4-Blade Prop, 1972 65hp Johnson?

[jay_merrill]

I like to cruise my boat at about 3/4 throttle, or less, for a good balance between speed and economy. After a bit of reading, it seems that 4-blade props are good for quicker acceleration to plane and perform better than a 3-blade at moderate power settings. I know that the 3-blade configuration is better for top end but I don't care about that because I rarely run at WOT.

I'd love to try a 4-blade prop for my motor (1972 65hp Johnson) but have never found one available for it. Is anyone aware of such a prop being made/sold for my motor?

 

[steelespike]

Re: 4-Blade Prop, 1972 65hp Johnson?

Not a prop expert but I think a good 3 blade would perform as good or better than a 4 blade.I think the 4blade would have too much resistance at cruising speed.If your looking for most efficient operation I think an efficient 3 blade and a flow meter is the best investment.its generally felt that speeds slightly above planing speed are best mpg/gph combination.But from my research that can be in a rpm band of about 1500 rpm.some setups just at planing speed and others signifigantly higher.I think with a 3 blade you would have a broader selection of props to pick from.I think you need to select a prop that puts your motor in its ideal rpm range through its entire rpm band not one designed for a specific rpm/speed relationship.
The experts here may have different thoughts and perhaps some prop selections.It would help if they had info on your present setup and max rpm and speed at wot.

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

Back when that engine was new, a 15 to 17" OMC OEM SS prop was very effective on 15-16' tri-hulls and bass boats. I remember the bro-in-law's 15' tri-hull had a 15" prop on it with a 60 John and it would snap your neck on hole shot. Popping Slalom skiers out of the hole was not a problem either.

Personally agree that a 4 blade is not the right answer for that engine.

I think if you really want some "control" the 15 would be it. Course it poops out early and won't be your big fuel economy leader. But that engine at mid throttle is very fuel efficient so it probably doesn't matter and you said you don't care about top end.

The 17 would be better for everything but control but you probably are looking for what I call control......small changes in throttle produce noticeable changes in boat performance and the boat holds this performance with changes in water and things; unlike a high pitched prop at the lower speeds where the boat bogs down, then a small throttle adjust and now it's going too fast, so another small change of throttle and now the boat bogs again type thing.

HTH

Mark

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

Thanks for the posts. I have a 15" wheel on it now (stock OMC 3-blade) and a 17" is too steep because the motor is on a 17.5 foot trihull. Even with the 15" prop, it only turns 5,000 rpm at WOT. I don't really want to go down to a 13" prop because my experience in doing this, is that it will result in more fuel burn for a given cruising speed.

One article that I read claims that 4-blade props tend to push a boat at a given crusing speed with fewer rpms, as compared to a 3-blade. They apparently also tend to hold a boat on plane better at slower crusing speeds than 3-blade props do. My usual day on the water involves a cruise through the bayous and lakes at about 18 mph because I like to go slow enough to take in the scenery and wildlife. I also do a lot of photography from the boat, so I am frequently slowing down to idle speed, with a later return to cruising speed. The final consideration, while not as frequent a need, is that I sometimes do SAR missions and a 15 mph cruise is a good speed for searching open water.

Thus far, I can't find any listings for 4-blade props to fit my motor. That may be because the performance improvement would be marginal and not worth the change, or it might just be because there isn't enough of a market to justify manufacturing them.

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

65 on a 17.5 tri hull is not much engine which translates to not much pitch. Don't remember if 5000 is the upper limit of that engine or not. Going to a SS prop will improve your efficiency with the thinner blades but they are usually cupped and unless you drop to 13P I doubt that you want to do that. Buttttt a SS 13 might not be a bad option.

I tend to agree that you don't find the 4's because of the lack of market.

Mark

 

[steelespike]

Re: 4-Blade Prop, 1972 65hp Johnson?

Thanks for the posts. I have a 15" wheel on it now (stock OMC 3-blade) and a 17" is too steep because the motor is on a 17.5 foot trihull. Even with the 15" prop, it only turns 5,000 rpm at WOT. I don't really want to go down to a 13" prop because my experience in doing this, is that it will result in more fuel burn for a given cruising speed.

One article that I read claims that 4-blade props tend to push a boat at a given crusing speed with fewer rpms, as compared to a 3-blade. They apparently also tend to hold a boat on plane better at slower crusing speeds than 3-blade props do. My usual day on the water involves a cruise through the bayous and lakes at about 18 mph because I like to go slow enough to take in the scenery and wildlife. I also do a lot of photography from the boat, so I am frequently slowing down to idle speed, with a later return to cruising speed. The final consideration, while not as frequent a need, is that I sometimes do SAR missions and a 15 mph cruise is a good speed for searching open water.

Thus far, I can't find any listings for 4-blade props to fit my motor. That may be because the performance improvement would be marginal and not worth the change, or it might just be because there isn't enough of a market to justify manufacturing them.

Your motor developes its power at 5,000 so you are right on the cusp.
For the health of your motor I would consider a 13 stainless would put you close to present rpm maybe 100 more rpm but a more efficient push.
Alternatively you could optimize your setup with attention to motor height.
It could improve ability to plane,more efficient cruising speed and higher wot speed and rpm.Increased rpm is better for your motors health.
If you can gain an inch in height you could get 2 to 3 mph with a corresponding improvement in operating efficiency.GPH(gallons per hour)will
remain about the same but mpg should improve.

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

The WOT range on my motor is 4500 to 5500. The motor had a 17" wheel on it when I bought it and would push the boat at 29 mph on the speedo. I didn't have a GPS at the time so I don't know the true speed. The problem that I was having though is that the motor was only turning at 4600 rpm with just me and a full load of fuel, fishing gear for one, etc.

I went to a 15" wheel and picked up the expected 400 rpm. The boat now does about 27 mph at WOT and it will hold speed with another person + gear. I don't mind the engine topping out at 5000 rpm because that is high enough to keep from lugging it and it is more fuel efficient. If I were to drop the pitch to 13", I would probably top out at 5400 rpm but the boat would go slower at any given rpm and burn more fuel.

I am thinking about buying a couple of more wheels - one 13" just to have in case I want to load the boat up with a bunch of people or gear. plus a 13" pitched up to 14". I would still love to try a 4-blade wheel but that is probably not in the cards so a 14", 3-blade might be a good compromise.

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

Don't go to a 13 unless it is SS. You may be pleasantly surprised at what the 13 SS can do for you. The 13 will definitely give you better control and hole shot. But on top end, the cupping and thinner blades probably will give you the performance you are currently getting with the 15.

Just for some numbers, I had a very heavy SS prop and decided I wanted to try an alum prop. I dropped from a 24P SS to a 23P Hustler Alum primarily because they didn't make more than that. Now Hustler is not just another alum prop. Other than thick blades, because of the alum, this prop has all the whistles and bells that any SS prop has.

You would have thought I would have picked up some rpm's by cutting pitch by an inch. Cupping was a wash as both props are cupped.

The alum dropped my speed 3 mph AND cut my rpm's (not raised them, cut them) a whopping 600 rpm's. Only thing I can think of is the thick blades made the difference.

Now you plug that into what we are talking about and I think the the 13 SS is your answer.

Mark

 

[Silvertip]

Re: 4-Blade Prop, 1972 65hp Johnson?

Your engine is right at the lower end of its recommended wide open throttle RPM band so you might as well say with anything more than your current load you are already lugging the engine. If it's lugging at WOT it is also lugging at any speed less than that. An engine that is running "more freely", even if it does rev a little higher, does not automatically translate to throttle opening therefore more RPM does not automatically translate to higher fuel consumption. You car going up hill reguires more throttle opening to maintain speed and RPM. Going down hill it requires less and RPM stays the same with less throttle opening. Prop your boat right for wide open throttle and it will be right for everything less than that. Forget the four blade prop -- the engine will probably die before you've paid for the new prop with any fuel saving you hope to achieve -- and it will die sooner if you prop it so it lugs more than it already is.

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

Mark, thanks for the info on the SS wheels. I hadn't really thought of them because I've never used one (most of my motors are older, pin drive models) and I guess I associated them more with top end performance. That's really "my bad" because I have read comments here and there about overall performance on the SS units.

I have no objection to a 13" wheel as long as it doesn't end up slowing the boat down a few more mph and costing me more in fuel consumption. I also don't think that I am going to find a 4-blade wheel for this motor, and it appears that a 3-blade SS prop may give me what I am looking for anyway.

As for current rpm range with my 15" wheel, it really isn't right at the lower WOT range. The acceptable range according to both my original OMC manual, and my local dealer, who has been working on these motors for 30 years, is 4500 to 5500 rpm. Since I am at 5000 rpm at WOT, I am right in the middle of the acceptable limits. My dealer's current advice is that the motor is fine with the wheel that is on it, unless I make a trip with a heavier load than I normally carry. Buying a 13" wheel to use in that situation would give me flexibility and sending another to the prop shop to be pitched up to 14" (the OMC/Michigan Match props don't come in 14"), would put me at 5200/WOT rpm.

However, it sounds, like a 13" SS might give me all the things that I am looking for in one wheel. That said, I think I am going to see if my dealer has a used 13" SS that I can try.

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

MWC used to supply bronz props for those "pin" drive engines but they weren't cupped. Cupping is an effective 1" of additional pitch noticed especially at the upper rpm's where you can get the most deflection of the water from it. That is one of the reasons for my response as I gave it. You run a 13 till you need that top end then you are running more like a 14.

Bronze was cheaper than SS also.

What you might do is to locate a Bronze and have a prop shop work it over for you, i.e. set it up for 13"P and cup it to an effective 14".

Mark

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

While the rest of my motors are pin drive, the 65hp is thru-the-hub. I'm still of a mind to try your idea on the SS wheels, especially if the cupped aspect gives me a "two for one" capability. The spare 13" wheel, along with a pitched up 14" wheel, would most likely give me the flexibility that I am looking for but it would be more complicated (having to change wheels), and not much different in cost by the time I paid to buy two props and have one pitched up.

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

Interesting point about your engine. Got tangled up in verbiage and forgot what engine you had. Don't know if I ever saw a thru hub SS or not. I ran an OMC teflon coated (bronze diffuser ring) on a '72 125 and it danced circles around the original alum prop. If your dealer doesn't have one might try prop shops. I know James took (may still take) trades on new props which would give him a supply of used ones.

Mark

 

[Texasmark]

Re: 4-Blade Prop, 1972 65hp Johnson?

Brain is neutral this am. "Don't know if I ever saw a thru hub SS or not" obviously should have been bronze rather than SS.

Sorry,

Mark

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

Well, you're doin better than I .... my brain is usually in reverse in the AM!

 

[ddaigle]

Re: 4-Blade Prop, 1972 65hp Johnson?

Just saw this post, any newer 13 spline evinrude/johnson prop should fit your motor. I have a 1970 60hp and a 1989 70 hp and I can interchange the props with no problem.

 

[Silvertip]

Re: 4-Blade Prop, 1972 65hp Johnson?

It took me aboujt 20 seconds to find a four blade prop. Right here on iBoats.

iboats.com/mall/propfinder/?manufacturer=Johnson&rotation=Standard&horsepower=65&search=Outboard&chart=15&pitch=13.00&engine_id=62&**********=861458636&*******=456021520

 

[Chaz1]

Re: 4-Blade Prop, 1972 65hp Johnson?

I have the exact same motor and I run a 13 1/4 prop with a 17 degree pitch. My manual states the a 19 degree pitch is the standard for this motor. My boat is a 16 ft. aluminum Starcraft. I am very pleased with the performance of the prop/motor combination. Your choice of prop(15 degree) is interesting to me. I suppose pushing a 17.5 ft. tri-hull is probably a bit harder on that motor than my Starcarft which is pretty light and using the 15 degree prop would make sense.

Bear with me, I'm learning this stuff as I go! I'm just getting into boating. I'll be sure to follow this thread. By the way I do have an original parts catalog so if you don't and need help in finding a part number, feel free to contact me. Chuck

 

[Silvertip]

Re: 4-Blade Prop, 1972 65hp Johnson?

Chaz -- prop pitch is measured in inches of pitch -- not degrees of pitch.

 

[jay_merrill]

Re: 4-Blade Prop, 1972 65hp Johnson?

Thanks for the offer re the parts manual. I usually look stuff up on BRP's site but if you ever scan your manual, let me know.

I had a 17" pitch prop on the motor when I first put it on my boat - its just too much pitch for it. The 15" would still be too much for most people but I operate my boat with one to two people in it most of the time, so I get away with it.

Thanks to the folks who came up with info on 4 blade props for this motor!