Zenoah Petrol Engine Hull-2

The Zenoah G231PUM and G260PUM are by far the most popular petrol engines around for fast competition boats and put out around 4 Horse Power. I have had many people ask for any details that I might have on them and whether I do a hull to suit, so with the help of a customer in Florida I spent some time and created a 4' Hull-2 version to fit them, in conjunction with a German MHZ gearbox which provides twin counter-rotating shafts fitted with two 60mm props from the single engine input shaft and it also provides a 1.3 to one up speed ratio to better match the engine speed and torque to the propellers.

There are other similar engines or Zenoah modified's based on brushcutter/chainsaw engines from Sikk, B.H Hanson's Quickdraw, O'Neill racing, Bonzi sports and many others that are also suitable and the modified and super modified engines can provide up to 7.5 HP at up to 20,000 rpm.

Of course you could just buy a brushcutter and take the engine off, and many people have done that, but my latest research says that many of the current brushcutter engines have a very large air blower, generally built into the front engine casting, to provide both forced air breathing for the engine (a simple supercharger) and a cooling air blast for the cylinder head and while this may be an option for a 6' or larger hull it would be very difficult, if not impossible, to fit many of the engines that I have looked at into a 4' boat.

I have almost finished the design for a 6' Hull-2 version on which I plan to use 5mm pitch steel reinforced Synchroflex / Breco toothed belts and pulleys to provide drive to three shafts, each fitted with a 60mm prop.

The biggest problem I have had with these two projects is getting definitive details on the parts, I asked Zenoah for a drawing of the engines, don't have one, they said! I asked MHZUSA for a drawing of their gearbox, don't have one they said! I asked B H Hanson and Bonzisports for mounting dimensions for their quick mounts, still waiting! (BH Hanson did send me some great Zenoah drawings though, thank you guys, but after I had finished drawing up an engine bottom casting that I bought off Ebay). 

So, as usual, I end up re-drawing everything, but to help all those out there like me I have put them on the website. So come on guy's, I don't want to copy your stuff, I want to make a terrific boat kit that fits them perfectly! J.D.

This is the 4' Zenoah hull-2 configured with the 25cc engine feeding into a Clutch, then feeding into either a belt drive pulley or a universal joint then into the MHZ gearbox and then two more universals to the twin output shafts (or these could be collets and flexible shafts). Two 60mm diameter props and twin angled rudders. The engine fits between laminated vertical  ply runners which end up 6mm thick and 5" or 127mm apart (inside measurement), these runners run virtually the full length of the boat and add enormously to the strength. The twin propshafts are 80mm apart to suit the MHZ gearbox. OK the top of the spark plug lead is above the day cabin roof but hey that can be covered by a dingy!

The 6' hull also has 5" engine mounting rails but only from the engine forward and has overlapping 8" rails aft of the engine in order to leave enough room for the three propshafts, pulleys and belts and it has the propshafts in the correct places for true scale and has all three rudders. The 6' keel and runners are too big for me to laser cut in my machine and also too big to ship Internationally, so I have split the kit into two parts, one part I will cut and the other part is cut outside on a larger machine. I am looking for a US based laser cutting company to cut part A for my US customers, these sheets are 1200mm (or 4') long x 300mm (or 1') wide by 3mm GL3 marine ply..

This is a view looking aft at the 4' rib and runner section and the gearbox and engine. The pull starter can be rotated 90 degrees to be easier to operate or it can be replaced with an electric starter which I am told is available from MHZ..

This is my new, more accurate, Zenoah engine drawing. (J.D. - This is actually the G230PUM, the 231/260 has less but bigger fins on the cylinder head casting) the new drawing is nearly complete and will replace this one!

This is a very accurate drawing I did of the engine casting that I bought off Ebay.

This is my drawing of the MHZ gearbox with a few assumed dimensions as well as some that we do know from the spec. Unfortunately the vertical offset (10mm or 14mm) is not exactly known (stated as 14mm by MHZUSA but that seems too large I think) and is probably neither 10mm nor 14mm but somewhere in between. I have tried to match the brochure picture using gear sizes that would match the stated gear ratio and the offset and neither seem quite right. Also the mounting hole sizes and distances are not known either! The width of 122mm fits within the 5" rails (127mm) with 2.5mm clearance on each side, presumably for some sort of packing or anti-vibration mounting material.

This is the lower engine casing overlaid with what I think I know about the B.H. Hanson and/or the Bonzi Sports quick release motor mounts. I know the vertical positions relative to the engine centerline and the distance apart are correct for the B.H Hanson mount but I can't get them to confirm the 1" (25.4mm) distance from the front engine mount to the front mounting hole, I have assumed this dimension as the rear mounting holes are almost lined up with the rear face of the pull starter unit on these great photo's they sent me.


This is a reasonably stock Zenoah (left) rated at around 3 HP and the B.H. Hanson Super Quickdraw engine (right) rated at 7.5HP and up to 20,000 rpm. These are both from B.H. Hanson and are fitted with their quick release mount. Note: the left engine is also fitted with a throttle arm setup to connect it to a throttle servo which would be mounted forward of the engine and a water cooled exhaust block. These engines are usually fitted with water cooled tuned pipe exhaust systems, a tuned pipe can increase HP by 30%.

The stock Zenoah G231PUM (22.5cc capacity) has an improved cylinder design, with 4 transfer ports, a low weight, high rpm piston with a thin 1.0 mm single piston ring design, it has a new improved combustion chamber shape that uses a compact 10 mm spark plug, a large bore carburetor with a manual choke plus a factory supplied velocity stack. This gives the 231 engine better breathing and higher rpm over the older G230PUM two port engine.

The stock G260PUM engine has the same features as the G231PUM engine but has an increased capacity of 25.4cc.

Both engines use the same output shaft shape and engine mounting points as the older G230PUM engine. Factory engine ratings: Stock G231PUM - 4.0hp @ 15,800 rpm. Stock G260PUM - 4.3hp @ 15,200 rpm. (Engine ratings measured with a performance tuned exhaust system).

Something to consider is that if the engine / boat is fitted with a brushcutter style centrifugal clutch to stop the propellers from spinning at engine idle speeds, the boat will need a separate water pump to cool the motor and exhaust system as water scoops will not work when the boat is stopped.

(C) Copyright 2006 - John Drain