Marinizing Rotary engine

Discussion in 'DIY Marinizing' started by greendeane, Aug 13, 2008.

  1. greendeane
    Joined: Aug 2008
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    Location: Eire

    greendeane G.D.

    Hi all, just wondering about the marinizing potential of a rotary engine.
    I am talking about a pistonless wankel engine like those found in mazda RX8s.

    Does anyone have any thoughts on the subject?
    Could a DIYer marinize?
    Would anyone want to marinize?

    I have no idea whether it would be a good idea to have one in a boat or
    not but it would be interesting to have a discussion on the topic.
    On the surface they do appear to have potential

    have high power to weight and power to size ratios,
    low vibrations and noise,
    No pistons or camshafts etc
    Run safely at very high rpm
    Rarely, if ever, seize.
    used by aircraft DIYers (must be somewhat reliable in a DIYers hands)

    Not much information about,
    No off the shelf conversion parts (I'd imagine),
    Cooling system 'seems' to be complicated.
    Probably lots of other cons i can't think of right now

    here is the mazda rotary wikki:

    and a nice animation:

    and the website of some guy who seems obsessed with
    automobile rotary engine conversion for light aircraft use

    I am in the process of ploughing through all the information I can find on the web
    and i'd like to have a look at an actual engine. One of my neighbours has an
    RX8 so i will see if he'll let me have a root around under the hood.

    Any opinions/ideas/facts out there?
  2. BHOFM
    Joined: Jun 2008
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    BHOFM Senior Member

    I believe the drawback is fuel consumption, that's why
    Mazda only uses it in the high end models!

    I may be wrong about this, but the older ones were famous
    for bad gas mileage!

    Another thing is they use the exhaust to preheat the cylinders
    and it must be hot or the engine does not run properly! I don;t
    think you could water cool the exhaust and make it work?

    I have no idea what I am talking about?
  3. TollyWally
    Joined: Mar 2005
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    TollyWally Senior Member

    This was done years ago by an outfit located on the Columbia River I believe. It was written up in National Fisherman. The benefits touted were the light weight of the rotary would add more capacity for carrying wieght etc.

    They were running these high revving engines with big reduction gears maybe 5:1, memory fails me as to the exact details. They made a few testbed experiments but for whatever reason the idea failed to take hold.
  4. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    I don't see any technical reason why it would be impossible. I have heard of Wankel engines being sold in factory-marinized configuration (not Mazdas, though) but have never seen such a setup.

    The Wankel, by its very nature, operates at higher RPM than a piston engine of comparable power. This does necessitate a rather large reduction gear if you are powering a boat with it- compared to the 4,000-5,000 RPM that a typical high-performance marine V6 or V8 will redline at, the Mazda Renesis engine will happily churn away at 9,000 RPM and, although Mazda does not endorse this, has been reported to comfortably exceed this speed.

    Fuel consumption is still a bit of an issue with Wankels; the RX-8 is known to be rather thirsty for its weight (although not too far off the fuel consumption of some of its piston-engined competitors).

    It is my understanding that the sealing issues that plagued earlier Wankels have been largely fixed in the new Mazda engines; however, this comes with a price- the tolerances involved are so fine that even a very meticulous, competent mechanic may not be able to perform internal servicing on the rotor, seals, and chamber.

    The water-cooled exhaust issue could be avoided, I think, by going dry-stack or water-jacketed with dry pipes.

    You might have a hard time gleaning anything useful from looking at a functional RX-8; the engine is relatively small and is tucked so far down and aft in its bay that it is pretty awkward to get to. If you could find one in a wrecking yard, or get underneath a working one, you may have an easier time figuring it out.

    The computer controls would likely be the trickiest part of the marinization, if you ask me.... I don't think the engine itself would need too many mods (flame arrestor, spark-proof starter and alternator, etc) but getting the computer to work right in the boat will be fun.
  5. greendeane
    Joined: Aug 2008
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    Location: Eire

    greendeane G.D.

    I think that the fuel consumption issue has been largely solved in the new RX8 engine.
    The gas mileage in the new cars isn't great by all accounts but not terrible.
    Some discussions on the internet suggest that mileage is not as good as quoted in
    Mazda literature, however.

    Interesting to see that it has been attempted before.
    Any installation would surely require big reduction gears as suggested above

    how about coupling to a jet drive?
  6. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    That's exactly what I was thinking! I have a 1965 Dorsett Sea Hawk 186 Jet Boat that originally had a 200c.i. Falcon Six (around 100 h.p. I think?).
    I built a Corvette 350 c.i. which is too heavy, too much power and drinks gas like I drink beer.

    I was thinking along the lines of a rotary because of the small size and weight, the free-revving ability etc. Maybe run a 2:1 belt reduction setup, it should be trubine-like in its smoothness. Any useful information would be greatly appreciated!

  7. SnappingTurtle
    Joined: May 2008
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    Location: Germany/Texas

    SnappingTurtle Snapping Turtle

    Hello Lt. Holden,

    I have seen several on the web.

    Here is one of them from some, gotta love em, crazy Aussies I think.

    20B Ski Boat
    - Hull - Kevlar 1850 bullet, Bravo 1 XR leg
    - Engine – X-Treme Rotaries 1962cc 20B (3 rotor) rotary engine, bridge ported, turbocharged, fuel injected, NRS Ceramic Power Seals
    - Horsepower – 800hp 9500rpm 615 ft/lbs torque (@ 22 psi)
    - Speed – max 120mph 9650rpm (top speed is now 142 mph)
    - Tuning – by Anthony Rodrigues, Maztech of Melbourne
    - Turbo – Garret GT42 1000hp
    - Intercooler – PWR barrel water to air

    Might be able to help with parts, but their website is being redone at the moment. boat footage.wmv

    The videos won't help with the details, but as proof of concept ...

    You might also try these State side guys for something a little milder:

    Rotary Marine Industries, Incorporated, Dept. TBM,
    P.O. Box 1676,
    Sandpoint, ID 83864


    They are marinizing a Mazda rotary engine rated at 175 hp at 6000 rpm.
    The engine is mated to a Dominator jet drive for a complete, marine-propulsion package.

    Hope this helps some. Keep us informed, WITH PHOTOS.

  8. RivrLivn
    Joined: Nov 2007
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    Location: US

    RivrLivn Junior Member

  9. fluffysheap
    Joined: Aug 2008
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    Location: Seattle, WA

    fluffysheap New Member

    I would not recommend using an RX-8 engine, they are highly dependent on their complicated EFI system. It could probably be made to work but you would have a much more difficult project for little gain. Instead I would recommend a 12A or 13B rotary from any 1975 to 1991 model car. Only the very early pre-1974 rotaries had problems! But you are very unlikely to come across one of these, almost everything you will find will be 1981-1991 which are all totally OK. After the problems were fixed the only thing that will kill a rotary is engine knock or overheating, either of which can kill one almost instantly. After 1991 the RX-7s were all the twin turbo style which were even more complicated than the RX-8 but less reliable.

    The advantage of the rotary is that it produces a lot of power in a very compact and lightweight package. However parts are more expensive.

    The 13B is slightly longer, takes different manifolds and has a little more displacement but basically they are the same deal. The 12A were all factory carbureted (discontinued after 1985) and the 13B were EFI from 1984, but they all adapt to carburetors pretty easily. 1986 and later went to distributorless ignition but you can put distributors on them if you want.

    The fuel efficiency thing is sort of overblown. Rotaries have bad fuel efficiency puttering around but they have good fuel efficiency at full throttle, and they can run all day at high RPM/full throttle.

    The rotary engines are actually extremely easy and simple to work with but you have to respect the quirks.
    * You need an oil cooler. About 30% of the cooling is done by the oil. This is a whole other thing to set up plumbing for. Aside from this the cooling system is totally ordinary. Do not overheat. Ever. Not even once. :)
    * You have to inject or pre-mix oil in the fuel like on a 2-cycle engine.
    * They run at very high RPM. You need lots of gear reduction. You can run them at lower RPM too but this will produce less power.
    * They are loud unless well-muffled.
    * Exhaust is very hot. You must use full stainless exhaust. Cars usually just convert to headers unless running a turbo. I would not recommend a turbo unless you are bringing the whole car EFI system with it, because turbo rotaries can be difficult to tune (in contrast to non turbo ones which are super easy). This doesn't mean you shouldn't start with a turbo motor however, though they are more expensive.
    * I am not sure what BHOFM is talking about with using exhaust to preheat the "cylinders." The exhaust goes right out the tailpipe and there are no cylinders. The only thing is that the intake manifold is heated with engine coolant to pre-heat the air coming in. That is not really special to rotaries and you can live without it.
    * Atkins rotary is a well known and reputable rotary shop and I would have no reservations about buying a marinized engine from them and dropping it right in. However their business model is not based on a low price strategy...
  10. bobg3723
    Joined: Aug 2005
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    Location: Crystal, MN - USA

    bobg3723 Senior Member

    I don't know how the differences in thermal soak can ruin a wankel, but could one for the purposes of reducing fuel consumption disable fuel going to the second chamber for smaller power output needs?

    If that's workable, will the ignition/power cycle from a single rotor shake rattle and self destruct itself?

    Just curious.
  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Somewhere around 1985 I bought a new Mazda RX7 and drove it for nearly 5 years. I remember bringing it to the dealer because there was an oil leak, which proved to be a ring nut on the giant oil cooler that needed tightening. It was the only problem I ever had with that car!
    The engine was a 13B with Bosch L-jetronic and a resonance box at the intake, emission control was done with this peculiar 'reactor' that was protected by a heat shield but still managed to scorch the grass under the car.
    The engine was as quiet as a V6 or V8, in fact it was only the red mark on the rev counter that showed this was no ordinary engine. No excessive oil consumption, maybe a little bit thirsty but it was a sports car like the Alfa spider I traded in, which was definitely thirsty and had cost me a fortune on repair bills.

    DIY marinizing such an engine can't be more difficult than any ordinary gas engine except the fact that you can't buy a water cooled exhaust manifold for it. But the shape of the exhaust is just a simple V with 2 flanges so it won't cost a fortune to have one made. As a lot of heat is removed by the oil flowing through the hollow triangular 'pistons' an oil-to-water heat exchanger with sufficient capacity is an absolute must, but that is available from several sources.

    The 8500 rpm from such an engine requires a 2.5:1 or 3:1 gearbox with an oil cooler, I see no problems there.

    Unfortunately there are not many RX7's on the market and the present owners of such a collectors item may be very reluctant to sell it.
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    FAST FRED Senior Member

    These are sometimes used in home made aircraft with success.

    Go to the EAA site and see how they modify them for cont duty.

  13. greendeane
    Joined: Aug 2008
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    Location: Eire

    greendeane G.D.

    Thanks for all the info guys,
    I'm a long way from engine choice still, never mind marinization. Very interesting stuff though. Keep it coming if anyone has any more info.
  14. Cheesy
    Joined: Aug 2008
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    Location: NZ

    Cheesy Senior Member

    A couple of points, these engines probably aren’t as light as you imagine, some of the other jap aluminum 4cyl engines are lighter and produce more power/better fuel consumption. The strong point of this engine though is that they are well suited to continuous duty. Obviously the newer the engine the better, a batman or cosmo engine with the turbo removed would probably be the go, these have the 2piece apex seals instead of 3 in the S5 rx7s. It would also be worth while to convert it to premix or modify the oil injection pump to use an external reservoir with two stroke oil

  15. pbjosh
    Joined: Jan 2009
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    pbjosh New Member

    I signed up just to reply:

    I have 2 marinized 13B motors from a Mazda Cosmo in my 45' Chris Craft. They were done on by the previous owners while the boat was parked in the Columbia River.

    The engines are quite a bit lighter then the original inline straight-6 that was with the boat. The waterline is 3" higher then the boat was orginally with a near half the fuel and full water load. They not only reduced weight by a large amount, they added quite a bit of space to the engine compartment. We are only half joking about adding a jacuzzi.

    The engines from a Cosmo are carburated, in this case with a 390 cfm carb each, giving about 175hp each (over the stock 150hp inlines.) They are ran through a 3:1 gearbox, but I think they places props that are a bit large on, since the engines easily run 6000 rpm (and up to 9000rpm) but with the drive setup, they do not spin more then 3500 rpm (on WOT.) A 5:1 gearbox or a smaller prop will be installed soon. I believe the props are not the 19" that were installed on it.

    The carburator setup is normal, with just K&N air filters. I want to replace the carbs though, they are older and a bit on the worn side.

    The cooling system uses an exhaust manifold built for the engine that does have the cooling water injected into it. Right after the exhaust there is a 3" hose, then the connection to the exhaust lines. There is no abnormal cooling compared to a normal marinized engine, and the engine compartment is far cooler then the one in my bayliner. The motor is cooled through an external heat exchanger. After we replaced the thermostat the engines stayed at 180 no matter what the load. There is an oil cooler, but it is tsmaller than the heat exchanger for the engine. It runs off the same intake line.

    They are VERY smooth, and sound fantastic. There is a deep burble out of the exhaust at the back, while a very interesting whine from the engine compartment. There is almost no vibration from the motors though. There was some in the drive line, but none with the engines. I imagine re-aligning the engine and replacing a motor mount will fix that.

    Performance is a bit spotty, because the gear reduction wasn't enough, or the props are too large. The engine should run at 5000rpm for proper power and for proper lubrication. No, the engines do not need to run 2 stroke oil, they use normal 4 stroke oil.

    The 45' 30,000 semi-displacement hull runs a tad under 20 knots, but very smoothly. While a 15 ton boat shouldn't be moved much by the motors, it is so smooth you would think it was falling down the water.

    If you have any questions feel free to ask. We had to update, clean off and tune them up quite a bit, till we realized the ignition coils were bad. The motors were fine though, and we go just about the same speed at any other Chris of that make.

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