bout the Interceptor 's Deep V hull design.
fast will it go?
important question. The interceptor has a hard chine, deep V planning hull, from
a design point of view somewhere between say a Sunseekert or Fairline (planning
hulls) and a Nelson or Aquastar (semi displacement) combining elements of the
two different hull forms.
The buttock lines are semi displacement in form, (i.e they rise up
towards the stern). There are a couple of reasons for this, a) to reduce
buoyancy in the transom so that she doesn't go down by the head at rest or when
traveling at displacement speeds. (This is due to the inevitable f/wd LCG of the
cab forward design). b) to give good handling in following seas, big wide deeply
immersed transoms cause the boat to yaw, especially at displacement speeds.
Interceptors underwater sections are deep V in form with a midships deadrise of
at least 22 degrees. Most Other hulls commercial hulls are medium V hulls.
Deep V hulls inherently require more power than shallower V hulls to give the
same performance, This is due to the angle of the planning surface to the
water, a flat planning surface provides the greatest lift and speed. You could
therefore say the Interceptor would be slower than, as an example a Bullet,
which is probably one of the fastest commercial hulls around, but the story
doesn't end there, The interceptor has a very slippery, low resistance hull
form, it has a reverse main chine and multiple spray rails which give
additional lift to the hull and reduce wetted surface area. These
design features of the Interceptor allow her easily driven hull to give
close to or similar performance as a lower deadrise and cathedral hull
types, of course it will all change the moment you encounter a head sea, when
the deep V hull design will leave everything else behind.
about economy? Especially when fully loaded.
the faster a boat is with a given power, the faster will be her cruising speed
and as a result the better her economy. But things start to get complicated when
you begin carrying heavy loads. In the 90's many skippers advocated the theory
of turning a large diameter propeller, and their reasoning was spot on. When a
boat is heavily loaded which is often the case with 12 anglers and a ton of
fuel, small diameter props lose there effectiveness. On the interceptor I
designed the prop to run in an Elliptical tunnel to reduce shaft angle and allow
her to swing a 25 - 28 " prop. Of course there is a limit to propeller size
as to large a propeller will, because of the massive torque cause the boat to
twist herself resulting in excessive heel when running. Shaft angle is also
critical, to much and efficiency drops.
she slam?, what will her sea keeping be like?
simplest way to determine weather a boat will slam is to look at how much dead
rise the hull has, and more importantly how much deadrise there is where the
hull will be hitting waves when at speed! Medium V deadrise hulls have typically
14- 15 degrees deadrise amidships, Cathedral or Gullwing type hulls have
similar deadrise but much wider outer chines, obviously these hull are not going
to cut cleanly through a wave, but will bump over it with high acceleration
forces imparted into the hull and crew. The Interceptor has a deep V hull
of 22 degrees amidships and 48-58 degress at the bow. Traditionally semi
displacement hulls have fairly steep deadrise at the bow and similar deadrise
amidships, which is what makes them so good into head seas, Whilst its
impossible to make a boat that wont slam in head seas without being seriously
compromised in other areas, the Interceptor by virtue of her deep V hull
gives a very smooth ride into head seas with little tendency to slam.
keeping! How will she handle?.
In designing the Interceptor I faced the same problem that must face every
designer, how to make her good into head seas but also good in following seas?
The two just don't go hand in hand. Fine sections forward with a deeply immersed
fore foot are best in head seas, but wait till you try to run downwind,
especially at speed, on this course she will tend to bow steer. Cathedral hull
types are very good in following seas. Why? just look at how much buoyancy
there is in their bow, it is hard for them to dig theirr bows in. But that
same virtue of a full bow is what makes her not so good in head seas. There has
to be a compromise somewhere! so in designing the interceptor I gave her fine
sections on the forward waterline (47 degrees) but with lots of flare to
the bow, the stem is well raked and the forefoot is cut away, the reason being
that when she buries into the back of a wave in surfing conditions she will bury
only to a certain point, that point being the upper deflection rail and where
the bow flares out rapidly. At this point the force of the wave will be spread
over a large area, the bow wont be deeply immersed and consequently wont act
like a fulcrum tending to swing the stern around. But probably the most
important factor in how a vessel will handle in following seas is her dynamic
stability. This is different from static stability, when a vessel is at rest.
Dynamic stability comes into play when, as example the boat is running downwind
in a following sea and begins to surf on the crest of a wave. At this moment the
bow and possibly half the boat will be sticking out over the crest and
unsupported. The boats effective waterline beam will in this moment effectively
be halved. If the boat does not have enough stability a yawing motion and heel
will be introduced possibly resulting in a loss of control and broaching. A high
GM figure in excess of 1.2m ensures the craft will maintain her course without
yaw in this scenario.
about head seas? Well she's still pretty fine up there but remember she's been
designed for higher speeds, how much use is a deep fine forefoot when your
running at planning speeds and that part of the hull isn't really impacting with
seas, more important is the deep V sections aft of the fore foot, to amidships,
as this is the area that's going to be hitting the seas when you come off a
wave, and this is where the Interceptor hull design pays off.
So what about stability? I've heard deep Vs aren't that stable.
probably comes from light displacement sports boats where the chine is often not
in the water at rest or only at the transom area. A flat bottom boat is probably
the most stable initially, but in some ways more uncomfortable as there will be
very fast acceleration forces in her initial roll, resulting in a snap
roll that in some ways is more tiring than a larger slow roll. The Interceptors
V sections flatten off at the transom to 18 degrees which helps stability and
importantly her chine is underwater right up to the shoulders,
further more the chine's themselves are reverse angle, causing the chine
to literally grab the water and prevent rolling, also there's a full length
keel, this is very important in giving a vessel good static stability. And
remember that because of the deep V sections, it is possible to position the
engine right down in the hull lowering the Vertical center of gravity, one of
the most important aspects of stability. That last point brings us to the next
question important to you angling guys.
fast will she drift?
important for angling boats, but once again its an area that's a bit of a catch
22 situation. The slowest drifting hulls are deep heavy displacement craft,
there weight and the fact that there's so much of them under the water means
that the wind doesn't move them much, so if a really slow drift is required, go
for a displacement boat! But that's not what you want. Boats that travel
fast across the water do so because there's not much of them under the water.
And planning boats drift fast because again, there's not much of them under the
water, especially at the bow. In addition, the fact that they have quite high
sides causes a lot of windage. The Interceptor has quite high sides, mainly
because I wanted a flush deck, and safe bulwarks! but the deep V sections
and larger keel mean that there is more grip on the water, especially at the bow
resulting in a slower drift. Again you can see that itís all a compromise.
about dryness? will the interceptor be a dry or wet boat?
is an area that has been given a lot of consideration. Obviously hard chine's
make for a dry boat and hulls of the semi displacement form without chine's are
very wet boats, on these type of hulls, even when there is a knuckle
incorporated, they are still wet, but even some craft with chine's are
still wet. The shape of the bow plays an important part, bluff bows tend to push
spray forward where it is caught by the wind and blown back over the boat, or if
the chine is to low then it isn't so effective. We have carried out a lot of
research by observing video footage of the chine and spray pattern in waves and
calm water, observing the effectiveness of the chine and trying to establish the
most effective position for them. This research showed that on our type of
craft, angling boats, where your carrying a heavy load at an economical speed
and with everyone invariably moved forward to keep out of the weather the boat
is not at an ideal attitude or angle of trim. Going faster would improve matters
but is often impractical, motion in a seaway becomes uncomfortable for anglers
and who can afford the diesel costs at 22 knots plus! At lower speeds, the
standard main chine's position becomes less effective. In a head sea
straight on the nose its generally ok with spray being thrown out and away, but
when your taking weather on the shoulder its often horrendously wet. The reason
I felt was that the main chine was, at lower speeds ineffective as it was mostly
immersed. Therefore on the Interceptors I incorporated a larger secondary cine
or spray deflection rail 280mm above the main chine to stop water riding up the
hull. A further design feature was to slightly down angle all chine's and rails
to actually deflect spray down rather than out, where it is just caught by the
wind and thrown back on the boat. A further point was spray rails. Bringing them
right up and around the bow mimicking the chine actually makes
matters worse as invariably they are to small where they fair in to the bow,
causing spray to be only partially deflected, thrown out and caught by
wind, much better to make them parallel and bring them forward only enough
for effectiveness at high speeds, this allows the main chine to do its job
Interceptors have a good beam to length ratio of around 3.2, but you have to ask
yourself this question. Do I require the maximum sea keeping ability, speed, and
economy or the absolute maximum deck area. Once again it's a compromise. A very
wide beam gives a greater deck area, but not the best sea keeping or
performance, the wider the beam, the less deadrise which equals slamming
and more power to push through the water. I felt that for a charter angling boat
sea keeping and economy combined with an adequate beam were the more
important design elements.
What about weight?
no excuses, the Interceptor is a strongly built hull. She is built to Lloyds
special service craft rules for scantlings and panel size and thickness. These
are the most stringent standards you can build a commercial hull to. This
results in a hull that can, within reason, be driven hard in rough weather with
out fear of structural damage. You only have to look at the video footage and
photos to see what Interceptors and Wildcats are capable of. Of course the
penalty of the strength is weight. But you have to decide what you want your
boat for. If is sitting around in marinas or fair weather sailing, then your
probably better off going for a lightly built pleasure craft. But if what you
need is strength and seakeeping, then the Interceptor has to be your choice. One
other point to remember is that at sea, size and displacement matters. The
bigger the boat, the better she will be in rough weather. And, all other things
being equal, such as VCG, freeboard and trim, the heavier displacement hull will
have better seakeeping in rough conditions. As a proven example, of all the
Interceptors ever built, the pilot boat, with its heavy 18,000 displacement has
the best sea keeping due to her heavy weight and huge stability, as opposed to
the standard 10,000-12,00kg displacement of the workboat versions.
there you go, a few thoughts about boat design that I considered important when
I designed the Interceptor.