Gas or diesel?

Discussion in 'Boats, Motors, Trailers and Towing Rigs Forum' started by Ruff, Nov 16, 2017.

  1. Ruff

    Ruff Member

    I've been looking at family cruising/fishing boats big enough for a family of 5 to spend a weekend (or more) out on the chuck. Most of the boats I've been looking at have the option of both gas and diesel power. My question is, what's the best way to go?
    You can generally count on around 25% more for a diesel as compared to going with gas but is the payback worth it? You can buy a lot of extra gas for the 20-30K that diesel is going to cost you.
    And what about maintenance? When it comes to trucks the cost of gas vs diesel repair is not even close. Same with boats?
    I run a long way every weekend (100-120 miles) so should this play into the decision? I need a boat that will cruise @ 20+mph so something like the Commander 30 is what I'm looking at. Is there something else I should be looking at? My kids are young (6 And under) and we have outgrown my 20ft aluminum. Once I pull the trigger I'll hopefully be keeping the boat for the next 20 years so should this be a factor on gas vs diesel? Let me know what you think. Thanks!
  2. profisher

    profisher Well-Known Member

    If you are doing mostly longer runs I would go diesel. A diesel engine takes longer to warm up so short trips don't suit them. They will save you on fuel and you will burn plenty of it in a gasser if doing the miles you mention. Diesel is safer, doesn't explode. Diesels hate dirty fuel, oil air to get the long engine life they are known for...use good filters and change them when you should. The only negative I'v e heard of is from huys who have them on outdrives...they produce so much torque that legs have not stood up as they should. This may have been fixed with the newer legs. The negatives are repair costs are higher, diesel is not offered at every marina and they stink.
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  3. Birdsnest

    Birdsnest Well-Known Member

    you forgot the drop the mic. lol
  4. bigdogeh

    bigdogeh Well-Known Member

    twin diesels... but I am biased.
    I wouldn't go back to gas now that I've had diesels for a number of years. Crazy fuel economy compared to gas. I do all my own maintenance so maintenance isn't an issue. I have mechanical fuel injection so very little electronics to have to worry about engine wise compared to common rail electronic fuel injection. My engines are fairly smokeless once they have warmed up a bit. The engine will run without the battery hooked up once it's running. Just need the battery to turn the engine over, then compression takes over. The newer electronic fuel injected common rail are pretty much smokeless. I have to admit, I do feel safer being in a diesel boat over being in a gas fueled boat. (explosion wise)
    The diesel may cost more initially, but the resale might be a bit better with a diesel.
    Profisher pretty much nailed most items with his post.
  5. Thunder21

    Thunder21 Active Member

    A 30 x 12 beam 13000 lb Commander with twin diesels burns the same amount of fuel as my 8000 lb 26 Hourston with a single gas engine . It's a no brainer
    trophywife, Sir Reel and Ruff like this.
  6. tubber

    tubber Well-Known Member

    I know the folks at Pacific Rim Charters have been displeased and frustrated with the availability of parts and the longevity of their inboard gas motors in some of their big (30'+) charter boats. Mercuries.
  7. bigdogeh

    bigdogeh Well-Known Member

    If I was in the market for diesels I would be looking for Cummins... Volvo's and Yanmars are quite a bit more expensive when it comes to parts availability. Cummins parts can be found anywhere and generally can be quite a bit less expensive.
    trophywife, Fox, Oly1 and 1 other person like this.
  8. ericl

    ericl Well-Known Member

    Join Use the advanced prop calculator to see potential fuel consumption for aa particular boat/engine combination. One of the most respected Diesel authorities is Tony Athens ( Tony feels boats under about 33ft/15000 lb are better with O/B's. Tony is in SoCal where they troll faster than we do. A diesel will troll all day on a few gallons or less at Salmon speeds. At about hull speed, fuel consumption of O/B versus diesel gets closer. Tony says diesel life is about 8000 gallons of fuel per liter of engine displacement - see this article:

    Use your 110 miles/weekend, divide 110 miles by 20 mph, to get hours/weekend, add in trolling hours, & chooses an engine that will last 20 years at this utilization rate. Iv'e looked at Yanmar's at your utilization it would probably take at least a year to break it in.

    I am re-powering a 33ft boat that has 2 old 200 hp diesels - will probably go with new diesels; probably not Cummins as they would add 1000lb of weight over current engines.

    All the modern diesels are using saltwater after cooled turbo charging. Dissimilar metals in the after cooler will require maint beyond what gas engines do.

    If you travel through heavy seas the torque of a Diesel will better keep you on planes & reduce fuel consumption.
    bigdogeh likes this.
  9. bigdogeh

    bigdogeh Well-Known Member

    I think the Cummins are around 1100 - 1200 lbs for the 6bt if not mistaken. Their larger displacement engines might be a bit more. That would make your engines around 600 - 700 lbs? Seems pretty light for diesel engines. Even the smaller D4.2L mercruiser diesels are around 950 lbs I believe... what kind of engines are you running ericl, may I ask? just curious..
    I would think a 33' boat would be perfect for 330 or 370 HP Cummins... or maybe even one of their newer, larger displacement engines.
    Last edited: Nov 22, 2017
  10. Rayvon

    Rayvon Active Member

    You might want to consider diesels with shafts,you may lose a little MPH ,but a lot less maintenance and pretty much bullet proof.
    trophywife, bigdogeh and turbomack like this.
  11. bigdogeh

    bigdogeh Well-Known Member

    diesels with arneson surface piercing drives...yee- haaa!

    not sure if this boat is using diesels though...

    Here's one that is... nice boat.

    Last edited: Nov 22, 2017
  12. ericl

    ericl Well-Known Member

    My current diesels are Volvo TAMD41B; 200hp at about 1000 lb. Original power was 350 Chevy at 850 lb or so.

    The Cummins common rail engines are around 1450 lb, If I replace engines, i want clean quiet Tier 3 engines. Using the boat diesel prop calculator, additional HP isn't worth the fuel burn to me as I am retired & I am more interested in cruising at hull speed versus going over 30 - I can go 25 now which is fine.

    FYI a Volvo D3 weighs in the 600's
    bigdogeh likes this.
  13. ericl

    ericl Well-Known Member

    Wanted to say more yesterday but I couldn't see well due to an eye infection.

    I have straight shafts. The prop struts do not look big enough to take a larger diameter shaft; which limits me to around 300hp or so. My prop clearance with 18" props is at the limit (12% or so).

    This info is also useful:

    Given a diameter of 18" & the prop diameter/pitch ratio's mentioned here, I can only harness so much power in a cost-effective manner. As a further FYI the dealer i will probably go with (Cummins & Yanmar) says the Cummins requires a very large diameter exhaust versus Yanmar, requiring all new exhaust that will be harder to fit.

    I also found this article useful:

    I took my typical fishing day of high speed running for 2 hours, trolling for six hours the calculated daily fuel consumption. Times how many days a year I will go this, times how many years I want to get outta new engines - I am 67 y/o. I then looked at the displacement in liters of available small Diesels & ran the numbers against Tony's formula for engine life (8000 - 16000 gallons of fuel burn per liter of displacement over the life of the engine.) I am sure this formula is not perfect, but it was far better than the rest of the internet garbage on the subject of small marine diesels. Being a true motor head who came up in the 60's, the "no replacement for displacement" rings true.

    I then looked at the EPA info on diesel pollutants & decided that a 100:1 reduction in particulate matter from the old stuff to tier 3 engines was something I wanted. I troll on these standing by the exhaust & will be taking my grandkid's out with me.

    Inboard gas is an alternative; maybe I'll convince myself to go that way. Modern Diesels go for around $100 US per HP. Gas is probably 40% or less. But over 20 years, how many gassers do I have to pay the R&R costs on ? The original owner of my boat had to have one of the 350's replaced under warranty at 70 hours. They were replaced by the Volvo Diesels at 14 years. I had a SeayRay I put a new 350 in it flooded & the engine was replaced by insurance with about 50 hours on it. The compression was already down a lot on a couple of cylinders. The cop's down here tend to run Ford's because Chevy engines don't hold-up to hard use, so for me that rules out a gas inboard.
  14. bigdogeh

    bigdogeh Well-Known Member

    I remember cummins saying they need a large exhaust diameter in their specs but I also remember Tony mentioning you can get away with quite a bit less. I thought it was 3 or 4" for some reason...

    this may be the line I was thinking of...

    "370 Cummins, with V-Drive with high mount turbo 3″ dry to 5″ mixer – One 90 degree wet bend – Under 10″ H2O at WOT – Very simple and safe arrangement!!"

    3" dry pipe isn't all that big. I think Cummins specs for 5" but I remember Tony mentioning you can get away with less.
    This is the link you were probably looking at, at one time. Some good info on exhaust stuff.

    my apologies to the OP, Ruff , for getting off track a bit on this thread. But if he is thinking of going diesel, their might be some good info in here. And one of the most important things is having the exhaust routed properly or you can kill an engine fairly quickly if you start pumping water into it through the exhaust. A really good thing to look for if buying used and you would hope a surveyor would point out anything that looks suspect when looking at the exhaust.
  15. captmike

    captmike Member

    good advice as I have joined boat and learned much about the KAD 44 volvo I have with a 290 volvo leg on a 26' Skipjack ,Yamaha kicker (the diesel does not like to run cold near idle) -very stable boat with engine below decks. I've had a good volvo mechanic who advises along with clean air,fuel, oil that this is not a ski boat-accelerate gently with a 260 hp diesel-3000 RPM-20 kn -6 gal/hr
  16. ericl

    ericl Well-Known Member

    Hey bigdogeh what engines do you have? Thanks for the info BTW.
    captmike; my TAMD41B's troll low & slow (a bit too fast for some situations. Biggest problem is trying to tweak the throttle a few RPM with manual controls. My gears won't take trolling valves & are 2 generations obsolete as far as parts are concerned. I'll bet the gentle acceleration advice is for the outdrive, not the engine. VERY nice mileage BTW.
  17. bigdogeh

    bigdogeh Well-Known Member

    I have 96' 6bta (p pump) Cummins atm but had the vm motori (mercruiser) d4.2L's previously. The d4.2L's were good in that they were virtually smokeless but when you compare to the Cummins they are very lightweight in design. I had a crankshaft break where the damper/balancer mounts. And some others (d4.2L owners) have had the same problem. The crank is just not beefy enough in that area. The Cummins is so much heavier built in that area and most others. Just no comparison really. The d4.2L was really good on fuel economy also. And I trolled on it all the time with no problems what so ever. actually had to give it a bit of throttle every now and then as I could troll almost as slow as I wanted... Just didn't have the confidence in that engine once I had broken the crank in that area. Decided to upgrade to Cummins. There's a reason they are used worldwide in all sorts of commercial industrial equipment.
  18. Sir Reel

    Sir Reel Well-Known Member

    I had the AD41P which is the predecessor to the KAD44 in my 26' Seasport. I was told cruising RPM was 200 rpm below WOT. My WOT was 3800 -3900 rpm so cruised at 3400 to 3500 rpm and about 25 knots and 5gph. I think your 3000 rpm cruise is too low to keep a proper load and temp on the engine. You must make sure you can make WOT fully loaded to make sure you are properly propped. By the way I switched to stainless steel props from aluminum and got 2 mph more top end and same rpm. C3's I think they were
  19. captmike

    captmike Member

    according to owners and mechanics 200 RPM below WOT (mine is 3900 RPM) is too much demand on this engine (google KAD 44 diesel) as I don't have a pyrometer I keep the engine temp at 180 F-3000-3200 RPM is about right. I too have C3 SS DP
  20. ericl

    ericl Well-Known Member

    Tony at SBmarine says he never saw a diesel fail from being ran too easy. My TAMD41B's are supposed to max out at 3800. My mechanic said to run them at 3200.
    There are those that feel 200 rpm below rated is too much for any engine. I guess you could run your engine at 200 rpm below rated & if it wear cylinders out sooner than you expected 200 below is too hard. It seems some engine mfgrs encourage this hard usage via a mentality of "you can use all the HP you paid for with our engines". Probably helps with sales & any bad results won't happen until much later.
    bigdogeh, some of those Italian diesels are marinized VW TDI diesels; were yours?

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