Twin Engines/Twin Rudders on a Mono?

[Dockhead]

Originally Posted by a64pilot

Maybe a hydraulic drive wouldn't have need for a variable pitch prop? Maybe RPM would be a function of load?

That could add back a lot of efficiency. Does anyone know?

Originally Posted by a64pilot

Hydraulics main drawbacks are $$$ and complexity. . . .

Really? Hydraulic systems seem pretty simple and cheap to me -- farm tech. I guess it's all relative.

[DeepFrz]

One thing to be aware of with hydraulic systems. You will need an engine running at greater than low speed (I don't know what speed exactly) in order to drive your thrusters while docking or undocking. You don't want a drive engine doing that. That is why many of the medium sized Norhavns run electric bow and stern thrusters. The newer thrusters, I understand, can take longer run times. Other boats run hydraulic thrusters with pumps on the main engine AND the get home engine OR the generator. Not likely in a motor sailer that you will need stabilizers so hydraulics are really not all that necessary

If I were contemplating such a boat as you are I would pay close attention to Nordhavn practices. Especially their motor sailors.

[Peregrine1983]

Interesting. I wonder if one electric motor/regen system and one diesel makes sense in this application.

[UNCIVILIZED]

There are a fair number of pleasure boats with hydrauliclly driven props. An acquaintence had a catamaran where both props were driven by the one pump. I think it may have been built by Ian Farrier. So as has been stated, you can mount the prop in many locations other than directly behind the engine. Some boats even have the prop in the aft end of the keel, & it's lined up to be dead parallel to the boat's WL.

Here's some info on Laura Dekker's boat, & it's for sale on sailboat listings too. LauraDekker.nl de Jongste solozeiler ter wereld! - â—¦ About Guppy â—¦
She's written a bit about how the smaller engine really extended her motoring range & efficency when the winds were nonexistent. This despite her enhance sail area due to larger rigs, & a Code 0 (which she loved).

Also, there are plenty of lifting keel, single or twin rudder, production & semi custom boats out there that are made out of aluminum. So there's little need to reinvent the wheel. Especially as building a boat from a blank sheet of paper is a monumental task, & requires barrels full of money.
One could modify a stock hull design to their own tastes, or even buy a pre-loved boat & have it rebuilt to meet requirements. There's no lack of quality aluminum boats that are a decade or three old, & in good shape. Though sometimes you have to hunt a bit to find them, & approach the owner directly about buying them.

[a64pilot]

I'm being argumentative, but a second motor is a get home motor. Good thing to have, no doubt, second source of battery charging etc.
However if you look at the BSFC which is brake specific fuel consumption of our little Diesels, you'll find it pretty flat, meaning that to make half as much power pretty much takes half as much fuel. There is no point to where you cut power in half, but fuel consumption only by 10% or so. Yes there is a point of max efficiency, but the graph is a gentle curve, not a pyramid.
If your after range, take the space and weight that a second motor takes up and put in an extra fuel tank, that will take you way further.
But you don't have a get home motor of course.

[Blue Stocking]

Robert Beebe told me, aboard PassageMaker in Bermuda, that he liked hydraulic because it allowed him to use all his power tools under water. Mind you, Dockhead's talking about ice cube country cruising.

[Lepke]

I farmed and commercial fished. I used hydraulics every day. It's powerful and reliable, but a motor to run a shaft at cruising speed would have large lines,a big pump and motor. Like diesel electric, it has a loss rate, too. Neither is as efficient as direct coupling to an engine.
When I was young, saw a number of old schooners with twins. Usually single rudder that looked to be original size. I think they were twins for reliability and because they had a smaller foot print than a bigger engine running a single prop.

[Panope]

I like the idea of identical twins with conventional shaft drives for the proposed sized yacht. However, if we are talk I think I would stick with a single rudder

[Panope]

I like the idea of identical twins with conventional shaft drives for the proposed sized yacht. However, if we are talking about a spade, I think I would stick with a single rudder for protection/fouling.

If we are talking about skew hung, then twin rudders would be great if the leading edge of the skeg has a gentle slope to deflect debris. Skeg would also fully protect the props. Might also get 'upright' beaching out of the deal.

I believe the efficiency losses of hydrolic drive are substantial and would drive me nuts.

Steve

[mikeod]

I have a question regarding hydraulic drive. I also may need to be educated on the subject. Isn't a standard hydraulic transmission just a pump and drive in one package bolted to the back of the engine with shaft attached. Wouldn't you just be separating the components.
Thanks
Mike

[seabreez]

Originally Posted by Dockhead

Continuing the thought --

I have two diesel engines on my present boat. A 100 horsepower Yanmar weighing about 250kg with the marine gear, and a 6.5kW Kohler generator weighing about the same.

So half a tonne of machinery.

Two somewhat smaller propulsion engines would weigh the same, give more maximum power (on two engines), and be loaded to a higher % of maximum at cruising speed (on one engine). Either one could be happy driving a 6 or 7kW DC generator head to quickly fill up the LiFePo batteries.

Other than props not protected by the keel, is there any big disadvantage to this setup? I like it more and more the more I think about it.

I have run twin engines (gas) for 31 years on a search and rescue vessel and we always go out in adverse conditions and many times early in the year and late fall we were the only vessel out there going on a rescue. The confidence running twins is reassuring. I never counted, but a few times we came home on only one engine. Running a vessel in sailing territory away from home or in a foreign country or on a long haul between ports is always a worry. On a second thought, diesel engines are very reliable. Good maintenance is the key to reassurance...

[a64pilot]

Originally Posted by mikeod

I have a question regarding hydraulic drive. I also may need to be educated on the subject. Isn't a standard hydraulic transmission just a pump and drive in one package bolted to the back of the engine with shaft attached. Wouldn't you just be separating the components.
Thanks
Mike

Hydraulic drive is a pump on the motor and a hydraulic motor wherever you want the drive force.
Nearly all Farm tractors and Bulldozers etc now are hydrostatic drive, which I think is hydraulic drive.
A good example of hydraulic drive is zero turn mowers, the John Deere I used to have was twin pumps and hydraulic motors, it was two separate systems so one couldn't contaminate the other.
There are of course efficiency losses, however it may be possible to recover those losses and the other benefits of hydraulics could offset the losses, plus this is a sailboat isn't it? I now more than ever understand the safety and luxury of having a dependable source of propulsion that is engine based so I'm not discounting it, but maybe the ease of being able to retract the drive mechanism for example may out weigh the losses inherit in the system?
If you had the money, I could see the possibility of a retractable, steerable saildrive made simpler if it were hydraulically driven.

[Sailmonkey]

Originally Posted by mikeod

I have a question regarding hydraulic drive. I also may need to be educated on the subject. Isn't a standard hydraulic transmission just a pump and drive in one package bolted to the back of the engine with shaft attached. Wouldn't you just be separating the components.
Thanks
Mike

A hydraulic gear on a boat is not a hydraulic pump and motor like is being discussed here.

The hydraulic gear is a very small hydraulic pump driven off of the input shaft. The hydraulic pressure is then utilized to operate the clutches in the same manner as a mechanical gear. The advantage is that the hydraulic gear can apply more pressure to larger clutch packs and therefore transmit more power to the prop.
There is no hydraulic motor in the hydraulic gear, when the clutch is engaged, the input shaft is coupled mechanically to the output shaft.

[mikeod]

Thanks I understand now, the hydraulic is just on the clutches I've always had mechanical gearboxes and didn't know.
Thanks

[KP44]

There were some monohulls made with twin keels. They are especially useful for grounding the boat and safely doing a bottom job during low tide.
Other than that, far increased wetted surface. ballast not centered but shared on two sides. Such ballast probably makes a tender boat. That is, reduced initial stability.

I personally don't see any advantage to a second engine. But to each his own. But why the twin rudders? Just a complex system with more that can go awry, and one rudder or the other will always be nearly unusable unless you are motoring. Again, much more wetted surface which is drag. If you really want the advantages of twin screws for maneuvering, I'd opt for a bow thruster instead of a second engine.
There is a reason there are so few twin engine, twin rudder, and twin keel boats. All decent ideas for certain issues but a disadvantage for most sailors.