Setting Up a Stepped Mono
By Allan Shillitto
About three years ago Dave Marles of Prestwich Models started producing a
stepped mono hull moulding for 12 cells in Epoxy glass - the 'Cheetah'. The hull
weighs in at approximately 200 grams. There were several 'new' concepts here in
the UK and due to initial problems neither this boat nor it's smaller sister the
'Targa' never took off sales wise.
These boats are still fairly little used in the UK for high end racing - a
pity as they are fast and stable when set up right. The boats have a fairly
shallow 'V' shape with two steps. The hull has molded in strakes on the front V
portion. The front step is angled so that the center section trails the sides by
about 1/2". The first step being 1/3 of the length from the transom and the
second step runs straight across the boat and trails the first step by 1/3 rd of
the remaining distance to the transom. It is set 1mm above a line from the first
step to the transom.
The original hull came with a set of C/F (graphite) strut/rudder mounting
brackets - these were the source of the first and ongoing problem as the would
distort allowing the rudder to touch the prop. When this happened the prop was
invariably destroyed - expensive.
The second problem was that the boat would roll out of a high speed corner
and land up upside down, because of the flex in the C/F components this was not
very predictable. This was when set up for 3 minute runs.
To add to the problems at this point the UK rules were changed to make the
runs 5 minutes plus mill time and final lap. (5m30sec ish). Having found the
right prop combination racing commenced - the C/F problem was nearly gone but
would still hit at times. Still there was enough room in between to work on a
reliable set up.
It was discovered that these boats do not like sideways spread weight - for
success the batteries must go alongside the drive shaft (we use flexi's). The
balance point is approximately 1/2" behind the front step. This means that
the motor must be mounted in front of the front step in order to achieve the C
of G. Having done this the boat became very fast. - but still suffered from whip
on the C/F supports. Ernie Lazenby had one of these hulls as well - I had nearly
given up when he rang one day and said 'I've fixed it'. Ernie replaced the C/F
bits with two Aluminum brackets cross braced by a bolt. The bolt locates the
strut on the central one and the rudder on r/hand offset one. The offset is 1
1/8th inches. The pivot point for the rudder is at the same rearward
point as the back face of the prop dog drive angled forward at the bottom very
slightly. The front face of the pro is 2 1/4" behind the transom.
Had Ernie sorted this earlier he may have won the UK mono2r champs, as it is
he came second.
Tip - the access hatch has to be cut in to the deck. Make it as wide as
possible - my new one is 5 1/2 inches wide , starts 1 1/2 inches from the
transom and finishes 9" from the transom - this due to lessons learned.
Recap:- The balance point is critical to performance, keep batteries near the
c/l to stop flipping. This means a long flexi with the motor in front of the
step. Prop front face is 2/1/4" from the rear of the transom. I'm using a
cut 442 Ernie uses a cut 215. Both boats are about the same speed and handle
well. The bottom of the pro boss/strut must be on an extended line from the
bottom center rear of the front step to bottom of the transom. These boats are
run in Europe with straight shafts - consequently the have down angle on the
prop - in the UK we are running them flat to the water surface.
The targa is a similar case except that it handled well. It initially ran
like a dog on direct drive and could not cut the mustard, people with no vision
dropped it. We now have the same expected run times as above. I fitted a Hughey
box with 1.95:1 ratio - this just fits under the front of the cabin molding but
needs care. Initially I used a budget modified 17 triple motor running a 642
prop. I get 7 minutes run time but have not fiddled as I don't see the need -
and haven't got to race this year that often. However since showing them the way
Ashley Wilson (jnr) has been running a Targa with similar set up he has missed
three meetings and suffered a couple of problems but is still lying 3rd
with a chance at second. I believe that Ray Fuller ran one of these boats on 6
cells against the hydros and blew them away. In the UK we run 7 cells!.
If you want some fun try a stepped boat - watch out for the C of G it is
critical to good performance.
Now have you ever seen a self righting 7 cell subsurface drive mono running
round a triangle. You want stressful fun try it. Want some info Badger Ian