Marine Propeller

[Gerry Mac]

I have located a rebuilt prop for my Johnson motor. However, my existing prop is 12 1/4 X 15 and this one is 11 1/4 X 17. The price is right and I want it only as a spare. Can someone advise what effect the smaller size and pitch will have on my motor, if any. Boat is 16 1/2 aluminum Pro Fisherman and motor is 50 HP.



Thanks



Gerry

 

[Andy Cabrera]

- Increase propeller pitch to lower top RPM wide open throttle (WOT) Full-Trim.

- Decrease propeller pitch to raise top RPM wide open throttle (WOT) Full-Trim.



If the propeller is too small, either in diameter or pitch, the engine can be damaged by excessive strain



So basically you will raise the RPM with the pitch but push more water with a 17" I really don't think it will really affect a jon boat too much

 

[Devon Fathauer]

smaller prop means quicker to get on plane.

it also means higher rpm which equates to decrease fuel efficiency. it wont have much effect on a 50 hp engine on a 17 foot jon boat though.

 

[Kevin Palmer]

Gerry, not sure if you know what the actual markings mean, if so, sorry in advance if I am insulting your intelligence..



A prop of 12 1/4 X 15 means that it is 12 and 1/4 inches in diameter and would travel 15 inches if you were to thread it into a block of wood for example. Obviously, it slips in water, but if you could thread it into a sold item such as wood or jello for example, the prop would move 15 inches after one complete revolution.



So, when we look at the 11 1/4 X 17, we are going down 1 inch in diameter but we have a steeper pitch creating it to travel 17 inches.



All in all, I believe the increased load of the pitch would make a minimal change over all to the engine RPM's.



I say this as the first prop has increased water resistance due to the diameter with a slightly less water resistance relative to the pitch. Looking at the 2nd prop, we loose resistance in diameter but gain it in the pitch.



Follow me????

 

[Gerry Mac]

Thanks for the input. Absolutely not insulted whatsoever. Some things I know lots about and some I know very little. This is one of those. I have always had leaky bath tub boats and sometimes run motors. As I get older I want to enjoy my fishing more and work at it less.



Coastiejoe you have explained it completely. Want to explain something else? I have a live-well in the boat with a pump. there are two holes on the transom that run into the live well and one of course a third for the bilge.

The question is: How do I use the live well. I do not normally keep fish but if one is going to die I will not waste it either.



Many thanks



Gerry



Message editied to add picture of "Loch Sloy"





[Broken External Image]:

 

[Jim parnell]

the live well should fill up when the boat is the water, and you put a plug in the drain hole INSIDE the livewell to keep water in the livewell should you catch a fish & want to take it home. There should also be a switch on the console that would say "RECIRC" or something similar, turn this switch on when you have fish in the livewell and the pump will recirculate the water in the livewell to replace the stale water or to oxygenate the water that is in the livewell, keeping the fish in better condition until you get home.



Since you say you don't keep fish unless it will die, you can use it for storage for items that won't hurt to be wet unless you put the plug in to keep water out, if you have a fish that is going to die, just put it in the live well and turn the recirc switch on, just remember to put the plug in before you load the boat on the trailer to keep water in the livewell.



did you get a manual with your boat, or contact the mfgr.

 

[Kevin Palmer]

What he said...



I have seen a few versions, but in most cases there is an in and a out. You do want to recirc the water in the live well as it keeps the fish oxygenated, aka alive...



In most cases, there is either a plug or some sort of a check valve for the drain side. I have seen where the recirc pump act ONLY as a recirc pump or also as the fill pump..



It is a matter of you playing with the switches to see the version you have..



Enjoy the boat..

 

[swshawaii]

The propeller is the single most important component on ANY powered boat regardless of type.

It will do more for performance and efficiency than any other "part" of your boat.

Ideally you should select a prop that will give you the maximum rated RPM at "wide open throttle", with the best acceleration, with minimum "cavitation". Boating 101.

Whether your setup is for fishing, diving or skiing, etc., ALL powered craft are "rigged" using these guidelines.

The manufacturers recommended "diameter" and "pitch" specs are only a suggestion.

The ONLY way you can determine the correct "wheel" is by testing in "real world" conditions with your normal "load". TEST the propeller in this manner if possible. From my past F-1 boat racing experience, I have never found the "optimum" prop with the boat sitting on the trailer.

BTW- I understand you are buying this as a "spare" wheel. Hopefully your boat was "propped" correctly when you "turned the key" the first time. Happy Testing.