83' 115hp Mercury, prop slip

[cyclops222]

Check out the antique race boat groups. They mostly use 2 blade props for efficiency & top speed. Hole shot is brutal sometimes if done to the extreme for speed. Power trim would be a big help. I will see if I can find the race boat associations to help you.

Web search...... old race boat associations...... still several around read & connect. BEFORE buying guesses
Ken at www.propgods.com Might be a big help also.

 

[Chris1956]

I have never seen a 2 blade prop for a TOP. There were old bronze ones for lower HP Mercs.

 

[jimmbo]

I have never seen a 2 blade prop for a TOP. There were old bronze ones for lower HP Mercs.

 

[Texasmark]

So, 3 blade hustler Alu at same 19 pitch you think Chris1956?

Some "guesstimates are that cupping adds 1" of pitch. Problem with attempting to verify that is that you would have to take a specific cupped prop, take it to a prop shop and have them flatten out the trailing edge of the blades and get back to the water under absolutely exactly (ha) the same conditions, and you to operate (ha) the boat in exactly the same manner.....fat chance on that!

 

[cyclops222]

Why do people just love to pick on a prop for more speed and power ?

 

[cyclops222]

THe engine is missfiring when at WOT load.
Check if the engine has the CORRECT part number sparkplugs and all are set to they correct spark gap. Do they all look very clean ?

 

[jimmbo]

Why do people just love to pick on a prop for more speed and power ?

I look at it as getting the Most out of the Boat and Motor. Also the faster it goes, the further it goes, on a given amount of Fuel

 

[Texasmark]

Why do people just love to pick on a prop for more speed and power ?

Because that's just what we do.

 

[QBhoy]

If you want to reduce prop slip and see better efficiency cruising…a 4 blade is not the answer at all. Their efficiency and slip reduces hugely anything upwards of slow planing speeds.
The issue you likely face with that particular set up, is inaccurate numbers from things like dash mounted analogue tachometers and pitot speedometers. Or at least one of the former. Just one inaccurate input in the data field (for calculating things like slip), means your result is also inaccurate. Think the guys previous to me pretty much have it covered already.

 

[lapl0007]

If you want to reduce prop slip and see better efficiency cruising…a 4 blade is not the answer at all. Their efficiency and slip reduces hugely anything upwards of slow planing speeds.
The issue you likely face with that particular set up, is inaccurate numbers from things like dash mounted analogue tachometers and pitot speedometers. Or at least one of the former. Just one inaccurate input in the data field (for calculating things like slip), means your result is also inaccurate. Think the guys previous to me pretty much have it covered already.

A few thoughts,
First is analog tach, and gps speed is my set up.

Second, so you're saying that a 4 blade would not be more efficient than a 3 blade? When you say:" Their efficiency and slip reduces hugely anything upwards of slow planing speeds" what are you even saying?

Third, let's pretend I don't have inaccurate data for me to figure slip. What would you do?

 

[QBhoy]

A few thoughts,
First is analog tach, and gps speed is my set up.

Second, so you're saying that a 4 blade would not be more efficient than a 3 blade? When you say:" Their efficiency and slip reduces hugely anything upwards of slow planing speeds" what are you even saying?

Third, let's pretend I don't have inaccurate data for me to figure slip. What would you do?

Hi. Yes. A 4 blade is never as efficient as a well chosen 3 blade.

 

[Texasmark]

Hi. Yes. A 4 blade is never as efficient as a well chosen 3 blade.

That's why Karl K. had 2 bladed props on all his engines for efficiency reasons.....fewer blades (over 1) the more efficient..... till he changed paint colors and started manufacturing 100 hp I6 engines for big, heavy boats. Some skeptics (I guess) claimed that his engines put out more HP than was stamped on the cowl....don't know about that. But when I was growing up Mercury outran everything going at the time.

 

[jimmbo]

Mercury's big inlines had gear housings that were more Hydrodynamically Efficient with Gear Ratios and Props that were miles ahead of what OMC was offering in the 60s on most of its V4s, hence one of the reasons Mercs could outrun the Competition

 

[lapl0007]

My goals are to increase a cruising speed fuel efficiency, decrease slip (which is roughly at 20%)

What 3 balde would be better at increasing cruising speed fuel efficiency? Or would a 4 blade be more efficient?

 

[ChooChooSnakeMan]

I had a 1985 Dynatrak Fiberglass 18' Fish and Ski with the 1985 Merc 115hp tower of power on it. It was my first boat and I kept it for 14 years. Never had a bit of trouble out of the Merc. or the boat. I ran a 19P stainless 3 blade prop and my boat would run low 50s WOT and was effecient at cruise. It would run right at 38-40 mph on gps at 4000 rpm. If your WOT speed is only 40 mph then there is a problem. As has been mentioned boat weight is key to speed. Especially weight up front will kill speed. It fights your prop trying to get the bow up in order to get the hull out of the water. Another thing I see is that often people don't trim enough or they trim too much. If the trim stays tucked down (under) for take off and is not moved much up when on plane then the boat hull stays in the water and results in a lot of prop slip. If you trim to high (rooster tail really high over engine cowling) then you are wasting thrust and speed will suffer. On most boats (glass not tin) it seems a good starting place for prop to pad height is to have the centerline of the prop shaft to be 3 1/2" below the pad. My experience is that aluminum boats need a little deeper prop. I'd suggest getting a known good 3 blade stainless prop and setting it at 3 1/2" for a glass hull or maybe 4 to 4 1/2" for aluminum hulls. Monitor your speed (on gps - don't trust the speedometer). Trim up slowly as you run in a straight line. Speed and rpms will increase until you either hit the engine rev limit (indicates you need a higher pitch prop like a 20") or you will still be below the rev limit but speed will start decreasing (indicates you have trimmed to high or not enough). It takes experimentation. If you can't get the engine to hit the max WOT rpm limit per the manufacturer and if you have a jack plate (I recommend a manual jack plate for optimum performance at least on bass boat style hulls) then only move the engine up in 1/4" increments. You will reach a point where your speed will decrease at the optimum trim setting. When you reach that height then move it back down 1/8 - 1/4" and you have found the best setting for your set up. A caution, when raising the engine or for that matter trimming it up while running, be sure and monitor the water pressure. If you get the water intakes too high the engine will just pull in air and kaboom can result. Good luck!

 

[jimmbo]

Running like this starved the Engine of Water, and the heat warped the Water Jacket Cover, resulting in the all too familiar water leaking from the Spark Plug

resulting in this being required

 

[flyingscott]

Is that the 18' ss Starcraft? If it is you're #s are way off.