Diesel Power

[raffshore]

I am a high perfomance boat builder in Europe. With our feul prices we have tons of clients who are asking us for dieselpower.
We want to use a bravo style drive, because almost every boat is equiped with this style of drive. The known problem is that these drive cannot handle the massive torque of a diesel engine, on top of that the powerband of a diesel is shorter then a petrol engine.

I decided to use my own boat as a test case, this is a waterski-race boat with a blown chevy 540 making +/- 800HP.

The dieselengine I decided to use is a cummins 12V 5,9 making 500HP at 3000RPM and 950 FTlbs at 2000 rpm.

If I would use a speed increaser gearbox of 1:2 on this engine, in theorie it could make after the box 1000hp at 6000 and 425ft lbs at 4000 rpm correct? This is actually very close to my petrol engine.

My questions :
Anyone already tried it?
Any leads on speed increaser gearboxes?

Thanks for the help,

Raf

 

[icrman]

If you use a speed increasing gear train. You will
effectivly lower the torque. And with zero losses through
that gear train. The HP would remain the same as the output
at the flywheel. But in the real world we have losses so
you will have less hp. If you go the other way and do a
reduction you increase the torque and slow the rpms, and
as before HP stays the same. Actually less.
So your step up box will just do that make more rpms. And if say the gas engine is a direct connection to the prop, and you need 500 hp, with a step up box, you will need more
diesel hp to do the same thing the gas engine does.

 

[the3car]

as mentioned by icrman, the HP will be the same or less, depending on friction/windage losses. The more important quesiton is how can you make up the loss in RPM with different prop? Given you are using the same drive, you should be able to prop up quite a bit. You might sacrifice low speed acceleration, but not sacrifice top speed.

I think any of the tranny makers would help you out on the 1:2 vers 2:1. Essentially your just flipping the tranny around.

Good point on the torque problem with the bravo....that's why so many of the BIG desiel boats, that go fast, use surface drives. Direct drives allow that higher torque..no gears

 

[rob768]

Whatever speed reducers or increasers you use, the actual power remains constant (of course). However, keep in mind that the propeller is designed for a certain combination of power AND speed. Changing speed means you have to change the propeller as well, or you'll wind up overloading the engine, or having an unused power surplus.

 

[MikeHalloran]

Forget speed increasers; there's no free lunch.

A typical 500HP Diesel will probably weigh enough to sink a boat that's intended for a racing 800HP gas engine.

Both Volvo and Cummins Mercuiser Diesel offer outdrives already matched to Diesel engines, with a warranty. I'm sure their application engineers would love to help you make a Diesel powered go-fast boat, especially if you can sell more than one.







Mike Halloran
Pembroke Pines, FL, USA

 

[UWH]

MikeHalloran is right - the power density just isn't there in the diesel vs. the gasoline engine.

Your boat won't be designed for the weight of the diesel and besides giving up 300 HP you'll have trim and possibly structural (and floatation!) issues with the boat.

 

[raffshore]

I am sorry to say but no, you are not right.
I build the hull's myself and we have 23ft boats with twin outboards on a hydraulic lift on the back, this is 500kg on the back (outside the boat).

When I builded the boat, I builded for inboard diesel, with all the structural strenght and flotation which it needs.
As for the power, also not correct. Currently on the BB chevy inboard we use a mercruiser 5 drive with a turbo400 transmission. In this transmission and drive we lose lots more power then in a bravo style drive. Also, the propellors we find are much more effective and have less slip then a cleaver style prop. Also the drag created by a bravo drive is less then a number 5 etc.

All the technical problems we have with the boat and drive we have resolved.

I was just wondering if there is a gearbox builder who has a speed increaser and not reducer.
If not we have one build here locally.
Thanks for thinking with me guys,

Raf

http://www.raffshore.com

 

[pawelmaz]

Hi Raffshore,

I think that the best sollution is use Cummins MerCruiser Diesel with Bravo III. Of course very important is prop selection but any good local distributor should help you. Poland is the biggest boat bulders in Europe and I notice that most of builders use MerCruiser drives. You can also select different gear ratio (e.g. 2:1).

 

[liviebudgie]

Before changing the drive train think about cavitation.
It could be possible to get what you want by using the deisel torque abilities and just fit a larger prop.
The gearbox should match best power from the engine or prime mover to efficent rpm of propellor.
Power should be matched to hull shape as this will finally dictate maxium speed of vessel.

 

[patprimmer]

I would think the only difference between a reduction box and a step up box is the labels on the input and output shafts, and the torque rating.

If you end up with relatively similar torque curves at the output shaft, you can run the same prop, maybe with minor tuning.

Like several others have said, you will lose a little power and not quite get the numerical ratio change in torque change due to frictional losses in the box. If the box is well made and well sized, this will only be a few % loss.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[waross]

You may want to post questions on some automotive performance sites. The automotive boys are getting a lot of extra power out of diesels with big turbo chargers and after market computer chips.
yours

 

[j2bprometheus]

The MAHLE website lists the weight of Cummins B5.9 liter engine as about 900 LBM (409 kg). This is actually for the on-highway version but the weight of the marine version should be similar.


A gasoline engine producing the same power should be lighter than the Cummins diesel.


j2bprometheus

 

[harrisj]

j2bprometheus - I think you're being a bit optimistic about engine weights. The fundamentals weigh the same but medium-sized marinised diesels are typically 10% to 20% heavier than the automotive version (even higher percentage on smaller engines). This is due to:

Engine cooling is generally built-in, using a seawater-cooled heat exchanger, often integrated with the cylinder head.

Exhaust manifold is usually water-cooled. Not good having red-hot exhaust components in the engine room.

Turbos are often water-cooled for the same reason. They can be quite big and heavy with an external water jacket.

Components such as oil coolers, transmission coolers, turbo intercoolers are all seawater cooled, and often mounted directly on the engine.

Seawater pump for cooling is usually direct mounted on the engine - not heavy in itself, but there is often a more complicated PTO drive train on marine engines dus to the large number of possible auxiliary drives.

All the cooling components tend to be made from marine grades of brass / bronze and cast iron, and they're not light.

Ancillaries such as fuel system components are usually bigger (and often duplicated for redundancy) because of the danger of sediment pulled through from tanks.

Since many boats (especially pleasure planing craft) run at WOT for extended periods, the manufacturers often de-rate the power output by changing the fuel mapping. So a 200kW truck engine may be only rated at 175 kW when it's configured for marine use.

I've known of several disillusioned boat owners buy a truck or car diesel and then try to marinise it. Get a Volvo Penta price list and you'll see what I mean!

 

[bdbat]

This thread started with the question from raffshore if anyone new where to obtain a speed increaser. And am also looking at about 2:1. I am working on a similiar type of application but with a turboprop. So if your reading this raffshore, please let me know what you did in the end. The engine modle I am working on will be 750 SHP and 2200 rpm and a max of 1790 ft-lbs at the propshaft. Of course I have the same problem with the RPM of the output shaft.
Anyone have any ideas for a speed increaser or where to look?

 

[UWH]

harrisj,

You're right about engine weights:

The Caterpillar 3056 Marine (205 bhp, 6 litre) engine has a dry weight of 1342 lbs.

The C-12 marine engine (490 bhp, 12 litre) has a dry weight of 2588 lbs.