Tug Efficiency - I/B Diesel vs Outboard

[NorthernFocus]

ALL hydrocarbon fuels will auto ignite. One example is a gasoline engine "dieseling" or "pinging" due to combustion occurring before the spark plug fires. It is true that the auto ignition temperature of gasoline is slightly higher than diesel but not enough to be meaningful. Either one sprayed on steel that is hot enough to visibly glow red will likely auto ignite.

Statistically there are far more fires on gas powered boats than diesel. That said, diesel is highly combustible when either vaporized or atomized. Modern common rail diesel engines like we all have in our Rangers have a high pressure fuel delivery system that will produce just such an atomized spray if the high pressure tubing cracks or a fitting comes loose. And diesel does contain a lot more BTUs per gallon and once ignited burns much hotter than gasoline. Not like that's going to matter to a guy on a boat that's on fire.

All that said, the most common fuel leaks are supply hoses, tank fitting, and filter housings. And there is one thing for certain. As unpleasant as it may smell, you can motor on home with a bilge full of diesel. With a bilge full of gas you better get off the boat before it finds a spark.

There must be some genetic, gender based sensitivity to the smell of diesel. My wife also has zero tolerance of diesel odor, either fuel or exhaust. That has forced me to develop very good hygiene practices when fueling, changing filters, etc. Liberal use of absorbent sheets contains the liquid and a spray bottle with a 10:1 solution of Dawn removes that tenacious diesel film quite well.

 

[Seaquel]

Barry, thanks also for your thoughtful comments. I realize I oversimplified the question of combustibility for diesel vs. gas, but Dan captures the essence in his comments about fuel in the bilge, which relates directly to our experience. I guess if we want to eliminate all risk we would never leave the dock, or even dare to commute on the freeway for that matter! 😀

The fuel tank on my SeaRay that leaked was aluminum.

I can understand your dilemma in choosing between two gorgeous boats! As noted, EVERY boat represents a series of compromises and what is best for one person will be different for another. Then, inevitably we will tinker and customize our boats to further reflect the myriad practical and aesthetic desires to where no two boats are rarely identical. And that's part of what is fun with Tugnuts in particular, to learn from each other and create what works best for ourselves, while gaining wisdom and new friends along the way!

-Mark

 

[NorthernFocus]

...High dB will eventually cause hearing loss and my R-25 would reach 93+ dB at 3800 RPM and even at 3000 RPM it reached 88 dB. The new R-27 is far less than this as I measured it during a short sea trial at varying speeds.

Here were my R-25 sound level measurements...

2010 R25 - Yanmar 4BY2-150
Mar 18, 2013
Calm and no wind.
Cabin door closed.
Side windows and hatches closed.
0 RPMs 50 dB
1000 RPMs 70 dB
1500 RPMs 75 dB
2000 RPMs 78 dB
2500 RPMs 83 dB (85dB door open 😱 not surprising)
3000 RPMs 87dB
3500 RPMs 93 dB
4000 RPMs 95 dB

Risk of hearing loss due to noise is dependent on a "time weighted average". In other words the louder the noise, the less time it takes to cause hearing damage. Though time weighted, the effects of non-damaging noise is not cumulative. If they weren't exposed to damaging levels then when your ears rest they recover. Hearing loss is indeed cumulative, but noise exposure is not.

US OSHA has set 90dB as the noise level for eight hours continuous exposure without causing hearing loss. I often hear people mention the 90dB OSHA limit but typically it is mentioned in the context of instantaneous exposure being unacceptable. Few people are aware of the time exposure component. Extremely loud instantaneous noise causes immediate damage. Personally I have hearing loss in one ear from shooting skeet and bird hunting in my youth. Back then no one used ear muffs/plugs.

By the way, my comments are intended to be purely informational. As others have mentioned both engine types have their advantages. I've already decided that if my boat were to require repowering, I'd bolt on a bracket and drop a pair of OBs on there. Suzukis due to the advantageous gear ratio and wheel selection. I positively drool over the thought of all that below deck space that would become available...

 

[Seaquel]

Is the 2010 R25, used in this decibel comparison, with the larger cockpit or larger cabin? This would likely make a big difference since a big impetus behind the layout update years ago in the R25 (and R27) was to get the entire Diesel engine outside the cabin (and we always run with the door closed).

It would be interesting to compare the decibels of the R27 Classic vs. new R27 OB. Regardless, another big benefit we realized in the move from the SeaRay (stern drive with engine sharing "space" with the nearby sunbridge helm) to the R27 was relative quiet. We converse, talk on the phone, and even listen to music on our R27 whereas none of that was practical on the SeaRay.

-Mark

 

[baz]

Dan: My wife cannot hear a thing without her having high-powered Hearing Aids (HA). With 80 to 95db and with her HA operational it's deafening for her and she has to turn the HA volume down which then makes it even more difficult for us to communicate when noise levels are that high.

To compensate for this noise in the past with our R-25 I've resorted to have to wear a small neck mic that allows me to communicate directly to my wife's HA via Bluetooth.

The above issues will be less concerning for us with the near 'noiseless' cabin sound levels with our new 2018 R-27 compared to what we had to endure with our previous R-25.

During my new R-27 sea trial of short duration I was able to note and record cabin noise levels and I can say with certainty the noise level (i.e., the dB levels) were many notches lower than what I recorded on my previous R-25. I refrain to post my dB numbers as I promised Jeff Messmer I would not make them public as the new R-27 I was on was Prototype-1 and was without many of the typical noise deadening materials people have on board such as blankets, clothing, bulk items in stowage areas up against the hull surfaces and so on. Most of the noise I heard was from the water splashing and running against the hull, and this was more evident when the R-27 was galloping along at 25+ mph. 😉

 

[Red Raven]

The dB level according to my iPad dB app on our 2014 R27 at 3500 rpm is 76dB with the widows and cockpit door closed. About 78dB with them open.

Curt

 

[baz]

Mark:

My previous R-25 (Classic) was a 2010 model and part of the engine was protruding into cabin space under the cabin's entry step. I went to great lengths trying to seal the cracks around this step to reduce engine noise in the cabin. It made little difference. The engine noise simply ran through the boats superstructure and into the cabin. With later R-25 models and the R-27 with inboard diesel the engine was positioned mostly out of the cabin area which presumably made some difference in noise levels in the cabin. However, If I recall, both of these models still had access to the engine via the 'step' in the cabin's entrance. I would speculate that the less engine noise levels in the cabin for these RT models was not really significant.... but obviously I dunno for sure.

One piece of advice I would have for the R-25 and R-27 owners would be to make sure the beauty plates on the engine (The Yanmar 4BY2 150/200 models) be thoroughly tightened down. If any are somewhat slightly loose the beauty plate can start rattling at certain RPMs and setup nasty harmonics which can be and will be very annoying.

 

[baz]

The dB level according to my iPad dB app on our 2014 R27 at 3500 rpm is 76dB with the widows and cockpit door closed. About 78dB with them open.

Curt

That is very impressive and comforting to know. Thank you. What brand of engine is in your R-27 (Yanmar or Volvo). I'm told the Volvo engine runs quieter. 🙂

My 2010 R-25 noise level measurements were all taken with my iPhone App at the center of the port side dinette table and held about 2" above the table surface to avoid any noise that might get transmitted to the iPhone's mic from being on the wooden table. I did at times move the iPhone to close to my head at the helm but the dB level barely changed.

 

[captstu]

The gasoline versus diesel discussion on noise is very interesting. One R-25 The engine noise is bearable. While I realize that is a subjective measure, starting the generator is unbearable. With the generator running we cannot sit in the cockpit and be comfortable.

I have a Honda portable gasoline that I run when I want air-conditioning and I want quiet. Possibly more soundproofing would bring the generator under control but I haven't tried that yet.

I do not think the generator is map functioning, just a single cylinder cylinder Diesel running at 3600 RPM.

The generator is even loud when we are sleeping on a hot buggy day with the air conditioning on.


Sent from my iPad using Tapatalk

 

[baz]

Outboard gasoline engine noise and inboard diesel engine noise in the cockpit area is another fair comparison to make as this is where people will also be sitting during a cruise and likely wanting to chat with each other. 😱

 

[NorthernFocus]

While dB levels can be measured quantitatively, if they are below dangerous levels(per my previous post) any other discussion is purely subjective based on individual perception. In contrast to what's been said so far on the topic, we run a R25 classic with Cummins 150 and aren't bothered at all. We spend most of our time trundling along at around 1500 RPM and easily converse at normal voice levels. When we do run at speed no question that elevated voice volume is necessary but not yelling by a long shot. At those times we don't discuss philosophy but communication is by no means difficult.

As has been mentioned already, much of the noise in small boats is difficult to mitigate. The hull transmits engine noise which is very difficult to dampen out. And at higher speeds hull noise can be as loud/louder than an outboard. In a small boat there is no doubt that an inboard engine is going to be louder in the cabin than outboards. All of the noise is transmitted to the boat. With outboards much of the sound simply travels off into the environment. After all sound is simply pressure waves which outdoors dissipate exponentially with distance from the source. Inside the boat the sound is transmitted in a much more lossless manner by the hull/structure.

It is only logical that an outboard boat of equivalent length should be quieter inside the cabin. Whether it is so in the cockpit isn't nearly as obvious. There are things that can be done with either design to mitigate the sound in the cockpit.

 

[jld]

I'm unsure about the diesel engine lasting longer than an outboard or having better resale advantage.

Respectfully, I would disagree with this statement.

A general consensus on most of the boater forums is that a well maintained inboard diesel engine has a life of about 5k hours, while a well maintained gas inboard engine has a life of about 2.5k hours.

If you look at Yachtworld (or another sales point), inboard diesel engine boats are usually priced for sale at a premium over identical boats with inboard gasoline engines. This should be expected, however, as the purchase price of a boat with diesel engine(s) is usually considerably higher than the same boat with gasoline engine(s).

Personally, if I'm looking at 8 - 10 year old boat with an outboard engine, I'm thinking I need to have money aside to repower. This is not true of boat with diesel engine(s), providing there are adequate service records and it passes an oil analysis.

My wild guess, however, is that in terms of a 2018 R-27 boat, the price of the outboard on the new style boat versus the inboard diesel on the older style boat may be neglible on resale, as buyers may see the value of the new style offset the lower value of the outboard engine.

Jim

 

[baz]

I'm unsure about the diesel engine lasting longer than an outboard or having better resale advantage.

Respectfully, I would disagree with this statement.

A general consensus on most of the boater forums is that a well maintained inboard diesel engine has a life of about 5k hours, while a well maintained gas inboard engine has a life of about 2.5k hours.

If you look at Yachtworld (or another sales point), inboard diesel engine boats are usually priced for sale at a premium over identical boats with inboard gasoline engines. This should be expected, however, as the purchase price of a boat with diesel engine(s) is usually considerably higher than the same boat with gasoline engine(s).

Personally, if I'm looking at 8 - 10 year old boat with an outboard engine, I'm thinking I need to have money aside to repower. This is not true of boat with diesel engine(s), providing there are adequate service records and it passes an oil analysis.

My wild guess, however, is that in terms of a 2018 R-27 boat, the price of the outboard on the new style boat versus the inboard diesel on the older style boat may be neglible on resale, as buyers may see the value of the new style offset the lower value of the outboard engine.

Jim

Woweee..... then assuming annual cruising hours is 100 hrs then diesel should be good for 50 years and gasoline OB good for 25 years.

If the outboard has been well maintained over the 8 to 10 years and service records show no more than 1000 hrs on the engine then I would see no reason for 're-powering' in immediate future.

 

[captstu]

The coast guard has begun testing diesel outboards on their high-speed chase boats. Depending on the results of their tests they beat purchase a block of Americanized diesel outboards opening the market for the rest of us to purchase a diesel outboard and have the best of both worlds.

 

[NorthernFocus]

...Personally, if I'm looking at 8 - 10 year old boat with an outboard engine, I'm thinking I need to have money aside to repower. This is not true of boat with diesel engine(s), providing there are adequate service records and it passes an oil analysis...

All things being equal it's a lot easier to determine the condition of a modern outboard propulsion system than an inboard.

Outboard manufacturers now have the benefit of a lot of history/experience (i.e.millions of motors) as well the benefit of advances in ignition systems, materials, etc. Overall they now produce some really well engineered and robust systems.

On the other hand in the relatively small HP models used in pleasure boats, there are very few diesel engines that are built for purpose. Most of them are marinized versions of automotive/tractor engines. And they have relatively limited production which means less opportunity to work the bugs out of them. Frankly the positive reputation that marine diesels have is mostly historical and not really earned. For example, Volvo, Cummins, and Yanmar all produce true built for purpose commercial marine diesels that are bullet proof and have earned the manufacturers great reputations. But those engines have nothing to do with (other than the name) the engines that we have in our little boats.

There is a lot of traditional/old school thinking on this topic. Change comes slowly in any case and seemingly even slower with the boating community and maritime profession. And that's reflected in on-line boating forums and by the market. So there is still a price premium on both new and used IB diesels whether or not it makes sense.

Having said all that, when it comes to boats, I'm old school myself. In spite of my technical BG, esthetics and nostalgia play heavily into my decisions with boats. After all we're on the water for the overall experience, right? And in that regard the hull/cabin design of our little tug and the purr of the diesel engine is awesome!

 

[baz]

Dan:

That was a good perspective on things, especially the one about the big genuine diesel engines vs. the smaller ones we have in our RT models.

What draws me to RT models is their focus of maintaining the nostalgia as best they can while including modernization and embracing new technologies to hopefully appeal to their current customer base as well as an anticipated new and younger customer base.

I have a family with ages that extend from 2/5/9 ys olds up to 39 and close to 50 yrs old for the adults. We had the R-25 (Classic), the R-21EC and soon to have the new 2018 R-27. The family embraces and likes all these models and the older ones are mainly concerned about up-keep expenses. They don't really have the nostalgia many of us here have for the tug's 'old look' and are impressed with the more modern look and it's new features that RT has now embarked on. They appreciate and accept the efficiencies and improvements that modern technology and new designs bring along.

 

[mcphersn]

Well, we've gotten pretty far from the efficiency topic. Oddly, I still haven't seen a plot of mpg by mph for the o/b vs. the in/b, which would be the kinda definitive answer. I'm betting that the mpg for a R23 between 25 and 30 is better than the R25 anywhere near the high teens.

That said, I confess that I put money down on an R21, but switched to the R23 after a sea trial. My ears rang for hours after a high-cruise run in the R21. By the way, some authorities put the threshold of hearing damage from prolonged exposure at 85db, rather than 90. That's well within the range of some reports of diesel noise. I've actually left the Yamaha idling by mistake a couple of times because it's so quiet at low rpm. At 25 to 30 high cruise, the loudest sound in the cabin by far is from the water, not the engine. To each his/her own.

 

[baz]

Well, we've gotten pretty far from the efficiency topic. Oddly, I still haven't seen a plot of mpg by mph for the o/b vs. the in/b, which would be the kinda definitive answer. I'm betting that the mpg for a R23 between 25 and 30 is better than the R25 anywhere near the high teens.
...snip...

Here you go.... and it seems that from around 10 mph to R-25SC's top speed of 17 mph the R-25SC gets a tad more MPG than does the R-23. At around 17 mph both R-25SC and the R-23 are getting same MPG with the R-23 going all the way out to 39 MPH using roughly what the R-25SC gets at 17 MPH.

...and you're correct about "I'm betting that the mpg for a R23 between 25 and 30 is better than the R25 anywhere near the high teens". 🙂

http://www.tugnuts.com/gallery2.php?g2_itemId=48728&g2_imageViewsIndex=1

When on a sea trial with the new 2018 R-27 traveling at 27 mph it was consuming 2.2 MPG which is what the R-25SC consumes running at 10 to 18 mph. So cruising the R-27 at 27 mph is using no more fuel than cruising the R-25SC between 10 to 18 mph.

The 'sweet spot' for the OB RT models is from 18 to 35 mph as its MPG is relatively constant at 2.2. It's also comforting to know that with calm waters and increasing the R-23/27 speed from 20 to 35 mph the fuel use is not increased above what is used at 20 mph.