Design help, making a tunnel

[clamsalot]

Good morning designers/fabricators

I'm looking for some imput from some of you brighter people about a project I'm doing. Here's the dilema. We have a 19 foot Carolina skiff. They are simply utilitarian flat bottom rolled edge boats. Because it's flat bottom, it gets into some "fairly" skinny water where we like to poke around.

But, because of the 70 hp outboard, we can't get "that skinny" or we drag motor skeg and then of course grind prop.

I saw a project in an old magazine where someone took this same basic hull and created a tunnel. By cutting out what appeared to be maybe a foot wide, by maybe 3 foot long piece of the floor, (and hull,starting at the transom) and then cutting out a matching squared hole in the transom, he glassed in an angled "cover" over that elongated hole.

He put his engine on a hydraulic jack plate. The theory was that as the boat progressed forward, water would be compressed into that tunnel and be "jettet" to some extent, out the back, and "up" approximately 4 inches or so above what would be the normal water line.

This actually allowed him to get on plane and lift his engine to the point where the prop was actually even with or maybe even slightly above the surface water level. By doing this, he claimed he could run on plane in less 6 inches of water. At trolling speed, he was only needed 10 inches to move.

I'm a pretty good glasser/ finisher, so it's not the cutting and layup that's bothering me, it's the mechanics/mathematics of getting the "right" size of this tunnel. I'm sure it's length/width/height has to fit a specific formula of some form, to make it perform correctly. Unfortunately in the article I read, none of that was discussed, just what he had done and how well it worked.

Would anyone be interested in giving their opinion on it?
Thanks

PS< Im new to this board and I've been blown away with the knowledge that's shared here. I went thorugh a six pack and many hours again last night reading old posts. Good stuff.

[tom28571]

There are lots of boats like this used in Florida. One manufacturer is at:

http://www.baycraftinc.com/mypage.ht...TunnelExplorer

I don't have any specific design information for you but your should search Google for "tunnel flats boats".

[Tim B]

http://forums.boatdesign.net/showthr...?t=8468&page=5 look at post #68

that is a true tunnel, and the trick there is to make the tunnel the same radius as the prop (no huge gapsat least), where the prop sits. If you're trying to get an outboard well up, then you could try a similar sort of solution with the tunnel radius a bit bigger. don't let the roof of the tunnel come above the static waterline, though. that's geneeerally bad news

The point of a tunnel is usually to get the engine output lower down. It's quite often used in mid-engined boats, and is generally helpful in getting the prop into the direction of travel, thus extracting a little more forward force. it may also let you get a bigger propeller on the same hull (again, advantageous). Do remember though that it will add wetted surface area, and that there will be a pressure reduction through the tunnel.

Hope this helps a bit

Tim B.

[clamsalot]

That Florida flats boat is almost exactly the idea I'm trying to achieve. As I said , in a magazine, someone else took a 17 foot flat bottomed Carolina and did the alterations, and they worked quite well.

I know they exist, such as the one you found, but I'm not sure about the "measurements" on a hull like mine. Thanks for looking

[clamsalot]

I you look at the application on the Florida flats boat, you see the height of the water tunnel is well above the static water level. In the article I read, that allows a stream of water to actually rise above static level and "jet" out the back, enveloping the motor in water and allowing you to hike the engine ( propeller actually) almost higher than the bottom of the hull.

This isthe type of modification I wish to try.

Thanks for the reply

[tom28571]

Originally Posted by Tim B

http://forums.boatdesign.net/showthr...?t=8468&page=5 look at post #68
don't let the roof of the tunnel come above the static waterline, though. that's geneeerally bad news

The point of a tunnel is usually to get the engine output lower down.

Tim B.

Tim, Most of the tunnel boats I've seen are just the opposite of what you derscribe. Most are done to make the engine draft no deeper than the hull draft and some have the prop shaft actually above the static waterline with the tunnel roof way above that. Those who design them say that the tunnel design is somewhat critical, which I can believe, but they do work.

[Tim B]

Tom, look at the geometry, if the ENGINE output is lower down then the shaft angle approaches horizontal. This is good news. I was using it to get a big prop in less draught on a deep-V hull. Also, yes, I can see it being critical, but I wonder if any have done any sort of analysis on it. Or perhaps, a few CFD runs? Have a think about it, a non-cavitating propeller in open water is easy. Introducing cavitaion is doable, but just setting up the case with a prop in a tunnel is hard.

As for allowing a "jet" of water to effectively increase the depth of water at the prop, that's not really what you're after (good excuse for energy loss (ie. drag)). What you're really trying to do is to put the prop behind the transom (so engine depth and draught are the same) then ensure a smooth flow of water to it. The only issue with this is that there is a possible cavitation problem, but personally I'd solve that with a larger anti-cavition plate on the outboard. It will be less draggy than trying to move the water upwards.

Tim B.

[tom28571]

Tim,

I see your point and had the same thoughts at first glance. All hydrojets have the same issue. They all need to lift every ounce of water before it is discharged. This represents negative lift at the stern.

However, your argument is not with me but with the Atkin's, Dave Gerr and the many others who have designed and operate these tunnel boats that lift water into a tunnel. Clearly they get an important advantage or these boats would be just a footnote in boat design history. The only difference between a tunnel and a jet is that the tunnel has an open bottom and the jet has an enclosed inlet and outlet.

[clamsalot]

Okay folks,

I've read the exchanges, and they are food for thought. But I'm still in a quagmire. I know thee has to be a "right" way and a wrong way to do what I'm going to do. I guess it's going to be based on hull width, draft, weight, etc.

I'm sure there are a few people here that would know the proper numbers to pull this off. Anyone want to give it a try? I won't hold you to it! LOL, actually, I'm going to do this, with our without the proper measurements to extract maximum performance, but I'd sure like a bit of input from those who's actually know

Thanks
Bob

[tom kane]

I have recently posted an image and a diagram in my photo album of a tunnel,that may be of interest to you.Hydrodrive it is called.I have travelled at 60 MPH + in a boat fitted with this design which has advantages.They were also used for many types of small boats.

[bananas]

Hi Clamsalot

My work takes me under some of the best large motor yachts (www.softcav.com). I have seen good tunnels and very poor tunnels. The best tunnel design I have seen resulted in a verifiable 2-Knot speed increase on a base of 38-Knots on a very large sportfisherman. The worst tunnel I have seen resulted in significant noise and hull damage. Speed change could not be verified.

A well designed tunnel captures the water aft of the spray line, in the region of planning that aligns the flow parallel to the keel. Yes, this works even in V-hulls. I have seen several photographs of tufted bottoms, and yes the flow does align parallel to the keel aft of the spray line.

The best tunnel entry is a gently raising (relative to the keel) rectangular entry. The area is ruled such that the segment above the propeller is captured as the rectangular section becomes circular. The tunnel should remain constant width and constant capture area. Shallow tunnels (15-30 Percent of propeller area seem to work best.

The tunnel reduces shaft angle (I have seen as low as 6.5-Degrees), and in the best designs provides hard water above the propeller. I have observed hard water in many tunnel wakes, even behind the rudder. I have built propellers which run at about 3-Percent of diameter away from the tunnel surface and they are efficient and quiet. That is no big adavantage however, I have run 2-1/2-Percent off conventional bottoms with similar results.

The worst design I have seen looks like a circular cutter has been run into the hull parallel to the propeller shaft. At speed, the flow separates from the increasing width and lets the propeller tip run in a region of vapor. You can see the vapor in the wake. The jet off the propeller tip produces significant hull damage and typically the hull must get protection from metal plates.

Hope these details help

Ciao

[clamsalot]

Bananas

Thaks so much for what you've said. I have read and reread what you've said, and it makes a lot of sense. So, can I ask another question.

Obviously you are talking about tunnels in inboard boats. I however am working with outboard power, mounted on a hydraulic jack plate so the height of the propellor can be varied by approximately 7 inches top to bottom.

Given the dimentions of the boat ( 19 ft long, with a floor width of 60 inches) I realize this is all guess work, and I'm not holding you to anything when I ask for input! But, I'm considering a rectangular tunnel, 14 inches wide. The tunnel will be 3 feet long, and taper up from the hull bottom, to 3 inches above the static water line at the transom.

I'm hoping that using those parameters, I will experience enough exhaust "jet" through the tunnel to allow my propellor to run almost to hull bottom height. Or at minimum, high enough that only the bottom arc of the propelor rotation extends below the hull line.

If I can accomplish that, I should be able to run on plane across water that is less than 7 inches deep. Any thoughts on if my dimensions will work?

Thanks again,
Bob

[tom kane]

Are tunnels fitted in a hull realy any different to what race boats achieve with out a tunnel? The motor is lifted to run the prop in surface piercing mode.
A tunnel operates the same way only the rear of the hull is placed further forward.The prop is usually run faster with a 1:1 gearbox.

[bananas]

Hi Bob

The tunnel length seems a bit short to me. If you make it a bit longer you can fill it in if needed. If it is too short it will be like starting over again! The width seems to be OK. Of course, we are both guessing.

Regardless of the length, the shape of the tunnel needs to be a very gentle S-curve. The curve needs to be tangent to the keel at the start (parallel to the keel) and tangent also at the end.

this curve should work

T=(X)/(Length) where X is measured from the transom

D=(Do)(1.0-3.0(T)(T)+2.0(T)(T)(T))

This curve will generate a nice smooth S, as measured from the transom and Do is the height above the keel.

The smooth entry will pull the water up until the speed gets high enough to separate the flow. Sort of like an upside down airfoil. The end of the curve will make the flow parallel to the keel just before the propeller.

Ciao
Max

[bananas]

Hi Tom

There is a very important difference between a tunnel and using propeller height to transition to surface mode.

The tunnel will keep the entire disk area of the propeller wet and reduce the draft. Most high speed sport fishermen run propellers too close to the cavitation induced thrust collapse load. At this load, additional power and propeller RPM will not produce additional thrust. Thus, because of thrust loading, the option to operate in surfacing mode and the attendant loss of disk area does not exist.

Fitting such a boat with a surface drive requires either a very large propeller (square root of two larger) or large cup to produce the required thrust load. The large cup will kill low speed performance so the surface mode is not generally successful for a high speed sport fisherman.

So the tunnel has found an important niche in this type of boat. A significant draft reduction, and because of cleaning up the flow above the propeller, often a very significant performance increase.

Ciao

Max