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Old 02-21-2008, 07:07 PM   #1
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Propellers

In the event this information might prove helpful or interesting*to someone, here is what we learned this week at Kruger & Sons marine propeller shop in Seattle where we took our two 22" diameter, 18" pitch (supposedly)*props in anticipation of having to buy two brand new props.* Kruger & Sons had been recommended to us by several people in the local marine industry as the best prop shop in Seattle if not the Puget Sound area.

Two years ago a different prop shop in Seattle had checked our props and said they were worn out and needed to be replaced.* After conferring with the yard we use, it was decided we could get another year or two out of them, at which point we would buy and install new props.

Kruger performed an initial visual and sound inspection of the four-bladed props*when we brought them in on Monday and said they were definitely*not worn out. They were pitched too coarse for our engines and*they could see*at least one slightly bent blade.* We've never hit anything that we know of so it may have been this way when we bought the boat.

They said they would repitch the blades from 18" to 16" as well as true up the blades, balance the props, etc.* This represented a major saving in dollars for us, which is good since the price of diesel will continue to climb.

They confirmed what I had been told---*fewer blades are more efficient than more blades, more blades generate less vibration than fewer blades.

They also said that, contrary to what I had been told, a three-bladed prop will generate more prop walk at low rpm than a four-bladed prop. I have been impressed with the responsiveness of our current props in kicking the stern around when maneuvering to a dock. So I guess three-bladed props would be even more responsive in this respect but we're not going to spend the money for new props just to gain this advantage.

Kruger & Sons carries two makes of new props in the size range for boats like ours. There are "economy" props from Taiwan or China*and "quality" props from somewhere else. As a point of reference, 22-16 props for our boat would be about $800 each for the "economy" prop and about $1,400 each for the "quality" prop. They did not say if our current props were from the same manufacturer who makes their "quality" props, but they did say that our current props are from a*high-quality manufacturer.

I asked if the finer 16" pitch would mean that we'd have to run at a significantly higher rpm to achieve the same boat speed. They said we would have to run at a slightly higher rpm BUT... the engines will not be as loaded down so we'll get the same boat speed at a lower fuel consumption. They compared the props*as they were currently set up*to running a car up a hill in too-high a gear.

I got a call today that the props are ready for pickup.* They said that whoever set them up before had*done a terrible job (their words).* Some of the individual blades were slightliy different in size than others on the same prop.* One prop was pitched an average of 18", the other an average of 17."* (There could have been a*legitimate reason for this, but the important point was that none of the blades on any one prop*were exactly matched in pitch.)* Some of the blades were bent slightly, not from striking something but from being improperly set up.* The hub of one prop was even out of true.

The blades are now all sized the same, they are all pitched the same, there are no more bends or "wows" in the blades, the bad hub has been trued up, and they have been computer scanned and balanced.

Since we are putting the same props back on the boat instead of a new set, it will be very interesting to see how they compare in terms of smoothness, performance, etc.* I'm curious to see if any of the*minor vibrations that*we simply chalked up to the boat being three decades old are no longer with us.

These props were on the boat when we bought it ten years ago, so we have no idea when they were installed.* Nor do we know who*set them up, other than it would have been someone in the San Francisco Bay area since the boat spent its whole life there until we bought it.

Anyway, if anyone in the Seattle area has questions about props, I would sure recommend at least including Kruger & Sons on their list of prop shops to talk to.
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Old 02-21-2008, 07:50 PM   #2
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RE: Propellers

I've done the computer scanned props and the difference is spectacular. You will be very pleased with the smooth ride.

Ken Buck
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Old 02-21-2008, 08:44 PM   #3
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RE: Propellers

Ditto to what Ken said.

I had my props re-tuned and computer balanced/checked two summer ago and was amazed at the increased smoothness, efficiency, and performance. You should be very happy with the results.

Gary

-- Edited by gns at 21:45, 2008-02-21

-- Edited by gns at 21:45, 2008-02-21
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Old 02-22-2008, 03:14 AM   #4
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RE: Propellers

They compared the props as they were currently set up to running a car up a hill in too-high a gear.

This is only true IF you are lugging the engine , running within 300 rpm of what ever you will get at WOT.

Higher pitch and NOT lugging will reduce rpm, noise and usually load the engine more efficiently .

The first 1000G used will tell.

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Old 02-22-2008, 08:11 AM   #5
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RE: Propellers

Ken,

Who did you have work on your props?* I'm hauling at Suldan's next week and taking the prop to Precision Propeller in Bremerton.

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Old 02-22-2008, 08:53 AM   #6
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RE: Propellers

Tell Jon Hi for me. That's the place! Good reminder, I have a 4 blade prop there to pick up. Now that the tides are at the right time I need to go on the grid and change to my new 4 blade. I'm all about smooooth.

FWIW Tacoma Propeller, the other close choice are cavemen beating props with rocks, IMHO. If by chance you need shaft work, ask Mark or Greg to suggest somewhere else.


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Old 02-23-2008, 10:29 AM   #7
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RE: Propellers

I'll see Jon on Thurs. and deliver your msg.* Before comitting with Jon I spent some quality time with*him last week.* It's easy to understand why you're recommending him.*

Thanks for the advice on shaft work.* I'll keep it in mind but hopefully I won't need to pull the shaft.* This is the first haul out since the pre-purchase survey so I can only hope there are no ugly surprises.

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Old 02-23-2008, 07:07 PM   #8
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RE: Propellers

I'm sure things will go well. The Suldan boys have been in the boat business together since they were kids. Their Dad Byron started the business and they took over when he retired. For those who don't know them, they are all in their 50's now. Assuming they started scraping bottoms when they were 12, that gives them a "lifetime" score of 40 years, each. If I remember correctly D'Antonios 20 years were considered a lifetime previously.

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Old 02-25-2008, 04:36 AM   #9
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RE: Propellers

Quote:
Marin wrote:

I asked if the finer 16" pitch would mean that we'd have to run at a significantly higher rpm to achieve the same boat speed. They said we would have to run at a slightly higher rpm BUT... the engines will not be as loaded down so we'll get the same boat speed at a lower fuel consumption.
Marin, that's interesting to hear that reduced pitch results in*lower consumption for same boat speed, but does it not also result in reduced top speed?*I appreciate that is not often a major concern for trawler owners, but I'm curious nevertheless.

My new boat has a prop that is very much Chinese made! I'm getting the top speed that I was hoping for during construction, but might one day have an expert tell me just how appropriate my prop spec is for my boat.

Incidentally, my boat pops out onto a semi(?)-plane at around 2700 RPM (max 3000 rpm), so if I lost a couple of knots at the top end by de-pitching my prop, I'd never get onto the plane. Once again, this is academic, 'coz I don't plan to be operating at that kind of speed (too often!).

Mark
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Old 02-25-2008, 11:13 AM   #10
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RE: Propellers

I think Mark's assessment of the subject is right on the money. Now, Mark, if you will just give me your transmission ratio, diameter and pitch of your prop, I will change mine out. I have all the other data that I need about your boat to make my decision.

Walt
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Old 02-25-2008, 09:28 PM   #11
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RE: Propellers

I won't ever go back to Kruger. They did'nt do the work they said. As for Marin, it's a ballancing act between propeller efficency and engine efficency. I think Kruger is right. Marin is going to loose some propeller efficency and gain some engine efficency. The engine makes much more power at higher rpm and the propeller increases parasitic drag by pushing the same water with a lower pitch at a higher speed ( propeller speed ). It's been obvious for some time Marin's been over proped burning 3 gph at 1700. It will be interesting to see his new numbers when he gets running again. The biggest change should, however, be his top speed.

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Old 02-26-2008, 10:58 PM   #12
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RE: Propellers

Marin - what is the gear reduction on your boat? I am curious as I am having a hard time getting my mind around all the prop theory with regards to our boat.
In our case the gear reduction is 3:1 so with the engine's sweet spot at about 1500 RPM our prop is rotating at 500 RPM which I think is quite slow compared to other vessels. It is interesting that my experiments with throttle settings over long runs (12 to 14 straight thru) we get the best performance all round at 15 - 1600 RPM which gives 7 knots and about 1.5 GPH.
How does this compare with your set up although I know you have twins I would like to compare as well as maybe get some idea as to what other full displacement hull singles are getting.
Have a great week
John Tones "Penta" - 36' ex commercial fish boat
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Old 02-26-2008, 10:59 PM   #13
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RE: Propellers

Woops, I forgot to add that the prop is a 32X24 on a 1 3/4 shaft!
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Old 02-27-2008, 11:00 PM   #14
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RE: Propellers

Mark--

A finer pitch prop will generate less thrust at a given rpm than a coarser pitched prop at the same rpm. So yes, you are correct in that if you reduced your prop pitch you would not be generating as much thrust at any given engine rpm as you do now. Of course nothing is ever this simple, and there are other factors like turbulence, cavitation, waterflow into the prop, etc. that can effect the efficiency of a prop besides just pitch. But looking at pitch alone, a finer pitch will require more rpm to achieve a specific amount of thrust than a coarser pitch.

However---- a finer pitch prop may allow your engine to develop more horsepower than a coarser pitch prop because the engine can turn faster (at least up to its governed speed). This is why many prop-driven aircraft have variable pitch propellers. For takeoff the pitch is set at its finest pitch so the engine can spool up to give maximum horsepower. In cruise the propeller can be set to a coarser pitch so it will move more air at a lower rpm setting for better cruise efficiency.

John---

I'd need to look at the actual plates on the transmissions, but the typical reduction on BW Velvet Drives is (I'm going from memory here so it may not be exactly right) 1.9-something on the port side and 2.1-something on the starboard side. Or it may be the other way round. In any event, the difference is due to the extra gear in the starboard side to counter-rotate its prop. We'll be back up at the boat this weekend and I'll try to remember to look.

Speaking of propellers, here are a couple of photos I took today in the yard where our boat is being worked on. This is not a new boat--- it's 16 years old--- but the owners keep it in immaculate condition. The yard manager told me today that the insides of the fish holds are all polished stainless. The thing under the prop is called a "beaver tail," and its purpose, according to the yard manager, is to keep nets and stuff out of the prop and also act as an anti-cavitation plate, similar in function although perhaps not as effective as a Kort nozzle. In any event, it increases the efficiency of the prop. This boat is apparently worked very hard and very successfully in a variety of fishing modes, from crabbing to longlining to net fishing.

-- Edited by Marin at 00:04, 2008-02-28
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Old 02-28-2008, 04:14 AM   #15
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RE: Propellers

Wow Marin, That thing looks like it has a barn door for a rudder! Out of curosity what is the approximate size of the boat and what kind of engine?
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Old 02-28-2008, 11:14 AM   #16
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RE: Propellers

I have*read in National Fisherman that Cummins engines are very popular in the Petersburg fishing fleet but I don't know if that's what Spicy Lady has.

*According to some information I found on a big*"historic fishing" photo site on the internet, Spicy Lady was built in 1992.* I don't know if she is aluminum or steel.*She is used for purse seining as well as other types of fishing so my guess is that her official length is 58 feet. This length limit gave rise to the term "limit seiner" for purse seiners working in Alaska. Here is a photo of her underway.

Spicy Lady is pretty typical of the multi-purpose fishing boats that work in Alaska.* The Seaview North yard where I took my shots has three or four of them in the yard right now.* I was attracted to Spicy Lady because of her immaculate condition, the huge bow bulb (the first time I've seen one on a boat of this type-- it could have been added) and the beaver tail under the prop.* The bare aluminum structure aft of the pilothouse is removeable.* I believe this is the setup that's used for long-lining, although I could be wrong.* As you can see from the attached photo, it is not used when the boat is rigged for purse seining.

-- Edited by Marin at 12:49, 2008-02-28
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Old 02-29-2008, 03:19 AM   #17
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RE: Propellers

Quote:
SeaHorse II wrote:

Now, Mark, if you will just give me your transmission ratio, diameter and pitch of your prop, I will change mine out.
Walt, my gearbox ratio is 1.96 to 1, like yours. My 4-blade Chinese prop is marked "650x600mm", but apparently the builders*sent the prop back to manufacturer for adjustment*before my trials, because the actual measurements don't match.

I'm embarassed to say that the actual diameter as measured by the builders and by my surveyor are way different (560mm versus 508mm) so, until I next take her out of the water, I won't be able to tell you for sure.

Thanks Marin. All very interesting.

Mark
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Old 09-15-2010, 05:56 PM   #18
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RE: Propellers

If I could ressurect this topic, I have a few questions for Marin.*

I have some noticable vibration at cruising speed (9 knots approx).** A quick dip under the boat didn't show anything in particular with either propeller.* When I had the boat surveyed in March, the starboard cutlass bearing was very worn and was replaced...the other was OK.* My best guess is that the vibration is prop related and I THINK it's getting worse, which leads me to wonder if the starboard prop is out of whack and, maybe, prematurely degrading the bearing.* It's not a horrible vibration, yet, but I'm concerned.

FYI...it's a 42' Cheer Man trawler with a pair of 28" Michigan props.* I don't know the pitch.* Twin John Deere 6068Ts running about 1,400 rpm at 9 knots.* The survey reported a 2Ĺ reduction.

So the questions...(for Marin and others)

Did you have a vibration issue or did you pull the props for other reasons?
Did Kruger do a good job and were you happy with the end results?*
Out of curiosity, how did you remove the props.* I don't really see myself trying to do this with the boat in the water.* I haven't used Scuba for 20+ years and, frankly, my snorkel under the boat was at the edge of my comfort level.*
Presumably, to have everything checked (props/shafts/bearins) properly will require a haul out(?).
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Old 09-15-2010, 07:00 PM   #19
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RE: Propellers

1. We did not have a vibration, at least not from the running gear. The boat is 37 years old and has plenty of sympathetic and harmonic vibrations but they are not caused by running gear issues. We pulled the props because we were told two years earlier that they were "worn out." They weren't but they needed a complete repitching and balancing.

2. We were very happy with the job Kruger did and so were the people who referred them to us.

3. We didn't remove the props, the yard did when the boat was out of the water. On a GB the props are held on by a couple of big nuts and a pin. I suspect it's a pretty standard arrangement. The ends of the shafts are tapered so they probalby used some sort of tool--- prop puller?--- to get the props off the taper.

4. At the same time we had one shaft replaced and one trued. This was mainly because we were switching to a much more substantial type of shaft coupler than the stock ones and one shaft was undersized a bit.

Vibration can come from a lot of things. Shaft/engine alignment is one. Shaft trueness (is that a word?) is another. An out of balance prop is another. A prop with a bunch of barnacles on it is another (we just had that happen the other month).
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Old 09-15-2010, 08:01 PM   #20
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RE: Propellers

Thanks Marin,

I might talk to Kruger and see if I can get (pay for) one of their guys on board for a brief run and a second opinion before I go any further. It may be nothing more than increased sensitivity on my part as I get used to the boat.
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