Real Hybrid

Good morning, I was wondering whether it is (above all economically) feasible to design and install a true diesel‑electric hybrid in the power range of 50–70 hp on a displacement vessel (medium‑heavy, D/L 320–400), with a 25" propeller and cruise rpm around 500.

I ask because the target (for this and for most recreational displacement boats) reports cruising use around 6.2 kn requiring roughly 20 hp, but with a necessary power reserve to cope with critical situations.

Sizing everything for 30 hp doesn’t make sense from a safety standpoint — too little margin. Sizing everything for 60 hp would be excellent for safety but unnecessary for 90% of time at sea, leaving the diesel under‑utilized (let’s assume we size the engine to run at 1,400 rpm with a 3:1 gearbox).

My question is: does a solution exist that allows a generator to supply an electric motor with about 20 hp for normal cruising, but that can be configured to provide the full ~60 hp to the electric motor for short peaks (wave climbs, extra loads, etc.), supported by a battery pack?

I hope I explained clearly in english.

Thank you.

 

Yes.
Simply hook a 60hp electric generator motor to the diesel and set regen at 1/3 his nominal power. A 60 hp electric generator motor can be set to regen anything from 1 to 60 hp (even more for short duration).

Or better, use a 30hp diesel/electric motor generator combo to charge a battery. This battery will supply 20hp to a 60hp drive motor 90% of time and 60hp when you need it (10% of time).

I use hp for simplicity, but in electric world we generally use the kW for power and kWh for energy (battery).

 

Ok thanks.

So the system would be: a 20 kW genset that charges a battery (of ?), battery powers an electric motor potentially up to the required Pmax.
When the motor load exceeds the genset’s supply it will be powered by the battery (until discharge) and then continuously up to the genset’s maximum kW?

Is this a system that, despite significant complexity, can bring advantages in terms of consumption and/or ease of use (ability to provide greater surge/acceleration for a limited time, maximum economy of operation for most of the time, ...)?

something like this already exists?

 

A battery of sufficient size to cover the time you think the emergency will last. If you need 70kW and the generator provides 20kW the deficit is 50kW. If the "critical situation" lasts one hour you need a battery of 50kwh, two hours 100kWh, three hours 150kwh, and so on until the battery size and weight exceeds the hull carrying capacity. This hard limit is exactly why DE ships use multiple generators instead of a battery bank. You can easily calculate the exact point where a second generator (+fuel) becomes lighter then the battery based on your chosen components.

There's also the transmission side of the equation, a VPP would be required if you chase efficiency.

Of course you can avoid the electric complication entirely, either with a dual input gearbox or by having two propellers each optimized for its engine power. The last solution is still practiced enough, either as dual IC engines (aka. "get me home" engine, often an outboard), or as a trolling motor (sometimes even battery-electric).

Simplest solution to your problem, two diesels (one small one big), each with its own propeller.
In the stated power range nothing makes sense economically, not even fitting a VPP to a single big engine.
The only exception I know of are sailing catamarans where owners choose to run on a single engine for economy, and that's only because you can't convince the manufacturers to deliver single engined cats.

 

Exactly. It's how some hybrid work.
The fuel remain your primary energy source. Other energy source can be sails or solar panels.

It's way more efficient to charge battery with a DC generator with proper voltage. If no, you need regular generator + a battery charger, so there is more loss.

 

This concept might not be as dependent on oil grid,

New electric boat motor uses wind and water to recharge its batteries at sea https://electrek.co/2023/04/05/new-electric-boat-motor-saildrive-servoprop-25/

Conceptually, battery charge up could be augmented with a pop up wind turbine generator while anchored, and a solar array could kick in a little more charge as well. Diesel doesn't offer the choice of making your own fuel.

 

I don’t really know.
Speaking of sailboats, with a 5 kW generator in the water you probably lose about 1 knot, which is a big deal when it comes to VMG. On top of that, you only get about 2 hours of motoring (maybe at 20% load) with batteries that are enormous and heavier than the diesel engine itself (and possibly even heavier than the fuel). Completely useless in bad weather or for passages that are little more than leaving the harbor and coming back the same day.

Obviously, this isn’t feasible on motorboats—why should I sacrifice efficiency for hours of navigation just to gain 15 minutes of electric autonomy?

Not to mention the costs, which are insane.

Electric propulsion might make sense if used as support, in my opinion.

 

Yes, operation in harsh sailing conditions, it's possible for diesel power to go down, as well. Mine are just some EV ideas- which of course might not work for everyone! Myself would study alternative routes and consult with references constantly during a sailing trip to minimize risks even if diesel fuel is used; as significant numbers of diesel powered boats have been lost under such conditions.

There may be over a hundred steps just in the process of making diesel starting with finding crude, extracting, shipping, refining, storing, marketing, political cartels, terrorism, etc. Some failure even for a short term at any step in an increasing unfriendly , inflationary $ world, might mean diesel availability issues.

My guess is the weight of lithium batteries is maybe around 30% of the next closest different chemistry battery contender, and they have considerably longer deep cycle life depending of the particular chemisty type, -but lithiums would be a BIG initial cost if purchased new, should many be needed- for an undependable/hazardous wind sail route. OTOH, cost of e-fuel might be around zero over many years of use, with e-power. Also generally lower cost of e-motor / repair/ keeping a spare for replacement/ maintainence, etc. Some motor e-boats are already feasible in applications involving short distances, such as ferries, I have seem some proposals where a carefully devised, flexible electric cable to the grid, is used for power instead of batteries.

The proposed hybrid may not be much of an improvement if any, than just going with a conventional diesel system. JMHO.

 

I actually build an electric solar catamaran and one of the key point to keep cost low is to reuse electric vehicle parts.
EV reach scrap yard and are often almost new (except the crash).
I have 250$ Nissan Leaf motor here and 102 kWh (2024 for 3500$) Cadillac Lyriq battery there... It's how I keep conversion cost low.

 

Really? care to share an example of that configuration sold after 1990? You could manage something like that with the old Perkins 4.236 thumpers, but they weight 1000 pounds marinized. You're going to want to look at faster speeds.

So first, your base case needs to be revised. Let's put a beta 62T in it with a 2.5:1 tranny, and compare the fuel burn with an appropriate prop - something like a 21 X 16. This will be much better than the Perkins @ 1400 rpm with the 3:1 turning a 25" prop. It won't be better over conditions ideal for the Perkins, but it will be better overall. And it will be 450 pounds lighter and take up less space in the boat. That additional space has to be built into the boat and providing it has a significant weight associated with it also. A good slip to aim for is about 20 ft/sec at full chat, and about 10 ft/sec at low cruise on the low side of hull speed. So slip numbers in the 40% - 50% are good for displacement cruising a plump 16,000 pound boat. Chasing lower slip numbers just doesn't pay in practice. You loose more in parasitic losses and weight penalties than you gain at the prop.

Bottom line is you are trying to solve a non-problem. The diesel isn't underutilized if you choose it and the rest of the system correctly. I'd look for 4 bangers if possible - 3 bangers will not have as much useable rpm range. And flatish props are fine for this duty. Three or four bladed props depending on resonance issues at cruise rpm.

As a point of reference, I had an old 27hp Universal in a 14,000 sailboat. But the cooling system was only good for about 16hp in the tropics. I had a folding two-bladed 14" prop that I could put in my pocket. It cruised on 3-4 hp on the intercoastal. You don't need 60 hp on 16,000 pounds unless it's a floating condo. We had single 80 hp outboards on 44,000 pound houseboats in the Florida keys and they would push into a hurricane.

So the idea of the diesel being under-utilized is a myth, or it is based on a naive set of choices that led to an unreasonably awkward comparison case. You're not doing anything that 1000's of others haven't done before you, and the solutions are the ordinary craft all around you.

 

It can be arranged so that the 5kw generator is only in the water when the extra 1 knot is not needed, say when there is more than sufficient winds. Removing drag can be done with a design using some kind of quickly retractable or tilt up outboard motor leg of the type sometimes used on sailboats. Or, a cable tow behind generator which can be quickly pulled aboard, making the appendage drag drop to zero.

Ideally free boat sail propulsion might work in a similar fashion to how a fixed wing airplane glider operates while seeking the best updrafts. Perhaps evolving to using AI real time, to help steer boats into the best wind to forward propulsion ratios for any particular destination. Batteries might allow for storage of enough wind energy, solar and maybe wave energy for when wind lies low. For those that like combustion propulsion methods, any surplus electric generation could be used to produce hydrogen fuel from seawater using the electrolysis process; hydrogen could be used in an adapted conventional engine or a fuel cell. Of course, even though these options are theoretically available, it doesn't mean that they are practical- because of complexity, insufficient gains, or cost to implement.

 

PhilSweet, thanks for the post.
I'm here to learn.

The Beta62T is basically the Beta60 with a turbo added.
To gain maybe 5 Nm at 200 rpm less.
Based on the charts (which I use for comparison), the 62T always consumes more than the 60.
The 60 reaches its lowest consumption (not BFSC, but liters per hour) at 1,400 rpm, and that’s exactly where I’d like to operate. The 62T consumes more.

Granted, these are all theoretical exercises—interesting for discussion but ultimately irrelevant in the real economics of building or operating a boat.
Still, I thought that setting the target usage with a low-rpm propeller and the engine running slightly below its BFSC (which still results in higher l/h, and that’s what you actually pay for) could be a good solution.

I’m also trying to understand the feasibility of a CPP (just as an exercise, of course), but there don’t seem to be any true CPPs available in the 24–25" range.

If someone wanted to push toward a “small engine” setup, then maybe an FNM 13 could make sense (variable geometry turbo, common rail, 215 kg with gearbox…). But it would have to run faster, and there’s no way around that: more rpm → more cycles → more injections → more consumption (even if each individual cycle consumes less).

 

You're not looking at this the way an engineer does. Let's look at heat rejection to the water jacket and heat conduction down the cylinder sleeve. There is a large temperature gradient in a diesel cylinder wall. It's hot up at the head, and a lot cooler down at the skirt. This is a result of adiabatic compression, and the fact that the skirt bottom isn't in the combustion zone at all. So heat conducts down the cylinder wall. This is bad for efficiency. The higher the rpm, the less heat migrates down the iron with each cycle. You don't want to quench heat at high compression. This is a large source of ICE losses, and it is only partly preventable. Increasing average piston speed is the most direct way to reduce this form of loss. Bigger diameter cylinders need to be thicker, so they conduct just as much heat. Piston count and stroke ratio have little effect here. Head design does somewhat. Using pistons with short skirts helps. And I want to emphasize this is a major issue limiting efficiency, it's not a small matter - it drives design.

A related matter is compression ratio. From the standpoint of the engine block, compression ratio is bad for efficiency. It exacerbates these temperature gradients. You want the pressure without the compression ratio - hence turbos. Lowering the compression ratio as much as possible in the block, and using turbo-compounding, can dramatically improve the thermal management of the iron block. It also reduces weight overall. Intercooling is also beneficial to efficiency. This is one of the reasons that the so-called 5 cycle and 6 cycle engines don't ever quite deliver their promised benefits - heat just keeps leaking away from them as they do their efficiency tricks. You just have to accept that some of the compression/expansion work has to happen outside the block if you want to limit these thermal losses and keep weight in check.

Additional efficiency issues involve the cooling system and auxiliary drives being designed for higher speeds. If your alternator doesn't charge your batteries at 1400 rpm (or you have to spend $2000 to get it to) you aren't saving fuel or money. The water jacket heat rejection will be off at 1400 rpm. No-one cares because the fuel burn is so low compared to design loads, but if you are using 20hp as the reference, the BSFC isn't going to be very good. Any time you find yourself looking in the corners of published charts of engine performance, you are asking for additional expenses. If you want cost effectiveness, look at the middle of these charts.

The marine gear losses are basically a constant fraction of rated torque. The deeper the ratio, the greater the losses. Using both an oversized tranny for the oversized motor, and also running a low speed shaft, adds large losses 100% of the time. These can easily eat 5% efficiency at cruise over what a correctly specced tranny would consume.

The Smallest 3:1 tranny I know of is the PRM 125. It's only rated for about 11hp at 1400 rpm coupled to a 4 cylinder engine on a displacement pleasure vessel. You'd want about 2500 rpm to get 20 hp through it. So you can't romp on the throttle or do a crash stop and you can't use your big prop. Realistically, you can't go much over a 25hp engine, and (you can't over prop a 25hp) in 3:1 ratio and still be able to do a crash stop. The first real practical gearboxes would be a ZF 25, 25A, 30M; and Km35, KM4 . The 25A can have a 2.714 ratio at a power rating of 3.37 hp/100rpm. This gives you plenty of headroom to over prop the the engine without damaging the gearbox. But it will waste a lot of power 100% of the time. You could run a 24 X 17 or 25 x 16 on it, though. But is it efficient? The prop costs $2300, the shaft will be more expensive, the gearboxes are around $3000 - $3500.

If you look at a Yanmar 4JH45 with a KM35A-2 tranny at 3.64:1, your prop drops to something like a 20 X 15. But the losses in the tranny drop by close to 50% compared to a KM4A, and the shaft bearing losses are less. These gains are in effect 100% of the operating time, which makes them really attractive. You have plenty of reserve power at 1600 - 1800 rpm to push against a wind at the torque peak of the engine. The prop is about $900 cheaper. The shaft is 1/4 smaller. The weight and size is reduced. There is little hope that you would ever recoup those savings employing a bigger prop at 20hp loads. All the auxiliaries will be happier at 1600 rpm. If you have a 35' waterline, the top speed is about half a knot less. But that is below your L/D of 250. If you have a 30' waterline, you won't know the difference. You will just go as fast as conditions allow burning whatever fuel you want to. The Nonsuch 33 weighs 16,000 pounds and usually has a 30 hp motor. The Hans Christian 33T can weigh 23,000 loaded on 30' of waterline and typically has a 40 in it at most, 3:1 gearing, stock prop is 20 x 16 or 21 x 16.

 

Very interesting, I will read with curiosity (and I will tring to understand)
Thank you for the time you spent on me.

 

Yes there are.
Finn propell https://www.nogva.no/fremdrift/propell/finn-propell starts its offerings at 450mm/ 17.7 inches prop diameter and 35mm/1.2 inches shaft diameter.
Vridbare propellere - Heimdal Propulsion Norway AS https://heimdalprop.com/produkter/vridbare-propellere/ starts at 600mm/ 23.6 inches prop diameter.
CP-EHW System https://www.westmekan.com/propeller-equipment/ehv-system starts at 25mm /0.98 inch shaft diameter and has two other systems starting at 40mm/ 1.5 inches shaft diameter.