Diesel outboard motors

Hanging big motors off the back is ok for planing boats, for yachts it doe'snt work, ive seen it done.

A massive 54 foot cat was fitted with 2x 25hp outboards ( not my Idea) and they would not drive in rough weather, the stern leaving the engines high and dry on crests. Besides the power being utterly insufficient they were virtually usless in heavy weather.

They were also swamped repeatedly--and removed.

 

Frosty raises the biggest "obstacle", apart from the manufacturing side, to large outboards regardless if diesel or petrol.

Having an outboard that is getting on the powerful side, shall be heavy. This concentrates a significant amount of 'fixed' mass aft. Unless the boat is tailored for ultra high speed, where the LCG is invariably 15-20% aft of midships, you'll have a tough job trying to balance the boat for normal operations.

So whilst having larger diesel outboards may be nice, however, if you require a bigger power unit for your required speed, you really need to look at the knock on effects. Having the bigger fuel tank aft will compound the issue. So place the fuel tank up fwd to balance it..well, when static that can be achieved (but not always), but what about when running and with half a tank....she'll trim badly.

So there is merit, as each design SOR yields a different solution. But having a requirement for a larger power unit outboard for diesel (or petrol) has limited applications owing to the type of hull form and speed and of course space/layout requirements.. Whereas an inboard generally does not suffer such immediate deleterious consequences and can be much more readily accounted for in the design - with increasing size of power unit.

 

Well the vessel I have in mind would easily carry a ton or more in the bows without blinking.

It was not the weight that killed it , it was simply the stern would leave the water in rough weather. The engines would then cavitate and then when submerged again took a while to clear and by that time they were clear again.

On an outboard you have only say 2 feet from skeg to cavitation plate. So if the stern raises from the water a mere 12 inches it would cavitate.

When in 6 foot waves (not that big) it was useless.

 

That is why my original idea was for drives that operated at least 4 feet underwater, but they would still give some problem in rough weather because sooner or later propellers where going to be in the air. Problem of weight is not problem, the engines would be place inward and hull would wrap around it like a catamaran.

The idea is to have engines pod removable via a crane. Engines could be in center of boat for all I care.

 

Frosty,

Your hull has a low Hp/Tonne ratio..thus having a hull form suited for the power ratio would...or should..easily accommodate weight up fwd. Whereas a high ratio, would not, as the LCG needs to be aft, and the hull form follows that SOR. (Fine bow lines and LCB well aft).

If your props come out the water in a following sea or head sea, you're going too fast for your hull form. Simple seamanship will dictate to slow down. If you wish to go faster in similar sea conditions, you'll have to change boat (hull) that is more suited for such conditions. Unless you simply have a crap hull form...

MD.
You're ostensibly describing podded drive system.

 

The boat in question was not moving. The speed was zero infact negative. Just the slop of the boat moving up and down raised the engines out of the water leaving any thrust impossible.

This is not my cat but a friend 10 years ago. This boat is huge , it had 12mm rigging.

 

And if you add a bit more complexity, you can rotate them as well. Just don't hit a chunk of ice and bend something otherwise things get bad.

Provided you can live with the whine and efficiency loss I'd say hydraulics were the way to go for this sort of system. As my opinion of hydraulics is, sooner or later it's going to leak and when it does, it's going to be messy, I don't think I'd do it, but it's certainly viable.

PDW

 

I have heard that Marine Diesel AB in Sweden is coming out with large Diesel outboard engines sometime in the near future. Does anybody know anything more about this?

 

Diesel is a stuggle but on kersosene they run great
The US military madated they only wanted to carry 2 fuels Jet a1 and Diesel ( MGO) Merc and OMC/BRP made engines to run in Jet A1 adn at a pich diesel, but that has so much carbon in it they wont run for long

 

Never heard of AB but if Honda or Yamaha said they were making one I might be able to muster some excitement

 

www.marinediesel.nu

 

The "multi-fuel" outboards that are sold by Mercury and OMC are similar in that they both employ direct injection systems. The Mercury uses the "Orbital" process where a small amount of air is injected to help atomize the fuel.

Combined with a reduced compression ratio, specific spark and fuel timing and a special oil, these engines work quite nicely on diesel fuel. The advantage is that they have the weight of a typical 2 cycle engine. The low weight is a result of having spark ignition, and the engine doesn't detonate like a diesel, so it can be lighter.

Since the fuel is injected after the intake port is closed, there isn't wasted fuel like in a carburated 2 stroke. Fuel consumption (SFC = lbs of fuel/hp) is about the same as a four stroke gas engine. A big diesel has better SFC, but really small diesels pretty much suck in terms of SFC. As a result, these new generation of engines are about the same or a bit better than a small diesel in fuel consumption. The lower compression ratio hurts the SFC, but it's still not bad, and a lot better than a 2 stroke with a carb.

Orbital did a lot of research with Mercury in developing the system and it appears to work quite well from what I have been able to find on the web.

Mercury developed its new oil (called Rejuvenate) for this engine, and found it really cleaned up old gummed up outboards, so now it's recommended for them to. This oil also meets all the requirements of the Japanese -D rated oils, so it's pretty good stuff.

 

Will Mercury/OMC provide normal engine warranties on these engines when running on Diesel? From what I have read on the internet they are designed for JP-5, JP-8 & Jet A-1 and can only burn diesel for a short period of time.
http://www.marineenginedigest.com/profiles/mercury/optimilitarymotor.htm

 

I had not seen that link. I had seen some research published by Orbital that showed the system was compatible with DF2, not sure why they limit the time on diesel fuel.

 

there is too much carbon in diesel so they smoke and coke up
jet fuel is what they were really aimed at