Pocket tunnel design HELP

[bahamabay]

We are working on a new boat model but are having problems with the pocket design. This is a flat bottom boat design and because it doesn't ride deep enough in the water we cannot get water in the pocket. This is not a full tunnel. We want to stay away from a full tunnel. A full tunnel takes away from the draft which we cannot afford. The pocket now is 22in. deep and about 20in. wide and 6in tall. The boat is 21ft. Can anybody help.

[bahamabay]

We use tunnels to pull water up in the hull so you can jack the motor up higher to run in shallow water.

[messabout]

Search for Wm. and John Atkin designs. They have produced some very workable boats with the feature you describe. See also a boat called Rescue Minor built more or less to Atkin design by the late Robb White. Rescue Minor has demonstrated the ability to run 12 to 15 mph in six inches of water. Not a fiction. I have seen the boat do so with my own eyes. It is an inboard of about 20+ feet LOA powered by a little Kubota tractor engine. RM is not a high speed boat so it may not suit your needs. I presume that you are talking about a "flats" boat with a large outboard and capable of terrifying speeds when there is sufficient water. The Atkins/White concept may be worth exploration even so.

[eponodyne]

Have you considered shaping about the aft third of the hull into a great big NACA duct? (http://images.google.com/imgres?imgurl=http://home.hiwaay.net/~langford/nacaducts/NACA_duct.jpg&imgrefurl=http://home.hiwaay.net/~langford/nacaducts/&h=1650&w=1243&sz=128&hl=en&start=2&tbnid=iXOpywjFGlYxKM:&tbnh=150&tbnw=113&prev=/images%3Fq%3DNACA%2Bduct%26gbv%3D2%26svnum%3D10%26hl%3Den%26sa%3DG) That one's out of Tony Bingelis' book The Sportplane Builder, but the formula for layout can be scaled up. I'd consider it as the ducts are purposely designed to move fluids around with very low drag.

[dick stave]

There is some information on the Bateau.com website. The phantom 16, and the extreme flats 20 have pocket tunnels. Chris Ostlind also designed a pocket tunnel I saw on Duckworks (Blackwater 16) which is a scaled down version of Capt. Henderson's flats boat. You might want to consider sponsons to maintain minimum draft. They really work well on my FLAT BOTTOM jet boat.

[dick stave]

Here they are.

[bahamabay]

The problem is that this is a wide flat bottom design with a outboard that only runs in 2-3 inches of water so I am trying to pull the water up into the boat so I can raise the motor up below the bottom of the boat or close to it. I am trying to do this with a pocket that is 22 inches deep. I can put a full tunnel in and get all the water we need but then loose the draft I have to keep.

[kengrome]

How much draft do you actually need to keep? Do you really need it to draft only 2-3 inches?

[Pericles]

Bahamabay,

You appear to be reinventing the wheel. http://www.flatscat.com/

Pericles

[bahamabay]

I want the boat to draft less than 6inches and run in 3-4 inches.

[kengrome]

I want the boat to draft less than 6 inches and run in 3-4 inches.This would require a boat that lifts while running at speed, and it also leaves out the Seabright skiff hull design -- along with its well-known seaworthiness in offshore conditions -- so I think the flatscat suggestion is more along the lines of what you need.

[Pericles]

Bahamabay,

Did you look at the specs?

21' Flats®Cat Beam 7'6"

Bottom 7' Weight 1350lbs

Gunnel Height 20" Transom Height
27"

Planing Distance 10' Take off depth 6"

Minimum running depth 3 1/2"

Bow rise on take off 5"

Boat Draft at Rest 4 1/2"

Boat Draft at Speeds 0-1/2"

Maximum HP 150

Recommended HP 90-130

Speeds 115 HP = 43.MPH 130 HP = 48 MPH

Just what you asked for.

Pericles

[Capt. Ken Owens]

The size of your hull needs a longer tunnel. I am a consultant with the Bateau design called XF 20. I came up with the design and notified Jacques Mertens owner and designer of the company.We put our heads together and came up with the XF20. Jacques did all the engineering and design work, The tunnel needs to be at least 36 to 40" long. 20" wide will work. From the bow forward area go from ) back 12" to 5" high,then rake back to the transom area finishing at 4" high. Install a 3" radius on each inside of the tunnel. This allows water to enter the tunnel and compress and forces it out the stern. Very simple. From the top of the tunnel to the top of transom you want 20". This allows the jack plate to come into play by raising the engine so prop runs in tunnel backwash.The Hendersen design is a proven hull design. I know Capt. Hendersen personally and have seen his hulls at work. We used his hulls as an inspiration and came up with our own design.Go to bateau.com and click on XF20 in the power boat section and see pictures.

[dick stave]

The pocket tunnel design on the Henderson skiff is interesting. I would be interested to hear from someone who is running one of these skiffs. I have done some experiments with jack plates on flat bottoms boats. The rule of thumb as explained to me is 1/2" of lift to 1" of setback to a maximum of 10" setback ( break even point ). I did not use low water pickups or expensive double cupped props and achieved some pretty skinny water results with no cooling or cavitation issues. I have also read that these tunnels are also sometimes vented at the top of the entrance with an elbow and tubing thru a ball valve and then discharged over the transom to alleviate priming problems. The way it was described is that you had released an anchor when the valve was opened. Good luck.

[kengrome]

I am a consultant with the Bateau design called XF 20. I came up with the design and notified Jacques Mertens owner and designer of the company.We put our heads together and came up with the XF20.Hi Captain Ken Owens,

I've designed an extreme flats boat that's even more "extreme flats" than most. It requires a simple surface piercing propeller drive, and the boat (including propeller) drafts less than 4 inches at 1000 pounds displacement. I'm wondering if you'd take a look at it and give me your opinion, assuming you're interested. Other's opinions are welcome too of course!


http://www.bagacayboatworks.com/linkfiles/flats15-67.jpg


Note that the top of the tunnel will be vented to allow compressed air to escape rather than be forced into the water in the tunnel. I expect this will keep the tunnel water more 'solid' with less entrained air.

You will probably also note that the boat is designed to use an inboard engine installed in the center console (not shown, nor is the rudder and other non-hull features). The boat is supposed to be like a 'scooter' with a big, wide, flat casting deck forward, and the rest of the boat with a flat deck 6 inches lower.

I like the inboard diesel for fuel economy as well as the fact that the stern is completely free from equipment, so you can walk around the aft end of the boat and fish from there with no obstructions to get in your way. I also plan a "trap door" over the prop to clear it without getting wet.

[dick stave]

I like it. What h.p. powerplant? Any speed predictions? More Hickman than Atkins. Whats happening with the Tolman seabright?

[kengrome]

I like it. What h.p. powerplant? Any speed predictions? More Hickman than Atkins. Whats happening with the Tolman seabright?I don't have any prediction for HP or speeds since this is not the typical boat I've designed. I hope it will plane on 20 HP at 1000 pounds and I suspect it will, but I don't know how fast it will be. It's a wide boat so it will have a lot more surface effect drag than the other boats I've focused on. I did this boat for a potential multi-unit buyer who has since vanished, so I went ahead and developed it anyways, for myself I guess.

This is just another version of a low-cost, low-power boat that might be a good alternative to someone else's high cost, high power boat. I seem to have a particular interest in designing boats for simplicity and lower than normal power requirements. My goal was to create a boat that would be cheap to buy, build and run, yet take 1-2 people fishing into all the same places they are currently going with their much more costly and "high-tech" scooter type boats -- and perhaps go into even shallower areas as well.

This boat has no propeller adjustments and none are required because of the way the water height is controlled in the aft tunnel and hits the prop at precisely the right place. Other boats like this put expensive high-HP outboards on jack plates to get the props at the right height in different running modes, and they still draft more because the props extend below the bottom of the boat. Their engines also take up most of the transom area so fishing is more difficult from the back of the boat.

Tolman Seabright is for sale as-is or at any stage of completion. My guys are working on it now and then in between other projects, but it is not at the top of my priority list right now.

[Capt. Ken Owens]

On the Bateau site there was a man I think from Texas that was redoing a hull very similar to your design.The power train was nothing compared to what you are thinking.. I am not sure if your concept will work or not.There are alot of design out there and a lot of ideas. I have a tunnel design in the back of my head that I have been thinking a lot about but not done anything with it.Biggest problem is trying to figure how to do it as a test part before initializeing it in a hull.There is a new jet drive(Inboard) on the market that looks interesting.It has a number of propellars in it like fins. Same design as the Dura Jet(outboard drive) made for the military. You can see this drive on www.smalloutboards.com Getting back to your design I am looking at a hell of a lot of work to fashion the tunnel and drive. ??? Not sure about it. If I can find the link on the inboard drive I will post it.

[Capt. Ken Owens]

Link to jet drive.
www.marinejettech.com/index.hyml

[kengrome]

I am not sure if your concept will work or not.Neither was I until I found this:


http://zf-marine.com/ZFR/Surface_Drives/Trimax/Images/2100I_450.gif

This is a ZF surface drive unit which appears to use virtually the same tunnel geometry I came up with independently in my flats boat. When I found this ZF drive info and picture online I was very pleased (to say the least) since it seems to confirm that my concept is a good one. Note the blue tube just in front of the prop which injects the engine exhaust atop the propeller to insure proper ventilation as required for this type of drive.

Here is the URL for more details: http://zf-marine.com/ZFR/Surface_Drives/index.cfm?PID=ABD (http://zf-marine.com/ZFR/Surface_Drives/index.cfm?PID=ABD)

I have a tunnel design in the back of my head that I have been thinking a lot about but not done anything with it. Biggest problem is trying to figure how to do it as a test part before initializing it in a hull.I live and work in the Philippines so hull construction is not a big expense for me personally. With so many Filipino craftsmen willing and able to work over here, hull construction is relatively fast as well. And I also build prototype boats for other people at a labor rate of only $2.00 per man-hour, with certain restrictions. I think I can cover my costs at this rate, although no one has taken me up on the offer yet so I'm not really sure.

There is a new jet drive(Inboard) on the market that looks interesting.It has a number of propellars in it like fins. Same design as the Dura Jet(outboard drive) made for the military. You can see this drive on www.smalloutboards.com (http://www.smalloutboards.com)I looked at it, but it still requires an outboard, and I'm sure it costs a bunch too, aside from the outboard itself. My goal is to use very simple systems, including an inexpensive (relatively speaking) non-marinized inboard engine to keep the engine cost down -- hopefully a diesel to minimize operating costs since high fuel costs are hitting the marine industry big time these days, or so I hear.

Getting back to your design I am looking at a hell of a lot of work to fashion the tunnel and drive. ??? Not sure about it.No, each panel is fully developed so they should all fit together nicely over a few molds. The drive shaft tube doesn't even need a stuffing box either because its inboard end is more than half a foot above the waterline. The lower end near the prop is above the waterline too, so I'm thinking of using an oil filled tube with sealed ball bearings on both ends. The ZF unit ends at the hull but mine would have the prop shaft and tube extending much further forward, all the way to the engine installed amidship.

[dick stave]

Can you get the surface drive completely tucked up inside the tunnel for total prop protection? It appears as the red line in the illustration is the profile of the boat bottom leaving half the propeller and rudder exposed.

[dick stave]

I had a look at the Jettech site ( Intellijet ). They claim that this system resolves the holeshot problems of conventional jets? Its pretty much universally known jets have a great holeshot (check out some jet drag boats). The big scoop diverter plate on the bottom resembles a shovel. Maybe I'm missing something here but I don't see the advantage. I don't see Hamilton getting run out of business with this one.

[kengrome]

Can you get the surface drive completely tucked up inside the tunnel for total prop protection? It appears as the red line in the illustration is the profile of the boat bottom leaving half the propeller and rudder exposed.In the ZF drive illustration. it looks like the lowest portion of the red line (at the bottom left) is the bottom of the hull. However, in my flats boat that line represents the bottom of the aft tunnel -- not the bottom of the hull. In other words, my design already has the entire surface drive tucked up inside the tunnel!

:)

[kengrome]

Here, I made a composite image by overlaying the ZF surface drive onto my flats boat hull profile. I think this will better illustrate how the ZF drive geometry matches my flats boat tunnel section rather well:


http://www.bagacayboatworks.com/linkfiles/zf-flats.jpg


Clearly my boat needs to be a bit longer -- or the tunnel should end a bit further forward -- to make the ZF drive fit properly, but these are minor changes during the design phase. Once these changes are made, it looks like everything else fits rather nicely.

[dick stave]

That would be a slick arrangement. Very few moving parts. Have you priced the zf drive? I'm sure the shaft could be adjusted to fit your tunnel as the stock length shouldn't dictate your tunnel design. This new design of you re's is very innovative. To eliminate the cost of large expensive outboards, hydraulic jack plates, and custom double cupped propellers is a step in the right direction. To gain wide acceptance I would predict a turn of speed in the mid 30 m.p.h. range would have to be achieved.

[kengrome]

Hi Dick, thanks for the feedback, I'm glad I'm not the only one who appreciates simplicity and lower cost ... :)

No, I haven't priced the ZF drive but my guess is that it will cost more than the hull. They probably don't make these drives in the small size this boat would need anyways. Besides, the ZF drive is more complicated than my drive system ...

Mine does away with the expense and complication of that "stuffing box" thing they use at the thru-hull. I plan to run the shaft tube all the way to the propeller, and just epoxy it into the hull -- much cheaper and easier than the ZF system, and no proprietary parts to deal with. I think I will also support the shaft tube with a solid vertical panel instead of leaving it exposed as in the ZF drawing. The shaft tube is above the waterline anyways so there's really no reason to leave it unsupported. A continuous solid shaft support will help to prevent things from getting wrapped around the spinning shaft, too.

Regarding your comment about the ZF shaft being adjusted to fit my tunnel length, I'm sure this is true. On the other hand, I have read that new designers of surface drives (like me) sometimes leave too little space between the transom and the propeller, and apparently this creates performance problems. Fortunately I learned about this before I designed the boat, but even so my dimensions are still none too long here. Therefore I'm going to move the "tunnel transom" (the panel the prop shaft passes through) forward in my design, enough to provide a bit more length here than the ZF drive allows. This should insure that I won't have any problems with the prop being too close to the transom.

This new design of yours is very innovative. To eliminate the cost of large expensive outboards, hydraulic jack plates, and custom double cupped propellers is a step in the right direction.I hope you're right. My thinking is to make it simple yet functional, which should lower not only the purchase price but also the operating costs and maintenance requirements. It won't be easy to damage the prop in this boat when it sits higher than the bottom of the hull, that's for sure. I think this feature alone might help owners to relax and not worry so much about a damaged prop.

To gain wide acceptance I would predict a turn of speed in the mid 30 m.p.h. range would have to be achieved.I think you're right about this too, which means it will probably need a bigger engine than the 15-20 HP I would personally be happy with. Then again I'm not a speed demon and slower speeds suit me just fine. Fortunately replacing a smaller stock inboard engine with a larger one should be very simple in this design. The engine is basically installed at deck level. I hope to put it under the center console, and as long as the console is big enough a number of different engines will do the job.

[dick stave]

If you can eliminate the "stress factor" of damaging propellers by tucking that drive well inside the tunnel you're onto something ( precisely why I went to jets). In the region where I live and fish the rivers are rocks and gravel. Nothing worse then running a boat constantly thinking about how much this outing is going to cost (takes the fun out of it). The flats boat guys must cringe every time they come out of the hole with that lower unit down with that stainless four blade in jeopardy. A mechanical seal in a simple housing would be a nice compromise for the stuffing box. I do think you will have to rake back the front of you're tunnel ( the aforementioned seal housing making the shaft penetration/mounting simplified).

[kengrome]

If you can eliminate the "stress factor" of damaging propellers by tucking that drive well inside the tunnel you're onto something (precisely why I went to jets).This is almost as protected as a prop can get without running a protective shoe beneath it -- as in the Atkin tunnel-stern boats. I can add a shoe or make it an option for those who want even more protection, but I think the raised prop alone will eliminate almost all the risk of prop damage.

Now that I think of it, I can probably extend the aft end of the shaft support into the water and down to the bottom of the prop, making it into a kind of skeg. This should divert the boat around most centerline obstructions and protect the prop a bit more too. But I think I'll try it without these added protection features first.

In the region where I live and fish the rivers are rocks and gravel. Nothing worse then running a boat constantly thinking about how much this outing is going to cost (takes the fun out of it).If others think like you do, then maybe I'm actually on to something here ... :)

The flats boat guys must cringe every time they come out of the hole with that lower unit down with that stainless four blade in jeopardy.I would think so, especially in places where the bottom is not soft. The prop in my design might still hit a high rock if the boat were driven directly over it on the centeline, but I think you would have to be really trying to make this happen, or else have more than your share of bad luck.

Another thing I want to do is protect the rudder(s). I am thinking of using two rudders instead of one -- each attached to one side of the vertical joints between the boat's transom and tunnel sides. This approach has several advantages:

1- It allows the use of shallower rudders since two shallow rudders should perform as well as one deeper rudder.

2- It gets the rudder out of the prop stream which might (in theory anyways) make the boat more efficient.

3- In comparison with a spade rudder (as in the ZF design), two hinge-mounted rudders will require cheaper components and be easier and cheaper to fix when damaged.

4- If one rudder breaks, you can still control the boat and get home on the other one.

The main disadvantages are that two rudders take more time and materials to build and control so the cost will be a bit higher, and they won't be balanced when hinged directly to the transom/tunnel corners so they will require more brute force to turn them.

I'm not sure about backing performance, but I doubt this boat will win any contests when backing anyways. I don't think any of these scooter type flats boats are good at backing, are they? It seems they are too optimized for high speed forward motion in very shallow water to be any good at backing.

If I ever decide to produce this boat I'm thinking about selling a stripped down model without a gearbox. Instead I'll use a V-belt drive with an idler pulley for neutral, and backing will have to be done via pole, oars, paddle, etc. Of course the regular models will have a F-N-R gearbox, but I think there are still people in the USA who might like to save $1000+ and use it for something other than reverse. Robb White never bothered to fix his reverse when it broke on his Rescue Minor. I think of reverse in a small boat as a convenience more than a necessity, and it's nice to be able to save money by making it an option rather than a requirement.

A mechanical seal in a simple housing would be a nice compromise for the stuffing box. I do think you will have to rake back the front of you're tunnel ( the aforementioned seal housing making the shaft penetration/mounting simplified).My shaft is set at a 3 degree angle as-is, so angling the top of the "tunnel transom" 3 degrees aft would be all that's needed to make the shaft tube pass through this transom at a 90 degree angle. Little details like these are easy in the design phase, and it is especially nice to discuss them with someone who understands my goals and who has an interest in what I've been working on. Thanks Dick.

[dick stave]

Just a quick thought . For reverse, a simple slide gate that dropped down behind the propeller ( similar to the reversing bucket on outboard jets) redirecting the flow forward might work. The rudder could rotate up on a shaft held down with a spring similar to sea kayaks.

[Pericles]

It's gone quiet on this thread. This might have been posted before, but even so, the Landau 20.

http://www.selectyachts.co.uk/landauhome.asp?ArtID=261

The website is quite useful and the L29 video nicely demonstrates the performance.

The twin tunnel design reduces river wash at low speeds and good speed on the plane. 120hp diesel and 35 knots.

Pericles