Considering a stainless prop for my 1984 Mariner 20hp

[onthewater102]

Right now with 2 people on the boat, 2 size 29 batteries, an 80# thrust Maxxum TM and the weight of decking, gear etc. (~450lbs between the boat, motors & gear) I'm getting about 20mph on my 10" dia 13" pitch aluminum prop (unsure of the make/model - probably original) running with a hydrofoil on the motor (zero bow rise, not giving that up.) When I run solo I can get about 25/26mph out of it depending on wind speed & direction, which is where I'd like to be with two people if possible.

Does the strength of the stainless allow for different cupping, or a more aggressive contour to the rake etc. allowing it to edge out a speed advantage over an aluminum prop of the same diameter and pitch? Otherwise I'll pickup a tachometer and just follow the prop matching guides most manufacturers offer to try to get a prop that will give me a bit more top end. Fuel economy with the 20hp is not a consideration - I hardly use any gas on most trips so if I use slightly more than hardly anything it won't make or break my bank.

However, speed is an issue as I fish a very long reservoir with two branches and to re-position from the shorter eastern slow moving arm (~150cfs natural flow) to the faster flowing main Housatonic branch (1500cfs natural flow) can result in a trip of 8 miles or more 1 way and I fish within the time limits of whoever is watching my kids that day, so if there's potential to make what is now a ~25 minute trip into a ~20 minute trip that extra 10 minutes saved (both ways) is huge when I've only got 3 hrs between dropping my kids off and needing to pick them up (trailering on/off & running up & down river). Unfortunately there are only 2 state launches I can use and they're ~1 mile apart on the opposing shores of the reservoir.

Here's the map for reference - 1 mile scale is in the bottom right most corner:

 

[DaleH]

SS props bite better and flex or bend less, so it is more that they maintain the desired shape throughout the OB powerband. Plus with thinner blades, they are more efficient.

Regardless of whether you buy one from them or not - as I'm not linking it for that purpoase (just a VERY satisfied customer) but this link from Turning Point props for YOUR boat and OB as YOU have it weighed down ... is really good !

See: https://forum.tinboats.net/viewtopic.php?f=4&t=37433

FYI, where SS props don't flex ... typically one runs a lower pitched SS prop than they would an aluminum one. That's why it is imperative to know the rated OB maximum RPMs and to prop it to run within 100-200 RPMs off that max number.

Too many use too steep a wheel and since "boats only get heavier" as the season goes on ... one could begin to lug the motor - and that is bad for longevity. I'd bet $$ that most rigs wear the wrong prop :shock: !

 

[onthewater102]

That's the prop. selector I was using too...I'm not sure how exact it is for my application (2002 alumacraft 1436LT / 20HP 1984/85 Mariner 20hp) as I've shifted the seating position forward ~5" from the front edge of the rear bench and converted it to side console steering, so the weight is carried a bit forward on the flat bottom portion of the boat. For my 1436 it was recommending a 10.5-13 but if I plug a 1448 in it recommended a 10.125-14 so I'm thinking that prop is safe on my motor. I'm going to see where I'm at with RPMs before buying this, but if I'm running on the fast side now I can get all the way to a 10.125-15 I think I could get the speed I'm looking for. If there is efficiency to be gained in the same prop size by switching to stainless I think I can definitely get to 25mph with a guest, which would be awesome.

The more I think about it the more I realize I need a tach to know exactly where I'm at right now with my RPMs at WOT with the load of 2 people in the boat. If it's bogging the RPMs down then perhaps a shorter pitch will get the RPMs up and close the difference between my 21 and 26 MPH observations.

 

[restornator]

get rid of the 24 volt trolling motor, that's overkill .

 

[onthewater102]

NO!!! Love it...a lot of lakes around here are electric only - the 24v really moves me along there a lot faster than the 55# 12v ever did. Were it not for this I'd agree with you that it isn't worth the weight & I could do just fine with a 40# thrust to save on weight.

 

[onthewater102]

Tested the speed on the fishfinder GPS vs the GPS in my phone (which agrees to the spedo on my truck) and the humminbird doesn't seem to be reading correct - it shows me doing 24 while my phone reads 21, so I don't think I'm going any different regardless of how many people are in the boat, but it also means that the 1 to 2 mph difference in speed to be gained from changing pitch isn't going to justify the cost of a $250 propeller...oh well.