Reply
 
Thread Tools Search this Thread
 
Old 01 May 2015, 05:08   #11
Member
 
Roflhat's Avatar
 
Country: UK - Scotland
Town: Isle of Lewis
Make: Ceasar Thundercat
Length: 4m +
Engine: Yamaha 70ces
Join Date: Jul 2010
Posts: 633
RIBase
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.
__________________

__________________
Roflhat is offline   Reply With Quote
Old 01 May 2015, 05:42   #12
Member
 
Country: Netherlands
Boat name: Tempo
Make: Scorpion 8.1 Mk1
Length: 8m +
Engine: Honda BF225
Join Date: Jun 2013
Posts: 30
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.
__________________

__________________
otilly is offline   Reply With Quote
Old 01 May 2015, 08:10   #13
Member
 
Country: UK - Scotland
Town: Denny
Boat name: Hebridean Storm
Make: Coastline
Length: 6m +
Engine: Honda 130hp
MMSI: 235107505
Join Date: Mar 2013
Posts: 841
Quote:
Originally Posted by Wazzajnr View Post
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.
__________________
Last Tango is offline   Reply With Quote
Old 01 May 2015, 12:16   #14
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
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 is offline   Reply With Quote
Old 01 May 2015, 12:18   #15
Member
 
Country: UK - England
Town: Southampton
Boat name: SMH Rib / War Shot
Make: Ribtec / Scorpion
Length: 4m +
Engine: 10hp Yam / 150hp opt
Join Date: Sep 2003
Posts: 1,894
RIBase
Quote:
Originally Posted by Wazzajnr View Post
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.
__________________
Searider is offline   Reply With Quote
Old 01 May 2015, 16:18   #16
Member
 
Country: UK - England
Town: Sussex
Boat name: Bombard, Y-162
Make: Aerotec 380, Y-Class
Length: 3m +
Engine: Mercury Mariner 15hp
Join Date: Jul 2010
Posts: 2,692
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?!
__________________
Max... is offline   Reply With Quote
Old 01 May 2015, 17:59   #17
RIBnet admin team
 
Nos4r2's Avatar
 
Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
Engine: Opti 225/Yam 85
MMSI: 235090687/235055163
Join Date: Jul 2005
Posts: 12,645
RIBase
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.
__________________
Need spares,consoles,consumables,hire,training or even a new boat?

Please click HERE and HERE and support our Trade Members.

Join up as a Trade member or Supporter HERE
Nos4r2 is offline   Reply With Quote
Old 02 May 2015, 01:55   #18
Member
 
Country: UK - Scotland
Town: Denny
Boat name: Hebridean Storm
Make: Coastline
Length: 6m +
Engine: Honda 130hp
MMSI: 235107505
Join Date: Mar 2013
Posts: 841
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
__________________
Last Tango is offline   Reply With Quote
Old 02 May 2015, 04:16   #19
Member
 
Country: UK - England
Town: Sussex
Boat name: Bombard, Y-162
Make: Aerotec 380, Y-Class
Length: 3m +
Engine: Mercury Mariner 15hp
Join Date: Jul 2010
Posts: 2,692
The demise was thanks to the EU emission regs banning their sale think!
__________________
Max... is offline   Reply With Quote
Old 02 May 2015, 04:26   #20
RIBnet admin team
 
Nos4r2's Avatar
 
Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
Engine: Opti 225/Yam 85
MMSI: 235090687/235055163
Join Date: Jul 2005
Posts: 12,645
RIBase
Quote:
Originally Posted by Max... View Post
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.
__________________
Need spares,consoles,consumables,hire,training or even a new boat?

Please click HERE and HERE and support our Trade Members.

Join up as a Trade member or Supporter HERE
Nos4r2 is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 09:15.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.