engine hp

Wazzajnr · 30 April 2015, 20:57

Been asking about getting my SIB on plane in the inflatable forum. Had some great answers and help there but it got me to thinking.
Why are boat outboard so weedy per cc. For example, my first motorbike was a Yamaha fs1e which was 50cc and made 4.8hp I the had various 125's which were restricted to maximum of 12hp (although they mostly made around 10hp) but were really de-restricted to give 16hp or so.
Now looking at most 6hp outboard motors (2 stroke) they seem to be around the 100cc mark.
So why is it that we only get 6hp from a 100cc motor.
Any thoughts?

Nos4r2 · 30 April 2015, 21:40

They've got to last longer and develop the power at lower revs with a lot more torque.

bmb779 · 30 April 2015, 21:41

HP isn't directly related to cubic capacity,for instance,a Tohatsu 20hp 4 stroke is the same cc as the 15hp and the 9.8hp carburettor jetting is what determines the HP in this case.

Wazzajnr · 30 April 2015, 21:45

Last longer? Not so sure. My 50cc Bike was 25years old when I got rid and still going strong. It made 4.8hp at about 5500rpm iirc. It had done 33000miles. Not sure how many hours that equates to but it a lot and 90% of those hours were spent at WOT and full revs!

Wazzajnr · 30 April 2015, 21:54

Yes, I understand it's not directly related to cc but there does Seem to be a generally lower power output per cc for boat engines than bike engines.
Just makes me wonder why there isn't more of an aftermarket tuning market for outboard motors. Mine is a 6hp 100cc which I know should be able to make 10hp really without stressing it

Nos4r2 · 30 April 2015, 21:55

I bet it had been rebuilt a few times in those 33k miles though.

Max... · 30 April 2015, 22:25

Yes but unlike your bike the outboard only has one gear remember.

bmb779 · 30 April 2015, 22:25

Is there a bigger HP model of the outboard you have,? If so check through the parts catologue and see what's different.Thats all I done and got 30% extra power by changing carb jets.

beamishken · 30 April 2015, 22:34

My son rides a kx85 which is 29bhp @ something like 14000rpm but you don't dare go past the recommended 20 hours for a new piston or your paying for a barrel too
People have tried bike engines in boats thinking they have great power to weight ratio but they just don't work as it's lots of torque you need for a boat.
In boat engines there realy is no substitute for cc ' s hence the popularity of big lazy American v8 ' s as inboards

bmb779 · 30 April 2015, 22:38

Is there a bigger HP model of the outboard you have,? If so check through the parts catologue and see what's different.Thats all I done and got 30% extra power by changing carb jets.

Roflhat · 01 May 2015, 09:08

As mentioned above, outboards only have one gear so the power band needs to be much wider, whereas in motorbikes/cars you can have a narrower power band and make use of the additional gears.
I'd say outboards are lower stressed as well, the engine components could handle more hp than the engine produces but the manufacturer wants the engine to be reliable. If an outboard breaks down you could be hours from land, possible life or death situation, if a motorbike or car breaks down chances are you're next to the road.

2 stroke bikes almost always use expansion chambers in the exhaust, giving much better back pressure and hence much higher power, but this is only within the narrow power band, usually optimised for within a small range of RPM.

otilly · 01 May 2015, 09:42

If you look at all the new cars nowadays you can see exactly the opposite of outboard engines. Very small engines with a 6 or sometimes 8 speed automatic and 2 or 3 turbo's stuck to it. Veeeeery small powerband out of those 1100 cc cars, but lots of speed.
Don't try using it for a big trailer though, no umph.

Last Tango · 01 May 2015, 12:10

Quote:

Originally Posted by Wazzajnr

Last longer? Not so sure. My 50cc Bike was 25years old when I got rid and still going strong. It made 4.8hp at about 5500rpm iirc. It had done 33000miles. Not sure how many hours that equates to but it a lot and 90% of those hours were spent at WOT and full revs!

Well at an average of 33mph that would be 1000hrs. I'd expect two to three times that from a decent outboard.

Searider · 01 May 2015, 16:16

An outboard spends much of its time at 50% LOAD/POWER or above. Typically cruising speed will equate to about 75% of maximum rpm and looking at the fuel consumption on my gauges this is half of the maximum fuel consumption, therefore half load/power.

It's also quite possible to run a boat engine at full power for extended periods - look at the run down to Yarmouth in January - everyone flat out all the way.

A bike or car will spend only a few percent at full power/load. An example, using the diagnostic software for my car I can measure airflow and from that get power being developed. On the motorway at 70mph my car is only making about 25hp - that's all that is needed to push the car along. It's VERY lightly loaded in normal use. therefore it can be built lighter - or tuned to produce much higher power, knowing that it will only be requested to produce this power for very short periods.

When you're in a hurry it may feel like you're "flat out" for a long time but in reality it's only a few moments. So, I don't think that the 33,000 mile bike will actually have spent 90% at full throttle. Even a GP bike won't have spent 90% at full throttle on the track!

So, a marine engine operates at high loads for long periods of time - so has to be robust.

A vehicle engine spends most of its time at very low load so can therefore be built lighter / produce more power for the same weight/size.

Searider · 01 May 2015, 16:18

Quote:

Originally Posted by Wazzajnr

Been asking about getting my SIB on plane in the inflatable forum. Had some great answers and help there but it got me to thinking.
Why are boat outboard so weedy per cc. For example, my first motorbike was a Yamaha fs1e which was 50cc and made 4.8hp I the had various 125's which were restricted to maximum of 12hp (although they mostly made around 10hp) but were really de-restricted to give 16hp or so.
Now looking at most 6hp outboard motors (2 stroke) they seem to be around the 100cc mark.
So why is it that we only get 6hp from a 100cc motor.
Any thoughts?

Has to be optimised to give high torque at low RPM - to get you on the plane - to deal with the issue you are having. If they were tuned for a narrow powerband, like a motorbike engine, you'd have even less hope of getting on the plane.

Max... · 01 May 2015, 20:18

This is a very interesting discussion, it's such a shame that the development of small 2 stroke outboards effectively ended (in the EU at least) a decade or more ago. I'd love to see if we could somehow have got to say a 25 or 30 HP but with the weight of a 26kg 9.8 Tohatsu?!

Nos4r2 · 01 May 2015, 21:59

Unlikely to be honest. The biggest issue is that you have to have a certain weight of flywheel/crank for the low down torque to get it to go into gear without stalling.

The little Tohatsu 9.8 is 170cc. Realistically you wont' get more out of that without expansion chambers or funky fuelling systems which you're not going to get on a low HP engine,without massively increasing the weight.

Last Tango · 02 May 2015, 05:55

I believe part of the reason for their demise was the disproportionate cost of fuel injecting the low HP engines. It is a shame because, with GDI I think there is still a lot of scope for two strokes particularly in power tools where the orientation the engine is used at is very variable and weight is critical. Maybe the market demand in that sector will result in lower cost injection hardware that could find it's way into small outboards

Max... · 02 May 2015, 08:16

The demise was thanks to the EU emission regs banning their sale think!

Nos4r2 · 02 May 2015, 08:26

Quote:

Originally Posted by Max...

The demise was thanks to the EU emission regs banning their sale think!

Which is sort of what LT said. Basically if one is produced that meets EU emissions regs then it's ok, hence his comment about GDI.

I think it'd take a lot of effort to make GDI light enough to be viable on sub 25hp outboards, for potentially very little gain on the part of the manufacturer.