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Author Topic:   New Prop/Little Improvement
LilSniffWhaler posted 07-28-2002 11:14 PM ET (US)   Profile for LilSniffWhaler   Send Email to LilSniffWhaler  
I [installed a] new prop on my 17 with a 70 HP. Prop [is] 13.25 X 17in. pitch. Runs good. Good pick up. Cruise at 4100 @ 27 MPH.

With the old prop, wide open was 4400 @ 32 MPH. Now it's 5000 @ 32 MPH. Is there something wrong?

Bigshot posted 07-29-2002 10:17 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
[Carbs] clean? Throttle going all the way to full? What brand? OMC were 5-6k WOT and Yamaha were 5-5500. I think Merc was 5300 or 5500. Try running a few tanks with Techron in it.
LilSniffWhaler posted 07-29-2002 07:50 PM ET (US)     Profile for LilSniffWhaler  Send Email to LilSniffWhaler     
What is techron? I have manual mix 50:1 mixture. What should I cruise? Right now I cruise at 4100 @ 27 MPH. Is my crusing speed right?
David Ratusnik posted 07-30-2002 09:27 AM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
Lil- Techron is a Chevron (gas station) product. Yam Ring Free is the same stuff I am told. Leans out the gas abit. Cleans carbs. David
Bigshot posted 07-30-2002 02:44 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Yup! Black bottle, About $8 available anywhere automotive stuff is sold.
where2 posted 07-30-2002 05:43 PM ET (US)     Profile for where2  Send Email to where2     
Used to be available everywhere automotive stuff was sold, then WalMart stopped carrying it. I had to go back to picking it up at the Chevron Gas Station. I drop a bottle of it in the tank of my VW every 6 months. Little VW rocketship has 150k miles on it. Never have used any in the boat. Boat is 5 years older than the 12 year old VW.
kamml posted 07-30-2002 05:48 PM ET (US)     Profile for kamml  Send Email to kamml     
Sounds like the lower unit might be to far down in the water, the hull could be giving you a little too much drag. What is the weight? What is the lowest planning speed? I pull 39 MPH GPS, (17-inch prop) cruise at 3000 rpm @ 21 MPH with a Montauk 90-HP with 3 aboard and 12 gals left in the tank. Sure there is more HP but your top end RPM should be around 5300-5500, not 5000. Seems like something is going on. Just my $.02. Ken
jimh posted 07-31-2002 11:54 AM ET (US)     Profile for jimh  Send Email to jimh     
[Edited thread for clarity--jimh]
jimh posted 07-31-2002 11:57 AM ET (US)     Profile for jimh  Send Email to jimh     
It might be helpful in asking people to evaluate the performance of the new propeller if you mentioned the dimensions of the old propeller (if they are known).

More information about your engine ("70-HP") would also help. Specifically, please give us the lower unit gear ratio and the recommended maximum operating crankcase speed.

Lil Whaler Lover posted 07-31-2002 09:21 PM ET (US)     Profile for Lil Whaler Lover  Send Email to Lil Whaler Lover     
Based on the reported performance levels, I would also recommend a compression test. Are all 3 cylinders within 10 pounds of each other. Dave
LilSniffWhaler posted 07-31-2002 10:38 PM ET (US)     Profile for LilSniffWhaler  Send Email to LilSniffWhaler     
Heres what i have going people: Montauk, New EVERYTHING including engine. Engine = 70hp. Johnson 1984 ...rebuilt...right at this very moment 33hrs only. Old Prop = 13.25 X 17...same as new prop but the old one was finger-tightened, had terrible vibration because of gash. but the new prop runs wonderful, just that it has faster pick the engine is not so far down in the water...wide open is 5000 rpm's @ 31.6 mph. .. and i have a 13 gal. gas tank
Schroeder posted 07-31-2002 10:53 PM ET (US)     Profile for Schroeder  Send Email to Schroeder     
Just to give a comparison. I have an 84' Montauk with a 84' 70hp Yamaha top speed is 38mph at around 5500rpm. The prop is stock I believe.
whalersman posted 07-31-2002 11:06 PM ET (US)     Profile for whalersman  Send Email to whalersman     
Sounds about right to me. I have a Montauk with a 1985, 70 HP Evinrude. I also have 2 different props that I try from time to time. One is a 13.25 X 17 and the other is a 13.75 X 15. Both props run about the same top end RPM 5250... The 15 pitch gets me out of the hole faster and runs about 32 mph top end. The 17 pitch prop is a little slower out of the hole but comes on strong a little later at about 33 or 34 mph........
A big gash in your prop with alot of vibration is no comparison to what you have now. What you need to look at is the prop you are running now. OMC 70 HP engines recommend between 5250 and 5500 RPM's to deliver the rated 70 HP.
I hope this helps.
jimh posted 08-01-2002 10:38 AM ET (US)     Profile for jimh  Send Email to jimh     
This is quite odd. From your narrative it seems that the same pitch propeller (17-inch) has produced the same boat speed (32 MPH) while turning at two different shaft speeds (4400 vs 5000).

I don't think this can actually happen. In actual fact there must be something wrong with your data, or the new propeller is slipping on the shaft already. If you went 32 MPH at 4400 then you should be making about 36 MPH at 5000 with the same pitch propeller.

bdb posted 08-01-2002 02:02 PM ET (US)     Profile for bdb  Send Email to bdb     
Somethinng is not right. I run about the same as you do at cruise (70 Evinrude, normal Montauk) with the same dimension prop. However that 17" pitch should get you to about 5500 rpm (where you want to be). At that rpm I'm running a reliable 38mph. Engine is mounted in 3rd hole down from the top.
Dr T posted 08-01-2002 11:49 PM ET (US)     Profile for Dr T  Send Email to Dr T     
It looks like there may be too much prop on the boat. Also, if it is a new prop, it may be cupped (and the old one un-cupped). A 17 in cupped prop will act like a 19 in uncupped prop. Try backing off to a 15 in cupped prop (or 17 uncupped) to see if you can equal the old performance. Also, change the trip to maximize the RPM.

Why I suggest this: In playing around with my '82 Johnson 35 at high altitude, I have discovered that the torque curve falls off very steeply. For example, at 5000 ft, I can run a 10.5 x 11 at 5500 rmp, but a 10 x 13 will run at 4400 rpm with about the same top speed and lower accelleration (and much less style). The externals of your situation resemble the performance difference I observed, and thus my suggestion.

whalersman posted 08-02-2002 01:33 AM ET (US)     Profile for whalersman  Send Email to whalersman     
There are a lot of variables. My engine is mounted all the way down in the first hole. I have no intention of moving it up as I would have to drill more holes in the transom. When I make a sharp turn, my eninge does not cavitate. I believe if the eninge is raised up to the 3rd hole like you have it, you would definitely cavitate in a sharp turn.
It depends on what you want or need to use the boat for. I travel up and down shallow , narrow rivers, and must turn constantly. If my engine was raised to the 3rd hole, I would be better off staying on the dock.
This is just from my experience.
bdb posted 08-02-2002 09:46 AM ET (US)     Profile for bdb  Send Email to bdb     

Actually, I don't cavitate in sharp turns. This is how the engine was originally installed by the dealer. This is my 2nd Montauk. The 1st was purchased in about 1980, was powered by a 90 Johnson, and that installer also mounted the engine "up." On my 21 Outrage the dealer also mounted the egine up, this time 2 holes. There are a lot of threads herein about mounting height.

I didn't mention earlier, but the prop I'm running is a SS Stilleto...don't know how that compares to yours, but they are the same size (13.25 x 17). My 70hp Evinrude is a 1988. And, I don't think I have any unusual weight or weight distribution issues. Fuel tankage is 2 13 gallon units under the RPS.

Hope this helps with your cunundrum, Joe. Good Luck

Bigshot posted 08-02-2002 12:08 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Tach could be off. Engine rebuild could be sub-par. carbs could be dirty or reeds worn(refer to rebuild above). Lower unit could be binding(doubt it). timing could be off(very possible, refer to rebuild above).

The reason I bring this up is "rebuilt" is a VERY open term. It could mean 1 new piston and a gasket set. It could mean a total rebuild. Questuion is WHO rebuilt it and WHAT was replaced. 90% of rebuilds I see they NEVER rebuild the carbs, starter, lower unit, etc and 98% never replace the reeds. These are VERY important. Reeds wear out, and I can guarantee on a 1984 they are not too strong. I would look into the "well being" of the engine first and not worry about the prop.

PS the 1984 70 runs 4500-5500 WOT. I believe 1986-present are 5-6000k.

LilSniffWhaler posted 08-06-2002 09:59 PM ET (US)     Profile for LilSniffWhaler  Send Email to LilSniffWhaler     
I don't know what it runs at 5000 rpm's at the carb is clean ...everything in the engine has been rebuilt HOWEVER, the guy put a bigger engine block in it for better fuel consumption and stuff...but the engine is fine and it doesn't slip...i get out of the water a lot easier with the new prop. So what else could be the problem? tach is ok and is wired direct.
bdb posted 08-07-2002 07:28 AM ET (US)     Profile for bdb  Send Email to bdb     
Hold the phones! All kinds of stuff seems to be changing or fogging the picture. The guy put in a bigger block? Next step up from your 3cyl 70 is a v4 90. Maybe you need to go back and talk to your rebuilder to clarify things. And take BS's post above about rebuilding with you. Good luck getting to the bottom of it.

Harpoon Harry, scratchin' my head

Peter posted 08-07-2002 07:39 AM ET (US)     Profile for Peter  Send Email to Peter     
LilSniff...How are you measuring your speed?
Bigshot posted 08-07-2002 09:45 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I think someone has been taking for an expensive ride or is just remembering what they want to. I have seen more "rebuild" scams than I care to mention...sounds like one to me or a bad rebuild. Whatever it is something is not right. Just because a tach is wired direct does not mean it is correct, etc.
where2 posted 08-07-2002 12:33 PM ET (US)     Profile for where2  Send Email to where2     
My $0.02: 1984 70Hp was crank rated, not prop rated HP. Anyone consider whether the Bore or Stroke changed in 1985 to get 70Hp at the prop? If it did, then rebuild blocks would typically only be the later style.

If I can go 38 Statute Miles Per Hour, that equates to 33 Nautical Miles Per Hour. If we're measuring speed with GPS between various users, we're comparing apples to apples as long as we're all using the same unit of measure.

As Bigshot says: Check the tach, once you rule out everything else. My tach reads high, your's appears to read low. Before the dawn of time, you could check the RPM on a turntable using a test pattern and a flourescent light. I suspect at an appropriate RPM, the teeth on the flywheel of the 70Hp Johnson will appear to stand still when viewed under a flourescent light powered by 60Hz AC. Anyone still remember how this works?

Bigshot posted 08-07-2002 01:53 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
you sure you are not talking about timing the engine?

The engines were different from 86? to present. More like a 1/2 of a v6. Not sure about pubic inches though. Should not make a difference, should kick more but and have a redline of 6k if block was changed. BUT! If block was changed and old carbs and reeds, etc were used, it might make it run like crap. I would get a second opinion or at least write down what the mech says so you can relay a decent message as to what was done and is going on.

Lil Whaler Lover posted 08-08-2002 07:08 AM ET (US)     Profile for Lil Whaler Lover  Send Email to Lil Whaler Lover     
The first round of OMC 3 cylinder engines were 49.7 cubic inches, 3.000" bore and 2.340" stroke. HP rated at 5000 rpm with a maximum range of 4500 to 5500. This applied to all engines from the original 55hp to the 70 hp. The 75 based on the block was rated at 5500 with a range of 5200 to 5800 rpm. These engines were loop charged.

The newer ones were entirely different engines made from "lost foam castings" etc. Fewer parts in the heads and stuff like that. They were 56.1 cubic inches, bore of 3.187" and stroke of 2.344". the hp was reached at 5500 rpm with a maximum operating range of 5000 to 6000 rpm.

To the best of my understanding there is no interchangeability of parts between the 2 powerheads. The lower units however are the same.

The switchover in blocks roughly coincided with the change from crank shaft rated horsepower to prop shaft rated horsepower. Normally used the same size prop on the same boat between the two series of ingines but the new series (prop shaft rated) really did pull 500 to 600 rpm more giving about 10% better performance.

Hope this clarifies the OMC 3 cylinder engines. Dave

bdb posted 08-08-2002 07:27 AM ET (US)     Profile for bdb  Send Email to bdb     
BS, great Freudian Slip in that last post! My sick brain will be flippin' that one around all day. Thanks for the laugh!

Harpoon Sigmund

Bigshot posted 08-08-2002 09:54 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I did not slip:)
where2 posted 08-08-2002 12:55 PM ET (US)     Profile for where2  Send Email to where2     
BS, It is a similar thought to timing the engine, but rather than timing the light to flash every time a particular cylinder fires, you have the light flashing at a specific rate, and determine whether the RPM of the engine is reasonably close to what is shown on the tach. (Some people here don't have the 3 spare tachs in their garage that you and I have...)
Bigshot posted 08-08-2002 01:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Only have 1 but my friend has a few:)

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