Starcraft needs the right prop........

[country cadillac 48]

Morning... I've got a 1984 18' starcraft center console with a 1979 70 hp serial #5352752 and a 4.5 merc kicker. I carry 2 deep cycle batteries, 1 regular car type battery, 2-six gallon fuel tanks and an electric trolling motor. I have 2 3bladed alu. props,first one has the numbers 48-73140-16 with a R in a square box opposite the number and I assume the R is the manufacture. With this number I'm not sure what the first 7 digits mean, the last two are the ptich, correct ? the second ones number is 031019 and that's a Michigan prop, 10" dia. & 19 pitch. The 48-73140-16 runs wot @ approx. 6200-6300 rpm with a gps speed of 31.2 mph. I know this is two many rpms. My motor should run between 5000 -5500 rpm. The Michigan prop runs 5900-6000 @ wot with a gps speed of 28.2 mph. I don't know what it came with from the factory. Should I go to a higher pitch prop, maybe 4 blade ?? thanks for your time and help.....................Ray

 

[steelespike]

Re: Starcraft needs the right prop........

I'm assuming a 79 Merc. 70 hp.?My very limited info indicates a 1.64 gear ratio.Do you know the gear ratio?

 

[country cadillac 48]

Re: Starcraft needs the right prop........

yes It is a merc, but I don't know the gear ratio....

 

[steelespike]

Re: Starcraft needs the right prop........

I'm pretty sure your gear ratio is about 1.64.Also pretty sure your tach is whacky!
I imagine the boat seems pretty sluggish with either one of those props.
I think your actual rpm is between 2800 and 3200.
There may be a knob on the back of the tach. The contacts may be corroded or it is set wrong.
Note the position and operate it a few times to clean the contacts. Check other setting I don't know what number it would be as it usually refers to stator poles not cylinders.

 

[country cadillac 48]

Re: Starcraft needs the right prop........

Well... the tach maybe screwy.....and it doesn't take off really fast, but I don't pull any skiers or tubes, just head out fishing. At idle it registers about 850 rpms. I'll pull the tach and try turning the switch thru all the positions and try another positions and see what happens. I'll get back as soon as I give it a try.

 

[country cadillac 48]

Re: Starcraft needs the right prop........

The tach has a 4 position switch, which I rotated back and forth. I started the engine and ran it about 1000 rpms and turned the switch from #4 position which it was on, thru the other positions and as I went down thru the other positions the rpm reading on the tach kept going higher and higher with out changing the throttle position.

 

[country cadillac 48]

Re: Starcraft needs the right prop........

And just a thought I had about the props....they both are used /reconditioned...so today I measured the dia. of each and the R brand was less than the 10" diameter by about 1/4" and the Michigan was right on 10". I still don't know what all the numbers mean on that prop, but not being 10" would cause the rpms to be a little higher than it should. Not sure any of this helps getting me closer to the prop I need but I did read here that for each increase or decrease in pitch by an inch will increase or decrease rpm by 200, is that correct ?

 

[steelespike]

Re: Starcraft needs the right prop........

Hopefully we can get the wot rpm and speed. You could get a induction tach like a Tinytach.It will work on almost any motor land or sea just be sure you tell them its for an outboard. My feeling is you will need an 11 or 12" pitch.
The 19" is way out of line. the biggest listed here is a 17.
The 19 would be used on a little light speed boat.

 

[country cadillac 48]

Re: Starcraft needs the right prop........

Well I guess I need to see where I can check out these induction tachs and check mine against it. Sure sounds like mine my be way off. Where do I check what they used when it was new for a prop?? Just want to say thanks for the help................

 

[steelespike]

Re: Starcraft needs the right prop........

Well I guess I need to see where I can check out these induction tachs and check mine against it. Sure sounds like mine my be way off. Where do I check what they used when it was new for a prop?? Just want to say thanks for the help................

The final prop selection would deped on the boat and typical load and activities.
My 50 merc with a similar gear ratio uses an 11" on a boat with an all up weight of about 1000 lbs.
Tinytach has a website and there are a number of similar tachs on ebay.

 

[country cadillac 48]

Re: Starcraft needs the right prop........

Thanks again for the info....I'll check out their site and when I have an accurate rpm I'll better be able to get the right prop......