have to try and return that 21 pitch prop. Thanks for all your help, guys.

[spudhauler]

Kab Dramin,

I am likely way more ignorant than you, and I have been dealing with similar issues as you regarding a 115 merc. What are the poles and how do you measure and/or adjust them. Thanks

[Dave]

"poles" is the adjustment on the tachometer; different settings for different motors. I believe there's a rotary switch on the back of the tach.

[Kab Dreamin]

Dave is right. The switch I had on the back of my tach had four positions labeled 4 poles, 8 poles, 10 poles and 12 poles. The correct position for a 115 hp merc is 12 poles. To make the adjustment, all you need to do is use a screwdriver or knife blade to turn the switch to the proper setting.

[Valv]

Here's a picture:

[buzzsaw]

What do you think the correct setting would be on a 150 HP 4 stroke Yamaha?? It seems like my motor should be able to go over 42 mph on a 1850 Crestliner sportfish... I read somewhere more like 49 mph should be expected??? However I do have a full tank of gas at the moment.

[Valv]

This adjustment is for tachometer not speedometer.

Usually they are set at the factory or at the dealer, I don't suggest everybody to mess with tach adjustments.

[marine_man]

Presuming you're running where you should be for RPM's (for your 150 the yamaha HSOforum shows 6,000) that's about all you're going to get for speed.. it seems a little low to me, but not by too much.. I'd say maybe closer to 45... but it's difficult to be sure...

What are you turning for RPM at Wide Open Throttle?

marine_man

[rtwills]

I think you may be runnin a little low too. I have a 1750 sportfish with a 125 mmerc 2 stroke on it , I am running 5300 rmp with a 13 1/4x 21" prop and am getting about 47mph with me alone and full tank of gas.

I do know my boat sppedo is way off so I would suggest using a gps for checking your speed.( my peedo reads about 6 mph slower than the gps)

[buzzsaw]

Quote:

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What are you turning for RPM at Wide Open Throttle?

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I'll have to check on the above question when I get out this weekend...

Also, I did check the speed on my GPS... Like I saw somewhere I should be able to hit 48, 49 mph approx... wierd stuff???