DPI American Dream hop ups

Improving the DPI American Dream

The DPI American Dream hydro is a basically sound model based on a high quality ABS moulding. As with all commercially available kits, compromises have been made in order to make it easy to produce and meet the necessary price points.There are a number of things that can be done to improve its performance and handling.

The improvements come in a number of areas and we will deal with them one at a time. We have used Darin Jordans version of the American Dream, Apian Jeronimo, as a basis for this article and we thank him for the pictures reproduced here.

Before starting it is a good idea to empty the hull off everyting you can.

Hull

The hull is a quality ABS mould however the moulding process itself introduces round corners. Round corners encourage water to stick to the hull and are a principle cause of excess drag. The kit already supplied ride pads for the sponsons in order to reduce the drag effect on them but it will benefit from other water contact surfaces being given a similar treatment. Darin has used angled plastic section around the corner and blended it in with filler. To avoid damaging the hull you need to be careful with the sanding. Darin has finished the rear section with primer as he finds it loosens up the hull by encouraging the water to detach.

A similar effect can be produced by applying a plate to the outside of the transom, this will produce a hollow between the plate and the hull where the hull rounds off which needs to be filled with lightweight filler.

The tunnel sides will benefit from similar treatment as will the rear ride pads.

Ride Angle

The angle at which the sponsons contact the water is critical to success. Not only is there a best angle for sponson contact but adjusting the sponson or rear deck height will change the effect of the tunnel. Now if we lift the rear of the boat then the sponson contact angle is automatically lowered. If you add a liner to the rear ride pads then you will have done this already. We need to get the sponson angle at between 4 and 6 degrees ( 5 is my preference) but when we have done all the work. So we will come back to the sponson pads.

Stiffening

Even though the kit hull has extra plastic parts applied to it to make it stiff, it will still flex under the forces applied to it. As it flexes the shape it presents to the air and water will change - and so will its performance. We can prevent this by inserting a frame inside the hull that will add the required strength and stiffness. The frame needs to be light and extremely strong and the best material for this is carbon fibre tube. Any tube made just from Carbon Fibre is very stiff but prone to split, to avoid this kite flyers use a cross between glass and carbon tube. You can tell the difference as the glass carbon mix is grey, glass carbon layers is black outside and grey inside and pure carbon is black right through. if you can, get the middle one. The bigger the tube the stiffer but we have space problems already so I suggest that you use 1/4 inch, 6mm or 8mm tube. You can see the frame inside Darins model in the picture below. the frame needs to be firmly fixed to itself and to the body. Use Epoxy or a small amount of GRP resin (THE ACTIVE INGREDIENT IN GRP RESINS IS STYRENE SO DO NOT USE GRP ON STYRENE BASED MOULDINGS. ABS IS A STYRENE BASED PRODUCT BUT THE BUTYL ELEMENT GIVES IT MORE RESISTANCE. CHECK FIRST OR BE SORRY LATER!) . Keep the hull an a flat straight surface and keep a constant check on it to make sure you do not introduce a warp. The hull MUST be flat and true.

Hatch

The hatch seal at the rear is not good and it is impossible to tape it. There are a couple of choices for improvement.

You can cut the rear portion of the hatch off, glue it to the body work and then glue in a plastic locator for the centre section. You will then be able to tape the hatch in place and have a completely waterproof hatch.

Alternatively you can provide a soft seal for the rear using silicon caulk. On the inside of the hatch there is a trough that runs round the rear of the jet tube. This is located above the rear upper bulkhead, filling this with soft silicon selant will really help to seal the boat here and enable you to tape the rest of the hatch up. To create the seal first you must smear the bottom mating surface with a release agent. There are specialised ones produced but Vaseline or KY Jelly will do the trick. you only need a very thin layer. Now clean off the inside of the trough and fill with silicon caulk so that it is just over full. Now press the whole set up in place, tape down and wait for it to set (24 hours). Once set you will be able to remove the hatch and will have a good repeatble seal left in it.

Drive Hardware

The hardware supplied with the model is of a minimum standard, it is not easy to set up, there are large surfaces to cause friction and there is a lot we can do to improve it. In my opinion the optimum position for a prop is at about 10% of the boat length away from the rear - this places it in clean water and improves it's performance.

Straight Hardware

I have nothing against straight shaft hardware, it is relatively cheap and effective, but once it is fitted there is very little that you can do with it adjustment wise. There are better options available than the one supplied. e.g. Gundert Drive units which have integral bearings,a central offset bearing in long units, a ground shaft and a precision integral motor mount. Similar units are available from H&M etc.

Hardware choices are discussed elsewhere in these pages so without going into it in depth, a fixed shaft is just fixed - no adjustments. It also present the prop to the water at an angle and that gives a degree of lift and a slightly lower rearward thrust. The lift must be balanced, generally this is achievable by moving the cells backwards.

As you will gather the set up is rather critical, also unless the replacement shaft is of the wire type the new outer will be too large to fit the skeg. It is just about possible to fit a 2mm bearing at the outboard end but you will have to sleeve the end to do it as the 2mm bearing has a 3.5mm od. You might consider lignum vitae! If we assume that the skeg is too small you will need to remove it - and the old shaft, if you have an integral motor mount you may need to remove the old motor mount as well. Resize the access hole in the bottom so that the shaft fits but does not bind. Now jig everything up so that the bottom corner of the new shaft outer is level with the bottom of the sponsons when the hull is at it's flat running angle. When you are satisfied tack it all together - check and when ghappy glue it all in place. Repair the visible damage with lightweight filler.

Flex hardware

In flex hardware I include flexible wire drives. Flexible drive hardware is the ultimatley adjustable hardware. It enables you to vary the height and angle of the prop and is my preference. It is also expensive. I do know people that have run straight shafts and replaced the shaft with a flexi shaft as a middle of the road upgrade. For this model I would choose 3mm or 125 or 130 cable sizes, this covers European, Fullers, Hughey etc. Beware they are mostly not interchangeable. Set the strut bracket either central or offset to the right by 5-8 mm. If you offset it then you should move the motor mount as running the shaft angled will increase the drag. Initially set the bottom of the boss of the prop on the same level as the bottom of the sponsons (with the hull in it's running position). Prepare and repair the hull as above.

Rudder

The supplied rudder does it's job but it is worth while replacing it with a CNC aluminium one, Fullers, H&M, Graupner. The positioning is not critical and on the original set up the rudder is on the left of the prop. Some people prefer to have the rudder nearer to a straight line behind the turn fin but this will increase drag on the RHS of the boat. The choice is yours

Turnfin

The turn fin is the subject of a lot of discussion. I have seen very fast AND manouverable boats running with long but short (1/2 inch) turn fins - these are obviously better suited to flatter waters. Others use different shapes of fin and the preference is personal- you will find a shape that suits you and your driving - and probably stick with it.

Before doing anything you should make sure that the turn fin is flat and parallel to the centre line of the boat in a forwards/rearwards direction. Tilting the bottom of the turn fin inwards slightly can help to hold the right sponson down in the turns, it is a fine adjustment - too much and you create drag - too little and the sponson lifts.

There are a couple of things that can help - sharpening the left side of the turn fin (rather than both sides), and/or hooking the bottom.

Hooking the bottom consists of bending the bottom of the turn fin over until it is parallel with the running water line this can help prevent the fin lifting out of the water. However getting the angle wrong increases drag significantly.

Sponsons

The sponsons support the boat whilst it is running as said above the best attack angle is between 4 and 6 degrees. so how do you measure this? Well you need a triangle. To create your triangle draw a line 305 mm (12 inches) long. at one end draw an line at right angles to this line. for 4 degrees measure up 21.5 mm (0.85 inches), for 6 degrees measure up 32 mm (1.25 inches). then cut the shapes out in wood or balsa. 5 degrees would be 26.75mm (1.05 inches). Jay Turner recommends Carbon Fibre (graphite) coating on the bottom and fine sanded with wet or dry, he says that this has minimum drag as the water runs flat and does not pool (no meniscus so surface tension and it is the surface tension that causes drag

Shaft

Straight shaft

Set the boat on a flat surface it needs to be set so that the front of the bottom of the boss of the prop just rests on the board and so do the front sponsons. This should be achievable - if not you will have to reset the prop position. Now measure the angle of attack of the sponsons by carefully inserting the triangle - if your selected triangle fits the ride is fin. If not you will have to adjust the angle by padding the rear of the sponson with extra plastic ride pad.

flex shaft

First we need to adjust the ride height of the boat. As a start point set the strut so that the bottom of the strut ispositioned so that the boat is flat. Now check the angles as above. An alternative is to set the bottom of the boat near flat, to do this measure the distance from the sponson bottom to the under deck, block the rear up to this height and set the strut on the board. Now measure the angle, and shim your sponsons accordingly. This gives you a good starting position. If you went for 5 degrees then dropping the shaft by 6mm (quarter inch) will decrease the sponson angle to 4 degrees (approx) and raising it 6mm will mahe the attack angle 4 degrees. This is where we get the advantage of adjustability from the flex set up - albeit at a cost

Mods

If you look at Darin's boat you will see that he has fitted side stepped sponsons, this helps at high speed by reducing drag and at lo speed by giving lift from both surfaces. The height indications above still stand. Another possibility is angling the outer edges up this could be say 4 degrees. This helps as the faster the boat goes the more lift it gets and the sponson automatically runs higeher with less in the water. This is self adjusting and softens the turns. The last possibility is a combination of both with the inner low side being at say 4 degrees and the outer side can be higher say 6 degrees.

Propeller

replacing the propeller is an essential step, there are a few choices. Octura small diameter plastic props are available and they are probably better than the one supplied. These are 1/8 inch bore props. next best are the Graupner 2318 or 2327 series props - these have a 4mm thread (you can purchase replacement fexi shafts with the correct end) and lastly it is really difficult to beat a preoperly sharpened and balanced Octure metal prop. The size is dependent on your motor so start small and work up - checking the heat of the motor and the controller as a guide. The motor may get fairly hot the controller should only get warm

Motor

Upgrading the motor will make the biggest difference. A modified 540 as low as 12 turns or an equivalent brushless set up will reap it's rewards but you must adjust the prop accordingly

You can see in the pictures that Darin has used a brushless motor and re-used the motor mount that was supplied. There is nothing wrong with doing this. Just make sure you can move your cells a bit so that you have some adjustment for the c of g.

And Finally

You will find that he boat reacts differently with different settings. You may need to put some down angle in the shaft to get enough lift to raise the rear of the boat. You will probably have to go through a range of settings searching for the magic set up. To achieve the top results you will have to time the boat over a fixed distance, you will have to recorde your findings and settings, and you MUST only change one thing at a time. Also remeber that when you find a setting that makes a big good difference - you will still have to run through the other settings. It takes time and patience but the rewards are immense......... Now you best paint it.... just remeber that paint is quite heavy ;)