black soot on transom

[ROB BONDY]

i have a 2001 40' sea bridge with 450 hp cummins 6cta. I ran the boat at about 1600 rpm for a slow trip and had black soot all over the back of the transom. Cleaned up the mess and ran home at cruising speed (2100 rpm) and the transom was clean. Is this something i should be worried about or just try to stay away from certain rpm where the turbos are quite kicking in?

 

[trit21]

Since the black soot on the transom is not there at cruise speed, it sounds like you need to stay away from 1600rpm as there is not enough air to burn the fuel at that rpm due to your load you are putting on the boat. You will also see that same 'soot' when you are getting over the hump while getting on plane but once the turbos kick in and she starts to breathe, no more soot or very little. Back her down or speed up and it should go away.

 

[Turtlesboat]

either run slower (hull speed) or faster (plane speed), that's where the engines are most efficient.

 

[alnav]

I had those engines on a previous boat. Seems to me the guidance I got from a Cummins tech was to try and stay away from running 1500-2000 RPMs.

 

[Ray 40]

I had same low rpm sooting on transom problem. My solution which eliminated 98% of the sooting is as follows.

Bought about 12" of white 3" irrigation thin wall type pipe at Home Depot. Cut 45 degree angle on both ends - cut into 2 - 6" pieces. Cut/slit the remaining pieces so you can squeeze smaller and insert into above water exhaust outlet above the water line at the stern. Slide it in to have the shortside of the 45 degree cut facing forward-the long edge to the back to catch the wind when you are moving and force the soot out and away from the boat. After fitting I adhered the exaust 'extention' with 5200 or something else - I don't remember.
Now I can run at 1400-1800 rpm for 45 minutes to an hour and then run it up to WOT or 2250-2500 for 10-15 minutes to 'blow it out'.

I cruised for more than 10 weeks this summer, washed the boat about 3 times while out, saw very little soot on stern- washed with regular boat soap - no harsh soap and re-waxing!

It works for me...hopefully my engines won't fall thru the bottom of the boat or some other catastrophe!

I forgot to add that the 'extentions' don't stick out far enough to hamper docking (fenders protect) and only a couple of people even noticed.

 

[Alex F]

As Tom and Mike have posted stay away from 1600RPMs. I'm almost positive that your boat is off plane at this RPM rate. If you she's lightly loaded and and you get her on plane with much greater RPMs and then back down to 1600RPM she might struggle but maintain to stay on plane. However, I don't know why you'd want to be in this range, b/c it's only costing you more money.

From my experience pretty much any planing hull boat has only two sweet spots (hull speed and cruise sweet spot speed). Take a look at this chart and you'll see what I mean.


As you can see, these numbers are for 480CE Cummins tested on 44DB. You might say that it's very different boat, but considering Cummins fuel burn table it's not much different and IMO your boat's sweet spot would be very similar.

Displacement Speed:
Our hulls are very close dimension wise. My hull speed is 7.4kts and just like you see in the pic I never push her over 1000RPMs, b/c that's going over the hump of the hull speed and the engines will need much more resources to go faster. So, you'll be wasting fuel by achieving poor results. A great example is by comparing 1000rpms to 1250rpms. You can see that at 1000rpms the boat is going 8kts (they might have larger props and very light test boat, but mine goes about 7.2kts-7.6kts range) and using only 4GPH total (both engines combined). But, the minute you push her to 1250rpms you only gaining 1.3kts (at best) but now you're burning TWICE MORE fuel (8GPH).

Cruising Speed:
This is very interesting point and a lot of people look at GPH number while forgetting that by going slower and staying at lower GPH fuel burn rate they take MUCH longer time to get from point A to point B, which translates in to MPG. I'm mostly focused on MPG as it's where the efficiency comes in to play the most. Based on my experience and learning from Cummins techs 450C love to cruise at 2300RPMs. I've tested all kind of RPM range and found that going faster is making the boat more efficient. A real numbers are, starting from 2000RPMs moving up to 2300RPM I gain 6kts. This is huge gain, IMO. My boat would love to run at 2400RPMs and it would gain another 2kts for extra 100RPM, but at this point we're crossing the "comfortable and safe" zone of continues operation of the engines. So, for this reason I have the "hard coded" spot 2300RPMs and this is what I consider the "perfect middle" in all respects (efficiency wise and engines health wise).

Depending on the props size your plane cruising RPMs might be slightly different (e.g. 2250RPMs), but the basic principal will remain the same. The minute you slow down to 2000RPMs or lower you're just paying more in the end, b/c you're loosing the MPG efficiency.

In regards to the soot, I only have very light coating that's hardly noticeable, after about 100NM run (obviously all at cruising speed). It only started late in this season and I think the biggest contributor to this is that my WOT has gone slightly down from 2650-2680RPMs to 2610-2620RPMs. Cummins suggest to stay closer to 2700RPMs, so I think I'll need to tune my props just a touch.

 

[CAPTAINJEH]

What's the formula for computing hull speed?

 

[Alex F]

I don't recall from the top of my head. But, this is something easy to find in google.

 

[Ray 40]

I thought the topic was 'sooting' not economy/efficiency.

I enjoy cruising between 7-12 knots - so will pull the throttles back to do so - 'sooting' was my problem and above was my
answer.
Enjoy

 

[Gofirstclass]

The formula for computing hull speed is:

Hull Speed = 1.34 times the square root of the length of the water line (in feet).

If a boat has an LWL of 30', the formula would be....1.34 x 5.477 (sqrt of 30) = 7.339 kts, or about 8.44 mph.

 

[Turtlesboat]

sooting is caused when the engines are being overloaded, running more efficient means less loading = less soot.