Props and Variable Gearbox ratios?

[ozflyer]

Hi Guys,
one for the prop engineers out there...
The aim is to properly load the engine similar to a diesel/electric drive to achieve low specific fuel consumption.
So is a gearbox ratio of 1:1.7 and 1:2.3 practicable?

Project data:
Power Trimaran, Loa 14m, L/B 15.3, Displacement 1.9-2.4 t, Kubota D1505 29 hp cont. @3000 rpm, Gearbox ratio: Suzuki Sierra 1.7 & 2.3 or Toyota Corolla 1.3 & 1.9, Prop dia. 400-420 mm
The aim is to cruise at 11 kn with 20 hp (2000 rpm) as well as eco-cruise at ~ 8 kn and still load the engine. This setup would also allow to "pick" the gear for varying sea and wind conditions.

[TeddyDiver]

Instead of variable gearbox ratios it's better to use variable pitch in the propeller. That is a CPP.
BR Teddy

[ozflyer]

Hi Teddy,
Yes that would be one way to solve the problem. Typically CPP's are not as efficient as a result of the increased cord thickness and hub diameter (least for smaller vessels). They are also susceptible to prop-strike and mechanical damage. Besides there is the costs... this would certainly blow my budget and possibly result in a divorce and worse - no boat...
Well everything in live is a compromise, hence my question with regards to the use of a car gearbox.
Cheers Joe

[FAST FRED]

A car gearbox would allow the use of say 2-1 3-1 for normal higher inshore speeds at higher RPM.

And allow the unit to have the equal of a "cruising prop" and be well loaded at say 1500, that would not happen with normal single speed gears.

A stack Exhaust gas temp gauge would be required , about $100 from the home made aircraft folks.

FF

[MechaNik]

I am not opposed to this system for loading an engine, but would there be much of an efficiency gain expected with this system?
I have heard of two speed gearboxes on the likes of planning hulls when used as sport fishing boats. Then there is substantial difference between loading when planning and trawling.
With the setup in question..a diesel engine rated cont@3000rpm will not offer very good specific fuel consumption down around 1000rpm. Do you have a spfc/power/rpm plot for that engine?

The only time a fixed propeller could be more efficient than a CPP is when at a set design without variable?

[ozflyer]

Fast Fred, correct that is the intension.

MechaNik, apparently there is considerable fuel savings to be had. Some modern large ships use diesel-electric drives for exactly this reason. With regards to the specific fuel consumption at a particular rpm, they are only quoted for the specified used torque at those rpm's.
However if you don't load your engine, at whatever rpm, your not using all the available torque and therefore your specific fuel consumption is higher than you find in the performance graphs.
For example if your prop is matched to your engine and you using 30 hp at 3000 rpm (Kubota V1505) and your now only using 1500 rpm, assuming at thereabouts half boat speed, your now only using about 25% of the available torque.
http://www.dieselenginemotor.com/kub...r05/page7.html (specific fuel consumption)

[MechaNik]

Ozflyer, what you're doing doesn't really have much similarity to diesel electric. Diesel electric gets it's efficiency from running variations of multiple prime movers at an optimised constant speed near constant load. Then not having a transmission or drive line helps with this.
An engine is best kept loaded as said. However there is a minimal loading and rpm point too where there is just not enough heat generated etc that can shorten it's life dramatically too. If the graph ends at 1600 maybe that is it for this engine? You can see that specific fuel consumption is best at 2200-2600rpm and is rising steadily below 1800rpm.
What I am saying is that if these are the loads you want to run at most of the time then maybe you should choose a different engine. ie smaller.

[ozflyer]

The original primary aim of the diesel electric drive was to reduce specific fuel consumption. This is achieved by running one or multiple engines at close to their lowest relative corruptions. I'm speaking from practical experience. I've done this very successfully with a single engine on one of my previous vessels. So the concept works with only a single engine on a small scale. I can give you the details if your interested.
With regards to the operating temperature, you are correct, running the engine below operating temperature will shorten its live and increase specific fuel consumption. However operating temperature is typically achieved by the use of a cooling water thermostat.

While I'm happy to discuss the behavior of diesel engines, this unfortunately does not answer my original question.
Is a gearbox ratio of somewhere 1:1.7 and 1:2.3 practicable? This is really a propeller question.