4JH80 SPP Steerable Propulsion Pod: Feedback?

Wondering if anyone on this forum has one of these units in a displacement hull and could provide feedback on how they are to live with ?

I have been looking at a troller style vessel and wondering about optimization of prop / rudder appendage drag. Since a displacement motor yacht is operating in primarily 2 modes (low speed maneuvering and cruise at 6-7kt) it seems a conventional rudder is not ideal for either condition. Low authority at low speed and higher drag at cruise because of the high speed slipstream off the prop 24/7/365.

I was wondering whether using a steerable saildrive (to get best maneuvering) combined with dual outboard rudders which do not run in the prop wash might be a bit more optimized for cruise drag ??? Since cruise drag has the biggest impact on range and the amount of fuel to be carried.

I know that the engine in this pod is one of the new common rail engines, so fuel consumption and noise should be much reduced compared to older mechanical injection motors.

What are your thoughts ?

 

The rudder is most effective in the prop wash. Why do you see that as a problem?

 

Would a rudder that is outside the prop wash not have adequate authority at cruise speed of 5-7kts ? That would seem to be how all sailboat rudders work when the motor is off ?

The prop wash over the rudder is needed only when maneuvering, at cruise speed the higher speed prop wash over the rudder is only increased drag and fuel consumption.

Im investigating if I can come up with a propulsion/ rudder configuration that is more optimized for both operation modes and not the old standby which is not optimized for drag.

The simplest and least expensive configuration would be a conventional shaft drive system with 2 rudders which are outboard of the prop wash. Combined with hydraulic bow and stern thrusters for extra authority in maneuvering. The rudders could be fairly neutrally balanced and have an airfoil profile for reduced drag. They could also be splayed outward in a similar manner to bilge keels so that each rudder is close to vertical with the vessel rolled to one side or the other.

Next up the list is a steerable pod drive, so that eliminates the stern thruster but adds a considerable amount of complexity to the drive system with 2 90 degree gear sets, extra seals and gear oil, the rubber diaphragm that keeps the sea out and the independent pod steering system. I dont care much for the complexity and potential corrosion issues. If the steering system on the pod were to go out, I dont know what options one would have to manually center the leg so one could keep motoring ?

 

Can you provide any evidence that "the old standby which is not optimized for drag"? If your statement was true, every race boat, which is optimized for least drag would be a wrong design. As I routinely tell people who claim their ideas are better because they are different, square wheels are novel but totally suck.

 

Gonzo, I think you would agree that planing racing powerboats have very different needs to economical long range powerboats. The relative sizes of the props and rudders for either would not be comparable.

If one compares racing sailboats, again apples and oranges since most sailboat racing rules don't allow motoring. So for sailboats generally, the engine comes into play when back in port or when there is a need to make headway directly into the weather (trapped against a lee shore).

The fact that a lot of very elaborate work is done to optimize the propeller and rudder of commercial cargo vessels, for which fuel is the primary operating cost, suggests to me that even on much smaller displacement vessels there "could" be significant gains to be realized with the prop/rudder configuration.

I have recently read a paper on just such a project I think it was with MAN, I will attempt to find it and upload it here.

OK, it was a retrofit project. Just optimization of both prop and rudder with majority of rudder unchanged.

 

I must support Gonzo's scepticism here; there is no general evidence for the cited statement. On the contrary, there is massive evidence for a positive influence of a well designed rudder in the propeller jet stream. The rudder is functioning as a stator vane, that redirects the swirl, thereby creating a lift force on the rudder. This force has a forward pointing composant when the rudder is correctly designed together with the propeller.

Now, in the case of the sail drive, the rig in front of the propeller works as a "non-swirl-straightener"; it still has some rudder effect when maneuvering. It has the effect of redirecting the incoming flow into the propeller disc when turning, which is positive for the propeller efficiency. But the main steering effort comes from the azimuth thrusting of the prop itself. The conclusion is thus: there is nothing to be gained by adding rudders here, particularly not outside the propeller jet.

 

Finally found the video I was looking for. Its called a "gate rudder". Claimed 14% fuel saving compared to a conventional rudder.


Paper study prior to build and sea trial
https://pdfs.semanticscholar.org/b0d2/a961fe4581f5ef577bbf21ddd87506e92626.pdf

 

If the claim was true, every commercial ship would be using it. That would save them about $9,000 a day on an average cargo ship.
Fuel Costs in Ocean Shipping - More Than Shipping https://www.morethanshipping.com/fuel-costs-ocean-shipping/

 

So this is a fraud by the Japs and a UK university ?

 

Scholastic claims are not necessarily fraudulent. Very often they extrapolate from experimental results, which is not realistic in normal operating condition.

 

In the video they claim 30% fuel consumption reduction in actual service, greater than the 14% measured in the sea trial.

 

The Motorship | New ducted propeller design offers fuel savings https://www.motorship.com/news101/ships-and-shipyards/new-ducted-propeller-design-offers-fuel-savings

"The world’s first Gate Rudder system was fitted to the container ship Shigenobu, which entered service in 2018. The voyage data from the vessel’s first nine months of service reveal that this recently patented system could provide energy savings of as much as 30 percent, compared with the voyage data of an identical container ship Sakura, fitted with a flap rudder. The results suggest the innovative Gate Rudder arrangement holds significant potential to replace the conventional propeller-rudder systems to achieve a step change in EEDI compliance."

 

There have been twin rudder arrangements in use long before this setup, I actually wonder what the patent is covering. In fact, functionally the "Gate rudder" is just a slightly refined variant of the old "Kitchener rudder". One of the properties of this is the ability to influence the propeller flow at different propeller loadings; it acts as a "variable nozzle".

Now, if a nozzle will work or not (ie increase the propulsion efficiency) is a function of the balance between shaft power, shaft rpm and advance speed as expressed by Taylor's classical loading factor Bp [=(power)^½*rpm/Va^2.5]. Highest efficiencies are associated with low Bp-values without additional paraphernalia. But when functional design limitations lead to increased Bp, some kind of flow guidance, like nozzles, twin rudders or stator vanes may compensate for the increased losses to some extent.

This is what you see in the voyage data here; but be very careful about the basis of comparison. There is no info given as to the actual propeller loading of the benchmark vessels. If they have design issues limiting, f.i. the propeller disc area, there is an inherent non-optimum Bp, which may benefit from flow guidance equipment. The remarkable "improvements" reported may thus be based on low starting values for "the naked propeller", not necessarily resulting in exceptonally high efficiency in total.

In this world, you have to be very careful when "new, phenomenal" equipment is presented with big fanfares. Statements regarding general 30% improvement in fuel economy without information about the base-line, should cause some bells ringing loud and clear.

Back to the original thread subject: could this technology improve efficiency and/or steering in an azimuthing sail drive? Efficiency-wise it depends on propeller loading in the actual installation. Generally, Yanmar have been designing for quite big propeller diameters, i.e. low loading, corresponding to high propulsive efficiency. So: naah the increased drag from the added area will destroy the efficiency of the low Bp-propeller.

Ok, what about the steering then? Rotating the drive will generate full propeller thrust in any horizontal direction; no rudder can compete with that!

 

The claims are always in the same language. They use words like "may" and "could" which means they don't have data to convincingly proves their claims.

 

The 2 ships, identical except for the rudder have only been in service since early 2018, and the data reviewed was for the first 6-7 months of service over the same route. I thought that a 14% improvement in economy would be very good and worthwhile incorporating into a new long range vessel. One wonders whether the 30% number incorporates a slightly slower cruise speed, since that would have a huge impact.