2 part Propeller Question

[Virginiaboater]

I recently got a 1978 55HP Johnson. The prop on this motor was 11 3/4 x 17, which seems to be the recommended size for this motor.<br /><br />My problem is that even though the motor seems to run great, my max RPMs at WOT won't go over 4100-4300, which is very low. <br /><br />My 1st question is: if I go up to a 19 pitched propeller, that should increase the RPMs. Right?<br /><br />My 2nd question is this: I have been looking for a 11 3/4 x 19 propeller, and they don't seem to exist. Could someone recommend an alternate prop size? I am primarily interested in top end. Hole shot performance isn't something I'm concerned about at all, really. I just go fishing. I go a fair distance at 3/4 throttle and come back. That's pretty much it. No pulling skiers, or anything else, for that matter.<br /><br />What's the biggest diameter I can work with? The smallest?<br /><br />You get the point.<br /><br />Gracias!

 

[Dhadley]

Re: 2 part Propeller Question

Going up in pitch will decrease rpm not increase them. The higher the pitch the harder it is to turn. The problem is youre looking for 1700 rpm. Thats more than a prop change.<br /><br />Lets start with the basics. Are you sure the tach is reading right?

 

[Virginiaboater]

Re: 2 part Propeller Question

Dhadley, thanks for the reply. <br /><br />Hopefully I have a little more specific info. I took my prop down to my local dealer, and he told he it was a Stainless Steel prop with measurements of approximately 13 x 21. According to the mechanic, this prop has no business being on this motor at all (he wasn't even sure how it fit in the splines). I want to give this motor to my son with a suitably sized smaller boat. It's a sweet operating motor, I remember it fondly from trips my dad and I took over the years. It has great compression, spark, starts on about two reveolutions of the starter, and is exceptional on gas usage. I tried a smaller prop I had for another Johnson I had years ago, and while it wasn't the exact size listed in my old manual, when I tried it on the old 78 and the boat it's on in the water, my RPMs went right back to 5100-5500. <br /><br />I've never seen a motor taken better care of than this one. It may be from 1978, but it looks and works as if it were only five or six years old.<br /><br />So, keeping your reply in mind, does 11 3/4 x 17 --as listed in a very old manual I have--sound like the correct size for this motor? <br /><br />If so, AND THIS IS MY BASIC QUESTION, NOW, what do you think are the high and lower limits for pitch on a prop for it? The mechanic didn't really want to work on it, the business is more into newer motors and he was pretty rude about it.<br /><br />All I want to do is put the "correct" prop back on the motor, and then try to understand the "pitch" variable a little more (what going up or down in pitch a little does).<br /><br />Thanks so much for looking at this again.

 

[Dhadley]

Re: 2 part Propeller Question

Well, your dealer has some learning to do. No wonder he doesnt want to work on it -- hes not familiar with it. Your era motor has the same spline count as larger 3 & 4 cylinder motors. Thats why it fits. <br /><br />OK, with your other prop youre getting much closer to the right rpm. We could still use a bit more but we're headed in the right direction. The next step is to figure out what pitch it is. Look on the hub and see if there are some numbers. Anything. That will tell us what it is. <br /><br />OK, as for your questions about pitch -- check out the faq's here and look at what Kenny has posted. Itll give you a lot of detail but basically pitch represents, in theory, the distance (measured in inches) a prop will travel forward in one revolution with no slip. Obviously from that we know the more pitch you have the harder it is to turn. So if we want to gain rpm, we reduce the pitch -- or load -- so the motor can make rpm easier. Remember pitch is always measured in inches, not degrees like some folks think. And we also know that no prop runs at 0% slip.<br /><br />Theres lots of other factors such as drag (from the hull and/or gearcase), rake, slip, hull design etc.<br /><br />Now, heres another idea. To gain rpm you can raise the motor on the transom. That reduces gearcase drag and allows the prop to run closer to the surface. There will be a point at which a given prop will start to vent (suck air). That point or X dimension, will not be the same for all props. <br /><br />Since you are taking the motor off this boat and putting it on your sond boat we probably dont want to get into buying props for this boat. How soon are we doing the swap?<br /><br />One last "food for thought" -- motors used to come with a prop from the factory. People refered to it as a "standard" prop. There is no such thing. The motor came with a prop and the dealer was supposed to exchange it out for the correct one when the motor was rigged. Most dealers didnt do that. They thought that the prop that came with the motor was the "correct" one for all boats. OMC eventually quit shipping props with motors for several reasons but that was one.

 

[Virginiaboater]

Re: 2 part Propeller Question

A lot of great info. Thanks.<br /><br />So, let's see...the prop I have now (remember it's borrowed from another motor)is a stainless steel prop that measures 12.75 inches, if I make a circle all way around with one of the blades; or 6.35 (rounded up) inches from center of hole to single blade tip. As I said in my previous note, the mechanic who looked at this prop said the pitch was 21; however, the prop clearly has a 17 stamped into its exterior body. <br /><br />OH, I also rechecked my tach reading to make sure I was giving you the right info. I was a little off in the RPM reading on this stainless steel prop I'm using. The tach itself was out of adjustment. The max RPMs for my motot is listed as 5300, and the SS prop is now giving me 4500-4600 RPMs.<br /><br />So, if I need to increase my RPMs at WOT to 5200-5300 (what is says in the manual) from it's present 4600 RPMs, then that's an increase in RPMs of approximately 700. <br /><br />Does that mean a decrease in pitch to 13 would put me about right if I got new prop with the same 12.75" diameter? <br /><br />Thanks.

 

[Barnacle_Bill]

Re: 2 part Propeller Question

Va Boater. First of all here is an excellent article on props. http://boatfix.com/how/props.html <br /><br />You are not very far from an excellent prop shop. Bay Propeller on S. Military Hwy in Chesapeake. Take your prop there and talk to Jerry. He will tell you exactly what you have and what you need. Be sure to have your figures (rpm) with you. He will also give you a loaner to be sure you get the right prop. Seriously, this guy is good and will go out of his way to get you straight.

 

[walleyehed]

Re: 2 part Propeller Question

I disagree with the "Target" RPM being the Rated power RPM....this would be saying "Taget" 5000RPM for an engine with a range of 4500-5500.<br />That's bad news....lugging if you will.<br />If your manual calls for 5300 max in 1978, I'd shoot for atleast 5600-5700.<br />I won't get into details on why I say that because I know Dhadley will cover that.....