Sound baffling for the motor

[DaveC426913]

Has anyone tried any mods for quieting the motor?

Honda BF50.

I sit at the helm and motor a lot, and it can be quite exhausting to yell over the motor on a several hour trip.

I tried cutting down a big Rubbermaid container and placing it in front of the motor to hopefully direct the sound sternward but it had no discernible effect.

Blocking the under-seat gap does not really do much either, since my ears aren't down there.

[Gotro]

Soon I am to attempt to build just a square plywood over the motor . Not sure how to connect it yet, might make the plywood fold in half so it’s easier to store. Really confident it will reduce some motor noise . Will post some pics soon

[NiceAft]

I know your pain.

I have a 2003 Honda 50 HP four stroke. It’s an extremely quiet motor as long as it isn’t going full throttle. I’ve done fifty mile runs but kept the RPM’s down, and with it, the overly loud noise, by keeping the speed to no more than 10 MPH (8.6knots).

[DaveC426913]

kept the RPM’s down, and with it, the overly loud noise, by keeping the speed to no more than 10 MPH (km/h).

10MPH? That's 8.5 knots.
That's in the high range for me. (Full throttle for me is 11 knots).

My top comfortable cruise speed is 7knots, and I keep it at 5 if I want to have my hearing recover.

[NiceAft]

This may sound strange, but the M’s captains seat sits appreciably higher than the X’s. I’m just wondering if there is some sort of loud resonance with the lower seat?

[Be Free]

kept the RPM’s down, and with it, the overly loud noise, by keeping the speed to no more than 10 MPH (km/h).

10MPH? That's 8.5 knots.
That's in the high range for me. (Full throttle for me is 11 knots).

My top comfortable cruise speed is 7knots, and I keep it at 5 if I want to have my hearing recover.

I have the BF40. It is, for the most part, a detuned version of your engine so my measurements should be applicable to your setup. For clarification, do you have a BF50A (3 carburetors) or the BF50D (fuel injected)? Do you know the specs on your prop?

My boat is loaded pretty heavily. I'm sure I'm close to the top end of the norm and I'm seeing much better "numbers" than you are reporting. Below full throttle I don't have any issues with noise. These values are fully loaded, no passengers, no ballast.

idle 2.00 kts
1/4 4.50 kts
half 7.00 kts
3/4 12.00 kts
Full 16.00 kts

[DaveC426913]

This may sound strange, but the M’s captains seat sits appreciably higher than the X’s.

I've actually been thinking of modding my X's seat to have a low hump. I find the seat a bit low, and it would be nice to have a comfy spot when on a heel.

[DaveC426913]

I have the BF40. It is, for the most part, a detuned version of your engine so my measurements should be applicable to your setup. For clarification, do you have a BF50A (3 carburetors) or the BF50D (fuel injected)? Do you know the specs on your prop?

I don't know. But I can find out tonight when I go see the boat if you tell me what to look for.

idle 2.00 kts
1/4 4.50 kts
half 7.00 kts
3/4 12.00 kts
Full 16.00 kts

My top clocked speed was 13.5 knots. (Maybe I should get a BF40 like you! )

My throttle hits the capstan before to maxes out. That's proably a good thing. I'd hate to think what I'd do if I could push it higher.

[Be Free]

Take the cover off the engine. If you see three carburetors it is a BF50A. If you see no carbs it is a BF50D.

You will find two numbers stamped either on the hub or on one of the blades of your propeller. They will be in the form ##X##. The first number is the diameter and the second number is the pitch. Another important data point is the number of blades (most of us have 3) and the material (stainless steel or aluminum). If it is shiny it is stainless, if it is painted ( or anodized) then it is aluminum (most common). Is there any damage on the blades (nicks, curled edges)?

Do you happen to remember what the engine RPM is at full throttle? A ballpark number is good enough..5000? 6000? 7000? With a (possibly) lower than expected top end and an overly loud engine my first suspicion would be that your propeller does not match your boat requirements. You could also have a "spun" hub that is slipping at high speed (slightly less likely).


BF50A with carbs. The big black plastic thing to the right is the air filter box. It also acts as a muffler. If it is missing you might be getting more noise than usual.


BF50D. The injectors are under the vertical silver part. Another clue is the little fuel filter/water separator just left of the orange dipstick. Only the "D" model came with the filter here.

[NiceAft]

There are many variables to top speed and RPM's.

My can also hit 17 kts, but no one else is on the boat, and the boat is empty; other wise, my top speed is 12 kts. At 12kts I show the RPM to be just under 6,000 with a fully loaded boat. Fully loaded means a cruise of about two weeks in not very choppy water, etc. That's when I keep the speed to no more than 8.7kts.(to conserve fulel on long runs) unless I need to make it to the next marina by a certain time, or the weather dictates. I have two 12 gallon tanks, and carry a 20 liter Wavian Jerry Can.

[Be Free]

There are many variables to top speed and RPM's.

My can also hit 17 kts, but no one else is on the boat, and the boat is empty; other wise, my top speed is 12 kts. At 12kts I show the RPM to be just under 6,000 with a fully loaded boat. Fully loaded means a cruise of about two weeks in not very choppy water, etc. That's when I keep the speed to no more than 8.7kts.(to conserve fulel on long runs) unless I need to make it to the next marina by a certain time, or the weather dictates. I have two 12 gallon tanks, and carry a 20 liter Wavian Jerry Can.

I totally agree. I'm trying not to overwhelm him with too much theory right now. Remember, the problem he's trying to solve is noise. If he's having to drop to 5 knots to keep the noise in a comfortable range something is wrong. The fact that he can only get 11 knots at full throttle points in that direction as well. If we determine that excessive RPM is the root of his noise problem we can start to narrow in on the cause of the high RPM.

The numbers I posted were measured fully loaded for cruising, no ballast, no crew. I would expect around 12kts on your boat under those conditions if you run with the ballast in. Also, if you are topping out just shy of 6000 rpm at full throttle you have the right prop (assuming maximum speed was your priority).

[DaveC426913]

Take the cover off the engine. If you see three carburetors it is a BF50A. If you see no carbs it is a BF50D.

I don;t know what a carb on an outboard looks like but that loks more complicated than mone.

You will find two numbers stamped either on the hub or on one of the blades of your propeller. They will be in the form ##X##. The first number is the diameter and the second number is the pitch. Another important data point is the number of blades (most of us have 3) and the material (stainless steel or aluminum). If it is shiny it is stainless, if it is painted ( or anodized) then it is aluminum (most common). Is there any damage on the blades (nicks, curled edges)?

It's 3 and painted. I'll check the pitch.

Do you happen to remember what the engine RPM is at full throttle?

Tachometer is not hooked up.

my first suspicion would be that your propeller does not match your boat requirements. You could also have a "spun" hub that is slipping at high speed (slightly less likely).

Oh. How do I check either of those?

[Be Free]

I know you were looking for answers regarding the engine noise but based on your other comments I don't think you have a noisy engine at a reasonable RPM. It sounds like you have a normally noisy engine that you are having to run at a higher than normal RPM for the speed you are going. The water is your muffler. Most of the sound (and exhaust) come out underwater through the center of the propeller. Even with the cover off the engine should be fairly quiet.

There is a small exhaust port on the back of the engine but it does not normally handle the majority of the exhaust. If something were to block the hub exhaust more would come out of the small port but I can't image anything that could plug up the hub that would not be very obvious.

I just though of a very long-shot item but it's easy to check. Make sure that all three of your spark plugs are tight and not leaking. A loose plug would hurt your top-end speed and can be noisy (very noisy if it come out completely!)


The red arrow is pointing to the fuel filter / water separator on the fuel injected model. If you have this then the comments below regarding the carbureted model don't apply.


The red arrows are pointing at the carburetors. The green arrow is pointing at the air box / air filter. If the air box is missing or loose then you will get a lot more noise out of the engine. This would not hurt your top end speed though.

So far we know you have a three blade aluminum propeller. That is the most common prop for this engine. Without a working tachometer I won't be able to give you an opinion on whether you have the best propeller but when you find the numbers I may be able to tell you that you have the wrong propeller.

Propellers need to have a way to protect the expensive parts of the engine when you hit something hard. Back in the "bad old days" they had a shear pin. This was a soft pin that went through the end of the propeller and the drive shaft. If you hit something hard with the propeller the shear pin would break (shear) removing the connection between the drive shaft and the prop (usually before anything got too bent or broken). I can't tell you how many times I've changed a shear pin hanging over the transom praying that I would not drop my last spare.

Modern propellers have a rubber hub between that couples the propeller to the drive shaft. This rubber will break and slip if you hit something hard. This is commonly referred to as "spinning a hub". Unlike the shear pin, which was either broken or not broken, the partially spun hub can sometimes still allow the prop to spin if you don't push it too hard. That may be the reason you are not going very fast at full throttle.

You may be able to id a spun hub by putting the engine in gear (but not running!) and then see if you can turn the prop by hand. If the prop turns easily while the engine is in gear you probably have a damaged hub.

Another possibility is that your prop is not slipping where it connects to the engine but it is slipping in the water either because it is the wrong size or the wrong pitch. That's what the numbers may point to.

[mac n cheese]

Anyone here have the transom filler mod from BWY and does it work? I considered it but I fish from my alot and need that space when standing up when landing a fish. It would be just more clutter to deal with removing it all the time.

[NiceAft]

On my I have used my companion way insert for that.

I found that it worked to soften the sound somewhat, but because it interfered with raising the motor, it wasn’t worth it. On an there may be more room there?