TOPIC: More Prop or Less?

Still playing and attempting to tune to get it all out of my '57 Evinrude 35 on the 14' Wolverine. I am not sure where to turn now. I do not have a tach, so that doesn't help me any. About 2 weeks ago, I had a passenger in the boat, so I could play with the motor angle. Starting with the motor in the 2nd angle pin from the transom, I can get a speed of around 26.5 mph on the GPS. It seemed as if there was more to get out of this motor. So I placed it in the 3rd setting away from the transom and speed went up to around 27.5 mph. I felt I was on to something and then tried it in the 4th and final position away from the transom. Speed increased to just over 28 mph at around 28.3. But, the boat felt as if it wanted to porpose. So I put it in the 3rd setting and even though I lost about .5 mph, the boat felt trimmed out better. Today I take the boat out with my Dad. Water conditions and wind were the same. BUT, this time, with the motor in the 3rd position, she would sound as if it were occasionally cavitating and the RPM would pick up suddenly for a very short period and distance and then go to a normal RPM for another 100 ft. and do it again. So I placed the motor back to the 2nd position from the transom and all signs of cavitating stopped. I also ended up with a top speed of around 26.5. It just seems as if the motor is really revving freely. I currently have a new Propco 10 1/2 X 12 prop on it. It has never hit anything but water, since it was put on in July. The reason I said that, is I was wondering if the hub was slipping and I don't think so at all, since the change in motor position stops the cavitating. Do I need more prop dia. and or pitch, or less? This boat is fairly flat bottomed, not a deep "V" at all. Weight of boat is around 325 lb., the motor is around 130 lb.there is the usual amount of gas/anchors/etc. on board and the passenger load would be around 350 lb.s all the times I have tested it. The cavitation plate of the motor is 1" below the 1/2" thick x 1 1/2" wide, keel strip. Any ideas?

On my boat it is almost flat from just under the front seats. Now with a 35hp motor I get around 25 and i'm up at 4500 feet. The only time I might get a little cavation is on tight turns The motor is in the 3td hole out. Try putting gas tank and battrie in the vary back. Another thing to try is have you passanger move a far up front at first and moving back as you get up to speed. If it still sounds like your cavating look over the transom and see if you getting a lot of bobble in your trail each time it sounds like its cavatating
Mike the pirate

Pin hole placement is different on every boat as the transom angles are not the same.When you are right on the edge of it "blowing out"the slightest change in weight can put it over the edge.In your case it could be the difference in weight between the two passengers.Also the porpoising is a result of the trim,either tabs / more hp/moving all weight forward or lowering your pin will get you away from that reaction. With your top speed pin placement any variance in water/wind and weight conditions are going to result in cavitation/ventilation.So you will be better off one hole down.You probably are not going to get any more speed out of her by re-propping.That's by guessing the total weight/drag and HP involved. You need to go to a bigger motor.Without a tach and the known max RPM on your engine you are whizzing into the wind by asking us to recommend a prop.Prop hub is not spun.YOU have a 54 year old motor that probably has the original pistons in it.What is your compression compared to what the manual calls for? By today's HP rating and again the condition of the motor it is is probably equal to 25-27hp. SO your speed is outstanding with that fine 57.

Don't forget . even a little weight can make a big difference. I can feel a change when my dog walks to the front of the boat. She is just over 40 lbs.

I am just attempting to get a period rig to get all the performance it has and deserves to get out of it. I have no interest in buying more horsepower. It is a matching dated boat and motor and the boat is rated for 35hp. Boat and motor had a complete redo/restore last winter and this spring. I happen to be an OMC fan so that is why there is the '57 Evinrude on it. The factory Evinrude shop manual (4th edition, last revision Feb. '58) does not give a compression or psi for the cylinders. Only information is on ring gap and piston clearance. When the motor was assembled around April, it had a psi of around 125psi on each cyl. The battery and fuel tank are in the rear of the boat in their respective holders at the transom. I was hoping with the information I was able to give it would give a clue on the setting of the motor angle or the prop size and pitch. I now see where the original prop. the motor was sold with, would have been 10 3/8" x 13 1/4" instead of the 10 1/2" x 12" I am currently running. I can see also without a tach, there is no answer on trimming it out. Sorry for the questions, but I certainly do appreciate the responses.

As far as the original prop goes the unknown pops up again.What was it set up for? 14-17'wood,glass or aluminum hull? You can't go by that because of the variances involved.You stated it sounded like it was revving up pretty good.If that is the case find a cheap tach and find out what rpms your running at before you grenade her.Try the JEGO forum here[maybe get the rec. RPMs] or Ebay for a tach,which is worth it weight in gold in the long run.Lets you know for sure where your at and warns you if you have a problem developing.Say your top RPM is 5000 and now it's running 4800,hardly noticeable without a tach and not enough for a speedo to detect.With a tach you know something is not right.I am not trying to come across as a hard ass,just trying to get to the root of your dilemma.

Dave, the "symptoms" you describe in each pin hole in your first post are pretty much the same results I get with my Starflite. The second hole is where I run it now, for the least amount of problems and best ride overall. Third hole is two MPH faster, but will experience cavitation in corners or on hard take off. Fourth hole no increase in speed and the prop "blows out" in any corner, or any hint of hard take off. One of the things that makes PT&T so nice is the ability to adjust to fit the conditions and what mood you're in. (Just cruisin' or "hammer down".)

If your cav. plate is 1" below the keel, you may pick up a little speed by raising the motor an inch or two. How much speed you pick up - I don't know. (Maybe a couple MPH) Is it worth the hassle - up to you. Just my two cents, hope it helps.

Without a tach of some sort, you're really just shooting in the dark. Those Big Twin Johnrudes are rated at 4500 RPM and don't like to rev much past 5000 before they start eating crankshafts.

That said, there's a fair bit you can do seat of the pants, just by watching and listening to how the boat performs. Most rigs end up being a compromise. You prop it and set it up to do "pretty good" across a broad range of loads and conditions. Your second pinhole is likely going to be that spot, as the third ventilates (not cavitates) too easily at that setting. Adding your Dad as the passenger made the second a bit touchy, but you may be able to counteract that by shifting some weight in the hull (tank, battery, etc.) Finding a bronze prop of the same size that you can ad some cup to may also help alleviate the ventilation. A word of warning - bronze props are much heavier and put extra strain on the shift components. Shift at absolute lowest idle speed!

- Scott

As far as the original prop goes, I am only going by the shop manual and sales brochures on what they were advertised what the motor was to be sold with. I have no idea, what the original intent, the motor or what boat it was set up for. I will need to look for a tach. Not the easiest to find as according to the Airguide information says they were only used with this application for a very short time, according to their information. I have always had a passenger in the boat with me. All were adults and probably weighed within about 20 lbs. difference from each other.

I may not be using the word cavitation in the right sense. What I am hearing is it sounds as if the motor loses bite and revs up and then catches bite again. If there is a better word for it, I need to learn that word.

The proper word for what you are experiencing is 'ventilation', which is the propeller sucking air from the surface of the water. 'Cavitation' is something else entirely - referring to pressure bubbles developing on the propeller blades, although it is often incorrectly used in this situation as the sensation of the proeller losing it's bite is similar.

For a tachometer, you might be much better off finding an inexpensive "Tiny Tach" for testing purposes... Some multi-meters have a tachometer setting, too.

Wow, that appears to be the answer. Not only do they appear to be able to be used on a magneto system, which seems to be alittle unusual to get, but the others as well. Never heard of such a thing. Like it because I can use it to tune my other boats and motors as well.

The proper word for what you are experiencing is 'ventilation', which is the propeller sucking air from the surface of the water. 'Cavitation' is something else entirely - referring to pressure bubbles developing on the propeller blades, although it is often incorrectly used in this situation as the sensation of the proeller losing it's bite is similar.

So is the "anti-cavitation" plate really an "anti-ventilation" plate then Scott? I get so confused !!! Sorry I mis-spoke guys........Think I'll just refer to it as "prop blow-out" from now on.

Hi Mark,
That is correct. Often referred to simply as an AV plate...
- Scott

I offer a bit of caution about the "tiny tach" I purchased one many years ago to use on outboard motors when setting them up to the boat. Is asked the munufacturer if the tiny tach was compatible with multi-cylinder engines, and was told that is was. Well, that was no my experience. I tried using the tach on a V4 and it would not read correctly. The Tiny tach (at least at the time) had a lead that the user wrapped around the HT sparkplug lead. The wire detected the high voltage through the rubber insulation by induction, no direct contact with the core. Because the engine was multi cylinder, the tach could not give reliable results - not even close. The lead was picking up stray EMI (electro magnetic interferance) from the magneto, distributor, and the other plug leads. Since I do electronics for a living - I instantly knew what was happening & why the tach would not work. I tried to use coper braided shielding on the pickup wire, but it didn't help enough to make the thing work. Tiny Tach would not let me return the tach, despite that I was told that it would work for my application. I was not very happy with them. If others here have had better luck - then my hats off to you.

As for the performance you are getting from the little old 35 - I think it's quite good. Your speed is right in there with what I have experienced in similar sized boats with that engine. Remember it's only 35 HP at the crankshaft, and the stock props and the lower unit were not designed for high speed performance.

Hmmmm. does anyone have experinece with using a Tiny Tach with something like my 35hp? This is a magneto ignition and there is no generator, in case there would be EMF's coming from that. The more I read into this thread, the more concern I have about over-revving the motor and wiping out the crankshaft. No, I don't run it wide open very long. I usually get her planed out on full throttle and then back it down to about 3/4-7/8 throttle which has me running around 26 mph. I am hoping to run this rig the rest of my days and as trouble free as possible.

Just a suggestion,ever think of a inexpensive multimeter that has the tach feature. It would give you the numbers and you wouldn't have to drill holes in dash or run wires.Once you dial in your prop and or rpm's your good to go.It also has multiple uses around the shop and home.Check local supply stores or ebay.Just make sure it does two strokes as many are 4 stroke 3-4 cylinder and up.Sometimes you can google mfg of multimeter and obtain correct info.Expect to pay 30-50$.

Certainly a multimeter would be great along with the other uses. Hate to be an extra effort in for me, but have you seen one like in Radio Shack? Like the Tiny Tach, I never heard of these either.

Hey, did some searching and I didn't find one at Radio Shack, but I did fine one available at Harbor Freight! This sounds almost better than the Tiny Tach!