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.

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

Ken Buck

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

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.

FF

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.

Ed Bruette

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.


Ken Buck

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.

Ed

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.

Ken Buck

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.

Click to expand...

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

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

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.

Eric Henning
30 Willard
Thorne Bay AK

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
Sidney, BC

Woops, I forgot to add that the prop is a 32X24 on a 1 3/4 shaft!

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

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?

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

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.

Click to expand...

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

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 2½ 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(?).

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).

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.

You may be able to pinpoint the area and source of vibration yourself using this technique that was taught to me by a "experienced timer" and friend.
Take a "rocks" glass (4-6 oz?) made of real glass and fill half way with water. Then while underway place the glass on various areas of the hull, deck, etc. The action of the water in the glass will let you know what you are at the source, or the area most affected.
I have used this method to find and solve a couple of vibration issues in the past. Perhaps it may help you.
At least it's a great excuse to take a boat ride.
Hope it helps.

Tonic,Do you know anyone else that has a boat like yours? If so and if you can take a ride in it and see if it is feels alot like yours. All kinds of things can cause vibration. Loose rudder bearings, resonant parts of the boat like a bulkhead, the side of the cabin or an area of the bottom. The length of a propeller blade and the material it's made of, the length of the propeller shaft between bearings ect ect ect. Have you ever noticed the windows shaking on the Wash State ferries? That's resonance. Propeller shafts are troublesome. They need to be strait and aligned. Sorry about this but there's no perfectly straight and aligned shaft * ...but a good shaft will come close. I worked in a machine shop that built propeller shafting and the stock we got was never straight when we got it. The machinist must beat on the shaft w a big hammer just so after very skillful measuring w a dial gauge. It takes lots of experience and skill to straighten shafts. And if a shaft was beat on and measured all day one could probably still measure some crockedness or "runout". *So they never get them perfectly straight but they usually get them close enough so other vibrations on your boat will be much worse than the very slight crockedness of your shaft. That shaft is called a straight shaft and you would be happy with it. A slight runout on your shaft and a slight misalignment could cause a vibration that one can feel and vibration is not vibration in that one skipper may take a friend out on his boat to show him his vibration and the friend may say "what vibration". I don't care for Kruger and I think a boat mechanic would be a better person to take out on your boat. Don't expect instant results. Vibration problems are frequently illusive and take time to solve.

Hello Marin,
You said that you were happy with the prop work you had done a couple of years ago, so does that mean that it took care of any vibration? Could you tell a difference in anything? Speed, fuel consumption, handling? Also if you don't mind, I am wondering what the cost of this work was, other than the shaft work. I don't think I need this type of work but you never know what will happen.

Thanks,
Bob

Another cause of vibration is the prop riding on the key which throws the prop a bit off centre. It doesn't seat properly on the taper
The gear/shaft coupling can be mismachined also. Or a loose or maladjusted engine mount causing misalignement.

If the vibration has only shown up recently then the most likely causes are propellor fouling or damage.

After my repower, I repitched my props to take advantage of the 1/3 increase in hp. I had a spare set of props, so started with them. When I put them on the boat, the added pitch dropped my rpm at 8.3 knots cruise from 2750 to 2150 and I noticed a decrease in fuel consumption (contrary to what Kruger told Marin, but exactly what my Volvo mechanic predicted.) Top speed increased by only .5 knots, as the hull shape would require another doubling of hp to go much faster.
I took the other set of props in and had them re-pitched, and in the process, I was told about all the dings and bends in a set of props that had given me no trouble and had no vibration issues.
The boat needs ot be out to pull the props. Youo may need a prop puller, so have one on hand. One of my props came off easily, one shot with a small sledge hammer after the nuts were removed. The other needed a puller and the application of a lot of torque on the wrenches before it "popped".
One of my sets of props weighs 1.5 times the other. maybe the "Chinese" v "better" difference. the both seem to drive the boat effectively.
Presuming you are in Seattle, I don't know the prop outfits. In Vancouver, Osborne is the superior prop yard, but others also do good work.

reefdrifter wrote:

Hello Marin,
You said that you were happy with the prop work you had done a couple of years ago, so does that mean that it took care of any vibration? Could you tell a difference in anything? Speed, fuel consumption, handling? Also if you don't mind, I am wondering what the cost of this work was

Click to expand...

We had the prop work done almost "accidentally."* Not because we thought they were the source of vibration.* We had been told the props were worn out (yes, they can wear out over time).* After asking a whole lot of people in various aspects of the local marine industry who the best prop shop in Seattle was and getting the same answer every time--- Kruger and Sons--- we took the props in there to get replacements.* (I know Eric doesn't like Kruger but he's the only person I've heard from who doesn't and I don't know the particulars of the problem he had.)

They determined that the props weren't worn out but they were horribly set up with all sorts of problems stemming--- they said-- from the methods whoever set them up last had used.* (This would have been prior to our buying the boat.)* They explained in great detail what had probably been done but while it all made sense at the time it was way over my head and I've forgotten most of it.

They charged (two years ago) $350 each to completely rework the props.* We had been prepared to spend $1700 to $2000 per prop for new ones so this was happy news.

The reworked props made no difference whatsoever to the various vibrations our boat has and had when we bought it.* What the reworked props did was eliminate the tendency of the boat to wander off to one side due to assymetrical thrust, and we idle between half and one knot slower now because the props were repitched one and two inches down respectively.* So instead of 17" and 18" they are now both 16".* At cruise power the EGT readings are a wee bit lower now because the engines are not working quite as hard as they were.

We also had one shaft trued and one replaced.* We had both of the original shaft "setscrew" couplers replaced with much larger and deeper "split couplers" which hold the shafts much more securely and consequently do a better job of maintaining shaft alignment.* At the same time we did this we had the cutless bearings replaced.

None of this work--- props, shafts, couplers, cutless bearings, and the requisite engine alignments--- made any difference in the vibrations in the boat.* But vibration was not what we were doing all this work to correct.

We've not noticed any obvious difference in fuel consumption but we don't have a FlowScan or any accurate sort of fuel-use measurement system on board.* We have a fuel gauge on the day tank and that's it.* But it doesn't look like we're using fuel any faster or slower than we always have.

The boat is extremely maneuverable (in my opinon but I don't have a lot of comparison experience). In large part this is because we have four-bladed props instead of the original three-bladed props. The more blades you have the more pronounced the prop walk will be, and in a twin you want the maximum prop walk you can possibly have with regards to low-speed or no-speed maneuvering.* I have noticed no change from the re-worked props in the maneuverability of the boat with regards to splitting the prop thrust or working thrust on one prop only.

Prior to our having the prop work done--- when we thought we needed to buy new props--- I had called Kruger to find out what sort of information they needed to properly size the new props.* They wanted me to bring in the rpm reading for each engine at WOT, they wanted to know the type of engine, its horsepwer and its maximum rated rpm, they wanted to know the gear ratio of the transmissions (they are not the same, port and starboard, on a GB), and they wanted to physically have the old props.

Neither engine would get near the maximum rated rpm at WOT.* GB typically overpropped its boats to give a bit more speed for a given amount of fuel burn since the engines they were using--- even the old FL120--- had the power and torque to deal with this up to a point, but we didn't have the original props on so whatever GB had set up on our boat when it was built didn't apply anymore.

Armed with this information--- plus the fact that Kruger has put props on probably 8 million GBs--- they had the information they needed to rework our props once they determined we didn't need new ones.* Had we needed new ones we were going to go back to three-bladed props because they're more efficient than four bladed props.* But as it turned out, we still have the four-bladed Michigans on the boat.* Which is great for maneuvering, less so for efficiency.

And that is pretty much everything I can tell you about that.




-- Edited by Marin on Saturday 18th of September 2010 11:08:16 PM