Engine room heat



Your posting caught my attention. I write this to be helpful. I am not trying to be a smartass. I am only trying to help.

I believe that the ambient temperature of the engine room (ER) will not reach much higher than about 60C. If I am not mistaken it takes the breaker reaching 75C or higher to possibly cause the breaker to open, but I am not sure what temperature the French-made Dirupter's open.

The operating temperature of the main engine and Onan should be not much higher than about 80C, regardless of how long they have been running. The exhaust elbow reduces exhaust temperature to about 50C. The ER input and output blowers move enough air to change 100% of the ER air in a few minutes. Are both working?

Of course, the above temperatures assume that the raw water heat exchange system is working. Sufficient raw water flow is the #1 reason you will have heat issues and you cannot tell by looking at the exhaust to determine if it is sufficient. Measure the temperature of the oil filter with a laser temperature gun after the engine has operated long enough to reach operating temperature. 

The two blowers (input & output) are also very important.

Some hidden water flow issues can possibly be:
Clogged sea chest including the connection through the sea chest to the sea
Clogged hoses in the raw water pathway, in & out.
Clogged heat exchanger.
Clogged elbow and on Onan also the exhaust manifold.
Clogged oil cooler for ZF 25 transmission.
Raw water pump defect or worn impeller. A raw water impeller will lose substantial power when blade tips become worn.

If I were you I would check further to see if the cause of the high ER heat was possibly caused by something not operating as designed.

Again, I hope this helps you and/or any other owner.


CW Bill Rouse 
Amel Owners Yacht School
+1 832-380-4970 | brouse@...
720 Winnie, Galveston Island, Texas 77550 
Yacht School Calendar: www.preparetocastoff.blogspot.com/p/calendar.html


On Sat, Apr 9, 2022, 09:03 Ralph Heilig <ralph.heilig@...> wrote:
Hi David,

for me the nice spot for the D3C is between 1280 and 1340 rpm about 4 lt.  1420 rpm is also a sweet spot. Propeller is Autoprop H3. Before longer Motorpassages, I dive and clean the Prop. 

A couple of years ago I made a nonstop 100h Motorpassage with rpm 1640 -1720. I would not recommend it, because the whole Engine room gets pretty hot. Even the watermaker did not work anymore, because the fuses were too hot. Since the fuses in the Engine room are thermal fuses, you might get trouble with other equipment because of the hot fuses...

Also important for me is a balance between speed, noise and vibration. Over 1600 rpm it's getting noisy. Over 1800 it's getting very noisy AND a lot of Vibrations. When I go over 1800 rpm it feels for me that the boat is nearly falling apart (-: Strong Vibrations.

You will get a good feeling, what you and your boat "likes". At some rpm, the water level stick is producing vibration noise, At some rpm, the oven produces vibration noise and so on....

I think every boat has a slightly different "nice spot" without any annoying vibration or noise. My sweet spots are 1280 - 1340 and 1420. I rarely cruise under 1200. There is also one rpm area, I think it's around 1000 rpm, where my D3 does not hold the rpm. Thats the area where I think the turbo is Opening, but when opening the rpm goes up and the electronis are closing again which results in permanent rpm wandering up and down. 

The Idle rpm is about 700-750.

Maybe someone has the same: When running Idle, my 24V Mastervolt alternator is not charging. It starts charging at about 800-850 rpm. Any Idea how to get it charging at Idle? My Volvo technicians had no Idea. 

Santa Isabell
A54 #144

Ralph Heilig

Hi Bill,

thanks for your Information. 

This was and is a Diruptor Problem that only occured once (three years ago) after motoring 3 days non stop with higher rpm. In my case the Diruptor was so "preheated" that after 10 min use of the Watermaker the fuse triggered. Afer arrival and cool down, all back ok....it also never happend again, since I never did this kind of long high rpm crusing...

I had a complete raw water check. All ok. Last year I had to clean and decalc the Onan heat exchanger.

But do you have any Idea how to get the 24v Alternater charging at idle? That's a little annyoing...I don't know it it was always like this or when it started, but I recogniced it about two years ago.