Pod Performance in Rough Water

Does anyone know how pods perform in rough seas. I am interested in Zeus Pods specifically, but the question of performance with IPS is still valid. I can't find any info about this on any company website or literature. My hunch is that they do lose some stability and might have a tendency to surf down the back side of waves in a chop, as apposed to traditional shaft drives that really "dig-in" in any wave conditions. Another hunch, is with the weight distribution change, from the strut on a shaft drive, might make some boats more stern heavy, as apposed to the weight balanced amidships in classic shaft drive configurations. This might make the pods have more "dig-in" ability as they do sit deeper and the load is balanced there. In terms of performance in variable conditions, this is where I see the IPS, being forward facing, and Zeus, rear facing in a tunnel, might perform differently. Because Zeus is in a tunnel, it would have less "dig-in power" than the IPS.

Any thoughts?

 

I'm not sure what you mean by perform in rough seas, you need to define this more....since their function is ostensibly no different to any other typical outboard installation, except if it is in a recessed tunnel in the hull, but not all are. Some are just 'below' the hull per se.
As shown in this 'typical' installation:
http://marine.cat.com/cda/files/1131285/7/Cat Marine Zeus Pod Drive Brochure.pdf

 

If by Pod you mean a transom extension of say 18 in on which the outboard hangs ..then yes I have experience with a stabicraft 5.6m made in newzealand ... www.stabicraft.co.nz .....when I came to sell it at 3 years old got a lot of people ringing saying had the welds cracked where the pod joins the transom .....well may be because I did get unexplainable water in the hulll ....these are used as rescue craft and for the filming of big willy ......should be top spec ..plate hull ..7 water tight compartments ....so I think there is a lot of stress on the pod .....Have changed to something smaller .....ha ha

 

"...a lot of people ringing saying had the welds cracked where the pod joins the transom.."

Only two reasons for that
1) poor load paths, ie the structural design wasn't "man enough"
2) Fatigue.

Of course ignoring the obvious like major strikes etc

 

I dont think the computer driven welder was at the wrong ange as it seemed to have more penitration into one of the 6 mmm sheets ...had only been used at sea a couple of time ...those with problems were going 50 miles offshore every day and had had it re welded twice !!! oops

 

Re-welding it will just make it worse.

If these are continual problems, then it is just poor detail design. The design load case scenarios have not been ascertained/established and as such the expected forces which are shirked from the pod to the hull have not been taken into account. Then, on top of those 'static' load cases, it needs to be fit for purpose, ie welded joint in sea water and fatigue. The static design stress is reduced by around 90% in such an environment.

 

I think the IP was concerned with the Cummins and Volvo pod drives that project below the hull just forward of the transom. They can be independently controlled for maneuverability, and supposedly if one hits a log or somesuch at speed, the pod will break away without holing the hull. The Volvo IPS has forward facing propellors, while the Cummins Zeus looks more like a conventional outdrive lower unit with rearward facing propellors.

 

Narwhal is right in that the IP was referring to steerable pods under the aft hull.

I also don't know just what he mans by "digging in" in waves or surfing on the backside of waves. Must mean surfing on the frontside of waves. I doubt that a skeg and shaft drive will respond as well as a pair of pods from either Volvo or Mercury. Both are very responsive and can direct steering thrust much more effectively than a skeg and rudder when the bow digs into the backsideof the wave ahead. In fact, that is when the skeg is least effective since the boat slows down just as it hits the wave ahead.


I assume the question is how to best avoid a broach in following waves. In my opinion, an outboard, outdrive, IPS or Zeus pods of adequate power should be much safer than a skeg and shaft drive in such conditions.

 

Having surfed down the front side of waves on a conventional prop driven boats. I would think the zeus drives would do ok, plenty of surface are to help track down the wave when surfing.

With a prop/rudder configuration, steerage is maintained even if vessel speed accelerates beyond what the propulsion package can take credit for. I.e. When I was 18 I dropped out of college and bought a big bristol bay gilnetter, flew up in march dropped it in the water and headed for false pass. Long story short, 12 miles off shore with a 55 sw coming out of the bering an occasional gust much higher. water depth 12 fathoms, sea height 20' + on a conservative side, with an interval that didn't seem possible. I cruise at 8knots and was hitting 17 surfing down the backs/fronts of about every third. Only broached and got the juices flowing 3 maybe 4 times In the longest port moller/ false pass trip I can remember. Was greatful to have a big rudder and lots of reserve power.

From all the pictures I can find of the zeus they have quite a bit below the waterline to help the boat track. If you are surfing and broaching the best
bet is to slow down, and utilize your reserve hp to assist when you hit the next wave and need to accelerate out of the hole so to speak.

Jdging by the pics of all the boats they are placed on, I would think that if you were in a storm where surfing was going on, there would be larger problems than surfing.

The mental gymnastics of how it would preform are fun. Best solution is probably to stay the heck out of storms that create waves that you surf down. Took me a while to learn the old adage stormwarning+small boat= don't go fishing.

your mileage may vary

 

When I say perform in rough seas, I mean the vessels ability to make head way safely and at a decent speed while maintaining a steady, dry ride. With Shaft drives there are big props that are almost constantly in the water in moderate to rough seas propelling you forward, "digging-in", and therefore keeping the boat stable. I am wondering if in certain sea conditions, following seas especially, if the Zeus Pods have sufficient distance from the hull to be able to support and create a steady, dry ride. My hunch is that in a following sea, there is a possibility the pods might be completely out of the water during the time that the boat is riding on the top of the wave. With traditional shaft drives this doesn't happen because they do sit relatively far from the hull, allowing them to be in the middle of the wave in most seas (following, quartering, etc.). If the pods were ever not in the water, it would create lots of noise, most likely be not good for the drive systems, and possibly create a loss of control.

 

"...With Shaft drives there are big props that are almost constantly in the water.."

This implies that the boat is driven 'within its limits' in rough seas. So, why would you do anything different with a pod driven vessel?

Digging in, in your defintion just means they are working as they should be...pushing the boat forward!

Also "rough seas"...covers a multitude of definitions.

 

In conditions YOU can stand:

like any other propulsion!

 

Fully agreeing with Ad Hoc and Apex1 above, I might add that there is a slight technical difference in that the (leasurecraft) pods are restricted in terms of propeller diameter. The Volvo and Zeus pods are working with very high propeller disc loads, that can make them more sensitive to flow disturbances.

Take for instance the situation when a vessel in a rough following sea is trapped in the through between two waves. There is a huge amount of air in the water as a result from breaking waves and the vessel coming surfing down. Add to this the counterrotation of water particles leading to a moment of very high thrust and low apparent advance ratio. In this situation a bigger diameter prop may not loose its grip, while the small dia pod prop will need a rpm reduction in order to get rid of the air collected due to the centripetal action of prerotating fluid.

But note that this is NOT a matter of pod or straight shaft arrangement; just a design limitation with two of the actors on the market.

 

I took my Zeus boat up the west coast of NA from Mexico to Canada and back this year. I did not have the opportunity to run in "rough" conditions. 30 knots of wind and 10-12 foot seas only (upper end of small craft advisory conditions). Apex is correct, the drives work just fine with no control problems as any speed that the crew is comfortable with.

Going north we had a bit of bumpy water where we reduced speed and altered course about 10 degrees to make the motion more comfortable. No big deal really.

Coming back south we had two periods where we were in 6-9 waves with 25-30 knots on the quarter. From our cruise speed of 9.2 knots the boat surfed to 15+ regularly and we hit a peak speed of 18.6. The autopilot did a better job of controlling the boat than we could. We never felt the boat was in danger of broaching.

As others have noted, the Zeus system pods have a decent lateral area. At docking speeds in neutral they are acceptable rudders, about the same as conventional planing hull rudders (too small at low speed).

The Zeus drive back down with more control than conventional drive/rudder combinations on other planing V hulls I've driven.

 

RHough,
Where are you in Mexico? I am currently in Puerto Escondido and will be heading to La Paz in January.
Chuck