Tunnel removal

[JBM]

I have an old Furnell Flyer D boat that has a 1/4" X 15" X 36" tunnel that I want to fill in. I am using the boat for a 125 and possibly for 350ccmh also.
Im looking for recommendations on how to fill the tunnel in.
Thanks

[David Weaver]

Why?

A lot of good 125's have a small tunnel. How wide is the bottom?

[JBM]

Tunnel

The boat has a 35" bottom. What is the reason to have a tunnel? It was recommended to me to remove it.

[Shane_B]

If you really want to remove the tunnel, there are a couple of methods to fill in the tunnel from my perspective. Quickest way may be to remove finish from tunnel (to bare wood) and then epoxy in a piece of 6mm okoume that is cut to fit tunnel (looks like front of tunnel is tapered vice a 1/4 drop off if I'm not mistaken so you'd have to taper the okoume with a belt sander and use some West System filler #406 to fill any voids). Other way is to remove the current tunnel piece, install 1/4" strips on your cockpit stringers, and then install new bottom piece - either just for the tunnel portion or entire afterplane... Just a couple ideas from a rookie builder - I'm sure more experienced builders have other options for you.

[racerv24]

Tunnel

Our 125 hydro has tunnel boat performs perfectly. What is the reason for removing tunnel?

[David Weaver]

To tunnel or not tunnel

JBM,

I am not a boat builder, but all my PRO hydro's from a 125 in the 70's to my 250 today have a similar tunnel in the back. That is probably 8 boats, including 3 Pugh hydroplane's. If builders like Gary, John Yale, Leigh and others believe it is a good idea, it might worth considering keeping it.

David

[Bill Huson]

Like a few others I'm not sure why your mentor would like the tunnel filled, but maybe a dead flat bottom is better on a 125. I have taken a balsa block and band sawed end grain pieces off it, epoxied the end grain slices into the tunnel and ground them flat with the bottom. Then I soaked the end grain balsa with slow kick WEST resin and sheathed the entire afterplane in 6 oz fiberglass. No science there, 6 oz is what I had. Did a few boats that way when the new OMC A rules banned tunnels in the ASH class many moons ago.

Having been there done that gooey mess of grinding/sanding I'm liking Shane B's idea, just saw the tunnel ramp out, shim up the stringers, and epoxy in a new chunk of 6mm ply.

[JBM]

Can anyone explain to me what the tunnel is supposed to?

[Shane_B]

I'll jump on the grenade as it was explained to me (others, if I'm dorked up - please chime in, I'm no expert by any means) - the tunnel canalizes the air under the boat into a narrow path thus creating a pressurized area, a 'bubble of air' that helps the aerodynamics of the afterplane (providing lift in turns, etc.). Someone else told me that it also allows a controlled spillage of air at the stern for boats at higher speeds. How well it works depends on how well it is integrated into the design of the boat... some designs it is effective and some designs it isn't effective at all. At least that is how it was relayed to me... can anybody else elaborate on the concept?

[Tom Cronk]

Tunnel

John
Give me a call when you have time,home all day.
Tom
863-665-5449

[epugh66]

I'm just a grocery clerk,so I'm not up to date on boat design. Having a tunnel allows packed air to escape out of the back of the boat instead of blowing out around the air traps and making for a wobbly running boat. Typically, you can run in rougher water with a tunnel. Having a tunnel also means there is less wetted surface which is always a good idea. All the cool kids are building boats with tunnels and all the OPC classes run boats with tunnels.

Those are the ideas for it, now here are some against. Lazy boat builder, rules violation, weak engine, lazy boat builder, rules violation or you like runabouts.

In about 2002, the hot trend was to fill in or build with out the tunnel. Then some Europeans showed up with boats with lots of lift and tunnels that ran well, cornered well and may have just been faster too.

My recommendation, as a grocery clerk, is to run the boat and see where you are. If the boat drags it's butt, see if your "check engine light" is on. If you know you're getting max RPM with a good prop, then look at the boat. The key is to finish every heat of racing. I've never scored a point on the end of a tow rope, but I've lost a few.

There's alot of sarcasm intended in my response, but only for comedy relief. Just run it and see where you get. The tunnels can be filled in by the ways mentioned above if needed.

[Merc1]

Mr. Pugh said it right! Just run it and see where you get. Otherwise you are putting the cart before the horse.

[Dave M]

Just My Opinion

Tunnels help with rough water. That said, it is slower than a flat bottom in most stock/mod classes in my opinion. Not much, but it is.

Eric has the best idea on filling it in. I used closed cell phone that I normally use for sandwhich cores glue it down and grind down the way I want it, easy to sand, then lay over some glass and done.

But the way Eric described seems easy. Considering the 125CCH is a single cylinder engine, and if the balance is upset it could cost you, I would leave the tunnel in for some stability. You don't have another cylinder to pull up the other if needed. This is just my opinion. I would defer to some more knowledgable Pro oriented builders. Also, a 1/4" deep tunnel is not very much, and I doubt you will notice much difference either way.

[DeanFHobart]

Balsa Wood

I have an old Furnell Flyer D boat that has a 1/4" X 15" X 36" tunnel that I want to fill in. I am using the boat for a 125 and possibly for 350ccmh also.
Im looking for recommendations on how to fill the tunnel in.
Thanks

John,

Awhile back, when I had 125 equipment, I also filled in the tunnel for the same reason.

I used Balsa Wood. You can get it in 1/4" thickness, then sand it down flush with the remaining surface for the taper. Then use Epoxy Resin for the top coat..... or if you want, the really thin Fiber Glass cloth. And, of course Epoxy Resin to bond it to the boat. It would be best to Vacuum Bag it if you could. Vacuum Bag equipment is not that expensive.

Balsa Wood is really easy to sand.

Be sure to test it before and after, to see the difference.

Good Luck,

Dean Hobart............................................ .........................

[PRO-MOTIONRACING]

I should have read the post by epugh66 before I typed this, then I could have posted... Yup, what he said.

I have always thought that the tunnel allows for controlled spilling of air off the back of the boat allowing the back of the boat to remain settled while simultaneously allowing the use of full length air traps as tracking rails.
I once saw a racer spend a lot of time over the winter filling in a tunnel only to find at the first race that it lifted the back of the boat and made the front dig in, they cut the tunnel back out that night.
I have also seen racers cut off the last part of the air traps trying to get the back to settle down only to then install little metal tracking rails later.

I would think with that light motor unless the boat rides bow high or is to much of a tail dragger I would keep the tunnel. I have seen tunnels narrowed using 2" strips of wood.

For those reading this for fun and information. If you watch Video in slow motion some times you can see and hear a boat that is going through a continuous cycle of packing air. lifting the back, spilling air off the back, the back drops, over and over. I think this is what tunnels and step bottoms help avoid.