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Old 04-28-2011, 09:26 AM   #21
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RE: Do new engine mounts equal less noise and vibration?

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Idle is 600-700. You definitely want to get yourself a handheld infrared tach and calibrate on a regular basis your tachs. You can probably pick one up at Harbour Freight. I think mine costs me about $30


Thanks Daddyo!
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Old 04-28-2011, 01:59 PM   #22
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RE: Do new engine mounts equal less noise and vibration?

Most engines have a sweet spot where they seem to run, sound and feel smoother.* For our 671 its around 1200 to 1400 rpm. Our 671 is hard mounted direct to the structs. I usually sit on the pilot house table that is bolted to the floor when my BUTT starts vibrating I know it not in the sweet spot. So instead of replacing mounts*first try changing the rpm.*
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Old 04-28-2011, 06:04 PM   #23
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RE: Do new engine mounts equal less noise and vibration?

Just replaced my old 1982 Lehman 120 engine with a rebuilt long block (incl head and valves). While the old one was out, dropped in new motor mounts. The ones I bought were the standard motor mounts from American Diesel ($35 each).

So far there seems to be a SIGNIFICANT difference. Just ran the engine through the break in hours and will run it for 15 hours this weekend to get to our home dock in Rock Hall MD. I'll be monitoring the feel of vibration on this two day trip.

R.
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Old 04-28-2011, 06:27 PM   #24
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RE: Do new engine mounts equal less noise and vibration?

Most boats are run at the sweet spot. Keep in mind though that the sweet spot has nothing to do with what rpm one should run. It's just a comfort zone. I suspect that most engine vibration felt is not so much the engine but hulls, bulkheads, rudders, prop shafts, windows, exhaust pipes and many to any other flexible part of the boat. Much of the time it's easy to find what's vibrating by going around in the boat while underway and feel flat surfaces and other things that can be resonant to the engine. I have a 4cyl boat that is quite smooth so you guys w 6cyl engines should be able to fix most all vibration issues.
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Old 04-28-2011, 06:51 PM   #25
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RE: Do new engine mounts equal less noise and vibration?

With my last engine and old motor mounts, the vibration was felt on the fly bridge but not in the main cabin (for the same RPM).

I used to look at my St Croix davits while running. I could see them vibrating and shaking at certain RPM then smooth out at others.

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Old 04-28-2011, 10:36 PM   #26
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RE: Do new engine mounts equal less noise and vibration?

Hi Flywright

Can you tell me what prop pitch and diameter you are using. My engines reach max rpm very quickly but they top out at 2350 - 2400. You seem to get to 2800, there must be some diff somewhere. I have the normally asprirated 4.236 which are rated at 85hp each, is that what you are running?

Thanks

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Old 04-29-2011, 05:34 AM   #27
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RE: Do new engine mounts equal less noise and vibration?

I suspect that most engine vibration felt is not so much the engine

All engines will vibrate , it is common for the engine mfg to attempt to place the worst vibration below idle , and above peak RPM.

Sadly not all work , so soft mounts built for a boat , not a universal or gen set (must accept thrust) style are required.
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Old 04-29-2011, 02:22 PM   #28
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RE: Do new engine mounts equal less noise and vibration?

Mappy, The 4-236 Perkins is rated;

82hp at 2800rpm

194 ft lbs at 1460rpm

So at wide open throttle (WOT) you should run 2800rpm.

500rpm over propped is way overpropped.

Probably need 2" less pitch.
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Old 04-30-2011, 04:59 AM   #29
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RE: Do new engine mounts equal less noise and vibration?

The 4-236 Perkins is rated;82hp at 2800rpm,194 ft lbs at 1460rpm.

I would use 2500 as MAX for , and contemplate proping to get down even lower.

These are not really robust engines.

What is the usual fuel burn per hour?
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Old 04-30-2011, 08:41 AM   #30
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RE: Do new engine mounts equal less noise and vibration?

Quote:
ralphyost wrote:


Just replaced my old 1982 Lehman 120 engine with a rebuilt long block (incl head and valves). While the old one was out, dropped in new motor mounts. The ones I bought were the standard motor mounts from American Diesel ($35 each).

So far there seems to be a SIGNIFICANT difference. Just ran the engine through the break in hours and will run it for 15 hours this weekend to get to our home dock in Rock Hall MD. I'll be monitoring the feel of vibration on this two day trip.

R.

Ralph,
I look forward to the follow up. My mounts appear fine but are at least 6 years old. How did you original mounts look?
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Old 05-01-2011, 06:23 PM   #31
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Do new engine mounts equal less noise and vibration?

Forklift -

Here are some pics of my OLD motor mounts taken in 2008.

http://tinyurl.com/3sjlrm2

I could not tell by inspection that the old motor mounts had anything wrong with them. I just decided that since I didnt know anything about the history of them being changed, I was going to do it this year. Then when I had to get a new engine, that was the logical time to do it (when the old one was pulled out).

The new engine runs very nice. Everytyhing is very smooth. But at 1800 I can feel vibration when I am steering from the main cabin. Up on the fly bridge I dont feel it much at all at 1800 RPM. At 1700 RPM, its dead smooth. Normal cruise for Lehman 120 is 1800.

Ran the boat yesterday from Greenwich NJ on the Cohansey River up the Delaware River, through the C&D Canal, then down the Chesapeake Bay. Normally we run as long as we can and stay overnight somewhere on the hook - Still Pond, Fairlee Creek, etc.

But we had the tide PERFECTLY timed yesterday. Left the boat yard at 6:45pm and made it all the way to our home dock at Rock Hall MD at 4:30pm. Thats about 80-85 miles - averages 8 kts for the day.

Changed the oil after we got in. Next weekend I will adjust the valves and retorque the head bolts.

R.


-- Edited by ralphyost on Sunday 1st of May 2011 06:26:24 PM
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Old 05-01-2011, 08:17 PM   #32
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RE: Do new engine mounts equal less noise and vibration?

Thanks for the update Ralph. I have a different style mount on my 135's, but after your experience I may swap out a set at the time and assess the improvement. Well done on your trip home! Sounds like everything was going your way!
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Old 05-02-2011, 12:48 PM   #33
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RE: Do new engine mounts equal less noise and vibration? _ CORRECT PROPS?

Eric wrote:

Mappy, The 4-236 Perkins is rated;

82hp at 2800rpm

194 ft lbs at 1460rpm

So at wide open throttle (WOT) you should run 2800rpm.

500rpm over propped is way overpropped.

Probably need 2" less pitch.

Hi Eric

I checked with Flywright who is running the same engines/boat he gets max RPM of 2800 and uses 2100 - 2300 for cruise. His survey says his props are 20 x 18.

My boat has 20 x 16 props, max throttle position give me 2350 rpm. The engines rev to 2350 very quickly. Is it possible that the throttle controls are not going to full fuel flow? Or could my tacks be out by 20%.

It does not make sense to me? Do I need 2" more pitch?

Roger



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Old 05-02-2011, 01:55 PM   #34
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RE: Do new engine mounts equal less noise and vibration?

Fortklift -

Last fall when I bought these new engine mounts,I called American Diesel to order them. Bob Smith and I discussed it. They sell two basic types, with one being more expensive than the other. Bob counseled me that he couldnt exactly say there was a definite benefit to the more expensive ones and convinced me to order the regulars.

R.
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Old 05-04-2011, 08:36 AM   #35
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RE: Do new engine mounts equal less noise and vibration?

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I get a pretty rough idle below ...* oh ... 800-900 RPM, or around there.* What's the trick to drop the idle RPM and keep it relatively smooth?
*Any thoughts?* Anyone?
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Old 05-04-2011, 12:22 PM   #36
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RE: Do new engine mounts equal less noise and vibration?

... "800-900 RPM, or around there. What's the trick to drop the idle RPM and keep it relatively smooth?"

First "trick" is to borrow a hand tach and calibrate your setup.

If you drive the tach with a cable or its own pulse generator it may not be very far off. 10-15%

BUT if the signal comes from a simple alternator pick up , a calibration is in order, as first step.
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Old 05-04-2011, 10:15 PM   #37
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RE: Do new engine mounts equal less noise and vibration?

Mappy,

Maybe your governors are limiting the top rpm. I'm very suspicious about the "The engines rev to 2350 very quickly." Does the last 5 to 10% of rpm gain as quickly as at 3-500rpm less? I recommend you (or your mechanic) check the governor. As a preliminary put your boat in neutral and go carefully to WOT. Your engine should come up to about 3000rpm. If it does not get it adjusted and check your throttle linkage for full throttle travel. And does Flywright have the same gear ratio? My engine does the last 200rpm slowly and the last 100 to 50rpm very slowly. I'll bet your governor is at fault AND if your gear ratios are the same as your friend you will probably need the 2" pitch increase.
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Old 05-05-2011, 10:57 AM   #38
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RE: Do new engine mounts equal less noise and vibration?

Thanks Eric

I shall investigate and report back. I am in the midst of an instrument upgrade and re-org for the lower helm so I won't get to the govenors etc for awhile.

Roger

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Old 05-05-2011, 10:04 PM   #39
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RE: Do new engine mounts equal less noise and vibration?

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Roger, what RPM are you running at? I have the same boat/engines and have for the most part run at 2350 RPM for 8 Kts. That's what the PO did and I have followed his advice.

Two weeks ago, I spent a week on the boat and had the time to try other power settings. I found that 2100 RPM cost me about 1 kt with MUCH improved sound levels and vibrations. Conversations were much less strained in the salon during cruise. I probably have aging mounts, too, but found this reduced power to be a great improvement.

Keep us posted on your findings and decisions.
*That is only a 2600RPM engine. *Running them at 2350 is running the P*** out of them. *Like the OP said, that engine is happy around 1800RPM can certainly run well at 2000....but once you get over 2200 I think you are running some life out of them....Just my opinion as having owned a 4236 for 6 years.
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Old 05-10-2011, 02:38 PM   #40
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RE: Do new engine mounts equal less noise and vibration?

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Baker wrote:FlyWright wrote:
Roger, what RPM are you running at? I have the same boat/engines and have for the most part run at 2350 RPM for 8 Kts. That's what the PO did and I have followed his advice.

Two weeks ago, I spent a week on the boat and had the time to try other power settings. I found that 2100 RPM cost me about 1 kt with MUCH improved sound levels and vibrations. Conversations were much less strained in the salon during cruise. I probably have aging mounts, too, but found this reduced power to be a great improvement.

Keep us posted on your findings and decisions.
*That is only a 2600RPM engine. *Running them at 2350 is running the P*** out of them. *Like the OP said, that engine is happy around 1800RPM can certainly run well at 2000....but once you get over 2200 I think you are running some life out of them....Just my opinion as having owned a 4236 for 6 years.

Baker, thanks for the advice.* I agree that it's working them harder than necessary, but I figured it was about 84% of 2800 RPM, not 90% of 2600 RPM.**Both engine*RPMs match at 2800 RPM WOT.* All I've read says 2800 is WOT on this boat.* At 2350, my average hourly fuel burn over about 300*hours of operation is 3.75 GPH (1.875 GPH per side).*

Here's a link to a similar (brief) discussion:* http://www.marineengine.com/boat-for...ne-application

I am rethinking*my past practices*and will probably settle in at around 2000-2100 RPM as a new cruise RPM target.* My calculations show 2100 RPM is 75% which seems more in line with what I SHOULD be running.* I expect to get about 7 Kts at that power setting.
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