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HST Tunnel
Extensions/Trim Tabs
Randy,
I currently own my second HST (with this being my fifth Hydrostream) and
have a question regarding the "Tunnel extensions", for lack of a
better term. What was the original intent of these? On my first HST, I had
good luck with mounting a piece of 12 gauge stainless steal plate under
these using countersunk screws at the leading edge and installing them so
that they were flush with the top surface of the tunnels. I then mounted a
piece of angle to the back of the extensions with a series of 3/8"
bolts and lock nuts to provide a degree of downward adjustment. This
set-up seemed to provide the boat with a little more lift at speed by
limiting the amount of air exiting the tunnels and in turn packing more
air under the boat. I was preparing to make the same addition to my newest
HST and was wondering if that was something the factory did when racing
these boats or am I just out in right field. Another question I have relates to the
different bottom designs that these hulls were offered with. My first boat
had the optional "Anti-Spin" design which I believe was added after the hull was popped from the mold. With
this option, the outboard sides of the tunnels where cut and the bottom
edges tilted in towards the center pad. This boat cornered like no other
boat I have ever driven and would turn circles around any STV! My current
HST unfortunately does not have this option. What I was curious about was
the acceleration and top speed performance difference between these two
hull designs. The "Anti-Spin" bottom obviously had more wetted
surface but would it pack more air and provide additional lift? Any
experience with these? By the way, John just re-worked my 3-blade Mazco
like you suggested to Barton with the Viking and added, as he calls, it
"acceleration" (rolled the leading edge) he also added a good
amount of cup to the ends of the blades providing me with some additional
bow lift. My current set-up only has 10" of set back and can use all
the help it can get in the bow lift department.
Thanks in advance for your comments, Mike
Mike,
Having owned pleasure and race HST's I can say a few things about them.
I'm probably going to get in trouble for this but here's the HST story.
The running surface of the HST was copied from the Stoker Sport Tunnel. I
know this because my dad towed the boat back from California. It was
rolled over at Hydrostream and templates were made. A friend of mine made
the templates. The tunnel plates were Howard's version of the turn buckle
cav plates that Stoker used. Do they work? In all the years and all the
boats I've found they are more trouble than they're worth. The boat works
better overall without them. On the race boat which were half the total
weight of the loaded production boats they helped pop the tail loose after
exiting the corner. The main problem is that Howard didn't put any
compression in the tunnels. That is why the anti-spin sponson didn't help
the cause. By taking more material out at the rear of the boat the tunnel
lift got worse. My first Mod-VP HST had anti spin sponsons. I didn't like
it as I had to compromise the set up to make it competitive. Don't get me
wrong, I commend Howard for trying something new, but it didn't make the
boat a world beater. Anyway the second boat was much better without the
anti-spin thing. We found that balancing the boat to have it's center of
gravity more toward the middle of the boat and running 4 to 5 inches of
set back was the fastest. Most people run 12, 14 and 16 inches of set
back. This compromises the overall boat. Now they have a big teeter-totter
that has to have the gearcase jacked to the moon. I'll be faster every
time with a properly set up HST.
There was also 2 versions of the HST sponson. One had the Stoker sponson.
(the anti-spin was a piece of clay inserted in the mold) The other had 2
lifting strakes on the outer chines of the sponson. The second version
really helped the boat accelerate and turn more predictable.
Believe it or not we ran the propshaft below the bottom about one half to
three quarters below the pad. Jon Graff's HST ran over 100 like this for
years. I'd work with your set up to gain bow lift. The plates will kill
the bow lift as they run too wet on the production HST's. They will force
the bow down until you get over 90. They may keep you from getting there.
Keep them off till you have gained maximum performance. The three blade
Mazco is a great prop for the HST but try running deeper before adding
cup. Cup always slows you down if you can get by without it.
Try some things and see how you do.
Good luck, Randy
Please submit
whatever questions you have to IHRTechTalk@yahoo.com.
All questions and answers selected will be displayed on this page as they
come in. Note: though all boaters are welcome, priority may be given to
I.H.R.
members. Randy can not be held responsible for any advice
given. Though his information and expertise is second to none, he
has no control over what you do with your boat. It is up to you to
boat safely and act responsibly, and his advice is only to be used as a
guidance for your high performance boat/motor of which you are the one
responsible for the risks involved.
Thanks,
Mark C.
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