prop size?

[miichael18]

I posted while back about changing my prop i have a 1974 70hp evinrude with currently a 14x16 prop. It tachs 5400rpm book says 5000rpm wot. I was going to change to a 14 x 17 but i was looking at them and most are 13 1/4 not 14? If i go to 13 1/4 would i lose speed or gain? What difference would 13 1/4 x 17 and a 14 x 17 have? I'm thinking if i up the pitch and drop the size of prop my rpms will still be the same. the 13 1/4 prop will spin faster than the 14.

 

[miichael18]

Re: prop size?

They have..

Michigan Match Aluminum 14 x 17 $120.99

or the vortex on sale for 94.78 but its 13 1/4 x 17

Michigan Vortex Aluminum 13 1/4 x 17 $94.78

 

[walleyehed]

Re: prop size?

The 3/4" change in diameter will have very little effect.. this is all taken into account when designed, so don't get hung on diameter.
Don't go anything less than 5500 either...in fact 5600+ would be better.
It's lugging the heck out of it at 5000, and it doesn't like modern-day fuel at 5000 either. Keep the R's up and the engine will last ya much longer.

 

[miichael18]

Re: prop size?

my manual says 5000 rpm wot. Right now with 14 x 16 it tachs 5400. I cant find a 14x16 to put back on it thats why im asking which one do i need. here are my options.

13 1/4 x 17 or 14 x 17.

Im not replacing the prop to gain speed im replacing it because it is very bad shape but if i can increase the mph it would be great.

 

[walleyehed]

Re: prop size?

OK, first, there are very few 16P props available, but there are some.
#2, I have extensive experience in what RPM these engines like...I am very much aware of what your manual says...we've been selling the engines long before even that, and the fuel we had in 1970's was very good fuel compared to what we call "Fuel" today...
To keep you engine alive for a long time, prop it for 5500+. 5000 is NO LONGER the recommended RPM.
A good quality 15P SS like a Stiletto would likely pull as hard as your 16.

 

[miichael18]

Re: prop size?

I would rather stay away from stainless steel its just a fishing boat and speed really isnt that big of a factor. If i drop down to a 13 1/4 and up the pitch to 17 it would prob equal out close to a 14x16 wouldnt it?

 

[miichael18]

Re: prop size?

was looking at this.

http://iboats.com/mall/propfinder/i...93958&cart_id=119476691&engine_id=65&pf_new=1

It says Do NOT use Gearcase Adapter Ring supplied with propeller. what does this mean? does the prop already have a hub on it i just bolt it on and go?

 

[walleyehed]

Re: prop size?

SS is not only for speed, it's more efficient, as in better gas milage.
You're getting hung up on the diameter issue...STOP!!!
13-1/4" vs. 14" will be un-noticable...it's designed that way and will have NO affect in general on your RPM.
If you go with a 17 pitch, you'll be well below the RPM you SHOULD be at, and problems will follow.

The ring that comes with that prop seals exhaust from leaking around the hub....you don't need it.

 

[miichael18]

Re: prop size?

OK ill go with the 13 1/4 but here's the thing. My prop now isn't cutting water very well its all pitted and dinged up. I'm afraid if i put a 15 pitch i will be way too high. I'm thinking a 17 pitch would be fine because a new prop would cut the water better and it says a 2" increase will lower rpm 200-300 rpm. I'm not lowering the rpm but maybe 100 then with a 17 pitch and since it will cut water better it will prob still put me around 5300-5400rpm.

 

[Crownie2]

Re: prop size?

The prop you are looking at should come with the XHS hub system - as shown in the picture - if so, that is a good price...

Bob

 

[walleyehed]

Re: prop size?

1" in pitch is worth about 200RPM if you are at the top now...1" increase if you're near the bottom of the range will likely result in 300+ drop.
You will also lose hole-shot.
a new prop will have better leading edges, yes, but new also means better technology and it's likely going to pull harder, beings it's more effficient. If it won't swing the 17 at 5500+, you have the wrong prop.
You can always add cup to add efficiency and drop RPM a tad, but you can't remove cup/lower pitch of a given prop because you also REMOVE the efficiency built-in. ALWAYS best to error on the low side and add cup if needed. If that isn't clear enough, ya better do what ya want anyway...