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Old 09-16-2010, 04:21 AM   #21
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RE: Propellers

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.

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Old 09-16-2010, 09:06 AM   #22
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RE: Propellers

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.

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Old 09-16-2010, 09:45 PM   #23
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RE: Propellers

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.

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Old 09-16-2010, 11:05 PM   #24
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RE: Propellers

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.
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Old 09-18-2010, 09:23 AM   #25
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RE: Propellers

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.
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Old 09-18-2010, 09:49 PM   #26
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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
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

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