Cockpit noise level over 1800 RPM

1984 Mainship 34, cummins 6bt 210 engine.
Under 1800 rpm or so, noise level in cockpit is quite low and a non issue, but if I go over 1800 rpm the noise level in the cockpit increases dramatically. Any other MS 34 owners experience this? Possible issues could be the bearing in the bulkhead under the sliding door or possibly the cutlass bearing. Or is this "normal" for this boat. Looking at the prop shaft, I see no vibration when this occurs and it "seems" fine, but the dramatic increase in noise does concern me. Ideas?

Do you have enough clearance between the prop and the hull? It should be at least 10% of the prop diameter.

Was the prop clean when you hauled? You would be surprised how much noise a barnacle or 2 can make. If the prop was clean and you do have your 10 plus percent clearance maybe time to have the prop reworked. Ck the cutlass while you are there.

What kind of noise? Ours (Perkins 6-354) gets louder above 1800 because of the turbo but we run at 1650 most of the time so it isn't a issue. On a previous boat the noise in the cockpit increased dramatically as the speed increased. The problem was the wheels needed balancing & they were causing the hatches to rattle in addition to a hi pitched vibration which caused a harmonic effect back there. After the wheels were balanced, the noise was gone except for the drone of the exhausts.
Somebody on here, Jay Leonard I believe, had to reinforce the hull above the wheel with more fiberglass laminate to get rid of the prop vibration on a Mainship 34.

Yes it could be vibration due to limited clearance between prop and hull. This is common on repowered Mainships.
What is the tranny ratio? What prop do you have? Do you know the clearance?
You can isolate the vibration source with an easy test.
Take a "rocks glass" (real glass not plastic), fill halfway with water. While underway place the glass in various spots. On the cockpit floor directly above the rudder (just aft of the hatch). Open the hatch and place it right in front of the rudder, which is directly above the prop. You will see the water bouncing in the glass at various levels based on how much vibration is there. If it bounces here, then it is likely that the hull is "oil canning" meaning the hull is flexing due to lack of adequate clearance between prop and hull. This is common among repowered Mainships, especially if you have a 1.5:1 tranny ratio.
When I tested mine, the glass itself actually bounced and spilled the water out.
The only cure if this is the cause is to fair the keel ahead of the prop, and reinforce the hull from inside above the prop (most modern Downeast hulls are reinforced for the same reason).
Of course this assumes your cutlass bearing is good, alignment is good, shaft is straight, etc.
By the way fairing the hull is perhaps the easiest way to gain performance. I faired mine the year before I repowered. I gained 250 engine rpm from this change. Picture below.
If you would like further details please send me a PM Good luck!

What kind of water lifts do you have? Is this a new development?

Or is it exhaust noise? Could the wet exhaust be transitioning from a point where there is always water baffling, to having enough exhaust flow that you essentially have straight through exhaust?

I think the first step is to figure out the source of the noise, which can be no small feat. I really like the water glass test.

Thanks for all the replies so far. I'm planning on a haul out soon to do a bottom job. I will check out the prop and test the fairing, replace anodes, bow thruster etc. If nothing turns up at that time, I'll go into more "expensive" surgery, or just not go above 1800 rpm

To reinforce the hull is cheap if you can do half-a$$ fiberglass work.
Scrap plywood, some 1 1/2 ounce mat and a gallon of resin is all you need.
It just has to be functional, not pretty cause nobody looks in there.

And you can do it with the boat in the water.

A long time ago I ran covert boats similar to PBRs that had twin Detroit Diesels and jets. At idle the engines couldn't be heard. At full speed only heard at very close distances. The engines had double water mufflers that had limited restriction.
Besides exhaust noise, much of the engine noise can be transmitted thru the hull. In most military engine setups, the engine is directly bolted to the hull structure. But in the boats I am describing, the engine and jet was isolated with floating mounts that stopped a lot of noise that was common in similar boats. The mounts were so flexible, someone standing alongside the engine could push it sideways. Also all the connections to the engines were flexible like fuel hoses, etc.
Just some ideas.

The current crop of US military riverine assault vessels have a noise spec of audibly undetectable at 5 knots for 30m. Do not ask how I know

Lepke said:

A long time ago I ran covert boats similar to PBRs that had twin Detroit Diesels and jets. At idle the engines couldn't be heard. At full speed only heard at very close distances. The engines had double water mufflers that had limited restriction.
Besides exhaust noise, much of the engine noise can be transmitted thru the hull. In most military engine setups, the engine is directly bolted to the hull structure. But in the boats I am describing, the engine and jet was isolated with floating mounts that stopped a lot of noise that was common in similar boats. The mounts were so flexible, someone standing alongside the engine could push it sideways. Also all the connections to the engines were flexible like fuel hoses, etc.
Just some ideas.

Click to expand...

Amazing noise spec!
Love the water glass test too.