Diesel Comparisons

[NorCal Boater]

I have been looking around for a diesel powered boat and I have a couple of questions.

There seems to be 3 choices: Detroits, Caterpillar and Cummins.

Are some makes more fuel efficient than others if all things are equal?

In a gas boat, more horsepower is often more fuel efficient than less horsepower. (bigger engine doesn't work as hard as smaller to maintain RPM, etc.) Does that hold true with diesels?

Is one manufacturer more costly to maintain than others?

I am currently looking at 2 models. Late '80s 46' Express and early '90s 50' Sundancer. I see both of them with Cats usually at 375 hp and the Detroits at 550 hp. It seems the 46 at 27,000 lbs would be more efficient with the big GMs.

Thanks in advance for the help

 

[tmhudson2]

In general, diesels don't like to be run at light load for very long; this is less true with newer high speed lightweight turbocharged diesels than big heavy old iron. But the mordern diesels have more parts turning at high speeds with low clearances and tight tolerances, so expect maintenance to be somewhat higher on them. Also, electronic fuel injection systems don't lend themselves to owner maintenance, whereas the old mechanical fuel injection systems can be owner-maintained if you know what to do and have the right tools.

But (there's always a but . . .) the newer diesels are generally more fuel-efficient, regardless of make. Electronic control of the fuel injection timing also reduces smoking when throttling up, and the characteristic diesel clatter.

The modern diesels are also more finicky about needing clean (really clean!) fuel, so paying attention to filtration and making sure your fuel tank doesn't collect gunk is far more important.

Before you buy any used boat with diesel engines, take oil samples from each engine to be analyzed. This is the first step to finding out if you are buying problems or not, and relatively inexpensive.

As I get older, I find I love the sound of a diesel doing its work.

 

[fwebster]

If you are considering pre '96-'96 then any discussion about electronic diesels isn't applicable since only mechanical engines were available then.

While you have some good questions, the answers shouldn't be the criteria for engine choice because they are not the important points. Detroit 6v71'2 and 6v92's were 2 stroke diesels that have not been produced in several years. Parts that were plentiful are getting scarce and expensive. Five years ago, you could do a top end overhaul on a Detroit for about $15-1800/ cylinder; now it takes $5-7000/cylinder. But, the fact that rings the bell louder is that 2 stroke Detroits usually require a top end rebuild at something like 1700-1900 hours. A lot of the Detroit powered boats available today are about ready for someone to bite that bullet. Don't let low prices get you excited until you consider a $40,000+ top end overhaul.

Similarly, the 3208 Caterpillar isn't produced any longer either. The reason is enviornmental in that Caterpillar couldn't get the 3208 clean enough to meet the late 90's emission standards. These are great engines and Caterpillar services parts a lot better than most engine makers, but there is a little problem with the 3208......The cylinders are not sleeved. Without liners, when the engine wears you only choice is to pull the engine out of the boat and rebuild it, assuming it isn't worn too much to re-bore. Since you usually can't rebuild one, the 3208, as good as they are, have always been called a "throw-a-way" engine. But thier unit injection system is superbly simple and the 3208 is known to last a long time........I know of a pair in charter boat service in Florida that has about 17,000 on them with no major repairs....yet.

Detroits are smokey, leaky beasts........Cats are better in the smoke and leak department, but neither will hold a candle to the next generation of Cat engines......the 3116/3126/3196 engines, which are very clean and virtually smoke free engines.

Another factor worth considering is that Sea Ray learned a whole lot about building big boats from mistakes they made in construction and materials choices in the late 80's and early 90's. For that reason, a better choice would be a mid 90's or later boat with either Caterpillar or Cummins engines. The 80's 460EC and the 90's 500DA are really old designs that are heavy and have dated styling.

For my money.........I guess you'd have to say I put my money where my mouth is.......one of the best boats Sea Ray ever gave us is the '95-'99 450DA. They are plenty big for a couple or small family, yet small enough to handle easily and thy are powered by lower hp diesel engines like the 3116/3126 Cat and a few have 6CTA Cummins. Our boat cruises at 22 kts and burns about 23 gph. Pull the throttles back to about 2200 rpm and the fuel burn goes down to 17-18 gph. The 6CTA burns slightly more fuel.

Well, that is enough to get you started or prompt more questions.............

 

[rondds]

frank
what about the shift over to cored bottoms in the mid-late 90s? which bigger SRs might be affected by this and how much of a consideration is this to a used boat buyer? granted an effective hull survey will bring the issue to light, but what about once you own the boat and THEN water-logging issue develops?

 

[fc3]

Volvo makes marine diesels that are exclusively made for marine. Yanmar also makes marine diesels. MAN for large yachts, etc etc.

Back in Feb 2003, Powerboat Reports tested Caterpillars against Cummins diesels. They were installed in identical ugly boats. One of these ugly boats had the 6CTA8.3 engines. The other ugly boat had 3126B engines. Both advertised 450 HP, but the 6CTA8.3 were 430 brake hp, while the 3212B was a true 450 brake HP.

Both engines turned rated RPM, so the props, though not identical, were correct.

They found that the cummins boat was quieter, especially at lower RPMs. At 1000 RPM the 6CTA8.3 measured 73 dB while the 3126B measured 80 dB. That's a huge difference!

On the other hand, the 3126B boat was quicker, faster, and burned less fuel. For example, at 2500 RPM, the 3126B boat ran 1.5 knots faster while burning 7 GPH less fuel. That's $35/hr cheaper at today's prices. The 3126B also burned less fuel than the 6CTA8.3 when both boats were running the same speed across the board. The 3126B boat also accelerated faster reaching 22 kts in 12 seconds vs. 20 seconds.

PBR sent their test results to both engine manufacturers. The Cat engineer replied "I can't see any reason to argue with your numbers." The Cummins engineer said that he would expect the Cat to outperform the Cummins engines "Their fuel system is more advanced than the Cummins, which leads to better fuel economy."

One more thing I forgot. The Cummins engines were a bit less expensive, too. Probably quieter and less expensive won over better performance and economy at brunswick (you know, the junk bond company) and Sea Ray.

Best regards,
Frank

 

[Skip]

I think Dom's burn numbers in his 410DA bear out Frank C's post. My 420DA is a bit thirstier.
Since you can't get CATs in Sea Rays anymore, the choice for 03 and later boats is Cummins, or Cummins.
52 LOA and above will get you MANs as an option.
The 420 is my first diesel boat, and I am very much still in the learning mode. What I do know after 360 hours in two and a half seasons, and doing all my own annual services each fall, and spring recommissioning, is that my mechanical Cs are reliable and easy to work on, a superb match to the 420 hull, and if I don't push them at 90% or more of WOT they are pretty economical given that they are pushing 14 + tons of 420DA up and down the river and the bay. I can also get parts from the Cummins warehouse in 24-48 hours, and though pricey, I can get a Cummins/Onan tech on my boat with not much waiting. I have never owned CATs, so I have no basis of comparison. What I do know is that I will not go back to a gas boat. For the way my family and I run a boat, diesels make the most sense.

regards
Skip

 

[fwebster]

Ron,

A lot has been written about Sea Ray cored bottoms but there have been very few "actual sightings" of boats with core problems. Almost any time someone tells you about one, the same 400DB photos show up and its problems were hull sides not the bottom.

I am of the opinion that if one is looking afor a new boat, he should know what its construction is and he should require his surveyor to take moisture readings on the hull and deck as a part of his due-diligence. However, I certainly would not discount a perfectly good boat design because it has cored construction.

The guys at Sea Ray and our dealer (an independant non-MM who is a member of a buying group of 20 other similar dealers) both say that on the few boats they they know about with hull problems, the cause can be traced to either lay-up chemistry (not core problems) or a hole being cut or punched in the cored areas and not properly sealed by whomever did the cutting/punching.

And you bring up an interesting point.......fixing wet areas once you find them. I think this is an area loaded with unknowns and suppositions. An area with a wet core does not turn to the consistancy of a sponge. Honestly, as long as the area is reasonably small or reasonable accessible, a core repair isn't that big a deal. You remove or dry the core and fill the area with epoxy, seal it up, gelcoat the spot if it is above the water line, bottom paint it if not and go boating. Personally, I would prefer knowing that the previous owner had a problem, found it and fixed it because it would indicate that someone who knows boats was involved in the maintenance of the boat if they even found a wet spot in the first place.

This whole wet core thing is a non-event for me.......but I know boats and I would use a surveyor with moisture detection equipment if I were a buyer.

 

[NorCal Boater]

Thanks for the responses guys. I will keep doing research and looking with all of your great advice in mind.

I will be in the DC area next week for business and I am taking time to look at 2 46' Expresses. Both with 550 Detroits. The boats are the same price but one states the engines were rebuilt 600 hours ago. If the Detroits are a 1700 hour engine TBO and it was done and done properly....? Also, I don't buy boats without a full survey from a reputable marine surveyor. My last name is Murphy; why take chances.

 

[fwebster]

On 80's 460EC's be sure you look at these areas:

1.Cockpit sea bases tend to rot
2. Cockpit side walls are plywood and tend to rot
3.Cabin windows tend to leak
4.Limber holes in the stringers are filled with PVC siliconed in place. This is a trouble spot that can cause stringers to rot. The pvc plugs should be removed and the holes sealed with epoxy or better still, filled with fiberglass tubing that is glassed in plced.

 

[NorCal Boater]

Since my last post I have found a few early to mid 90's 50' Sedan Bridge boats obviously for more money. Almost all from 90-95 have Detroits of various sizes. It seems that 96 was the magic year to stop using the GMs as most boats 96 and up have Cummins or Cats.

So many to choose from.....

 

[tobnpr]

I can only comment on the CATs; my 390 has the 3208's and I'd want more power in a boat that's a couple of tons heavier. I've got a 22 kt. cruise, I'd bet the 46 would only cruise in the high teens; but if that's OK for you it might be a good choice. Factory reman blocks and parts are still readily available. CAT made the 3208 up to 475 hp, but they have a much shorter lifespan when hopped up to that output. The 3208's at the lower output (375) have a "bulletproof" reputation.

I'd suggest you join Boatdiesel.com if you're serious about a diesel boat- there's a lot of knowledge on that board.