Jet auxillery power?

[sirgordy]

While surfing the net I ran across a SurfJet. It's a surfboard with a very simple jet drive that was made in the 70's.

I have a 17 foot open sailboat and I don't want a small outboard dangling off the transom and an inboard would be out of the question.

The SurfJet engine is a two cylinder, 17 hp unit that produces about (I think) 160 pounds of thrust. This is somewhat more than I need to get away from a ramp and down a short canal before sailing.

The SurfJet unit looks like it could be mounted 'bout anywhere with a tube for the jet to exit the transom.

I don't need steering, I have a rudder.

I don't need reverse, would be nice, but not worth the trouble or expense.

Now, if I had the powerhead from a 5 hp (or so ) outboard and a centrifugal pump why wouldn't it work?

Ok, I can hear you thinking it wont be efficient. How efficient does it have to be? Four knots would be acceptable. Five or six knots would would leave me giggling helplessly as I 'sail' past my friends.

It wouldn't weigh much. It could be mounted next to the centerboard trunk, out of the way. There would be no protrusion below the boat. I wouldn't have to extend my massive self half out the rear of the boat just to start it.

Question:

How do I know what size pump to use and where would I get one?

[gonzo]

Have you found a pump yet? There were a couple of outboard manufacturers that offered jet pumps. Yamaha was one of them. I think they only made larger motors only though. How about an old jetski. Kawasaki's smaller 440 engine could work.

[pkoken]

The problem as I see it is that jet drives are at their most inefficient at slow speeds. You are trying to move a (relatively) heavy displacement sailboat with a very small, very high velocity jet of water.

If you are really just trying to get in and out of the marina, my personal favorite for small sailboats is a high thrust electric trolling motor. You can cruise around for quite a while with a decent Deep Cycle battery aboard… it’s light, simple and you even get reverse.

[Guest20100203]

I have just designed a rudder mounted trolling motor setup for a 23' sailer. The power head (controls) are removed and mounted under the tiller, but attached. The shaft and motor are attached to a pivoting bracket that is bolted to the rudderhead.

Underway on power, the motor is lowered and locked into position, free to turn with the rudder. Steering is still done with the tiller and the controls are at hand. When sailing, the motor bracket is unlocked and lifted clear of the water, where it is locked in the up position. It's a quite simple device and works well with the 60 some pounds of thrust MK motor. The brackets and pivot assembly don't touch the water, just the motor and shaft. Basically the shaft is pivoted well above the LWL allowing the motor to swing down and get some bury. The lock is a captive spring loaded pin on a handle. This engages holes on the rudderhead (up and down) to lock the motor position. It took some trial and error to get the swing and hole locations just right, but could have been figured out on paper with less re-bending of material.

The two wires coming from the motor, up the shaft is sealed with a boot, some foam and ends at a plug on the aft deck, where the control head picks up the wiring.

The rig can be removed quickly for racing or whatever, by pulling the pivot and stowing the motor and shaft assembly.

[Tohbi]

any pics of your electric drive?

[Guest20100203]

Sorry Tohbi, I don't have photos nor a digital camera. Picture a couple of plates of aluminum, sandwiching the rudderhead at the top, just below where the tiller attaches and a pivot bolt running through holes in the plates and a hole in the top of the trolling motor shaft. The shaft going to the motor was bent a bit to square up the prop shaft with the LWL. The motor and shaft assembly align directly behind the rudder about 16" below the surface. This is held in position by another plate of aluminum, through bolted on the shaft about half way up from the motor. This plate slides past the port side of the rudderhead as the assembly is lifted up or down. In the down setting the plate aligns with a hole in the forward end of the rudderhead and a quick release pin (the type used for rigging) is inserted in the aligned holes. The same happens in the up position using a different hole near the rear edge of the rudderhead.

I wanted to be able to steer with the motor and rudder at the same time, greatly improving the low speed handling of the boat. Not having to get some way on before you can steer is a big asset in tight marinas and docking. Mounting the motor with the least amount of trouble and linkages seemed only logical. Finding the correct places for the brackets and holes for the locking pin was time consuming, but easy once I started using hose clamps to hold the brackets on the shaft until the location was established. Once established, I drilled for fasteners and through bolted everything.

This is a kick up rudder arrangement and has yet been tested with a shot from a log or bottom strike. The motor would be in the way as the rudder blade tried to rise up, maybe taking out the prop.

I couldn't figure out a way to keep the kick up feature, lock down the motor and have both turn on the same plane, with the same effort, while under power. In the up position the motor doesn't prevent the kick up feature.

I thought I could place the motor next to the rudder and fix it's position, but this would basically be the same as a transom mount for a trolling motor and there are plenty of choices for this feature.

The only flaw I can find are the holes in the rudderhead (teak in this case) will egg out with use and time. At that point I'll drill over size and insert a bearing, maybe a S.S. tube of the correct ID and epoxy it to the rudderhead. The other flaw being the before mentioned lose of kick up rudder while under power. I can live with the compromises made in this setup. It required 3 strips of aluminum plate, 4 through bolts of varying sizes, a handful of washers and nuts, a quick release pin, a couple of sheet metal screws to hold the control head to the bottom of the tiller, a two pin connector (I used a bulkhead connector) for the connection to the control head and a rubber boot to cap the end of the motor shaft where the wires come out. I did spray some expanding foam down the shaft to seal it up.

I hope this describes it well enough to figure out. If you email me, I can send you a drawing of the assembly as I haven't figured out how to post stuff on this damn machine, while on line yet.

[SuperPiper]

PAR, I am also interested in your installation. I had never considered mounting the drive on the rudder. That is rather ingenious!

How does one convert pounds of thrust to horsepower? The units for horsepower are ft-lb/sec.

I am assuming that the power from an electric outboard is only a fraction of a horsepower. What is the maximum speed expected from the motor? What is the range expected per typical battery? Can I expect to run against a current or motor a mile out to the racecourse?

[gonzo]

There is no direct conversion. Pounds of thrust depend on many things besides HP. For example, propeller efficiency.

[Guest20100203]

Gonzo is quite correct in that there is no direct "conversion table" to convert pounds of thrust to HP. A general rule is 80 pounds of thrust is a single horse power. Some think this a conservative estimate, others think this a bit heavy.

I had a 108 pound motor on a Catalina 22 that pushed it fine at most times I'd need to use it. You're not going to bash into a steep wave pattern with a half a gale blowing, but for getting around anchorages, away from the dock or mooring it's a light weight, reliable and inexpensive way of propulsion. It sounds right too . . .

Do yourselves a favor and run two batteries if you motor very much. I, personally sail away from whatever as often as I can, I feel I have more control. Short tacking up a creek or to open water to me is a challenge I rarely turn down. I love to come into the docks at the ramp with a bone in her teeth and swing her around in a boat length, luffing up and stopping just as my fenders kiss the dock. It scares the hell out of the power boaters and impresses the sailors. This is so much more fun when I've a newbee aboard, they just have to trust me when I ask the fenders be put on the side of the boat that is away from the dock that the boat seems to be coming into . . .

[Guest20100203]

Lets try this one. This is a revised version of the motor mount. It has fewer parts with the removal of the lock bracket. The upper bracket holds the pivot and locking pin in the same bracket.

I'd use 1/8" plate aluminum for this bracket.

http://boatdesign.net/forums/attachment.php?attachmentid=1018&stc=1

The earlier version of this used another bracket, some flat plate stock several inches below the pivot bracket. It bolted to the motor shaft and lead forward along the port face of the rudderhead. The pin holes where in the rudderhead. It may provide a little more support, but this setup needs very little as you can hand hold the motor by the shaft over the side of the boat with reasonable control.

[Blether]

Are you still afloat ? :-)

How about something from here ? http://www.osengines.com/index.html. You could use an aero engine and prop and be a 'swamp boat' that can even motor up on the beach, or put inboard back into the question with a boat engine. No heavy batteries, and the propeller's more efficient at slow speeds.

Second-hand availability is high and there are plenty of support forums online.

These guys http://www.hobby-lobby.com/index3.htm, I heard have small folding props, though I don't see them listed on the site. If you're really set on the water jet, they say their jet drive JM1001 http://www.hobby-lobby.com/jetdrives.htm rates up to a .90 glow engine - i.e. maybe 3-4bhp.

[WJohnson]

Your question I think is a good one. I also am interested in moving a large mass using a jet drive for reasons of weeds, shallow water and protrusions.

Does a large diameter slow turning, high prop angle jet drive give large push at low speeds? Physics says the efficieny of a jet or rocket is the difference of speed between the expelled material and the speed traveled WRT the medium.