BYYB Forums

Ok, I admit....I'm a unique person (more unique than most).

I'm working on a project.....

A modified Dragonfly II that I'm planning to put foils on.

The boat is going to be converted to an inboard.....which will be a 150cc motorcycle engine. I want to do water cooling, but I do not have a water cooled engine right now. So to start...I'm going to use the air cooled one I have.

I'm working in Autodesk Inventor changing the design for my purpose. I have to admit, I'm an not a marine engineer. More of a shade tree designer.

The design is coming along, but I have found a few things I didn't plan. Like a water cooled exhaust with a lift type muffler.

Also, finding a prop in the size I figure I need is also a problem. I figure a 8 x 8 prop, or 9 x 8 prop or there abouts to start.

The project should be interesting......even if I do waste my time. I figure I will learn something from the experience.

I hope it works out.......

J Long

Not so strange.  I have seen projects like this before.  I have a few questions though.

There are numerous boats called a dragonfly, both power and sail.  Could you be more specific; give us some specs. length, beam, maker, so on.

Is this engine going to be completely open to the air, or enclosed?  The difference significantly affects the modifications you will have to make to the engine to use it on a boat.  Contrary to what you may think, dumping any old engine in a boat may be life threatening.

I'm not saying it can't be done, in fact it has been done.  But there are considerations.

What kind of foils, fully submerged, surface piercing, ladder foils, ?

Data. Not enough data.

Peter,

Well...I didn't want to make a two page post the first time out......so I did leave much of the significant data out. I figured if anyone was interested, I could complete the picture.

The design is the dragonfly II from http://www.svensons.com/boat/  

Heavily redesigned of course(foils, seating, engine mounts, upper deck, transom)

The boat is about 10 ft long, about 4 ft wide.

The engine is not going to be enclosed in the first test (air-cooled version). I am going to duct cooling air with baffles around the cylinder to keep the engine from cooking it's head when on the move. There will also be a fan in the ducting when the boat is sitting still or moving slowly. The next engine will be water cooled, if the design works out. It will also be 50cc's bigger.

I am planning a firewall behind the engine to prevent heat tunneling under the upper deck to the passenger(s).

The front foil is planned to be submerged with a surface follower, and the rear foils surface piercing. (like the Sportfoil design on the rear). The front will be of similar design to the "aqua-skippers" which have been popular for the last year or so.(human powered hydrofoils). It will be a lever surface following design. The front being submerged to prevent accidental ventilation of the front foil on any waves. No one want's the front of their boat to come crashing to the water surface at any significant speed.

I plan about 8-12 inches of total lift which should take the craft through most lake waves. Displacement wake waves will probably require a slowing of the craft.

I am designing in Autodesk Inventor, and the design will be freely distributed, if the design proves to be safe.

That should paint a pretty complete picture of what I'm working on. I have a few things to figure out....like if the boat will bank correctly with foils. If the front foil strut will be a rudder, of if there will be a rear rudder.

I'm hoping one of the few people I have contacted about the Sportfoil PDF will get back with me. I really want to read it again, to insure I do not have an improper leaning boat in a turn. (I lost the PDF in a recent hard drive crash.....after having the PDF for several years.....go figure).

J long

Sounds interesting.  10 feet is a bit small but there's no reason I can think of why it shouldn't work.

Just thought I would add a caution about the engine. There is a specific reason why I asked if the engine was going to be enclosed. Using a motorcycle engine is fine if it is out in the open, but if you put it in a box it's a whole different ball game.  Then you have to start thinking about compartment ventilation, ignition protected electrical equipment (starter, alternator, distributors, etc), USCG type fuel hose, marine wiring and so on. Also a flame arrestor, and if this is a carbureted engine, a marine carburetor ( I don't think they make them for motorcycle engines). Marinizing an engine is a big task and can quickly become more expensive than just buying a marine engine.

Just some thoughts.

The engine is not going to be enclosed on the top.

Also.....when moving to a water cooled version....a marine carburetor of a similar size engine should suffice to be legal.

I knew about the ventilation and other items.

I do appreciate the finer points about the necessary regulations, and you probably do know more about them than I.

Most everything is pretty standard when dealing with an inboard. The engine specific problems, I am prepared to deal with.

But still I do appreciate the points you make.

J Long

Even if it's got an open top you'll need to make the marine conversion on this engine. The reason is gas fumes are heavier then air and will collect in the lowest portions of the box, enclosure, bilge, etc. where one spark can cause an explosion (seen this several times, it's not a good day, trust me). The starter makes sparks each time it's used, as does the alternator. Fumes can come from several locations in the carb, so make the conversions.

As far as productive advise, you may want to hop over to another forum. www.boatdesign.net is gear to this type of thing, which many other s doing the same thing. Of course we're love to have to keep us updated over her as well. I'm known as PAR over there.

That is great advice.....I do appreciate it.

I will bounce over the the other forum...and scout around to see what I can find.

I will need ti find out what differences there are in carburetors......and possibly modify the one I have for testing. I know about spark arrestors ..... so no big thing there.

J Long

The basic difference between a marine carb and an automotive carb is venting. Automotive carbs are vented to the atmosphere.  Marine carbs are not.  If any fuel is spilled from a carburetor, such as a stuck float bowl, the fule flows out the vent on an automotive carb.  But on a marine carb any spilled fuel is not allowed to go into the boat. So they are usually designed to have the vent in the throat so that fuel will go down the throat and into the manifold.  There are also differences in jetting and the acelerator pump.  It's better to just buy a marine carb than try to convert an existing one. 

here's a page that gives a general explanation of the differences between auto engines and marine engines, many of which are not evident just by looking at the engine. http://newboatbuilders.com/pages/fuel-2.html

Paul's advice is good.  venting needs to be from the bottom of the compartment. That is why the exhaust hose on a boat (the one that has the blower) goes to the bottom third of the compartment. Gasoline fumes collect at the lowest point, they need to be actively exhausted from the compartment. 

Well.....I don't see why the overflow line could not return to the tank.

The carburetor on this engine is side draft....so no accelerator pump to deal with. With a spark arrestor, and running the overflow correctly (back to the fuel tank), I do not see where any problem will be caused.

I was planning a "bilge blower" in the engine compartment. I feel not only is it regulations....it is a must for any engine that is enclosed (boat or otherwise).

James L