Outboard engines

[Newboy]

I don't mean me leaving one behind at the slip, but the technology itself.

 

I look at my Honda 600cc and can't but wonder why outboard engines are so cumbersome.

 

It produce a tad under 100hp but is only 600cc, while most 100hp outboards are around 1400cc. Yes I know hp aren't the yardstick as torque which is what needed to push the boat/bike along, but if bike manufacturer do built something small and powerful, why can't the outboards?

 

Also why is there no gear (other than f-n-r) on outboard? Maybe it's unrealistic to have 5 forward gears , but 3 even 2 would be nice.

[great white]

The gear part of this may be due to the position that the gearbox and gears are in, on the prop shaft. in the water at the bottom of the leg

 

It has to convert from vertical to horizontal movement have a clutch and forward and reverse gears and have minimal drag as the boat goes through the water.

I would have thought that getting more gears into the box would not be possible and connecting up a gear change mechanism that would be reliable also very difficult.

 

I do not know why there is such a differance between bike and outboard engines

 

But a 75 hp 4 stroke in an outboard seems tiny compared to one in a car.

 

Charlie

[Coddy]

Hi Charlie

 

The reverse applies for a diesel, my car engine is a 1.9 turbo and produces 125+ bhp but revs up to 4500 rpm, boat engines seem to be huge for a modest bhp but the revs are much lower.

 

I suppose it all come down to torque in the end.

 

Coddy

[Member Removed]

being a mechanic by trade i learnt all about the guidelines manufactures follow its trying to find a happy medium of reliability against performance. you could build a 2000cc engine with 120bhp like a zetec and would never put excessive stress on it but slap a few mods on it ie lightening larger valves different cams etc and you could get as much as 180 200 hp but it puts a lot of stress on the engine thats when things either break or need very high maintainence. also an engine can be designed to produce very high torque figures and still only produce low bhp.

[Brian]

Boats don't require a 4 speed gearbox, cars etc. require a range of gears so they can build up to top gear without stalling. Prop slip takes care of it with a boat.

[duncan]

Not sure about the prop slip thingy - I don't get much with a large duo prop untill the turbo /revs combination is giving near max power capability, and even then it's slight.

 

Re OBs I think you are more on the right lines with gearing issues than torque requirements as a core difference.

 

Gearing allows you to optomise engine output to use - taking the upgrades to that Zetec refered to above the major HP enhancements are going to be at the higher revs (where bby definition you get more HP but thats another thing....) and nover a narrower band. To transfer this to a boat you are going to need a larger (bulky/drag heavy/performance limiting) gearbox just to stand still performance wise so why bother?

 

Outdrives and OBs with adjustable trim are effectively providing an element of effictively gearing (sort of) by directing thrust and of course surface drives are able to utilise the ultimate controlled prop slip to achieve the same thing.

 

Generally car engines produce less power at lower revs but rev higher (producing better HP) because they deliver drive via variable gears.