Yet another prop question

[imported_JD__]

I ordered a Johnson/Suzuki 140 4-stroke to replace the 115 Merc 2-stroke I had on my boat. I have a Tracker Avalanche (stretch-formed) aluminum boat 18 ft. w/modified v-pad @1400 pounds + 410 pound engine. With the 115 (348 pounds) and a 22 pitch Laser II it would run 46-47 with me alone and 1/2 tank of gas as per GPS. The dealer I used has only mounted the 140 on pontoons & deck boats. He seems to be as much in the dark about propping as I am. Anyone have a clue as where to start? 22, 23, 24? Anyone out there with first hand experience using the 140 4-stroke on a bass type boat? Anyone dare guess how fast my boat will go with the 140 and correct prop?<br />Thanks,<br />JD

 

[walleyehed]

Re: Yet another prop question

I may be full of crap, but I'd say #1, you were over-proped based on speed figures you gave for a 22" prop on the 115, so I'm willing to bet you won't be far-off going with the 22" on the 140...<br />If you're propped for the correct RPM range, I don't think you will see more than maybe 2mph gain..

 

[Dhadley]

Re: Yet another prop question

Since you already have the 22 Laser why not start there?

 

[imported_JD__]

Re: Yet another prop question

Walleyehead, The 115's aren't hgh RPM engines. The published Merc 115 2-stroke RPM range is 4750-5250, gear ratio is 2.07 to 1. Actually the 22 pitch prop was suggested by Tracker for this application. This engine was running @5100-5200 rpm at wot and running 46 mph (gps). It was right in the speed range published by Tracker. The Johnson specs are: 140 HP @ 5900 RPM with a 2.38 to 1 gear ratio. Published RPM range for the Johnson is 5600-6200 RPM. <br />Dhadley,<br />I sold the 115 for a very good price so I let the prop go with the engine. Thanks for the input guys, I suppose I will have to wait and see.<br />JD

 

[walleyehed]

Re: Yet another prop question

JD, I'm not going to say you were proped "wrong", but 5200 by yourself and half fuel is still not "right".<br />Very few people understand you actually want slightly higher than max WOT range with the load you had....say 5500-5700 RPM would be MUCH better because your engine will be at the top of the range with a load....hence at 5200 WOT, you are actually lugging the engine with a load.<br />I understand the gear ratios, and took that into account when I posted what I did.<br />The 4-strokes may not like continuous operation at max WOT, but you still have to prop correctly to get the best performance and economy.<br />If you prop the 140 for max allowable rpm with just you and half fuel, it won't be the right combination.<br />The best method would be to fill fuel tanks, add a buddy or 2 and your gear, and prop to hit the upper end (within 200rpm) of the RPM range.<br />Just pull back to max when by yourself, or a little lower for cruise.<br />Good luck,<br />Wally

 

[imported_JD__]

Re: Yet another prop question

Wally,<br />I'm talking about Mercury 115 2-stroke 4 cylinder engines. The ones that idle on 2 cylinders and run on 4.<br />I appreciate what you're saying BUT... 5250 is the max RPM with this 2003 yr. model engine as per Mercury. It would still turn 5000 rpm with 2 people and a full tank (41 gal) of gas but would only run around 42-43 mph. The rev limiter wholehartedly agrees. It just wont turn over 5250 RPM. I tried a 20 pitch prop on the same engine. The hole shot improved but it suffered on top end, still 5200 RPM. The 115 is not a performance engine per-se and not designed to do 5500-5700 rpm, so I was told by a Mercury tech. I have a 2002 yr. model 115 on another boat, a modified V design that weighs about 275 pounds less than the Avalanche. It won't turn over 5100 RPM with a 19 on it and no load other than my big butt and 10 galons of gasoline.<br /><br />Thanks for your help.<br /><br />JD

 

[walleyehed]

Re: Yet another prop question

Sorry, wasn't aware the 115 had a rev limiter set that low....guess your best advice would come from the dealer in this case...<br />Good luck

 

[walleyehed]

Re: Yet another prop question

This engine doesn't have a rev-limiter, at least not below 5700RPM, so as I said before, you are incorrectly proped.....<br />Another item not mentioned that would help, is whats your altitude above sea-level.

 

[imported_JD__]

Re: Yet another prop question

walleyehed,<br />Elevation is 1000'. I copied the specs below from the Mercury web site. <br /><br /> http://www.mercurymarine.com/mercury_115_classic <br /><br />Mercury 115 Classic<br />Specifications<br />HP @ Prop: 115<br />kW @ Prop: 85.8<br />Max RPM (WOT): 4750-5250<br /><br />According to my Clymer manual #B727, the 115 has a RPM limit module which is connected to the CDM's stop circuits, ignition switch and the trigger coil. If the engine exceeds the pre-programmed limit, the limit module detects this from the trigger coil and momentarily shorts out the ignition, limiting speed. What is the pre-programed RPM? I don't have a clue . However, I was told by a re-builder and a Merc mechanic not to try and exceed the max RPM with the 115 unless I wanted to buy a new power head. Both said the 115 is not a high RPM engine, that it was not designed to rev up like the XR6, EFI and Optimax. If pushed beyond its limits it would fly all to hell.<br />JD

 

[walleyehed]

Re: Yet another prop question

Well aware of the Classic 115, as I researched it as well....5250 is the top of the rpm "RANGE", not the limit..At your altitude your engine is only putting out 110hp-not 115, so I still strongly believe you are Over-proped.<br />I also understand the gear ratio, as 2.07:1 is about the same as the standard 2:1, and I can tell ya first-hand that running at 873ft above sea-level with a 115 on an 18ft'r, 19" is about right, with 21' on some light boats.<br />My thoughts are, you are not "hitting" a rev limiter, you have simply reached the limit of HP your engine will produce with That prop.<br />There is another gent on another post running a 19" on a light boat @ 5700 on a 2003 115 classic, but pulls back to 52-5300.<br />I do have some experience with this, and I'm not trying to lead you astray, just trying to help you get the most.....

 

[imported_JD__]

Re: Yet another prop question

walleyehed,<br />Thanks again for the advice. Didn't take it in a negative way, just trying to go by the book.<br /><br />JD