What prop are you running on your "Rude 70 2 stroke??

[joetheis]

So I bought a 1991 Grumman UF 184 SF- 18' fishing boat.
1991 "Rude 70 H.P 3 cyl 2 smoker hangin off the back.
Got a good deal, in good shape, runs great.
Season is over and now I get to "play" with it.
Looking at the prop............
Part # on it is "39118" it's a 13 3/4 pitch 15" dia. When I went to the "Rude dealer, this isn't OEM part #???
It get's on plane with 2 adults and cruises well at 5000, 28 mph. (got a hyrdo foil on it too).
(I dunno top RPM, it got on plane at 5 grand, why push it?? I'm gonna think top RPM is 5500, 6,000).
With 4 adults, naaaaaaa will not stand up and run, (I know, it's a 70 hp, a little small in the HP department-but it's good on fuel).
So I want to either get a 4 blade or 1?2? step smaller 3 blade to kinda sorta get on plane 4 people. Top speed not important-o, just level out loaded up.
But I'd like to know what the orginal prop was-maybe that will work???
Thanks,
Joe
Beer gives you muscles
Wine- wisdom
Water has bacteria
You decide!

 

[racerone]

They generally install a 17" pitch prop for general use.------It is assumed that the 70 will be installed on a 16' boat...........A 17" will not work in your case and likely you need a 13" pitch to get up on plane with 4 people.

 

[F_R]

So I bought a 1991 Grumman UF 184 SF- 18' fishing boat.
1991 "Rude 70 H.P 3 cyl 2 smoker hangin off the back.
Got a good deal, in good shape, runs great.
Season is over and now I get to "play" with it.
Looking at the prop............
Part # on it is "39118" it's a 13 3/4 pitch 15" dia. When I went to the "Rude dealer, this isn't OEM part #???
It get's on plane with 2 adults and cruises well at 5000, 28 mph. (got a hyrdo foil on it too).
(I dunno top RPM, it got on plane at 5 grand, why push it?? I'm gonna think top RPM is 5500, 6,000).
With 4 adults, naaaaaaa will not stand up and run, (I know, it's a 70 hp, a little small in the HP department-but it's good on fuel).
So I want to either get a 4 blade or 1?2? step smaller 3 blade to kinda sorta get on plane 4 people. Top speed not important-o, just level out loaded up.
But I'd like to know what the orginal prop was-maybe that will work???
Thanks,
Joe
Beer gives you muscles
Wine- wisdom
Water has bacteria
You decide!

Are you sure you don't have those numbers backwards? Seems to me it should be 13 3/4 dia X 15 pitch. OMC/BRP # 391198
Next size down in pitch would be 13 3/4 dia X 13 pitch. #765181 If that won't get it up, you may need a bigger motor.

 

[steelespike]

You need to have a prop that puts the motor near the top of the wot rpm range.
My info indicates the motor developes its hp at 5500 rpm.
In the spring we need you to do some lightly loaded wide open throttle runs.Get us the rpm and a gps speed.
While the foil may help you plane it may be a bandaid over a poor setup.
If the foil is in the water at speed it will slow your top end both from drag and the boats reaction to the foil.
Being under powered you may end up needing a foil or something like smart tabs.
The numbers you posted 5000 @ 28 don't add up. It produces s slip of 4% Possible but highly unlikely when running at a slightly reduced throttle.
Slip is the measure of the actual forward motion of a prop. Typical slip measures in the low double digits. I usually expect 12 or 13%.
Heavy boats usually have higher slip, as you reduce speed slip usually goes up.
Speedometers and tachs are allways suspect when numbers don't add up. The gps speed is allways accurate.You just need to be aware of
currents,wind and tides as with any testing.

 

[Watermann]

IMO you're just dropping cash on band aids rather than springing for the cure. That size boat would need at least a 90 HP to get what you want out of her. Having a spare prop is always a good idea that most people don't think about. The lowering pitch could cause the problem of over revving the motor when not laden with extra people so a good accurate tach would be essential.

 

[joetheis]

Tanks!
A spare prop is a good thing to have up the Thousand Islands! (there is a LOT of "hard water" @ where I am, (Clayton)).
The 5000 rpm was off my tach, the 28 mpg was off the bass pro fish finder, (has the wheel in back, no GPS), but it matched the speedo.
That day the water was like glass and you could really see what the boat could do.
(The Grumman is just shy of #900, motor maybe close to #300, 20 gallon tank in the bow, 2 adults#400 with gear).
I installed the foil when I drag her over to my place, (I had it from my other boat). Never tried it with out it, but it made my last boat run so much better.
The boat comes from factory with 70 2 smoke horses hanging off the back.
(I'd like to boost it up to 90, but the motor runs so well)
I just installed a Raymarine dragon to go along with the Elite 4 HDI I took off the other boat, so I'll get a GPS speedo reading in April?
My last boat was a real dawg, the P.O. had installed this HUGE pitch prop to "try to make it go faster, (I was surprised it didn't take the motor out, the load on it was sooo bad. Ended up going down 4 pitch sizes and made it a different boat)
Just wondering IF this prop is right pitch and did or did someone just toss this on to.....
Joe
If it was easy, everyone would be doing it!