yamaha outboard out of oxygen

[AlexMD]

hi,
I am new to the community and new to boating. I just bought my first sailboat, a Neptun 25 which is a 25 foot boat equipped with a Yamaha outboard engine that sits in a sort of “engine bay” below the cockpit (so not hanging at the AFT of the boat where usually outboards are). This is how the boat was built.
Engine model is Yamaha FT8DE - 8HP four stroke.

During my first motor sail with the boat I discovered 2 problems with the engine. Here is the first one:

1. when starting/boat staying still, or at slow speeds, the engine stalls after a while. I did some troubleshooting and it seems that it is running out of… oxygen? The reason why I am saying that is because the upper exhaust of the engine pushes a lot of smoke in the engine bay, and that, besides being unpleasant for whoever is in the cockpit, also I think starves the engine of oxygen. If I lift the wooden deck floor (which provide access to the engine) then the engine would not stall and run OK. But then the entire cockpit becomes very impractical (nowhere to step), and also all the smoke really comes in the cockpit.
   Any ideas what I could do? Can I build some sort of exhaust for this engine?
   Here is a picture with the engine running (but having the cockpit floor removed so that it could run

 

[99yam40]

use your sails instead of the motor.
looks like a really bad design.
most of the exhaust should go out the prop hub unless it is just idling
or you have a bad gasket

 

[fatlenny]

A lot of outboards have an exhaust relief up out of the water. I agree with the previous response that this is a really bad design. If it is possible I would locate a way to cut an air inlet powered by a marine type bilge blower fan that will suck in fresh air when ever you are running this outboard, then see if you can find a place to cut out a hole to route the exhaust fumes that build up out away from the cockpit.

 

[AlexMD]

use your sails instead of the motor.

That's a bit harder in the marina during docking maneuvers or low speeds, which is exactly when the smoke accumulates and the engine stalls. But thank you for your advice, in general it is super good

If it is possible I would locate a way to cut an air inlet powered by a marine type bilge blower fan that will suck in fresh air when ever you are running this outboard,

This is a very good idea. I have a few holes between the engine bay and the cabin (which now allows for smoke to go into the cabin). I'll see if I can install some fans there to push air from the cabin to the engine bay.

then see if you can find a place to cut out a hole to route the exhaust fumes that build up out away from the cockpit.

The big question I have about that is how to design this exhaust system. Like does the hose that I will put in (between the engine bay and outside) needs to have a certain shape or angle? Any advice about the design would be great!

 

[fatlenny]

The big question I have about that is how to design this exhaust system. Like does the hose that I will put in (between the engine bay and outside) needs to have a certain shape or angle? Any advice about the design would be great!

Well as this wouldn't be an "exhaust pipe" in the normal sense that would be taking a lot of heat, I would say just a vent pipe of some sort. I would think a 4" flexible dryer vent hose would work in this application. I wouldn't use the plastic type. With the positive pressure you would create with a blower fan introducing fresh air, where ever you put it in this engine bay should work as long as it is not possible for it to ever be below the water line or water to get into it. With water intrusion possible I would suggest that it has no low spots in what ever route you may take as this would hold water and most likely plug the vent. You could cut a hole in the cover and have a removable chimney that you only put in place when needed and stow when not in use. I wouldn't be able to "design" or give advise other than what I have already mentioned without being there to look over the entire setup and possibilities. I hope you do understand what I am proposing may be considered by others as "jerry rigging" but if you can keep things out of sight when the engine bay is closed and it works for your needs, I don't see any issues with it.

 

[airshot]

I would think a powered exhaust would be better than a blower. Anyway to make a 3-4" dia exhaust outlet then use a bilge blower to suck out the smoke ? By sucking out the exhaust, you will be drawing in fresh air to the surrounding areas. Just a thought......

 

[fatlenny]

I would think a powered exhaust would be better than a blower. Anyway to make a 3-4" dia exhaust outlet then use a bilge blower to suck out the smoke ? By sucking out the exhaust, you will be drawing in fresh air to the surrounding areas. Just a thought......

I thought about that and believe you would be correct that this would be more efficient that way but I was thinking about the longevity of the blower unit. Sucking only fresh air should be no issue for it. But sucking oily smoky air, and any water that may splash around may damage the blower quickly. I am not sure of the durability of these blowers.

If they are good for this environment, I agree. use in a way to suck the dirty air away from the compartment and maybe cut fresh air vents to allow more air in if needed. But I would worry about my noted concerns above.

 

[airshot]

The blower on my 1983 Starcraft is original if that is any indication of life expectancy...

 

[flashback]

Replace the cockpit access cover with a piece of grating, teak or expanded metal. The poor motor needs to breath and I also bet it gets pretty hot in there..

 

[fatlenny]

The blower on my 1983 Starcraft is original if that is any indication of life expectancy...

ya but,,, its not sucking or moving oily exhaust fumes or splashing water.

 

[99yam40]

ya but,,, its not sucking or moving oily exhaust fumes or splashing water.

and possibly no salt water environment