Weird Vibration at 5 mph GPS

[Fisnfool]

1965 Starcraft Holiday, Merc 110 4cyl inline.

The deck on this boat was replaced in 2004. I do not know if engine moust were replaced.

Before the season ended I started getting a vibration that rattled everything in side tackle lockers and the forward hatch even rattled.

This only happened at about 5 mph as read on a gps. Not above or below around 5mph. Boat runs fine no miss or other problems, and will even hit 30 mph gps if I have to get back in fast. Normally only run about 2/3 throttle max. Timing checks OK. Mechanical advance on the distributor seems to work. It advances the timing mark as I accelerate. ( on the water test)

I am thinking, gimbal bearing, U-joint, Engine Coupler, or Engine Mount.

I am going to pull the stern drive later and work on the problem this winter.

Any Ideas on what failure would normally cause this boat shake?

 

[triman]

Re: Weird Vibration at 5 mph GPS

Others are more qualified to discuss possible mechanical reasons for a vibration, but just to throw in a curly one.
Is it possible for there to be some harmonics involved between the engine and the propellor than only manifests itself at a slow speed?
Remembering that most ships have a "critical speed" range where if the engine is run for too long at that rev speed the engine MAY self-destruct.
I also had a need to change my propellor on a V8 sterndrive after a 3-blade had a problem. I threw a brand spanking new 4-blade on her and couldn't handle the vibrations at 3 speed ranges. Went back to a new 3-blade and the vibrations disappeared.

Just random thoughts.

 

[Fisnfool]

Re: Weird Vibration at 5 mph GPS

Thanks for the tip. I was thinking that at this speed it may be a harmonics problem with the aluminum hull.

But I have had other boats that ran at this speed without a problem. 5 mph is the posted speed max from the marina launch point to the mouth of the river.

This rattle makes it sound like the boat is shaking apart. So I run at 4 mph while in the river.

Waiting for other replies.

 

[ErieRon]

Re: Weird Vibration at 5 mph GPS

Try putting a different prop on, like a spare, or borrow one. Results will eliminate or confirm if it's a prop issue. Vibration is usually a prop or prop shaft issue, but could be a number of other things. Can you watch your engine/mounts while someone else pilots at 5 mph? Also check the lag screws where the front mount is secured to the bottom of the boat (if applicable).

 

[Fisnfool]

Re: Weird Vibration at 5 mph GPS

Thanks. I actually have a spare complete stern drive with prop that came with the boat. Original owner lived in a warmer fresh water area and did not like any down time for routine stern drive service. (must be nice to have deep pockets)

Got a second engine too but that block was cracked when he lived up north.
(stripped all the parts for spares and I sold the block for scrap)

I can switch out the stern drive after I put a new impeller in the second one.

 

[ErieRon]

Re: Weird Vibration at 5 mph GPS

Hey Fisnfool,

After I posted about switching the prop, I started thinking that, that is an awfully low RPM for the prop to vibrate everything as severely as you described. I would still check it, but how is your engine running at that low RPM. I know my 4 cyl is pretty jumpy at idle and low RPM sometimes. That, combined with mount issues may be a point of focus too.

 

[Fisnfool]

Re: Weird Vibration at 5 mph GPS

Idle is rock steady on the tach when checking timing. The vibration is only;y at the mentioned speed range an the tach is still steady.

That's why I was thing engine mount or U-Joint.

Switching out the outdrive will eliminate U-joint and prop. Could an engine coupler vibrate this bad too but only at a particular speed?

I have to lift the engine to change the Alternator / water pump belt, so I can can check the mounts and coupler then.

 

[ErieRon]

Re: Weird Vibration at 5 mph GPS

Idle is rock steady on the tach when checking timing. The vibration is only;y at the mentioned speed range an the tach is still steady.

That's why I was thing engine mount or U-Joint.

Switching out the outdrive will eliminate U-joint and prop. Could an engine coupler vibrate this bad too but only at a particular speed?

I have to lift the engine to change the Alternator / water pump belt, so I can can check the mounts and coupler then.

Sounds reasonable and like a good plan. Engine coupler Not too sure if it would cause something like that. Usually they're good or bad, but you never know. Most are rubber encapsulated, so if it was at the beginning stages of failure...maybe.

 

[Fisnfool]

Re: Weird Vibration at 5 mph GPS

Thanks. Maybe Since I already have the outdrive off, and a coupler is usually good or bad, I'll just raise the end of the engine enough to change the belt.

I can do that without pulling the engine. I do not have an engine hoist, and it is a hour and a half round trip to borrow one. So that would be three hours counting the return of something I may not need.

I'll check to see if the local Auto zone rents the hoist under there free rent program. They are only 10 minutes down the road.

If not and I need to pull the engine, I try a tool rent place. My be cheaper than fuel for three hours.