Cutwater and Ranger Ventilation

[BB marine]

After much thought in this ventilation subject I thought I should look into it more. Ranger/Cutwater support team insist there is not an issue with ventilation and heat in the engine Compartment. Their response was ,we have built these boats for years and have engines out there with thousands of hours and have never had any engine failures or electronic failures do to heat because of lack of ventilation in the compartment. Their designed ventilation system of using no air circulation or vents and exclusively relying on the air gap between the hull and cockpit liner to be more then a sufficient means for ventilation and compartment cooling has given no issues. I agree there should be sufficient air for induction of the engine. I also agree that a boat used in cool climates and cool water temperatures with an open cockpit,the design is adequate. It does help with drier air for engine air induction which is important. My question is, what happens when a Cutwater or Ranger owner adds a full cockpit enclosure to gain some added livable space reducing some air movement ? What happens to that engine compartment temperature when running in warmer water, warmer ambient temperatures ? What happens if cruising at hull speed with the engine running at 1980 rpm consuming 160 CFM of air for induction and thermostat maintaining the engine at 180 degrees. I can't speak for others but I know what I experienced.Very warm engine compartment temperatures.My next question is what happens to the inverter that is being used in that compartment generating heat and cooling fans blowing to try to cool it ? What happens to the relays, the MCC, the 180 amp alternator generating heat because it is generating amperage to feed the inverter and charge the house batteries ? What temperature increase is experienced if I was running a generator in the compartment? How does that warm air get out of that compartment? I'm not sure ! Does the Cutwater / Ranger ventilation system rely strictly on the engine induction to pull air in and exhaust air out of the compartment?
When I asked these questions to Cutwater/Ranger support team response was "we haven't had issues with its so it doesn't concern us"." If you want to add fans go ahead and add them". The next advise was" you should reach out to a Volvo for an installation manual if your worried about it". I did. Volvo has a complete section for their Ventilation requirements. There is an explanation of where to install the ventilators how much air is needed to supply the engine and how much air is needed to provide adequate ventilation for compartment cooling. These specifications are based on the engine HP. I added the 220 hp into the calculations provided by Volvo. The results showed sufficient available air for engine induction and cooling. The results also showed there was insufficient air movement in the compartment because there is no way for the air to get out. With the engine running there is a negative pressure in the compartment. The designed system is not a passive ventilation system with ventilators forcing air in a inward direction and and forcing air out with a outward direction.The air is pulled in around the air gap of the interior of the cockpit by the engine. Volvo specs for a 220 hp require 405 CFM of air movement coming in and 405 CFM going out for proper cooling. The requirements for engine induction are dependent on engine demand and are separate from the air for cooling ventilation . Example a 220 D3 at 2000 rpm 169.5 CFM, of air for engine air induction is required, moving and changing that quantity of air, 3200 rpm-328 CFM, 4000rpm-392 CFM, 4130rpm-406 CFM. If I run my engine WOT I will meet Volvo's minimum requirement of air movement in the compartment for proper cooling. The compartment needs this air movement at all times when running at any speed the temperature of the engine is consistent with 180 to 200 degrees F regardless of engine speed.When at 2000rpm cruising for 6 to 8 hrs moving less then half the required air for cooling I would expect the compartment temperature to get warm and it does. Volvo rates their engine HP at 77 degrees F. for maximum output There is a small percentage of HP loss for every 18 Degrees F above 77 degrees F. My calculations were -(6% loss in performance and fuel economy while running last fall not much but measurable ). Volvo recommends that engine compartment temperatures at sea trial be no higher then 36 degrees above ambient using 77 degrees as standard ambient. They also recommend that if temperatures are found to be consistently at 113 degrees F or above to derate the engine. When I emailed my findings to Cutwater/Ranger support team the response was," we have been doing this for 12 years and never had an issue with the ventilation in our boats " There is nothing we will change or plan to change on how we install the engines but respect what you are trying to do to extra protect your boat and engine. While I disagree with them I have to respect there experience and expertise they build the boats. I probably have nothing to worry about but I'm still adding a true passive ventilation system with engine fans to the boat. I will use the fake exterior vents and install exterior ventilators with fans that will run while the engine is running. The fans will be for continuous duty rated at 5000 hours These are standard engine fans used by many boat manufactures for engine ventilation.




This is a few pictures. It doesn't look like much but I have a lot of time invested in this and the rest of the system. All thru deck connections are epoxied in then sealant applied exterior and interior all connections have life calf sealant applied. a 3/8 drain is used that is run to the bilge . I don't expect water to get in but if it does it has a drain system I am trying to put the system out of sight but efficient. I will post more pictures when everything is completed.
Brian Brown
Cutwater 26
PORT-A-GEE

 

[KnotLost]

Thank you very much for this post. I agree with your thinking and don't think your efforts are wasted. Also the installation looks very nice. Your gains in lowered engine room temperatures I believe are worth the effort, for the reasons you stated in your post.

I do think Volvo is being a little cautious with it's desired engine room temperatures. I don't know if many diesel engine rooms are below 113F, especially in warmer climates. I've boated where the ambient temp is that high. (don't ask)

 

[BB marine]

I looked at the requirements of the ABYC for Diesel engine installation; I posted the items pertaining directly to my post.

H-32 RECOMMENDED PRACTICES AND STANDARDS COVERING VENTILATION OF BOATS USING DIESEL FUEL.
b. Storage Batteries - Compartments containing storage batteries shall be vented to provide for the escape of hydrogen in accordance with ABYC E-lO, "Location and Installation of Storage Batteries".
e. Combustion Air - space provided for supplying combustion air shall accomodate the air requirements required by the engine manufacturer(s) for each propulsion and auxiliary engine in that space. These openings may also function as means of providing natural ventilation.
f. Additional Uses of Ventilation - Power or natural ventilation is not required on a diesel boat, but may be used to control compartment temperature. Power ventilation may also be used in the machinery space for odor control and personnel comfort while servicing equipment.
g. Ambient Temperature - For design purposes, the ambient temperature of machinery spaces is considered to be 50° C (122°F) and of all other spaces is considered to be 30° C (86° F).

H-32.S. INSTALLATION
e. Ventilation openings shall remain outside of weather enclosures.

I do think Volvo is being a little cautious with it's desired engine room temperatures. I don't know if many diesel engine rooms are below 113F, especially in warmer climates. I've boated where the ambient temp is that high. (don't ask)

ABYC is requiring 122F and using 86 as ambient allowing 36F above . Volvo is using 77F ambient and 113F both are 36F above their ambient. Volvo is not requiring the compartment temperature to not exceed 113 F. They are stating if the compartment temperature is always running 113F or above the engine should be derated with respect to HP. Volvo's engine compartment calculations ( that I used) for ventilation sizing were done by using 86 degrees ambient. 86 F as a base line with 36F max running. A max increase to 140 degrees after shut down. If these can not be met added ventilation and fans should be used. My boat reaches these temperatures and goes slightly above in the compartment.

After reading the ABYC requirements I feel that the Ranger /Cutwater design is adequate. Except the design is less effective or not in compliance with the ABYC if a cockpit enclosure ( weather enclosure) is used. My opinion is the added outside ventilation is needed if a full enclosure is used. Having vented air come in from outside the enclosure. Warm air , odors and gases vented outside the enclosure will help in ventilation and cooling of the compartment also be compliant with the ABYC. I realize that for the average Boater this is not a concern. I plan on doing extended cruising and believe that this additional ventilation will help the longevity and performance of the Volvo D3, add safety , and piece of mind while cruising.


Brian Brown
Cutwater 26
PORT-A-GEE

 

[captstu]

Ventilation and cooling are two aspects of air flow that might be considered separately.

The Mase family of generators, used by Ranger for quite a while, is totally intercooler cooled, the air in the compartent is used again and again with only a small amount of combustion air being sucked into the case.

You may, if you are concerned about derating your Volvo engine, introduce an intercooler between the turbo charger and the intake manifold. These typically reduce the temperature of the combustion air significantly and, at least on Diesel trucks, are a major part of the process of retaining full power at high ambient temperatures.
Take a look at https://www.fcpeuro.com/products/vo...Fh3bTb5Ac-tK25uvYivrmB9ZNXO1c5CRoC50QQAvD_BwE

/Stu

 

[BB marine]

Ventilation and cooling are two aspects of air flow that might be considered separately.

The Mase family of generators, used by Ranger for quite a while, is totally intercooler cooled, the air in the compartent is used again and again with only a small amount of combustion air being sucked into the case.

You may, if you are concerned about derating your Volvo engine, introduce an intercooler between the turbo charger and the intake manifold. These typically reduce the temperature of the combustion air significantly and, at least on Diesel trucks, are a major part of the process of retaining full power at high ambient temperatures.
Take a look at https://www.fcpeuro.com/products/vo...Fh3bTb5Ac-tK25uvYivrmB9ZNXO1c5CRoC50QQAvD_BwE

/Stu

The Volvo D3 is after cooled. It has an exchanger that uses sea water to cool induction air after the turbo. Higher temperatures in the compartment effect more then just induction air although that is important according to Volvo.As you have brought up Volvo and many other car and truck manufactures have introduced intercoolers and cold air intakes. They are now not pulling air into the engine from the engine compartment because of heat. They are pulling the air in from outside the compartment and also adding intercoolers to get better performance. At 77F air peak performance out of a D3 . Electrical devises including generators, alternators, relays,ECU's,and MCC's work better and have increased longevity if operated in a cooler environment.

As far as derating my engine. I would never do that. Thats why I'm adding ventilation. Bring cool air in pull warm air out.

You boat in Florida . Have you ever checked compartment temperatures ? That's the true test Boating in Florida waters. Curious !
Brian Brown
Cutwater 26
PORT-A-GEE

 

[BB marine]

I'm posting the last pictures of the install. It doesn't look like much but it was a lot of work. I hope it pays off.
Brian Brown
Cutwater 26
PORT-A-GEE

 

[BB marine]

I'm posting an update on the ventilation install. With only two, 2 hour trips I do not have much data but I am already leaning towards the additions have helped but are not enough. Lake Michigan water temperature is 50.7 degrees, air temperature first run was 60 degrees, Running at mixed RPM, WOT to 1950 rpm engine compartment temperature started climbing 116F and I turned the blower on and it maintained 115F. The next trip we cruised 20 miles at 3400 rpm Air temperature 72F and water still at 50.7 ( water temperature is based on weather buoy close to our location). At just over an hour of running 15 mph The compartment temperature climbed to 126F I then turned blower on and it maintained 126F never dropped but never got any warmer. We had the enclosure removed just Bimini top up. So air movement was as factory spec plus my side vent and blower running. I know I'm jumping to early conclusion but 12 F increase outside ambient and same sea water temperature I increased 10F engine compartment temperature. For the midwest these are cool summer temperatures for souther boating waters these are cold conditions. When we shut down I had the blower running for 15 minutes and the temperature finally came down to 106F at this point I was comfortable with shutting the blower down. I think I'm going to add additional outside ventilation to the compartment. I worry about cruising in southern waters with 85F water temperature and 85F air temperature. The effects of running with high compartment temperatures are not immediate, electronics, sensors, relays, alternator, and the engine internal components can have their longevity compromised. ABYC installation guide lines state any engine compartment temperatures above 36F above ambient of 86F (122F) should have additional ventilation installed or air movers. Engine shut down with heat soak should not exceed 140F or additional ventilation or air movers should be installed. Volvo states if a compartment temperature continually runs at or above 113F the engine should be derated. Volvo is not stating this as a max temperature 113F. Volvo guide lines are the same as most engine manufactures 120F to 130F is the average of all that I looked at. With all of this I am not stating that Ranger or Cutwater boats have inadequate ventilation I assume they have engineers that can prove me wrong on that. I also know that every boat is different.I will say that the ventilation in our C26 was marginal at best and with additional that I installed it still is. I would like to know if others have monitored there compartment temperatures so we can compare data. It may just be the Cutwater 26 that has higher temps. Or just our C26!

 

[ALICE J]

I have been concerned with cooling my alternator on my Ranger 27 , because a cooler alternator will put out more amps before it ramps down. A while back a fellow had placed a blower fan that pulled air from the transom opening in the cockpit and plumbed it to be blown directly into the alternator. Not sure what the CFM was on the blower fan but I believe it was a 3 inch duct. Curious to know your thoughts if this might achieve the results you were seeking.

 

[Brunski]

Hi Brian,

Nice install! I was wondering, have you considered starting the ventilation fan before starting the engine, and keeping it running?

 

[BB marine]

I have been concerned with cooling my alternator on my Ranger 27 , because a cooler alternator will put out more amps before it ramps down. A while back a fellow had placed a blower fan that pulled air from the transom opening in the cockpit and plumbed it to be blown directly into the alternator. Not sure what the CFM was on the blower fan but I believe it was a 3 inch duct. Curious to know your thoughts if this might achieve the results you were seeking.

I'm reluctant to force air in the compartment. There should be a slight negative pressure to prevent fumes migrating into the cabin. Diesel powered boats do not have a sealed compartment between engine compartment and cabin. For the alternator I have a 4" vent directly in front of the alternator.I actually feel this is working well for cooling the alternator and electronics on the engine. All electronics Alternator , relays, fuel pump, and ECM are all mounted on port side in-between my outside air vent and my engine compartment exhaust fan. The big issue is the engine air induction intake on a D3 is in the front of the engine starboard side with very little room for air movement and cooling. It is located in the furthest point from any out side air. The engine is pulling air from the designed Transom air gaps over the 180 F engine and pulling hot air in. My temperature sender is located directly in front of the engine air filter. I installed the outside vent in the front of the engine with hopes that this would provide cooler air for induction. It may be helping but it is not enough. The original engine used-( Yanmar engine ) I beleive has the air induction at the rear of the engine which would be a much better option the air doesn't have to travel over the hot engine to be used for induction. The D3 air induction is in the hottest part of the compartment with no way for it to be vented except thru the engine. Unfortunately R&D (research and development ) is hard to do at this point the boat is designed and built. I am doing D&R ( develop and research). I feel I need to install another 4" vent to the Starboard side near the engine intake similar to what is used in the automotive industry and now is very common in the marine industry ( cool air intake )

Hi Brian,

Nice install! I was wondering, have you considered starting the ventilation fan before starting the engine, and keeping it running?

I may try that . My plan was the fan would be used for slow operation and heat soak after the engine shut down. I would like to have natural ventilation when the boat is moving at speed. I have used it while running at speed and its function has worked but hasn't helped enough.

There is plenty of air for induction but not enough air movement for cooling. Performance suffers slightly but I believe longevity suffers more. My opinion!

 

[captstu]

Brian, You said your original engine was a Yanmar. What is it converted from a Yanmar to a Volvo because the Yanmar had a problem?

My Yanmar 4BY2 is overheating and we can’t find the problem. I’m considering replacing it with a Volvo but Andrew says that is not possible. What was done to do your conversion?


Stuart Bell
Ranger 25: Shearwater
(561) 352-1796

 

[BB marine]

Stu, sorry for the confusion. My boat came with the D3 220hp. The earlier models were delivered with 180hp Yanmar. I was just stating with the air intake located in the rear of the engine (Yanmar) would be better location for the Cutwater vent design. The D3 intake is in the forward corner of the compartment in front of exhaust manifold and turbo.

I wouldn't give up on your Yanmar you'll get it figured out. Good luck.

 

[Chimo]

Brian

Please allow me to follow up on your posts from earlier in the year. I did install a remote thermometer in the engine compartment of our R25 SC close to the air intake. After running now for a couple of months I have seen the temperatures climb as the weather heated up, just as expected. I'll convert my notes to American units for ease of comparison with yours. With an ambient temperature of 76 deg F and running for an hour at 2500 rpm the engine room temperature ran around 105 deg F. Slowing down to 800 rpm naturally reduced air flow and temperature climbed close to 118 deg F before cooling off fairly quickly. I have no real concerns in this climate.

I was interested how others may find temperatures with higher rated engines. The D3 family is (I suspect though the local Volvo guys will not confirm it) mechanically uniform with changes in electronics and fuel system to change ratings. I've listed the various options below showing torque at nominal rpm.

HP KW RPM Torque (Nm) Torque (Ft Lb)

110 82.06 3000 261.22 353.96
150 111.9 3000 356.22 482.68
170 126.82 3000 403.71 547.03
200 149.2 4000 356.22 482.68
220 164.12 4000 391.84 530.94

Torque is a good indication of mechanical load on the engine. One interesting thought is that the rated torque for the 150 and 200 HP units is the same. Would it really cause increased wear on a 150 unit to run at WOT of 3000 rpm since it will happily run at 3600 cruise rated 200HP. Just food for discussion.

 

[BB marine]

With an ambient temperature of 76 deg F and running for an hour at 2500 rpm the engine room temperature ran around 105 deg F. Slowing down to 800 rpm naturally reduced air flow and temperature climbed close to 118 deg F before cooling off fairly quickly. I have no real concerns in this climate.

I would be real happy with those temps. The sending unit I installed is Approximately 6" in front of my air intake. My run times are 2 hours minimum. It takes over an hour of run time to start significantly raising compartment Temperature. The block temperature cruising maintains 180 F. The heat dissipation from the block slowly raises the compartment temperature. I start seeing 118 F after an hour running at 3350 rpm. From there it keeps climbing. I will list 5 different runs all in cool lake water and cooler air ambient temperatures. This is normal temperatures for the area we boat in.

Water temperature, Air temperature, Compartment temperatures
50.7 F. 60 F 116 F
50.7 F 72 F 126 F
56.0 F 65 F 127 F
59.0 F 76 F 138 F
63.0 F 78 F. 148 F
68.0 76 F. 146 F

When I bring the RPM down to 2250 the compartment temp drops a few degrees so does the engine temp to 178 F. If I run the blower the compartment temperature drops slightly but not enough to make a difference. I put my hand over the blower discharge vent very warm air is being discharged. I only added 1- 4 inch vent and it is bringing air in, the only way for the Warm air to get out is thru the engine and the 3" 150 CFM blower I installed. Acording to the Volvo engine installation manual for the D3 this is not near enough ventilation for the compartment. I have spoken to Volvo this is their response.


You need to follow our technical data as reference for the engine room temperature.


Best regards,

Technical Support - GC
Volvo Penta of the Americas
1300 Volvo Penta Drive
Chesapeake, VA 23320


If those values are correct, it is out of our recommendation and is not possible to predict what could happen.
The engine must work according to our measurements and recommendation.


Best regards,

Technical Support - GC
Volvo Penta of the Americas
1300 Volvo Penta Drive
Chesapeake, VA 23320

This is the measurement and recommendation sent from Tech support- The temperature of the inlet air at the air filters must not be higher than +25 °C (+77 °F) for full power output. During sea trial the air temperature in the air filter should not exceed 20 °C (36 °F) above ambient temperature. Volvos requirement sent from Tech Support- For compartment ventilation - "To keep the engine room temperature down to the permitted values, a great deal of the radiant heat must be transported out of the engine room, in other words be ventilated away.
The same dimension must be chosen for the inlet and outlet ducts to achieve low flow speeds and low noise levels."
The Cutwater or Ranger does not follow this requirement. I have asked for sea trial results of the C26 but never received a result. The response I did get was is there is nothing to be concerned about.


Longevity for the mechanical component of the engine is not a big concern of Volvo. Performance is a concern also longevity of electrical components. The last conversation that Volvo had with the Volvo dealer that is working with me " Not at all concerned with longevity. Very concerned with performance". The advise given to me is add more ventilation to maintain the rated performance of the engine.

We are taking a trip this Fall, Grand Rivers Kentucky to Knoxville 650 miles warmer water and warmer ambient. I will monitor temperatures then. I would be interested too in what temperatures other D3 owners are seeing. I know the D3 install in the C26 is very tight . The air intake is in a very tight location. The air coming from the transom air gaps has to pass over the engine to get to the intake, the exhaust manifold and turbo are right there too.

Would it really cause increased wear on a 150 unit to run at WOT of 3000 rpm since it will happily run at 3600 cruise rated 200HP. Just food for discussion.

Thats a good question. I don't know the answer to that question but I will say this. I would feel better about the longevity of a 150 hp running at 90% of WOT RPM then running a 220 hp at 90% of WOT all the time.

 

[NorthernFocus]

... One interesting thought is that the rated torque for the 150 and 200 HP units is the same. Would it really cause increased wear on a 150 unit to run at WOT of 3000 rpm since it will happily run at 3600 cruise rated 200HP. Just food for discussion.

Assuming the 150 and 200 share the same engine core then there wouldn't likely be any issues with crank, rods, pistons, etc. However it is likely that some of the top end components are different and would be stressed on the smaller engine. The first item that comes to mind is the turbo which has to be sized appropriately for the rated power of the engine.

 

[NorthernFocus]

Brian, don't forget that in addition to inlet air temperature there is a spec for cooling water temperature. On an engine with after cooling you can tolerate higher inlet air temperature as long as there is adequate after cooling. As you know what the engine cares about is manifold temperature/pressure.

Of course that's only half of the issue you're trying to solve. Cooling water doesn't do anything for the electronics, etc. Well at least nothing good...

 

[Chimo]

Dan, I hadn’t thought about the turbo. Great input, thanks.

 

[dclagett]

FYI - Currently the water temperature on the Choptank River off the Chesapeake Bay is 91.5 F and the air temperature has been in the low to mid 90s F. In these conditions my cruise coolant temperature at 2650 RPM stabilizes at 187 F after about 15 minutes and after about 1 to 1.5 hours rises to 189 F and stays there for hours. I don't have an engine compartment temperature sensor, however I will add one prior to my next trip and provide that data.

It would be interesting to hear form someone with a high time (1000+) VP engine on a Cutwater/Ranger Tug operating in hot conditions on their experience. Equipment failures, etc.

 

[BB marine]

Brian, don't forget that in addition to inlet air temperature there is a spec for cooling water temperature. On an engine with after cooling you can tolerate higher inlet air temperature as long as there is adequate after cooling. As you know what the engine cares about is manifold temperature/pressure.

Of course that's only half of the issue you're trying to solve. Cooling water doesn't do anything for the electronics, etc. Well at least nothing good...

Dan,
These are good points. The Max water temperature for incoming cooling water passing thru the after cooling is 86 F. The advantage of running the boat on Lake Michigan is we will rarely see water temps above 75 F. The warm air being inducted in the engine has less effect on performance because of these cooler water Temps passing thru the after cooler . Although I have seen a slight loss of between 110 Rpm and 150 RPM after a few hours of run time with compartment temperatures above 130 F. WOT 4130 with compartment temps 100 F to 105 F Ambient air temp 76 F. Same day WOT 3980 to 4020 compartment temps 146 F (ambient 76 F) I'm still running in the recommended rpm range of 4130 to 3900 RPM. My concerns are when I'm running the river systems and southern waters where water temps are closer to 86F or at 86F . Last year's run on the river system is when I noticed the loss in fuel economy of 10% to 12% and very warm compartment temperatures. I will be more aware of the issue this year and will monitor it to come up with a plan as to how much more ventilation I need to add to put the engine in Volvos designed conditions.There is an air temperature sender located in the charge air pipe this monitors charge air going into the intake manifold. The sensor will flash a code if intake air reaches 176 F. There is also a pressure sensor in the same location. These temperatures and pressures can be monitored with a break out box and a VOA using resistance readings to equate to temperature .

The other issue to higher compartment temperatures is the increased chances of electrical components failures. The newer common rail engines have more electrical components then mechanical moving parts. Then take all the other electrical components in the compartment, inverter, battery charger, isolation relay, generator, auto-pilot pump and motor. Read the installation manual for all of these components. What is a common statement-• The location to install the unit has to have a normal air temperature between 0°C – 60°C
(32°F-140°F). The cooler the better, or overheating may result. The unit should be installed away from the battery in a separate well ventilated compartment. Quote from KIsea charger/inverter. I am told by Fluid Motion industries representatives that all this is taken into consideration in their builds and there are no concerns. Fluid Motion has built a lot of boats with many happy customers with no complaints of this issue so they must be right. I still question it and feel that adding a true ventilation system to the compartment will help in performance and longevity.

 

[NorthernFocus]

There is no doubt that operating equipment within it's design window is optimum. I will also say that in my career as a machinery engineer 99 percent of the equipment that I worked on operated outside its design window, sometimes good sometimes bad. And with 50+ years on the water and having been aboard literally hundreds of boats, the only ones I've ever seen with forced air ventilation(and let's face it passive systems are ineffective) in the engine space have been large commercial vessels. And in those cases the ventilation is provided for crew safety. So basically both the impacts on horsepower and the effects of elevated temperatures on alternators, generators, and various other electric components have long been accepted in the industry, either by design or by default.

That said, the marine industry as a whole tends to operate well behind the technology curve. Change comes very slowly. And the points you bring up about longevity of electronic components on modern diesel engines are valid. Maybe you're just ahead of the (industry) curve and improved ventilation for cooling purposes will become a norm some day.

Just as an aside, in my youth I worked on boats built for Gulf coast oil field crew relief service. When we wanted to squeeze every bit of HP that we could out of the old Detroit two strokes we would open the deck hatches directly over the top of the engines so they could breath.