Diesel Outboard 5hp 25kg

[lw395]

for use on a small fishing dinghy, with about 100 hours use a year ? It's the lower flashpoint of petrol than diesel that concerns me.

My priorities for an outboard are

1 starts every time if fuel is there and the o/b has been serviced properly


and way down the list,
2 exhaust noise
3 mass


A diesel o/b of about 4 - 6 hp would suit me fine.

There are lots of small diesel engines for scooters. All we need is for Mattnj to conceive an enthusiasm for a diesel outboard, and all our problems will be solved.

When i last bought an outboard, one of the main attractions of the Yamaha 2hp 2 stroke was that SWMBO could start it reliably.
The light weight is also magic with a rubber boat. We can easily carry boat boat and o/b up the steepest of beaches etc. That has its uses. Being able to easily pass the o/b up to the deck of the yacht matters too, 20kg is a lot for SWMBO to lift at an awkward angle on a pitching boat, whereas 12kg (?) is a one hand lift. That means less dents in the deck!
In the 2-4 HP range, cost of fuel does not really figure much.
There might be some benefit in sharing with the yacht's fuel, but equally there is something to be said for having a separate supply when your diesel gets the bug. The reality in my case is that I can 'borrow' a few litres of petrol from my motorbike for the outboard.
Diesel is safer, but petrol is not that dangerous. Petrol fires are really very, very rare.
Except for forum members' bonfires of course.


Noise matters. I don't like other people using excessively noisy o/b's and I want to be able to have a VHF conversation while using mine (funny how that's become obligatory while phoning and driving a car has been banned!).
What might have more future is a small RIB (posh yacht tender size) with an inboard diesel and Z-drive. Using one of the modern small car diesels? Alfa 2.4 JTD perhaps!

Or of course, with the modern batteries, maybe electric outboards will be more practical?

 

[Lakesailor]

So one of these would be just the job:-

http://www.goodsdirect.net/fhdiesels.htm

Ahem. Let me draw your attention

 

[TQA]

Can you tell me how you know that 150cc is the limit? This would be very interesting for me.

This was taken from a lecture I used to give on alternative engine types and I am pretty sure that my source was a Society of Automotive Engineers research paper published somewhere in the 80s. The topic was diesel combustion chamber design. There is no absolute cutoff point. The engine will be harder to start and the exhaust emissions higher as the cylinder size diminishes.

I no longer teach in that field but maintain an interest and still read around the topic whenever I see something.

The 800cc three cylinded diesel fitted to the Smart car is the smallest diesel in automotive use and has very low emissions. This is due in part to the sophisticated turbocharger management system incorporated in the ECU.

There are a number of small single cylinder diesel engines produced in China but the smallest that I know of in commercial production was around 300cc capacity and weighed 40kg in aircooled format. Please do not take that as gospel as I am plumbing the memory banks on this.

I note with interest that Lakesailor has found a site offering small single cylinder diesel outboards with the Star name. The smallest seems to weigh in at 63 kg. I wonder if any forumites have seen one in operation?

 

[Lakesailor]

I would imagine that once seen, never forgotten!

 

[reeac]

small diesels

I have a 16 bhp., 3 cyl., Kubota compact tractor which is 762 cc. swept volume and works very well so that's 254 cc. per cylinder with no sophisticated gizmos - just glow plugs for cold starts. It's a pretty old design [20-30 years?] so I would expect them to be able to reduce the cylinder size by now.

 

[Tranona]

I have a 16 bhp., 3 cyl., Kubota compact tractor which is 762 cc. swept volume and works very well so that's 254 cc. per cylinder with no sophisticated gizmos - just glow plugs for cold starts. It's a pretty old design [20-30 years?] so I would expect them to be able to reduce the cylinder size by now.

No. the smallest cylinder in the current Kubota range (at least those marinised by Beta and Nanni) is 240 for the 10 and 14, 300 for the 16 (twins) and 240 for the 20 (3 cyl).

 

[ProDave]

Interesting thread.

About the only real interest I would have in a diesel outboard is the fact you can buy diesel from many boatyards, so refueling on a long trip is easy.

By contrast, petrol usually has to be bought from a car garage, and they are rarely right next to a mooring.

So planning for a long trip with a petrol motor means carrying more fuel, or more detours to get it than a diesel engine.

 

[Neil]

I think the discussion around diesel outboards around the 4HP mark, for a tender, is probably about the wrong target, for all the reasons already mentioned. However a diesel OB in the plus 150HP range would be more of an attractive prospect. Many of the aforementioned difficulties recede to the point of practicality, and fuel consumption is more important? who wouldn't like to get more than 0.5 mile per litre. So why hasn't it been done?

Probably because the biggest market is the US, and for the same reason why diesel cars have little market penetration; The standard of diesel fuel in the US is poor, and petrol is so cheap. Thats why BMW and Merc use the Bluetec oxidisation technology to get round US-diesel emissions.

 

[Boathook]

I think we are at the stage where it is possible to make a lightweight diesel outboard that runs quietly, without smoke.
We have the common rail and piezo injector systems for efficiency and the modern diesel can be boosted to produce similar hp to petrol engines.
These technologies have made it possible for us to have diesel cars that run just like petrol.
I could imagine that a state of the art diesel engine boosted with a small supercharger could easily produce 5 hp. The size might only be 50cc. The outboard needs a total weight of around 25kg.

A conventional diesel needs roughly 400cc for 10 hp and so in any sort of outboard form, is going to weigh roughly 60 kilos or more, produce smoke, noise and vibrate.
Yanmar used to make 18hp and 30hp outboards but they ran just like what is written above and weighed lots. They stopped making them as they could not satisfy emission requirements.

I know many people will say that it will be too expensive etc etc, but lets forget that point for the moment.

So is it possible to engineer a 25kg 5 hp diesel outboard that runs reasonably quietly and satisfies the eu emission regulations using the latest diesel technology?

I cant see any reasons why this could not be done now.

Do we have any engineers or anyone with experience in this area who could comment on what Ive written ?

As far as I'm aware Yanmar still make the diesels outboards at 27 and 36 hp? French marine engines on the East coast were advertising new ones recently as a one of 'import'? just wish I had purchased one 15 years ago .....

 

[mattnj]

the smallest Diesel i have seen (and have bought, i have one sitting in my garage) is a Yanmar L40, which is 200cc and 4hp, engine alone is about 25kg.

It is also very hard to start from a resistance point of view, (it actually starts easilsy, but only if its not bolted down well!)
If it was in an outboard, on an inflatable, it would be a nightmare.
Then there is the decompressions lever faff.

One of the reasons for the 200c limit from what i have been told from my mates at Ricardos (diesel engine people) is the supply/regulation of the fuel becomes almost impossible, you end up with such tiny amounts of fuel, the whole injector, pump system just becomes too small

 

[AngusMcDoon]

...preferably a 9.9 hp...you would probably use 2 liters diesel per hour

In calm conditions my petrol Yamaha 9.9 four stroke outboard achieves 22 Nm/gallon when motoring at 5.5 knots shoving a 31' boat around. Works out at 1.1 litres per hour.

 

[saltwater_gypsy]

Quote: One of the reasons for the 200c limit ...is the supply/regulation of the fuel becomes almost impossible, you end up with such tiny amounts of fuel, the whole injector, pump system just becomes too small.

Interesting point from mattnj BUT the individual cylinder size on multi cylinder engines may not be much different. I think its that the power to weight ratio of a diesel is impractical for a small outboard.
Have you ever seen the flyweel on a single cylinder Saab.....massive but very necessary to smooth out the power delivery.