11 Spline Prop on early 1970's 50hp Mercury

[MidWestSkiBum]

I have a 1973-ish Mercury 50hp 4 Cylinder on my 1973 GW Invader I just purchased. The prop that is on it is probably near original and it shows. I have no idea what the pitch is exacatly...based on calculations, I am guessing it is a 15 since the boat does about 30mph (by speedo). The tachometer has been broken so I am assuming a 5300 rpm WOT since its an old engine and the prop is not smooth.

The propeller should be replaced since there are chunks missing and it is bent up. But it is an 11 spline prop which I cannot find for sale on the internet. I tried ebay but only found low pitch props.

I would like to go to a 17 or even 19 pitch prop to get into the mid 40s since the boat seems to hook up very nicely and the prop is well submerged at full speed. But I think the chances of finding an 11 spline 17 or 19 pitch prop are slim to none.

Does anybody know where I could find the prop I'm looking for? Or better yet, is it possible to convert the drive shaft to a 13 spline so that I can use the props I have in my quiver for my Mercury thats on my Dux inflatable?

 

[MidWestSkiBum]

Re: 11 Spline Prop on early 1970's 50hp Mercury (edit: 1966 Merc 500)

Re: 11 Spline Prop on early 1970's 50hp Mercury (edit: 1966 Merc 500)

..turns out its a 1966 Merc 500 engine.

 

[pdreher]

Re: 11 Spline Prop on early 1970's 50hp Mercury

There is a prop shop here in Houston,TX that has old mercury hubs and things. Reasonable priced also. I don't work for them or anything like that just have done business with them in the past. If you want there number let me know they ship anywhere in the U.S.

 

[steelespike]

Re: 11 Spline Prop on early 1970's 50hp Mercury

Please read the stickies at the top of the page and the helps on the iboats prop store page.It will help you understand prop selection better.
Motor must operate within its rpm range5200-5600.If you had a 15 at 5,300( barely within your rpm range)and changed to a 19 your rpm would be ABOUT 4,500.
with no hole shot and overall poor performance.
You shouldn't make any changes without a tach(tinytach is reasonable and easy to install).A gps is more accurate for speed but a speedo is better than none.You can't guess about prop size/rpm and make any kind of prop selection.I think you will find the prop number under the prop nut.Probably smaller than 15.I have a 66 500 on a Crestliner 18' utility 375 lbs.If I remember right it is a 12 or 13" pitch.
The props are out there; there is a post here he is waiting for 2 11 spline props he has ordered I assume used.

 

[MidWestSkiBum]

Re: 11 Spline Prop on early 1970's 50hp Mercury

Read the stickies and helps...Im pretty familiar with prop selection also. I got lots of practice while tuning my 14' Dux Hammerhead inflatable (tunnel hull performance inflatable) and have a library of performance props that I switch often depending on use.

steelspike, what speed do you get on your 500? Also, at what RPM?

Did some more testing this weekend and found that I am not getting full throttle every time because of the throttle cable which is why rpms are low (..got a new tach.. ~4,900 RPM right now @ full throttle) plus I think that the drag on the prop from chips, dings and bends is also limiting RPM some and not turning all the energy into forward motion.

I have looked all over the prop and there is no marking for size. The only thing I know is that there is a 2nd prop it came with in worse shape that has less pitch. I will go out on a limb to believe that is the 12". Using the very unscientific method of eyeballing, I placed the prop in question between the small one and a equal sized known 17" and it appeared to be directly between the two.

Doing a little math on the below numbers:
Speed: 29mph
RPM: 4,900
Gear Ratio: 1.83 (please correct me if I'm wrong on the ratio steelespike)

I got the following results:
a 12-pitch is a .05 prop slip
a 13-pitch is a .12 prop slip
a 15-pitch is a .24 prop slp

Considering the motor has planing fins on it, the lower unit stays well submerged underway and the boat is a 10' GW Invader with near flat hull with very thin fiberglass (I dont imagine the hull weighs any more than 200 lbs ..if that)...The 12-pitch seems way too low of a slip for a junky old prop. The 13-pitch seems about right for a decent prop but the 15-pitch seems about what I would expect to see.

People I have talked to who have driven these boats in the past recall 50hp speeds nearer the 40mph+ mark with the right prop (math says that would be the 17" prop. Maybe a 19" prop if driven purely for top end.)

Tonight I will measure the pitches to get more official.
http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement

 

[steelespike]

Re: 11 Spline Prop on early 1970's 50hp Mercury

With that further boat info Yes your prop might indeed be a 17 or perhaps more. I had my boat in the water when I first got it and while the performance was adaquate I wasn't up to my other 77 18ft Crestliner utility
w a 88 50 evinrude. A comepletely unrefined setup that is as it arrived from the marina. It leaps on plane and runs fast enough that I'm not comepletely comfortable running wot for extended time.(60 years of boating) Never clocked it but surely 40+.I gave it to my son.The 500 is on a 57 Crestliner 18ft utility. Keep in mind it sat for about 7 years (very carefully stored by Original owner). And I've only run it sparingly one summer.It is signifigantly slower than the 77,probably about 35.I feel it wasn't reaching max rpm. Some day I'll get it back in the water and get a tach on it and check the speed with a gps.

 

[MidWestSkiBum]

Re: 11 Spline Prop on early 1970's 50hp Mercury

Well I pulled out the protractor, calculator and compass and the results were surprising.

The prop on the boat currently is a 17" pitch prop. The spare prop that I have is a 15" prop.

What was surprising was to see how prop construction has changed over the years. Since the 1960's era props are pretty straight forward and easy to calculate, the pitch seems true. However when I used the same method on some of my newer props, I got:
Stamped 17" pitch measuered at 26"
Stamped 19" pitch measured at 24"

So it looks like if I can figure out what is causing those last few RPMS to disappear and I can get a clean prop, top speed should go up pretty quick!

..the hunt continues for a 17p or 19p 11-spline..

Anybody have leads?

Is it possible to take the hub from the 11-spline and put it in one of my props that has 13?

 

[steelespike]

Re: 11 Spline Prop on early 1970's 50hp Mercury

You may want to ask a prop shop about changing hubs.
That would be very interesting to know.If I ever get my boat back in the water I'd like to try a Turning Point Hustler on the Merc 500.I'm too cheap to go stainless but I bet the Hustler could make it really go.