DSH "tunnel" spec?

[ram95]

Anybody want to comment on the amount of "tunnel" which is legal for D Stock hydro bottoms? I think I have read the APBA technical description but when I wander around the pits, I see boats which appear to have more tunnel than is allowed. Maybe I'm not measuring it correctly. Any comments, including those of a lewd or lascivious nature will be accepted with an appropriate level of gratitude.

Alex 12A

"My mind is made up... don't confuse me with the facts" Newton, when penning out the 3rd Law.

[modhydro]

Tunnel Rule

Alex,

Can you please post the rule that you are refering to? I didn't think that there were any rules on the tunnel depth.

I'm wondering if you are thinking about the airtrap rules???

This is the excerpt from the 2007 stock technical manual pg 6

3. The exclusion of airtraps shall only be allowed so long as the airtraps are not
greater then 5/8 inches in depth at the aft end of the boat and 1-5/8 inches in depth
at the point 18 inches forward and, in addition, are not any wider (ie: thickness
which is measured transversely) than 7/8 inches each.
4. The previous definition of “airtrap” in number 4 above shall not be construed to
include any wetted surface, which obviously does not serve the purpose of trapping
air.

Thanks

Steve Roskowski

[ram95]

I guess I was reading in the 2007 rulebook on stock outboard hydro hulls where there was wording to the effect that the bottom couldn't deflect upward in by more than a sixteenth. However, when I went on the APBA site and looked at the rules the same wording wasn't there. I just figured I was missing something or that I wasn't looking in the right place. You comments confirm what I suspected, ie that there isn't really any spec for tunnels. Just like finding the legal propshaft depth and DSH weight (tohatsu)... you just have to know where to look. I got tossed out of a DSH win at the scales in Lockhaven because the inspector was under the impression that I needed to weigh 490. I thought it was 480 but figured I must have read the rules wrong, so being the nice guy I am (errr ahh... that I TRY to be), I didn't contest the matter and threw 12 pounds of lead into the boat. I'm trying to be a little more cautious now. Thanks Steve, very much for your response.
Alex

[Bill Huson]

I seem to recall . . .

a "tunnel" rule whereby one couldn't race a tunnel boat (like OPC). Can't recall if that applied to stocks or PRO boats, but I think it was for PRO boats.

As for tunnels, either ramp style or Karlson style, I think the only classes that can't use a tunnel are J and A.

[ram95]

Thanks Bill,
Guess I was reading too much into it. My hull has a rocker built into it and quite a bit of lift. One of my ideas is to flatten out the rocker and leave a tunnel area in the middle. I sure didn't want to go to all that effort only to find out it was against the rules.

And Thanks goes out to everyone else who patiently educated me on the subject.

Alex

[Bill Huson]

AHA, a DSH with a rocker!

Thanks Bill,
Guess I was reading too much into it. My hull has a rocker built into it and quite a bit of lift. One of my ideas is to flatten out the rocker and leave a tunnel area in the middle. I sure didn't want to go to all that effort only to find out it was against the rules.

And Thanks goes out to everyone else who patiently educated me on the subject.

Alex

I worked on one of those many moons ago. Boat had put a few drivers in the drink. Same type of wreck, go bow high, drop left sponson and *STUFF*! First I sawed thru the bottom and did a tunnel ramp deal about cockpit wide (Bezoat style). Then I used thin pads of end grain balsa epoxied to the non-ramped part of the bottom which were ground down flat - removing the rocker - and covered the balsa with 10oz glass cloth. A little more sanding, some varnish, and the boat behaved itself.

[ryan_4z]

Bill is correct, the rule quoted above only applies to the J and A classes.

[ram95]

I like the "end grain balsa" suggestion. I was trying to think of a light weight way to create a flat surface. I'm going to flip the boat over and put the straight-edge to it. I may try cutting the air trap by an 1/8th and testing it once more this year before 'ice-in' but, I suspect the "flattening" operation is going to happen. Alex

[pops67g]

Cutting is a "last resort"

If you stop to think, the little bit you might trim off the air trap will have minimum effect compared to changing the contour of the bottom. My DSH has a "tunnel" the last 4 feet of the bottom or so. It surely releases some air while running, but it also reduces wetted surface in the rear to improve acceleration off the corner. Any time one of my boats has run squirrelly has been because the bottom is no longer flat. Often brand new boats are farther out of whack than you would think. When I got the last new one I spent two solid weeks getting the last 4 feet straight. It was pure grudgery, but when it went in the water the results were pure heaven.

[Dave M]

Beware Balsa

I like the "end grain balsa" suggestion. I was trying to think of a light weight way to create a flat surface. I'm going to flip the boat over and put the straight-edge to it. I may try cutting the air trap by an 1/8th and testing it once more this year before 'ice-in' but, I suspect the "flattening" operation is going to happen. Alex

If you elect to use the balsa core make sure it is sealed very well. Balsa soaks up water better than any sponge. Your boat could gain a lot of weight unexpected if the balsa is exposed. A lot of builders use it. If you do, simply make sure you encase it with cloth of some sort, and don't skimp on the epoxy when wetting it out.

[Bill Huson]

Good advice . . .

If you elect to use the balsa core make sure it is sealed very well. Balsa soaks up water better than any sponge. Your boat could gain a lot of weight unexpected if the balsa is exposed. A lot of builders use it. If you do, simply make sure you encase it with cloth of some sort, and don't skimp on the epoxy when wetting it out.

Although the end grain balsa squares for rocker removal are around 1/4" thick (depending on how much rocker) so saturating is easy. Apply liberal coat of epoxy, set all the balsa squares in place, let cure. Wait until you sand the balsa flat before drooling epoxy on the top surface (actually the bottom but boat is upside down), easier to sand when bare. I usually lay tyhe glass cloth when wetting down the top. Might need a putty of epoxy and microballoons at the ends to fair it in.

Next Boat Fix lesson: Fixing a rotted out transom on a fiberglass pleasure boat with a chain saw! It's fun!

[modhydro]

Balsa cautions and misconceptions

Mis-used balsa can soak up a lot of water. Properly used balsa will absorb less that plywood!

The bad knock that most people have on balsa cores is a throwback to its early days of polyester open layup where it may or more likely may not be in full contact with the fiber and resin it was meant to. In this case water will run between the unattached skins and core and soak the core thoroughly.

If the core is bagged or otherwise held tight to a skin, it simply cannot happen. The cross-grain absorbtion is VERY minimal to the point of non-existant.

Bear in mind that if you soak up the grain heavily, you no longer have the benefit of a lightweight wedge to create your tunnel. You simply have a virutally solid resin pad at great expense and weight. A better method might be to get it sanded to your finished surface and apply a light primer coat and let it cure. Sand this and apply a second coat. This will help seal the end grain and not fill your pad completely and add unnecessary weight. Once you have one or two primer coats, you can apply the glass or whatever fiber you choose.

On this topic itself, I never apply cloth with the first coat of epoxy. It is just too easy to have a more porous grain of wood soak up resin and leave a starved laminate robbing it of strength. I roll on a primer coat to seal the grain, wash and sand the area and then wet out the cloth. I think this is a far better method.

Just another opinion. Hope this helps guys out.

Steve Roskowski