Graupner - ARROW - IC version!

Running name the "ARROW"
Deep-Vee Offshore racing boat for twin electric motors or IC with 3.5cc engine.
Accessories...80 (Engine, conversion pack & fuel tank) + radio.


I eventually sold the
SHG SHADOW to raise money for a new kit. My Hydrafibre Predator, "RAVING MAD", was running well so now I wanted it to have a worthy stable mate. The Graupner HYDROSPEED with it's ABS hull and electric surface drive worked well, so how about an IC boat with a similar setup. I decided to go for technology with a light weight hull rather than brute force and ignorance. I was going to do this by using ABS hull, an IC engine with surface drive, but can you fit an IC engine in an ABS hull? I hadn't personally seen it done before so mine was going to be the first I would see!. Someone told me that "ABS boats can't take an IC engine. The vibrations, heat and power will fracture the ABS". I took the bait, bought the kit, a  second hand rear induction HGK 3.5cc engine, an Irvine power pipe and "slowly and surly, I drew my plans against them".


At the time I started this boat deep Vee boats were still wining nearly all offshore races particularly an Italian genius Fabio Buzzi with his CESA 1882. The ARROW is a long elegant deep Vee thoroughbred boat, a near-scale copy of offshore racing craft very popular in the European & American offshore racing scene and a real mean racing machine.

Looking the kits on the market at the time, there wasn't much choice in ABS Deep Vee kits that would take an IC engine and surface drive, even now the ARROW is one of only a handful of boats kits that can. The ARROW is designed for twin electric motor setup using a Graupner HYDRO II surface drive strudder, unit similar to the one used on my HYDROSPEED but bigger and stronger. There is a manufacture's twin electric motor/drive pack  available for the ARROW.  It comes with gearbox, shaft, strudder and propeller, It's a good unit and I've seen it used in several other boats. The Graupner catalogue also has a conversion pack to enable the ARROW to take a 3.5cc IC engine. This was exactly what I was looking for.


The basic kit comprises of hull and deck mouldings in ABS and is very expensive for what it is, a few bits of plastic, a couple of fittings and plan sheets. All main parts are moulded in ABS and measures 820 x 210mm. The plans are very good but not many fittings and no running gear come in the basic kit. For propulsion the electric motors unit costs 60, or the IC conversion pack 50, engine not included. I wasn't going to pay either of those prices and prepared myself to do the IC conversion from scratch. I did find a model shop that had the IC fitting kit, but most of the bits had been nicked and only the wood parts were left. I bought what was left of the fitting kit for a fiver and counted myself lucky that I wouldn't have to make up all the woodwork myself. The profiles of the wooden sections are included on the plan sheets if you want to make your own.

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Construction was going to be a long job so to motivate myself I cut out a photo from the side the box and stuck above the work bench to give me the incentive to carry on. I only used Stabilit Express or a good Superglue on an ABS boat although I now also recommend Quantum, made by Holdtite & 'Devcon' 'Plastic Welder'. Rough the surfaces to be glued with sandpaper first to ensure a good bond. Start be marking out and cutting the hull components, deck etc. just the same as the HYDROSPEED kit.

All of the internal work woodwork, engine, radio etc. must be fitted  before the deck is glued down. Engine and radio rooms are constructed by installing wooden bulkheads which gives three separate sections. This makes the bow and sides into a an airtight  compartment. There is plenty of room for buoyancy in this area using two part of plumbers foam. The foam is applied with the deck off in small
amounts and make sure that it doesn't push the sizes out as it expands. If it does, a sharp knife can be used to trim the foam back down the hull sides, also trim the top down so it doesn't quite touch the deck when fitted. The foam will be heavy but as it's going to be massively over powered anyway so the extra weight will add to the boats stability and strength. Make sure that the wood is thoroughly fuel proofed inside and out after fitting. Don't apply the foam until the engine installation is complete. ( Be careful with Plumber / Builder foam - see

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The designed method of fitting the engine makes it very difficult to take out for service and maintenance, so I modified their mounting method to make it removable. The base of the engine was made as shown one from aluminium plate to the same dimensions might be better. Anyway I made the engine plate and fixed them to the base frame via rubber engine mounts available from Prestwich, SHG etc.

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This whole unit then was made to sit neatly on the hull floor but is not glued down.  The final position of the frame was marked out and ply wood spacers made up to pack the space between the frame and the engine room sides. A 4BA bolts can then pass through the frame, spacers and side walls to a spiked nut on the side wall. The outside walls needs to be reinforced with an extra layer of ply around the spiked nuts to prevent them splitting the wood. Make this joint well because all the  stress from starting and running is transferred through these points. I used ordinary slotted cheese head bolts to hold the frame but to undo these and remove the engine took ages and required triple jointed fingers, these needed to be replaced with hex nuts or Allen bolts. Even so with this setup the engine comes out in less than 5 minutes.

With hindsight and now having built other boats, I have found better ways of mounting an engine such as in shown in the GLOW ENGINES Section - Engine Mounts. Coat the inside of the engine room with liberal coats of fuel proofing after construction is complete, have I said that already? A suitable tank e.g. a SLEC 'RED', can be fitted crossways right up next to the radio box bulkhead and wedged in with a block of polystyrene etc. I always use a filter in the feed line.

I used 4BA  for the main shaft but 2BA would be better. Use lots of Stabilit around the transom exit of the shaft as this will be covered by the radio room floor and not be assessable after building. The shaft needs an oiler tube, fit this as close at you can get it to the front bearing to make sure it also gets oil. The main shaft can be connected via a Huco universal joint, (UJ) to the engine and then through a Graupner UJ on the strudder. The Huco UJ are very strong but I had doubts about the power handling capabilities of the small Graupner UJ with the brute power of a the IC engine. ( Eventually the Graupner UJ did require replacing but only after I ran the boat aground! It can do it's job if it's not abused.  )

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Graupner's own strudder is well suited to this boat but other manufactures are now supply strudders particularly the Americans who also supply a large range of struts, rudders and running gear, some can now be purchased from UK outlets. The Graupner HYDRO II strudder is made from glass reinforced nylon and is tilt adjustable and very though but not indestructible. Take extreme care when fitting the strudder as the slightest misalignment will cause all sorts of problems that will be very hard to correct. Connect both shafts via the strudder coupling when marking the pilot holes of the strudder bracket. Constantly check the freeness of the shafts when marking and drilling the holes. It might be a good idea to strengthen the transom with thicker ply than indicated on the plans. I used 1/4 inch ply, loads of Stabalit and bolted the strudder on with M3 Allen screws and Nylock nuts.


I rearranged the radio compartment so that the servos are higher than on the plans so that the connection links are simpler to the engine and strudder. The two servo sit on aluminium rails fitted across the beam of the boat. This arrangement allows me to monitor the bottom of the radio box for water ingress. The receiver is fixed next to the servos on the rails and the batteries to sit in foam underneath. A Robart watertight bushing takes the throttle linkage through the radio room bulkhead to the carb on the front of the engine via at bell crank on the front bulkhead and then another link back the throttle arm 9see 1st below). It not an elegant link and did get in the way when starting the engine but it works and works reliably. The two links to the strudder are achieved by trial and error. The lid of the radio box supplied in the kit is white ABS, it's better to make one from Perspex sheet so you can see what's going on in there. The radio aerial need some thought, I fitted a hollow straw type aerial to the cockpit hatch but the wire keeps getting in the way, it would be better having a permanent home on the hull or on the transom where the wire can be fixed down permanently.

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Part of the original plan for this build was to install an Irvine power pipe on the engine but, it soon became evident that there wasn't enough room to fit it. Irvine power pipes are small tuned pipes that boost an IC engine's output by up to 20%, (- see Tuned pipes. ) A standard aeroplane type silencer was then fitted with silicon tubing running up to a 10mm brass outlet pipe that runs from engine room, through the radio box and out the transom (photo 1). This pipe will get very hot so when planning out the radio box, keep it clear of the servos & wiring etc. I fixed my exhaust in with Stabilit but  this gave way due to the heat of the exhaust, this needed resealing with silicon tubing and sealant around the pipe at the bulkheads is better.

Two other tubes run through the radio box, the water inlet and outlet pipes. I used 1/8 brass pipe and bend them out of the way of the servos, the water pick up exits the transom on the centre line, I don't like external fittings at funny angles. The water pick-up is part of the strudder unit and a short length of 1/8 brass tube pushes in to the hole and silicon tubing take up the gap to the inlet tube. Don't glue the pick up tube just yet as it will need some adjusting for the best water pick-up.

I then became concerned about the strength of the hull deck joint in case  of a collision so a SGH rubbing strip was fitted all the way around the hull and meets at the transom. Warm the strip in hot water to make it more supple and keep the rubbing strip taught at all times until the glue  has set, I used large crocodile clips and clothes pegs to hold it on while gluing, use Evostick or Superglue.

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With the engine, running gear and radio in place in the hull the deck can now be fitted. Again fitting the deck is a hard job and needs to be planed out meticulously and a couple of dry runs are in order. Again I used Stabilit Express, but as the joint is long and Stabilit has only 15 - 20 minutes working time don't hang about once you've started. A lot of care is required to seal the bulkheads to the underside of the deck because if water gets in behind the wood you'll never get it out again. A really difficult bit to seal is the front wall up under the deck. I used a dentist type mirror and a small spatula to check to see how bad a job I was doing. Once the joint is complete and left to cure overnight an extra filet of Stabilit to seal the outer edge and provide more strength.

To improve handling the corner of the transom and hull has to be made completely square to stop water creeping up the stern and causing drag. A few layers of masking tape is stuck over the bottom overlapping the transom, Stabilit then fills the slight cavity created by the tape and the hull. When set remove the tape and a completely smooth hull with a overhang beyond the transom should be left. File and sand to a square edge.

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Remove everything that's removable i.e. strudder and engine and seal of  the engine room opening with masking tape. Put a blob of 'Blue tack' or 'Plasticine' over the shaft end and more tape around the in / outlet pipe and tubes. I used car spray paints, cellulose based which were available  in any number of colours. I chose Aquamarine metallic blue. A cellulose based varnish must be used to protect the paint work from the fuel. Lightly sand the ABS surface to key the surface and then wash it down with soap,  water and then rinse thoroughly. Use very thin coats, if the coats are too thick the varnish may  wrinkle the top layers. Wait 2 or 3 days for the paint to dry before applying the varnish else the two coatings may react to each other and blister. Funnily enough I forgot to fuel proof this boat and the paint stood up very well but I did eventually have to be repaint the hatch after it became a little scratched and tattered. The gold trim is Solatrim and again trimmed and applied by my artistic mate Brian. Always make good friends with someone with a skill, equipment or money!

Fit the throttles and cleats but these will be knocked off after a few runs but they look good while they are there. Stronger throttle arms could be made from map pins with the plastic heads. Paint the heads before fitting them and make a good job this is where a lot of people look first for some reason.

The plans show the hatch is to be fixed down with hinges at the front,  don't bother, they make the front sit high off the deck, which looks unattractive and it doesn't work properly anyway. The old elastic band job is best. Fix hooks (in wooden block) to the underside of the hatch and another to the hull floor, a band secure the hatch down, I actually tied the band down around the main shaft. Strengthen the front and rear edges of the hatch with Stabilit express and scraps of ABS, it'll get bashed a lot.

I actually made a proper stand for this boat because none of my other stands would cater with the narrow beam of the boat. The plan shows you an outline to make a stand but it could be made better if the sides are extended further up around the hull wall for a more secure cradle. The stand was made up from some old shelving. Complete the water and fuel tubing to the engine. Oil the two shafts and charge the radio, and that's it. Construction which took about 72 hours including all the modifications, the electric version can be built in around 12 hours if you know what you're doing.


A Graupner 40S red surface propeller was fitted. Surface drive propeller  are not the same as ordinary propellers and don't let anyone tell you that they are. Graupner's props are made from glass filled nylon and subject to bending under stress, they wear out after a while and not the absolutely the best shape but they are much cheaper than metal ones and work very well, so they are perfect for sports models. Metal ones are better, but have you seen the price of them?

The tank was filled with straight castor and the radio switched on. On starting the HGK .19 ( 3cc) engine it started to rev up higher and higher, I thought it was out of control or the mixture too thin or something, so I stopped it in the usual way, finger over the carb intake, watch you don't burn your self on the cylinder head. Making the mixture richer did nothing to slow the engine down, eventually the mixture was so rich
that the motor was 'four stroking' only firing every second  revolution. On thinning out the mixture again, the engine still ran fast but seemed stable, so I settled on this setting and readied myself for our maiden voyage.

On the water she revved, ranted and raved but didn't do much else. The propeller splashed about a lot but only moved forward at a fast walking pace. "Well that's not very impressive!" It seemed the propeller wasn't 'biting' the water. A few other propellers were tried but the general consensus of opinion from other club members were;

"What's he got on the water now?"
"Something not right with his boats again!"
"Seems to be 'over revving.'"
"Anything good on TV tonight?"
"When do the pubs open?"

It was back to the drawing board. The HGK engine has a very high tick-over speed and would appear to be best suited to run on a gearbox or work with a tuned pipe. A high top end is good for racing and usually these engine don't like to run at low speeds. On a sport type boat low tick-over and steady acceleration is more desirable for reliable running and slow cruses pass the jetty to impress the public. This engine showed no signs of being tamed and burnt out several expensive glow plugs to-boot. I never manage to obtain any sort of respectable speed even after a lot of fooling around so after a month or so I retired the HGK engine in search of a better engine.


I had a discussion with John at Red Baron Models ( now gone the way of the Dodo) who runs multi boats. He  confirmed my conclusions and told me that 'HGK engine do run very fast, that why it was sold with the small tuned pipe'. You need a more tame engine and strangely enough we have just the engine you need an Irvine 25!" . You guest it, I had to sneak more expensive bits in to the house and pass the 'Mrs'.

The Irvine sport 25 (4cc ringed) is a lovely engine. It started first time every time and ran smoothly and predictably. Their ABC engines are extremely powerful, I had one fitted in my next multi boat that you will soon read about and the boat was very fast. Can I have  a free engine now Irvine?

The engine mount needed only a little modification and I was glad that I hadn't made the engine a permanent fixture as shown on the plans. The radio link was remade and was now much simpler as the engine has a front induction carb ( see 2nd in dia. above). A 40S propeller was refitted, the batteries charged and it was back down to the lake by the next weekend.


The first run with the new engine was much better. Now the bows were  lifting right out of the water, in fact lifting too much. The front end was bouncing up and down like one of those nodding dog you used to see in the back of cars. I put some weight in front of the engine (small spanners etc.) and this improved slightly but not enough. The angle on the strudder was increased so now it was no longer parallel to the hull but the inclined downwards. This temporally cured the porpoising but as I experimented with different propellers and needle settings and performance increased the bouncing would reappear.

I now know that this problem is caused by an incorrect centre of gravity and my boat's C-of-G  is a little too near the transom. You can either add correcting weights towards the front or fit trim tabs. An advantage of fitting trim tabs is that they are quickly adjustable so that a workable setup can be quickly achieved. Trim tabs extend the practical length of the hull thus moving the centre of gravity forward, adjusting the angle below the hull line lift the stern up and tilt the bows down. SHG make a nice set out of plastic with turn buckles for adjustment. Try annealing them in boiling water for a few minutes to ease the bending area, bend while still warm. Doing this is meant to ease the internal moulding stresses but I've never had any trouble with any of mine without doing this. You can simply make you own trim tabs from brass, stainless steel etc, and use clevies to adjust them.

I traced around the tabs on the transom, removed the paint and stuck them on with Stabalit. It's important that they are perfectly in line with the hull bottom so as not to cause any turbulence. Back at the lake, a slow run showed that the tabs didn't produce any noticeable drag as I had feared. On opening up the throttle the engine made a lot of noise, revved to a worrying peak then the propeller seemed to take grip on the water and she shot forward at a terrific rate.

Just a few adjustments to the trim tabs and about 1lb in lead in front of  the engine and up against the front bulkhead ended all the porpoising problems. I added the lead because I didn't want to turn the tabs down  into the water causing excessive drag. The best strudder angle is about parallel to the hull. She rides high on clam water with most of the hull out of the water on rough days the trim tabs are raised to
prevent the bows jumping up off each wave. If I was going to build another ARROW I would use lead sheeting in the bow compartment to improve the balance and maybe eliminate the need for trim tabs. As it stands now the balance point is 260mm from the transom.

On a straight run she could keep up with the Raving Mad before the tuned pipe was fitted and even now the ARROW will give her a run for her money. You can't turn sharp corners but offshore racing boats aren't meant too. If you try a sharp turn at speed the bows drop, it starts to  turn then spins a spectacular pirouette in a explosion of spray but is hard to turn over. Quite impressive but it doesn't do your nerves or
the boat any good!


After about a month of successful weekends of running and tuning a problem set in. It would run OK until at very high speed then the engine would suddenly stall without warning. On recovering the boat it would start again immediately without any hesitation. It would run at half speed all day long but cut after about 10 seconds at full throttle. Everything was checked, re-checked and checked by others and found nothing amiss, still the boat wouldn't run at full speed. The engine didn't scream or splutter before dying which would indicate a fuel problem, it would just shutdown as it I had closed the throttle. Someone suggested an exhaust problem, maybe there wasn't enough back pressure at high speed as the exit had quite a wide opening. The aero silencer was removed, a stub manifold fitted along with a neat, small in-line silencer. No change. Different fuels were tested, the same result. Was the engine over heating? A additional water pick-up was fitted on the transom which resulted in the boat always being turning  towards the pick-up. The pick-up was removed at the lake  before it came home, at least the boat went straight without it.

Eventually late one summer evening after she had stalled again but this time near the jetty, I managed to recover her fairly quickly. The throttle servo  was twitching like mad!?!?!?! At last I had found the problem. Back home in the boat room the radio box was pulled apart and found to be full of mildew. It took hours to clean it all up and repair but nothing needed replacing and I had found the problem...... Back on the water it was exactly the same. DRAT!

Now I was really scratching my head. I came home moaned, groaned and blamed my wife for the problems on the boat and went upstairs and sulked. "It MUST be the radio!" The radio box was striped out and completely rebuilt, throttle servo replaced and sealed with  silicon. Again the same on the next run, 10 - 15 seconds and that was my lot. Fuel?!?!! "That's it, fuel flow problems." A new larger tank fitted. Same. A smaller tank fitted. Same. Pressure feed system fitted. All the same. By now I was suspecting my wife of sabotage while I was asleep!

"Well I know it worked OK to start with, so what has changed?" Back to  basics, engine, silencer, fuel, tanks, radio, servo, water cooling, shaft, bearings and my wife all had been checked or replaced, everything except the PROPELLER. I had replaced the propeller  during tuning session for a larger one (42.5S) to get more trust while test a higher Nitro fuel and I hadn't thought of going back to the smaller 40S I had originally used. A 40S was refitted and the problem disappeared instantly. Surface piercing boats rev much higher than submerged types because only half the propeller is actually in the water when working properly. A propeller that's far too small will allow the engine to over rev, thus over heat and a propeller that's too large will will overload the engine also causing it to over heat!

That may seem obvious now but this is only because I've told you the  answer. At the time it didn't make any sense at all and took three months to discover. The original 40S prop was fitted and the ARROW has ran brilliantly ever since ...........apart from a few instances of bad navigation!!! On a video that a friend took for me, when I accelerate the ARROW, it looks like the video has been speeded up as the acceleration is absolutely  phenomenal!


The Irvine engine's carburettor gave several problems mainly coming loose a couple of times. The attachment for the carb is two grub screws through the engine body on to the carb throat. On the old style carbs the body and throat are made from glass filled plastic, thus when you try to tighten the  screws into the neck to get a good grip, the neck squashes inwards reducing the intake area. The newer type carbs have a metal collar to give the grub screws something hard to bite into. If you get / have an old style engine, there is two things you can do, (1) note where the screws grip on the neck and mill a slight recess, about 1 or 1.5mm, thus even if the neck works loose it will still be held in position, and (2) clean the engine, carb and grub screws neck and silicon sealant the two together. Be careful not to get any silicon inside the engine or carb.

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The problems with condensation and mildew in the radio box were due to the hot exhaust pipe passing through and heating up the air tight compartment. A white mildew formed on everything in the radio box that had to be cleaned out more than once. This was cured by leaving a small vent hole in the lid in which a silicon sump tube is drop to drain out any water, but none does get in.


The ARROW is probably an easy kit to put together in the electric version but the IC version is more of a challenge. I have run the ARROW against RAVING MAD many times and have only gone home when we've run out of fuel or careered out of lake! This is probably the most successful experimental boat that I have ever made not only because it's impressive speed or that the ABS hull still shows no sign of distress or the easy start engine with the IC engine or it's good looks but mainly because this deep vee boat handles surprisingly well. I'm glad that I put in a bigger engine than recommended as the hull design has no problem handling the extra power.

I have seen another ARROW in Kellers of Norwich that also has an IC  engine, an ENYA 19 and he says it also runs really well. Well that make at least two happy Graupner ARROW IC boat owners. I wonder if that guy in Norwich wants a race?....

Graupner have designed a very good  kit, well worth buying, HIGHLY recommended.

Electric version
Tuned pipe
Metal propellers
Radio controlled trim tabs
Scale fittings

Irvine .25 engine
Home-made engine mount with rubber blocks
Huco & Graupner couplings
40S Graupner surface propeller
Manifold + in-line silencer
Red SLEC fuel tank, pressurised system
Accoms 2 channel radio

Value ........................... 4 (Quite expensive hull and fittings)
Kit Quality .................... 9 (Up to usual Graupner standards)
Kit Design .................... 9 (Internal layout well thought out)
Ease of Building ........... 6 (IC tricky, electric would be much easier)
Finished appearance ..... 8 (Few decals so needs nice paint job)
Handling ....................... 9 (Very stable even at high speeds)
Other examples of the Graupner Arrow - mainly from Ebay.

With the successful completion of the ARROW project I was foolishly became  over confidant and thought I could build and run on any type of model I wanted. Hydroplanes are the fastest class of real and model boat and it just so happens that an aeromodeller had one sitting in his garden shed doing nothing. After a little haggling we arrived at a deal and the
hydroplane was mine.


"Well  all  of this is just my opinion,  but what do I know!"