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Propeller sizing
trickydaze	#1  Posted on 02/13/13 - 5:57 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	17' outrage with 120 hp johnson, I got two props with boat and i have always used the 12-3/4" x 21 p stainless, works well in mid range to top end but it seems to lug a bit like at 2000rpm and under. rpm is like 5600 rpm and a bit more when trimmed up. My other prop is a 14" x 19p much better at lower rpm but top range is closer to 6000rpm anyone know if this is ok? I believe top range is 5500 to 6000rpm, But trimming up at w.o.t can push it passed 6000rpm like 6200 rpm. Fuel economy seems much better with 19 pitch. I do a lot of lower rpm cruising on intracoastal waterways. Any input on this appreciated.
Tom W Clark	#2  Posted on 02/13/13 - 8:02 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	What is the recommended WOT operating range of you Johnson 120? What year is the motor? What year is the boat? Is the motor a Long shaft or Extra Long shaft? How fast does your boat go with the 21" pitch propeller? How fast does your boat go with the 19" pitch propeller?
trickydaze	#3  Posted on 02/14/13 - 4:39 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	Boat and motor are 1991. Not sure but I think rpm range is 5500 to 6000. I do lose a couple mph top end with smaller pitch. It seems as long as Im not out racing I should be ok with 19 pitch and I dont trim up to high on w.o.t. Plus with more people in the boat the 19 pitch seems to respond at bit better coming out of the hole.
trickydaze	#4  Posted on 02/14/13 - 4:41 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	Oh and it is a short shaft 20" I believe.
Tom W Clark	#5  Posted on 02/14/13 - 9:06 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Well, you didn't answer my question about speed, so it's hard to gauge whether your 19" pitch propeller is performing well or not. I looked up the 1991 Johnson 120 HP Long Shaft. It has a 5000-6000 RPM recommended WOT range and a 2:1 gear ratio. Unless your propellers are slipping very badly your boat would be going in excess of 50 MPH which I find hard to believe, so it may be your propellers are slipping badly. I would expect a 19"-ish propeller to be the right fit for your motor, but you should not be exceeding 6000 RPM. If you are running an aluminum propeller, then you are certainly leaving performance on the table.
Joe Kriz	#6  Posted on 02/14/13 - 11:23 AM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Just for your reference. 15" is a Short shaft 20" is a Long shaft 25" is an Xtra Long shaft
trickydaze	#7  Posted on 02/16/13 - 1:37 PM
Member Posts: 10 Comments: 0 Joined: 02/02/13	How do I know what my motor shaft length is? Im thinking its a short shaft?
Joe Kriz	#8  Posted on 02/16/13 - 1:41 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Model number. J120Lxxxxx or something similar. The J would be for Johsnon The 120 would be the HP and the L would be for long. It might have something in front of the L like an T or whatever, but you get the idea. Here is my model number from my Evinrude 150 Model Number = E150TXCO E = Evinrude 150 = HP T = Trim and Tilt I believe X = Xtra Long 25 inch shaft. CO = year = 1985 Edited by Joe Kriz on 02/16/13 - 1:44 PM
trickydaze	#9  Posted on 02/17/13 - 7:22 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	Tag on motor reads 1991 J120TLEIE I guess means tilt with long shaft, good deal thanks. Now as for prop. This is strange because I was on Michigan wheel website and recommended size for 15 to 17 foot boat was 17 pitch? and for 17 to 19 foot it said 15 pitch. But this seems to low of pitch for my situation. Unless like prior post from Tom said maybe some slippage going on. Any one else have this same boat and motor package and what are they using. When i got the boat the 14 x 19 aluminum was on it, I changed to the other thinking better performance because of stainless. I don't know the exact speed but it does cruise good at 4000rpm with the 21p stainless but the low end seems to lug and fuel economy seems worse. Maybe I should try a 19p stainless? do these props come cupped or is this an option? Edited by trickydaze on 02/17/13 - 7:44 AM
Tom W Clark	#10  Posted on 02/17/13 - 9:37 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Yes, you have a 20" (Long) shaft motor. That is the shaft length the Outrage 17 requires. ...Michigan wheel website and recommended size for 15 to 17 foot boat was 17 pitch... I find that hard to believe. The correct pitch for a given boat depends on quite a few things, the HP of the motor, the RPM range of the motor, its gear ratio and the anticipated top speed of the boat, to name a few. Just as all 15-17 foot boats are not the same, all propellers are not the same. Propellers are not generic pieces of hardware. There is far more to propeller selection that just the pitch and then choosing aluminum or stainless steel. An intermediate gearcase motor like yours has only a fraction of the propeller options as the large gearcase motors. Still, there are literally dozens, if not a hundred or more, different models of propellers to choose from half a dozen or so different manufacturers. You still have not told us exactly which propeller you have now. Brand? Model? Diameter? Part number? Condition? I am not sure it really matters as neither seems to offer very good performance. I think the better way to have started this thread would have simply to have asked: I have this boat, that motor. Which model of propeller should I choose? As always, I do not recommend aluminum propellers for anything but small outboards or as emergency spares. There is no economy in selecting an aluminum propeller. ALL modern outboard propellers for 40 HP and up are cupped. Have been for decades. For an intermediate gearcase motor, you could spend as much as $500 on a new stainless steel propeller, but you really need not spend more than $250. You can considerably less on a perfectly good used propeller is you know what you are looking for. I think the advice to have an existing aluminum propeller you own modified by a prop shop to add cup is really bad advice. You can't make a silk purse out of a sow's ear. No, I am going to ask this question for the third time: How fast does your boat go? Edited by Tom W Clark on 02/17/13 - 9:53 AM
trickydaze	#11  Posted on 02/17/13 - 10:44 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	21p stainless sst not sure of part number right now, runs 41mph at 5800rpm. The19p aluminum runs 38mph at 6000rpm never said i wasn't happy with that. Just looking for a little better in the bottom with more load and maybe boost fuel economy a bit because I do a lot of low end cruising with this boat with friends in Fl. I got the picture, Ill stick to the 19p for what I want to use it for, I was just looking for a little insight on the matter. Thanks for the info. And double cupping is an option for cavitation problems I did find that out. My 34' Scarab comes up off low end a bit slow too with twin 454cid, 425hp engines and 23p stainless quicksilvers but I don't cruise at low speeds with that boat, 65 mph at 5200rpm. Oh and I did mention my props sizes in first post on prop sizing. Thanks again Edited by trickydaze on 02/17/13 - 12:54 PM
Tom W Clark	#12  Posted on 02/18/13 - 8:10 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	OK, now we are getting somewhere. A 21"pitch prop running on your motor at 5800 RPM and 41 MPH is 29 percent calculated propeller slip. A 19" pitch prop running at 6000 RPM and 38 MPH is 30 percent calculated propeller slip! If your tachometer is accurate and you are measuring speed in MPH on a GPS, then something is very wrong. Depending on the model of propeller used, we would expect calculated propeller slip on a boat like yours to range between -5 percent and 15 percent. But now I wonder if your motor has a 2:1 gear ratio. The LONG shaft Johnson 120 uses a 2:1 gear ratio, but the EXTRA LONG shaft 120 uses a 2.25:1 gear ratio. If we recalculate the propeller slip with 2.25:1 as the gear ratio, the slip declines to 20 and 21 percent, still w a y too high. Either way, I'll repeat what I have already said, neither propeller is even close to performing well and I would not spend a dime trying to modify either one. It is time for a new well-fit propeller. And I am not sure you are getting the picture. Quicksilver is not a model of propeller, it is a brand of propeller. Mercury makes several different propeller models under the Quicksilver brand. While it would be inappropriate to discuss on Whaler Central, I am sure your 34' Scarab could see tremendous improvements in speed and fuel economy with a pair of modern well-fit propellers. If there are dozens of choices for your Johnson 120, there are hundreds of propeller choices for your 454s.
trickydaze	#13  Posted on 02/18/13 - 10:03 AM
Member Posts: 10 Comments: 0 Joined: 02/02/13	Man I wish Wolverine propeller would have told me that when they custom fitted the props for me, but I'm sure they have no idea what there talking about. : ( And I'm sure finding the right prop for this little 120 hp can't be rocket science. Well its been nice chatting with ya anyway.
Tom W Clark	#14  Posted on 02/18/13 - 7:55 PM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	It's not rocket science at all. If you want to get your boat moving at 45 MPH like it should you need only buy the right stainless steel propeller for it. If you are happy struggling along at 38 MPH, by all means, stick with your aluminum 19" pitch prop. You'll save yourself the $250 a new prop would cost.
Tom W Clark	#15  Posted on 02/19/13 - 7:37 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	I'd start with a 13-1/4" x 15" Stiletto Advantage 4.25, but what do I know?
EJO	#16  Posted on 02/19/13 - 10:44 AM
Member Personal Page Posts: 669 Comments: 6 Joined: 11/25/12	Tom you seem to know your props so here is the question. What prop should I use on my little boat. 2008 Montauk 150 with 60HP Merc Bigfoot. First I want to get good fuel mileage and get out of the hole quick for my "daily" use. Secondly what prop should I use for best top end speed. I'm not telling you what SS prop I have now I just want to know what you recommend for the 2 scenarios above. Thanks in advance. Edited by Joe Kriz on 02/19/13 - 10:45 AM Skipper E-J m/v "Clumsy Cleat" a 2008 Montauk 150
Tom W Clark	#17  Posted on 02/19/13 - 11:05 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Good question. Start a thread to discuss this and we'll see what we can learn. The topic of this thread is propping a Johnson 120 on an Outrage 17.
Tom W Clark	#18  Posted on 02/21/13 - 7:50 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	I was just reviewing my notes and found some data from a friend who has an Outrage 17 with an Evinrude 115 on it. After he propped it with a 13-1/4" x 17" Stiletto Advantage, he reported a top speed just under 50 MPH! That is such a huge delta between 38 or 41 MPH and almost 50 MPH that I have to wonder if there is something wrong with your Johnson 120? The right prop can make a HUGE difference, but not that huge. Edited by Tom W Clark on 02/21/13 - 7:33 PM