Retractable Marine Drives

Many sailors would like to have a 'retractable propulsion system' to eliminate that excessive drag of conventional fixed systems. There have been a number of different systems worked on in the past few years, but mostly custom systems that have not reached any production status.

I even suggested one utilizing a 'rim drive propulsion unit' ,
http://www.runningtideyachts.com/dynarig/Section_8_and_Rim_Drives.php

.....but development efforts on these rim drive units dropped off significantly during the depressed yachting market following 2008.
Just to clarify, the is a destination between the '(bow)thruster units verses the propulsion units. The thruster market has progressed along, but little work has been put into the propulsion units.

I was just doing a little research on retractable propulsion units and ran across this mention:
http://www.shippingandmarine.co.uk/article-page.php?contentid=15939&issueid=454

Anyone know where we might find some photos and more details on these units?

 

I find space on boats sometimes limits a lot of good ideas. Even on large yacht space is at a premium. It is far easier to let it just hang out than take valuable engines room space. There has to be a significant benefit for the usage of the space.

 

Well, there's always the Mirage drive ;-) :

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I wonder if one (or more) of these could be retractable:
http://www.voith.com/en/products-se...sion/vsp-voith-schneider-propeller-10002.html
These are not new and quite a few are in use, though I'm not sure they are made small enough. I wonder if they could be made small enough for human powered boats? I don't know what the linkage is like, but it seems to me if you could align all the vanes fore and aft, the drag ought to be pretty low even without retracting them.

 

I thought my concept was a minimum use of space, ....particularly if you viewed it from a profile view:



....and of course that was if they could produce a reasonable size rim drive PROPULSION unit

 

Piston Boxes for Retractable Rim Drive

BTW, the tops of those 'piston boxes' that housed the rim drive units were above the load waterline of the vessel, so the rim drive units might be extracted from the vessel for servicing even while the vessel was still afloat in the water.

 

Vitters Shipyard Retractable Thruster Pods

https://www.youtube.com/watch?v=GMhNtCpoA94

This was just recently posted on another forum.

I still have a few questions:
1) Why continue to utilize the 'thruster' designation?
2) How do they direct drive it?

Nice clean look, particularly with door closed while deployed as well as retracted.

 

I can understand the use for a drive with less drag than a feathering prop for a sailboat , or perhaps on the get home of a power boat, but, what purpose does retractable drive serve on a normal power boat?

Many canal boats have used wells over the prop for clearing lines .

 

Don't know that it serves much purpose on a powerboat?

Perhaps the steerability aspect of the Vitters unit might be an attraction.

 

BTW I just received an answer from OYS that their units were:, " Hydraulic in deployment, rotation and propulsion."

 

Hydraulic, they would have to be or you would have nightmare mechanicals. Also Hydraulic 20% power loss, what is efficiency of unit against hull in the stream. Somehow doesn't look power efficient for daily use.

 

"efficiency of unit against hull in the stream"
What is that?..... I don't understand your wording.

"power efficient for daily use"
I don't understand that either? Yes, I realize there are losses in the hydraulic transmission of power, but sometimes those have to be accepted if it leads to an acceptable overall design option.

BTW, you do realize that I was speaking of the hydraulics of the Ocean Yacht Systems unit rather than the Vitters one. I do not yet know details about the Vitters design.

 

If we get an old 200 hp diesel and put it on a shaft, with a good prop, we get X propulsive force. If we make ithe run a generator that then feeds electrical motor or hydraulics we lose say 20% in conversion. That is 40 hp lost to conversion. Now you only have 160 hp to propel boat. That is a huge loss.

Then propeller efficiency, we know what a modern prop is capable of, does anyone know how efficient rim props are?

 

Rim prop thruster data

Here's some good data.

http://www.marinepropulsors.com/smp/files/downloads/smp11/Paper/FA2-1_Yakovlev.pdf

 

https://www.youtube.com/watch?v=Te_xejpriFM

an interesting video with other links

 

The goal here is not that "sailors would like to have a 'retractable propulsion system' to eliminate that excessive drag of conventional fixed systems" but rather to have
a "system to eliminate excessive drag of conventional fixed systems.

Looking over many websites on retractable drives, they appear to be complex and extremely expensive. Adding in a large generator to drive the rim driven propeller, the energy efficiency drop can be significant.

Certainly there are some benefits of rotating pods but at a large fuel and expense cost

Instead, I would consider then an axial flow jet pump, with the intake mounted flush with the bottom of the hull. A direct coupled or engine above jet, (chain or belt driven)to optimize space. The only issue then is to then be able to close off the intake while underway which then takes care of "excessive drag of conventional systems"

There might be another way to even do away with having a hydraulically activated intake blocking device but instead just a mechanical/hydraulic, manual activated plate that would cover the discharge of the jet. So the intake and impellors would be immersed. There would be some drag then between the relatively stagnant water in the intake area and the water moving past the hull. If Baekmo is around perhaps he might comment on this. Blocking the outlet side of the pump would be easier than a moving plate to cover the intake of the pump.

I had been told several years ago that Hamilton jet was experimenting with an intake area that could be altered on the run to optimize thrust. At higher speeds, the intake area changed to get more thrust.

With a rim drive on a cat, you need a large single engine or two, a single or twin generator (more cash and loss of energy efficiency) and an extremely expensive pod/rim drive/ steering mechanism.

And do not forget that when under power the retractable rim drive unit is still exposed below the hull making shallow water transit an issue

With two jets, you would need two smaller motor, no generator, two inexpensive jet pumps (probably axial flow, two or three stage) which might cost $3,500 US each.

The steering could be accomplished by the jet nozzle if the outlet was right at the back or depend on the regular rudders.

If the outlet was right at the back of the hull, the reverse bucket would allow thrust reversal for docking.

If the nozzle could not be effectively placed at the back of the hull, you could install a reversing transmission ahead of the pump, albeit at extra expense. I believe that a company called Moose Boats from the Pacific northwest of the US used a reversing transmission. I think the main reason was to flush any debris that got into the intake.