overpowering your vessel

Scott, have a look at Sparkys graph and tell me what speed you think is "full operating speed".

I don't understand your comment, "best to run at full operating speed" then make the change to "optimal RPM for that engine"

And of course "Running the engine below operating speed is bad for diesel engines" How so with electronic diesels?

I am just trying to find the point on the graph that you are referring to

 

Barry,
Excuse me for using two different phrases to mean the same thing. I understand. However your question about where on that curve you want to be, it is even labeled right there.

However this chart is one specific setup with engines producing 1000+ horsepower. Although the concept is the same, it's usually matching the engine to hull design and prop picked by the designer. The question is really how many horsepower do you want in order to run the boat at cruising speed with the engining turning at designed RPMs so you aren't running the engine under throttled and under loaded.

 

Then the larger engine was running at higher efficiency than the smaller one.

 

Perhaps we've run a little off course here. The BEST way to look at the OP's issue is to due some reverse engineering. I can assume from the recommendation of the designer low RPM is referring to shaft/prop speed not engine which makes sense on a full displacement. Unless, you go large displacement and high low end torque this would be heavy and expensive.

This appears to be a one off custom construction by viewing the links. So you have nothing to compare to performance wise. Also as this will be a new craft you will be subject to Tier EPA standards. As such, determine the biggest prop you can fit, speed the hull wants to travel, this will determine gear ratio and hp to drive it at the sweet spot.

And yes you will find it difficult to find a modern (electronic) 200 HP or less in the marine market. It is just not cost effective for manufacturers to research, engineer and EPA certify without pricing themselves out if the market.

So to answer the original question; Yes it may be possible to select a larger engine with proper gear ratio and achieve the desired performance as long as it meets the manufacturers operating recommendations for warranty purposes. Find a good prop shop and have them run some numbers. And choose an engine that is affordable to buy, run and maintain. My last advice would be to seek dealers in your area that can give you the most support for service and parts.

My current build I could have easily gone with a 500 +HP but found the overall expense fuel consumption and maintenance a little on the pricey side. An upfront cost of $8,000 for an extra 100 HP was the deal breaker. Good luck and keep us posted.

 

Hymar EU project gives some good information about diesel fuel efficiency at low loads. Here is one of their presentations: http://www.hymar.org/upload/LIBS_130110.pdf

Note the SFC curve at 1500 rpm and SFC map with propeller curves drawn on top of it. Even though the engine is rated to have maximum power at 3000 rpm, it still can give quite good efficiency at 1500 rpm on propeller curve, if propeller is not undersized.

The maximum power is 55 kW at 3000 rpm. Using just 10 kW at 1500 rpm still gives 260 g/kWh, which is less than a matched propeller would consume at full power (3000 rpm 55 kW) and the same it would consume at 2770 rpm and 42 kW. 1500-2770 rpm the the best SFC on the propeller curve is just under 250 g/kWh thus only a few % better.

Actually the propeller curve shows over 70 Nm at 1500 rpm and less than 250 g/kWh at 12 kW.

So using this 55 kW engine at 10 kW would be at least as efficient than using 10-20 kW engine at 10 kW. Usually the bigger engines have lower SFC at same specific loading, which makes the bigger one even better.

It would be even better to use the 55 kW engine at 20 kW 1500 rpm, but that is not possible with a decently matched fixed pitch propeller.

 

More efficient at 1500 rpm 10 kW than at 3000 rpm 55 kW, thus less fuel per kW. If a 10 kW engine would have an identical SFC map, it would consume more at 3000 rpm 10 kW than the 55 kW engine at 1500 rpm 10 kW.

A 10 kW engine will most likely have higher values in its SFC map, thus the difference is even bigger advantage to 55 kW engine.

 

Interesting post... I think there is a lot of variables in efficiency, hull design, diesel duty rating, reduction, even the power required by the prop at certain rpms and the amount of wasted power the diesel produces to turn the prop at that rpm . In full displacement hulls (even in displacement hulls one could find many variables) from my experience the more continues duty diesel will tend to be more efficient. Example, I would lean towards let's say a 100hp 4l diesel then a 150hp 2.5l. Asking less power out of more cubic inches will reduce rpm, heat and vibration and increase longevity. And as far as efficiency let's say the bigger diesel can run continuous at 90 percent without​ damage, it makes it easier to shrink the gap between the extra power of the diesel and the requirement of the prop. Ultimately if one had the money I would tend to think with the technology today diesel electric would be the most effective way to power a displacement hull. Just my opinion, I hope I don't step on any toes! Haha

 

Take a look at the HYMAR EU project reports. Not easy to make diesel electric more efficient than diesel. Here is one of the reports: http://www.autoprop.info/Latest_News/pdfs/HYMAR.PDF

Here the cross-over problem is shown in detail: https://www.unols.org/sites/default/files/Calder1_HybridEfficiency.pdf

 

From a simple seat of the pants perspective, I have seen many marine diesel motors damaged by running too hard, while I have not encountered any that have suffered catastrophic damage by running under rate.
The electronically controlled motors should be the real champs at running well below rated output.

 

I've been running diesel engines of all sizes since the late 1950s. Bigger older diesels, depending on gear ratio, props and many other things, can get better mileage at low rpm than new engines running designed speeds. Many old engines are much more reliable than that the new electronic controlled engines of today. Simple injectors, Detroit Diesel no injector pump at all, no circuit boards or electronic sensors to fail at sea, and so on. Years ago I ran a packer with a Washington diesel that topped out at 250 rpm. Pushing an 80' x 20' x 9' draft, maybe 150 tons, consumed 6 gallons an hour at 9kts. As a commercial fisherman, years ago, trolled for salmon and tuna. Engine ran at idle all day for salmon (about 700 rpm) and maybe 1/2 throttle for tuna w/o problems. It was common for engines to go 20 or more years between overhaul. So what damage to the engine are we talking about... My current boat, 83', about 80 tons burns 8.5 gallons an hour at 10kts in ocean swells. And that's not its' economy speed. Detroit Diesels.

 

It will be easier to get the instructions to build a nuke weapon , but what you need is a fuel map for your engine.

The eng. mfg are VERY reluctant to part with real information , but the map is the only way to set up a vessel for efficiency.

Most eng sellers will only give a HP and prop theory graph, basically great to make a water ski puller , or sport fish that operates at WOT .

For a cruiser they are basically useless

The fuel map will look like a set of bulls eyes ,

The center eye is the lowest fuel burn , the rest are less efficient.

Good hunting