Mount a clean, quiet, fuel efficient 4 stroke outboard on the transom of your sailboat
Installing a retractable four stroke outboard on your sailboat
with a retractable mount that eliminates prop drag while sailing
by Adrian Biffen
The venerable old Volvo 2 cylinder inboard diesel was seized
when we bought Serenade, our 30' sailboat. I've never liked the smell of diesel fuel, or the exhaust emissions, or the vibration and noise of high compression diesel engines, so for me this was a good thing. I decided to convert the engine compartment into useable stowage space, and get all the noise and emissions out of the boat altogether by mounting a retractable four stroke gasoline outboard on the stern transom.
She was well suited to this application, having a forward slope on the transom of about 20 degrees, allowing me to easily raise the engine completely clear of the water, eliminating propeller drag while sailing. I also liked the idea of eliminating any electrolysis and fouling while at dock, since the engine would have no contact with the salt water when retracted.
I was concerned about upsetting the balance of the boat, since any weight applied away from the longitudinal center of buoyancy can affect the pitching motion of the hull (and other parameters), so I looked carefully at the outboard marketplace before choosing the Nissan 9.8 hp 4 stroke (with the long shaft and
remote control electric start options). I've had Nissan motors before and they have proved to be reliable, quiet, fuel efficient and well engineered (they make 4 strokes for Merc, and are the same engines, see them at the WestMarine.com
website). It has 'low oil', 'no oil', "over-rev" and 'overheat' warning systems, and a 3 star emissions rating. At 85 lbs, it was the lightest engine I could find in its class, so the impact on hull trim would be minimal. The fuel from the built-in tank is passed through a large fuel/water separation filter, with a replaceable filter element.
I didn't want to have the engine tilt, and I didn't want to have to wrestle with fiddly release mechanisms, typical of standard outboard mounts, that tend to jam just when you need to deploy engine power the most, so I set out to design a sliding 'car' system that would move the engine smoothly up and down the stern, holding it in a vertical position. I wanted it to be a gravity based system so that the retracting could be easily motorized later, with the ability to be cut loose to drop instantly in case of an emergency.
Another very important factor is the need to have the engine as deep and as close to the stern as possible, to minimize any chance of the prop coming out of the water, causing the engine to race and over-rev in rough seas - a common problem for many stern mounted sailboat outboards. This design approach accomplishes just that. One very nice related feature of this engine is that it has an over-rev limiter that automatically throttles back the engine and triggers an audible alarm and trouble light if does exceed maximum RPM. I haven't had this happen from the engine leaving the water, but it did happen once when I was motor sailing and the wind picked up considerably, accelerating the boat and suddenly taking the load off the engine (still in the water), while running at maximum cruising speed (6 knots).
I tried several approaches that worked reasonably well, such as a plywood board sliding in 2 parallel aluminum 'u channel' sections (in the picture above), but the biggest problem I ran into was the rotational torque that exerted a twisting force on the mount, even just from the weight of the engine at rest (not to mention the additional torque applied when it was actually pushing the boat). This factor created too much friction for it to slide down by itself and I was constantly having to push it down with my pike pole to get it into the water - not a great thing when you're under sail, single handing. It also created very heavy loading on the assembly when underway, and I had to reinforce it to prevent it from failing. Retracting it was never a problem, as I used a standard boat trailer winch for lifting it that had plenty of pulling power.
I finally realized that I had to throw some money at it and buy a suitable frictionless bearing system to overcome the sliding resistance caused by the unbalanced weight. I wanted something that was corrosion proof, so I started to look at travelers and sail track systems that could be used for this application.
At first I figured that two pieces of parallel sail track with 4 traveler cars (2 at the top, 2 at the bottom) would do the trick,
but when I priced them out and found that I would have to mortgage my home to raise enough money to cover the cost, I went back to the drawing board. I mulled it around for a while, but couldn't find any better industrial alternative that would still be corrosion proof.
Finally, I realized it could be done with just one centered track and one car, with the top of the engine mount fastened in the center to the car, and the bottom of the engine mount riding on a pair of wheels on an axle. This brought the price back into a realistic range, but I ended up using two cars on the single track for an extra measure of safety.
It turned out that Harken makes a well suited product for this purpose, the 2726 Small Boat CD Car w/Shackle. It had a S
) of 500 lbs. and a breaking strength of 2700 lbs., well within my needs, especially when using two of them. I immediately ordered them as it met all the criteria I had:
-The Delrin ball bearings supported the loads in all directions.
-It was made of anodized aluminum and was thus reasonably corrosion proof.
-The two screws that hold the shackle in place could be replaced with hex head bolts and used as solid attachment points, and the shackle could be used as a third extra safety device.
-The price was not too bad, about $85 each for the cars and $65 for the length of Harken 2720 track.
So far this solution has worked very well, meeting all the requirements discussed so far. It travels up and down smoothly, stays vertical so the fuel float bowl doesn't drain (i.e. instant start every time), comes completely out of the water when retracted, and sits at the correct depth when deployed (depth can be adjusted while underway, depending on how many people are in the cockpit, and to adjust for fuel/water and other load changes). I made the wheels by hack sawing a rubber bow roller into 2 pieces, and used stainless steel bolts as axles. It works well and meets my corrosion resistance requirements.
The hull performance is enhanced, and I can often outrun newer yachts when sailing, simply because they lose 10 to 20 percent of their speed due to propeller drag. Below is a sequence of pictures showing the engine at different points of its vertical travel.
The fuel vapors, exhaust and noise stay completely out of the boat, and the original built-in fuel tank can carry a huge amount of fuel, although I only carry a quarter of a tank to save weight for short trips of a week or two. I also now have a cavernous ex-engine compartment, located very close to the center of buoyancy, and I have filled it with what I run out of first - not fuel, but potable water ... she now can carry an extra 30 gallons of water, over and above the regular tanks, with room to spare.
Merc (Nissan) 4 stroke kickers can be ordered through the WestMarine.com
|Here, the outboard is fully raised, completely out of the water. This also helps when it needs servicing as you can reach the engine from above while still inside the boat.
|Now the outboard is about halfway down, and the prop has just submerged. Although it is difficult to see, the hand winch is bolted onto the stern railing and is very accessible from the cockpit (as is the remote control).
|Finally, the outboard is fully lowered into the water. This picture was taken with the boat empty, but it sits at the proper depth as soon as anyone is on board.