Single Diesel Driving Twin Hydraulic Props

[Arthurwg]

I believe I've read somewhere of a system that uses a single diesel engine to power a hydraulic unit that drives twin screws, one in each hull of a catamaran. I would assume that each prop could be used independently, giving great maneuverability. Anyone know anything about this? Sounds like a great way to go.

[Chuck34]

I've never seen or heard of it, but I often thought that is a system worth exploring. A single engine boat with 2 props, each with their own transmission. Obviously, the manueurvability would be way better than a single screw.

[a64pilot]

The only reason you would want to do this is if there is benefit in placing the engine in a location that isn’t conducive to running a drive line, other wise direct drive is real tough to beat.
Some hydraulic pumps are very noisy, not all but some, so of course avoid whichever ones are noisy.

[smj]

I had this setup on our old Cherokee 35 catamaran, very noisy and inefficient. Ended up pulling the diesel and hydraulics and replacing with a Honda 50 outboard. Quietly burned the same amount of fuel while making 1.5 kts. better cruising speed.

[Montanan]

Very inefficient as to transmission of power from the prime mover [engine] due to the losses associated with the pump and then duplicated losses at the turbine end. Gear or vane pumps will operate at 85 to 90 efficiency at peak power and lesser at reduced loads. And there are additional losses in the transmission pathway, pipes, bends, hoses, couplers, both returning to and from the engine powered pump. So figure .85 times .85 maximum efficiency power transmission efficiency from the diesel engine - 72%. There also may be the need for a heat exchanger to cool the hydraulic fluid which heat exchanger will add resistance and friction, so now you would be dropping down to the upper sixty percent overall efficiency. Direct drive is much more efficient and has fewer components.

[44'cruisingcat]

I encountered a Fastback cat that reportedly had hydraulic drives. (Didn't see them myself. )

When we saw it it was motoring quite slowly, but incredibly noisily.

[Sand crab]

Vetus makes this system and they have been doing it for years.https://www.vetus.com/en/power-hydra...ropulsion.html

[Lepke]

Another way is diesel-electric. But that is also inefficient compared to direct diesel drive. I had a WWII built, 100' tug with twin engines each running a DC generator and two DC motors running the reduction gear on a single prop. I could run on one engine. It was a great system for maneuvering control. I had many positions between stop and flat out. I could turn the prop so slow, you could swim between the blades. But you didn't want trouble with the old technology that controlled the generators and motors.
Tugs like that still working had all the DC generators, motors, and control gear replaced with 2 direct drive diesels. And the old stuff was more efficient than diesel hydraulic would be. Larger commercial fishing boats usually have a large diesel dedicated just to running the hydraulic pumps that run the gear or net motors.

[snoozy814]

This is the system we have on our boat currently (60 hp turbo Kubota Nanni diesel to twin hydraulic props)….SUCKS!! Cooling it is a bitch as it will overheat and lose power big time. Looking to repower ours from this system.

@smj, how do you find using an outboard? transom mounted or on a sled? We've looked into this, sail drives, z drives, IO drives. Tough to figure out what is going to work. Have heard w/the outboards you lose power on a cat in rolling seas and not looking to have the same nightmare of not being able to get out of harms way by being underpowered.

[Chuck34]

Originally Posted by snoozy814

This is the system we have on our boat currently (60 hp turbo Kubota Nanni diesel to twin hydraulic props)….SUCKS!! Cooling it is a bitch as it will overheat and lose power big time. Looking to repower ours from this system.

@smj, how do you find using an outboard? transom mounted or on a sled? We've looked into this, sail drives, z drives, IO drives. Tough to figure out what is going to work. Have heard w/the outboards you lose power on a cat in rolling seas and not looking to have the same nightmare of not being able to get out of harms way by being underpowered.

Engine cooling is pretty straight forward as long as the you have good water flow. Any problems are an easy fix unless the water jackets inside the engine are clogged with rust and corrosion.

Inboard/outboard...I've never heard of anyone who had switched from OB to IB say they missed their OB and wished they could go back.

[smj]

Originally Posted by snoozy814

This is the system we have on our boat currently (60 hp turbo Kubota Nanni diesel to twin hydraulic props)….SUCKS!! Cooling it is a bitch as it will overheat and lose power big time. Looking to repower ours from this system.

@smj, how do you find using an outboard? transom mounted or on a sled? We've looked into this, sail drives, z drives, IO drives. Tough to figure out what is going to work. Have heard w/the outboards you lose power on a cat in rolling seas and not looking to have the same nightmare of not being able to get out of harms way by being underpowered.

We’ve had various cats with different outboard setups. On the Cherokee we cut a hole in the bridgedeck and mounted a central outboard in a pod. We have a catamaran now with a single outboard on a sled and also a cat with twin outboards in lifting pods, advantages to both.
I believe you own a Kelsall? Have seen Kelsalls around 38’ with both a single centrally mounted outboard on a pod and also twin outboards.

[smj]

Originally Posted by Chuck34

Engine cooling is pretty straight forward as long as the you have good water flow. Any problems are an easy fix unless the water jackets inside the engine are clogged with rust and corrosion.



Inboard/outboard...I've never heard of anyone who had switched from OB to IB say they missed their OB and wished they could go back.

I’m wondering if maybe it’s his hydraulic system that’s overheating?

[Cheechako]

It can be done. Fishing boats use hydraulic power for many things. But a lot of it is not continuous use like motoring. Hydraulic is inefficient due to a lot of energy going to heat, Not to mention the mess. A maintenance supervisor who worked for me once noted: "If it ain't leaking, it ain't hydraulic".
It seems a big generator and two electric drives would be very cool but I'm not sure how the numbers work out on that. No transmissions, no mess, no fancy valving. I think it requires too big a generator for extended powering. Let's say you have a 40 ft cat and need two 15 HP electric motors. That's 30 HP or about 24k watts needed in really rough numbers right? A 24k generator is pretty big.

[a64pilot]

Many earth moving pieces of equipment, farm equipment and now even zero turn lawnmowers use hydrostatic transmissions, which is really just a fancy way of saying hydraulic.
However I believe the monstrous stuff is all electric, maybe electric takes huge sizing to work the best?
Hydraulic works, but isn’t as efficient as direct drive, both trains and big Earth moving stuff the transmission is problematic as the power transmitted is so great, and electric and hydraulic requires no transmission, and I think that is why it’s used.
I suspicion that the US Navy actually paid for the development of electric trains, but perhaps the train came first

[Montanan]

Originally Posted by Chuck34

Engine cooling is pretty straight forward as long as the you have good water flow. Any problems are an easy fix unless the water jackets inside the engine are clogged with rust and corrosion.

Inboard/outboard...I've never heard of anyone who had switched from OB to IB say they missed their OB and wished they could go back.

Overheating ranks No. 2 in the list of most common problems with hydraulic equipment. Unlike leaks, which rank No. 1, the causes of overheating and its remedies are often not well understood by maintenance personnel

Why do Hydraulic Systems Overheat?
Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. A hydraulic system’s heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as:

PLtotal = PLpump + PLvalves + PLplumbing + PLactuators

If the total input power lost to heat is greater than the heat dissipated, the hydraulic system will eventually overheat. Installed cooling capacity typically ranges between 25 and 40 percent of input power, depending on the type of hydraulic system.


The hydraulic system requires a quite large cooling heat exchanger which adds significantly to the system resistance of flow of the hydraulic fluid and thereby reduces the efficiency of such hybrid drive schemes and note that there is a second pump to bring in the cooling water, or in the alternative a fan / blower to bring in ambient air for cooling which is yet another parasitic loss.

On a vessel the cooling exchange is typically done via the water and not ambient air so the temperature of the coolant is relatively constant.

The average power conversion efficiency of total hydraulic systems [engine / pump / turbine impeller] is about 21% but then one has to reduce that percentage yet again by the inefficiencies associated with the propeller of the vessel. I would hazard a guess that one is doing well if achieving a system efficiency of 14 to 16%.