Tunnel Drives + Thrusters versus Zeus and IPS?

How about waterjets?

I don't like waterjets as they're really only efficient at cruise speed and even at that not usually as efficient as propellors.

The zues and IPS are generally 25% more fuel efficient than traditional props and shafts as well, but they are a little more exposed than them.

Props and shafts in tunnels are still somewhat exposed to logs and such. I have never had an issue docking with them and neither has most Captains.

Don't forget surface piercing. Lots of choices. S.P. and jets have issues as pointed out. Another issue with them and with IPS / Zeus is the availability and cost of maintenance & repair. Mess up a conventional prop, shaft or strut and it can be fixed fairly quick and almost anywhere. Not so much with any of the other propulsion choices.

Tunnels won't help you all that much with submerged logs and such unless you're lucky enough for them to be below your draft, which is the main benefit of them. They enable you to run in more shallow waters. BTW, I've run a lot of boats with tunnels, and I've never had problems maneuvering. I think your friends may just need to practice more.

I think they are much more efficient than a propeller that hit a log...

Yeah, and they can be knocked off of the bottom of the boat just as well and it creates a huge hole in the bottom, such as with Octopussy. Maybe not as easily sheered off but it has happened.

With IPS or ZUES if you hit a log, the drive sheers off cleanly as it's designed to do with minimal breakage and there is no water intrusion. I have seen this happen on a yacht someone else was running. They hit a log in the ocean and sheered 2 of the 4 bolts, limped into port, dove under the boat and removed the bolts, installed new ones and were on their way. Problem is if the drive sheers off and you cannot recover it.

If you hit a log at cruise speed, chances are your propulsion will be the least of your worries. I think surface drives would have the best results in this situation because they are above the bottom of the boat and might get away un-scathed.

Tunnel props or pod drives

It depends on the size of the boat and the area of usage. For planning boats up to about 45 to max. 50 ft, where the builder will give you the choice between (Volvo) Twin Duoprop Z-drives and Twin IPS or inboard engine with conventional shaft and V-drive, I would always go for the VP Z-drive. The Duoprop Z-drives are exactly as effective as the IPS-drives and in combination with a bowthruster, the Volvo Penta joystick docking system works also with the Duoprop drives. The Z-drive saves even more space in the aft hull and in case of contact with the bottom or floating objects, the damage to the boat and drives is far less dramatic. The biggest disadvantage of the Z-drives up to now is, that neither Volvo Penta nor Mercruiser or Yanmar have developed steering systems and software for tripple or even quad installations. The only tripple installation, I have seen, was in conjuntion with tie bars for the center engine. Bud with this method, you are loosing the docking mode.

The higher effectiveness of an IPS or Zeus system (for the below the hull part) in comparison with a standard shaft and prop is mostly based on the better thrust vector being perfectly parallel to the flow plus some advantages of the counterrotating props. The lower fuel consumption is only based on the fact, that Volvo is taking the higher thrust into account with a 20 to 30 % smaller engine, i.e. Volvo IPS 1200 = the thrust of an 1200 HP shaft and prop system provided by a 900 HP engine. The weight of such a system is not much lower than a system with reduction gear, shaft, prop and rudder.

Example: Given a 85+ ft boat with twin MTU 16V2000 M93 (2.400 HP each) and ZF3060 or TwinDisc MGX-5147 plus shaft / prop and rudder system, the complete drive and steering system has a system weight of app. 22.000 lbs.

The same boat with quad Volvo IPS 1200 (equivalent thrust of 1200 HP but only 900 HP engines) has a system weight of app. 20.300 lbs. If you take the lower weight of the IPS version for more fuel plus the lower fuel consumption, you will have a far better range or more speed due to lower total weight.

On a boat with a 3600 HP setup, i.e. quad IPS 900, the better setup would be the tripple IPS 1200 as far as system weight is concerned (as done by AMG with his 80 ft design). And this system would be heavier than a conventionell system with twin MAN 12V-1800.

But you have to be sure about the water depth below. In uncertain waters, IPS is not our favorite system. Our retractable protection bar in front of the IPS drives is only helpfull at moderate displacement speeds. At planning speeds it would to rip open the hull and damage the IPS drives even more.

My son is half way through his second season with his quad IPS boat and had no issues with the engines and drives. Bud the boat is maintained by a full time bosun and the engines have an anual operating time of only about 350 to 500 hours. Average fuel consumption is much lower than a conventionell drive train, because of the smaller engines and at displacement speed, 2 engines can be switched off. My son would buy the same setup again, both the boat and the IPS. Will see what the future will bring.

Operating this big quad IPS boat is really easy. Docking and maneuvering in close quarter is not difficult at all. Even my grand son can dock it . Performance is great, very low noise and vibrations and no smell or soot because of the through drive exhausts. The special IPS autopilot works great and the after sale installment of the Interceptor Trim System was one of our best ideas with this project.

The only fear is hitting an hard obstackle at high planning speed, most likely the end of the boat or at least of everything in the watertight engine room. And using the DP mode with swimmers in the water is a no go. I would rather swim in the vicinity of sharks than close to the rear part of the hull with those 4 monsters operating in DP mode.

I have not much experience with planning hulls with tunnel design but our commercial (inland) cargo vessels with tunnel design are far less maneuverable when going backwards due to the tunnel design. But those vessels have mandatory Schottel pumpjets in the bow area with independent engines.

I think you are right, this boat is really flying and is highly maneuverable at all speeds...

Engine room size

A quad IPS installation is not that bad, as far as engine maintenance or repair is concerned. You cant take a bike tour around the engines but on our 21 ft beam boat, the mechanic can stand and work between the engines.

The engine room hosts the 4 x 13 Liter inline 6 engines with short axle and drive, 2 x Fischer Panda 25 KW Gensets, Chilled water AC, watermaker, all electricity lockers and boxes plus starter batteries and a large array of fuel filters (total of 12). And a little workbench .

The stern has a large engine room door, where the complete drive train can be pulled out by a crane. The only disadvantage is, because of the engine room being far aft and while retaining full standing height in the complete engine room, there is no tender garage possible. But the Williams Rib lives on the aft sundeck without any stability issues.

As we have had no grounding jet, it is most likely only a fear. The only thing we have been told is, that the shear off of the Volvo IPS pod drive is not as flawless and easy to repair as with ZF Zeus system. Hopefully it will never happen. I know, the Volvo Penta Duoprop is no problem in this regard.

Volvo Penta Z-drive

My wording might have caused some misunderstanding. We call the Volvo Penta Aquamatic system a Z-drive in Germany.

Since we are at it, here is our triple installation. There is plenty of space except above the mid engine, since we have a garage for a Williams 385 RIB. But the garage can be lifted out for major engine works or removals.

The draft is reduced a little as there is a flat recess for the mid IPS. (D=1700 mm).

It seems to this point a lot of the limiting factor has been horsepower per unit. Now that has changed a lot over the last few years. Now we're up to 900 hp or what Volvo likes to claim is the equivalent of 1200 hp on an inboard. To many of us the idea of having to have four engines to achieve 3600 hp isn't appealing, although in some ways 4 is easier than 3. Then combine that with the construction issues of designing a boat appropriately for them, or redesigning existing boats. Just having to put them all aft changes what we're used to, changes weight distribution, requires a lot of rethinking and re-engineering.

There was a time I would have said no to Zeus and IPS. Now it's slowly becoming where it depends on the boat to me. If there was a boat in a range I really liked and it had IPS, I would no longer dismiss it. On the other hand, I'm not to the point yet I'd seek it. At this point I haven't had to choose as I haven't been looking at the type boats that IPS is most often used in. If I was in a different area, cruising different waters at different speeds I'd consider them. Still in some ways they are fixing a problem I don't really see myself having as I'm quite happy with traditional drives. But I also think some boats with IPS are great. There are some builders who have taken the combination of their boats and IPS to new heights. They make it attractive. To me the worst thing that ever happens to IPS is when a builder just takes an existing boat and decides to offer it as another option without really designing for it. Hinckley and Hunt seemed to do that on their boats. But many builders have put a lot of effort into building great boats with Zeus or IPS.

As they evolve and the choices continue to grow, as bigger engines are added, I'd think the market is going to continue to increase.

Captholli, I think you are right, stern drive is the better english term for it. But we most likely can agree, that if a stern drive tears off, it will not make a hole in the bottom of the boat .

Pod drives

Volvo has versions of their IPS system available for semi displacement hulls and the drums are telling, they are working on a steerable pod drive for displacement hulls and an IPS 4 with their 16 Liter engine, means IPS 1500 / 1800 ?

But the pod drive will come to smaller displacement yachts also. As posted before, Schottel is downsizing both on their mechanical driven pod drives and on their electrically driven pod drives. Mainly designed as quad drives for river cruise vessels, they are very suitable for smaller yachts, where designers and naval architects would gain much more flexibility for the internal layout.

The problem, I personally have with mechanical pod drives (in our commercial applications) with long shafts and heavier engines in the center of gravity, is the neccessary bending of the shaft with several couplings. This setup needs care and perfect construction work and is trouble-prone (picture 1).

The small twin propeller with the engine gearless directly in front and at the same level as the thruster gear is perfect for a smaller displacement yacht. Very effective propulsion and maneuvers like the IPS system (picture 2).

The best system IMHO is the small electrical version the Schottel Combi Drive. The same effective twin prop with the electric motor directly mounted on the vertical shaft (and out of the water). A simple system, where the gensets can be placed at the designers discretion (picture 3).

More than 40 river cruise ships in Europe are using these systems in twin, tripple and quad configuration with great success (picture 4).