HST Tunnel Extensions/Trim Tabs

***Archived from the original "TechTalk" section of the HydroStream.org Site, October 10, 2003***

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