Vibration between 1800-2100 rpm

[vickyb]

My boat was recently surveyed 3 weeks ago when outside the water and shafts and props were found to be fine. However, when in the water, there is a wicked vibration/shimmy at 1800-2100 RPM that goes away at lower RPM's and when higher than that.... checked the engines by keeping the floor panels open when running and they run perfectly. Interestingly, cannot feel that vibration much when sitting down below near the engines... so its definitely not in the boat. Is that just normal as a "bad rpm spot" for this type of boat? Similar to my motorcycle that vibrates violently at low RPM but everything seems to "tighten up" once the throttle is open (Harley Davidson of course)

Thanks in advance for any insight

[bud37]

A question.....did this happen on the sea trial or just start now ?? If this is what you are seeing on the very first time out then there could be a running gear issue....we need more info.

Considering your previous issue with the fuel, this could be a continuance. That rpm spot is when the engines are just starting to get under heavy load powering up, could be engine missing there. You can not reproduce that loading when in neutral. It is very hard to diagnose these things without being there.

My advice, regardless of the above vibration is to change out the filters as you said and keep the load off those engines till you burn off that fuel, that means if it were me I would run the boat at hull speed around (1600 rpm or less) ,no more, unless you find something in the fuel filters when they are changed and the problem clears. There are injectors/plugs etc to consider and as you found out they don't like low power/batteries

It is not normal for that, if all was well it would be smooth all the way up and down the rpm/loading range. Did this show up on the sea trial ?

[vickyb]

Thanks for your response. The fuel filters were changed. I had a marine mechanic at the engine when I did the full run up and he did all the checks on the engine. It was perfect. No knocking or vibrating at all.

[Viper]

So many possibilities. Certainly the engines can be causing a vibration. Simply listening to them though if that's all that was done, is not enough to conclude that they're running "perfect." When you say "...the shafts and props were found to be fine" how was that conclusion reached? Were the props sent out to be tuned? Going by eye will not reveal subtle tweaks that can cause vibrations. Were the shafts dialed? Were the strut bearings inspected? Have the couplers been detached from the trannys and the alignment checked? Until you do all that, you can't eliminate the running gear or alignment as possible causes.

Sometimes assemblies on board; panels, cabinetry, appliances, etc. can also contribute to vibration, as well as furniture, mounted accessories, etc. Had a customer that spent most of a summer trying to source a "vibration" in his cockpit. Spent a lot of dough checking the running gear but got nowhere. Turned out to be a spare anchor he had stowed in the bilge below, a vibrating transom door, and the swim platform ladder. One of the keys here is to determine what category the issue falls under; is it due to a mechanical deficiency as noted in the first paragraph that will actually cause the hull to vibrate, or is it acoustics resulting from something bouncing around. Either category can present itself more at certain rpm ranges than others.

[Helmsman]

Source of the post My boat was recently surveyed 3 weeks ago when outside the water and shafts and props were found to be fine. However, when in the water, there is a wicked vibration/shimmy at <a href="tel:1800-2100">1800-2100</a> RPM that goes away at lower RPM's and when higher than that.... checked the engines by keeping the floor panels open when running and they run perfectly. Interestingly, cannot feel that vibration much when sitting down below near the engines... so its definitely not in the boat. Is that just normal as a "bad rpm spot" for this type of boat? Similar to my motorcycle that vibrates violently at low RPM but everything seems to "tighten up" once the throttle is open (Harley Davidson of course)

Thanks in advance for any insight

Had the very same issue. The props were bent in my case. Just enough to cause the vibration between 1800 and 2100 rpm. Got them straightened and no more issues.

[vickyb]

Thanks again for the last two responses. Will verify the items by process of elimination.

[BarryK]

I had metallic noise and vibration from one engine that I thought was a bad transmission. It turned out that the engine needed a tune up! I replaced the distributor cap, rotor, plug wires, and plugs and the problems went away. My Crusader engines are on rigid engine mounts as opposed to rubber mounts in cars, so the engine cannot vibrate without shaking the whole boat. In my case it was worst at low rpm.

Barry