Exhaust Hose; 1.5" or 2" ?

[John, CD28]

With my new Beta 13, I can run 1.5" or 2" exhaust hose. Are there any advantages to one over the other? Would 2" have too much volume, and greater risk of back-flooding the muffler & engine? Would 1.5" hose have faster exhaust gas velocity, pushing the smaller water volume out faster? Would 1.5" have more back pressure, harmful to the new diesel? Am I over analyzing this?

All opinions much appreciated,
John

[Tim]

One thing that might determine which size you choose is whether you're attempting to connect to any existing fittings, and which size they are. Obviously, sizing your new hose to match any existing fittings would make sense, but this may not apply.

Larger hose reduces back pressure, which is good in general, but 1.5" hose is adequate for that small engine. Smaller hose and smaller fittings tend to be less expensive, so that might be a good reason to choose that since you're given the option.

There's no real pitfall to using the larger 2" hose, other than its additional cost and the additional cost of fittings (through hulls, waterlifts, etc.) to accommodate it.

[bcooke]

I think it would take a real engineer to know for certain which is best. Considering that the manufacturer says 1.5" or 2" I would say whichever suits the other fittings in the system.

A larger diameter hose means less constriction and better breathing for the engine.

A larger diameter also means the exhaust gasses don't have to move as fast so they transmit a lot more heat on to the exhaust piping on their way out which increases corrosion and wears the exhaust system out faster. Exhaust piping is a consumable part. It doesn't live forever.

The engineers that design the motor look for a balance between these two outcomes.

For the size of motor we are talking about I would have to think 1.5" is plenty big enough . My Honda Civic motor is very efficient with 1.5"

-Britton

[Figment]

I would think that there is little (if any) reduction in backpressure with the larger hose in what I assume will be a waterlift exhaust, because the larger hose belches heavier slugs of water.

I do think that larger hose and a larger muffler will be quieter, however.

I think that your decision might be made for you when you try to fit a muffler sized for 2" hose.

[Peter]

IMHO, it's my understanding that oversized pipe actually decreases engine efficiency, as the engine requires a certain amount of back pressure to run properly.
My Yanmar 2GMF 13 hp exhaust manifold outlet is only about 1" diameter (if my memory is correct) and uses 1 1/4" pipe for the riser. From the end of the riser to the Vetus WLOCKL40R muffler I used 1 1/2" ID hose (that can be persuaded over the 40mm muffler inlet using a bit of rubber lube), and 1 1/2" hose to the riser at the transom.

[bcooke]

Peter, you might be right about that. I am a bit foggy about the engineering details. I don't think the engine requires a certain amount of back pressure so much as it is designed to work with a specific amount.

I DO know that going up too much in size can cause your exhaust manifolds to glow cherry red and melt the manifold gaskets in no time flat. Not that that is a possibility with a 2" hose of course.

-Britton

[John, CD28]

Much thanks all!

The old Volvo MD7A (also 13hp) had 2" hose. The bronze exhaust thru hull is 2" bronze.

So, I'll order a muffler with 2" fittings, and some new 2" exhaust hose, and move forward.

Any suggestions for mufflers? I'm thinking maybe a Vetus LP, or Vernalift, or Naqualift. Any advantages or preferences in this group?. Others? The quieter the better.

Thanks again,
John

[Tim]

If you have space, the fiberglass Vernalift waterlift mufflers are a nice choice. They're bulky, though, particularly in 2" size.

The plastic Vetus ones are fine, but they can melt if for some reason the cooling water flow is stopped. But they are small and versatile, and inexpensive, so they often fit the bill.

[John, CD28]

Assuming this motor lasts 30 years like the old one did, there's a good chance there will be cooling problems along the way. I don't want to compound a cooling problem with a melted muffler too. Maybe Vernalift is the way to go.

Thanks Tim.

John

[Tim]

I should add, in case it's not clear, that I have no personal problem with the Vetus units. I have one, I've installed others. But in each case, I chose it simply because it fit in a tight space where others would not.

Given my druthers, I'd choose the fiberglass versions every time. Realities don't always allow it.

[Peter]

Good Old Boat Magazine had an interesting article on how to make your own waterlock muffler in their Issue# 54-May/June 07.

From GOB articles index:
"Exhaust ingenuity: Paul Ring's friend, Ken Simmons, had a problem with his Pearson Vanguard: How to fit a properly sized and correctly positioned waterlift muffler in the engine area when replacing the Atomic 4 with a Yanmar. Paul's elegant custom-built solution was to build the muffler from epoxy and have it (built as a V-shaped unit) surround the prop shaft. Paul explains how he designed, built, and tested this muffler."

[feetup]

The diameter of the exhaust for internal combustion engines is not related to the "back pressure" so much as it is to the velocity of the exhaust gasses. The engineering gets pretty complex but the concept is that a moving column of any fluid has inertia and the quicker you move it up to a point the more it tries to keep moving, and the more it will create negative pressures between the "pulses" of the cylinders firing, in essence trying to 'suck' the exhaust out of the cylinders when the next exhaust stroke comes. Having a large diameter passage allows the gasses mixed with water to meander along, allowing the water to "puddle" and get blown out when the puddle gets too large. A smaller diameter passage keeps the gasses moving briskly and causes the cooling water to exist pretty much as spray, (except in the trap) which makes it move better, and the spray also destroys the accoustic qualities of the tube better, making it quieter. There is no benefit going larger than the exhaust fitting on the engine.
You can however restrict too much, but that is obvious.

Feetup

[John, CD28]

I'm seeing wet exhaust hose with wire, and some described as "soft exhaust hose" without the wire. The price is pretty close - es there any significant difference? Any advantage or disadvantage to either?

John

[Tim]

When you have bends in your hose you need the wire-reinforced type to prevent kinking and collapse.

I don't know what "soft" exhaust hose is, but in general exhaust hoses are called "hardwall" because of the makeup of the hose itself, with or without wire. Whatever you use, be sure it's marked as "wet exhaust hose" (the exact markings vary brand to brand) and you're ensured of the correct product.

[Figment]

When you have bends in your hose you need the wire-reinforced type to prevent kinking and collapse.

but don't let the fact that the hose is reinforced to withstand bending fool you into thinking that it bends...

[Tim]

...don't let the fact that the hose is reinforced to withstand bending fool you into thinking that it bends...

How true, how true!

The most bendable reinforced hardwall exhaust hose is the corrugated type, such as this Shieldsaust- C. The corrugations make bending almost easy.



Shields Hose

[John, CD28]

Ok, my muffler and hose arrived this week; I went with the Vetus LP, as it is narrow, and looks easy to fit into tight places.

I was curious about this wireless wet exhaust hose from Trident, and it looks pretty neat. It's rated for 250° and bends easily. As it bends, it assumes more of an oval shape than round, but it doesn't collapse. If you bend it too much, It looks like the oval flattens out to a point where it would restrict flow, but it's obvious as it happens.

This Trident "soft wall" hose came from Defender, and prices about 10% higher than Trident wired exhaust hose.

Best,
John

[John, CD28]

I'm really concerned about the 2" hose now.

If I did the math correctly (pi x radius squared?), the 2" hose has 1.7x the volume of 1-1/2" hose. This means 1.7x the water volume will be sitting in the 2" hose between the muffler and the top of its rise after the muffler. These are not dry exhaust systems; I suspect the greater weight of this larger mass of water would actually increase back pressure, rather than decrease it, as this water has to be pushed away in the muffler to let exhaust gas past the outlet within the muffler.

Here's my concern; When the motor stops, the water between the muffler and the top of the rise falls back down into the muffler. Are these little 0.8 gallon mufflers large enough to hold all that water? If not, some of it floods into the engine, correct? I'm thinking the 1.5" hose with its lesser volume has a real advantage here, and is plenty large for my little 13hp diesel.

Of course, the absolute volume of water between the muffler and the rise depends on hose length to the rise. How far back are you guys putting the high point in the exhaust run after the muffler? I know the shorter the better, but is looping it up at the transom too far back, creating too much volume for the muffler to hold?

I really don't want to kill my new engine first day out. Any and all input much appreciated.

John

[Tim]

How far below the engine exhaust outlet is your muffler? It should be well below the exhaust outlet and/or riser, if equipped. It's this vertical distance directly downstream of the engine outlet that provides that backflow protection when you stop the engine, on top of the volume of the muffler, etc.

In other words, the exhaust hose should be able to drop a certain distance between the engine outlet and the top of the muffler; sometimes this is accomplished with the positioning of the muffler (i.e. well beneath the engine), and sometimes you need a high riser/loop to accomplish this. It all depends on where your muffler is located relative to the engine outlet.

The high loop at the transom discharge is to help prevent backflow of outside water into the system through the final transom exhaust outlet, not to limit how much exhaust water might flow backwards when the engine is shut down. Two different height differentials, two different functions.