What rules to follow to make a Tunnel Hull

[teddy dauphinee]

Ok, here is a question for the tunnel hull group.

To race tunnel hulls I know you need an outboard motor. What about hull design?

Can I put a motor on a cat hull and race it in tunnel class?

Would a cat hull be faster than a tunnel design?

Are there specific rules for hull design??

 

[topfuel443]

Ok, here is a question for the tunnel hull group.

To race tunnel hulls I know you need an outboard motor. What about hull design?

Can I put a motor on a cat hull and race it in tunnel class? Yes, you can run a cat in the tunnel hull class with an outboard.

Would a cat hull be faster than a tunnel design? Not necessarily. There are many differences in tunnel hull design when compared to cat design. Obviously the main factor being engine placement.

Are there specific rules for hull design?? Yes, but it is very open. See below.

Here is the only rule pertaining to tunnel hulls as noted by the IMPBA rule book, section "K" rule g.

g. Tunnel Hulls: The tunnel hull will have two continuous keels or "sponsons", thus creating a tunnel or air trap along the length of the hull.

If you are talking about NAMBA rules, I am honestly not sure.

~James

 

[David Moyle]

Hey guys,

Here in Australia we have the same rules as from the IMPBA rulebook, which are quite simple and open. The NAMBA rulebook seems to be a lot more specific in what is and isn't allowed in tunnel design. The main limiting factor which stands out to me in the NAMBA rules is in section 19.D.3.h: "No hull will be allowed to have a recessed or pickle fork bow which exceeds 30 percent of the overall boat length."

I designed my current tunnel hull with a pickle fork length of around 40% (to help with stability at speed). I'm ok with AMPBA (Australian) and IMPBA rules, but wouldn't be legal in a NAMBA event. It just depends on where you intend on racing (if at all).

Hope this helps!

Dave

 

[topfuel443]

Thanks for the info Dave! I know NAMBA has its own thing, i've just never had much reason to look at their rules on the subject.

~James

 

[Mark Poole ModVP]

Namba rule books states that the boats in the OB tunnel class should remain as closely as possible to the design of a full size OPC tunnel boat.

Someone tried to race a gas OB cat in the gas tunnel class at an IMPBA race recently and was turned down. That would be the wrong call unless the rules for Gas OB tunnel specifically state that a cat cannot compete.

So, put a full length cowl on a cat and why is it not a tunnel boat??

 

[Mark Poole ModVP]

As far as design info.....stay away from compound curves anywhere on the sponson bottoms. Best example I could point you towards is an HTB 340/360. These boats handle gobs of horsepower and eat up rough water.

 

[topfuel443]

You are right Mark, that was the wrong call as far as the IMPBA rule book is concerned, but on the flip side Gas Tunnel is not an official IMPBA class. Ultimately its up to the host club to make that call due to the lack of class rules, but if those special restrictions were not listed on the race flyer it should have technically been allowed to run, in my opinion.

Cowl or no cowl.

~James

 

[teddy dauphinee]

Reason I am asking is that there is a boat hull in my basement that has a cat bottom but a round nose front end. I was wondering if the steps on the sponsons would be the determining factor to being allowed or not.

The rules don't specifically say they need to be smooth from the front to the back?

 

[HTV Boats]

The original full sized tunnels were round nose designs. As for steps in the NAMBA rulebook they are very specific on the distance apart (5") and depth not to exceed 1/8 inch. IMPBA does not address this but I am sure the intent and purpose is to not allow multi plane surfaces such as a hydro. The basic principal is the boat riding on the rear of two sponsons. Versus something like a canard that rides on 3 points. Both rule books should be updated to include verbage on "stumble blocks" which are indeed another riding surface but allowed universally in the hobby. Be nice if IMPBA would adopt the NAMBA verbage and clean this up, along with gas rules. Guess tunnels are still second class citizens in this hobby though they seem to still bring numbers to races.

In full size outboard racing basically OPC is divided into V-bottoms and non V-bottoms (sport classes). A hydro or cat could run in a "Sport" or "tunnel" class. Maybe that has been updated in APBA or CHAMP boats defined as specifically for tunnels. Muti steps are allowed and the focus is advancement of design. This has transfered over to production mono's and cats that have stepped surfaces for efficiency and accelerating speed.

As for reinventing the wheel just take your hull set it up have fun and worst case if not a tunnel it becomes a hydro for model boat racing purposes. Put a photo on here of what you have so we can look at it.

Mic

 

[topfuel443]

Regardless of the perceived intent of any rules, we have to adhere to them as they are written. A persons interpretations of rules are often different, this is where long more complicated rules cause more headache than they are worth. I got into a heated 'discussion' about the sport tunnel rules a couple years ago, as the way they are written it allows for some modifications to be made that some deem illegal. It really boils down to the way someone interprets the rules on a lot of them. In my case, their opinion did not agree with mine, so it was squashed as they were the deciding power. The rules aren't perfect, but they work 95% of the time.

Of course, anyone who wishes to change, add to, or amend the rules needs only to submit a proposal to the board for review. I am thinking of writing one to allow oilers in the sport class. Makes no sense (again, to me anyway) not to allow them.

Teddy, it 'sounds' as if your hull would be perfectly legal in IMPBA. If you post a picture or two, we could tell you for sure.

~James