Vibration and Cavitation

[patzfan4eva]

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,

 

[GFC]

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.

 

[Nomad Willy]

Props are so lightly loaded on trawlers it's hard to imagine your vibration is cavitation.

 

[Larry M]

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.

 

[boatpoker]

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" ?

 

[Baggiolini]

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.

 

[patzfan4eva]

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.

 

[Marin]

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.

 

[patzfan4eva]

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.

 

[Marin]

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.

 

[patzfan4eva]

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.

 

[FF]

"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?

 

[skipperdude]

Also if you can. Look at the back pressure on the exhaust. Excess back pressure can lead to lower rpm at WOT.

SD

 

[Nomad Willy]

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.

 

[alormaria]

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?

 

[Delta_JimS]

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.

 

[koliver]

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.

 

[Marin]

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.

 

[Delta_JimS]

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.

 

[Delta_JimS]

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.

 

[Marin]

Jim--- For a given engine/shaft rpm, the finer the prop pitch the less water it moves and the slower you go.

Our marina has a "5 Knot or No Wake" speed limit. Or it might be 4 knots--- I never look at the sign anymore.

Our engines idle about about 650 rpm (1973 SW tachs are not the most accurate instruments on the planet).

Before the props were re-worked, our speed at idle was just under 4 knots. With the props correctly set and pitched down an inch our idle speed is 3.2 knots or thereabouts by the paddlewheel sensor. Anyway, whatever the numbers, it's noticeably slower.

Now we never had to bump in and out of gear before, but the slower idle speed we have now is nice. We carry that power right up until we're turning into the slip.

As to the engine loading, putting a finer pitch in the prop blades is a bit like dropping to a lower gear in a car when you go up a hill. You don't go as fast but the engine is working less at 2000 rpm in third than it is at 2000 rpm in 4th.

 

[psneeld]

Overpropping is for the educated cruiser...if you know what you are doing then that's that...for newbies...bad idea.

For those that say "no" in all instances have to remind themselves of their own limitations before they impart those limitations on others.

 

[Delta_JimS]

Jim--- For a given engine/shaft rpm, the finer the prop pitch the less water it moves and the slower you go. Our marina has a "5 Knot or No Wake" speed limit. Or it might be 4 knots--- I never look at the sign anymore.

As to the engine loading, putting a finer pitch in the prop blades is a bit like dropping to a lower gear in a car when you go up a hill. You don't go as fast but the engine is working less at 2000 rpm in third than it is at 2000 rpm in 4th.

Marin,

Then I generally had the right idea on your point. Somehow though in my mind, at cruising speed and plenty of extra horsepower, the greater rpm of the diesel (with reduced pitch) would have more effect on the fuel burn than the loading factor.
In my particular case, I do not do many 100's of miles per year, so the difference of a few extra gallons of diesel is as much a benefit as a negative in that I can keep fresher fuel.

 

[Marin]

My guess is that the fuel savings from the reduced load on an engine due to some pitch being taken out of the prop(s) is negligible, as you say. Depending on the kind of engine the reduced load might be beneficial to its service life. Or not.

The biggest benefit in your case sounds like it would be the reduction in idle speed.

Grand Banks boats were all over-pitched from the factory, at least in the first few decades. This made all kinds of sense given the power of the engines and the relatively low cruising speeds people went with them. Like driving in a bit too high of a gear, over-pitching can give you more speed for a given rpm, so as long as an engine is not overworked in this process, you get a more economical boat at cruise.

On the recommendation of the prop shop we had our four bladed props (not original to the boat) pitched down an inch not to lower our idle speed but because the shop said that as a general rule, for the same diameter prop on a given boat, a four-bladed prop should have one inch less pitch than a three-bladed prop to get the same performance and economy. The four-bladed props on our boat were set--- badly--- to the same two pitches the original three-bladed props had been.

 

[Third-Reef]

I also have turbo volvos tmd41a. The only issue I see with the 2000 rpm cruse is that you are probably not creating any boost. I produce some smoke until I get over 2200, I typically cruse at 2500. In one of Tony Athens articles he references an approximate 10% increase in efficiency on an engine with boost. Turbo engines should cruse in a rpm range where they produce boost or the turbos carbon up I believe.

 

[Nomad Willy]

Delta JimS,
You said your Volvo "tops out at 3600rpm". Something very important you haven't mentioned is what your engine is rated at. RPM wise. At what rpm does your engine develop it rated power. That is the rpm that it should attain at WOT. And one should check to make sure the throttle and cables are advancing the fuel injector to max. One should change pitch to achieve rated rpm.

That was good information from Third-Reef but his cruise rpm may be a way different loading on your engine at the same rpm unless you both get the same rpm at WOT.

Quite a few mechanics support over propping but I've never heard an engineer say anything positive about it.

If your boat is way over powered and over propped you will save some fuel if you stay under loaded and over propped but it's hard to tell when you are and when you aren't. You may burn LESS fuel by rated propping w higher rpm and less load. In most all cases you'll experience more noise. And I think the engine may last longer making more noise. MANY here disagree w me here so be amused by this conversation and seek an ENGINEER'S opinion. I think over 95% of them will say prop to rated rpm. Talk to the engine manufacturer or an engineer at the distributor. Very few commercial applications (perhaps none) will ever resort to over propping.

 

[Delta_JimS]

Thanks Eric,

Appreciate your input. Based on my own understanding and input from Marin and Third-Reef, I am definitely in favor of trimming back my pitch. Hope to get lower idle speed as well.

I have included a scan here of the engine datasheet for the TAMD40B.(if it is hard to read, I can send you a better copy if you provide an email addr.)
As you can see, it states "Propeller Shaft Output" at 3600 rpm. I have never taken her over 3500 rpm frankly because that rpm still makes me nervous enough at this point. I still have some throttle arm left to go, so I have no doubt she will do that. Like I mentioned, I have plenty of power out of that diesel and am plowing water big-time at that point at over 9 kts.

The other reason for not running above 3200 much is because of that bad vibration that I attribute to the prop balance - she is shaking pretty hard at that point.

TR makes another good point to support my upping the rpm by reducing the pitch which will allow running more in the Boost mode while cruising. I certainly agree with that approach. For the limited amount of yearly cruising that I do at this point, fuel burn is a lower consideration. It would actually be a plus to be able to run fresher fuel.

 

[Nomad Willy]

If it were mine I'd definitely take her to WOT just long enough to stabilize the rpm so you know what it is. I'm not there so don't know how bad the vibration is so be very cautious. If the vibes are really bad you should fix that (at least to a tolerable level) before going to max rpm.

No I can't read it. You should also go to WOT in neutral gear w no load at all. If you've never heard of checking the rev-limitter (governor) it may sound crazy but like the full throttle full load test this should be done carefully and only top out enough (up to about 300 maybe 400 rpm). If you reach 400rpm over the rated rpm your governor may not be working right. Ideally it should top out about 200 above rated rpm.

Then go to a good prop man and tell him "I'm getting ___ rpm and this is my prop. My engine is rated at ___ rpm. What can we do to make the WOT ____rpm?"

 

[Marin]

Then go to a good prop man and tell him "I'm getting ___ rpm and this is my prop. My engine is rated at ___ rpm. What can we do to make the WOT ____rpm?"

This is some of the info the prop shop we use asked for. So we took the boat out and got the numbers they wanted. Most Grand Banks were over propped-- deliberately-- by the manufacturer. We still are over propped a bit, I think. We had an inch taken out of the props because they are four blade instead of three blade but had been pitched (badly) to the boat's original three blade specs.

 

[BruceK]

Delta Jim, How clean is your running gear? Is the pic of prop and rudder before or after cleaning? There looks to be remains of barnacles on the prop, and barnacles on the rudder.
I recently hauled out, not for antifouling, the hull a/f was near perfect but the shaft/prop/skegs were heavily fouled with shell, a yard had used the same soft a/f as on the hull. I cleaned up and applied primer and hard a/f which cured vibration and loss of speed that had gradually developed. That could be your main problem, but a prop tune up can`t hurt.
I may apply Prop Speed to the running gear next time.