Prop talk Drive-system essentials

Radio Control Boat Modeler, Jun 2004 by Eyrich, Rick

Q I'm in the final planning stages of constructing my first RC powerboat, and so far, I've learned a lot by reading the threads on the RC Zone boat forum (radiocontrolzone.com). I want a nitro or gasoline deep-vee monohull, but I'm still unclear on one point: in the posts about drive systems, the terms "surface" and "subsurface" keep coming up. What are the differences between them, and which one would be better suited to my project boat? [email] Randy Hamilton

A Before we decide on a drive arrangement for your powerboat, let's review both systems. In a surface drive, which I recommend, only one prop blade "cuts" the water as the hull is moving. Surface drives are the choice of most nitro/gas racers because they allow the use of a larger propeller than a subsurface layout, and this means more straightline speed using less engine power. Surface drives also prefer full-throttle action, so they can be more difficult to turn at less than wide-open speeds. A well-placed rudder unit can improve this.

A subsurface-drive propeller is completely submerged, and because of this, it provides both great acceleration and turning ability. It's also easier to set up the rudder because it's centered on the hull's rear transom plate or is underneath the boat itself. On the downside, a subsurface drive creates more drag, and if your prop blades are too pitched, the boat will be prone to cavitation (air pockets form around the prop and reduce its efficiency).

As for your nitro/gas deep-vee prop choices, you'll likely use a non-lifting design, regardless of the type of surface drive you use. With a guide such as the one that's available from Octura Models (and also online at various websites; just type "Octura prop" into your search engine), you'll easily determine which non-lifting, 2- or 3-blade metal props are suitable for your final hull/engine combination. In general, your boat's powerplant will quickly let you know whether the prop is in the right size range, as it will either bog down or rev up like crazy as soon as you launch the hull! In your quest for the perfect propeller, one trick is to closely read the "race results" sections in RC Boat Modeler, as they always list the winning boats' prop specifications. When you've purchased a prop or two, be sure to balance them before you run your hull; few metal props come balanced from the factory, and an uneven prop can quickly destroy the hull's driveline.

To prevent your driveline from breaking, be sure to use the right size of flex cable: at least 3/i6-inch diameter on a nitro hull and at least 1A inch on a gas boat. If you use a thinner cable, you're asking for trouble because it might break when your boat is running wide open across a lake.

Now, armed with this drive/prop information, you need to decide which brand of drive hardware you want. Speedmaster, Warehouse Hobbies, Marine Specialties, Aeromarine Laminates and Ballantine Hobbies all sell surface- and subsurface-drive setups, and these companies also have excellent setup notes in their catalogs and on their websites to guide you through installation. RC Boat Modeler is a terrific source of driveline-mounting techniques: boat reviews and how-to articles usually include a detailed description of how to best position the surface drive of your choice on your project hull.

When you've chosen your engine, hull and drive setup, send a post to the forum detailing your assembled pieces, and it's likely that several of the regular members will quickly jump in and give you a host of helpful and supportive hints, so you'll be sure to end up with a killer project boat!

Aeromarine Laminates (302) 628-3944; aeromarinerc.com.

Ballantine Hobbies (863) 612-0555; ballantinehobbies.com.

Marine Specialties (713) 468-6070; marinespecialties.com.

Octura (847) 674-7351.

Speedmaster (901) 396-7485; rossisales.com.

Warehouse Hobbies (863) 699-1231; whobbies.com.