Gonna buy a SS prop!

[Kustrud]

I'm going to bite the bullet and try out a SS prop.

Recently switched form an aluminum 10x15 to a 10x13, didn't lose any top end really and boat is much quicker off the line. I don't have a tach but have a good ear and it sounds like its revving up all the way, and don't think I'd want to rev it any higher as well. When switching to a SS prop, should I get the same size? Go down 1 inch? Go down 1 inch but add cup, same size with cup, Etc.?? I figure I will use the SS prop for big lakes/fishing and the AL for running timber/rivers to duck hunt.

 

[KMixson]

The 10X15 has more bite than the 10X13. That is why your RPM's are higher with the 10X13. I would choose the 10X15 to get the RPM's back down a little. You said there was no difference in the top end speed so I would rather have it a little slower off the line and less RPM's on the run. As for changing from aluminum to stainless steel I don't know if that will affect your RPM's any. Someone on here may be able to give you some insight on that.

 

[Kustrud]

I'll rephrase it some, with the 10x15 the motor did not rev as high and when you put two people in the boat my speed dropped down to like 21-22, which I think is too much and was lugging the motor. That's why I think the 10x13 is perfect, should pull a load a lot better and sounds like it revs to where it should be able to......I just don't know if I should stick to 10x13 in SS or go down to 10x12 or 10x11, everything I have read says the RPM's might drop some but not enough to justify going down in pitch to a 12 or 11. Just trying to clarify if people think a 10x13 SS will be my best bet to try first?

 

[Skiffing]

You should really consider putting a tach on it.

You can get an inductive tach on ebay for $15 free shipping.

 

[Kustrud]

How accurate are those Chinese inductive tacks?

 

[Boat2fast]

I have used three different tachs for checking RPM on the water at speed.

The MacTools Smart tach is a good idea. The antenna reads the sparks right out of the air. Just lay the antenna next to the hood and read rpm. No tools, no hookup, no nothing. The device just needs to be set for cylinders and cycles. Nice idea, poorly built, looses sensitivity, and eventually stops working. Not designed for bouncing around the floor of an alum boat. Antenna is fragile. Unit is expensive to refurb once out of warranty, and takes a long time.

The MacTools timing light with a tach function is another good idea but, the high voltages and solid copper ignition wires on outboards zap the poor thing into total confusion. After I did this a couple of times I read the directions. Operator is supposed to substitute a resistor core plug wire when using this tool on outboards. So much for that.

The TinyTach is the unit that seems to have the best track record so far. I find it a little inconvenient to have to find a ground bolt as this requires a tool to be present for hook-up. The other wire wraps twice around the plug wire. Make sure to keep the wire far as possible from the other cylinders coils/wires. You'll read double/triple rpm. Wrap no more than twice or the induced voltage will knock out the instrument. You will have to call their tech department to find out how to re-activate the unit.

Old Evinrude dealers had a neat tach that worked off vibrations. It had little fingers that would vibrate at different frequencies. The fingers were marked for the RPM at which they would vibrate. The mechanic would just hold the unit firmly against the engine and read the speed from which ever finger was vibrating. Old timer showed me this one day many years ago and it stuck with me.

All things considered, I recommend the Tiny Tach for checking rpms on tiller boats at speed. Once hooked up correctly they require the least attention to get a reading...and they are the least expensive way to go. Buy a genuine TinyTach as I would imagine there are knock-offs. Buy the cheap version, the options turned out to be useless to me. Buy the one with the long wire in case you want to use it from a console. Specify the marine unit.

Hope this helps.

 

[Pappy]

Those are nick-named "Vibra-Tach". They are a reed style tachometer and are deadly accurate. I still have mine from years ago when they were issued to us. They are still available but are wicked expensive. If it weren't so difficult to post a picture in here without having to jump through resizing hoops I would post one.

To answer the various questions about pitch and cup - there is no definitive answer. Kind of like your post on Jack plate vs trim system. Don't get me wrong...I think it is great that you are seeking out knowledge on this subject and have a desire to learn.
Your boat/engine combination, center of gravity, and performance expectations will dictate what needs to be done as far as a prop goes. I think this is about the third different way you have asked this question now. Might be time to actually send off a prop and have it cupped as has been discussed before in your posts.
At this point you are still flying blind without knowing exact RPMs and two-way average speeds with the same exact test load each time. Without this basic knowledge it would be very difficult to provide a definitive answer.


You can see that my old Vibra Tach has been used! Still works wonderfully and it is right on the money as well.
On the left is a Tiny Tach. I keep both of these in a small Pelican Case for when they are needed.

 

[Kustrud]

That makes sense Pap, and I do appreciate the shared knowledge from you guys. I think I'll order a tac and check my RPMS before ordering a prop. Actually dead-centering my motor greatly helped with the venting issue. My plan is jack plate/TT (still don't know which one haha) and a SS prop for when I'm not duck hunting in timber or a shallow river. Pappy where did you say my RPMS should be with just me and a light load? I'm fairly certain it's there but I'll check with a tac to make sure. 5500 or so what you said previously with a light load?

 

[Boat2fast]

Look on your ignition module. If it says SL6100 next to the part number, that means it has a speed limiter built in for 6100 rpm. At that rpm the engine will start breaking up as the limiter interrupts the ignition to control RPM. Could be SL5900 for 5900RPM or whatever.

This is the only time I trust my "ear" when it comes to RPM. When I hear the engine bouncing off the rev limiter, I turn right around and go get a bigger prop...then see what happens. Comparing one prop with another, use a good tach and a good gps. You can see exactly what one prop does compared to another. If you don't hear the rev limiter cutting in you're in the safe-zone even if you're not in the optimum-zone. If you do NOT have a rev-limiter in your powerpack, then a tach is a must.

Props can vary widely. Don't ever assume one prop is the same as another identical prop just because it has the same part number. There's usually a difference. I checked three identical 17P props one day. They ran 5400,5500, and one ran 4600rpm. All three were new props with same part number. Obviously there was something wrong with one of them.

Often a stainless prop will turn MORE rpm than an identical aluminum prop just because the blades are thinner and more efficient, even if the stainless prop is cupped. You should also see a gain on the GPS. I expect to see at least one MPH gain going from aluminum to stainless.

If the boat runs good with the 13aluminum...takes off good and doesn't hit the rev limiter then your probably OK to buy the same size stainless. Try to find an OMC stainless 13p new in a box...not a later BRP because its an imported unit. Look for an older one.

Also, never over-prop the engine. It makes for a sluggish holeshot. It overloads the engine at full throttle. It could make it run hot.

 

[Boat2fast]

Your engine max rpm is 5500. Operating range is 4500-5500. With a light load, try to get on the upper end of the range. If you run a little high, that's OK. The 30hp engines run 5300-5800, same internals.

 

[Kustrud]

Got it!