Evinrude 33 hp prop size?

[sgtmac]

How do you determine the nominal prop size for a particular engine? I don't have a tach on my 33 hp Evinrude so I am not sure what the top end revs are. I have 10.5 inch, 10.75 in, 11 in, and 11.25 old props that I use but cannot see much of a difference after stopping and changing them out. I also don't have a manual(that would probably help).
Thanks,

 

[jay_merrill]

Re: Evinrude 33 hp prop size?

The selection of props for you motor is limited these days. They were available at up to 14" pitch at one time. Try the various online stores that sell "Michigan Match props.

Prop selection is best accomplished through the use of a tach and matching prop pitch to the manufacturer's recommened WOT rpm range. If you want to go that route, there is a company called Tiny Tach, that makes a 2-stroke compatible, inductive rpm gauges.

http://www.tinytach.com/



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[F_R]

Re: Evinrude 33 hp prop size?

How do you determine the nominal prop size for a particular engine? I don't have a tach on my 33 hp Evinrude so I am not sure what the top end revs are. I have 10.5 inch, 10.75 in, 11 in, and 11.25 old props that I use but cannot see much of a difference after stopping and changing them out. I also don't have a manual(that would probably help).
Thanks,

The greatest difference between the props that you listed is less than an inch in pitch, so you won't see much difference---as you said. I doubt you are getting any more than 500 RPM difference out of them, from the smallest to the largest. That's too close to tell by ear without a tach.

The 33 was shipped with a 10 1/2 x 12 three blade prop, which was suited for the great majority of boats that it was likely to be used on. Beyond that it was try-it-and-see.

The OMC options were:
11 x 9
10 3/8 x 11 1/2
10 1/2 x 12
10 1/2 x 13
10 3/8 x 14
All were 3-blade

You came up with some strange sizes. Must not be OMC.

 

[psteurer]

Re: Evinrude 33 hp prop size?

If you really want to get this right, you would need a tachometer. It is a good investment to ensure that your engine is running in the correct rpm range. You could probably pick one up on eBay for less than $50 including the tach cable. A new Teleflex tach and connecting cable would cost around $100. The Tiny Tach is good idea too but I like to look at my tach when I am boating so I can cruise at a fuel savings rpm.

You have quite an array of props. The numbers you see on the prop are the diameter (first number) and pitch (second number). The pitch is the important number. F_R said the prop your motor came with had a 12 inch pitch. That would be for a "normal" boat. If you are running a heavier boat than normal (like a pontoon boat), then you need to lower the pitch. If you are running a fast speed boat, you need to increase the pitch. The rule of thumb is for every 1 inch in pitch change, you change the rpms by 200-300. Lowering the pitch, the rpms go up. Increasing the pitch, the rpms go down. A tach will allow you to fine tune this. Without a tach, I suppose you could also look at how easier your boat planes. If it takes a long time to plane, then you could try lowering your pitch.

 

[jay_merrill]

Re: Evinrude 33 hp prop size?

He's going to have to use an inductive or optical tach on that motor - it has no rectifier. While I have seen a few optical tachs that "read" the teeth on a flywheel, they are expensive, so the Tiny Tach seems to be the most practical route to me.

That said, most people don't bother with a tach on these engines. As F R said, you just try different props and "guestimate" rpm by listening to the motor and finding a good balance between acceleration and top speed.

http://boatpropellers.iboats.com/Ev...r=1968&**********=463944988&*******=769007551



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[sgtmac]

Re: Evinrude 33 hp prop size?

I am using this motor on a 16 foot Loweline that I use for duck hunting; it is usually loaded down with 3 people and a bunch of gear. So, using a prop with less pitch than 12 will result in more power to move a big load and using a prop with pitch greater than 12 will result in higher speeds if the boat is empty of all the hunting stuff, correct?
How does the diameter of the prop fit into this? And would four blades be better than three(not sure if one is available)?
Thank you,

 

[jay_merrill]

Re: Evinrude 33 hp prop size?

Since you are loading the boat heavily, you'll want something on the order of a mid-range pitch to a lower-range pitch. The problem is that you won't know exactly how low to go on pitch, without doing some experimenting. Without a tach, you'll just be guessing, based on how high the engine seems to be revving. On the other hand, if you had a tach, you could tell exactly how high it is revving and would know if it is operating within the proper WOT range (I don't remember and can't find my 33hp service manual - I think its about 4500 - 5500) recommended by OMC.

The eventual selection will end up being a compromise, as it always is with a boat prop. In effect, you are driving a vehicle with one gear. Because of this, you have to choose a pitch (select a gear) that will meet a balance of needs - accelleration and top end. In doing so, you will optimize neither.

On the thought that a higher pitch will increase top end, that is a yes an no situation. If you run the boat with a light load, you might get more speed with a higher pitch. If you operate as you describe, you most likely won't. This is because the motor will be overloaded and will not achieve a desired rpm. What you will do in this case, is to "lug" your engine. That, in turn, will cause your motor to run at high manifold pressures, which will waste a lot of fuel and possibly damage the engine.

If my memory serves me correctly, there were 4 blade props for these motors, but I doubt you'll find a new one. If you decide to go that route, try fleabay, or a similar site.


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