Extreme Prop Ventilation or something else?

[bsutravis]

I've got a question about a potential prop issue I'm having with my '77 Mercury 115hp Tower of Power. I'm having an issue where when the engine RPM's hit around 2000 all the sudden the engine begins to rev and I basically lose forward momentum in the water until I back off the throttle. Over on the Mercury section of iboats I posted this problem since I initially thought I was dealing with a lower unit issue. I've marked my prop and it's definately NOT a spun hub issue. The prop that is on the engine is a 3-blade aluminum prop with the numbers 48-32746A3 21P stamped into the side of the prop. On the center hub it has 48-32746-2 stamped and when measured it has an 11" diameter. The boat is a 16' Fish and ski open bow.

I put the prop in question on after spinning the hub on the original prop that was on the motor last summer. The boat originally was my Grandpa's and he gave it to me before he passed away so I don't know the history of this prop that I currently am dealing with. Grandpa was notorious for picking up used items and this prop is definately used.

My questions are: Would it be possible that the prop is not the right size & pitch for a Merc 115 and it works ok until I reach a certain RPM and at that point the prop fins lose their bite with the water and thus my engine revs with no increase in speed? I don't honestly think I have a lower unit issue since the boat goes into gear just like always, with a nice crisp engagement into Forward gear AND I never lose Forward gear when the RPM's shoot up, I just have to back off the throttle until I am sub-2000 RPM and then the prop is able to propel the boat forward. The lower unit oil is clean and has zero metal shavings. Obviously I need a new prop if nothing more than to have a spare, but I'm hoping that the prop I have on the motor is NOT correct and it's the source of my problems and at that point I'll gladly purchase 2 new props!

Thoughts?

 

[steelespike]

Re: Extreme Prop Ventilation or something else?

That prop appears the diameter is very small.Should be around 13" What was the diameter and pitch of the prop you replaced?
You may be able to have it rehubbed.

 

[bsutravis]

Re: Extreme Prop Ventilation or something else?

After re-measuring the prop is actually 12". I don't have the original prop that spun the hub to compare sizes, etc...

 

[steelespike]

Re: Extreme Prop Ventilation or something else?

While it may not be your entire problem the diameter still is a little small 21'"3 blade props here at iboats
are 12 3/4 " or bigger. Before ordering a prop check the height of the motor.
The antivent plate (just above the prop) should be about even with the bottom of the transom.
Sometimes higher. Depends on the prop and boat.
If the height seems reasonable and nothing is disturbing the water to the prop we'll assume a prop might improve things.
The Solas props seem to have good results. Pick a prop with a good full diameter.

 

[bsutravis]

Re: Extreme Prop Ventilation or something else?

Am I only limited in size to what will fit between the prop shaft and the antivent plate? Would you go as big as possible? Would you drop down to a 19 or 17 pitch as well?

 

[supercub1974]

Re: Extreme Prop Ventilation or something else?

I have a post started with the same problem if you wanted to take a look. Can you please post your results if you come up with a fix?

 

[steelespike]

Re: Extreme Prop Ventilation or something else?

A prop listed for your motor should fit. Sometimes its necessary ti change the trim tab/anode to gain some clearance.
A 21 does seem a little big but then agin the boat seems reasonable size.Has the boat been setup like this for awhile?
What height is the anti vent plate?If we can lower the motor to get some good hard numbers it will be a lot easier to decide on a prop.You might try putting a person or some added weight in the stern to see if we can get past the venting long enough to get some numbers.

 

[steelespike]

Re: Extreme Prop Ventilation or something else?

You should really start your own thread to avoid confusion, the mods may move this.
You'll find a thread starter tab at the top of the main prop page.
I'm pretty sure your 21" prop is too much pitch but without wot rpm and speed its hard to suggest a size.
Maybe you can find a dealer that will let you try some props with your intension to buy from them.
In aluminum a nice agressive prop like a Turning Point Hustler might help and a 19" to start with.
It may be a vented prop but I think the holes will have plugs.A vented prop would just aggravate your problem.

 

[bsutravis]

Re: Extreme Prop Ventilation or something else?

Yesterday I got a larger diameter prop with a 19 pitch.... I'll post the results after I run it this evening. Keeping my fingers crossed!

 

[bsutravis]

Re: Extreme Prop Ventilation or something else?

Amazing! The 19 pitch on the bigger prop solved my problem!!! I was running on the river which only allows me a short distance to run WOT so I didn't get any top speed data, but with the new prop I ran right past 2000 RPM and the boat ran great! I knew in wasn't the Lower Unit, but just couldn't figure it out. Amazing what dropping to the 19 pitch did!!!! Thanks for all the help guys....now I can try to fine-tune and get my old Merc running like new again.