Ducted Propeller on Retractable Outboard for Sailing Yacht ?

[Seabury]

I am currently at the preliminary design stage of my next boat and have a few ideas that seem quite clever to me, but might be completely bonkers to those of you with a bit more experience. Any feedback would be gratefully received, and if you have to be brutal, I think I can take it !

The concept is based on a folding keel (similar to the Dutch builder Jongert's concept for their larger yachts, but much simpler manual operation), twin rudder sloop with an outboard motor in a well at the aft end of the cockpit which is both steerable and retractable. Because there must be some form of plate attached to the bottom of the drive shaft / skeg to lie flush with the bottom of the boat when retracted, I thought I may as well use some form of ducting around the whole prop to increase thrust when manoeuvering and reduce aeration / cavitation when under way.

My questions are, has this been done, if not, is it because it is a dumb idea, and if it isn't, how does one go about getting specifications for the correct size, thickness, foil profile etc. of the duct to suit a given outboard ? I am initially drawn to a Honda 4 stroke about 25 HP - is this going to be low enough geared, have a large enough prop and have enough power ?

I want to go with a steerable outboard as I would use the engine to maneouvre in / out of the dock, up and down rivers and in emergencies, but otherwise try not to use power - it is a sailing boat after all ! It also frees up a lot of space where it is most useful - in the cabin, is easier to maintain, and means only one type of fuel is needed on board (dinghy outboard will be petrol too).

Initial design parameters of the yacht are LOA less than 12 m, LWL 9 m, Bmax 3 m, Total Displacement 8,000kg, Ballast 3,000 kg, canoe draft about 0.70 m, overall draft (keel down) 2 m.

This gives an extremely fine yacht with quite large overhangs - mainly for style, but also because I need room aft to mount a vane self-steering gear (if you are going to hang such a vulnerable thing 2 or 3 feet out the back, you may as well extend the hull around it to protect it and cover up some of the workings). The folding keel is because of the area I sail in now, the twin rudders are to allow the outboard to sit between them, reduce draft and provide stability when drying out. Construction will be some combination of strip plank / cold moulding / foam core with grp outside coat.

I look forward to being shot down in flames !

 

[Guillermo]

I don't see why theoretically you cannot add a plate to the end of the outboard leg, to keep hull fair when retracted. I do not know if it has been done before, but there are also other solutions available. Have a look at:
http://www.atomvoyages.com/projects/outboard.htm
(Although this one doesn't allow to steer the motor)
Cheers.

 

[FAST FRED]

Since the power is great enough to ignore the extra drag under power , I doubt if a fairing plate for motoring would be noticeable at all.

A faired hull piece locked in would probably help sailing in very light conditions.

A Kort ring or similar only helps pull at low speeds..

Picking an outboard that can swing a good sized prop would do more for thrust with no risk.

A 16 inch prop will eat 25 hp well , if you can find an outboard with enough reduction gearing.

The ability to steer the OB would be worth more when docking than an extra bit of push.

FAST FRED

 

[PAR]

Outboards make for lousy secondary sailboat propulsion systems on boats over a few tons. They have to be hung in the end of the yacht, which is a rotten place for the weight of a heavy 4 stroke, they don't like living in wells all that much and can compromise the accommodations in the cockpit. These aren't insurmountable issues. Of course, it's been done before and some successes recorded, but generally, you'll do better with a more conventional arrangement.

If you don't want to go the inboard/bow thruster route, then diesel/electric, hydraulic, electric hybrid could be of some use.

A retractable electric pod, with closing doors to seal the hole for sailing could be fitted. I suspect a steerable sea drive sort of device, maybe with folding or feathering prop could be engineered and a host of other options. When venturing out into the different types of propulsion systems, particularly the ones that have limited experience, you'll have more difficulty sorting out problems.

To directly answer your questions, sure you can have an outboard, in a well, that is steerable, is fitted with a vortex ring, retracts into the hull and of course has garage doors that will close off the hull as it leaves the water all in one kind of thingie. How much engineering energy, you want to commit to this device(s), will determine it's success.

 

[Seabury]

Thanks for the link, Guilllermo - basic idea of the installation on Taipan is the same as mine, but compromised because it is a retro fit. The website indicated that a fully steered outboard was the original aim but could not be made to fit in the available space - as I am starting from scratch I hope to avoid such a compromise.

Fast Fred - Totally agree with you about a larger prop being the best solution given availability of reduction gear etc., but a larger prop would have difficulty with mounting height and given even a slight swell, would probably break the surface. I was hoping that the duct / shroud would help here, and with the steering, but any benefit in thrust would be a welcome side effect, rather than the principle goal. I guess a pair of shaped plates top and bottom to fair into the hull when the unit is up or down would probably do the job, rather than adding the full duct ?

PAR - Yes, I looked at diesel electric hybrids, but the complexity puts me off, and the consensus (on this forum, at least) seems to be "watch this space" - great in principle, but needs more development. Also, most of this development does seem to be directed towards fat, high torque, shaft driven electric motors installed in the conventional position usually occupied by the diesel.

Even if the outboard in a well idea is flawed, it does at least rely on tried and tested technology with the only tricky bit being a complicated mounting system. But given that I will have twin rudders and a vane self steering gear to link up anyway, lifting / steering the outboard could be incorporated relatively easily. As to weight, I was hoping that it would be lighter than a comparable diesel inboard, and with judicial redeployment of other fittings, this would not increase the moment too much.

As for outboards in wells generally, are there any other problems apart from ventilation that I should be worried about ?

 

[FAST FRED]

As to weight, I was hoping that it would be lighter than a comparable diesel inboard,

True, but an OB is hard for distance cruising , no large alternator , no mechanical refrigeration , no emergency engine driven bilge pump.

FAST FRED

 

[PAR]

As designed, an outboard, slung under or hanging off the stern of a displacement boat, bash the water to a froth, as they struggle to propel the vessel to hull speed. The prop is ventilated across most of it's blade face and is operating in less then desirable conditions, because it isn't "eating" enough water to become efficient. This usually results because the props are small in diameter and heavily pitched for a particular speed, which may be several times the displacement speed of a sailing cruiser. It's sort of like asking a F1 racer to live comfortably doing 45 MPH, carrying several times the weight it was intended to drag.

You can re-prop the outboard, but will be limited to a diameter under it's ventilation plate. Even if you cut the ventilation plate off, the engine will still spin the bigger prop at small diameter prop speeds, likely cavitating the bigger prop, which will get expensive quickly.

Efficient displacement props, are bigger diameter and spun much slower then an outboard.

I designed a sailboat with a well mounted outboard last year and had similar difficulties. It wasn't tied to the steering, didn't have doors on the hull, nor a vortex hoop, but did attempt to address the other issues, typical of this installation. Ventilation was solved with a generous gap around the hatch cover, over the well. Heat will be ducted and blown out by a fan. I designed it to act just like an outboard boat, but the boats transom (rounded) extended past the engine's transom, enclosing it. Now this was a smaller boat then yours (20' LWL), but the principles apply. The hole in the bottom of the boat was above the LWL and shaped to offer as little drag under sail as possible. It still offers very little secondary power (12 VDC at 3 amps), no power take offs (fan belts) for other devices, etc. This little boat didn't require much in this regard, but a serious cruiser needs much more.

Believe me, we'd love to toss the old, cranky inboard over the side, in favor of a better system and they are slowly becoming available, but we need brave souls, to take on the challenge and prove the technology and concepts. Where's Howard Hughes when you need him . . .