Prop size / RPM

[StriperChaser]

My 96 Johnson 112 SPL runs well at 4900 WOT but I've lost some bottom end making it slower getting on plane. A friend suggested my old prop may be the problem although it appears OK with only a few slight dings. I noticed while looking at new props that most outlets suggest a 13 1/4 X 17 prop for this motor. I've been running a 14 X 17 the dealer originally installed. How much difference would dropping the prop size from 14 to 13 1/4 make in RPM's and bottom end? How would it effect top end? I could stand to gain a few more RPM's at WOT. Thanks in advance.<br /><br />StriperChaser.com

 

[walleyehed]

Re: Prop size / RPM

#1, even though you are in the range of recommended top RPM, 4900 is too low.<br />5500 should be your target and that's going to take a lower pitch prop.<br />The 4900 is also telling me why it doesn't want to come out of the hole good-It's over-proped.<br />The 14X17 OMC prop pulls about 150 RPM lower than the 13.25X17 at WOT. Top speed is the same, maybe a tad slower with the 14X17.<br />I know this engine quite well, and I would recommend you go to a 13.75X15...this will get you at 5200-5300, which I believe is as low as you want for WOT.<br />OEM P/N, 391198.

 

[StriperChaser]

Re: Prop size / RPM

Thanks for the advice. Never realized the prop was a 14" until just recently. That's just what the dealer had installed. When the engine was new, it did quite well with this set up but I guess wear and tear through the years has changed the performance of the engine.

 

[Dhadley]

Re: Prop size / RPM

With a 14" diameter you should be able to raise the motor a bunch. That will increase your top rpm and overall performance from holeshot to top end.<br /><br />5500 is a good target but I would look for 5800 if it were mine.<br /><br />At 4900 the rings may have started to coke up. A good decarb job would be an excellent idea. Compression test before and after.

 

[rickdb1boat]

Re: Prop size / RPM

StriperChaser<br /><br />It's not really the diameter that's going to help much, it's the pitch that controls your RPM's for the most part. As Walleye said, the 15", along with the change in diameter, should at least get you in the operating range for your motor. You get about 200 RPM's change for every inch pitch that you go up or down. So going down 2" in pitch will gain you 400 RPM's right off the bat, which will get you to 5300 RPM's, plus the change in diameter will help, too. Walleye is right on, although, that would definetly be as low as you would want to go on the RPM's, as was stated. Just thought I would confirm what is good advice.. BTW, what boat and size are you running?

 

[StriperChaser]

Re: Prop size / RPM

The boat is a Seaswirl Striper 1730 Center Console (17.5') I'm carrying a 30 gal bait tank, (4) batteries, 40 gals fuel, plus gear, so it's loaded down pretty good. I've done a decarb recently and run SeaFoam regularly with the gas. Last compression check had all cyl between 110 - 115. How much top end will I lose with a 15" pitch? I'm lucky to get over 40 MPH now with the 17".

 

[StriperChaser]

Re: Prop size / RPM

walleyehed, you were dead on the money with your recommended OEM part #. I bought that prop and tried it this weekend. It was like having a new engine on the boat. It pops out of the water like a cork. Is there any prop, i.e. SS, 4 blade or another that can possibly give back the 5 to 8 MPH top end speed I lost? Also my RPM's at WOT increased to 5200, how can I get that to the 5500 that has been recommended. Thanks again for your help.

 

[sanmarc]

Re: Prop size / RPM

I just put a 112 spl on my 16 ft Glastron and I too cannot get over 5000 rpm. The prop is SS with holes in it (for better hole shot I think) and it looks new. The boat planes quickly so I guess this is a good prop? How do I find out what size it is? I do not see any writing on the prop that says size etc.<br />Thanks.<br />PS-Boat does 47 with this prop...is that good with this motor?

 

[Iceking]

Re: Prop size / RPM

Walleyhead,<br /><br />Sounds like your the man when it comes to a 96' Johnson 112 SPL.<br /><br />So I ask the questions: I have a 2003 Bone Boats 18 cape island (18'-6", 1600#, 48 gallon fuel, 30 gallon live well.<br /><br />With two adults, full fuel, livewell and 100 gear, the boat is a slug getting on a plane, even with the trim tabs down.<br /><br />Currently, the prop is a 13 X 19. RPM at WOT is 5100, and top speed is 41 mph. <br /><br />What do you suggest to increase planning, I'm not too concerned with top speed, just getting on a plane quicker.<br /><br />Thanks.

 

[walleyehed]

Re: Prop size / RPM

You need to be turning 5800RPM at WOT...<br />First, is your engine setting clear down, mounted in it's lowest position on the transom??? If it is, we need to take the top 2 bolts out, loosen the bottom 2 and raise the engine 1 hole at the top, reinstall and tighten bolts and go test. we may see only 75 RPM per hole but it's a start.<br />Your figure calculate out to about 10-1/2% slip ratio...not bad, but I think we can bump that down a bit. after testing each hole, you will get to a point where at #3 or 4 it may ventilate if no jackplate is in use....if it ventilates in #4, drop it one, if it does the same in 3, drop it to 2. We want as high of mounting heigth as possible without ventilating in a straight line.<br />See what the rpm is at this point, and I'm going to say we need to either add a jackplate, or drop in pitch and go with a high quality prop that we may be able to run higher.

 

[walleyehed]

Re: Prop size / RPM

You need to be turning 5800RPM at WOT...<br />First, is your engine setting clear down, mounted in it's lowest position on the transom??? If it is, we need to take the top 2 bolts out, loosen the bottom 2 and raise the engine 1 hole at the top, reinstall and tighten bolts and go test. we may see only 75 RPM per hole but it's a start.<br />Your figure calculate out to about 10-1/2% slip ratio...not bad, but I think we can bump that down a bit. after testing each hole, you will get to a point where at #3 or 4 it may ventilate if no jackplate is in use....if it ventilates in #4, drop it one, if it does the same in 3, drop it to 2. We want as high of mounting heigth as possible without ventilating in a straight line.<br />See what the rpm is at this point, and I'm going to say we need to either add a jackplate, or drop in pitch and go with a high quality prop that we may be able to run higher.

 

[Iceking]

Re: Prop size / RPM

I'm going to try a 13/17 prop first and see what happens. Then I will begin to adjust the mounting height. Thanks.

 

[Dhadley]

Re: Prop size / RPM

Iceking, you might think about doing it the other way around. Dont spend money on a prop just yet. Work with the set up first -- its basically free rpms. In most cases dropping to a smaller pitch will reduce top speed also.<br /><br />If you raise the motor you will reduce drag throughout the entire rpm range. Your holeshot will improve, your cruise speed will improve, your top speed will improve as well as your top rpms.

 

[RaginCajun]

Re: Prop size / RPM

Hey Guys, I read all your comments, and you seem to know what you are talking about. So here is my problem. I just inherited my dads 1983 Cajun Bass Boat, and it has a '83 Evinrude 150 V6. It sat in a boat slip for 2 years and wasn't run. I took it out here on my lake and it just doesn't seem to want to go. Yesterday I was trying to keep up with some schooling stripers, and I bet it took 1/4 mile for the thing to build up enough speed to plain out. Just doesn't seem like it has any power. When I was a kid I remember it had so much power we had to put a "dolfin" on the lower unit to keep the rear end in the water. Any ideas ? Could it be the carbs ? The engine was totally redone a few years ago by a 30 year seasoned pro, and was dino'd at almost 175hp...not now... any thoughts ?? OH, I noticed the spark plu wires (where the wire connects to the plud) was a little rusty, could this make for the major power loss ?

 

[RaginCajun]

Re: Prop size / RPM

Also, you guys were talking RPMs.... at full bore, what do you think I should be pulling for RPM's on this 1983 V6 150 Evinrude... I know it is a 24 pitch. Right now I think I am getting 4000.

 

[walleyehed]

Re: Prop size / RPM

by walleyehed;

We want as high of mounting heigth as possible without ventilating in a straight line.<br />See what the rpm is at this point, and I'm going to say we need to either add a jackplate, or drop in pitch and go with a high quality prop that we may be able to run higher.<br />

What I'm getting at here, is what Dhadley is saying...I'm just looking at options AFTER we maximize the current set-up. <br />Don't mistake what I said as just trying a lower pitch, again, as Dhadley said, that's a last-ditch effort.

 

[Dhadley]

Re: Prop size / RPM

Welcome Cajun!!<br /><br />If its been sitting that long I'm amazed it even started. And amazed even more that you didnt score a piston when it did run. Yes, youre exactly right -- the carbs are the first place to look. They need to be cleaned and have new kits installed. Remember, on a non-oil injected motor if the carb is gummed up and not supplying gas, it aint supplying oil either.<br /><br />The next thing would be to get rid of any old gas. If you put new gas on top of old, that doesnt count as fresh. Get rid of it.<br /><br />Then I'd replace the water pump kit. Just because we dont know what shpe the impeller is in from sitting dry all that time.<br /><br />And remember -- Champion plugs only!<br /><br />Oh, the 150 and 175's use the same powerheads from the factory.

 

[RaginCajun]

Re: Prop size / RPM

OK, so spraying some of that "choke and Carburator Cleaner" into the intake won't do it will it.... Is this cleaning job something I can do myself ?? I'm pretty technically inclined. Worked on small craft in the Marine Corps, just all my motors were Diesel Engines..haha. Also, you caught me, I put new gas on top of the old gas... I think I have run probably 2 tanks worth thru her already.... but maybe I should drain it all out... I tried the fuel treament route, probably didn't do anything... Can you explain to me how the spark plugs work in this motor.. there is no "L" on the end of the plug.

 

[RaginCajun]

Re: Prop size / RPM

Oh, the water pump..... It pumps water really good, got a good jet coming out of the engine... and it wasn't dry, the lower unit was in the water the whole 2 years... suprised it wasn't clogged with algae.

 

[RaginCajun]

Re: Prop size / RPM

Well guys aren't going to believe this... I have had this boat for about 3 months now, and I use it about every day, but most of the time I am just at idle speed.. Well this morning I took her out and the lake was white capping like crazy (3 foot swells) anyway I tried to open her up again, and same thing... took about a quarter mile to open up to about 3000RPMs. We got about half way across the lake and hit a big swell and all of a sudden she took off like a bat out of hell... RPMs jumped up a little over 4500 and I had to actually ease off the gas... we got in some calm water and stopped and fished a while with the trolling motor, and when we got ready to go I said "I wonder if that fixed our take off issue"...and sure enough, You hit the gas, and she jumps up and goes just like she was new... No idea what happened, but it must have jarred something loose that was messed up. But anyway I am still gonna take your advise Dhadley and get the carbs cleaned and pump out that fuel tank. i just hope I don't undo what ever fixed itself...<br /><br />OH, quick question. According to my tach I was pulling about 4500 RPMs at WOT and the trim up, is that about right for this V6 150 ? I heard you guys talking about 5500. Am I good to go ?<br />Thanks For all your help.