Tuesday, February 14, 2012

Building the endoskeleton of the Spencer Sea Hawk/Manta Ray Roboboat Platform

(Update April 15, 2012:  Note that after extensive field testing there has now been a revision in the Sea Hawk endoskeleton for greater stability and for easier placement of 2 liter soda bottle pontoons under the wings.  This involves replacing the "front float support" with a copy of the "rear float support" and the installation of new braces.  New directions will follow, but for now consider purchasing at least 4 more 90 degree elbows, 4 more 45 degree elbows, 4 more T's and an additional 2 foot length of 1/2" PVC pipe! Thanks!)

The original Brent Spencer sketches for a PVC pipe version of his  Sea Sparrow/Sea Hawk, allegedly found on anaerobically preserved papyrus scrolls in an antediluvian  ship-wreck near the Red Sea.

 (Please note: Brent Spencer, creator of the Sea Sparrow surface craft for harbor surveillance,  changed the name to "Sea Hawk" somewhere in the early part of the year 2012 AD,  so we are changing accordingly. The plans below are the property of Brent Spencer from Motion Picture Marine, who owns the copyright to the Spencer Sea Sparrow and Spencer Sea Hawk and Spencer Manta Ray robotic surface vehicles and is generously allowing PORPOISE to use and modify these craft and his concepts for educational purposes).

  The name Sea Hawk has a venerable tradition in both the Navy and in Motion Pictures (to say nothing of nature itself!).  Now when we are building and operating our  roboboats we can put on the marvellous soundtrack music to the eponymous 1940 Errol Flynn movie (scored by Erich Korngold) and get into the swashbuckling spirit of high adventure that maritime robotics is all about!
(A great youtube video of Korngold conducting an Australian orchestra playing the overture can be found  here).

And here is the list of Parts you will need to build your very own Sea Hawk Manta Ray:

The Endoskeleton:
Constructed from 1/2" diameter PVC, identical to that used for the Sea Perch so as to create an "evolutionary" pathway from "fish" to "marine mammal" or "water bird".  (STEM tie-in is the biological concept that evolution is conservative and that organisms develop by mutation and natural selection of old parts re-purposed for new tasks in new environments. Thus the Sea Hawk/Manta Ray is created from the same skeletal materials and motor/propellers as the Sea Perch!)

1/2 " PVC pipe: 7 pieces of 2 foot length.
7 @ 0.94c = $6.58
The endoskeleton will require at total of 147 inches of 1/2" PVC pipe, ideally obtained from 7 pieces of 2 foot long (24") 1/2" diameter PVC.  You will need 1 for the bow, 3 for the main frame, and 2 for the wing float, totalling 6 pieces, but you will need an additional piece to get the last 6 inches for the two motor mounts (3 inches each) and have at least 9 inches left over for modding the Sea Perch frame for removable motors. 

T- fittings: 5 @ 0.34c = $1.70 for boat frame only; 8 @ 0.34c = $2.72 for both boat and Sea Perch
 (you might as well buy a 10 pack for $1.98 and save some money; you need 5 for the boat frame = $1.70, but while you are at it, you can use three extra to mod your Sea Perch frame for it to use the same removable/exchangeable motor mounts)

45 degrees: 13 @ 0.46c = $5.98
90 degrees: 5 @ 0.46c = $2.30 (you might as well buy a 25 pack for $4.75 since you can always use more 90 degree elbows!)
90 degree three-ways (aka 1/2" PVC Side Outlet): 6 @ $1.23 = $7.38

Male screw adaptors (aka 1/2" PVC Male Adapter SXMPT) for three way elbows: 4 @0.25c = $1.00
1/2" PVC Cap Slip: 1 @ 0.25c = $0.25
1/2" Plug SPG: 2 @ 0.62c = $1.24

Loctite Gel Super Glue: 1 @ $2.98 (we use this in initial stages rather than PVC glue because it is easier to pull apart if mistakes are made).

Total cost for endoskeleton parts: $6.58 + $1.70 + $5.06 + $2.30 + $7.38 + $1.00 + $0.25 + $1.24 = $25.51 + tax (0.0925 = $2.35) = $27.86

(Buyer beware: Local hardware stores charge substantially more for PVC.  At Lincoln Hardware (1609 Lincoln Blvd. Venice CA 90291) the prices were $1.79 for each 90 degree 3 way PVC side outlet,and  $0.79 for each 45 degree elbow -- about 2X the price from Home Depot.  We purchased 13 of the former and 17 of the latter to make two boats at Venice High around the corner and those parts alone cost $39.91! Lesson learned: make one run and purchase in bulk from a wholesaler, even if it is farther away!)

Bow and Main Frame:

A vector art reconstruction of the ancient Spencer scrolls done by marine robotics archaeologists in the modern era... top form is the bow, bottom is the main frame

The ancients used an obscure measurement system based on their body parts, where a "foot" was approximately the size of the average human's foot at that time, and "inches" the approximate size of an opposable thumb.  We translate these arcane dimensions into the modern decimal system of "meters" which makes calculations much much easier!

Bow -

23.5 inches (59.69 cm; we use an even 60 cm) of 1/2" pipe (Use one 2 foot, or 61 cm  PVC piece and cut); 1 45 degree elbow, 1 90 degree elbow, 1 1/2" PVC cap

  1. A: 1 - 1/2" PVC Cap
  2. B:  1 - 8 1/2" pipe (8.5 inches = 21.59 cm; we use 21.5 cm).
  3. C: 1 - 45 degree elbow
  4. D: 1 - 8 1/2" pipe (21.5 cm)
  5.  E: 1 - 90 degree elbow
  6. F: 1 - 6 1/2" pipe (6.5 inches = 16.5 cm)
Main Frame -

55 inches of 1/2" pipe (Use 3 2 foot PVC pieces, i.e. 72 inches,  and cut; keep the 17 inch remainder for rear float support); 5 T fittings, 2 90 degree elbows, 4 45 degree elbows.

  1.  G: 1 - T-fitting
  2. H: 2 - 6" Pipes (These form the BOW) (6 inches is 16.24 cm, we use 16.25  cm)
  3. I: 2 - 90 degree elbows
  4. J: 2 - 4" Pipes (4 inches = 10.16 cm; we use 10 cm)
  5. K: 2 - T Fittings
  6. L: 2 - 3" Pipes (3 inches = 7.62 cm; we use 7.5 cm)
  7. M: 2 - T Fittings
  8. N: 2 - 7 1/2" Pipes (7.5 inches = 19.05 cm; we use 19 cm).
  9. O: 2 - 45 degree elbows
  10. P: 2 - 3 1/2" Pipes  (3.5 inches = 8.89 cm; we use 9 cm)
  11. Q:  2 - 45 degree elbows
  12. R: 1 - 7" Pipe (This is the STERN.  7 inches = 17.78 cm, we use 18 cm)
Wing Float (# of pieces are for both sides) -

One of the wing floats rendered as vector graphics; note that the ancient plans call for two wings, an early example of biomimicry perhaps... Note that each wing float has a "front z" 45 degree elbow and a "rear z" 45 degree elbow. Archaeologists believe that these were the attachment points for the front and rear float supports respectively...
A team of paleontologists (who normally work on reconstructing dinosaur bones!) were tasked with turning the ancient mariner's sketch into a three dimensional model, using Blender, so that it could be more easily understood by the layman.

Subtotal: (This is the material needed for BOTH wings!)
39.5 inches of 1/2" pipe (use 2 2 foot PVC pieces, i.e. 48 inches,  and cut, keep the 8.5 inch remainder for front float support); 4 90 degree three-ways, 4 male screw adapters for 90 degree three-ways, 4 45 degree elbows

  1. S: 4 - 7 1/2 " pipes (~ 19 cm)
  2. T: 4 - 90 degree 3-way
  3. W: 2 - 3" pipes (~7.5 cm)
  4. X:  4 - male screw adapter for 90 degree 3-way
  5. Y: 4 - 1 3/4" pipes  (~ 4.5 cm)
  6.  Z: 4 - 45 degree elbows
Front Float Support  (From Z) 

The front float supports appear to have attached to the Z 45 degree elbow on the front part of each wing float. These seem to have  held a cluster of three plastic water bottles used for flotation. The ancients seemed to have built their boats from the flotsam and jetsam that their wasteful society discarded into the water...

Need 2X below to complete right and left front float supports.

8 inches (2 x 4") of 1/2" pipe (salvaged from the wing float pipes remainder); 2 45 degree elbows, 2 90 degree elbows, 2 1/2" PVC plugs.

  1. A1: 1 - 2" Pipe ( ~ 5 cm)
  2. A2:  1 - 45 degree elbow
  3. A3: 1 - 2" pipe (~ 5 cm)
  4. A4:  1 - 90 degree elbow
  5. A5: 1 - 1/2" PVC Plug
Rear Float Support (From Z)

Archeologists have reconstructed this rear float support and assume that the motor depicted was a variant of  the Sea Perch motor, attached to a cut piece of PVC pipe and bound there with leather (similar to the way Wookies on the planet Kashyyk  blend traditional organic materials with galactic federation supplied industrial materials) or with electrical tape. In later models it appears that a T was added so that the both wheels and motors could be added to the craft, enabling it to perform amphibiously.

Need 2X below to complete right and left rear float supports.
 15 inches (2 x 7.5) of 1/2" pipe  (salvaged from main frame pipes remainder); 2 90 degree 3-ways, 2 45 degree elbows, 2 1/2" PVC plugs.

  1. B1: 1 - 3" pipes (~ 7.5 cm)
  2. B2: 1 - 90 degree  3-way 
  3. B3: 1 - 4 1/2" pipes (~11.5 cm)
  4. B4: 1 - 45 degree elbow
  5. B5: 1 - 1/2" PVC Plug
Motor Support

Need 2X below to complete left and right motor mounts.
6" of pipe (you will need a separate 2 foot piece of PVC for this unless you can find a 1 foot piece or some scrap; it doesn't hurt to have the extra though, since you can use 9 more inches of this  two foot piece for hacking your sea Perch for exchangeable motor mounts!); 2 male screw adapters. Hacksaw.
  1. B6:  1 - male screw adapter for B2
  2. B7: 1 - 3" pipe (this will receive the  half cut two inches in to accommodate the motor canister; ~ 7.5 cm)
 Cut pipe  2"  horizontal to mount motor.

The finished endoskeleton:


6 small bottles (16.9 oz or a little more than a pint) for the front, bundled like a pyramid under the "hawk's beak".
6 large bottles (1 liter bottles; you need 2 bundles of 3 1 -liter bottles  for each wing, also bundled like a pyramid under the wing).

Brent recommends "Smart Water" bottles because they have a symmetrical contour and are smooth, creating less drag. Er... and because they are "smart" looking.


Corrugated Plastics sheets http://corrugatedplastics.net make the skin of the roboboat.  We use the 4 mm (3/16") sheets that are 36.5" X 24". They cost $3.69 per sheet.

 We use two sheets, one from which the basic platform is cut out and another, with the grain going cross-wise (giving strength and reinforcement, like in plywood) for the wings. Color is your preference; One sheet of each in your school colors could be fun!

Putting it together in the Real World:

 Note the addition of a 90 degree 3 way side outlet for the motors; now the Sea Hawk uses a total of 4 motors -- two facing to the rear with propellors on them and two facing out to the sides with wheels on them. 

In this way the Sea Hawk can be used in the classroom as well as in the water. The code for manoeuvring the craft can be tested out on land (particularly handy for schools that don't have easy access to water, but useful for everybody since waterproofing is another matter entirely and should not hold up the programming aspect of your robotics effort!) .  So now the Sea Hawk is an Amphibious Vehicle! The inspiration came from EZMoney Harper and his dad Roy when they attended the ASNE (American Society of Naval Engineers)  conference in DC and saw the vehicles on display.  EZ said,"this is what the Sea Hawk should be!", so we made it so!

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