Thinking about turning my flat boat into a tunnel hull

[Smokepole27]

Im looking for any tips advice measurements etc that I would need to turn my fiberglass 14 43 flat boat into a tunnel hull.. Rivers I hunt on are very shallow at times and I need to run in as low as 8 inches to a foot of water. Is it easy as just cutting out the bottom section of my transom about 4 inches high 8 to 10 inches wide and 12 inches deep and beveling the front piece of the tunnel (furthist away from the transom) and fiberglassing it all in then getting a jack plate?

 

[Kiwifisher]

Re: Thinking about turning my flat boat into a tunnel hull

With fibreglass virtually anything is possible. The question would be how would a mod like that effect the handling and stability and would it achieve the desired outcome. An expensive lesson if it does not.

If you decide to do it, please post pics and details, I would like to follow your progress.

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Well I really want to do it this year but! Hunting season is about to open up and dont really have the time and was wondering if anyone has accomplished it... I dont see it being too hard or technical I hope! How ever high you cut the bottom of the transom that would be how high I would have to extend the transom on top side, or how high the jack plate would have to go... Then Im guessing for every inch up youd have to cut 2 inches deep so im thinking about raising it around five inches and that should give me the heighth I need. Dont really know though. I have a friend with a jet ski with no paper work and am thinking on turning it into a jet boat...

 

[Alpheus]

Re: Thinking about turning my flat boat into a tunnel hull

I have a friend with a jet ski with no paper work and am thinking on turning it into a jet boat...

Where did it go?

Stolen???

 

[oops!]

Re: Thinking about turning my flat boat into a tunnel hull

hi....welcome.

im sorry.....i dont understand....is there something i am missing?

you have a flat bottom boat.......(that is the shallowest draft you can get) ... and you want to turn it into a tunnel?

why?.......the two out board sponsons will be lower in the water than your current configuration?.

is it the drive hanging low that you want to change....no problem.....put on a jet.

other wise....i just dont understand

thanks for helping me
oops

 

[tinkeringwackyone]

Re: Thinking about turning my flat boat into a tunnel hull

I agree with oops, a flat bottom boat is the shllowest drafting boat. if sponsons were added it would make the boat heavier and draft lower in the water until it planed out. even then it would still be lower.

a jet would be ideal as the jet does not stick below the hull.

but could possible eat up a pump in that shallow of water.

how about an air boat?

good luck

sail on.... tink

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Yall have me all wrong. And companies do make flat bottom boats called tunnel hulls. Google them they are a flat boat with a jet type tunnel in the middle on the bottom of the boat to force water out higher on the transom where the motor when on plane will be virtually completely out of water when adjusted correctly. I am starting this project tonight it will not be the extensive tunnel due to hunting season here but after season I will come back and do the tunnel hull the way I want it....

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Tunnel works great but need a pros advice, do I need to lift the motor higher? She cavitates unless I trim the motor 2 knocks forward on the transom mount...

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

These 2 pics are with the motor trimmed up or in. Do yall think I need to lift the motor up?. Im guessing that the curl of the water is pushing the back of the cavitation plate down and causing the bow to cavitate, what do yall think or anyone with any ideas... Id hate to have to run around with 150 lbs of concrete in the bow of the boat or have the trim tilted in and risk of hitting another log and tear my pinion gear and prop up again... Also the motor pulls to the left, steering the motor to the right what could cause that. Ive checked for center also level on the transom, a friend told me to check the jack plate on the transom for sinking and Ive found none. Could it be because of the tunnel?

All in all she runs a heck of alot better!!! Had a 60 hp mercury 16' walking me yesterday and when I got to the launch he was asking me If I had a little *** behind my boat, I had no clue what he was talking about until I noticed his 60 and he said he was wot. Im guessing im runing around 35 mph now vs the 27 I was before!!!

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Where did it go?

Stolen???

No not stolen, the jetski with paper work flooded during katrina and after the storm the guy sent off the paper work and my friend that bought the jet ski layed on his but till the guy just fell off the face of the earth... And Im talking about 2 to 3 years after he bought it!! Oh well not too worried about the paper work now... But sure am wanting to make a jet boat! id got a hell of alot faster and in alot shallower water than anyone else that goes where I go!!

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

At second glance it seems what I need to do is lift the motor up one inch and trim the top out about a quarter of an inch....

 

[69rrvert]

Re: Thinking about turning my flat boat into a tunnel hull

Hey Smoke, looking at your pictures, the tunnel looks really short. Flat bottoms are only so good. It you want to run shallow, you have to have tunnel and jack plate. I would say it should be a minimum 3' to 4' long. I am not sure if the water would have time to fill the tunnel before it exits. The width and depth looks pretty good though. Mine is 3 1/2' long with a vent to keep it out of a vacuum. I would have made it longer but I would have to cut into a rib. I built the tunnel in my Alweld and it is a totally different boat (all good). As for the cavitation, look into getting a "Shallow Blaster" for the cavitation plate on the motor and something similer. The one on my motor is home built but I used a Shallow Blaster for measurments. It does a few things. It helps with hole shot, helps with porpoising, and it holds the water from the tunnel around the prop, which I think will cure your cavitation problem. I also run a hydralic 6" jack plate with a 6" set back plate mounted to it. I am good for about 5" to 6" of water while on plane. You might also take a look in the "boating" section of www.2coolfishing.com/ttmbforum/ . Everyone on there runs a boat with some sort of tunnel and there are also several flats boat builders on there as well like www.youtube.com/user/scbfactory#p/u . His boats run shallow and fast. A few other boats you may look at for ideas, it the Trans Cat and the Flats Cat. Thanks, Steven

Pic of my motor setup and home built shallow blaster:

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Hey Smoke, looking at your pictures, the tunnel looks really short. Flat bottoms are only so good. It you want to run shallow, you have to have tunnel and jack plate. I would say it should be a minimum 3' to 4' long. I am not sure if the water would have time to fill the tunnel before it exits. The width and depth looks pretty good though. Mine is 3 1/2' long with a vent to keep it out of a vacuum. I would have made it longer but I would have to cut into a rib. I built the tunnel in my Alweld and it is a totally different boat (all good). As for the cavitation, look into getting a "Shallow Blaster" for the cavitation plate on the motor and something similer. The one on my motor is home built but I used a Shallow Blaster for measurments. It does a few things. It helps with hole shot, helps with porpoising, and it holds the water from the tunnel around the prop, which I think will cure your cavitation problem. I also run a hydralic 6" jack plate with a 6" set back plate mounted to it. I am good for about 5" to 6" of water while on plane. You might also take a look in the "boating" section of www.2coolfishing.com/ttmbforum/ . Everyone on there runs a boat with some sort of tunnel and there are also several flats boat builders on there as well like www.youtube.com/user/scbfactory#p/u . His boats run




What type of a vent do you have on it? Im haveing one issue with it when I get on plane its shooting too much water or doing something strange and water sprays to my left and to my right from the plate above the cavitation plate... If I hit a couple wakes then it ll run perfect and pick up 5 mph faster and the water will just go straight back from the prop.. Does this sound like an adjustment or because I dont have a vent on the tunnel, and if so how would I install it?

Also Im getting plenty of water out of the tunnel just cant put my finger on how to correct that issue..

 

[rcam97]

Re: Thinking about turning my flat boat into a tunnel hull

Your motor is mounted too low. When you set the motor back farther like you have with the jack plate you have to lift the motor higher than you would if the motor were closer to the transom. What I'm trying to say is that when the motor is farther back it must be mounted higher or the water will go over the antiventalation plate which is what I think is your problem. If the motor were mounted directly to the transom you would want the antiventalation plate level with the bottom of the hull, but the farther back you mount it the higher you must mount it. I think an 1" to 1.5" higher would do the trick.

Hope this helps.

 

[Smokepole27]

Re: Thinking about turning my flat boat into a tunnel hull

Yeah I figured I had to go up some more but itll have to wait till after the hunting season due to I have to rebuild/replace the transom. If I go Higher witht the jack plate then Ill rip the top of my transom off...(the transom is soaked with water right now) So itll have to wait till febuary.

 

[69rrvert]

Re: Thinking about turning my flat boat into a tunnel hull

Those are the same symptoms that I had with my boat. At first, I thought that I had a bad coil that kept going out. My boat would speed up and slow down depending on chop. What was happening, is the small ripples on the water would allow air to get in the tunnel and break the vacuum. The motor would gain about 500 rpm's and run real good and then the tunnel would pull it back down. Lower the jack plate too much, and it would flood the back off my boat. When I added the vent and the shalow blaster the motor, it was like riding in another boat. Something else you may consider is getting a good 4 blade prop with an aggressive cup. The prop that I am running is made by Power Tech. As for the vent, I am not sure how you can do it with wood. Aluminum makes it a lot easier. I will have to agree with another post on the jack plate. If you are looking for the additional ability to run in shallow water, then the hydralic is the way to go. If you are looking for speed, then it probably would not be very cost effective because you will not gain that much on the top end. The top two jack plates on the market is the Power Lift by CMC and the Flats Jack by Bob's Machine Shop. The Power Lift is integrated into one unit with the pump mounted inside of the jack plate. All you do is install the plate and run the wires to the console. With the Flats Jack, you have to mount the pump inside of the boat and run hoses to the jack plate but it is a little faster. I am running the Power Lift on my boat. If you have any questions, let us know. Thanks, Steven

Picture of the vent taken from the stern looking towards start of tunnel:

Picture of the vent inside of the boat:

Small plate over the vent hole to prevent water from spraying up the pipe: