What are you neutral RPMs and your in gear RPMs? Is there a split between sides? If they drop below the recommended RPM in gear, that could cause some vibration, and a split can get an uncomfortable resonant frequency going.

I have the same issue on my CAT powered boat, but engaging the engine sync almost completely eliminates the issue and adding a touch of throttle to get them back around 750 RPM, makes for a butter smooth ride.

 

 

Check that your "slow idle" switch isn't engaged; that causes quite a bit of additional vibration (on my boat feels like a rumble) but at the frequency of the engine's revs. Also the engine "RPM +/-" switch will raise and lower the idle by 25 RPM per click. So, probably need to make sure your in gear idle is correct at 650 RPM.
If you have a "low frequency" vibration, like half the engine's rpm's, then look at shaft alignment or for a bent shaft (which would be rare).
Is the issue on one or the other drivetrain or both?

 

 

I’ll check to see if there is a split when I’m on the boat next trip. Thank you for the ideas.

 

I’m in the ICW alot with the slow idle switch engaged and it is more pronounced in that condition. When I bump up past idle it’s smooth. I’ll isolate each engine to see if it’s one or both. It’s somewhat comforting to hear that it’s not abnormal. Thanks for the response. I live in Arkansas and the boat is in Palm Beach Gardens so I’ll have to wait until my next conjugal visit.

 

This has been discussed among QSM powered 480DB owners before. Mine has always had a rumble/vibration at idle speeds in forward, one engine is worse but I can't recall which one at the moment. Seems a lot better once everything is warmed up.

We also experience a lot of turbulence while pivoting, more than I've noticed on other models. I've had the cutlass bearings replaced and alignment checked so I assume it's just a characteristic of the hull design.

 

I had this issue when I first got my boat in 2017. It was just the port engine. I added 4 gallons of sea foam and it cleared itself up. My only thought was injectors needed a little cleaning. It’s been running smooth ever since. Knock on wood.

 

I also experience a fair amount of vibration at idle, in neutral or in gear. I did have both motors aligned recently, which helped a bit. The mechanic stated that big diesels are known to vibrate...

 

I have another inquiry and probably will also start another thread but wanted to ask you for comments. I suspect exhaust leaks after reading the Seaboard Marine article on the effects of over loading the engines with a prop with too much pitch (from the factory). The pics in his article look like my engines that only have about 750 hours on them although they are 13 years old. Have any of you reduced prop pitch and if so by how much? Replaced exhaust components and/or turbo? Cost? I called Bobby Soles Propeller shop in West Palm and he said going down from 28x35 to 28x34 would reduce load 6%. Is that enough? Your boats are heavier than mine (36,000lbs). There are computations to figure this out but I thought someone with a similar boat may have already been down this road.

 

Exhaust leaks in these engines are commonplace; just monitor and if it gets bad time to do something about it. Look under the heat exchanger and atop at the mating surface between the engine's head and exhaust manifold. Leaks will be indicated as black soot traces. It is, however, important to get heat out of the exhaust system; heat is what causes the exhaust components to fail.
Regarding the propellers, it's not about reducing load per-say but rather getting the engine into a better engine fueling combination. The objective is to try to get 19 or less GPH at cruise RPM (1800 - 1900). With all of that said the engine's load is proportional to the fuel consumption and consequently can be used to tailor the pitch on the propellers. My boat on the pins was turning 2300+ RPM and at 1950 showing 90% load with clean bottom and full fluids. I'm pulling this from memory; there is a thread in this forum where we worked through the numbers on several DB's. I determined it needed to add another 75 to 100 RPM which would consequently reduce the fueling requirement at cruise RPM (not as much load on the engine at the cruise RPM's). So to get the extra RPM my prop's were set at just over 31 inches. Currently, with clean bottom and full liquids, the boat cruises at a hair under 40 GPH and 75% load at 1800 RPM which I think is about as good as it will get. Some do better some worse. The interesting thing is my speed at cruise RPM actually did not change....

 

Keith,

I’m running a 2006 52DB with 12’ Highfield With 40 HP Honda tender weight in at 735lbs with lift on the stern. The 52DB is heavery the the DA but I’ve not looked st the exact numbers. I started out with SR factory spec to my knowledge 28”/32”pitch working towards Tony Anthens recommendations.

After three modifications prop wise I’m now st 28/28.5”. I would recommend taking anything less than 2” out of your pitch if you really want to make a change. 1” reduction IMHO starting from where you are is pretty insignificant change as your seldom going to hull wise always clean running gear, hull and load wise.

I could go on but suggest you go back and follow some threads on the over prop conditions, fuel burn & explore EGT installation to monitor your Exhaust Gas Temperatures. One of the biggest high heat conditions comes when running lower RPMS in rough sea conditions as well as other GPM burn rates.

I drop 14 GPH from where I was at cruise RPMS of 1900 which obviously reduced engine load but more importantly heat on the engines. Engine loads currently running under 70% and even at 2000 still below 75%.

It’s the heat and fuel burn numbers that will tell you a lot of where Tony wants you to work to. Since you’re running predominantly in SF you’re already bring in pretty warm/air to your engines. Cummins states the engine intake air temperatures should not exceed 30 degrees above ambient air temperatures. 95 degree EAT + 30 degrees equals 125 deg. I see hear of many people not running engine room Blowers before, during the entire run time as well as not long enough once the engines are shut down. Heat in the engine & water in the fuel are two of Diesel engines major killers.

Hope this helps some!

I've read most of Tony's articles on QSM11's and he emailed me this morning that he can't tell me what prop reduction I need without running the boat at 1900 rpms and checking fuel flow. For scheduling reasons I am trying to make a prop adjustment before heading out on a 1200 mile trip so my current thought is to reduce to 33.5". This pitch is based on the recommendation of the prop shop who says that will result in a 9% load reduction. My concern is that is not going to be enough since some have reduced as much as 3.5" to get the desired result.

I do use my blowers before start and don't turn them off until I shut down the engines. I'll change to running them 10 minutes after shutdown. Good advice on the forum. Many thanks, Keith

 

Keith,

I’m running a 2006 52DB with 12’ Highfield With 40 HP Honda tender weight in at 735lbs with lift on the stern. The 52DB is heavery the the DA but I’ve not looked st the exact numbers. I started out with SR factory spec to my knowledge 28”/32”pitch working towards Tony Anthens recommendations.

After three modifications prop wise I’m now st 28/28.5”. I would recommend taking anything less than 2” out of your pitch if you really want to make a change. 1” reduction IMHO starting from where you are is pretty insignificant change as your seldom going to hull wise always clean running gear, hull and load wise.

I could go on but suggest you go back and follow some threads on the over prop conditions, fuel burn & explore EGT installation to monitor your Exhaust Gas Temperatures. One of the biggest high heat conditions comes when running lower RPMS in rough sea conditions as well as other GPM burn rates.

I drop 14 GPH from where I was at cruise RPMS of 1900 which obviously reduced engine load but more importantly heat on the engines. Engine loads currently running under 70% and even at 2000 still below 75%.

It’s the heat and fuel burn numbers that will tell you a lot of where Tony wants you to work to. Since you’re running predominantly in SF you’re already bring in pretty warm/air to your engines. Cummins states the engine intake air temperatures should not exceed 30 degrees above ambient air temperatures. 95 degree EAT + 30 degrees equals 125 deg. I see hear of many people not running engine room Blowers before, during the entire run time as well as not long enough once the engines are shut down. Heat in the engine & water in the fuel are two of Diesel engines major killers.

Hope this helps some!

 

As stated there is lots of info on this board and boatdiesel regarding the issue and potential fixes. I reduced 1" pitch and took some cup out to get to where I am now but every boat is different. We cruise around 1900 RPM burning 38 GPH with loads in the low 60's and EGT's of 900. My port engine has an exhaust leak that I'll live with for now but the damage was done before these steps were taken.

 

The prop shop guy said that reducing 1” would increase rpm about 80 and he would not reduce below 33.5 which is about 120 rpm. My thought is better a little too much than not enough. I hardly ever push the throttle to the pins and the governor should prevent an over speed. In your case taking out 4” is a lot. How did that effect your speed?

I saw the EGT port and have considered getting the probe and gauge. Flying airplanes I monitor EGT or TIT which have standard limitations so it makes sense in some respects. However you don’t control fuel mixture on boat engines like you do on planes. I recently had the smart crap replaced with vessel view and need to confirm my fuel flow is calibrated correctly.

As for the blowers, the Owner’s Manual says to run the bilge blower 4 minutes before starting an engine or generator, when operating below cruising speed and when the vessel is at rest with the generator running to disapate heat in the bilge. I probably over use them.

 

I should also mention that we have a 11’ Boston Whaler center console with a 25hp on the swim platform that decreases performance. It weighs 750bls.

 

I was always told that 2" up or down is the most you can move pitch. Anybody heard this?

 

Scans attached - props were definitely 28X35 to start with. The 52DB is a heavier boat than the DA. I would have thought we would see slower speeds at the cruise RPM's however speed was about the same albeit I had the bottom redone when the props were off.

 

This is from the parts manual; however, my spares are 35". Strange.....

 

My props were 28/32 when I started and ever had any engine load issues. Currently running 28/28.5. GPH is the key per Tony and I’m not there 100% but very close. The biggest EGT problems we saw before the addition of the dingy was jumping over 1000 degrees when backing off to 15-1800 RPMS in rough seas. That’s where the High EGT we’re showing up. 1800-2000 none issue witH EGT and the tender. Remember when you’re upping you’re RPMS you’re pushing more air with your turbos through the engine.

 

Wow that's a substantial reduction. The DB is 4000lbs heavier so that will make a difference but I have a tender on the swim platform that weighs 775lbs. I read Tony's articles again and he says to reduce prop enough to increase rpms 150-175. The prop shop guy tells me that will be about 28X33 for my boat. I hope he is reducing enough. We'll see.