71 Johnson 50 hp Initial Prop?

[jbuote]

Hey folks!

In August 2016, I bought my first boat. I've gone through it (with help from the other forum topics here) and so far so good!
​Took it out last Sunday for it's first splash. One big issue I had, was vibration. At about 1/4 to 1/2 throttle, there was a pretty good vibration, so I backed it off and puttered around. I knew the prop was dinged up pretty good, but apparently it is worse than I thought.. That's why I just puttered at that point. Didn't want to shake the gearbox apart..

​Everything else on the test went ok, so I'm willing to put more time and resources into her.

​When I started, the PO of the boat had almost EVERYTHING backwards. I've determined that PO probably gave up and slapped it all together to sell.
​Therefore, I'm not sure I should trust the prop size that's on it now..

Given that, I'm wondering what a good starting point is for a prop?

The boat is a 1967 Cruisers Inc. 1532 Camero tri-hull, 15' 1" LOA open bow.
​It has a 1971 Johnson 50 hp outboard on it.
​WOT Rpm is 5500
​I have no idea the top speed is/should be/desired...
Primary usage is for fishing... (freshwater, local lakes. NOT ocean or salt water..)

​I'm looking for overall performance. Point A to Point B, then sit and fish.
​Not interested in towing, skiing, tubing... .Don't need a fast hole-shot, or getting on plane instantly..
​Not too interested in going as fast as I can either.. Just enough to get on plane and enjoy the journey, with reasonable performance and fuel consumption.

​Just a good baseline, overall performance prop to start out with.
​
​I just don't know if I should trust the prop size that's on it.. For all I know, PO slapped any old prop on it for the sell...

I appreciate it in advance!

Joe

 

[Sea Rider]

If current prop was not tached before when new/used whichever comes first, won't know if performed excellent, good or awful. In that case try a prop selector and go for the suggested prop, that's your starter prop. Can go from there fine tuning wot revs playing with pitches. Anyway as usually loaded, selected prop must run at least towards middle wot rpm, but max wot revs is preferred,

Happy Boating

 

[jbuote]

Little update...

​Most prop selectors I find online don't go back to 71' for my engine...

​I did some research and then crunched some numbers regarding hull speed, prop slip, LU gear ratio etc....

​What I'm coming up with, is the engine performance curve seems to show that 4500 - 5500 is the WOT range..
The engine HP curve chart, shows max HP at 5000 rpm.

​Given all that, I think I've come to using either a 14 X 17 prop (which was on it, but beat up) or a 13 1/4 X 19 prop...
Both should give about the same performance as I understand things now... (still learning though.. Open to suggestions...)

​I do know that once I've chosen an initial prop, it WILL be the wrong prop! (unless I get really lucky! lol)
I'll still need to tach it, and check for what I really want..
​The whole point of this post was to get suggestions and/or confirmation of an Initial prop to start off with..

​This initial prop will most likely end up being my "Spare" after dialing in what I really want..

​Does this sound reasonable to you experienced and seasoned folks?

Thanks!

 

[Sea Rider]

That's the issue buying pre owned combos, as owner could have prop it right for his application which could be much different than yours. Is it possible to send current delivered prop to a prop shop and have it rebuild ? can tach that prop as usually loaded and check if engine runs at least middle to max wot revs. Besides will know which diam/pitch is it, from there can play with other +,- prop pitches till one is dialed spot on for your boating needs.

Need a tach to check wot revs, if without one, combo is going straight to nowhere land..

Happy Boating

 

[Maclin]

With a 50hp engine the performance between a 17 pitch and 19 pitch will not be close to the same. If you think the previous owner ran it with a 17 then rebuild that one or get another 17. I am stuck on a 13 pitch, or a 12 even. A 15 foot fiberglas trihull with just a 50hp on it would be struggling I feel with anything over that. I Had a 15.5 foot fiberglas trihull, with a 55hp, and did not like how it performed with anything above a 12. We did tube and ski, but only tubed with the 12 on it. An 11 worked out best for skiing with passengers, just sayin'. You "mileage" may vary

 

[jbuote]

Hnmm.. Ok... That's interesting information Maclin , Thank you!

​Curious though... In all my research, I've seen it stated a few places that if you go down an inch in diameter, you'll need to go up about 2" in pitch for proper engine load.. (RPM)
​So a 14 X 17 should be ROUGHLY equivalent to a 13-1/4 X 19 right? At least in regard to RPM.. Maybe not in performance of the prop as you suggest..

Sea Rider: Not sure it would be worth having fixed.. Don't know the cost to fix it, but imagine it's at least 50 bucks. New ones I'm looking at are here on iBoats for about 100... I'll check for prop shops locally (if there are any) and find out cost for sure of a repair. Thanks for that tip...

​This kind of info is exactly why I've asked the questions! Thanks!!

 

[Maclin]

No, pitch is pitch. More diameter/blades can aid in lowend push, also give more speed at same RPM by lowering slip% but does put more load on an engine. Sometimes same pitch with more swept area can lower RPM's because all the slip goes away, an indication that engine never really could spin that pitch.

Conventional wisdom usually "says" your statement the other way around.... Starting from a known "good" prop pitch, When going up in diameter or blade count need to go down 1" in pitch.

 

[jbuote]

Yeah.. I thought that's what I was saying... All else equal, if a 14 X 17 prop was the "good" prop for the engine, then if I reduced diameter by and inch, I'd have to increase pitch by 2" to maintain the RPM in the WOT range..

​Sounds like you're saying the Conventional wisdom is 1" change in diameter, needs 1" change in pitch, where I thought it was 1" change in diameter would require 1.5 - 2" change in pitch. (Smaller diameter, more pitch and conversely, larger diameter, less pitch.. to maintain rpm in the WOT range...)

​Also, I've now read that in general, it's a good idea to get the largest diameter that will fit the engine, and then fine tune with different pitches to get your desired results. It looks to me like I couldn't go much bigger than a 14" diameter anyway, so NOW I'm thinking I should use the 14" diameter, and just decide on my starting pitch.. Either fixing the prop I have, or getting a new one...

​Would that be a fair statement?

 

[David Young]

Buy a factory repair manual. Our old motors were shipped from the factory with propellers. My factory repair manual say my 1977 70 hp motor was shipped with a 17 pitch prop with the option of choosing a 15 pitch prop if you wanted that one.

​With your motor and boat you might do around 28 mph for top speed. Just a guess

 

[Sea Rider]

There are far more pitches to play with same X diam, than the opposite. It's what prop manufacturer wants to play with, unless you count with a prop factory to make your own custom props. Wouldn't worry so much about diam, pitch is all, dial a prop for OB to run at least middle to max wot rpm factory range as loaded.

Happy Boating

 

[jbuote]

Ok.. So I'm ready to place a prop order, but now I have a question about the prop info here on iBoats....
​The prop I'm looking to start with is:

http://boatpropellers.iboats.com/Joh...t_id=381337696

​It says if current prop is non cupped, go 2" less in pitch... All my figures were not accounting for cupped props.
​Is this listing trying to tell me the prop I'd be ordering IS, or Isn't cupped?
​I think it's saying the prop I'm looking at is cupped..

​How do I know if the prop I have is cupped or not? It's pretty mangled... (sigh..)

 

[Sea Rider]

Non cupped props endings are same diam as prop body, cupped has larger diam at end. Your current 17 prop is a cupped or non cupped prop ?

Happy Boating

 

[jimmbo]

How do I know if the prop I have is cupped or not? It's pretty mangled... (sigh..)

The trailing edge of the blade will have a little flip or wedge effectively increasing pitch. A curl or wedge at the tip increases the props rake.

Now days most props for engines like yours are cupped. The props in the early 70s, especially the AL types were rarely cupped.

 

[jbuote]

I don't think the prop on it now is cupped, but.. Here's a pic of it. Maybe I'm wrong? LOL
​It's pretty nicked and dinged, and you can see quite a bend in the top blade in the pic..

 

[jimmbo]

Run your fingers across the blade in the direction of the arrows, does the blade feel like it does a little flip where the blue arrows point to, ?


The area with the red circle does not count as rake cupping

 

[StingRay_90V4]

I would think that the 50 hp will not spin a 19 pitch prop well at all. I would get a 17p or maybe even a 15p. Just my 2 cents.

 

[Sea Rider]

Personally would repair current prop at a prop shop, that should be your starter prop as you already have it, or would you preffer to buy say 15-13 less pitch props blindly ? Have not posted any wot numbers, without that data current prop or new ones will be wild only wot rpm guesses. Need OB to run at around 5K if that's the max wot rpm range for that OB.

Would prefer investing 50 bucks and respairing actual prop that investing 100 on newer ones, in both cases, won't know what will their wot rpm be without a tach...

Happy Boating

 

[jbuote]

jimmbo So that tip isn't rake cupping? LOL!!!!

​So I checked as you stated, and I found that where the blue lines are, I felt no ridge/flip etc.. The red lines indicate where I felt some kind of a raise.. Very subtle, so not sure it it's because it's damaged, or because it's cupped.. This is my first boat, engine, and prop, so I don't have a point of reference on that..



​IF it is actually cupped with that subtle ridge, then I guess I need to understand how they're marked... ​ I added the hub stamp showing it's a 14 X 17.
​Does this mean it's a 17 cupped so I consider it a 19 or is it really a 15 cupped so it's stamped a 17?
​I'm sure I'm not making sense, so bear with me.. .LOL

Sea Rider : I haven't posted any wot numbers yet, because I haven't gotten to wot yet.. lol...
​Too much of a vibration just before 1/2 throttle and I didn't want to push it. Haven't had it back out on the water since.. Want a reasonable prop first, and found a cracked coil.. Waiting on coil to arrive...
(Edit: Also, pretty sure it's got a spun hub on top of the aluminum damage... Do you still think it's worth trying to repair?)

​I did get and receive an inexpensive inductive tach to use. Actually doing an ongoing review of the little thing as I go good or bad..


StingRay_90V4 : Yeah, I heard that too.. I think the highest I'll go is 17 at this point, but I'll have to test and tach to know for sure...

​So that's pretty much where I'm at on this as of now...
I appreciate ALL the input, and continued help!!!
:joyous:

 

[Sea Rider]

Just realized there's a huge missing portion on third blade, surely will account for excessive prop vibration, check with a prop shop if fully repairable and at which cost, will need to include prop hub if it's already going or already at kaputt land. What do you plan doing if it's not worth being repaired ?

Happy Boating

 

[jbuote]

Well, I was planning on buying a new prop... They're only about $100.00US..

​So far, my research on prop repair costs, would be about 50 for the aluminum work, and another 35-50 for the hub being replaced. So total of 85-100 to fix it, or about 100 for a brand new one.. haha!

​That's where this whole thread came to life.. Just trying to figure out what a good baseline prop would be, KNOWING that whatever I got would NOT be exactly right for my application, unless I got VERY lucky! LOL...