OPINIONS WANTED: prop size

[KAIROS]

I installed a 2YM15 recently, with the 3.2 to 1 reduction gear. In calm water, in gear, my full-throttle maximum rpm is 2900, which is 76% of the no-load maximum (intermittent, not continuous) rpm for this motor (3830 on my tach).

"John" the diesel mechanic tells me that I should get a maximum rpm of about 3450 under load, under way, which is about 90% of the no-load max of 3830. He says I need to get my prop re-pitched so that my engine is turning about 550 rpm faster at full-throttle (3450 - 2900 = 550).

When the prop is adjusted, John says I should cruise at about 2800 rpm which is about 80% of 3450. He says until my prop is fixed, I should cruise at 2300 rpm which is about 80% of my current maximum loaded rpm of 2900.

QUESTIONS:

1. Does John know what he is talking about? If not, what are reasonable numbers?

2. At full throttle now, I can run for hours without overheating. And, at full throttle now (2900 rpm) I attain more than theoretical hull speed. Is the engine really working too hard if it is not overheating?

3. John mentioned that until I get my prop modified, I should cruise at about 80% of my current max. rpm of 2900, which is 2300. Given my existing prop, if the engine is happy at 2300 then why modify the prop so it turns faster to attain the same speed?

THANKS!

Notes: My sailboat is 28' and 9500 lbs, with a full-length keel. Hull is clean. Prop is a 14x11 3-blade "Campbell Sailor". I sized the prop with the prop calculator at boatdiesel.com and with the manufacturer's input.

[willmarsh3]

I should start out saying that I'm not a professional diesel mechanic but I have worked with them enough (taking care of one and installing one in a boat) to have some basic knowledge.


Is this your engine: http://willyfogg.com/p5499418/2ym15_...diesel_engine/

the 14hp at 3600 rpm version?

I have a Westerbeke 30 B Three pushing my 32 foot boat with a 14x8 three blade Campbell sailor. It uses a 2.5 to 1 PRM 80 transmission so it turns the prop a bit faster. The boat is between 12000 and 17000 lbs (another story). I'm going to guess 13000. At full throttle it will go 6 knots which is not quite hull speed of 6.9 knots. The prop manufacturer selected this one based on what I told them.

As for your questions:

#1 I would say yes. That is what I have seen over and over in my research on engine installation.

#2 I would say you are working the engine too hard but I would like to hear what others say too. My guess is that you'd get fewer hours before a major repair or overhaul.

#3 sounds like a good idea.

[paladin]

I just ran some numbers....
Assuming your boat to be 26 feet on the Waterline....
I read the engine as 14 hp at 3600 rpm.....and you didn't say 2 or 3 blade prop.....I went with 2 blades.......
BUT! Using your figure of 2900 rpm.....gives you a shaft rpm of 906 with a slip ratio of .51.....
Your speed would be 5.8kt's and the best prop works to 17.5 inch dia. with a 17.84 pitch which will use 12 hp from the engine.....use this as you will...

[KAIROS]

Will,

That is my motor. On my calibrated tach the max no-load rpm is 3830. The continuous rated rpm is actually 3489 according to the manual.

[KAIROS]

Paladin,

Prop is a 14x11 3-blade "Campbell Sailor". My waterline length is 22 ft.

[Breakaway]

Kairos

Your mech is on target. A good rule of thumb for any engine is that when properly propped, "normally" loaded boat, it should turn within ten percent of its rated max rpm. Doing so ensure the best efficiency and longevity. An engine that is overpropped like yuors seems to work fine, but its actually "lugging" working harded than it has to to get the BOAT SPEED you want. You do not need to use the extra ENGINE SPEED. But propping for correct WOT ensures best perfromance throughout the power band. (throttle setting).

Re-pitching is one answer. As a rule of thumb, every inch of pitch is equal to about 200 RPM. So for your application, you need a prop with 2-3 inch less pitch. Find a good yard or prop shop to work with.

Kevin

[KAIROS]

...Re-pitching is one answer. As a rule of thumb, every inch of pitch is equal to about 200 RPM. So for your application, you need a prop with 2-3 inch less pitch...

Kevin

Thanks Kevin.

Can I (just as easily) have the diameter reduced instead of repitching?

The 14" diameter is at least close to too much based on the size of the aperture. I feel it may actually be too much in my installation because the rudder/tiller vibrate considerably underway. As rpm increases, I have to fight the rudder's and boat's tendency to swing to port too. At full throttle keeping the tiller amidships eventually causes a cramped muscle (and causes drag).

Jack

[paladin]

Using your new numbers and trying to calculate for a 13 inch 3 blade ain't cutting it....
I come up with 5.39 Kt but the recommended prop diameter wants to be 16.5 inches with a 15.8 pitch....you need a bigger hole to work with.

[KAIROS]

...recommended prop diameter wants to be 16.5 inches with a 15.8 pitch....you need a bigger hole to work with.

But, wouldn't the engine lug more swinging an even bigger prop?

[willmarsh3]

It won't lug the engine if it is pitched less. Plus it will be more efficient than a smaller prop. The downside is that tip clearance with the aperture will be less which can increase vibration. On a wooden boat it can clear the caulking out of seams.

Boatdiesel.com has a good prop calculator.

[KAIROS]

It won't lug the engine if it is pitched less...

Will, The post I was responding to suggested a bigger diameter AND more pitch. My prop is 14x11. Paladin's calculator suggests 16.5 x 15.8.

I am at the limit now, at a 14 inch diameter. I said above....

"Thanks Kevin. Can I (just as easily) have the diameter reduced instead of repitching? The 14" diameter is at least close to too much based on the size of the aperture. I feel it may actually be too much in my installation because the rudder/tiller vibrate considerably underway. As rpm increases, I have to fight the rudder's and boat's tendency to swing to port too. At full throttle keeping the tiller amidships eventually causes a cramped muscle (and causes drag)."

[Breakaway]

Kairos

Due respect to Mr Paladin--I've lurked here for a few years and have read many of his astute posts. However, I disagree. Larger dia and pitch will only see you losing more RPM.

That engine is avalailable with a higher gear--2.6:1 that could be the very expensive answer. Before I went that route, I'd consider another pretty good rule of thumb: an inch of diamter equals 2 inches of pitch. So if re-pitching doesnt get you where you need to be, you might try a smaller diameter wheel. Personally, I'd hope less pitch works, as larger diameter is better for thrust.

Find a good prop guy and work with him--cajole him to let you try a different wheel or two.

Thats my two cents.

Kevin

[peter radclyffe]

Thanks Kevin.

Can I (just as easily) have the diameter reduced instead of repitching?

The 14" diameter is at least close to too much based on the size of the aperture. I feel it may actually be too much in my installation because the rudder/tiller vibrate considerably underway. As rpm increases, I have to fight the rudder's and boat's tendency to swing to port too. At full throttle keeping the tiller amidships eventually causes a cramped muscle (and causes drag).

Jack

how about fitting a kort nozzle, an aerodynamically shaped tube around the prop, it should stop your boats tendency to go to one side as insted of the power being dissipated to the side, all the thrust is aft, like a grain auger
i havent got the whole thread but this may not work if your prop is partly in the rudder, as there may not be clearance

[willmarsh3]

Will, The post I was responding to suggested a bigger diameter AND more pitch. My prop is 14x11. Paladin's calculator suggests 16.5 x 15.8.

I am at the limit now, at a 14 inch diameter. I said above....

Sorry if my post was confusing. I see where you mentioned the 14" limit.

In Dave Gerr's "The Propeller Handbook" he suggests the use of rake or skew to offer increased tip clearance and reduce vibration for the same effective diameter and pitch.

I agree with others to go talk to a prop shop.

Good luck.

[brad9798]

Maybe you need a valve job ... or rings ... or head gasket!

[KAIROS]

The Wind-Up: The Campbell sailor prop is not typical (http://www.westbynorth.com/CampbellSailer.aspx). The only prop guys on the island smirk at it.

So, when I next haul the boat, I'm sending the prop back to the manufacturer. They're going to get me the additional 500 rpm I need by both reducing the diameter 1" (will also cure vibration, maybe), and reducing the cupping.

Here it is installed:

[wizbang 13]

Can you borrow a 14" by14" regular wheel? Can you sharpen and fair the arpeture (back and front)? Ultimate would be to modernize the shaft tube ,move the cutlass bearing inside the deadwood,then you could break out the sawzall and open it up, move the wheel 6" fwd and carry a 15" dia. This way, more of the "shark bite" would be in the deadwood,(preferred) and you could close up part of the rudder hole.The vibration is probably from too little clearance and that chunky hunk being hit by quickwater, but the pulling to the left is something elce.I live in Gig Harbor and would lend you a wheel, but my boat is in the BVI(with 3 spare props) I hate to see you go even smaller with the dia.What you have now with the low rpm is like having a high gear in your car. In calm wind (flat road) it's fine,but when you start bucking wind and waves (going uphill) the engine will lug, blow smoke and loose power. Picking up rpm is like downshifting .

[KAIROS]

....I hate to see you go even smaller with the dia....

I'd prefer not to reduce the diameter, but we can now drive the boat above hull speed with a 3.22/1 reduction gear. I'm alright with loosing a bit of power if I don't have to sawsall the boat. I really don't like cutting away parts...the boat cringes if she sees a sawsall.

...Picking up rpm is like downshifting...

That's why we're reducing the prop size and cup/pitch, to get the rpm up to spec at full throttle.

Thanks.

[John B]

Kairos , giving away diameter is not good. Diameter is what gives you traction in a seaway.. we've all seen those boats sitting in a chop just hobby horsing rather than punching through... undersize props just slipping rather than biting.
If it wasn't for the vibration and the hard helm , I'd say just live with it and don't stress it up . Its easy to tell when you are and lower revs makes for a more pleasant experience. However if the helm is that hard then there's a significant paddlewheel effect going on and on that basis I tend to agree with the others , de pitch the thing. Honestly , you'll really regret chopping diameter.

[KAIROS]

....I tend to agree with the others , de pitch the thing. Honestly , you'll really regret chopping diameter.

I'm not sure why it matters how the prop in 'de-powered'. In the end, the engine turns faster because the prop does less work. I do see how a larger diameter is more efficient, expecially with inches-width of horn timber up stream. But, isn't my prop just too efficient now?

[John B]

No, its just pitched too much.

[KAIROS]

No, its just pitched too much.

please explain why the problem is, specifically, that it is pitched too much. Why is pitch the key, and not the diameter or the cupping or anything else that would make my rpm lower.

Why is pitch the problem?

[John B]

Pitch is like gearing kairos. Every inch of pitch is an inch further travelled on each revolution, so if you're overpitched its like lugging up a hill in top instead of shifting down a gear.
Diameter gets you traction or bite. Props slip and the grail is to limit that slip in percentage terms, Its not something you give away lightly . Again to use an automotive analogy.. Its wheelspin. It'll work alright until the load comes on but as the resistance rises , the less effective it'll be.

[KAIROS]

Pitch is like gearing kairos. Every inch of pitch is an inch further travelled on each revolution, so if you're overpitched its like lugging up a hill in top instead of shifting down a gear.
Diameter gets you traction or bite. Props slip and the grail is to limit that slip in percentage terms, Its not something you give away lightly . Again to use an automotive analogy.. Its wheelspin. It'll work alright until the load comes on but as the resistance rises , the less effective it'll be.

Helpful analogies.

[Hwyl]

My gut feeling screams "leave it alone"

[KAIROS]

My gut feeling screams "leave it alone"

Mine too. But, the engine is new and I'm caught up in trying to make it happiest. Most motorheads say it should turn over 500 or more times a minute faster than it can now under load at full throttle. Otherwise the poor little 14 hp Yanmar will work too hard and die young.

[Candyfloss]

Even if we do leave it alone, your description of her handling characteristics makes for pretty uncomfortable reading. Surely we could address that? Is this just a propeller-in-aperture phenomenon? Could some simple fairing improve matters?

[KAIROS]

Even if we do leave it alone, your description of her handling characteristics makes for pretty uncomfortable reading. Surely we could address that? Is this just a propeller-in-aperture phenomenon? Could some simple fairing improve matters?

YES. I am coming around to this idea...even before you said it, I swear. The rudder was repaired and reinforced just downstream of the prop. Several through boltheads and nuts are still protruding from each side of the rudder. I planned on fairing over it with epoxy/microballoons (easy to crack off if we need to), but didn't get to it.

The new prop was used only after the rudder repair, so we don't know how it would have worked out before. I thought the unfair rudder might be partly responsible for the vibration, but did not connect it with the reduced RPM. Now I'm thinking......just fair the rudder and see what that does.

Could several 3/8 bolt and nut heads on the rudder, 12 inches or so DOWNSTREAM from the prop, reduce max. rpm?

[John B]

No I don't think so, but they could have something to do with the vibration. Plus of course you have that hard helm thing going on. That, as I said before , is an indicator of too much pitch ( causing excessive paddlewheel effect) or.. there's an idiosyncracy of that particular prop or prop design ( to do with the cupping)

I nearly agree with Hwyl about leaving it and that was my first thought ...but the helm issue( which was first noticed after fitting that prop right?) means to me it'd be worth taking to a prop guy and saying 'take some pitch out I need 300 or so extra rpm , and balance it while you're at it'. Which any good prop guy would automatically do anyway.

[kc8pql]

Could several 3/8 bolt and nut heads on the rudder, 12 inches or so DOWNSTREAM from the prop, reduce max. rpm?

Not very likely. You're just overproped. Campbell Sailors load up a lot more than a conventional prop because of the airfoil shape of the blades. I have one too, on a 25,000 lb boat, transom hung rudder, prop in aperture. I don't get any vibration at all but I need to have some pitch taken out. My max RPM is only 3000 on a 3600 RPM diesel. In other words, it's lugging and that's not good.

[KAIROS]

Thanks for the answers!

It is a fairly unique prop. That's probably why the boatdiesel.com prop calculator was so far off. The manufacturer did his own calculations and did agree with mine, though.

Sad to admit that I am glad to hear someone else overpropped their boat with a Campbell Sailor prop.

[kc8pql]

Thanks for the answers!

It is a fairly unique prop. That's probably why the boatdiesel.com prop calculator was so far off. The manufacturer did his own calculations and did agree with mine, though.

Sad to admit that I am glad to hear someone else overpropped their boat with a Campbell Sailor prop.

I used the manufacturers recommended size, which is as you say, a good bit less pitch than a conventional prop, but apparently it's still to much. On the other hand, it's not uncommon to have to screw around with prop size on any new engine install. None of the formulas can take into account all the possable variables.

[donald branscom]

What John did not tell you was that if you re-pitch your prop and get 550 rpm more, that you will have more torque and a cooler engine.

[Lew Barrett]

If you would really like to know how your engine is doing at best cruise you might also like to measure your exhaust gas temperature under load at standard cruising rpm. That will tell you with good precision how close to ideal the load on your engine is.

[MiddleAgesMan]

Lot's of good info to consider here. I would take a conservative approach: re-shape the aperture and rudder for better flow (which will likely eliminate vibration) and reduce the pitch, leaving the diameter alone*. Changing the pitch can be done and re-done and un-done but changing the diameter works in one direction only.

*This assumes your tip clearance follows the rules which I do not recall. From the picture, however, it appears you have enough.

[Breakaway]

Tip clearance should be 10-percent of diameter, IIR.

[Lew Barrett]

Re vibration: Assuming your engine and tranny are OK, have the wheel balanced. Check the shaft for true (run-out). Check alignment
in the water. Look at any bearing surfaces, and engine mounts and make sure they are up to snuff. Most of these checks are visual inspections by somebody who knows what they are looking at. The propellor needs to be spin balanced, you can't just look and decide unless the blades are very thin and worn, in which case it's time for a new wheel anyway. If you get re-pitched, then a balance job should automatically come along with that.

I know you realize this already, but your mechanic is spot in with good advice, and John B knows a thing or two about this stuff.

There just isn't much more to it than that. In respect to vibration, the wheel is the first thing I'd be suspicious of if everything else is good.

[KAIROS]

If you would really like to know how your engine is doing at best cruise you might also like to measure your exhaust gas temperature under load at standard cruising rpm. That will tell you with good precision how close to ideal the load on your engine is.

I read that somewhere. I thought of hooking up a real gauge (more than the idiot-light and alarm on the engine panel). Maybe I will. I am a sailor though (except when under power ), and it grieves me to spend my time out there keeping track of engine temperature too. I suppose trying to get some answers via this forum is a way around that, though not as sure.

In other boats, I sometimes would not use the engine at all for an entire season. Especially since the boats were on a mooring. I'd run the motor just to keep it alive. A power cruiser guy like you enjoys the motor part of it much more. It's an expensive, necessary (now) evil for me (to his face I address him as Mr. Yanmar and wait on him hand-and-foot).

[KAIROS]

Thanks All. Yes, lots of very helpful responses.

To clarify the vibration issue, I think it is really just the rudder that is vibrating. Sorry I did not make that clear. Everything else in the drive train and the engine mounts, etc. Is brand new, and in good order. The prop is new too, and was balanced when istalled. I did not hit anything with it, I assume. The vibration is from day 1 after we got the boat sailing again.

So, here's what I'll do, then use the boat after each (I really do believe in trying to hold variables constant to evaluate one change. In this restoration though, almost EVERYTHING changed at once).

1. Fair the rudder downstream of the prop. The strap for the pintle plus some other bolt/nut heads do protrude. These did not protrude when we had the old, minimal 2-blade prop, and we had no notable rudder vibration. However, when that 1-cylinder Volvo ran, with no flexible engine mounts, it would have been hard to tell. Most everything vibrated...even the masthead.

Also, have the prop pitch reduced so the engine can spin it more easily. Evaluate cupping and any asymetries.

2. If needed to further relieve rudder vibration and torquing/turning, reduce prop diameter and increase pitch to get about the same rpm.

3. Punt.

Photo below shows the strap and bolt/nut heads I need to fair over. The rudder is offcenter here, so the prop looks (too) close to it. The cupping is also obvious here...

[John B]

Perfect.

In the meantime ,just watch out for any signs of the motor loading Kairos. That would be temperature of course and any sign of unburnt fuel exiting the exhaust.. dark smoke or fuel residue on the surface of the water. Like most things its not when it happens occasionally( a bit of emergency power), but over prolonged use that causes the problems.
You know , an engine manufacturer has to make his product so that it performs properly over a wide spectrum of uses. So they won't sign off an installation for warranty purposes unless it meets their criteria. Making the load revs being one of them, correct backpressure (not too small a pipe) etc.
In other words, they have to specify to the lowest common denominator. Someone who recognises when an engine is loaded and doesn't over fuel it by driving it too hard with too much throttle for the conditions can operate an installation like yours without doing damage.

[KAIROS]

Perfect.

In the meantime ,just watch out for any signs of the motor loading Kairos. That would be temperature of course and any sign of unburnt fuel exiting the exhaust.. dark smoke or fuel residue on the surface of the water. Like most things its not when it happens occasionally( a bit of emergency power), but over prolonged use that causes the problems.
You know , an engine manufacturer has to make his product so that it performs properly over a wide spectrum of uses. So they won't sign off an installation for warranty purposes unless it meets their criteria. Making the load revs being one of them, correct backpressure (not too small a pipe) etc.
In other words, they have to specify to the lowest common denominator. Someone who recognises when an engine is loaded and doesn't over fuel it by driving it too hard with too much throttle for the conditions can operate an installation like yours without doing damage.

Thanks John. That sounded just like my doctor last month.

Now I can actually go do something else, like raise the new mainsail to see how they did re-cutting it (does every damned thing need to be endlessly tweeked? Stupid question).

[KAIROS]

A few months later......

Hauled out today. As I said above I will first fair the rudder surface downstream of the prop to see if that is a source of the rudder vibration and torquing to one side at higher rpm. If that does not produce results, next year I'll reduce the prop diameter somewhat.

If I'm forced to reduce the diameter because of clearance, that would allow the motor to spin the prop faster, and may take care of the rpm issue (can't get to rated operating rpm). The prop clearance at top and bottom of the aperture is 3/4" (14" prop). The part of the rudder I will fair is around that pintle-strap reinforcement at right. I'll make it slippery but repairable.

[MiddleAgesMan]

Tip clearance of 3/4 inch is about half of the recommended 10 percent (of diameter). If the prop is 14 inches tip clearance should be 1.4 inches if my mind is working right.

That may be most of the problem right there.

[KAIROS]

Tip clearance of 3/4 inch is about half of the recommended 10 percent (of diameter). If the prop is 14 inches tip clearance should be 1.4 inches if my mind is working right.

That may be most of the problem right there.

That's what I thought originally. Above, months ago, forumites convinced me to experiment some before 'sacrificing' the diameter. So, I'm going to fair my rudder this year.

[paladin]

Breakaway...I was really recommending nothing, but simply recalculating using the constantly changing information that was being supplied.
Without ALL the data, one cannot make reasonable assumptions, I merely was trying to show how the working parameters changed as say, an inch was removed from the diameter or pitch from the prop and what would be affected.

[boattruck]

Kiaros, Have skanned the thread and will say this, I agree with your mechanics recommendations, those are the pretty standard rules of thumb, but I very much doubt the engine will suffer noticably in the short term. It sure should be on your shorter list of to-do's though.
There may possibly be a grain of truth to your local prop shops opinion of this type or brand of prop, I have seen clients buy the newest most revolutionary widgit, and suffer all sorts of grief while the manufacturer of the widgit in question will blame every thing up to sunspots for the ensueing troubles, trying to shift blame away from their product, mind you, I have no knowledge positive or negative about this particular prop, but if I had to send my prop through the mail to get it trued, pitched and balanced, it would raise a flag in my mind.
The clearance issue also needs to be addressed, perhaps you should be working on an inch smaller prop solution, I have no unreasonable fear of the sawsall, but from the pic's I dont think you can gain the clearance you seek going that route.
Just out of morbid curiosity, when you throw the whole story, with all the data and pics you have, as well as the prop you are now using, what does your local prop shop recommend and how much would it cost to try it?
Sorry, more questions than answers, but remember this is all part of the process of repowering, and when it is done it will work basicly unchanged for many years, Cheers, BT

[donald branscom]

If it was me I would listen to the experienced marine mechanic.

[KAIROS]

I had wanted to experiment a bit doing incremental changes to boat and prop, but I decided to just go with the advice of the prop manufacturer (one very likable old guy in Nanaimo BC). He designed and manufactures these Campbell Sailor props.

He's going to reduce the diameter 1" total and reduce the pitch by one, to get me my additional 550 rpm underway, full throttle, calm water. The diameter reduction will alleviate some of the vibration transmitted up the rudder to my hand. I also faired over a pintle strap and some bolt heads/nuts just behind the prop on the rudder.

I just wanted one more issue crossed off the list! However it is when it goes back on, that's how it will be. I'll let you know. I might miss some of the power, but it wasn't right for the boat or motor the way it was.

Thanks all.

Here's an MIT study result of the drag of these props:



And here it is as originally installed:



Here is the rudder fairing over those knarly bits......prop is off getting tuned up (or down).

[boattruck]

Kairos, Sounds like a good working plan, let us know how the prop modifications effect your rpm and the vibration situation, Best, BT

[MiddleAgesMan]

I think you're on the right track. Inadequate tip clearance causes two things as far as I know, but neither is serious in the short term: It will cause vibration and it will wear away any nearby coatings at an accelerated rate. Longer term it can wear away the stuff your boat/rudder is made out of.