Black smoke from the diesel = too much pitch for the prop?

[Duncan]

There's a thread running in the Buy/Sell section, regarding black smoke on the transom for some 25-28' Cape Dorys.

I recall seeing similar discussions, and the consensus seems to be that "they are underpowered, and shouldn't be run at full throttle". I can understand how throttling back reduces demand on the engine, but this rationale seems backwards to me.

If the prop is pitched correctly, shouldn't the engine be able to run at full revs? If it's underpowered, then those full revs won't drive the boat at hull speed, but there still wouldn't be any black smoke.

I've experienced the same thing on my (Yanmar YSE8 - powered CD 27), and I believe that the problem is not that they are underpowered, but that the props are over-pitched.

Another CD Board member sold me a lower-pitched prop, but I haven't tried it yet. Can anyone correct me if my theory is wrong here?

(edited to fix a typo)

[JWSutcliffe]

Black smoke in diesels is a symptom of incomplete combustion and is typically due to one or more of the following: too much fuel, not enough inlet air, restricted exhaust or incorrect injector pump timing. Having an overpitched prop will prevent you from reaching the engine's maximum speed before the injector rack is at full fuel delivery. Pushing the rack further will dump excess fuel into the engine and produce black smoke. The key here is not reaching maximum rated speed. If you can reach max speed, the prop is not overpitched. Stated differently, if you can push the rack further and still increase engine speed, then the engine is not maxed out.

My Universal M25 soots up the transom slightly every season, and I can reach full rated speed without any problem, indicating that my prop is sized correctly. I tend to blame it on the long, restrictive exhaust system.

[Duncan]

Black smoke in diesels is a symptom of incomplete combustion and is typically due to one or more of the following: too much fuel, not enough inlet air, restricted exhaust or incorrect injector pump timing. Having an overpitched prop will prevent you from reaching the engine's maximum speed before the injector rack is at full fuel delivery. Pushing the rack further will dump excess fuel into the engine and produce black smoke.

Thanks, Skip, this part is very clear, and confirms what I had previously understood.

The key here is not reaching maximum rated speed. If you can reach max speed, the prop is not overpitched. Stated differently, if you can push the rack further and still increase engine speed, then the engine is not maxed out.

There is no tach on my Yanmar YSE8 panel, so I don't know if the engine can achieve max rated revolutions.

What I can say for sure is that the engine can't achieve max speed without black smoke. If I "push the rack further", I can get a little more speed, but also a lot of smoke. (The engine was completely rebuilt, so for now I am ruling out engine condition as a cause).

[tjr818]

There is no tach on my YSM8 either. How hard would it be to install a tach on a diesel?

[Duncan]

There is no tach on my YSM8 either. How hard would it be to install a tach on a diesel?

I've heard this tach recommended: Tiny Tach link.
(You can click on "installation instructions" at that address).

There are endorsements for it from Cape Dory owners here (link)

I also found this good CD-27 Prop thread (link), which contains a particularly helpful post from Andy Denmark.

I think I will try out my 3-blade, lower-pitched prop next season, and post some feedback.

[Matt Cawthorne]

How fast is your boat moving when you start getting a little extra boat speed and a lot more smoke? If you are close to what is commonly referred to as hull speed, then you should expect this behavior. Without a tachometer you want the prop pitch be high enough that the propeller demand curve exceeds the power available curve so that the engine can't exceed it's rated RPM. High prop pitch also keeps the engine running slower when you cruise along, and keeps the cylinder temperature higher. The bad news about over pitching the prop is that when you need full power, you can not get all that the engine has to deliver. The goal, therefore is to get the propeller demand curve to match the power available curve exactly at the maximum rated RPM with a clean prop. I would venture a guess that this condition is rarely achieved. Once barnacles get hold of the prop the power demand curve will go up anyway.

Matt

[ilanaverbuch]

Hi- Does anyone know which 4-stroke engine fits in the outboard engine well of a Cape Dory 25, preferably with a remote control? Right now I have a 2-stroke 8 HP Mercury outboard engine which just died and won't start. Thanks so much.

[Duncan]

How fast is your boat moving when you start getting a little extra boat speed and a lot more smoke? If you are close to what is commonly referred to as hull speed, then you should expect this behavior.

I can get about 4 knots in most conditions, but anything much faster than that starts causing black smoke.

From memory, 4 knots on a 20 foot waterline requires about 4 hp, so I don't think I am getting full use of my 7 hp engine. I reckon I should be able to get 5+ knots if the prop is pitched right.

[Matt Cawthorne]

Duncan,
I would agree that black smoke at 4 knots is premature. Is your prop clean? Barnacles can affect it's performance greatly. A comparison between your boat/prop and others of the same size and engine would be useful. What you need to ask other users is what engine/transmission ratio/prop pitch and prop diameter combination they have on their boat. A lower transmission ratio is equivalent to higher pitch. Right now my money is on a dirty prop or an over-pitched prop.

A built in tachometer is useful, but if you want to consider another tachometer option, I bought a photo-tachometer off of ebay a few years ago. I used it to calibrate my built in tachometer. it was reasonably inexpensive. All that you need to do is put a small piece of reflective tape on the engine pulley, point the unit at it while the engine is running and you get the RPM.

[Duncan]

Duncan,
I would agree that black smoke at 4 knots is premature. Is your prop clean? Barnacles can affect it's performance greatly. A comparison between your boat/prop and others of the same size and engine would be useful. What you need to ask other users is what engine/transmission ratio/prop pitch and prop diameter combination they have on their boat. A lower transmission ratio is equivalent to higher pitch. Right now my money is on a dirty prop or an over-pitched prop.

A built in tachometer is useful, but if you want to consider another tachometer option, I bought a photo-tachometer off of ebay a few years ago. I used it to calibrate my built in tachometer. it was reasonably inexpensive. All that you need to do is put a small piece of reflective tape on the engine pulley, point the unit at it while the engine is running and you get the RPM.

Thanks again, Matt, for your thoughts. The prop is very clean, since I'm in fresh water.

I am pretty sure this is the original prop, and I've heard black smoke/low speed stories from other CD 27s with the YS Yanmars.

I can update things when I haul out in a couple of months.

[Matt Cawthorne]

If you are thinking of getting a tachometer (or a photo tach) do it now and gather some data. It will help in deciding what pitch is correct. ie, motor at 3 knots, measure engine rpm, then 3.5 and get rpm...etc until you get black smoke.