Prop sizing (more info)

[monk-monk]

Lets start this topic over, but with more detailed info: My boat, a 1975 Glastron V 178...1050lbs hull only...1977 mercury 1500 outboard 300lbs...3 people onboard with total weight of 400lbs...cooler full of ice and drinks 40lbs...12 gal of fuel at 6.75 lbs per gal = 80lbs...anchor 10lbs...misc gear 15lbs....this all comes out to about 1800/1900 lbs....boat barely can get on plane with a 14" dia 13 pitch prop....I changed to a 13 3/4" dia 15 pitch prop and get basically the same results...with both props the motor will RPM to about 4300 RPM (1500 RPM short of max RPM specs for motor)....I have to trim the motor way up to get it RPM'ing, then I can lower it slowly and get on plane...once on plane it screams (still tops out at 4300 RPM)....my motor is in top operating condition...thanks for any suggestions!

 

[jestor68]

First off; a 115 Merc should be able to do about 44 mph at 5400 rpm turning a 19 pitch prop with a boat of your stated weight range(according to the Merc prop selector).

So it doesn't sound as if your Merc 1500 is performing up to snuff.

Got any compression numbers? Just because a motor runs smoothly doesn't mean it's making power.

Timing set correctly. If the timing is only slightly retarded, it'll really hurt performance.

 

[steelespike]

I have to agree there is either something wrong with the boat or the motor.
You could be down a cylinder and not realize it except for the lack of power.
You have 3 carbs if one is off it will affect 2 cylinders.Timing is critical both for the health of the motor and performance.
On some the timing even retards slightly at wot.
The solid state ignition can be damaged from poor handling of trouble shooting.
I think your 13" prop is about the size you would find on a very hard to push pontoon setup.

 

[Frank Acampora]

Agree--- I don't think it is a prop problem. While the 1500 in good condition should make that boat fly, it simply may not be running well. Check compression ratio and timing. Replace the fuel pump diaphragm and valves and if the engine has an accelerator pump replace that too. And while it is a long and tedious job, remove the carbs and clean them

 

[Chris1956]

Monk, I had that exact motor on my 16' speedboat. I normally ran a 21" pitch prop and could pull 2 skiers. Something is wrong with that combo. That motor has a habit of not accelerating at all, if the pitch it too high. For example I could pull 2 skiers with a 21P prop. I could not get above idle with a 23"P prop and 1 skier.

Empty out the boat, and just you take a ride and post some WOT RPM and speed numbers. You should be running a 19P prop or thereabouts, IMO.

 

[monk-monk]

Okay, I see a pattern here, with all the replies...Thank You! First thing I want to say is this: I am a very good mechanic, I have been through my entire fuel system on this motor, i have the carbs rebuilt to the NTH degree (primary, secondary float levers set exact specs with new needles and seats) I have performed a compression check (but not real recently)...my timing is set per the original merc manual with a dial indicator, finding TDC and going through the .464 timing procedure, etc....I have a factory manual..i read it intently....these motors are some of the most finicky things...BUT, now i agree, something has to be "letting down" in this motor...everyone that has replied is saying the same thing...i'll go searching, gang...i have another 1500 that i have completely rebuilt and it's sitting on a stand right now...im gonna swap motors, i have more faith in the motor i built...i will report back....Thanks for all the advice!!!

 

[Chris1956]

Monk, Check the spark plug wires for arcing to ground. Check the cylinders for water infiltration. Those two are insidious problems.

 

[monk-monk]

Okay, i did some exploring on my motor today. I am going to start a new topic in the outboard repair category. To all those who helped me here in the prop section, please follow my thread over in the repair section! Thanks!