Auto Diesel (Yeah, again, but...)

Discussion in 'Propulsion' started by Filmdaddy, Jul 21, 2005.

  1. Filmdaddy

    Filmdaddy Previous Member

    I know that there have already been umpteen postings about using automotive diesels and gas motors in boats. But one thing I have wondered about has never been addressed. Everyone assumes that the auto motor will be run at 90%, high-revs, pedal to the metal speeds. But if the motor is to power a displacement hull, and the prop is around a 20/20 (give me some slack here, OK? I'm not really familiar with the right terminology), then why not assume instead that the motor is going to run at, say, 2000 rpm, through a transmission, to reduce the prop speed to, say, 500 rpm?
    Continuing this thought, everyone goes on about the expense and turmoil of marinizing an auto engine. The safety requirements, no argument. But the rest - raw water cooling, bronze plugs and the rest - make me think. Why not just stick on a couple aluminum radiators, say in a lazarette, complete with high-volume fans and considerable screening to promote good airflow? It seems to work pretty well in tractors, and I have run a diesel tractor for fifteen hours under appalling condition of high speed/high torque requirements.
    I am astounded at the cost of anything from paint to motors that has the word "Marine" in the brochure anywhere. I am also not interested in what is "traditional", only in what is practical. Remember - marine tradition used to dictate that we wore wool all summer, bathed once a year and got flogged for everything.
     
  2. fcfc
    Joined: Feb 2005
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    fcfc Senior Member

    The trick is not only the cooling.

    Check bowman heat exchangers. or keel cooling. On steel boats, some simply weld on the length of the keel half a pipe.

    It is the whole engeenering : starter, alternator, bed mount, fuel supply, air supply, exhaust, soud insulation, vibrations, etc, etc ... You have a bunch of points. not all complex. But you cannot afford to miss a single one.

    Another point, it is hard to find a non "depolluted" automotive engine. It means that in the injection system (diesel or gasoline), you will find a bunch of electronic systems rather undocumented and absolutely not marinised.

    The next point is the gearbox.
     
  3. Filmdaddy

    Filmdaddy Previous Member

    fcfc...
    Yeah, OK, I understand that there are complexities. But I still want an answer to the basic question - why not use a (cheap) automotive engine in a displacement hull, transferring all of the relevant engineering - starter, alternator, etc. - from the auto into the boat? If the motor is driving the prop through a transmission... and yes, I am assuming a thrust bearing or something similar... why would this kind of system not work?
    Aircraft fly with Subaru engines and 1/2 VW engines and Vortec engines. They are able to do it because of reducers that cut the prop revolutions per minute to a fraction of engine speed. Is there a reason why boat people can't use the same philosophy?
    The concept is the question. The details are for later.
     
  4. woodboat
    Joined: Nov 2003
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    woodboat Senior Member

    Why does one choose a displacement hull? Most I have seen want to travel, have utmost reliability with good fuel economy. Would you set off across the ocean in a home marinized auto diesel? What kind of transmission do you propose to use? It is very common to have gear reduction units on marine transmissions to reduce prop speed and allow you to swing a bigger diameter. I think I have 2:1 in my boat and am swinging 19X16 props. I will say that companies like peninsular have successfully marinized truck diesels so it is possible.
     
  5. Filmdaddy

    Filmdaddy Previous Member

    Woodboat...
    The concern here, and my question, is whether the auto engine would actually have to run at 70-90% throttle all day to successfully propel a displacement hull. I think that, by using an appropriate transmission, you could reduce the engine RPM to a point where the engine wouldn't be required to slam along at high throttle for long periods. By keeping the engine RPM down, and still being able to move the boat with some efficiency using a cheap, easy to work on and maintain power system, you get the best of both worlds.
    As to marinizing an engine, why?
    Why not lift an engine from, say, a pickup, complete with transmission, install it as a air-cooled unit in the stern of a large boat, and motor off? No electrolytic corrosion, no sacrificial anodes, no high-priced "marine" components (beyond those required for safety, of course), for an installation that is inexpensive, easy to service, and, if you choose the engine and transmission carefully, able to be repaired and maintained anywhere in the world.
    All the threads that I have read have come back to the high-throttle question, sometimes with a lot of sarcasm and indignation. Everyone cites the high RPM requirements of the marine engine. For a planing boat, I understand completely. But a displacement hull is different. And nobody seems to address that difference.
    That's what I'm asking.
     
  6. SAE140

    SAE140 Guest

    Ok Filmdaddy - I'm on your team with this one ....

    I'm currently designing an ocean-going sailboat which has as it's basic design brief that nothing, but *nothing* used in it's construction will have a 'marine' tag attached. Why ? Caused I'm hacked off with the "marine" price-tag rip-off, and the poor quality of gear which is passed-off under this label. This boat will form the basis of a book. More on this if anyone's interested. I now have everything sorted, with the singular exception of a gearbox.

    Ok - so, staying with engines - yes of course it's feasible, you just need to be selective about the donk you initially choose. Stay small. The smallest European auto diesels are in the 1.4 to 1.8 litre range, and typically produce around 50 bhp at 4 to 5,000 rpm. Their max torque tends to be around 2,000 to 2,250 rpm - so you could expect around 25 bhp at that speed, which is perfect for a 30 - 40 ft sailboat, with the upper rev limit for propulsion held at 2,250 rpm where bore-glazing shouldn't become a problem. I'm talking non-turbo, 100% mechanical units - any electronics are simply indicator sensors, and are not involved in engine control.

    Cooling - not a problem on a steel boat, simply use freshwater keel cooling using the original water pump. Ensure the thermostat is still fitted to prevent over-cooling. Use large bore tubing. Make-up a water-cooled exhaust manifold and pipe it straight from the keel, so that pre-warmed water is then fed to the engine jacket. Use a dry exhaust.

    Air-cooling an engine with a radiator is perfectly possible, but it's not something I'd want to do - you'd need to use forced-air ventilation, and that would mean moving a lot of salt-laden air around.

    Marine alternator & starter ? Don't bother, use standard auto kit - just keep inflammable gases out of the engine bay, and keep the bilges dry. Carry replacements.

    Transmission - this is where I'm currently working on ideas. Even s/h marine gearboxes have crazy prices in Europe, but admittedly they do provide an ideal solution.

    One alternative is to use an auto gearbox, but they have the clutch in the wrong place, and you'll be gear-crunching unless a prop-brake was used. Also might need cooling. All-in-all - messy.

    A second alternative is simply to have a direct drive from the engine to the prop. In the above case this would mean propping for 2,250 rpm. My tables give: 6 knots/25 bhp/2200rpm = 12.6" x 7.7" @ 40.5% as opposed to 6/25/800 = 22.9 x 15.5 @ 51.9% - so sure, a big slow prop is better, but not having a gearbox is possible - depends on how much you intend to use it, and for what purpose. It'd be hellishly awkward to use for manouvering in a marina, but ok in open water. Needless to say, a thrust bearing would be required as soon after the stern gland as possible. Auto prop-shaft or drive-axle flexible joints fitted to the shaft make life easier, and are essential if you're fitting flexible mountings. If you do fit flexible engine mounts, then ensure that if they fail the engine cannot break free. A loose engine in a seaway might ruin your day.

    A third alternative is to run the engine straight to a variable-pitch prop. This would be very much as above, but with the added bonus of giving you neutral and reverse, as well as optimising the pitch angle when underway. You could make your own - I've heard of it being done.

    I'm currently working on an even simpler method of providing FNR from a direct-drive single solid shaft without the use of any kind of gearbox, gearing, sprockets&chains, pulleys&belts, layshafts, prop-wash control or variable geometry prop - but I'm still handicapped with too many revs, so a commercial 2:1 reduction unit might still be called for.

    So in answer to your question, despite the vocal dissent which usually accompanies this topic, my opinion is that for displacement boats the concept is indeed sound, and would suggest that reading the classic "Marine Conversions - vehicle engine conversions for boats" by Nigel Warren, Adlard Coles, 2nd ed. 1982, ISBN 0-229-11678-7 might prove beneficial if you intend to seriously pursue this idea.

    Regards

    Colin
     
  7. woodboat
    Joined: Nov 2003
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    woodboat Senior Member

    First off you are assuming that the experts that tell you that you need 75-90% throttle are correct. I will agree that some engines need to be run this way or else they carbon up. We had a cummins turbo diesel in a lift truck with an auxilary engine for lift. This engine idled for 12 years without issue while the man worked in the bucket. 12 years, 8 hours a day, five days a week. Also my ford diesel in my excursion runs around at part throttle, I do not expect it to carbon up nor does ford.
     
  8. Filmdaddy

    Filmdaddy Previous Member

    SAE140...
    I was considering installing a very heavy flywheel, appropriate diameter and tooth pattern, and running an automatic, perhaps from a truck for the gearing and shock-absorbing qualities. Agree completely with regard to the Euro diesels. And, as I am paranoid about through-hulls, I will go a long way to making air-cooling work. Aluminum radiators, as used in motor-sport racing, have high capacity, and very good heat transfer. And there should be a lot of room for a dry exhaust... large can-in-a-can style, with plenty of insulation... without the charring potential of smaller units. The electric fans, thermostatically controlled, that JC Whitney sells, seem like an indicator of the way to go. BTW - mobility in a marina can be helped by Phil Bolger's idea regarding trolling motors. He actually has a bow well designed into one of his boats just for that purpose. I won't go that far, but a 65 pound thrust long shaft Johnstone would make a great low-speed, easily controlled tug to get around tight corners.
    Woodboat...
    Glad that someone who knows has spoken up. I read the archives, and it seems as if everyone is an expert, and all the experts come up with that same 70-90% figure. As if every boat had exactly the same needs. I've had a little experience (not modest - really not much) with farm equipment, and the diesels in those things just crank along year after year, under the most appalling conditions. I did some plowing in wet soil with the plow accidentally set too deep, and, although the tractor went through a ton of fuel (relatively) it never overheated, never bogged, never complained. And this was over eleven hours or so of high-RPM, high-torque operation.
    LOVE diesels!
    HATE marine diesels $!
     
  9. woodboat
    Joined: Nov 2003
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    woodboat Senior Member

    I think the problem with marine diesels is most are engineered to operate at 75-90%. This does fit most boat needs. Even displacement hulls, they simply reduce the size of the engine so that it matches hull speed to HP. As an example my friends 30 sail boat has a 12 HP diesel. They have EFI on marine gas engines. It won't be long before all marine diesels have electronics and they will be able to perform well from idle to WOT without carbon build up.
     
  10. Filmdaddy

    Filmdaddy Previous Member

    woodboat...
    Aha! A light dawneth! Thank you for the clarity!
    So my initial assumption - that the engine doesn't have to operate WFO all the time - is correct. And a properly sized engine, run at a sufficiently high RPM to overcome glazing and carbon problems, should be perfectly feasible on a boat.
    An interesting thought. Horizontally opposed engines (boxer types like the VW and Subaru) are very comfortable at high RPM. Perhaps that's why they are used in planes, and a boating magazine recently sponsored a contest to design a boat around one. Makes me wonder about building a tender with a VW that would scream along...
     
  11. gonzo
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    gonzo Senior Member

    Do the numbers. After you put all that work and parts it is still cheaper and better to buy a used marine engine. If you want to experiment and have the cash to spend, that is another story. Howver, the most economic route is to use standard equipment. Many companies have already spent millions figuring out the best setup.
     
  12. Filmdaddy

    Filmdaddy Previous Member

    Gonzo-
    I'm glad that you weighed in, since you have been one of the most vocal (well, you know what I mean) proponent of avoiding auto engines.
    It's hard to see how a used marinized engine would be cheaper than a used auto engine, given that I can get my hands on a pretty good used diesel engine... and transmission... for about $700. I am not going to run the engine WFO at all... in fact, the engine will hopefully never get over 2500 RPM. So why bother with any marinizing beyond the safety stuff, which I agree is essential? If I am going to air-cool the engine, assuming I do all the right things to set that up, why spend thousands of dollars on sacrificial zincs and cam regrinds and wet exhausts and all the rest?
    As I see it, I am not making extraordinary demands on the engine, certainly a lot less than I have done on other engines in cars, trucks and tractors. The environment in the engine compartment is going to be at least as favorable for the engine as in a SUV trapped in traffic on the LA Freeway for six hours.
    For a planing hull, I have no argument with your position. But a displacement hull is another kettle of fish entirely, since the role of the boat, and the demands on the power train, are totally different.
     
  13. PowerTech
    Joined: Feb 2005
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    PowerTech Senior Member

    If enjoying your self out on the water with out alot of BS is your thing you may want a good marine diesel.If tinkering around with funky ugly homemade junk yard crap is your thing go for you will have plenty to do.It's a cool hoby I can dig it.
     
  14. gonzo
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    gonzo Senior Member

    For starters you'll have to make some kind of thrust bearing setup for the shaft. Then, if you don't intend to run the engine at rated power, a marine engine of less power would be lighter, cheaper to buy and operate. Check ebay and traderonline among others. There are running engines of 25-45 Hp for about $1000.
     

  15. Filmdaddy

    Filmdaddy Previous Member

    PowerTech - Thanks for the comment. Can't say that I agree, but you probably have way more experience than I do. I'll probably futz around with the junk yard crap, at least for a while.
    Gonzo - good points. I would ask how easy it would be to replace/repair something like a Sabb in a backwater far from the beaten path, whereas a Chev Vortec replacement part can be found on almost any corner or in any junkyard.
     
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