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usually vicious circle of increasing penalties, (more fuel for
greater range requiring more power that consumes the extra
fuel), has become a spiral of benefits (Fig. 7).
Table 1 Superior Seakeeping – Fox 86 seakeeping data,
bulb vs. no bulb
Seakeeping:
The added appendages (bow bulb, midship blisters, and bi-
foil) and slick hull form also give superior seakeeping char-
acteristics. Not only can the vessel operate economically at
a higher speed, but also there is no loss in the degree of
comfort. The reduced bow wave height means that when
the bow enters a wave there is less water being pushed
aside so there is less water to come on deck. In fact, clients
have confirmed that this form produces a very dry boat over-
all. A recent article in Showboats Magazine (March 2006)
says
“He (the owner) wanted the efficiency and seaworthi-
ness that a bulbous bow provides by reducing pitching. Bray
Yacht Design organized tank tests, which showed that the
bulb and a redesign of the cutwater would provide a 12-
percent reduction in resistance, allowing Impetus to make 9
knots (instead of 7 knots) i
a 9-foot sea.”
Of all the boats that
we have retrofitted
with bulbs, it is the
reduction in pitching
that is noticed the
most by the owners.
It can be seen from
this table (Table 1)
that with the addition
of the appendages
the vessel has virtu-
ally the same degree
of comfort at 10
knots in sea state 3 as it does at 8 knots in the
same sea without the appendages. In fact, an in-
crease in speed to 12 knots causes very little in-
crease in pitching motions. This 2-knot speed ad-
vantage continues to be true at sea state 5 as well.
The addition of the appendages allows a 2-knot
increase in speed with no real increase in motions
for the vessel. (Sea state 3 is defined as having a
scale significant wave height of 3.5 - 4.0 ft. (1 – 1.2
Continued next p
Fig. 7 Performance Comparison
Table 1 Superior Seakeeping – Fox 86 seakeeping data,
bulb vs. no bulb
Seastate
Speed
(kt)
Bow
Accelerations
(g - rms)
CG
Accelerations
(g - rms)
Stern
Accelerations
(g - rms)
Config D - No Bulb, No Bi-Foil
SS 3
8
0.247
0.091
0.124
SS 3
10
0.275
0.109
0.145
SS 3
12
0.273
0.115
0.159
SS 5
6
0.281
0.106
0.148
SS 5
8
0.349
0.138
0.185
SS 5
10
0.374
0.159
0.21
Config A - Bulb and Bi-Foil
SS 3
10
0.254
0.109
0.138
SS 3
12
0.256
0.122
0.154
SS 5
8
0.298
0.127
0.164
SS 5
10
0.352
0.16
0.2
Fig. 8 Fox 86 - Stability on Waves