# question about tunnel hulls



## jeffm66 (Sep 14, 2010)

Doesn't a tunnel hull boat draft as much as a regular hull at rest? Can a 6" high tunnel allow you to raise the motor more than that to get on a plane? I am trying to figure out if the tunnel is worth it if I can get a boat that drafts less 18" including motor without a tunnel? Not inclunding cats.


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## spurgersalty (Jun 29, 2010)

Here my duck hunting/shallow water fishing rig, you decide after looking at a few. My tunnel is 5", and my cavitation plate is right at 3" or a little more above the top of the tunnel.


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## [email protected] (Jun 25, 2005)

Yes it will draft the same tunnel or no tunnel. The tunnel will allow you to get on plane in less water though and run in less water than a non tunnel boat. For example a Haynie 24 HO can get up in about 18 inches of water and it doesn't have a tunnel. The bigfoot which is the tunnel version will get up in a foot of water. The bigfoot is a tad bit smaller boat by about a foot but you catch my drift. A tunnel hull will hurt speed and fuel economy though so there is a trade off there.


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## Jeff SATX (Jan 18, 2011)

Most traditional v-hull boats have a continuous v from bow to stern. That continuous v sticks that much further in the water (just a few inches) than if it were gone due to being missing because of the tunnel cut out. Flat bottom boats are different; if both flat bottom hulls are similar (skiff and maybe a RFL), both will draft the same regardless of tunnel or not. Pocket tunnels would be the same draft on a v hull boat as one without one because of the 90% continous hull. Jack plates will also have their place in the equation, not so much on draft, but on hole shot. If I wasn't sitting on my couch, I'd post some pics of different hulls and their draft lines.


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## RedFly (Mar 22, 2010)

If you compare the same boat that offers a tunnel as an option, you will answer this question I think. Every case I've seen the same boat compared, the non-tunnel hull consistently drafted less at rest than the tunnel hull version by a few inches (not too much unless 2"-3" makes a difference for you). This holds true with Hell's Bay, Maverick, and East Cape Skiffs. This is why Florida style poling skiffs don't comonly have tunnels. They can tilt up their motor and poll over shallower water without a tunnel. Tunnel boats draft with their back end deeper in the water because there's less surface area displacing the weight back there (while at rest/drifting). Most tunnel boats have sponsons designed to hang out the back of the boat to help alleviate this problem, but because their back end sits lower they often suffer from needing (slightly) deeper water to get on plane since their back end has less area resisting the wheelie affect.

The biggest obvious difference is how much shallower a tunnel boat runs when on plane. The jack plate can lift the prop even up above the bottom of the boat in advanced tunnel designs such at the New Water Curlew while running on plane. Pocket tunnels do the same thing just not as dramatically as a full tunnel design.

The biggest drawback to running with the tunnel is the wetted surface increases greatly on plane, and even more so whith sponsons. The more wetted surface dragging the water, the slower the boat at full speed. This speed problem can be helped when running over deeper water if your tunnel is correctly vented, allowing air to fill the tunnel and requiring you to lower your jack plate to keep the prop wet thereby reducing the wetted surface but requiring your prop to be lower to pick up speed/effeciency.

Am I making any sense? 

Texas has lots of huge shallow flats compared to Florida's bigger tidal swings and Florida's more prominent channels which allow easier drifting/poling into water plenty deep enough to plane out of.


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## fishminer (Jan 4, 2006)

*Tunnel*

I currently have a Tunnel. My next boat will not. My 110 HP acts like a 90 HP. My previos boat was a 16' with a 110 HP that ran circle around this one. My cousin had a Bluewave with 200 HP and it acted like a 150 HP due to the tunnel. Just my 2 cents......................


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## TooShallow (May 21, 2004)

If you don't plan on running and stopping in skinny water (< 10 inches) then you probably don't need a tunnel hull. Most bay boats will run in 10-12 inches without a tunnel if set up correctly. The problem comes when you get shallower or stop and try to get back up. As for the draft, it's all about weight and displacement. A tunnel will decrease draft slightly due to lost displacement. The other factor is speed. Tunnel hulls are slower than standard hulls and usually this is minimized by using more HP on a tunnel hull.


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## Stumpgrinder (Feb 18, 2006)

Typically tunnel hulls turn for crapppola and have no /very little reverse either. Just some things to consider.

Dont buy a tunnel unless you absolutely need one.


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## skeltowt (Aug 25, 2010)

*Tunnels do it shallower*

We have had two ShallowSport boats for the last 22 years. _*BIG*_ tunnel. The drawbacks have been mentioned, poor turning, drop in top end speed. But I challenge anyone in a typical non-tunnel hull to follow me while roaming around Port Isabel, Mansfield even back lakes around E Matty etc. If you do not intend to go shallow, you do not need a tunnel. If you intend to go shallow, by all means get one.


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## NewbieFisher (Jun 3, 2009)

never been to a physics class, but if you take a piece of plywood (4 x 8) and lay it in water, it will float. all 20 pounds of weight, it will still float but will be X inches below the surface. correct?
now take same piece of plywood and remove an area the size of the common sized tunnel and box it in just like a tunnel and add same 20 pounds, wont the plywood sink to a lower depth till the upper part of the tunnel help support water?

of course this is all at rest. everyone knows that once its in motion, the tunnel will run shallower


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## spurgersalty (Jun 29, 2010)

NewbieFisher said:


> never been to a physics class, but if you take a piece of plywood (4 x 8) and lay it in water, it will float. all 20 pounds of weight, it will still float but will be X inches below the surface. correct?
> now take same piece of plywood and remove an area the size of the common sized tunnel and box it in just like a tunnel and add same 20 pounds, wont the plywood sink to a lower depth till the upper part of the tunnel help support water?
> 
> of course this is all at rest. everyone knows that once its in motion, the tunnel will run shallower


Good point, I agree. But my jack plate gets my motor up higher than the amount of loss in buoyancy (draft).


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## NewbieFisher (Jun 3, 2009)

spurgersalty said:


> Good point, I agree. But my jack plate gets my motor up higher than the amount of loss in buoyancy (draft).


what does that have to do with sitting at idle/drifting? the bottom of the boat is still in the same lower level.


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## spurgersalty (Jun 29, 2010)

It means the motors not stuck in the mud further hindering my forward movement.


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## NewbieFisher (Jun 3, 2009)

even on a flat or v bottom without a jackplate, youd be able to trim the motor high enough to get it off the bottom.


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## spurgersalty (Jun 29, 2010)

But trimming causes more downward force on the rear end, shoving it further in the mud, and creating more drag.


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## NewbieFisher (Jun 3, 2009)

oh. i understand you know. your talking about getting on plane from a shallow water start.


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## troutsupport (May 22, 2006)

Turning... The desperado tunnel will turn on a dime.. gives that boat rails.. 

yes, there is a drop in top in speed.. but the desperado hull is stepped and it get's it pretty good. no, won't keep up with an extreme or any v that's designed for speed, but it's an advanced cat tunnel. 

that's one thing to look at, a lot of tunnels are just a v hull with a slice out the middle... sure it's a cat / tunnel.. but it wasn't designed from the start that way. 

I do agree with your last statement.. if you intend to go shallow, get one. They are also safer since you can run with the prop tucked into that tunnel and there is less chance of hitting an obstruction or reef.


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## JohnHumbert (May 21, 2004)

*Simple, really...*

How much a boat drafts is strictly and totally dependant on two factors: The weight of the boat and how much surface area is touching the water. Nothing else figures into it, period. Larger, heavier boats can draft shallow that a smaller lighter boat if they have more surface area. Case in point, my current 21' RFL floats shallower than my old 18' RFL, and the 25' RFL floats shallower than my 21'.

How shallow a boat will get up is dependant on MANY more factors - but primarily lies with hull design. Flatter hulls will generally get up more shallow and speed/accelleration is major factor. Case in point, the reason my TRP gets my boat up so fast has NOTHING to do with "prop lift", which is really a fiction. The TRP, with it's twin props, has amazing accelleration - within a few feet it pushes the boat so fast that the hull begins to plane. However, as I said - MANY factors go into this.

To get ANY boat up, you have to push it. To push it the prop needs to be in the water. Can't get around that. Tunnels give ZERO advantage when the boat is at rest. Even with my RFL, I still have to lower the jack/prop enough so that it is sitting the water.

Here is where props come into play. Some props "bite" the water better than others. Some props require to be fully submerged to get any bite at all, some can be 3/4 submerged, and some - like the twin props on my TRP - can only be 1/2 submerged to get enough bite to start pushing the boat. Lots of factors go into the prop that control "bite": pitch, cup, size, blade design, etc.

Naturally, the more prop in the water - whatever the design - the more it will bite. This is where tunnels come into play.

The theory/operation of tunnel design - and there LOTS of different type of tunnels - is that the tunnel creates a stream of water that is thrown toward the motor. The more efficient the tunnel, the more water is thrown back. In most cases, this stream of water can/is directed in such a way that the stream of water is ABOVE the bottom of the hull.

When this happens the prop can be raised so that it can get the "bite" from the artificially created stream. In extreme cases, like with the RFL, a sufficient stream of water is throw above the surface of the water so that the prop is totally above the surface, and in many cases above the bottom of the boat.

But - to get this advantage - the boat has to be moving, and moving fast enough to create a large enough stream for the prop to take advantage of. Again, this is where accelleration comes into play and the reason why tunnel boat SHOULD be powered a little stronger than non-tunnel boat. You need to get that quick push to start the tunnel action.

There are dozen of boats, hull designs, tunnel design, motors, and prop configuration out there. You can take the SAME boat and put different motors on it and DRAMATICALLY different hole shot performance. You could also have two identical boat/motor setups and change out the props and get DRAMATICALLY different hole shots.

Most tunnel experts choose props that give more PUSH (lower pitch) so that they can get the accelleration and better hole shot. They sacrifice top-end speed (higher pitch) to get the hole shot. You can, and I have experimented with, put a high pitch prop on a tunnel boat and get them to go SUPER fast, but hole shot sux and it could take 100 yards to get the boat on plane. But MOST traditional design flat-bottom tunnel hulls, get really, really squirrelly and hard to control when they go over 50mph.

It's a beautiful thing when there is a perfect matching of hull, tunnel, HP, and prop. There are some great marriages that have been found over the years. For me, my Majek 21' RFL, with a 150 HP TRP is just one of the perfect combinations. Shallow draft, unbeatable hole shot, and a top end about 48mph. But I still get beat to death in a heavy chop.


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## OxbowOutfitters (Feb 17, 2006)

I have a tunnel.. its ok, but its slow compared to non tunnels..even with over powering it by 30hp... it does however get up quicker with the bigger (Push) motor...they do have pizz poor turning radious & they porpus when there maxed out.. trim tabs dont help either.. my next rides gonna be a cat...


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## cva34 (Dec 22, 2008)

*Tunnel?*

16 flat bottem tunnel and pontoons..40 hp John.With a level and trusty yard stick.I leveled boat in driveway took these measurements.With jacked all the way up (the way I run)the center of the prop is even with bottem of boat so from bottem of boat to bottem of skag is 7".The boat floats in about 5" of water with the motor tilted up......That says it will float in 12" and not touch bottem:::::I also took these measurements which would be a typical set up with no tunnel..Running the motor all the way down with jack plate to where cavation plate would be even with bottem..Boat would still float in 5" but to skag bottem would now be 14" below bottem of boat That says it will float in 19" and not touch bottem...Hope thats clearer than mud...CVA34


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## MapMaster (May 25, 2004)

Good discussion. I have seen better answers here than I got at the Houston boat show last weekend.


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## Red Line (Jun 9, 2009)

I have a 21' Majek Tunnel Hull Boat, the only thing I don't like is it's top speed. I'll take the slower boat to run shallow over a cat, skiff or flat bottom becuase it handles the chop way better than these other shallow running boat. I have not had any problems getting out of 8" inches of water to date, if the bottom is soft, it will get up. If the bottom is hard sand, then I just putt out. The other pro and cons have be listed on this thread already.


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## gray gost (Jul 8, 2010)

one thing to consider. with good tunnel when motor runs aground boat does not float. nothing like seeing your boat leaning to one side on mud flat. with out tunnel, boat will normally float


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## pipeliner345 (Mar 15, 2010)

skeltowt said:


> We have had two ShallowSport boats for the last 22 years. _*BIG*_ tunnel. The drawbacks have been mentioned, poor turning, drop in top end speed. But I challenge anyone in a typical non-tunnel hull to follow me while roaming around Port Isabel, Mansfield even back lakes around E Matty etc. If you do not intend to go shallow, you do not need a tunnel. If you intend to go shallow, by all means get one.


Amen x 10 !......i went and got one


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## Jeff SATX (Jan 18, 2011)

Slow day at work...

Draft @ rest:
24 VDR - 12"
24 VDRT - 12"
Pathfinder 22 - 12"
Gulf Coast Vside - 10"
Haynie 24HO - 12"
haynie 23 BF - 10"
Haynie Z21 - 9"
Bluewave 220T - 8" (bluewave being a little generous there?!)
Bluewave 220 - 10"
Bluewave 24 Pure Bay - 12"
Bluewave 20 VBay - 12"
Shoalwater 22V - 9"
Shearwater 23 - 12-13"
Majek Xtreme - 10"
Majek Illusion - 8"
Majek RFL 25 - 8"
Majek RFL 21 - 8"
Majek RFL 18 - 7"
Maverick 18 - 9"
Hewes 18 - 10"
L&B Boca Grande 20 - 10"
L&B Back Water 20 - 10"
my 16' alumacraft duck boat - 6"
my 230tv exploder - 10 maybe 12" with it's big arse

granted these are mfg specs, i think this is a pretty good list of vhull, tunnel, flats style, and flatbottom hulls, and while looking at the numbers, i see three things - Typically boats of the same model with tunnels draft shallower (look at the bluewave! just kidding... can anyone verify that???), typically the smaller models draft shallower as well despite displacement, and all those boats draft pretty darn shallow reguardless! i guess it comes down to how shallow do you wanna run. 

we need some real world numbers and spreadsheet for this. :hairout:


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## railbird (Jan 2, 2009)

JohnHumbert said:


> How much a boat drafts is strictly and totally dependant on two factors: The weight of the boat and how much surface area is touching the water. Nothing else figures into it, period. Larger, heavier boats can draft shallow that a smaller lighter boat if they have more surface area. Case in point, my current 21' RFL floats shallower than my old 18' RFL, and the 25' RFL floats shallower than my 21'.
> 
> How shallow a boat will get up is dependant on MANY more factors - but primarily lies with hull design. Flatter hulls will generally get up more shallow and speed/accelleration is major factor. Case in point, the reason my TRP gets my boat up so fast has NOTHING to do with "prop lift", which is really a fiction. The TRP, with it's twin props, has amazing accelleration - within a few feet it pushes the boat so fast that the hull begins to plane. However, as I said - MANY factors go into this.
> 
> ...


I agree with 98% of what is said above. My only difference is at extremely shallow running conditions, i would run as slow as i could without rubbing bottom to give the tunnel adequate time to fill with water. WOT in 3 inches will overheat you or blow out all the water in the tunnel, there is so little water to pull from under the boat, it needs time to collect it. 15mph is much better than 45mph in this situation. John explained it the way i see it otherwise.


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## Fishtexx (Jun 29, 2004)

I agree with John ad Railbird. I also experience losing water in the tunnel when making a turn in shallow water at speed, if I slow down or straighten up, the tunnel will refill and engine temp will go back down and water pressure will return. Great discussion!


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## redfish203 (Aug 10, 2010)

I own a poling skiff and tested true world draft on several manufacturers when buying. It is amazing the amount of BS they put out for true draft numbers. There are a few manufacturers who advertise honest draft for skiffs. All you do to measure draft is load the boat the way you fish (1 or 2 people, gas ice etc), mark the transom with a grease pencil at the water line, take it out of the water and measure with a tape. Tunnels will always draft more when added to the same hull/weight, its just physics, but they are great for other reasons. Its fun to watch a salesman squirm when you show him that his 6" draft is really 10+.


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## spurgersalty (Jun 29, 2010)

My little Southfork tunnel drafts about 5" or less. It's 15' and 42" wide. 

Now granted I don't have a water pressure gauge, and the 25hp ain't much motor, but comparatively i guess its similar. 

This rig performs the best in shallow, shallow water by drilling the throttle. It does sound like it cavitatates and probably does, but it helps to get the rpms up and build speed. 
When on plane, it throws a near perfect tunnel allowing me to run in next to nothing (water wise).
Also, I do have 4" trim tabs which really help, at least on this rig.
I know this is apples to oranges considering your looking at a much larger bot but I feel its pertinent information.


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## jeffm66 (Sep 14, 2010)

Good discussion, I don't think I need a tunnel. Where I fish if the skeg for the motor was above the bottom of the hull I would have rash marks all over my hull.


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## spurgersalty (Jun 29, 2010)

I will say this, yes theirs only 2" of a blade hanging below my hull, but there use to be a lot more skegs also


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