Twin Engines/Twin Rudders on a Mono?

[Dockhead]

What do we think of this?

Some of the gorgeous and obscenely expensive Laurent Giles designed Moodys of the '50's and '60's had twin engines.

I have always envied cat drivers for their redundant propulsion.

Advantages:

1. Redundancy -- safety.

2. Maneuverability.

3. Better loading of the engine and economy at cruising speed (run on one engine).

4. More power when you really need it.

5. Better loading of the engine when you use a main engine as a generator.

6. Matches up better with twin rudders.


Drawbacks:

1. Cost

2. Bulk -- need bigger engine room.

3. Weight

4. Props not protected by the keel

5. More drag from two props.



What do we think about this? I'm thinking it might actually not be a question of two engines vs. one engine but two propulsion engines versus one propulsion engine and one generator. Especially if you have LiFePo batteries, I think you can easily give up the genset if you have twin propulsion engines. Just put a really large DC generator on each of them. This is more versatile and safer -- both generation and propulsion are fully redundant, which is very appealing. Deleting the genset greatly mitigates the cost and bulk issues.


We are talking about a 60 to 65 foot custom metal boat made for rugged high latitude sailing.

[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.

[UNCIVILIZED]

Some of the downsides are that you'll catch more crab pots via the prop & rudder locations. And twin rudders can be a pain in the arse unless the hull form necessitates them. Such as a wide flat stern, which when heeled would cause a centerline rudder to lift out or lose it's end plate where it meets the hull.

You could go with 2 power plants of different sizes, much as Laura Dekker did. So that the smaller one just sips fuel, & is sufficiently sized to power you over great distances in light air. Along with operating at close to full load driving auxilliary components when not in propulsive mode.

On her boat the 2nd, smaller, prop shaft was slightly offset from the main one, & didn't protrude so much as to be a full on crab pot catcher.
It's another option anyway. Though, yes, you lose the parts redundancy feature if yo go this route.

What'll work (best), & what won't work well will be much more apparent once the role that the boat is to fill is defined.

[Pete7]

Why not add twin lifting keels?

Two engines leads to a corridor down the center of the yacht and the engines mounted each side. Don't Beta do an marine engine with genny attached?

http://betamarine.co.uk/portfolio-it...id-propulsion/

Pete

[Dockhead]

Originally Posted by UNCIVILIZED

Some of the downsides are that you'll catch more crab pots via the prop & rudder locations. And twin rudders can be a pain in the arse unless the hull form necessitates them. Such as a wide flat stern, which when heeled would cause a centerline rudder to lift out or lose it's end plate where it meets the hull.

You could go with 2 power plants of different sizes, much as Laura Dekker did. So that the smaller one just sips fuel, & is sufficiently sized to power you over great distances in light air. Along with operating at close to full load driving auxilliary components when not in propulsive mode.

On her boat the 2nd, smaller, prop shaft was slightly offset from the main one, & didn't protrude so much as to be a full on crab pot catcher.
It's another option anyway. Though, yes, you lose the parts redundancy feature if yo go this route.

What'll work (best), & what won't work well will be much more apparent once the role that the boat is to fill is defined.

Well, the role of the boat is well defined in the other thread -- in two words, high latitude, long distance adventure cruiser, which is supposed to have very, very good sailing performance but is also supposed to be a very good motorboat. 60' to 65'. Metal.

One other configuration I think about is similar to what you are suggesting. A single main engine -- probably a 150hp Yanmar, which is big enough to drive the smallest Hundsted propeller I lust after. It would be centerline mounted.

Plus a "wing engine" which would be just big enough to drive the boat well in calm weather at 75% power. Maybe 40 horsepower?

The wing engine would also be the main generator -- attached to a large DC generator (but I would put one of these on the main engine too).

The wing engine would drive through a fully retractable shaft and fixed prop -- a la Open 60. Big advantage of this is it would catch no pots and cause no drag while retracted. Downside is you can't put a very big wheel on one of those.

Downside compared to regular twin engines and props is you lose the maneuvering advantage, lose the shared parts.

[poiu]

There are other options as used by motor yachts.

A main engine and a wing engine, similar to what Uncivilized mentioned. That is how Nordhavn trawlers often do it.

A main engine and a generator which is able to power a motor on the main shaft. This is how Selene trawlers often do it.

The wing engine has less drag than equal twins and could possibly be retractable, so pretty small. If you make it a genset or a combined hydraulic power pack genset then it earns its keep better. It would save having a hydralic power pack and a bow thruster possibly, though 2 engine motor yachts often have them too. If winches were hydraulic then you save a bunch of heavy motors. Downside from hydraulics is oil leaks and hose bursts. Can be very messy.

A reasonably sized genset is needed for it to work and if you give appropriate attention to power requirements only the smallest genset is needed, so it may be hard to make this work without oversizing the genset. Note the weight of the genset is less than half in the engine, so your weight comparison of engine and genset isn't comparing like with like. A genset and engine with large alternator should be lighter than two engines with twe genset heads and two prop shafts, gearboxes, couplings and props.

With the twin equal engine set up you can have folding instead of feathering props and that will give more protection for them. A mini skeg before props is sometimes used too for further protection.

With the twin powering engines or engine/gensets you could also use them as twin hydraulic power units for redundancy. A hydraulic engine driven pump is small and light.

I think you need to know how big the extra drag is as a factor from two props. Likewise the weight and cost issues from all options are important to lay out. A best option may jump out from that.

Really, I can't see a clear good choice. Every option has pros and cons.

[Dockhead]

Originally Posted by Pete7

Why not add twin lifting keels?

Two engines leads to a corridor down the center of the yacht and the engines mounted each side. Don't Beta do an marine engine with genny attached?

http://betamarine.co.uk/portfolio-it...id-propulsion/

Pete

Yes, I think about lifting keels.

And I think about twin keels also.

This is getting into the territory where the NA is already needed -- it's getting over my head technically.


I like all the stuff Beta make, and I am familiar with their hybrid gear.

Attaching a DC generator is not rocket science. A big question is belts? Or layshaft?

[Dockhead]

Originally Posted by poiu

There are other options as used by motor yachts.

A main engine and a wing engine, similar to what Uncivilized mentioned. That is how Nordhavn trawlers often do it.

A main engine and a generator which is able to power a motor on the main shaft. This is how Selene trawlers often do it.

The wing engine has less drag than equal twins and could possibly be retractable, so pretty small. If you make it a genset or a combined hydraulic power pack genset then it earns its keep better. It would save having a hydralic power pack and a bow thruster possibly, though 2 engine motor yachts often have them too. If winches were hydraulic then you save a bunch of heavy motors. Downside from hydraulics is oil leaks and hose bursts. Can be very messy.

A reasonably sized genset is needed for it to work and if you give appropriate attention to power requirements only the smallest genset is needed, so it may be hard to make this work without oversizing the genset. Note the weight of the genset is less than half in the engine, so your weight comparison of engine and genset isn't comparing like with like. A genset and engine with large alternator should be lighter than two engines with twe genset heads and two prop shafts, gearboxes, couplings and props.

With the twin equal engine set up you can have folding instead of feathering props and that will give more protection for them. A mini skeg before props is sometimes used too for further protection.

With the twin powering engines or engine/gensets you could also use them as twin hydraulic power units for redundancy. A hydraulic engine driven pump is small and light.

I think you need to know how big the extra drag is as a factor from two props. Likewise the weight and cost issues from all options are important to lay out. A best option may jump out from that.

Really, I can't see a clear good choice. Every option has pros and cons.

Thanks; this is very interesting.

You have inspired me with the thought that I should be studying trawler practice -- that's a hot tip which had not occurred to me.

I will start looking at Selene trawlers -- hybrid drive with a separate generator could be interesting and efficient. I wonder if the inefficiencies of the mech-elec-mech conversion offset the advantage of using the smaller genset engine? Maybe not if you're driving a really big wheel designed for the regular main engine. As I understand, that is the big advantage of diesel-electric drive on some big ships (including my friend's 60 meter ex-military motor yacht) -- because of the torque, you can drive a much bigger slow turning prop.

As you say, tradeoffs wherever you turn. But it's fun!

Concerning the problem of oversizing the generator -- note that this problem is far easier to solve, if you have LiFePo batteries which have almost unlimited charge rates.

If you air condition, then you face the problem of generating continuously, but without air conditioning, you can mostly live on battery power with regular charging. I will have an extra large battery bank (probably 12 or 15 kW/h) and a gang of inverters, so the generator won't need to be matched to my consumption -- just to my charging capacity.

[a64pilot]

If you can get a transmission with a PTO, then you could drive the generator off of the PTO. An advantage if a PTO is when it's off, it's off. Generator etc is not turning accumulating hours.
Then the Graduate level of a PTO would be maybe to run a hydraulic pump and from the hydraulic system run anything you want to, generator, separate propulsion shaft, windlass, etc.

I had no idea Laura Dekker's little boat was so advanced as to have a get home engine.

[Dockhead]

Originally Posted by a64pilot

If you can get a transmission with a PTO, then you could drive the generator off of the PTO. An advantage if a PTO is when it's off, it's off. Generator etc is not turning accumulating hours.
Then the Graduate level of a PTO would be maybe to run a hydraulic pump and from the hydraulic system run anything you want to, generator, separate propulsion shaft, windlass, etc.

I had no idea Laura Dekker's little boat was so advanced as to have a get home engine.

Definitely, if you're going to run the generator from an engine which is used for anything else, you need to be able to disconnect it.

There are different ways to do that. With a belt drive, there are different kinds of clutches you can use. With a layshaft, it can be a dog clutch, which is simple, cheap, and 100% efficient.

Hydraulic drive for a generator sounds scary, but maybe I need to find out more about it.

I know that many workboats have hydraulic drive, so it can't be all that bad or all that inefficient. If the wing engine drove a hydraulic pump, I guess you could drive all kinds of different stuff from it, including a propeller shaft, and as Poiu mentioned, a mongo bow thruster. Including a mongo windlass/capstan. It's pretty intriguing actually.

Actually carrying on further with that thought -- why not drive a rotating drive leg with hydraulics? Then you get a steerable stern thruster in the bargain? I have been on old ferries in Finland that had steerable hydraulic legs as their main propulsion units, driven by a hydraulic pump driven by a Perkins 4-108. One of them was like 50 years old and apparently extremely reliable and good in service, according to the owner. Made in the U.S.A. Need to find him and ask him who made the gear.

We discussed retractable drive legs here: http://www.cruisersforum.com/forums/...ve-174581.html


Maybe this? http://www.oceanyachtsystems.co.uk/t...ters-hydraulic

Wonder if it can be used for propulsion, and not just as a thruster? I guess 33hp would be decent sized wing engine for a 25 tonne yacht.

[skipmac]

I have long been intrigued with the potentials for a hydraulic system on a boat but have never used anything hydraulic beyond some farm equipment IE tractor, log splitter, etc so not sure about all the potential downsides on a boat.

Some potential advantages

- Two identical engines for all the advantages already mentioned especially redundancy and duplicate parts.

- Engines can be mounted anywhere, any orientation, sound insulated and on isolated engine beds to reduce noise and vibration.

- Can run hydraulic pumps off both engines but have only one prop so can use one engine for motoring or two for the extra power.

- Can run hydraulic to all sorts of auxiliary items like generators, thrusters, pumps, winches.

- Several advantages to mounting the prop and shaft.
Can mount the shaft lower in the boat so shaft can be more horizontal and work more efficiently.
No issues with engine to shaft alignment.
Can use a shorter prop shaft.

Disadvantages

- Lose the dual engine maneuverability but can offset that with thrusters.

- There will be power loss in the pumps, drives and hoses so less efficient, fewer mpg cruising, etc

- Have heard comments that hydraulic drives can be noisy but no experience to confirm or deny this.

- Have heard comments that the hoses and fittings can be leaky. This is not born out by my experience.

[Dockhead]

Originally Posted by skipmac

I have long been intrigued with the potentials for a hydraulic system on a boat but have never used anything hydraulic beyond some farm equipment IE tractor, log splitter, etc so not sure about all the potential downsides on a boat.

Some potential advantages

- Two identical engines for all the advantages already mentioned especially redundancy and duplicate parts.

- Engines can be mounted anywhere, any orientation, sound insulated and on isolated engine beds to reduce noise and vibration.

- Can run hydraulic pumps off both engines but have only one prop so can use one engine for motoring or two for the extra power.

- Can run hydraulic to all sorts of auxiliary items like generators, thrusters, pumps, winches.

- Several advantages to mounting the prop and shaft.
Can mount the shaft lower in the boat so shaft can be more horizontal and work more efficiently.
No issues with engine to shaft alignment.
Can use a shorter prop shaft.

Disadvantages

- Lose the dual engine maneuverability but can offset that with thrusters.

- There will be power loss in the pumps, drives and hoses so less efficient, fewer mpg cruising, etc

- Have heard comments that hydraulic drives can be noisy but no experience to confirm or deny this.

- Have heard comments that the hoses and fittings can be leaky. This is not born out by my experience.

I think I might be willing to take the plunge with hydraulics. They are so ubiquitous and so well developed, the parts are so generic, and the systems are so easy to get repaired in case of need. Basic farm technology -- that's the kind of technology which is good to take to sea.

I wonder how much efficiency is lost in a hydraulic pump/motor setup for propulsion?

[canyonbat]

Here is one with twin engines and twin rudders and a lifting keel!

https://www.sailmagazine.com/boats/b...seaward-46rk-2

[a64pilot]

I think a decent generic hydraulic system is about 85% efficient.
I believe a mechanical transmission is about 90% efficient, it's not close to 100%, if it was then it wouldn't need a cooler, there would be no heat.

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

Hydraulics main drawbacks are $$$ and complexity.
However hoses are very easy to build yourself, just need spare hose, fitting can be re-used if you use the more expensive ones from the beginning, sort of like rigging I guess.
Majority of hydraulic repairs are hoses and seals, both pretty primitive as far as mechanical aptitude is concerned, just keep the stuff accessible.
Small, very high pressure pumps can be noisy, but if you can drop the pressure some I think reliability goes way up and noise way down,
Aircraft, at least the ones I worked on ran 3000 PSI systems, but size and weight were huge drivers there, Farm tractors may run under 1000 PSI?

However assuming a hydraulic back up, likely a big boat would run hydraulic furlers, windlass, thrusters, steering, autopilot etc.

I have seen hydraulic furlers on a 40 ish boat myself.

[skipmac]

I think I might be willing to take the plunge with hydraulics. They are so ubiquitous and so well developed, the parts are so generic, and the systems are so easy to get repaired in case of need. Basic farm technology -- that's the kind of technology which is good to take to sea.

Exactly. This technology is the standard for farm equipment,
earthmovers and even commercial fishing boats, all applications where reliability, simplicity, ease of repair are critical. So repairs should be easy and available almost anywhere.

I wonder how much efficiency is lost in a hydraulic pump/motor setup for propulsion?

If I recall from my research which was some time ago the systems lose 10-15% so not trivial.