Mercury 115 Prop Pitch?

[alabamaboy]

I have a 1988 17.5' Astroglass bass boat, with a 115 mercury. I changed for a 19 pitch prop to a 21 pitch prop. I lost 10mph on the top end and killed my hole shot.

What is the best prop for my boat?

 

[HeavyObject]

Re: Mercury 115 Prop Pitch?

What were the WOT RPM's and (GPS)MPH with the 19? What about the 21? What's your goal/ need with the prop change? The 19 might have been the best prop.....

 

[alabamaboy]

Re: Mercury 115 Prop Pitch?

WOT with the 19 was 4300 to 4500, and a gps speed of 38-39 mph, I was trying to get more top end speed.

WOT with the 21 was around 3800, and 5 gps top end.

 

[haskindm]

Re: Mercury 115 Prop Pitch?

What is the rated WOT RPM for your motor? 4300 to 4500 sound like it may be a little low, so you may have already had too much pitch with the 19" prop. You need to prop your boat so that it reaches the rated WOT RPM with your normal load. Top speed will be what it is, you cannot increase the top speed by "over-propping".

 

[HeavyObject]

Re: Mercury 115 Prop Pitch?

How is the engine running? Looks like the RPM's were to low with the 19 so it won't pull a 21...

 

[alabamaboy]

Re: Mercury 115 Prop Pitch?

The engine runs fine, it just had a fresh tune up and carb rebuilt from the mechinac.

what effects would having a hydrofoil on the back have? It has a stringray xp, speed something. the kind that mounts with out drilling.

 

[steelespike]

Re: Mercury 115 Prop Pitch?

You went the wrong way with your prop.You were allready below the minimum
of 4,750 and should be running near the max of 5,250.
A 17 would put you at about 4,900.

 

[HeavyObject]

Re: Mercury 115 Prop Pitch?

The engine runs fine, it just had a fresh tune up and carb rebuilt from the mechinac.

What effects would having a hydrofoil on the back have? It has a stringray xp, speed something. the kind that mounts with out drilling.

I asked about the engine as you seemed to be having some issues a little while back. Had the compression checked? FYI, lugging the engine like you have been is bad for the engine.

How high is the engine mounted? The cav plate level or above the bottom of the hull? You could be dragging the stingray through the water at speed, costing you some mph/rpms with the 19p prop....

How heavy is the boat? Any waterlogged foam?

 

[alabamaboy]

Re: Mercury 115 Prop Pitch?

I am not sure how high the engine is mounted. Do I want the cav. plate to be pretty close to the equal with the bottom of the boat?

The boat is dry and all the foam is dry.

Yes I have checked the compression on all the cylinders they were good. I have had the motor in the shop since I was last having problems, and everything was fixed and checked out.

Thanks.

 

[15footdeathtrap]

Re: Mercury 115 Prop Pitch?

I also have a 115merc(tower of power) the prop that came with my motor worked very good, dont know what the pitch was?? but i sat around 5500rpm, i touched bottom with it and had to replace it, i was told to run a 21pitch so i did. The 21 droped my rpms down to 4500 so i replaced it once again with a 19, now im maxed out at 6000rpm. dont no what im gunna run next but that 19pitch has gotta go! For some reason sounds like you have a problem geting your revs up, btw im running it on a 15fibreglass speedboat

 

[kansassniper]

Re: Mercury 115 Prop Pitch?

I have the same boat motor setup, except mine is an 87. i dotn know what my prop size is or anything. What i do know is that my boat maxes out at 4100rpm. how do i tell what size and pitch my prop is? im looking for information and recomendations. Also what is max rpms?

 

[sschefer]

Re: Mercury 115 Prop Pitch?

A 17p prop is about as big as you want to go with that combo. Your WOT RPM should be around 5800. 5200 or below is lugging it and will eventually lead to engine failure. The top end will be whatever it is at 5800 it's more important to be able to get it into that range than it is to try and squeak more speed out of it. I ran a 4 blade 16 on a 115 behind a 17-1/2 foot aluminum and it was a perfect match.

Top speed on mine was 35 fully loaded with fuel and passengers. 30mph in high altitude (5900ft).

 

[kansassniper]

Re: Mercury 115 Prop Pitch?

what is the diameter of the prop that i need? also how do i know what prop i need? will any mercury prop rated for 115 or higher work?

 

[~Nickolas~]

Re: Mercury 115 Prop Pitch?

There are a lot of factors that affect your selection for a prop. I have experimented with a lot of props with 3 versus 4 blade, aluminum versus stainless, different brands, etc... Every boat is different depending on factors. I have a 17.5' aluminum bass boat with an Evinrude 60 and the best prop for mine was a Turning point aluminum 3 blade 19 pitch which got me to 5600-5800 rpm and 54mph on a digital unit with paddle wheel. The stainless 3 blade 17 pitch got me to 47 mph around 6000 rpm. I also used a 4 blade 15 pitch which gave me excellent control and the best ride and smoothest running prop of all but not the best top end. I really liked how the boat handled with that prop and the engine ran exceptionally smooth compared to the other props. So experimentation will get you were you want to be. If I'm just out putting around the 4 blade prop is what I like best. If I want speed than the Turning point prop is the ay to go.

I also have a 16' Tri Hull with a Mercury 115 on the transom and it came with a Solas 19 pitch 3 blade prop on it. It turns about 5800 rpm and on the stock in dash speedo runs around 47 mph which isn't too bad. Seems like it is going much faster and I know those pitot speedometers are not very accurate. I'm going to get a digital unit and put on it and see what it tells me. I also am going to try a 20 or 21 pitch prop and see what it does. If I can squeeze out a few more mph I'll be happy. I had a 7 mph gain with the Turning point prop on the Bass boat so I'll see what happens with this Tri Hull. Main thing is to stay up with the rpm's where the motor was designed to run. Props are like changing gears in your differential on a car. The lower the gear the more hole shot and the higher the more speed to a certain extent. You can experiment like I did but just stay in your rpm range and all will be good.

Nick