The adventure
09 Nov 2006
 

My first incursion in the kingdom of boat design. Will it run well? Will it float? We see incredibly ugly boats that do it. Why can't I?

The goal was to create a low cost boat for 7 cells and fast enough to pop the eyes of some nitro guys...
I will make two of them (I want to race against someone...) by the price of a cheap boat.

The materials needed are 3mm and 1mm ply, insulation foam (for the sponsons) cyano or wood glue and that's all.

The objective is to achieve a weight (complete with hardware and batteries) below 1Kg. The Targa is a small boat but weights 1150gr ready to run, so I think that this is a fair expectation.

Taking into account that the wet area is much smaller that a normal V hull, I think we will have a bullet in our hands. Let's hope it can turn OK, or we will have to dig for it after the first run.

The hardware  is a mix of Prestwich Model Centre (rudder) and Octura (strut, flex cable, shaft etc). If you want to know the available hardware from Octura, take a look at the Octura catalog.

What and who inspired me

The spark that ignited it

One day at the lake there was a guy with a Dumas Hawk 3.5 rigger with a MDS engine with pipe that could not setup the boat.
We tried to help him with advice about the prop (it was way too big) and the prop depth, but he replied looking rather angry with our daring attitude. We tried no more and went to our business. I had my Targa ready to go and he was really amused with the "toy". An electric boat for kids...
I put my boat on the water and he started to look more and more embarrassed with the speed of the "toy".
After that he was not so rude accepting advice and he even considered using a smaller prop (he was using a Prather S220). This was one of the problems, but there were others, like the building (poorly done) the rear sponsons, the front sponsons etc.
He's only concern was the pipe, the engine, the nitro content and he didn't care about anything else. From a plane point of view we all know that you can make a stable door fly with enough power. It was the same approach he had with the boat. The water spray was so huge that it was unlike any other rigger. I noticed after, that everything was on the water, not only the sponsons, but EVERYTHING.
An idea came to my mind - What if I build an electric rigger? Will it go faster than this duck?
This is the politically  incorrect explanation, but I'm just human. Had he been a nice guy on the first approach, and I never would have bothered to prove a concept...

The politically correct explanation

There is a site that you should visit if you like FE riggers (Rum Runner Racing Team) that led to believe that these boats can be fun and fast. Apparently the cost of such a boat can be low, and this makes it even more attractive.
Looking at some basic drawing guidelines from the Fast Electric Boats (from Traplet Publications) and some pictures from various sites this was the final result.

Building

11/11/1998

The wood cutting started. I'm using 1.2mm ply instead for the sponsons with foam interior. Using a computer for this is really nice because I just print the templates for cutting the wood. The box sides will be 2 mm balsa and 1.2 mm ply glued together. The initial plan of 3 mm ply would result in a very heavy boat. The resistance will be adequate and the weight will come down for sure. Only the transom former is a little thicker. The underside of the box is 2mm and 1.2 ply with epoxy and glass fiber for rigidity. The torsion on this kind of hull, especially in rough water, is really high and I have to make sure that the hull will hold without deforming or breaking.

11/24/1998

Some improvements were made on the design (I'll put the updated drawing here if time allows). The boat now has a hatch with a canopy. It was made with 1mm balsa and it will be reinforced with very light glass cloth. The canopy allows a gearbox to be fitted upright (I'm sure that I will want extra performance further on...) I'm trying to keep the box as narrow as possible and this invalidates mounting the gearbox on the side. If I did that the shaft would be off center or the box would have to be widened. This way the boat looks cool (if you like this type of boat) and I can keep the original dimensions of the design. All the formers were redesigned in order to save weight. They are just a frame of 3mm ply. The weight savings are important and the resistance is maintained (I had forget my planes with formers in ply...). I'm figuring how I can make a mount for removable sponsons. The booms are glued to the sponsons, but they will be removable on the hull side. If I break one or if I want to adjust the width of the boat (with a minor variation) I can do it easily. After some calculations the hull will be in the 150-160 gr. range (That's light!). It may break in the first run, but it's light!!

11/29/1998

The sponson booms will go into fixed aluminum tubes that go through the hull and are fixed with 2mm screws. The sponsons are complete and the box parts are cut and ready to assemble. The formers are hollow and the bottom of the box will be in 2mm balsa laminated with glass fiber and resin instead of the 3mm ply original. The weight gain is considerable and the resistance will be more than adequate. I'm trying to gain weight in every little detail. Starting to look like "half" boat. In this picture you can see a sponson, the booms in aluminum and the left side of the box (ply and balsa)

11/30/1998

Work is progressing at a very fast pace. Here you can see the complete front section. The booms, the outer tubes and the laminated balsa pieces that will help support any tensions on the booms.

12/01/1998 (Independence Day - in 1640 we got rid of the Spanish Kings! Bad move I guess...

Sides, formers and bottom glued together.

12/13/1998

The boat is almost ready for painting. I've made some mods as work went along. The reinforcements you see on the pictures above (for supporting the booms) were discarded. I really think I don't need them and every gram is precious. The top front is balsa instead of ply, and the distance of the sponsons from the hull was increased by 1cm to 6cm. After assembling the parts the proportions didn't look OK. The boat seemed too narrow and I think it will be more stable in the turns with the increased distance from the sponsons. The resistance to torsion is amazing for a hull this light (it is 155 gr. after being brushed with epoxy). I'll try to apply a very thin coat of paint to keep the weight low.

12/15/1998

Almost ready... Today I spent a couple of hours designing and building the turn fin and bracket. I've made them from aluminum after drawing them in AutoCAD (AutoCAD, I love you!).

In the picture you can see the mount and a mock up of the final assembly. The batteries will be over the aft boom in order to keep the CG in place. The bracket allows the horizontal and vertical adjustment of the fin.

12/17/1998

The motor mount made in aluminum. I was planning to make a modular motor mount like the Targa (last on the column), but since I had the material I decided to try and the final result was OK. Enough to put it on the boat. Additionally it will help dissipate the heat from the motor. Every piece of material that could be removed, was removed. I'm not going to spoil an almost perfect (in weight) hull with lots of stuff that add weight. In spite of using classic materials I'm achieving weights typical of much more sophisticated hulls. After cutting it I just bent the mount to the desired angle et voila. I can adjust it back and forth, because if I want to use motors with standard cans, they can be moved back a little bit. And after all, I really like the idea of being able to adjust everything to tune the hull.

And after all the experience I got from drawing the rigger I tried to draw a new workbench. To make it more difficult I decided to do it in 3D and after that render a couple of scenes. This is the result. What do you think? Neat, isn't it ? You can see the rigger on the bench!

2/17/1999

I finally have all the hardware for the rigger. The strut (OC6SBEPA-130) the complete flex and shaft, dog drive are from Octura. The rudder is from Prestwich (lighter than the Octura equivalent). The brackets are made from 90º aluminum in order to save weight. The last phase is about to start. After that I will plan the painting and decals. I got a heat transfer paper from Epson for Ink Jet printers that will do the trick. I'll just print what I want (mirror) apply it with an iron (the iron for heat shrink covering will do) and after that a coat of clear sealant and the boat is finished.

Running

It was run, but not by me...

One of my "NET Fellow Boaters" Juan from Granada - Spain, asked me a couple of questions about motors and propellers some time ago. He told me that he was going to build the XS for his son.

"estoy haciendole a mi hijo (12 años) una copia del modelo XS, ya que es él el que corre con ellas y yo soy su mecanico y constructor y el año que viene, si es posible, correremos el Campeonato de España con este modelo, si conseguimos ponerlo a punto para lo cual tu colaboracion y conocimientos nos serian de gran ayuda."

(Here's a somewhat faithful translation...)

I'm doing a copy of your XS model for my son (12 years old), because he intends to run it in the national championship if we can tune it. I'm the builder and mechanic and I would appreciate your help to do it.

What more could I do? I was flattered and answered the questions as good as I could. The result came on the following email:

"soy el que te pregunto hace unos dias por el tema de las helices . Bueno, pues he probado la XS que tu diseñaste y que segun me parecio entender en tu pagina qu eno sabias ni siquera que fuera a flotar, FELICIDADES, no solo flota, sino que ademas va como un tiro. felicidades de nuevo, no te mando una foto porque no la tengo pero en cuanto se la haga te la mandare. va tan deprisa que los brazos que sujetan los flotadores que eran de aluminio los dobló.  le puse un motor Dr. speed  14x3 y una helice graupner de carbono de 32.5 mm.. bueno Antonio en cuanto tenga unas fotos te las enviare."

Here goes the translation:

I'm the one that asked you about propellers. Well, I've tried your XS design and I seemed to undertand that you were not sure if it was going to float. Congratulations, not only floats but also runs like a bullet. Congratulations again. I can't send you photos yet because I still don't have them. As soon as I get them I'll send the photos to you.
It goes so fast that the aluminum tubes that hold the sponsons got bent! I used a 14x3 motor and a Graupner 32.5mm prop in carbon fiber. Well Antonio, as soon as I have the pics I'll send them to you.

Gracias Juan, lo mejor de todo podria ser un titulo nacional com el XS.

I love this Internet thing. If any of you guys want to try the design, just download it here. 

Kyosho Magnetic Mayhem

Good value for money, 22 turn, with a longer than usual can and armature (1cm more) at a price you can afford to burn one or two every week.
Excellent torque and suited for 7 cells, this will be the primary choice for the XS. I want to keep the prices down and this motor fits this goal perfectly.

Trinity D3, 17 Turn Triple

• It draws 7 Amp and 80 Watt free running (with the shaft connected, 2.7 with no hardware), and 27 Amp and 200 Watt when on the water (with the Targa). With the 2000 pack the theoretical run time is 4.4 minutes. Not bad (theory is always very nice...).

Trinity Speed Gems Quartz, 19 Turn Double

The disease is starting to strike!! Exotic stuff, the need for speed, wanting to embarrass the nitro guys...
If I don't like the motor options I've made for this boat and if I want to race it the only way to go is BRUSHLESS. I have two choices - KONTRONIC and AVEOX. The first one has a very favorable price for the motor and drive set, the second option seems to have a better performance. Miha Holc won the 97 World Championships with Aveox motors on the monos and riggers he makes and races. Decisions, decisions...

Present config.

Related Information

Astro Flight
Octura Prop Chart
Prestwich Model Centre
Trinity R/C Racing Online