Outrigger Hydro Turning Dynamics
December 2011
by, Marty
http://engine-analysis-software.com
The dynamics of
making an outrigger turn especially well (“Snap Turn”) are complicated. I have wanted to do a Technical Article on
this subject for a long time. Some may
not agree or understand my findings and opinions, but they are based on a long
period of testing and experience.
Usually I do a several part article, but this time I think I will make
it a complete Technical Article about the subject.
Important
Note: I want you to visualize the way that a
perfectly set up hull “snap turns”.
Imagine that the turn fin is the pivot point of the hull and when set up
correctly, you will see almost ZERO water come off the turn fin in the turns. The boat will PIVOT around the fin and the
amount of rudder throw will be so slight, that it will amaze you. I almost gave up on the “Snap Turn
Characteristics” because I found that I had to reduce the rudder throw so
much. With all the
previous rudder throw, the boat performed Horrible. Only after reducing the rudder throw did I
see what I was looking for.
Snap Turn
Defined:
A “Snap
Turn” is when the boat goes through the turns and appears to speed up as it
turns. In addition, the turn fin will
have almost zero water shearing off of it.
You will see no water high off the turn fin. The boat will be pivoting around the fin with
the rudder only moving the rear of the boat around the pivot.
This Technical
Article will be broken down into several main components including:
Important
Outrigger
Sponson
Physical Characteristics
Turn Fin
Characteristics
Rudder
Physical Characteristics
Prop
Characteristics.
Important Outrigger
Balance:
In order for an
outrigger to be able to “Snap Turn” consistently, the balance of the hull needs
to be such that when it hits a wave or any rough water the boat bounces level
with the water surface. No bow up
bounce, no transom kicking up, etc, etc.
It must be set up to be balanced.
Attack Angle of the Sponsons:
The attack angle of the sponsons are one of the most important factors
that contribute to a boat being able to consistently “Snap Turn”. If you have the attack angle set even a few tenth’s of a degree off, the turn will not be consistent. Some time the boat will snap turn and some
times the boat will wash out and not turn well at all. With higher attack angles, the boat will run
faster down the straightaway, but that will be more than offset by bad
turning. The turn fin will not be able
to stay hooked up consistently.
Sponson Stance:
The width of
your sponson stance (the distance between the inside edges of your sponsons) is
also very very important. This wide stance makes the turn fin have a
very long lever arm, so that it does not overpower the boat when you make a
left correction. It also makes the boat
remain stable in the turns because of the “long lever arm”. The boats remains
flat and level (side to side) with a wide stance. How do you know if you are too wide? If the boat “teeters” back and forth between
the right and left sponson in the straightaway, you are too wide. It really takes a lot of width to get into
this condition.
Flex of the Sponson Tubes:
If your sponson
tubes are super “flexi” you will have a boat that changes attack angle while
running and will be difficult to get consistent. Rigid sponson tubes are a must, if you want
to have a consistent running hydro.
Setting the Attack Angle of the Sponsons:
I have
previously talked in detail about the methods for setting the attack angle of
the sponsons using two different methods.
These discussions can be found at: http://rcboat.com/past.htm Taking time setting
attack angles accurately will reap great rewards for you in the way that your
boat performs.
Wheelbase of Rigger:
The wheelbase of
an outrigger (distance between the running point of the sponsons and the center
of the prop) is important to keep the hull in a stable condition without
rocking forward and aft. You will be
more prone to have a boat that rocks forward and aft, the shorter it is. The prop will be much more critical since it
generates the lift that creates most of the rocking. I prefer a longer wheelbase to keep
stability. You will see a much better
race boat if you have considerable wheelbase.
On my newest boats, I am tending to lengthen the wheelbase a great deal.
Weight of the Outrigger:
Physics will
indicate the importance of having a light weight boat. The lighter the boat the faster it will
accelerate. The lighter the boat, the
more pitch you will be able to pull effectively. The lighter the boat the easier it will
launch a big fast prop. Weight is a super
important factor that contributes to an impressive and well developed
outrigger. Weight is an area that I
spend a great deal of time on. I don't
allow ANY excess epoxy in building, I use light weight materials, I use
aluminum screws and nuts, I lighten all components by milling out unnecessary
materials and I don't put a ton of paint on the boat. Any weight that you can save in the building
process is going to make the boat better, assuming that it is designed and
refined as indicated. NOTE:
I have never seen a boat that was too light, which was refined and well
designed. Not sure “too light” is
possible…..
Running Friction of the Mechanical Components:
The running
friction of the running components (driveline, strut, alignment, etc) must be
smooth and free. Any bind here will
totally destroy your efforts at refinement.
The smaller the engine class, the more effect increased friction will
have.
Radio Installation:
Radio
installation is an often neglected component of the great outrigger. The linkage between the servo and the runner
assembly should not have ANY
slop. You should not be able to feel any
slop in the linkage when moving the rudder assembly with your hand.
Sponson Physical Characteristics:
Sponson Width:
The running
surface and dihedral of the sponson also contributes to the correct attack
angle required to make the boat “Snap Turn” consistently. For example, the Crapshooters that Stu Barr
makes have narrow run surfaces and take a few tenths more attack angle to make
the boat run consistently well without bouncing excessively. The boats that I run and the Roadrunner have
a wider running surface and take less attack angle to get this
consistency. What angles you say? On the boats that I run and the Roadrunner,
you can't go wrong with attack angles of 3.0 degrees on the left sponson and
3.2 degrees on the right sponson. On the
Crapshooter that Stu builds you need more, like 3.6 on the left and 3.8 on the
right. I can't say on any of the other
hulls, as I have not worked with them.
If you have stats that convince you that you have the optimum attack
angle figured out, let us know. Why the
difference between the left and right sponson attack angles? The right sponson has .2 more because of the
turn fin drag to keep the boat running straight. The width of the sponson will be the major
controlling factor in how well the boat runs in rough water. The wider the sponson, the more reaction it
will have off of a wave or wake. You
want your sponson width to be as narrow as possible to get the attack angle
working where you find turning consistency.
The balance of the boat will also have a big effect in the rough water
characteristics.
Sponson Toe-in:
Having a little
toe in for the front sponsons is important.
I personally use 1/16” toe in on the right sponson and none on the
left. What this does is make the boat
track straight. There will be no pulling
in of the boat under acceleration toward the center of the course. The boat should run totally neutral with a
little toe in.
Sponson Dihedral:
In years past,
dihedral and stepped sponsons were the rage.
There is not doubt that sponsons with dihedral or steps are faster on
the straightaway, but a flat sponson with no dihedral is better for heat
racing. The boat stays much more stable
and consistent with the flat bottom sponsons.
Sponson “Belly”:
This term will
be foreign to most. If you can imagine,
the running point of the sponson is the contact point with the water. If you take a straight line with an attack
angle of 3 degrees forward toward the front tip of the sponson you will have a
sponson without any belly. If you
imagine the same line going forward about 3 or 4 inches and then having an arc
to the front tip of the sponson, that is the amount of
belly in the sponson. Some belly is
really good for running in race water.
Too much belly will adversely affect the running of the boat at launch
and at slower speeds. With a lot of
belly, you will have to add attack angle to get consistency. For straightaway, a sponson without belly is
best as it stays cleaner than anything.
Some dihedral here is beneficial for SAW.
Sponson Tops:
The tops of the
sponsons are important to the boat staying in a level condition as it
encounters rough race water. Unless you
have some angle of attack on the top of the sponsons, they can get to the point
of no return if they get up in the air from a bounce. Once the sponson bounces, it creates more
attack angle and air gets under the sponson.
Unless you reduce speed, the angle increases until the boat blows off
the water. A little (3 degrees +/-) of
attack angle on the tops of the sponsons will eliminate this problem.
Sponson Sharpness:
The edges of the
front sponsons should be sharp and never rounded. The sharper the better.
Sponson Finish:
The running
surface of the front sponsons is very important in it’s
finish. A close friend of mine, builds boats that always run very well. They appear to not need much horsepower since
they are so easy to push. I asked him
one time why and the reply was very interesting. Sharpness of edges, sanding
TOTALLY flat on the running surfaces and increasing to 1200 grit paper for a
final finish. All this sanding
and flattening is done AFTER painting and will pay BIG DIVIDENDS.
Turn Fin Characteristics:
Depth of Turn Fin:
The depth of the
turn fin has a great deal to do with how the boat acts when you turn left. It is important also in keeping the boat
hooked up in the turns. If you have a
boat that wants to flip up on its top when you turn left, you have a fin that
is too deep. The area of the turn fin
can be maintained by making the fin wider rather than deeper. This will allow you to shorten the fin and
reduce the bad effect when turning left.
Straightness of Turn Fin:
The relationship
between the turn fin and the inside edge of the sponson is very important. The fin should be perfectly in line with the
inside edge of the sponson. Take a
straight edge and be sure that there is no warp in the fin and the fin is
perfectly in line with the inside edge.
Turn Fin Contribution:
All of you have
seen boats that turned amazingly well and actually seemed to pick up speed in
the turns. I first saw this
characteristic with a boat that Steve and Jack O’Donnell brought to the Indy
Hydro Masters in the 80’s. That boat was
beyond awesome. It had a funny flat turn
fin that was really deep and angled in.
Boat had a
Turn Fin Stiffness:
Any flex in the
turn fin will negatively affect all your efforts at refinement. It took me many years to finally find a light
weight solution to making the turn fin rigid.
I use a piece of 1/8” thick flat carbon plate, sandwiched between the
turn fin and an aluminum doubler. It makes the fins TOTALLY rigid and will not
bend any under load.
The area of the turn
fin that is under the surface of the water while running has a point where it
represents the center of pressure of the turn fin. If you look at the fin, you can imagine,
fairly accurately, where the effective center of pressure of the fin is. You may have to draw a water line to help you
visualize the center of pressure. It is
the effective point at the center of the area of the fin. Once you put a point at the COP, you will
want this point to be in front of the c/g of the boat with ½ tank of fuel. On most hulls, the COP should be between ¼”
and 1” in front of the c/g with ½ tank of fuel. You will have to slot the turn fin to find
the “sweet spot”. Run the boat, move
the fin a little and keep trying until you find the sweet spot.
Rudder Physical Characteristics:
Rudder Length and Width:
The length of
the runner is also important in keeping the boat running flat and
consistent. You want the rudder to be
about as short as possible and still do the job. For example, on a .21 hydro, the rudder should
be about 2 1/8” below the center line of the shaft. That is also a good reference for larger
boats. Just take that measurement and
add ½ of the diameter difference between a 50mm prop and the prop that you will
run on the larger boat. For example: a 50mm prop is 1.9” in diameter and for my 67
boat a H32 is about 2.5” in diameter. So
2.5”-1.9” = .300” ½
of .300” is .150” so lengthen your rudder that amount from the base of 2
1/8”. That will get you close. I use all Speedmaster rudders and the ratio
of width to length is consistent between sizes so this work out closely. A note:
for Straightaway, you use a MUCH shorter rudder in order to lessen drag. If your rudder is to long, the rear of the
boat will be lifted by the rudder and the boat will never run well. Having some lag in the rudder will make it
even worse (bottom tip of the rudder further back than top of rudder blade).
Prop Characteristics:
Prop Being Used:
The prop is one
of the “Black Arts” of our hobby. A prop
that is super fast and has a ton of lift is difficult to get consistency
with. An example: Several years
ago, I watched Stu Barr run his favorite class (40 Hydro). He spent an entire season working with the “V
Series” Octura Props. His boats were
rockets, but he could not get them tamed down and have consistency for heat
racing. He had to drive wide and fast to
be able to race that prop. Also, in
rougher water his boat would not stay hooked up very well. Now, he is back to a more consistent prop in
the “1400” and “1600” series. He can run
on the buoys and snap turn with consistency.
Maybe gave up 7 or 8 mph but can race much much
better. I personally run large diameter
props on all my race boats. The reason,
I want the prop to stay “hooked up” in race water so that I have consistency
and good acceleration out of the turns.
CONCLUSION:
I hope that this
discussion will be helpful to you in running and tuning your outrigger hydro. Many of the things discussed can be applied
to other type hulls including: Scale Boats, Sport 40, Sport 20, and similar
design boats in the gas classes. The
same physical characteristics should pay dividends for you in these other
classes. I use the items discussed on
all 3 and 4 point hydros.