Smaller or larger diameter prop **Results Added**

[Stumpalump]

I've narrowed my prop choice down to two based on availability. They are both OEM props. One is a stainless 11 1/4" X 19 pitch and the other an aluminum 12" x 17 pitch. I'm leaning toward the smaller diameter higher pitch but never experianced a diameter change before. What's effect does diameter have on it?

Boat is a very light 1993 Aluminum 18' Valco Bayrunner with Evinrude 48 special . Wot RPM 4500-5500. Current recently damaged prop was a an old 17p aluminum running 5900 @ 34 mph on a cold day with empty load. Cruise has been is 24 mph 4600 rpm.

 

[jimmbo]

Well since your engine currently running 400 rpm above the recommended max a higher pitch prop is in order. However, you mentioned one parameter which could affect your selection. On a very cold day an engine usually puts out more power and selecting a prop from that data could leave you with less than happy performance come next summer. What kind of RPMs was the engine getting during the summer?

 

[Stumpalump]

Well since your engine currently running 400 rpm above the recommended max a higher pitch prop is in order. However, you mentioned one parameter which could affect your selection. On a very cold day an engine usually puts out more power and selecting a prop from that data could leave you with less than happy performance come next summer. What kind of RPMs was the engine getting during the summer?

It runs a solid 5700 plus rpm most days. 5400 was the worst I ever saw with a heavy load on an Arizona 112 degree day. Some versions of the 2 cly jonnyrudes were rated to 6000 so the old Michigan wheel 17 worked OK if I watched the revs. It was really old and I'm sure bagged out aluminum prop that's been filed clean. Boat just ran fine for 355 miles on Lake Powell this week but the trim broke on the way home and prop hit the street. I ran it at 4400-4750 rpm 99% percent of the time depending on wind and fuel load. Any slower and it would fall off plane real easy when it hit wakes or wind puffs. I was thinking I could stay at that RPM or less with a new larger diameter 17 omc aluminum or stainless omc 19 in 11 1/4p and pick up cruise speed. I understand pitch and Aluminum vs stainless but the change in the diameter in the equation is messing up my decision.
I guess I could ask the difference in an 11 1/4" X 17P aftermarket and a 12" X 17P OMC prop from iboats? I have an oportunity to buy an out of production 19 P and it only comes in Stainless. What would you buy?

 

[jimmbo]

Somehow my edit didn't post. It stated: In a lot of prop lines, as pitch goes up, diameter will get smaller.

It is not really something to get too concerned about, let the prop engineers worry. There is a lot in a blade design, Pitch and Diameter are just two parameters. I've had Merc props where the dia was greater on the SS prop than on the same pitch in AL. The SS wasn't a performance prop(this was in the early 1980s) but the blade shape, blade areas and rake were slightly different compared to the AL.
Some of the 2 cylinder J/E engines are rated to run 5000 - 6000, but their power/torque curve is different than your engine with its 4500 - 5500 rpm range. Running at speeds above the recommended range, the engine is actually making less hp than at lower rpm.

You said you have a light boat, a smaller dia prop should serve it well.
Make friends with a dealer and see if he will let you try out a couple of props with the intention that you WILL buy one from him, also that if you prang it, or even scuff the paint, you will buy it.

 

[ahicks]

I've had my best luck running props that allow the engine to turn at max rated rpm. Here, you know what the ratty 17" was turning. If you got a new one you'd have some assurance you're going to be very close.

We can predict what your engine will turn with a 19" prop on it based on the 17" performance. What is not so predictable is how the boat/engine combo will work out for you while turning the calculated rpms. That's a calculated guess that might leave you wishing you had your 17" back....

Agree, for our purposes here, let the engineers worry about diameter. For general knowledge, consider the props you might see on a tug boat and a blown fuel nitro drag boat. Both will absorb 1000hp easily......

 

[Maclin]

This has been hinted at, but if you are still kinda hung up on Diameter, consider that out at the edge of the blade there is not much of a swept area change with 1/4" or even 1/2" or so radius changes. As mentioned, blade shape factors like pitch, rake, cupping are the main players.

 

[Stumpalump]

.... Here, you know what the ratty 17" was turning. If you got a new one you'd have some assurance you're going to be very close.

We can predict what your engine will turn with a 19" prop on it based on the 17" performance. What is not so predictable is how the boat/engine combo will work out for you while turning the calculated rpms. That's a calculated guess that might leave you wishing you had your 17" back....
......

That's the issue. I should have mentioned the boat is rated for an 80hp but the 48 works well. I'm more of a joy rider. Next adventure will be to cover the 100 miles or more on Lake Mead. It would need to comfortably turn the 19 at cruise RPM. I'm afraid the 19 might feel boggy and am afraid cruising at 4600 rpm for hours with the 17 may be too high. Pick a poison I guess but think I should roll the dice on the 19. Is 4600 rpm plus too high to cruise at?

 

[ondarvr]

The motor can run all day at 4,600 RPMs.

The important part is making sure the 19p is still allowing it to reach the upper end of the RPM range. Comparing different props, especially made from different materials is just a guess at best.

 

[Stumpalump]

The motor can run all day at 4,600 RPMs.

The important part is making sure the 19p is still allowing it to reach the upper end of the RPM range. Comparing different props, especially made from different materials is just a guess at best.

That's good. I'm gonna try the 19p stainless and if I have to throttle up to stay in the sweet sounding part of the powerband it should work. $152 shipped. If I drop back to a 17 then Iboats sells 17's with most expensive one being the OMC for $129 shipped. I may want the 17 for heavy loads in the summer anyway but with the cool air on its way the 19 will be fun. I'll post up exact performance comparison numbers for reference.

 

[Silvertip]

Why would you want to "lug" that wonderful engine with a 19P prop (forget the diameter in this situation as it doesn't help anything). I've owned that same engine and had it on two different boats and tested props including stainless, cupped, non-cupped and always came back to a 11-3/4 x 17 or a 12-1/4 x 15. Let that engine sing at 5500 - 6000 rpm. It will last longer than being lugged with a 19P prop.

 

[Stumpalump]

Why would you want to "lug" that wonderful engine with a 19P prop (forget the diameter in this situation as it doesn't help anything). I've owned that same engine and had it on two different boats and tested props including stainless, cupped, non-cupped and always came back to a 11-3/4 x 17 or a 12-1/4 x 15. Let that engine sing at 5500 - 6000 rpm. It will last longer than being lugged with a 19P prop.

You are correct in your assumptions but as Jimmbo pointed out and I experienced on this 48 and almost identical 60 hp version, the power curve is different. The 60 would start to fall off under 4700 but scream to the moon. The 48 will chug along at 4300 before it falls off the power band but it needs more hull speed to stay on plane so I run it higher. I owned both engines on on similar boats at the same time. A 16' Bayrunner and an 18. The 48 on the larger boat was better than the 60 in every way except it would sing when I rung its neck out. I chose to kept the 48 on the 18 footer over the 60. Weird I know but I like my boats dialed in for long runs. Boat, motor ,fuel and gear only weigh 850 pounds. I am with you that the motor should not be bogged. I'm hoping to run it at 4600-4700 rpm but at a higher cruise speed. The final straw in the decision was based on how it ran at elevation of 3500' on lake Powell. It over reved a little bit. My next 100-200 mile trip will be to see as much of lake mead at 1300' in much colder air as I can over thanksgiving. More air and colder means more power. The 19p may be just right. Thinking like you and considering the Arizona Summer's I also boat in, I'm betting I own a fresh 17 for it by then as well.

 

[ahicks]

That 200 mile trip sounds like the perfect time/place to pick your favorite if you have the 17 and 19 with you. For that matter, you could learn the attributes of both, and mount up the one on that best suits the mission you'll be on for any particular day! Truth be known though, I'll bet you settle on one as a clear favorite for overall use, and my next bet is that it will be the 17"....

 

[Stumpalump]

No Title

18' Valco Bayrunner with Evinrude 48 SPL. Full fuel, cooler, dog, anchor and regular gear.

With the 19 pitch SST prop it ran 34.4 mph @5500RPM. 95 degree air temp, 70 degree water, 1700' elevation. It stayed in the 34 range except in ruff water where it ran 32-34. It bounced 35 a few times at 5600. Time to fast plane in 8.5 seconds but it jumps right up. Cruise RPM dropped by 500 to 4100RPM at 24.5 mph. I can run it a little faster to 27 plus before it starts sounding like it's working so my cruise is just where I needed it. It's super quiet on plane now because it's not screaming. I'm super stoked! I gained all sorts of mid range drivability and it does not ventilate on take off if I forget to trim down. 35 mph in an 18' with a stock 48hp. Aluminum boats rule! Fwiw I bumped timing up and down a degree with no worthwhile change. 89 octane Pennzoil TCW 3 @ 45:1 ratio. Does the slip look right or could the tach be off a little?

 

[jimmbo]

You sure you have the gear ratio correct? I know some brochures give the ratio as 12:29, I'm very certain the 44 inch 2 cylinder loopers were 12:32, the 3 cylinder loopers were 12:29., and I think someone got lazy/sloppy in the proofreading.

 

[Stumpalump]

No Title

You sure you have the gear ratio correct? I know some brochures give the ratio as 12:29, I'm very certain the 44 inch 2 cylinder loopers were 12:32, the 3 cylinder loopers were 12:29., and I think someone got lazy/sloppy in the proofreading.

No I'm not sure and the web lists both numbers depending where you look. I just counted teeth on used eBay gear sets and find both but they may use any old photo. Is there a way to tell? I could spin the flywheel but that's not easy when the ratio is so close. Maybe the chart tells the answer because when I run 12.32 I get this:

 

[jimmbo]

What is the actual Model # of your engine?

 

[Stumpalump]

No Title

What is the actual Model # of your engine?

E48ESLESA 1990 48hp long shaft.
Parts break down show 12:29 so I guess it is 2:42. 16% seems like a lot of slip?
http://www.iboats.com/Gearcase/dm/cart_id.289825835--session_id.558291345--view_id.1551259

 

[Texasmark]

The motor can run all day at 4,600 RPMs.

The important part is making sure the 19p is still allowing it to reach the upper end of the RPM range. Comparing different props, especially made from different materials is just a guess at best.

50 years + of boating has taught me that 2 strokers like to run. If you want to lug get an undersquare 4 stroke diesel.

 

[QBhoy]

Jeez...you know, what ever the slip and all that is...you aren't doing half bad in terms of speed for the engine and boat eh ?
I'd think that your slip isn't too bad for the set up...if it's what I think it is.

 

[Stumpalump]

Jeez...you know, what ever the slip and all that is...you aren't doing half bad in terms of speed for the engine and boat eh ?
I'd think that your slip isn't too bad for the set up...if it's what I think it is.

The only concern I have with that much slip is that the tach may be off. If I crunch numbers with a more moderate slip number I'm still OK with the revs but like others in the thread I like a boat to spin up. That keeps the mid range really responsive and they sip fuel at cruise. Next time my buddy is in town I'm going to borrow a tach calibrator. Using a Fluke meter to do it does not work all that well because jumps around too much.