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Lets talk Diesel engines

Old 06-09-2021, 12:47 PM
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Default Lets talk Diesel engines

Ok all I know this could be a can of worms I don't want to open but I am going to try and see what happens here. I have plans to repower my 42' Post with a set of diesels. Let me start by saying I don't have the ballers budget that I am sure most people that own a boat this size do. I will say though that I am not scared to drive a ways to pick something up to save some cash and not scared to search a little. I will likely end up with a set of Pull outs that I am going to have to drive a ways and pick up. I will also be doing all the work for the same reasons mentioned above. I currently have a pair of great running 350 HP crusaders with velvet drives. I want to repower to diesel strictly to remove the explosive properties that come with gasoline from my boat. I definitely don't want to lose any performance when I swap. I would also still like something that I can get around fairly easy for maintenance and repairs. So with all of that said I am curious on recommendation from all of you. What you would install and why? It appeared that these also came with 6V71s, 8V71s, 6V92s, and 8V92s Detroits. I have been looking around mostly at these since I am pretty comfortable that if they came with them there shouldn't be much of a fitment issue when I swap to them. I have never owned anything with a Detroit diesel so I know nothing much about them. I know the people that love them are diehard and the people that hate them are just as diehard. I am not stuck on them either way. In another post I made someone was talking about installing C12 Cats and how I was going to spend $100K to repower the boat. Well I promise you that I will not spend $100K to repower! 1) I don't have a $100k to spend and 2) there are to many other options out there to justified me spending $100K on a repower. I have found many fresh built engines in the Detroits for $12K and less. Someone also mentioned installing C7 CATs I don't know a lot about them with exception to the one truck I owned that had the C7 in it would drop all the way down to 25 mph pulling my backhoe and my 6.0 Ford would run circles around it pulling the same backhoe. I have been all over the web looking and seeing tons of different engines and trying to figure out the size, weight, HP, TQ, and well just as much as I can. Its become a bit over whelming at this point. I figure information from people that have actually dealt with them would help me narrow down my options. I found a pair of 450 HP 3116 CATs that someone has for sell as well. These appear to be small enough to fit in the area. So now that I am done rambling what information would you offer? What engines are good options for what my plan is? Thanks in advance for any and all information you can provide.

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06-09-2021, 01:03 PM
mystery
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Hard to read but you want to repower simply because of gasoline's explosive properties? If so, sell current boat, find a boat with diesels.
Old 06-09-2021, 01:03 PM
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Hard to read but you want to repower simply because of gasoline's explosive properties? If so, sell current boat, find a boat with diesels.
Old 06-09-2021, 01:13 PM
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The Detroit v-blocks are big and heavy. They make the 454's look like a lawnmower in the bottom of an empty swimming pool. And did I mention heavy?

And most diesels are at least a few inches taller than a gasser, and most diesels run offset gears where prop shaft is below crank, unlike a Velvet Drive. So you need to be sure of how much clearance is above engine and below hatches.

This is a big project. I've managed quite a few. Usually best to sell old boat and buy another with this already done, or came with factory diesel.

If serious, look for 6BTA 5.9 370hp or QSB 5.9 380hp. Relatively compact and not much heavier than 454. A few inches taller and about a foot longer.
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Old 06-09-2021, 01:19 PM
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gonna have to agree with mystery on this one. Unless you're planning on doing the majority of the work yourself, you are most definitely going to spend 100K to repower. It's not just engines. It's reduction gears, it's the entire fuel system being redesigned, it's the engine beds needing to be moved/redesigned, new shafts/struts/props, potentially cutting out the aft bulkhead and taking the salon apart/putting it back together.......it's a huge job. You'd really have to be married to that particular boat to go through with it.

As far as engines go, I have heard Detroit 2 stroke parts are becoming hard to source....others can reinforce or refute that. CAT C7s are good engines but the high hp models (450hp) didn't have great reputations. I'd think a set of factory remanned Cummins QSC 500 or 450Cs would be a good option for that boat. Big money though, I'd guess at least 25K per.

If you haven't already, join up over at boatdiesel and perhaps someone over there has done a similar repower and can give you some insight.

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Old 06-09-2021, 02:00 PM
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Those CAT 3116's are not 450 hp - the highest rating they had were 350 hp. The C7's are electronic versions of the 3126's and only came in 461 hp models so I have no idea what HarborRd is talking about the h. Cummins 450c (non-electronic C-Series) as remans would be a good choice, but they are $42k each with a ZF280A gear attached.
If you can get a pair of C12's with ZF325A gears on them for $100k for the pair, do so as the C12's are $72k each and the ZF325'S are $14k each.
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Old 06-09-2021, 02:31 PM
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First let me apologize for the rambling above. Sometimes I get to typing and think of things as I am typing. If I don't get it typed out I will forget it. So everything doesn't flow smoothly into printed text as it seems while its in my head.

Yes I will be doing all of the work myself. I am pretty handy with a wrench and not scared to cut, grind, weld, or glass anything. There will be zero cost for labor. I am just getting into the whole boat world from the car world so I am sure there is plenty I don't know but the only way to learn is to dive in. As for the whole sell the boat and buy one with diesels. I am not sure if you read the part that I don't have 42' boat money over here. I got this boat for a steal at an auction is the only reason I could afford it. I had plan to sell it but got to thinking how I have always wanted one of these and it was a end goal deal. If I sell it I am not sure I would ever be able to afford it. If I had $150K to spend on a boat then sure I would just sell this one and find one. Right now this one is paid for and doesn't cost a dime to take my time sourcing parts to make it my dream boat.

Back to the engines though, Yeah I have not had the chance to actually measure to see how much more space I have. I know there is room above the engines to go up but not sure how much. There is plenty of room forward of the engines as well. Just don't have numbers yet. As I was looking I saw lots of the Cummins engines. Didn't really look much at them as I hadn't heard anyone say anything about them in the marine world. I know they are great engines for trucks. I will have to check into them since there seems to be a large amount of them out there. Thanks for the suggestions.
Old 06-09-2021, 02:35 PM
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For a fraction of the cost and work of a diesel conversion you can rebuild the gasoline engines, ventilation, fuel supply and delivery systems with quality components. Gas is fine when done right.
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Old 06-09-2021, 02:38 PM
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Originally Posted by LI32 View Post
Those CAT 3116's are not 450 hp - the highest rating they had were 350 hp. The C7's are electronic versions of the 3126's and only came in 461 hp models so I have no idea what HarborRd is talking about the h. Cummins 450c (non-electronic C-Series) as remans would be a good choice, but they are $42k each with a ZF280A gear attached.
If you can get a pair of C12's with ZF325A gears on them for $100k for the pair, do so as the C12's are $72k each and the ZF325'S are $14k each.
When I first started looking at the 3116s I saw this on the Wikipedia page that they only went to 350hp as well. So I assumed that the person was either wrong or had some type of mods done to the engine that made him claim this. It ran me away from them either way. As I am going through the 100s of engines that I looked at today I came across another single 3116 that had a picture of the engine plate that clearly stated 450 HP as well. I just assumed that the chart I saw might have been wrong.
Old 06-09-2021, 02:48 PM
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Originally Posted by Milehog View Post
For a fraction of the cost and work of a diesel conversion you can rebuild the gasoline engines, ventilation, fuel supply and delivery systems with quality components. Gas is fine when done right.
From my understanding you can't get the coast guard to sign off on your charter boat if its gasoline. Now granted this is what I was told by boat captain in Florida so take that for what its worth. If that is true that should say something about the safety of gasoline on your boat. I know it could be made safer but the fact will still remain there is a potential boom underneath the salon and between me and the exit to the boat. Couple that with the fact that the main breaker box is mounted into that compartment and it a no thank you for me.
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Old 06-09-2021, 02:48 PM
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Make sure you really research this. We are talking engines, new transmissions, new mounts, props, shafts, probably struts, stuffing boxes, controls, gauges, fuel filters, tank cleaning, in my case generator too. Then you have to account for the additional weight, how you are going to mount them, pulling the old engines, putting the new ones in, renting a crane or lift to do this.

I researched this extensively and there was no way I could make it make any financial sense when I thought of all of the things that need to be replaced, added and fabricated.
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Old 06-09-2021, 02:49 PM
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The Cummins mentioned above are good choices. I like Yanmar as well
Old 06-09-2021, 02:56 PM
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CAT made the 3126 in 420 Horse Models, maybe up to 450 as the C7? Those 3126 @ 420 are perfect in a 40 Ocean. Not sure how much heavier a 42 post is?
Old 06-09-2021, 02:58 PM
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If you don’t have 100 clams to start , don’t start. That’s the entry point for a diesel swap. Just wait till you find out there was a reason someone sold you a pair of used diesels.
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Old 06-09-2021, 02:59 PM
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Are you open to buying a set of low hour used diesels?

a friend of mine is getting rid of a pair of 450hp mechanical volvos, with gears. I can get the specs if you like, or if anyone else is interested.

he was gonna put them in his egg harbor
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Old 06-09-2021, 03:00 PM
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On the budget side of the equation I'd be looking for some decent 350 hp rto cummins for a few reasons.

Cheaper than all other options to own.(rtos will need work sooner than later)
Light weight. (A 454 is tiny compared physically to a c12 or 8v92)
closest rpm to power as gas ( you'll likely be closer on shaft sizes than a big bore diesel)
Cummins parts seems more readily available than the yellow paint options for the DIY guys.

I love old Detroit diesel engines. But reliabilit parts suck, most of the good Jimmy mechanics died of obesity related issues 10 years ago. They are heavy, puke oil, and are noisy. This as someone who has an operational 8v71 that I hope to nurse along a little while longer.

Going big will end up needing, bigger shafts, different props, shaft logs, packing, couplers transmissions etc. Bigger exhaust, fresh air flow, fuel lines, fuel filtration....

Truth be told this is like asking for suggestions of 10 dollar solutions to a 40 dollar problem.

Repoweres are expensive, and it's the little things that add up that kill you. Especially in twins as its everything x2.
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Old 06-09-2021, 03:00 PM
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It is pretty much the same as doing a truck conversion. Let’s take an early 90s f250 it comes with either gas or diesel should be an easy swap but there are many parts that are different the rad support is different for the diesel radiator and the gas rad support is missing the second battery tray. The gas gas tank is only plumbed for a feed and no return line the trans crissmember is different the rear gear is different it all adds up quickly along with the time to change same with the boat going to need room for four batteries instead of two might as well also put a diesel generator in if it has a gas one. It all adds up quickly just really think it out before doing it
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Old 06-09-2021, 03:00 PM
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Before you start looking for engines, make sure your existing shafts are adequate size, changing to bigger shafts is a much bigger (more expensive) than buying engines. Also make sure it's viable to actually remove and install engines without cutting the boat to bits. Trying to match performance of your old engines with diesels is not going to be easy, especially when talking "old" Detroit diesels, I would put that idea way to the back of your mind, to get anywhere close to current performance is going to require more modern, lighter, higher revving diesels, and that means money.
Old 06-09-2021, 03:02 PM
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We've had 2 Post 42's in the "family." Buddy of mine's dad has one with 6-71N's, my late brother had one with 454's. Fuel consumption-wise, they were very similar. Also, performance-wise they were very similar. I agree with the above, the conversion is going to cost a ton of money, even if you do it yourself, and you'll never recoup the investment. Going with used engines, you never know what you're going to get. You'd be better off with a brand new pair of 454's if the current engines are worn out. Most folks would avoid gas engines in this size/weight boat, but it's flat at the stern, so planes easily. Gas engines actually work perfectly fine on this platform.

I'm pretty sure the one my friend has will be going up for sale soon. It has 6-71N's that may have 20 hours on full rebuilds, which pushed well over $20K. It needs a bit of TLC, but mechanically it's ready for another 6,000 hours. Might be something to consider.
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Old 06-09-2021, 03:04 PM
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Fix that boat up, sell it, buy a diesel powered boat.

Otherwise you are truly going to convert that boat to a hole in the water and dump your money (and maybe your marriage) into it.
Old 06-09-2021, 03:15 PM
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Huge HUGE...project your taking on.... as someone previously pointed out, it's not just a repower...it's a complete REDESIGN....with all attendant costs.

I have a 1970 Striker twin diesel s/f...love it. Twin 6-71 Ti's @ 450 a side. Repowered in 1988. Original's were 6-71 naturals. I bought the boat with the TI's. I have nothing bad to say about the current setup...I can loaf along at 9 kts almost idle and burn less than 10gph....or run at 1800-2000 at 18-20 kts at 34-37 gph.
A few things ....DD like to run with their tongues hangin out....matter a fact, I make it a point to run her hard for a bit on the way in after long low power settings.... got to keep em clean or they start to drool all over.
If your a fisherman...nothing..I mean NOTHING...seems to raise fish like the ol twin DD...and they sound awesome too lol.

Maintenance....pretty straight forward. Keep your cooling system up to snuff, maintain your turbo's IF ya have em...and run decent oil. If you maintain them, and they are cheap to maintain IF you stay in front of it...they are beyond reliable, inexpensive, and trustworthy. Easy and cheap to rebuild in place, parts are available, and you don't need 3 freakin degrees to work on them.That being said.... GOOD old school DD mechs are getting hard to find. That knowledge base is slowly dying off unfortunately.

If it were me....given your situation...I'd try and find a suitable hull currently powered with the diesel of choice. It will be far less time trouble and expense in the long run.... BUT....don't be scared of 6-71's... there are times I'd swear I'd marry em if they had breasts.....lol.

If your really willing to take this project on...do your homework first...think it all thru. Everything your about to do will be 3x's the time, cost, and effort as you originally plan for. And keep in mind...A homebuilt repower will have an effect when you sell the vessel down the road.

Not trying to say don't do it...just be VERY aware of what your really getting into. That extra 25k you'll end up going thru to repower can buy an awful lot of existing boat.
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