I'm new to the sport. I bought a 26cc Zenoah mono 6 months ago, and have learned quite a bit about props, porting etc. I have just ordered an Insane G4 Hydro hull and need to pick an engine.

I want something that I can practice a lot with, (without wearing things out too quickly), but race competetively. Although I was thinking if something a little more exotic, I am gravitating back towards a conventional Zenoah G260 as the most 'practical' starting point.

Logic tells me that like with other motor sport, the cheapest horsepower is extra CC's. I don't understand why everyone doesn't immediately go from 23cc to a 29cc cylinder, (once they decide to go open class). I would have thought that a mild port job at 29cc makes for a less temperamental and longer lasting engine than a wild port job at 26cc? What about adding the longer stroke crank as well, (although I guess at a point vibration increases)? What am I missing, why not go straight from 23cc to 31cc?

On the two bearing V's single bearing crank issue. Do two bearing Zenoah cranks bend at 6.5hp? If not, why go to 4 bearings?

If building the engine at 31cc makes sense, is there any point going beyond a mild port job at my level of driving experience?

It's worth adding that here in Australia, it seems that it is only the 23cc engines that are heavily restricted to original components, and that you are pretty much free to change anything on 'open' engines,(capacity, cranks etc).

I have ran my Insane G4 for the last two racing seasons with a RCMK 254s EVO modified by Scott and I am very happy with it's performance. Boat runs in the 70 mph range with no problems. There are faster ones out there but I prefer the boat handling vs all out speed. The hull also handles rough water and wind very well. Don't think you could go wrong with mod 26cc RCMK or Zenoah.

Thanks Mercury94, I'll probably stay Zenoah just because it's easier to get parts here. It seems that there are just more classes in the US so the Zen 29cc cylinder is not widely used?

A lot of us in the Western U.S. race under NAMBA rules and class structure.
We at least here locally ONLY race the G-1 class engines within all the hull types.
G-1 is modified stock production cast head & case engine of 25.99 cc or less.

So ... build a stroked or big bore Zenoah 260 or RCMK 254 you have an engine you can't race.

**If and when the wants and desires have enough folks wanting to run the Now G-1 hull class's under G-2 specs ( 35.99 cc limit ) will you see the mass's who race under NAMBA rules using them.

The G-2 class for YEARS never had any decent engines available so all the effort has gone into the G-1 class, now with Stroked cranks and Big bore cylinders becoming the rage this may change ?

** Do keep in mind what SANCTIONING body you will compete ???? .... The IMPBA gas engines rules have a LOT MORE room to mix and match manufacturer parts to build an engine. Where as NAMBA "G" class's MUST USE only those parts and pieces from original engine manufacturer
Only way under NAMBA rules you can mix and match manufacturers parts is under the "GX" class thats puts engines built this way in with Quick Draws, J&G's or other exotic and billet parts motors.


Just an FYI ...

Scott, thanks for the excellent info. The regulations in the US seem a bit odd based on the available motor options. With motorcycles, (my son has recently raced the Australian MX series), classes have significant capacity differences, ie 250cc and 450 cc. I just don't get the 23cc, 25cc thing in gas boats. Logic would say two classes, one up to 25cc, (with modified stock parts to keep the entry cost low),then a second unlimited class up to 35c for those that want to experiment and push the limits. Sounds like too many classes for a relatively small sport?

In our club we often run "open" class with 26cc,30cc,etc. A good handling boat and good driving will only do so much for you when a more powerfull boat just drives away from you. Having racing with a engine class system keeps the racing much more fun. The competition part comes in and you have to work on your boat and driving to stay ahead of the other guys. I would do as Scott suggested and contact your local club and find what rules and class they run. I would build the boat with a engine of the same capacity for the class you are racing with.

Interesting issue for someone new to the sport, (without a history of why it is like it is). The Australian rules seem to make a lot of sense, one engine class up to 25cc, (based on original manufacturers parts),then a second up to 35cc pretty much unrestricted. The hulls probably need to be limited to the same length for both classes, (so that a full 35cc engine pays a penalty for it's physical size increase). If this was universal, Zenoah etc would change from 26cc to 29-31cc for their second engine size. It would seem that there are currently too many possible engine classifications to be good for the sport overall?

The real issue seems too few boaters compounded by not enough time in the day too even run all the class's if each hull also had different engines as well.

A few years ago an average 2 days of racing was 135 to 160 boats. Take that number having 6 boat heats, 4 rounds per class and your days pretty full indeed !!
Some still race 8 boat heats which can speed things up but DNF rates go up too having retrieve boat picking up the dead killing time.

If you had LOTS of interest and LOTS of clubs with personnel willing to set up and take down racing venues more races could be run on alternating weekends .... Sadly like most sporting venues the few do most of the work. Somewhat a paradox

I'm getting it now, only just realised that you said ONLY ONE engine class, (G1), in your previous post. At the meeting I went to here, they ran two engine classes in the popular hull types and I thought that was normal. Now that it is well known that 31cc is achievable, as you suggested it possibly makes sense to move the limit up to 31cc?