Can i use a motorcycle engine?

Discussion in 'Boat Design' started by 65chill, Dec 3, 2008.

  1. 65chill
    Joined: Jun 2008
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    65chill New Member

    I would like to build a small 15-16 foot vintage wooden sports cruiser. But with some good performance. I have thought of using a small block chevy but would like to use a modern sports bike engine (1000cc) or similar that i have knocking around. Many years ago i saw some very small 8-9 foot hydroplanes using yamaha 250cc and 350cc motorcycle engines. my question is, why do more boats not use water cooled motorcycle engines. Are ther any good performance calculators, formulas or software. I guess it is a question of torque and the motor i have in mind has around 150hp and 90lb/f torque. All this is packaged with a clutch, six speed box and a weight of 75kg.
    :idea: Thinking outside of the box :idea:
    Thats where all the best ideas came from....
  2. marshmat
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    marshmat Senior Member

    Hi 65chill,

    Welcome aboard :)

    In terms of engine architecture and performance curves, motorbike engines are actually remarkably similar to the engines used in high-performance PWCs (jet skis). Lightweight, and they like to run at very high RPM in order to produce their rated power.

    Motorbike makers often seem to have no qualms about mixing metals and using exotic alloys in their engines. That's fine when you're circulating a 50% propylene glycol / water blend, but tends to cause corrosion issues when saltwater gets involved. So that's one issue you'd have to address; a closed cooling system would add weight and leaving it open might promote corrosion.

    Another issue is longevity. Boaters, on the whole, tend to want an engine to start and run reliably for fifteen, maybe twenty years before needing serious work. Sportbike riders are renowned for their love of tinkering, tuning and rebuilding every thousand or two hours. And even if you ride it really hard, the bike isn't spending more than a few minutes, maybe an hour at a time if you're on the track, at anywhere near its top rated power. The boat will be demanding that kind of output for hours on end, every time you take it out.

    I think if you can size the boat, prop and engine so that the engine (in the boat) is running at a similar RPM and power output to what it would be expected to provide in the bike, it could work.

    You will, of course, have to marinize the engine in accordance with applicable codes, regulations and standards. Certain safety precautions must be applied to gasoline engines and their associated fuel and electrical systems in boats, that do not matter in a car or bike. (An example: Gasoline vapours are heavier than air, so they pool in the bilge, where in a car they'd float harmlessly away. That means forced ventilation, spark-proof starters and alternators, flame arrestor, etc. to stop your boat from exploding when you turn the key.)
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  3. Tcubed
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    Tcubed Boat Designer

    Thanks Matt for replying on a subject that has had me curious for awhile.

    It seems to me that motors in general shorten their lifespan dramatically by being operated at high RPMs .

    So i had the concept in reverse - On what type of boat and with what objectives would adapting a motorcycle engine make sense?

    If the objective were economy and smallish cruising power boat (not flat out sport boat) , then i think it could be sensible.

    What especially appeals to me with the motorcycle engine is that it comes with a chain sprocket already so it is very easy to get whatever gear ratio is desired with an appropriately sized sprocket on the prop shaft. That and they are very simple engines ( at least the older ones) and very cheap to obtain. (at least here in puerto rico)

    The gear box is surplus to requirements though. If only there were two speed motorbykes, that would be perfect.

    What i'm getting at is to arrange it so you don't run the engine at peak performance RPMs but rather at RPMs that are most conducive to fuel economy and engine longevity.

    Another thing i've been wondering about is why not just a basic air cooled engine? It would need a fan of course and some generous ventilators, but it would get rid of all the problems associated with pumping seawater through the well as gasoline vapours pooling in the bilge of the engine compartment.
    Last edited: Dec 3, 2008
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    The problem of the heat exchanger is a false one. It's not very heavy nor complicated; you can make even one in fiberglass and copper or aluminium tubing. Ok a 90/10 or 70/30 nickel copper is better but in a smal boat is acceptable. The rule is never have salt water inside an engine, as salt deposits at only 50 celsius degrees, the poor engine has to run cold at 45 degrees. When you try to marinize a car, truck or bike engine designed to run run at 90-95 degrees (gas engines) or 78-82 degrees (diesel) and cool it directly with salt water it goes wrong.

    There is also a big loss of efficiency, the better examples are the Mercuiser 5.0L and 6.2L, which cooled with raw water are big inefficient suckers, and cooled with glycol mix because very efficient with a specific consumption of about 200 gr/HP/hour, if you use a BravoII with the biggest (in diameter) propellers possible.

    I have marinised a Honda NTV (called also Revere) 650cc twin with no special problem, I kept a dry scape cooled by a strong blower so the installation was simple. The Honda NTV is unbreakable, it can run hours at 7500 RPM and is able to make 300 miles at max speed (8,000 RPM) so 3 hours WOT in a german highway. 200,000 miles with just maintenance is common.

    Look also to the Subaru engines, and the Suzuki Geo or Metro. They are often used on planes. The Subarus have very high reputation.

    The Ford Ranger 2.3L 145 HP all aluminium, 120 kg goes well but you have to change the computer as it has a bug in low RPM. It's very easy to install a marine gear box. Such engine costs 2300 US$ at Ford performance, but you can get brand new at 1500 US$. The engine is pretty strong; it's able to make a complete year of rallies in England at 220 HP 7,200 RPM with just a change of cams and computer, all the remaining of the engine is origin from the factory.

    It's just for fun, as it's no more produced but the Citroen GS 1300cc flat four air cooled 65 HP (72 with a freer exhaust), engine of about 40-50 kg, has been tried for a ULM in the eighties. They took an ordinary new engine put on the bench, run it smoothly 10 hours, change the oil, and went for 100 hours non stop at max RPM (6,000 RPM) and max power (72HP). Change of oil and spark plugs, 100 hours again. Not problem. So car engines, the good ones, may be used on boats... Imagine; 4 days and 4 hours the engine screaming...

    The best is to take an engine from a GT motorcycle as it has a better curve of power and better torque. Sport engines with very light flywheels can become bothering because they stall easily and have not stamina at constant RPM. A 150 HP 75 kg engine is a sport one and won't last, these engines are designed for about 50,000 km in street use, a Subaru would be a better option.

    Do not forget the security: the US Coast Guard rules are rational and well explained.
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  5. MikeJohns
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    MikeJohns Senior Member

    Good points Ilan

    I have met a few subaru engines in smaller sports boats usually the EA81 and EA82 using the automatic bell housing mount and the adapter plate de-fingered with a damper in its place.

    I'd never run these sorts of engines raw water cooled always use a heat exchanger and as Ilan.. says heat exchangers can be very simple, a common cheapy is a copper pipe inside a larger polythene pipe pump the fluids in opposite directions.
  6. kenJ
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    kenJ Senior Member

    Motorcycle transmissions don't normally have reverse. Might have to look at an ATV transmission if you need to back up.
  7. Ike
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    Ike Senior Member

    Look here for info on Coast Guard rules on engines While there, take a look at the pages on electrical and ventilation. Marshmat is right. The first PWC engines were all basically 2 cycle motorcycle engines. They have changed a lot over the years but if you go find an old Yamaha or Kawasaki PWC and pop the hood it will look really familiar. However, some of the rules for fuel and electrical were waived (the Coast Guard gave the manufacturer a grant of exemption) for certain things.

    The thing to keep in mind though is, this will not save you any money. It'll be a cool project and fun when you get it finished but after you make the necessary mods the price will probably be more than just buying a marine engine.
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I agree totally with you Ike on the 2 points:
    - Security-Safety Rules. US Coast Guard rules are not made to bother the poor boatbuilder, they are the result of the analysis of hundreds of accidents involving carburants and the means to prevent it. These rules are very good and the best thing to do is to stick with them.

    - Cost. Marinising an engine involves a lot of work, custom pieces, expensive failures and often poor results. It needs also good mechanical knowledge and a good well equipped shop.

    If it's a fun project, ok it's better than getting drunk or sniffing cocaine, and it can be very amusing.
    If it's to save money, it's generally a mistake.

    I'll add that the last generation of 2 Strokes outboards with direct injection like the Optimax series are million times better in reliability and gas consumption than the old 2 strokes motorcycle engines.
  9. Frosty

    Frosty Previous Member

    65 There is no laws to stop you having a bit of fun in Uk.

    You may be too young but In Norfolk broads was the Alluminium Albatross boat builders. These things were as popular as Ford escorts round Windermere.

    They had 100E or 1500 or the Coventry climax.

    I bought one without a motor when I was 19 . That summer I fitted in a cross flow 1600 it was driven from the front pulley and had no reverse, you pointed it in the direction you wanted to go jumped in and pressed the starter,--as the originals boats were.

    I made an oil cooler with 1/2 copper pipe and elbows, this was in the sump winding backwards and forwards as many times as I could. It was fed by a bath plug type inlet with a scoop on it so when the boat moved forwards water went through the cooler, through the engine and out of the side where you could see it. It was too good and the engine did'nt get hot enough

    The cast exhaust manifold was replaced with one from a scrap yard made of mild steel off a 1600E called a banana manifold, this was easier to drill 4 holes for 4 1/4 injection pipes fed by a washing machine pump driven off the front pulley by the shaft ( the belts didnt last long).

    The carb was a standard ford down draft Zenith and mechanical pump with no mods what so ever.

    So thats it I think. It was great Ski boat and we skied all summer with only belt changes. I remember towing boats back to the slip on more than one ocassion. You would soon know the belt had gone as the exhaust got noisy.
  10. Bruce Woodburn
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    Bruce Woodburn Junior Member

    Modern sport bike engines are very durable. I just took apart my Suzuki 1100gsx engine with 120,000km's and it still has the honing marks all the way to the top of the cylinders.

    The "downtuned" litre engines used in sport/touring bikes have more conservative cams and broad powerbands. They don't have to be redlined to get the torque. My 1100 Bandit has a torque curve that is almost flat from 6000 to 1100 rpm. And the engine is cooled by a combination of air and oil: there are oil jets spraying on the hottest engine innards with a low pressure, high flow oil cooler. I bet you could cool it in a boat with just an external oil pump and a big cooler.

    There are alot of bent bikes with salvagable engines. And alot of the bent bikes have very few miles, driven by those who couldn't quite cope with the learning curve.

    There are a few awkward things about putting a "crotch rocket" engine in a boat:

    These engines are exquisitly sensitive to modifications on both the induction and exhaust sides. Make sure you stick with the stock airbox and filter arrangement. I doubt you're going to use a stock or even aftermarket exhaust system, since they all exit under the motor. Maybe you could just mount it on the engine upside down so it arches over top ??!!?? Way Kewl.

    The gearbox arrangement isn't great for a boat. It will force you to mount the engine to port of the prop shaft, causing problems with the centre of gravity. And you're carrying a 6 speed gearbox transmitting (and loosing) power through two unnessisary pairs of gears. And none of those 6 gears is reverse.

    Have you considered a Mazda RX-7 engine? The early ones put out 120hp naturally aspirated and the last version (twin serial turbos) were about twice that, all in a package the size of a microwave oven. They sound like a chain saw and you have to deal with VERY high exhaust gas temperatures, but they are nearly indisructable and ideal to turbocharge.

    Have fun

  11. Frosty

    Frosty Previous Member

    Bike engines just don't have torque, unless you are talking a very large V twin.

    Boats need torque , torque and more torque.

    There is no gears on a boat so top gear is what you have to start off with.

    Take your suzi 1100 or your sakakaki and try to set off in top gear.

    A big V twin will.

    A bike motor would run a jet,--torque not as necessary, (jet ski)
  12. Chris Bretter
    Joined: Nov 2008
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    Chris Bretter Zaphod

    Hi all
    I am starting to build a 38 ft Wharram i would like to use a centre mounted Nissan SD22 with a 2-1 reduction box and a dropdown longshaft. All fairly simple Has anyone any ideas.Also perhaps a ducted prop with a rudder to assist low speed manouvering.You guys think out of the box.Rick you got some crazy stuff going on so cool.
    Regards Chris South Africa
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    FAST FRED Senior Member

    it can run hours at 7500 RPM and is able to make 300 miles at max speed (8,000 RPM) so 3 hours WOT in a german highway. 200,000 miles with just maintenance is common.

    Running flat out may be high in RPM, but not particularly high in HP.A boat is like hillclimbing with a sidecar , fat frau, and 1/2 ton of camping gear.

    Home builts with car engines ( Chevvy or Subaru) like most air craft recips will limit full take off power to 3-5 min and operate at 60% to 70% if the CHT/cooling stays in realistic numbers.

    The reduction from say 5000 rpm down to 500 -1000 for some propeller efficiency may be EZ with sprockets and chain or a toothed belt.

    Who needs reverse with a sport boat?

  14. Ilan Voyager
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    Ilan Voyager Senior Member

    Hi Bruce, sweet words of truth I've read. The Suzukis engines are exceptionally strong, and the Bandit a delicious bike. About reliability I wouldn't say the same of some brands...some have at 50000 km very tired camshafts...The problem is the center of gravity with chain sprokets. A GT with a shaft transmission would be easier to adapt, and why not keeping the speeds with a shifter, the clutch and the throttle...and for the reverse on a small sport boat a good paddle.:p
    Surely it will be fun.

    The big derating of plane engines come of 2 reasons: first maximal reliability as a engine problem on a plane is bothering (do you know why planes have propellers? It's for cooling the pilot, have you seen how the pilot sweats when the propeller stops?), and liability concerns. On a small boat making circles in front of the harbour high reliability is not a big problem.

    I've seen a long long time ago in a country far far away (Senegal) a shrimp fishing boat with all the engine and gear box taken from a truck. The skipper was sitting on the original seat, with the original wheel (using the assisted direction for the rudder), lever, pedals (no brakes however... ) and so on. It worked perfectly. This boat was a marvel of ingenuity.

    For the 38 feet Wharram sincerely a pair of 10 to 20 HP used 4 stroke outboards will do the job with less work and almost surely less money. And a lone engine with long shaft on a catamaran...not so easy. Catamarans are notoriously difficult to power with a lone engine for reasons I won't detail here. An engine in the middle of the platform of a light polynesian styled cata like the Wharram.... you'll get a problem with the rear beam.

  15. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    Good to read the latest comments.

    I will be building a 16 foot 'runabout' , 'skiff' , 'inter island speedboat', call it what you want. I was planning on buying a 15 HP yamaha outboard and that is what i designed it for. But that will set me back almost 1400$. So i'm now revisiting an idea i have long entertained, precisely what we're discussing in this thread.

    My idea was just like Ilan mentioned. Take the whole motor as is and link it to the propshaft with a chain. The design would need very little modification.

    I was thinking of using an old honda 450 engine, which i can get for about a hundred dollars and which is rated at something like 38 HP so i can run it comfortable for getting extended life span and better fuel efficiency (4stroke instead of 2stroke) than the outboard. The weight is very comparable too. I would put it in a soundproofed box with a generous air intake (water draining, of course) pointing forward and large air exhaust pointing aft. Linking a fan to the drive shaft is not difficult.

    The only thing left in my mind i'm not decided on is leaving transmission and clutch on or removing it. If i remove it i save weight and some friction, but i no doubt will have more machine shop expenses and i lose the flexibility of gears. (if i need to tow someone, it would be handy to drop a couple of gears)

    What would you recomend - leave it on, or take it off?
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