9
4354
13
2022
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SEP IP 60
MG_0004
M
in
Pos. B
Pos. A
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MG_0048
Increase tunnel length to protect
the propeller from water forces
when high-speed cruising.
Water
Forces
Increase tunnel length to
prevent a circular water vacuum
cavity between the propeller and
the hull of the boat.
STANDARD USE
FLAT BOTTOM HULL
HIGH-SPEED OPERATION
Do not allow the variable length of the tunnel walls to vary in length
excessively.
EG. the top tunnel wall is x 4 longer than the bottom wall.
Cavitation
Water flow must have
space to "straighten"
itself for best
performance.
The gear leg/ propeller(s) must
never extend out of the tunnel
Tunnel installation in Sailboats
Some sail boats have a flat bottom and shallow draft in the bow section. This can make installing the thruster as far forward from the boats main pivot
point diffi cult.
(Fig. 1)
.
However, it is possible to install a tunnel thruster in most sail boats, even when the hull does not directly support the fi tting of a tunnel.
Instead fi t the tunnel halfway into the underneath section of the existing hull. Strengthen it with a deflector/ spoiler directing the water flow around
the tunnel. This will allow installation of the thruster in the proper position on the boat, maintaining the reliability and space advantages of the tunnel
thruster.
This installation is being used by some of the world’s largest sail boat builders and has proven to give little to no speed loss during normal cruising.
This can also be an installation method for flat bottomed barges to avoid extremely long tunnels and large oval tunnel openings in the hull.
MC_0003
MG_0003
High water force
while underway
High water force
while underway
from wave contact
To protect the propeller from water entering into
the tunnel at high speeds (25 knots +) the
extended deflector/ spoiler must provide
protection from the front and the bottom. In the
event when the boat is traveling at high speeds it
will commonly pound upon the water surface.
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Water Deflection
MC_0003
1. A possible problem in sail boats or fast powerboats is that a non-rounded surface can generate drag from the back face of the tunnel, as it creates
a “flat” area facing the flow of water.
This problem can be solved in two different ways, depending on what is possible or easier to perform.
2. The best solution which generally reduces the most drag is to make a recess in the hull at the back of the tunnel. As the back face is removed
water can flow freely past the tunnel entry. The depth and shape of this recess will depend on the boat and the angle facing up/ down aft of the
tunnel insert. Normally it is angled slightly down because of the water flow on this area.
3. Making a deflector/ spoiler in front and underneath the tunnel can also reduce damage to the thruster and drag. The deflector/ spoiler will push
the water flow out from the hull so water can pass by the back face of the tunnel. The shape and size of this deflector/ spoiler will depend on the
hull shape. The easiest way of making the deflector/ spoiler is to retain a part of the lower forward area of the tunnel while installing the tube. Use
this area as support to mould a soft curve/spoiler shape from the hull.
4. The thruster propeller can spin (passively) producing noise while sailing or cruising as water is forced through the tunnel. Water-flow directed
through the tunnel at high speeds, during turning or as the boat bumps waves while underway can also damage the thruster.
(NB: As a rule, you should not see the back face of the tunnel when standing directly in front of the boat looking aft.)