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Old 01-20-2013, 11:00 AM   #1
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Vibration and Cavitation

HELP!!!!!

I have a 34' CHB TriCabin with a nasty vibration and cavitation problem. I'm running a 23" 17 LH prop. I'd like to hear from my CHB brothers of the size they are running before I switch, balance or tune my existing prop.

Thanks for the help,
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Old 01-20-2013, 11:17 AM   #2
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David, is the vibration something that just started, or did you just buy the boat?

If so, I'd get your prop balanced. I had that done on two boats I used to own and the difference was amazing. Find a shop near you that has Prop Scan--you won't be sorry.
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Old 01-20-2013, 11:27 AM   #3
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Props are so lightly loaded on trawlers it's hard to imagine your vibration is cavitation.
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Old 01-20-2013, 03:16 PM   #4
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David: I see you're in Marina Del Ray; warm, fertile water. When was the last time your prop was cleaned? The prop could easily be fouled within a month to give the symptoms you're describing. Grab the mask/snorkel and take a look. It's a cheap and easy first step. Just don't swallow any water.
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Old 01-20-2013, 05:29 PM   #5
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Quote:
Originally Posted by patzfan4eva View Post
HELP!!!!!

I have a 34' CHB TriCabin with a nasty vibration and cavitation problem. I'm running a 23" 17 LH prop. I'd like to hear from my CHB brothers of the size they are running before I switch, balance or tune my existing prop.

Thanks for the help,
What makes you say "cavitation" ?
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Old 01-22-2013, 01:48 AM   #6
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My vibration.....serious vibration, shaft moving all over the place at the transmission was a cutlass bearing. That one sucked, two house payments including the haul & a few other minor issues while she was out of the water.
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Old 01-22-2013, 11:49 PM   #7
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Thanks all, a quick run down of work performed so far.

Cutlass Replaced
Stuffing Box Repaired
Engine Overhaul
Prop Cleaned and Tuned.

Cavitation symptoms

White water behind boat
2750 Max RPM under no load
2750 Max RPM under load with no prop
1950 Max RPM under load with prop

Next step is to replace prop with a donor 4 blade 22 18LH for test.

Want to see what everyone is running with approximate speed and RPM range for comparison and aid in decision of new prop.
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Old 01-23-2013, 12:14 AM   #8
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Not sure what you're saying with "2750 rpm at max throttle under no load" and "2750 rpm at max throttle under load with no prop." The prop is the load so "no load" and "no prop" are the same thing.

As a general rule of thumb the engine should achieve its maximum rated rpm at wide open throttle with the boat's propeller installed and the boat underway. So if the max rated rpm of your engine is 2750 the engine should achieve that at wide open throttle with the prop installed. If the rpm is below that the prop is over-pitched.

I'm not sure you can do this test tied to a dock. I believe the boat has to be running through the water to determine if the engine will achieve its max rated rpm with the prop installed.

My suggestion is take your situation to a good prop shop. They'll want to know the horsepower of the engine, the max rated rpm of the engine, the rpm you get at wide open throttle with the prop you have now, and they may also want to see your current prop. From all this they can determine the best diameter and pitch for your boat. They may be able to adjust and tune your existing prop to these specs or you may need a new prop.
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Old 01-23-2013, 12:31 AM   #9
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Marin,

By doing the No Load test with the prop removed we were able to confirm that the issue is the prop. I was trying to show that my driveline issues aren't the cause hence the reason I'm seeking to know what props others with boats like mine are running.
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Old 01-23-2013, 12:52 AM   #10
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The fact you are only getting 1950 rpm at wide open throttle with you current prop would indicate that the prop is way over-pitched, too big in diameter, or both.

While we don't have a CHB we do have a pair of Lehmans with a max rated rpm of 2500. The single engine version of our 36' boat used the same engine and the stock propeller fitted was three blades, 24" diameter, 17" pitch. However, GBs, particularly the older ones, were deliberately over-pitched. So to achieve a WOT rpm of 2500 the prop would have to have a 16" pitch or thereabouts.

Don't know if this info's any use to you but it's one point of comparison.
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Old 01-23-2013, 01:00 AM   #11
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I have a 23 17LH Prop and was told by Michigan Prop that I need a 24 16LH and another source reccommended a 23 15LH so taking that into consideration and hopefully seeking current versions of fellow CHB owners prop size to make an educated decision on the next prop to solve my power, speed and RPM issue.
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Old 01-23-2013, 08:36 AM   #12
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"Over propping" is a good solution for most displacement boats that have a factory oversized engine installed.

Sounds like the PO knew what he was doing.

The usual simple technique to operate with this efficiency and engine life boosting is to run full throttle and pull back 10% or 300 RPM.

This will usually give the boat far lower RPM at long range cruise speed , and a lower fuel bill.

The usual cruise speed for a displacement boat is about the SQ Root of the LWL.

NEVER "Hull Speed", unless you are happy with a 300% higher fuel burn.

With a 34 ft boat and perhaps 30LWL that cruise would be under 6K.

What speed do you get at 1600RPM or 1700RPM?

What speed did you hope to cruise this boat?
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Old 01-23-2013, 12:20 PM   #13
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Also if you can. Look at the back pressure on the exhaust. Excess back pressure can lead to lower rpm at WOT.

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Old 01-24-2013, 12:07 PM   #14
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Over propping is not adult behavior.

As the Dude implies you probably have something wrong w your engine system. You might check you're throttle linkage as you may not be getting mechanically advanced to full throttle. It's hard to imagine someone over propping 550rpm.

Too many downsides to over propping and way too little advantage. My grandmother had a 1957 VW and her mechanic and owners manual promoted high engine speeds and light loads. The speed limit in the city was 30mph and of course grandma wasn't a speeder so she drove 29mph in 2nd gear most of the time. The air cooled engine didn't deliver very good gas milage as it was running cool all the time but the little engine lasted a long time and was trouble free.

Our trawler diesels have less oil cooling and "water" cooling when over propped and considerably more pressure on many important parts like pistons and cylinder walls and/or crankshaft and connecting rod bearings. Engineers designed our engines for a given rate of flow in the water jackets at specific speeds and loads. Same for lube oil systems. In trucks they do such things on a regular basis but only for short periods of time. Our trawlers (if over propped) will run hundreds of hours in chances of time spanning several hours typically with less oil and coolant.

This may not mean much or perhaps nothing at all but it's something to think about. Other things like fuel injection timing, amounts of fuel relative to loads and balanced w engine speed probably enter into the picture. And the best thing to think about is that probably all engineers will agree that to prop to rated rpm at WOT is the most recommended and responsible approach to the issue.

Wasn't going to respond as I've basically made this post many times and FF has made his pitch for overdrive on the sea as many times. But balance is usually good.
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Old 01-25-2013, 03:38 PM   #15
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I think my 34 MT has a 23 x 16 prop.


I had a vibration that was tough to find. It turns out my shaft was 'cork screwed'. Not bent but not right either. New shaft and removal of the shaft support and bearing fixed it. My old shaft was brass and very soft. New is one of the Aquamet stainless varieties. Now the boat will crack in half before the shaft bends.

2750 RPM? How did you ever get one of our engines up to 2750 RPM?
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Old 01-25-2013, 06:07 PM   #16
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David,

Not wanting to hijack your thread, but based on Eric's last comment, I need to come in with a couple of related issues of my own.

I also seem to be suffering from a lot of Vibration of late. Actually, I am not sure on how long this has been an issue, but I think it has gotten worse since I purchased the boat a bit over 2 years ago. I do not recall anywhere near this much vibration during the sea-trial or on the homecoming trip from SF Bay up to the Delta, although I do make more of a point of running the engine harder at higher rpm to heat up the diesel and flush the exchanger before returning to the marina.

Let me start off with two points. I am of the same school as Eric with regard to the issue of running the diesel at higher rpm when cruising rather that lower because everything I have read and previous history of problems with this engine being run too slow would support this approach.

Point two, full disclosure, I am running a Turbo high-rev Volvo TAMD40B. (OK, go ahead and laugh...) The bottom line is that this engine tops out at 3600 rpm WOT. The Volvo seems to run well, no smoke, no oil burn, starts easy and has lots of power, no stalling or hesitation.(just hope I never need major parts)

I do not intend to operate at WOT, only to make sure on occasion that the engine and drive system are still capable of it.
My expectation is that at hull speed of about 7.5 kt that the engine would be running at 80% - 85% of its maximum. I typically cruise at 6 7 kt.

My thinking is thus; I know where the top of the throttle lever is, and that about matches the 3600 rpm on the lower station gauge. At this rpm WOT, I am pushing water like a bulldozer at around 9 kt. The Volvo is a powerful engine and has no problem spinning that 4-blade steep pitch prop if I had trim tabs I could almost plane!
I think I am over-propped.

Eric, do you follow my thinking here and would it not make more sense to be cruising closer to 2800 - 3000 rpm, rather than the 2000 or so rpm that the boat appears to be propped to now? Depending on which previous survey I consult, the prop is either a 22" x 15, or 22 x 16. She is a 17,000 lb. boat. The engine is rated at ~ 155hp.

Now, back to the vibration issue. At my next haul-out I am thinking to have the prop pulled and hopefully cleaned up and balanced.( as well as service of the whole shaft) There is some debate as to whether this is possible, though being a great piece of bronze; I certainly would like to try with one of those PropScan shops.
In any case, even if I have to (heaven forbid) buy a new prop, I am seriously of a mind to have the pitch reduced enough to bump up my cruising rpm by at least 500 rpm.

I am in the process of getting educated on boat to prop engineering from some books I have and the Internet, but there is a lot of Experienced advice here and I thought it good to tap into it also.

Comments, criticisms, opinions?

Thanks.



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Old 01-25-2013, 06:42 PM   #17
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Delta Jim:

I used to have a pair of your TMD40s. I cruised at 7.5knots, at 2700rpm, burning 4.5gph (total).
My Volvo mechanic (he has Volvo Penta certification) knew me and my boat, as I had been coming to him for everything for 6 years, (1500 hrs) when he offered me a pair of TAMD41s. Part of the discussion was his suggestion that once I had the 41s, I could up the prop pitch and cruise at a much lower rpm, thus saving fuel and adding to engine life. Fuel savings paid for the swap and re-pitching within a few short years.
I now cruise at 2000 rpm and have gained speed to 8.25 knots (I try to avg) and my fuel burn is under 3 gph. In 12 yrs (1500 hrs) since the swap I have had no issues with the engines.
My boat also plows a huge wave if I put on full power, as even with trim tabs, I think I would need over 1000 hp to plane.
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Old 01-25-2013, 06:57 PM   #18
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Jim--- In our experience it can be pretty amazing what an experienced and reputable prop shop can discover about a propeller. I won't go into the details of what happened when we took our props to arguably the best shop in Seattle, but the bottom line is that instead of needing new props which we had been led to believe, the shop found that our props were horribly out of tune, out of diameter, out of pitch, you name it. But we had no prop or shaft vibration whatsoever and had not had since buying the boat some eight or nine years earlier.

So they reworked them extensively for a fraction of what it would have cost to buy two new props. They also pitched them down an inch from the factory pitch specs for a three-bladed prop (ours are four bladed) so now we go slower at idle and our engines work a bit less at cruise which means they use a little less fuel.

So a prop shop can find and fix some pretty impressive problems the boat owner may not have known were there.
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Old 01-25-2013, 07:25 PM   #19
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Thanks Marin,

You bring up another good point.(as usual)

I definitely would like to go slower at idle. The Volvo (bottom throttle) idles about 750 rpm, just above a stall.(which has happened) When I enter the marina fairway and then into my slip, I am constantly popping in and out of gear to slow down. Now, granted I am not very experienced, but I find the saying "if you think you are going too slow, you are just about right" most appropriate. I like a nice, smooth, calm entry into the marina and slip - no surprises, no wake, no damages, no marks.

Not sure I follow your point about less pitch but less loading at cruise and less fuel consumption. Do you mean as a result of the clean-up and balancing, or the change in pitch? I assume you mean the clean-up.
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Old 01-25-2013, 07:32 PM   #20
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Darn, forgot the other part again....

In addition to having to pop in and out of gear in the marina, not having the prop spinning also has an adverse effect on steerage. I find myself having to jockey forward and reverse to get good rudder control.
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