Prop Help: '77 115hp Tower of Power Pics Incl.

[bsutravis]

I've inherited this boat from my late Grandpa and I am in the process of fixing many things that have been neglected. One thing I notice while running is every now and then the prop seems to "slip" in the water and the RPM's will rev and you have to back off the throttle so the prop "grabs" the water again. Usually this happens in rough water or while turning a tight turn at high speeds.

I'm looking at replacing the prop so that is my question: What do you guys recommend? I know that's a loaded question so here's some background info:

'77 Mercury 1150 - 115 HP
Boat is an '88 16' - Fabuglass Fish n Ski
Prop Numbers: 48-32746A3 21P and is Aluminum

The boat is rated for 6 persons or 800lbs, 1350lbs total including persons, gear, and motor. The majority of use is running with a 2-3 persons weighing less than 450, occasionally putting the kids on a tub or skis but 90% of the time it's just taking me and my son from one fishing hole to the other. Currently I'm not sure what the top speed is but I can take my GPS and get an accurate reading on that. I'm just worried that the 21P is the reason for the RPM surge and perhaps I should go to a different pitch.

Thoughts???

Thanks!

 

[sutor623]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

Sounds like you have a spun prop hub. It will be hard to get an idea on how that prop performs on that motor with the hub slipping. You are going to need some tachometer readings and GPS speeds to get a good answer.

 

[sutor623]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

Your gear ratio should be 2:1. RPM range for that motor should be between 5500-5800 RPMs. With the standard 10% prop slip you should be doing around 55-60 MPH. Unless you are under/overpropped. Does it feel like you are going anywhere near that fast? Also your prop may be slipping a lot worse in the Upper RPM band and may be hard to notice.

 

[bsutravis]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

Not even close to 55mph... The dash speedo doesn't work (working on fixing that) but the last time I checked her top speed was with a buddy in a new Sea Ray and he said I was running 43-45 at WOT. I'm unsure about the RPM's.

I'll do a GPS speed and RPM test and get back with the results.

 

[sutor623]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

Ok, well that is something to work with. Assuming that you have the proper lower unit (2:1 ratio) installed, and that your speed reading was on cue, we can get some good info from this. Your motor is either running at 4500RPMs (which means it is overpropped) or it is running at a higher RPM range and your prop is slipping at WOT. SO a good tach reading will get us the info we need to move further.

Good Luck!!

 

[steelespike]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

I don't think its a spun hub.A spun hub will occur at a certain power strain and as it continues to occur will get worse and worse.
I'm pretty sure you have ventilation(cavitation) issues.The prop gets fouled with air.It is normal to trim down for turns to limit venting.
The anti vent plate(just above the prop) should be about even with or slightly above the bottom of the transom.
You want as little of the motor in the water as possible without excessive venting.Prop selection can help control venting and allow the best height.Prop efficiency is rated as slip.(nothing to do with a spun hub) an indication how good a prop moves through the water.
typically a boat like yours will have 6 to 10% slip, give or take a little.
You will need a working tach and gps speed to do an accurate evaluation.
My info indicates the gear ratio is 1.7:1
Is the prop in good condition?
With your tendency to ventilate your slip numbers may be high.

 

[jimmbo]

Re: Prop Help: '77 115hp Tower of Power Pics Incl.

Your engines rpm range is 5000 - 5500, the 140hp was 5300 - 5800. a 17 or a 19 would be a good choice for your boat. A stainless would have better grip on the water allowing more trim before ventilating.