Help! Newbie daft mercruiser outdrive questions

This is a production motorcycle engine with nearer 110 bhp/ litre. An average road use sportsbike engine of the type that should last 50000 miles with correct servicing- though not at WOT for much of this.

150-200 bhp/ litre would be a racing engine and you are correct, it wouldn't last very long at all at WOT and would need a rebuild every race. Like many vehicles, I assume, in development, production motorcycles are tested on high speed ovals or on engine test beds at WOT, flat out under max load and expected to last rather more than 4 minutes. But I'm ready to be corrected on this.

What is different for the motorcycle's engine about running at WOT at 175mph down an Autobahn or a high speed oval and running at WOT at the same rpm at, say, 45 mph in a boat? (Providing it recieves the correct cooling to operate at its designed operating temperature)?

I'm not being contrary, I genuinely don't know the answer to this and need to.

 

A motorbike (or car) never does more than a few seconds, or max. 1 minute full throttle. A car sometimes longer than a bike. And no production bike is designed for being run full throttle for more than that! (talking the bigger bikes)
A average 600ccm sportsbike has around 100hp, a average 1000 sportsbike has 150+, a Hayabusa has 200 hp at 1300ccm, none of them comes in "race" setup, all are able to stand 50.000 to 100.000 km without a problem.
But NO bike engine sees long Highway trips at full throttle ever! Traffic and drivers condition make it impossible.
If in doubt, feel free to test a production bike at 3rd gear WOT on a highway for some minutes.
(cars are a bit different, they do´nt usually have such power/Ltr.)

In a boat you´re almost always at the upper end of rpm. That was the reason for my (serious) doubt.

Regards
Richard

PS. maybe 4 minutes are a bit pessimistic, but are 6 much better?

 

Richard you're too quick shifting the focus to the engine. That way we'll never see those stunning pictures of a bike engine / stern drive marriage.

When Jakeeeef can provide the proper conditions, like a 150 mph airstream directed at the engine and its exhaust, the power pack may survive several hours of abuse.

 

I would disagree on this. I think any standard bike will (should) stand WOT for more than an hour. While for bigger bikes this is very seldom due to top speeds close to 300 km/h, the smaller ones do it more often. I had a 47 hp 250 ccm bike that I drove several times WOT for more than 6 minutes. And even more so 20-35 hp 125 ccm bikes that used to be common here and had a top speed of 120-155 km/h. OK most of these smaller ones where 2T, but I know several 4T bikes as well driven WOT for much more than 6 minutes without any problems.

The reported 110 hp/l is not far from a typical (sport) car engine. The 125 ccm 4T bikes used to have 12-15 hp so very close to this and they were driven very often WOT since 120 km/h is quite common speed limit here.

Nevertheless I'm also skeptical about a motorbike engine in a boat.

 

You can get a tailstock to suit an Alpha drive just the same as you can for the other Mercruisers II, III, IV, V, and VI drives.
It is just an aluminium bracket that bolts across where the rear engine mounts would be and has bearing holding a shaft that has the 4 bolt flange on the inside and is hollow to take the shaft from the stern drive on the other side.

My concern would be when you put the alpha into gear the lack of rotating mass and lack of torque would probably stall the bike engine.
Unless of course you bike engine was 4 harleys connected together or a Honda Goldwing flat 6...just guessing though.
I would have thought that starting with a watercooled shaft drive bike engine will make life lots easier.

 

Finally, someone is getting to the crux of the matter. First you don't need the 6 speed gear box, just stick it in 4th, maybe 5th and call it a day (which ever is closer to 1:1) the drive will give you a fairly decent reduction.

Your bike engine relies on torque multiplication (gears) to spin up serious power. Without the trans, you're out of luck here, so as Powerabout mentioned, the reciprocating mass of the engine will have to over come all the dead weight of the upper and lower gear sets, plus smash the prop faces through the water. Your clutch will not last long, that's to say the least.

A momentary ignition kill switch can help tremendously in this regard (getting her into gear without stalling the engine). Of course rigging this and other limit switches will make for an interesting Saturday afternoon (bring lots of coffee).

The main issue is the difference in output a boat drive places on the engine, compared to a road bike or car. In a car or bike the engine output is 10% - 90%. By this I mean the engine is working at full output only 10% of the hours it runs and at a greatly reduced amount 90% of the time. This is why a car engine can have a quarter of a million miles on it. A boat engine is a 90% - 10% beast and much like a stationary engine on a pump. It's producing 100% of available power, 90% of the time and very occasionally a much reduced output (about 10%). This kills engines quickly, especially lightly built, high revving, seriously geared up, short stroke, piston pumpers.

You see a marine engine doesn't need to rev up or have neck snapping throttle response. It just needs to lay down globs of torque at max output as long as it can, pretty much the moment you slam the lever forward. It's not going to roll to a stop at the next light, or back way off the peddle after you've reached cruising speed. The marine engine has two speeds and outputs, WOT at max power and idle, that's about it. Some will back the engine down from WOT to save fuel, but the engine is still under very high load. It takes reciprocating mass and big gulps of fuel for this.

 

That depends very much on the boat and its use. A high speed planing boat may well be used 90% of time at 50% rpm and far less than 50% of power. So not far from road use.

The same may even be true for a displacement boat. If you have the self-control to keep the speed well under "hull speed", you will not need much power from the engine sized also for bad weather. E.g. I use typically only ~40% of the maximum power (75% of max rpm) when motoring with my sailboat. It only has 3 hp/ton while many others have 2 times more and should still keep the same speed if they want to save fuel and avoid making too much waves.

 

Dont forget the boat engine is working harder regardless of the rpm
Just hook up a vacuum gauge to a car and then check the same engine in a boat and you will see why a car can do miles per gallon and a boat can do gallons per mile.
In a cruiser it basically drops to nearly nothing once you try to get onto the plane and bearly recovers.
The only setups I know where the vacuum recovers once on the plane is light weight (ski) boats for example.
Hence you could say the boat engine is always going uphill.

What did the original 928 Porsche add say, its cd. was so low it only required 15hp to pull it through the air at 55mph.
Stock 25hp race v bottom class boats only do 50mph in a boat I can easily pick up ( without the engine)

 

In terms of stalling the engine as it goes into gear.
My bike would stall if I crashed it straight into 1st gear at standstill and tickover like a boat engine can (if I could do so without wrecking the gearbox). The bike has a clutch so that the power can be fed in smoothly whether that be to get the heavy mass of prop, shafts etc turning in a resistive medium, or to get the bike rolling on tarmac.

In terms of clutch life, the clutch of a motorcycle (some are better than others) will last about 15000 miles worth of starts, some of them hill starts with 200 kgs worth of rider and pillion rider Thats a lot more mass to get underway than the mass of getting a marine drive unit turning. Due to prop slip the comparison is not between the comparative weights of bike and boat as the boat, unlike the bike does not have to immeadiately move as power is applied. I bet water will be a lot kinder on the clutch than tarmac.

Case in point: I had a marinised car engined ski boat some time ago. It was direct drive (forward and neutral) Ford Sierra Pinto engine with its standard clutch in the bellhousing to engage and disengage the drive. You could leave the boat at tickover and release the clutch suddenly (to release the clutch you just pulled a long lever out of a J gate and let go of it to bang the clutch immeadiately out). It didn't stall when I did this at tickover let alone with any revs. I expect the clutch is still going strong in that boat. OK a car clutch is stronger and its engine harder to stall, but if i tried to pull away in the Ford Sierra the drivetrain was originally in at tickover by sidestepping the clutch it would have stalled.

In terms of engine lifetime at WOT under full load, I'm waiting for some figures on this from motorcycle people. Manufacturers do test their engines in this way though I do not yet know how long they last. Part of the point of keeping the gearbox is to limit the full throttle use the engine is subjected to. Third gear and full throttle for 5 seconds to get her on the plane, then cruise in 5th or 6th at three quarter throttle for the rest of the day. I am under no illusions that the engine will not last as long as it would in a bike.

In answer to how I cool the engine. Cooling is less challenging in the boat than it was in the original bike thanks to the limitless supply of cooling water passing under the hull. It will be marinised, probably a keel cooler. Getting 155mph of air to pass through the radiators of the bike (most bikes are water cooled) is neither a practical option nor fortunately a necessity.

In answer to the chap who suggested I put the bike in third gear and ran flat out to simulate its engine's life in a boat: This is not a good analogy to using the engine in a speedboat. It will overrev, so either will blow up immeadiately or bounce irritatingly off the rev limiter if it has one. It would also be doing about 110 mph, so I would run out of road and lose my license before learning anything.

I have no doubt whatsoever that rigging a motorcycle engine as you would a marine unit with a single forward gear and crash gearbox would be a complete failure. What I wonder is to what extent would the addition of the clutch and 6 gears make up for the shortcomings.

What I am taking from this discussion so far is:
1) It MIGHT work, but will have shortcomings not least for engine longevity
2) Use a jet drive, not an outdrive
3) Make the boat small/ light/ easily driven hull

 

Sure going 90 km/h (55 mph) is much easier with a car than with a boat, but what would that prove. Most people would drive ~20 kn with their planing boats, while 90 km/h (55 mph) or even 120 km/h (75 mph) is just a normal highway speed.

Around here a ~6 m boat that has a displacement of around 1 ton and top speed around 45 kn with 150 hp is quite common. Just saw data for one with 150 hp Suzuki. Top speed 43.2 kn, 5800 rpm and 56.6 l/h consumption. The consumption/mile is smallest at 19 kn, 3500 rpm and 19.1 l/h. Thus the engine is running at 60% rpm and 1/3 power.

The same engine could be i a small SUV, which would be in a very similar loading at 120 km/h.

 

You had this answer!
5 minutes max rpm and your streetbike engine is shot, period! Go try it!
(use tape to hold the throttle, you will not like to be close to the dismantling engine)

There is no difference of course!
But you never, NEVER be able to run a bike of 100hp or more, for more than 1 or max 2 minutes at max rpm on a highway (not even in Germany). And the industry knows that, sure.

The smaller Bikes all have the highest gear designed to reduce rpm at highest speed (say to 12000 instead of 14.500), that makes sure they stand longer trips on a highway.
A sportscruiser often has no such reduction (by gear), but as you already mentioned, no one does more than 250km/h for more than a few seconds on public roads.
On racetracks you can watch them die every weekend if you like! The boys try to hammer down top times in lower gear at full throttle half of the time.

Regards
Richard

 

Jakeeeef, you are really rowing against the wind here, but that might make you even more determined to do the project.

I agree with Apex1 that the life expectancy of your engine will be rather short, especially with the throttle lever all the way down. But outboard engines share the same basic design and can live quite long when properly handled.

The cooling is something you underestimate. Your bike may be liquid cooled, but it needs the airflow as well to remove heat from the parts that are outside the cooling circuit.

I do not share the opinion that the engine stalls when put into gear. The mass in an Alpha-1 that must be accelerated is very small and located near the center of a few shafts, except for the prop blades which behave like a torque converter. The 928S in my garage has no flywheel at all and despite its slender lines is a heavy vehicle, but the engine never stalls when put in drive or reverse.

 

A 100 hp bike would have a top speed of ~230 km/h. Certainly there are people driving at those speeds on highways ocasionally for several minutes or even for close to an hour, which is all you get from limited fuel capacity.

A school friend of mine drove through Germany in the 80's with his Kawasaki ZX10 (http://www.bikez.com/motorcycles/kawasaki_zx-10_1988.php) and when he came back he complained that somebody was faster and he decided to order some tuning parts to get more power.

Most high power road bikes have their maximum power at 9000-11000 rpm. Only very few reach to 12000. Why would they use 14500 rpm?

 

have a look at your torque curve as the thing may never get on the plane unless you can change gears on the fly?

 

I am looking for part #45 in your image. can you give me some information about what book this diagram is from? any help would be greatly appreciated.