Pitch change for trolling

[Wet_Net]

Hey guys i am in need of some prop advice. I have a 79 25 foot cruisers inc gran bateau. Single 305 mercruiser and an alpha drive (I believe 1.5ish gear reduced). Boat runs 35mph gps at 4200rpm right now.

Currently we have a three blade prop 16" diameter 15 pitch. I found an "AX16" as well as a 15p cast in the hub.

What I am looking to do is put a prop with less pitch on the boat to increase our rpm while trolling for salmon on the great Lakes. I'm looking for more speed control and better oiling and charging by having a higher rpm. We are pulling trolling bags now to slow down to fishing speeds about 2 to 3mph. But at times depending on wind or currents we struggle to get slow enough. I would definitely keep using the bags with a new prop as well.

What effect would dropping down to say a 13 or 14 pitch 16" diamond 3 blade have on the boat? I don't mind losing some top end speed as we don't launch far from the lake. The boat planes off around 22/23mph right now and I typically cruise around 27.

 

[Sea Rider]

You're achieving 4200 wot rpm while lightly, medium or heavily loaded ? What's the max wot rpm factory stated for that engine ? Ideal for the engine is to rev around max wot range as currently loaded, once there can throtthle throughout the whole throttle range as preffered.

Depending oh how heavy that combo is and current load on board, going one less pitch while maintaining a 3 blade prop can produce somewhere 200-400 + revs. Whichever prop pitch you select must be water tested.

Happy Boating

 

[Bondo]

What effect would dropping down to say a 13 or 14 pitch 16" diamond 3 blade have on the boat?

Ayuh,..... Welcome Aboard,..... Have ya found any props with that small a pitch, for the Merc outdrives,..??

In my experience, 15" is as small as they get,....

I troll more for walleyes, used to do the salmon, 'n trout, but bought a house on the water with walleye grounds right outside the door,....

Drift socks are great, bigger the better, hung off the bow, 'n tied up along-side the hull,....
I also mounted a monster set of Bennet trim tabs on the barge,.... Way bigger than whats called for,...
Trim 'em Full Down, is like puttin' the brakes on,....

Wouldn't work for you, but I run a Torque-Shift prop,...
They're still 'round, Used, but no new ones anymore, the factory burnt,....
It starts out at 11" of pitch, 'n progressively shifts up to where ever ya set it, 26" is tops, 20" is 'bout as small as ya can hold it on the cams, 'n screws,....
My 4.3LX/ Alpha at 1.65:1 needed the smallest I could get, 'bout 20",...
I slapped on a 1.84:1 drive this year, but ain't had time to set up 1 of my torque-shifts to try on it,...
Hopein' to be able to open it up abit more,....

 

[Wet_Net]

I'd say maybe a moderate load. As I normally run the boat, close to full tank of fuel and three or four guys plus the fishing gear if I run it wide open it will generally hold right around that 4000 to 4200 rpm mark(34 35mph). Even this morning with just myself and my dad it only got to 4200. I did see about 38 at about 4500 one time when we were working on the motor adjusting the timing, light tank two guys and no gear. That was advancing the timing a bit more then it calls for.

I haven't had it over a scale yet, but when my dad owned it the boat was about 5000lb with an empty fuel tank(85 gallon). Likely a bit over 6000 now with my radar arch, gear, and the tank full.

The manual states 4200 to 4600 rpm WOT.

I guess I could stop down to the marine mechanic I had rebuild the carb and buy my parts from and see if I could borrow a prop for a test run before I buy. Or put a small outboard kicker on it, now that would be REAL nice lol

 

[Sea Rider]

Over advancing timing is bad for the OB as is synched to carb fuel entrance, fuel intake and correct timing advance works hand in hand, less wot rpm achieved with current load needs to be compensated with less prop pitch.

Happy Boating

 

[Rick Stephens]

I sure like having a small outboard for trolling. My boat is half the size of yours, so it took a lot less power to accomplish. I tried all the tricks to run off the sterndrive, in the end hanging an old 4 stroke Honda made all the difference. Typical salmon wants different speeds for lure presentations, different days or even different time of day. Sure is nice to just dial in 1 mph or 1.4, or 1.8 and so on. Best part though, I can pull up, shut down main motor, fire off kicker and drop lines in the water in about a minute. I used to spend more time tinkering with brakes than actually fishing, or at least it seemed so. I went with a 15 pitch 4 blade Solas prop for better slow speed handling, same reasons as you are stating. Worked like a charm while I was using drag devices and the main motor. That prop just sits on a shelf now, but served admirably while it was in use.