prop pitch on 2011 R21 EC

[fran uteg]

I have been following the factory notices and in one PDF concerning props the spec for a R21EC with a Yanmar 30 is an Acme 224, 13X15.5 cupped to .90. I ordered a 224 for a spare and today I pulled the prop that came on the tug (2011 R21 from Wefings) and the prop on the boat is an Acme 650 13X12.5 cupped to .60. Was there a reason that the 2011s came with a lower pitched prop? I am going to run the 224 and use the 650 as a spare but I was just wondering ....... Fran Uteg :?:

 

[SGIDAVE]

I have been following the factory notices and in one PDF concerning props the spec for a R21EC with a Yanmar 30 is an Acme 224, 13X15.5 cupped to .90. I ordered a 224 for a spare and today I pulled the prop that came on the tug (2011 R21 from Wefings) and the prop on the boat is an Acme 650 13X12.5 cupped to .60. Was there a reason that the 2011s came with a lower pitched prop? I am going to run the 224 and use the 650 as a spare but I was just wondering ....... Fran Uteg :?:

Hello Fran,

Marc may chime in with more info on your boat. I had the SAME situation...purchased a 21EC from Wefings...thought about a spare prop...and ALMOST ordered a more aggressive prop than the Acme 224 since my boat was getting 3750-3800 WOT RPM. Before ordering anything I pulled the nut off the existing prop and to my surprise it was an ACME 650 just as you describe above. In my case, I was able to acquire the 224 from Marc which is now my standard prop and the 650 is the spare (or high altitude) prop.

The 224 is definately TOO MUCH prop for the 21EC; WOT rpm usually 3200-3300 range. And the 650 is NOT ENOUGH (meaning it needs more pitch, cup, or dia). Fuel economy is better with the 224 than the 650. My feeling is that a prop a little less aggressive than the 224 but MORE than the 650 would be the "proper" prop for the 21EC based upon my experience and the rpm guidelines for WOT from Yanmar. I've talked to a tech at ACME factory and he gives rules of thumb such as (CAUTION - I'm making up these figures for illustrative purposes) +/- 50 rpm for every 30 thousands of CUP or +/- 100 rpm for every 2 inches of pitch. In my experience these these rules DON'T work. He DID tell me that the ACME props are easily "re-cupped" but not to let a prop shop mess with the pitch. I've not had either of my props adjusted by any prop shop. Taking out he cup from the 224 would be a good start; OR, adding more cup to the 650...ahhh, SO many prop options and so little $time$...

Merry Christmas to all!

/david

 

[rt11002003]

Isn't the "rated rpm" on the 30hp engine in the R21EC 3,400 rpm? If so, most recommendations are to have a prop that pulls the 3,400 rpm at the foremost power level position, ie. WOT, when the boat is fully loaded. Some folks also suggest starting out at 3,500 rpm to allow for the additional weight the boat will likely gain. The primary objective is to have the engine operate most effectively. All that being said everyone has their own objectives. 😉

 

[SGIDAVE]

Isn't the "rated rpm" on the 30hp engine in the R21EC 3,400 rpm? If so, most recommendations are to have a prop that pulls the 3,400 rpm at the foremost power level position, ie. WOT, when the boat is fully loaded. Some folks also suggest starting out at 3,500 rpm to allow for the additional weight the boat will likely gain. The primary objective is to have the engine operate most effectively. All that being said everyone has their own objectives. 😉

Hello Gene,

It depends on what you mean by "rated rpm."

Looking at the Yanmar manual, the 3ym30 engine is rated at 3600 rpm for "maximum rated power." Page 11 of the manual states "It is recommended that new vessels be propped so the engines can operate at 100 to 200 rpm ABOVE the Maximum Rated output power engine speed (3700-3800) to allow for some added weight and hull resistance. The engine should be able to reach the Maximum Rated Power engine speed (3600) under full load at all times."

It is based on these statements that I say the Acme 224 is TOO MUCH prop for the 21EC since only 32-3300 rpm is attainable. For me, the 224 is better than the 650, so for now, I use the 224.

The boat runs good, I'm not complaining...I view this as mainly an 'academic' discussion.

Fair winds and fast props,

dave

 

[rt11002003]

Hi Dave,

I was working from memory. The "max rated rpm" I quoted (from memory) could have been from another Yanmar I owned in an Albin. Or, more likely, it was simply a poor memory. 🙁

At least I had the additional 100-200 rpm over max rated rpm correct. My Yanmar manual is on my R21EC, so I couldn't refer to it. Of course, I likely have it somewhere on my hard drive. But, I'm sometimes lazy. 😉

Also for the benefit of all who don't know; the "max rated rpm" is on the engine data plate.

My experience with the 30 hp Yanmar is that maximum rpm is 4,000 to 4,200 rpm. That's without any load, in neutral. My first R21EC's Yanmar would sometimes turn 4,200 rpm, most of the time it was 4,050. Weird governor, or tach? The Yanmar I have now barely turns 4,100. I wonder what the governor limit is.

gene

 

[fran uteg]

I'm kind of sorry I poked this snake but after a couple of calls I got my answer which is pretty much what Dave said. According to Andrew at Ranger, the original 21ECs came with a 224 but it was determined after a period of time and conversations with Yanmar that the 224 on a loaded boat was not allowing the Yanmar 30 to achieve the target RPM at WOT. Ranger then started equipping the later 21ECs with the 650 and the diesel is now achieving the desired WOT RPMs.
A second call to Acme provided a guideline for determining prop selection for the Yanmar 30 with a 2.62 marine gear. The formula is on the Acme website and if you input all the data for the R21 you would come up with a prop somewhere between the 650 and the 224, just as Dave said. For each inch of pitch added you lose 81 RPM, conversely for each inch of pitch removed you will gain 81 RPMs ....... for each inch of diameter the same thing happens +/- 81 RPM ....... for each .030 of cup added or deleted there will be a change of 41 RPMs. My solution is that I am going to run the 224 for light day cruising and when my wife wants to bring everything but the kitchen sink on a longer cruise she can put on the mask and snorkel and hang the 650. Knowing this, the duffels should be a lot lighter.

 

[SGIDAVE]

I'm kind of sorry I poked this snake but after a couple of calls I got my answer which is pretty much what Dave said. According to Andrew at Ranger, the original 21ECs came with a 224 but it was determined after a period of time and conversations with Yanmar that the 224 on a loaded boat was not allowing the Yanmar 30 to achieve the target RPM at WOT. Ranger then started equipping the later 21ECs with the 506 and the diesel is now achieving the desired WOT RPMs.
A second call to Acme provided a guideline for determining prop selection for the Yanmar 30 with a 2.62 marine gear. The formula is on the Acme website and if you input all the data for the R21 you would come up with a prop somewhere between the 506 and the 224, just as Dave said. For each inch of pitch added you lose 81 RPM, conversely for each inch of pitch removed you will gain 81 RPMs ....... for each inch of diameter the same thing happens +/- 81 RPM ....... for each .030 of cup added or deleted there will be a change of 41 RPMs. My solution is that I am going to run the 224 for light day cruising and when my wife wants to bring everything but the kitchen sink on a longer cruise she can put on the mask and snorkel and hang the 506. Knowing this, the duffels should be a lot lighter.

Hello Fran,

To obliterate confusion, please EDIT your post and change the prop "506" to the correct nomenclature "650."

Second, the loss/gain of rpms associated with more/less pitch or cup DON'T hold up with the 21EC.

For example, the difference between the 224 and 650 props is 3.375" pitch and 0.030" cup. By Acme's calcs this should mean the WOT rpm difference should be:

3.375" x 81 rpm/inch = (approx) 273 rpm

0.030" cup x 41 rpm/0.030 inch cup = 41 rpm

273 rpm + 41 rpm = 314 rpm as the calculated difference between the two props.

My experience is the 650 gives WOT 3700-3800 (which is the "correct" rpm per Yanmar) whilst the 224 gives WOT 3200-3300.

REALITY is the the difference is 400-500 rpm, which means the calculated guidelines are off by 25-60%. Error of that magnitude cannot be called a 'guideline.'

When you look at Acme's website, both the 224 and 650 props are in their skiboat/wakeboard boat line of props. There's NOTHING wrong with that, but I suspect those type of boat have driveshaft speeds considerably HIGHER than our diesels. So, in an gasoline inboard engine with a WOT of 4000-5000 and assuming similar gear reductions, the propeller's rpm is MUCH higher than in our application...the guidelines of +/-81 rpm/inch pitch may actually hold up.

But enough of the engineering chatter...remember the 21EC is a GREAT boat as noted by many other in the recent thread (viewtopic.php?f=2&t=5716) and since the engine is RARELY operated at WOT, I'm not inclined to be excessively concerned with my 32-3300 rpm situation.

dave