DPV Construction

[Larry Horne]

I originally posted this under the DIY section. I've received some great input but wanted to toss this out to a wider audience and see if there was more comments priour to starting.

I am starting a PDV project this weekend. I plan on using a Minn Kota 55lb thrust trolling unit lower unit for the motor with a 3 blade prop (new $125 motor, $35 prop). I am going to hand form a composite body. I have a few questions to throw out to you all. If the motor gets wet, its $15 in parts to overhaul.........easy. I'm going new verses used because of contacts in the business and new will provide performace IAW specs.

1) Over all design. I have two options. Linear body like a Apollo or Torpedo. Or something where the trolling motor lower unit is shoved up into a battery compartment (trolling motor hangs out of bottom). I perfer the torpedo design. Has anyone any info on the Shakespeare Submobile? What is its depth rating? The Tiger Ray uses a trolling motor configuration and rates it to 125 ft. Any thoughts as to what depth a stock Minn Kota will test to? I read one thread here where one of you re machined the seal locations for additional seals? Since I plan composite material either design is doable. Thoughts?

2) Designing it for 150ft or around 70psi.

3) Can Minn Kota motors run dry or does the lower unit transfer heat via the housing for general cooling? My tech personl locally indicated they can run dry but wet is the preferable obviously. Thoughts here is that I can add a second degree of water sealing by extending the output shaft and running it through a water tight seal outboard of the trolling unit lower unit. Basically placing the motor inside a water tight compartment on the stern of the unit.

4) Batteries. I can locate a 12V 55Ah sealed lead battery. The trolling unit pulls 45A on high speed. Battery weight is 39#. Is there another option? This may provide 30 - 45 minutes of good run time before the voltage drops too much where repeated cycles will damage the battery. Are there better options such as runnig smalleer twin batteries? Thoughts?

5) Motor modifications. If I take the motor unit direct from the factory what improvements can I do to increase the possibility of getting greater that 100' out of it. They are simple units, double sping shaft seals, casing o-ring, and o-rings on the two studs holding the assembly together. My thoughts are to cover the heads of the bolts with a polyurethane sealant and maybe try improving the body gasket. not certain much more is doable unless I take the approach like one of the posters here and replace the shaft seals with better units. Thoughts?

Well this is enough to chew on now.

Any and all technical advice is appreciated.

Larry
VA

 

[Nemrod]

Yo, Dude,

Years back I built a scooter loosely from plans I got from PM. It is now in the back of my hanger somewhere lost forever. it worked, it was the Shakespere motor you mention. It generally leaked beyond 60ish feet. It was purchased new. The batteries available now are the gell cells, get two 18 ah and wire them together in parrallel for 12V and 36 ah. These batteries are very small actually, I bought a set for my Tekna DPV for 34 dollars each. I can give you the brand and part number if you need.
I scraped the aluminum body tube and went with PVC. The last version has the motor recessed into the rear of the PVC tube using the PVC tube also as the prop shroud with scalloped cutouts for the prop suction.
The trolling motor props are not optimum for DPV use. They are intended for moving a large boat very slowly at less than 1 MPH. They have a very flat pitch. The modern "Tekna" type prop has much more twist and is adjustable. You may be able to adapt one.
I eventually sourced a seal from a place in Houston where I lived at the time and it stopped the leaking--sort of. Don't remember where I got it.

No, running the motor dry will damage the seals.

The performance was roughly on par with the SeaDoo type units, nothing like my Tekna at all--which is very fast.

N

 

[Larry Horne]

I have the old PM plans for the original dinosaur. What a whale. I am however using the plans as a spring board to this project. I will take the advise on the props and batteries. Ever consider heating the props and increasing the pitch?

 

[Nemrod]

Larry, yes, sounds like your heading down my path. I built several versions, only the first was a somewhat similar to the PM plans. Each version afterwards improved and evolved. I am positive that there are seals that you can use now or that the currently available motor is improved as to the sealing effectiveness. I believe the propellers are a lexan type material. Lexan does not heat form well but it is worth a try. I finally found a propeller on a different trolling motor of fiber reinforced nylon that had more pitch, it was not of the modern weedless type.

Batteries of the gell cell type which are lead acid sealed batteries that gas off little hydrogen unless overly charged or charged to fast or loaded to heavily--hydrogen can explode-is a very bad thing---scooters like my Tekna use an absorbant catlyst that is replaced yearly. They are in a myrid of sizes and are often used for things like wheelchairs and power chairs and things like that. The batteries I am using are 18ah and in the Tekna are wired in series for 24V and 18ah. I am suggesting wiring two or three in parrallel to provide 36 to 54ah at 12V. They would fit in industrial PVC type pipe of 10 inch diameter. This is what I used on my last version. I machined the end of the tube once squared and trued for an X ring O-ring and spring clamps such as those often seen on camcorders. The nose section was made of .5 inch plexi layers that were turned into a slight curve and I had a 55w halogen lamp and reflector up inside the nose section as well. I used camera controls from Ikelite to operate the power switch, light and speed control. The rear section was foamed with the motor in place and then carved and glassed leaving the motor mostly as a permenant install. The tube was then cut at four locations with a half moon cut out to allow water flow to the prop. The motor wires passed through the PVC glued aft bulkhead and were potted with epoxy. The handles were attached to "wings" made of 0.5 inch PVC plate. I used a router to groove a .5 inch slot half thickeness of the tube wall and then glued the wings in place. Using a belt sander (on stand) I used remaining stock to make fillet gussets for the wing to body joint. I used lead shot bags and moving the batteries around to get balance. I came to the belief that very slightly negative was best.

Let me tell you this, several times over the last year Tekna scooters and parts were sold on ebay, just a few days ago a motor and prop assembly etc. The Tekna motor is the type used in all current high dollar scooters. It is a GE direct drive 24V motor that turns about 650 RPM constant. The motor comes with the aft housing usually which is very easy to fit into a PVC adapter. Speed is varied by adjusting propeller pitch, RPM remians mostly the same, amps draw of course goes up due to the greater load. The 24V motor does not get as hot or draw as much amps. The motors are often hot rodded with a "hot" wind, the wind on the Tekna was hotter I think than the Mako still in production but probably not as hot as say a Gavin. Again, best I can tell it is the same basic motor and prop and even shroud.

Here is another idea, scavenge a cheapo SeaDoo for the prop and shroud and attach it to the PVC tube. Junk the motor and use a GE 24V or even your trolling motor. As many of them as are laying around now some of them are bound to be dead and junked.

Modern trolling motors I think use a digital speed control, at least some do, this cycles the motor on and off to maintain a specific RPM for a given load.--many times per second. These are more efficient than cheap units with resistors etc.

Also there is a website with parts and service for Tekna scooters etc.

Hope this is helpful to you.

N

 

[Larry Horne]

Nemrod,

Any chance of you having any pixs of your past efforts? Sounds like a great effort. My Buddy here on the board just says "buy one". That takes the challenge out of it for me. I've worked on building two airplanes sine 1975 and complete restoration on 4 over the years and now I want to take that passion of design and build to the depths.

I'll keep hitting you for advise as I move forward.

Lar

 

[Nemrod]

Larry, hmmm, I too have built two airplanes and restored a couple and I am slowly building another now. My "remains of the day" are stored in a crate way in the back of my hanger (50X50 aircraft hanger) back home a near 14 hour drive. I am sorry, I tried looking through some slides but the only ones that show the scooter are pretty poor and do not actually exhibit it. This all took place for me between 1982 and 1986.
I now have two Teknas whcih I am also rebuilding. one was recently pool tested and I also had it on a short dive trip over to Arkansas. WOW. The Teknas have some structural issues that were mostly resolved in the current Mako unit but still, if you treat them gentle they work fine and they are fast and good to 130 feet. The structural problems relate to handling issues and such as that.
I tried to describe the one I last built as best I could but I cannot go home right now to take pics--lol.
If I built one now I would have the aft section remove as well. The "shroud" would also slip on and screw in or clamp so as to make foaming in the motor and all the sanding and contouring easier and the glassing of the foam. On the other hand I might dispenmse with the foamand just secure the motor with a bracket and then build an inner shroud of glass in two pices to fit around the motor and then secure with screws or clips. As soon as I find a junked SeaDoo I am going to grab it and then I may revisit that crate in the back of the hanger.
I don't think your going to find the performance of a Tekna in that rig unless you scavenge a Tekna motor and in that case you got a whole nuther animal. all those fancy 30000 dollars scooters are is a GE motor stuffed into a PVC or poly tube of 10 inch diameter. The props and shrouds are also the same as a Tekna design or very close.
Another thing I was thinking on doing since the SeaDoo units are small and light. Take the innards out and the shrouds and props and build them into a single tube as a push/pull! Think about that.
N

 

[Larry Horne]

Nemrod,

I'd almost take the time to go off topic and ask about the airplanes..................my 1st love but we're here to talk scooters..........

http://www.silent-submersion.com/

Have you seen this site? Go to the images section...........this site has basically given me the plans in picture form. The design is perfect for home construction. The water-tight stern section is perfect and the tunnel supporting the battery assembly. This system can be dublicated rather easily one the materials are located. I agree with your textual description of the build. 8-10 inch tube, water tight stern section holding motor with foamed and glassed fairing (for looks and hydro routing of source flow). Question: What is the trade name for the "draw clamps" shown in photos in the link? I've been to McCaster but don't seem to be able to locate the small ones http://www.mcmaster.com/.

Another product I think would be nice to use in the end plate fabrication is "solid surface counter top" material used in home kitchen counter top construction. Would need to fing 1 foot square scrap. Easily machinable with a router table.

OK......give me the airplanes...........mine: Mini Mac Volks Plane in 1974 (Dad built and helped, Kit Fox now ready for test flight, refurbs: Grumman Yankee, Eurcope, Cherokee 140 (owned currently). Ratings: Commercial, Multi, Instrument, Instructor.

Long Story..........got bored shydiving with those "nut cases".

Well I am off the next two weeks and am going to try to obtain the raw materials adn get started.........

Lar

 

[Nemrod]

I am only Private, Instrument, Commercial.

Airplanes, Grumman T-Cat (Yankee), Cardinal, Cub, KitFoxIV-1200, RV7A now and I built 90 percent and sold a RV4.
If you go to the KitFox message board and mention me I am sure there are a bunch who would remember me--lol. I sold her a couple of years ago to invest in the new RV project.

Let me study on those links, I assume you are aware of the DIY section here with a long thread about scooters. I think I saw your posts in there? I bought my clasps from AquaVideo, in Florida, they sold me a couple of sets. I really don't remember how much. I bought and still have a SVHS functioning camcorder in their housing and it used those clasps so I called them and they sold me some. I am sure there is a better source.
N

 

[MDTzak]

Do you think there is any way to run a pnuematic type "scooter". Possibly an HP hose from your first stage to run a piston type pump to run a crank shaft? just curious to weather or not that possable. thanks alot

 

[Nemrod]

MD, that is an interesting idea. Surprisingly there was thread here sometime back about an air powered scooter. Yes, there have been some. Thing is nothing is free and you would probably be amazed at how rapidly the air would be consumed running a small motor. The Tekna scooter has about .25/.3 horsepower I think so an air motor to be succesful would have to be equivlent output. Also, the deeper you go the more air it would use. A scooter with about 140cf tanks might be practical with a small piston motor. I would have to calculate the consumption for that. An air turbine using a small drill motor to run a geard prop might be fun also. Most of these use between 4 and 10 cfm. Say it is 6cfm at sealevel and 90 psi over ambient then at sealevel 60 cf would last 10 minutes, at 30feet approx 2/3 as long (90psi plus 15psi additional over ambient etc). To get an hour run time at sealevel then you would need 360 cubic feet or approximately six 80 cf tanks. Something like that. N