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[Spawn Sack]

I do understand what diameter and pitch mean as far as they relate to propellers. However, I am unsure what replacement propeller for my 4hp 4stroke would best accomplish the goal I am trying to achieve. More specifically, that goal is to lower the trolling speed a bit at lowest throttle. Right now the motor will troll as slow as about 1.5 mph. I'd like to lower to to 1 - 1.2 mph or so. Right now the motor is fine if I'm going into a bit of a headwind, but if flat calm I wish it would push my 12 footer a bit slower.

Prop the motor came with is 7 and 7/8 x 5 and 7/8

3 other prop choices for this outboard are 7 and 7/8 x 6 and 3/4, 7 and 7/8 x 7 and 1/2, or 7 and 1/4 x 5 and 1/2

All props are 3 blade.

I don't care about hole shot and top speed, just which one will troll the slowest at lowest throttle.

I know that decreasing prop pitch will raise RPM so I think this will make the boat troll faster. But then I think this prop also has less bite/torque in the water so maybe it'll troll slower at lowest throttle.   

Edit: after doing some more online research, it seems that the best choice for my desired outcome would be the 7 and 1/4 x 5 and 1/2 prop. Less pitch = slower trolling speed is what I read on several sites. I imagine I would get a slight increase in RPM, but sounds like still a slower trolling speed at lowest throttle.

[VAGAbond]

You are going in the right direction, smaller diameter and less pitch will give the prop less bite in the water.

The usual concern is at the other end of the range, choosing a prop to match maximum engine rpm  to full throttle.  There are formulas but they are approximate due to the infinite variety of hull shapes etc.  Typically you try a prop at full throttle, if engine speed is too low, try something with less diameter or pitch until the right engine speed is found.  If too high, try a prop with more bite to load the engine and slow it down.

For your case you are looking for a prop with less bite.  The risk is you might overspeed you engine at full throttle due to not having enough bite to the prop.  At the size of engine you mention you probably don’t have a tachometer so you would just have to take it easy on opening the throttle.

The other approach to achieving a slow trolling speed is to drag a bucket.  Low cost and it works.

[ShaunO]

I don't know about you, but I like cheap, fast and good solutions.  Have you considered a drogue?  Sometimes called a sea anchor to take a bit of speed off of your troll.  I once rigged up a 5 gallon bucket with some eye spliced line and it dropped a knot off my troll (25hp on a 15' aluminum).

[Spawn Sack]

Thanks for the replies guys. Ya I have a drouge for my river boat. It's a necessity at times when sturgeon fishing to keep the boat straight. Not really sure if I'd want to drag it or something similar lake trolling. IMO just one more thing to get wrapped up in a prop or fishing line, have to bring it in with a big fish on, etc. I have no doubt it would work, just not really for me for this application. I'd rather try the prop route.
I have an hour meter on the engine that shows RPM while running. Could monitor it to make sure the RPMs don't climb too high (manual says 5500 is max engine speed).
Think I'll try the 5 and 1/2 pitch prop and reassess.

[Spawn Sack]

So went to GA Checkpoint yesterday where I bought the motor. Parts guy was super knowledgeable/helpful. He talked me out of a different prop ($230 +tax) and into trying a "Happy troller" ($150 + tax) for my motor. The Happy troller is a plate that affixes to I believe the cavitation plate of the leg. When you want to use it you pull a cord which drops the plate down. When you want it out of the way you gas it and the plate pops out of the way. I needed the "Lil fella" size for motors under 20hp which they didn't have in stock, so had to order it in. Hopefully I like it. Buddy of mine used to have one on his 16footer with a 30hp outboard. He says he loved it and could troll down to 1mph with the plate down.\
Curious if anyone has one of these trolling plates and what they think of it.