Tunnel Hull help

I posted this on Mean Chicken, thought I'd post it here too with the number of guys running Jets now.

I have a 2010 Lowe Roughneck 1655D tunnel hull powered by a 2008 Mercury 90/65 jet outboard. I recently went through and replaced the floors and decks in .080 aluminum, and used 1.5 x1.5x.125 tubing for braces. Before the mods it had .125 decks and floors which made the boat pretty heavy. Then it would run 32-34 mph with an occasional 36 if the water was flat, and you couldn't make it porpoise. Now with only 2 batteries at the stern, forward console, 19 gallon fuel cell under the front deck, running empty I can only get 33 tops. To say the boat is light is an understatement, The boat drafts very shallow now, which was what I was after, but the lose in speed has me puzzled. New liner, good grating, smooth bottom. Motor runs like a scalded cat.
Before the mods, I could adjust the trim and get In the sweet spot, she would dig with no over spray. Now with minimal trim adjustment I get a lot of spray, then comes the porpoising. It will run with the motor tucked in the tunnel, but at 31 mph. It almost feels like the hull is stuck to the water, I know that some tunnels create low pressure areas causing a vacuum so to speak, and I'm wondering with the new weight distribution, if this is what's happening. I have already planned to move both batteries under the front deck to distribute more weight, the nose of this boat is very light even with the fuel cell under the front deck.
When I come off of WOT It almost feels like the hull stays "stuck" to the water for a bit and then breaks free and slides fairly easy. From looking at my tunnel and reading some topics here on tunnel dimensions, I'm wondering if some tunnel work would benefit this hull. I have since removed the strake you see inside the tunnel, but haven't had it back In the water yet. I don't have the tunnel dimensions with me but will measure them when I get home and post them later. Here are some shots of the hull when I had it flipped to do some bottom painting, and another of the hull when we took it out after the re-wire and before the rear deck. The boat rides great, has awesome throttle response, goes super shallow, but its not as fast as it was when it was heavier than now, Any thoughts or input is appreciated.

I went through and adjusted the reverse gate to make sure it was as tight as possible in the open/forward position, and took some more pictures with the motor at its running height. I've never paid it much attention until now but the tunnel is 6" larger than the shoe on the pump, 3" on each side.

The tunnel measurements are 36" long front to back, 19 1/2" across the front opening, 14" at the rear on the bottom, and 20 1/2" at the rear top. Is it just me or does that seem like a waste of space. Some of you guys know tunnels and I hoping it will be as simple as closing up the tunnel some, that's no problem I can handle that, I just need some direction from someone that knows a lot more about tunnels than I do. Thanks in advance.

Sounds like the weight balance was perfect and maybe you disturbed it? Are your rpms the same as they were?

I would get a few 80lb bags of concrete and move them around the boat to see if it helps before you actually start moving batteries, etc.

rotus623 said:

Sounds like the weight balance was perfect and maybe you disturbed it? Are your rpms the same as they were?

I would get a few 80lb bags of concrete and move them around the boat to see if it helps before you actually start moving batteries, etc.

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When I purchased the boat originally it was light In the nose like now, and I had the same problem, I moved some things around and installed heavier decks and floors, and it ran well that way, but was heavy and drafted a lot more. Now that its light again, it almost feels like it wants to run there, but with the batteries In the rear, I'm almost certain this changed the dynamics of the tunnel and created a low pressure causing it to pull the stern down more. The spray is ever present with a touch of the trim, something I'm not used too.

The concrete idea is a good one, but I have 2-30lb. anchors, a 40 lb. bait tank and a 200 lb. man on the front deck and this thing will still bounce your eye teeth out at WOT and I still get spray when I touch the trim. I'm not sure if re-working the tunnel will solve this but everything I know about this boat tells me its more than just the weight distribution.

since you changed some things around the boat,lost some weight.you may have to lower your motor a bit,not by trim but using a different hole.lower or raise the motor. Sometimes 5/16 of an inch makes a world of difference

With my Lowe 1652 jet tunnel and Merc 60/45 I was getting porpoising past 1/2 throttle. I put some extra weight up front and it helped but it didn't go away. I put on the transom wedges and that got rid of it. I have to run trimmed all the way in, if I start to trim up a little, I get spray back. I moved my console forward and put the battery under the console and it seems to run great this way. You tunnel looks a little odd, was it a prop tunnel that somebody modified for a jet? It also looks like your motor needs to be moved down a little, I run with the foot just under the top of the tunnel. I wonder if your pods are grabbing more now if your noes is up because of less weight. You might want to try the wedges to get a little more tucked in.

I'm going to agree as stated above that your motor looks like it is mounted a little high. On my 1756 I have a piece of UHMW between the exit of my tunnel and the bottom of my foot that forces water straight into the jet instead of it going between the jet and transom. Also, my jet has side fins on it to help force water in the jet. I will get some photos of how my jet is setup today or tomorrow. Not sure if doing this would help, but I have been able to run with 4 people a dog and gear over glazed sand bars that have 2 or 3 inches of water covering them.


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Thanks for all the replies....

I've went back and forth with the idea of welding up the tunnel, and just use it as a mod V/ flat bottom and this is the list of pro's and cons I've come up with so far and how I rate it.

Tunnel....

Drafts a little lower at idle. (Con)

Has to have some water to get UP on plane. (Con)

Loose 3-5 maybe more mph on top end. (Con)

Not very fuel efficient. (Con)

Will run in spit (Pro)

Non Tunnel

Gain another inch or 2 of static draft (Pro)

Should plane fairly easy from get go. (Pro)

Gain a little top end (Big Pro)

Better fuel economy (Bigger Pro)

Run motor lower on transom (Pro)

I really don't need to run 3" of water on plane, I would rather be able to idle through some tuff spots, Anybody know of any more Pro's and Con's? To weld this Tunnel up I'm gonna have to flip it, It will be tig welded in, no spool gun, I want to make sure its water tight. I did the math on the tunnel and if welded up I would gain another 1.34 cubic ft. of air, almost like another Pod under the boat. That will support another 83 lbs., that along with moving both batteries under the front deck should float this boat level in 4" of water.

Pros/cons sound about right. Since the tunnel is already there I would just leave it if it were me, but that's me.

I have had plenty of boats, some with tunnel and some without. The tunnel does rob many benefits, but I have run over rock ledges where I would have caused some SERIOUS damage if I didnt have a tunnel. I also have UHMW on my boat, which slows me down even more. Top speed with just me and a light load is 25mph. Me, one other person, and gear loaded up we are looking at 22-23MPH. This is on a 1648 with 60/40 4 stroke. IMO, I don't really want much more speed in the treacherous waters that I run in this rig. That's why I have 2 boats

rotus623 said:

Pros/cons sound about right. Since the tunnel is already there I would just leave it if it were me, but that's me.

I have had plenty of boats, some with tunnel and some without. The tunnel does rob many benefits, but I have run over rock ledges where I would have caused some SERIOUS damage if I didnt have a tunnel. I also have UHMW on my boat, which slows me down even more. Top speed with just me and a light load is 25mph. Me, one other person, and gear loaded up we are looking at 22-23MPH. This is on a 1648 with 60/40 4 stroke. IMO, I don't really want much more speed in the treacherous waters that I run in this rig. That's why I have 2 boats

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O.k. the numbers you posted for your boat carrying that weight brings me to my point, I'm only running 8 mph faster than you with a lighter boat and a 90/65 2 stroke. Motors not the problem, the power is there, Its almost like I cant get the hull to respond without trying to jump out the water. I can go from a dead stop to 33 in a few seconds, no problem, but when I try to trim it ( and I have always had to at WOT to get to the sweet spot) its like you throw an anchor out, I know its drag but why, the stern is lighter than its ever been. I'll tell you why, the a$$ is getting pulled down lower In the water, how, no clue but I do know before when it was heavy something made the hull "break" free but now its almost like the hull is to light to get away from it. I do know its got something to do with the tunnel.

I'm not convinced I'm welding it up yet, I'm going to try the things MT told me before I decide anything. If it will respond after the extension mods then I'll keep it. If not its got to go.

Running 8mph more with a 90/65 is probably about right, when comparing apples to apples. I wouldn't expect to get too much more out of it. I had a prop 75hp on a 16' aluminum boat and it would max out at 36mph with a light load, trimmed out. Just food for thought here.

You might take a look at venting your tunnel to release some of the suction it seems to be creating with the current attitude of the hull.

Try a google search for "Vented Tunnel Hull"

You will see what others have done. A creative person could rig up a temporary/test system using Ø1/2 PVC and a ball valve hanging over the transom turning under/into the tunnel with a 180 pointing the outlet towards the stern. Try to get that 180 outlet as far forward in the tunnel as you can and not have it stick out. The ball valve gets adjusted while under way to allow small amounts of air to enter the front of the tunnel to disrupt the suction.

The folks I know with flats boats with vented tunnels get them adjusted the way they want and never adjust it again.

Just a thought.

Rob

What did you paint it with and did it run fast after painting or are the drop in speed and the painting at the same time?

Dearl,
I have a Lowe 1752D with factory tunnel and have the same issues that you describe. I only have a 50/35 at the moment. The stern feels like it is sucked down, if I'm on the lake at WOT and I run over a wake sometimes the boat will break free...the bow will sit down and I will pick up 2-3mph instantly. You can feel the speed increase and I have gps on my garmin that verifies it.

I tried moving the motor up/down, all trim angles and transom wedges to no avail. I really think it has to do with the tunnel extension angle. On my boat the tunnel goes from 0 to 2.5" and is the same angle from start to finish. Looks the same as yours. I believe the section of the tunnel that extends past the transom should be parallel to the bottom of the hull. The way it is now, it is trying to pull the water up and in return sucking the stern down.

I'm currently rebuilding a 90/65 and waiting to put that motor on before I cut into that tunnel extension and bend it down to be parallel with hull. If you look at the top end tunnel manf this is how the extensions are setup. That's my .02

I also coated my hull with wetlander over the summer hoping it would help and didn't make a difference. Pic for reference, level is on the bottom of hull.

rotus623 said:

Running 8mph more with a 90/65 is probably about right, when comparing apples to apples. I wouldn't expect to get too much more out of it. I had a prop 75hp on a 16' aluminum boat and it would max out at 36mph with a light load, trimmed out. Just food for thought here.

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I dont buy it, 33 is not about right, like I said earlier I have had this boat up to 36 when it was 200 lbs. heavier.

Ebug said:

You might take a look at venting your tunnel to release some of the suction it seems to be creating with the current attitude of the hull.

Try a google search for "Vented Tunnel Hull"

You will see what others have done. A creative person could rig up a temporary/test system using Ø1/2 PVC and a ball valve hanging over the transom turning under/into the tunnel with a 180 pointing the outlet towards the stern. Try to get that 180 outlet as far forward in the tunnel as you can and not have it stick out. The ball valve gets adjusted while under way to allow small amounts of air to enter the front of the tunnel to disrupt the suction.

The folks I know with flats boats with vented tunnels get them adjusted the way they want and never adjust it again.

Just a thought.

Rob

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From what i've read you can only vent a prop tunnel, when you open the vent it pulls air inside the tunnel to break the low pressure area, this allows air to enter the tunnel and would probably end up cavitating the pump.

gabeskillzz05 said:

Dearl,
I have a Lowe 1752D with factory tunnel and have the same issues that you describe. I only have a 50/35 at the moment. The stern feels like it is sucked down, if I'm on the lake at WOT and I run over a wake sometimes the boat will break free...the bow will sit down and I will pick up 2-3mph instantly. You can feel the speed increase and I have gps on my garmin that verifies it.

I tried moving the motor up/down, all trim angles and transom wedges to no avail. I really think it has to do with the tunnel extension angle. On my boat the tunnel goes from 0 to 2.5" and is the same angle from start to finish. Looks the same as yours. I believe the section of the tunnel that extends past the transom should be parallel to the bottom of the hull. The way it is now, it is trying to pull the water up and in return sucking the stern down.

I'm currently rebuilding a 90/65 and waiting to put that motor on before I cut into that tunnel extension and bend it down to be parallel with hull. If you look at the top end tunnel manf this is how the extensions are setup. That's my .02

Click to expand...

Sounds like we've got the same problem, i agree with you on the tunnel textension being parellel to the hull, A guy on another forum gave me the same recommendation on how to set it up. After weighing all my options, I still think Im going to close it in. I like the idea of doing away with that issue all together, plus the Pro's out weigh the cons hands down.

I took the boat out yesterday morning, lake was slick as glass, new liner, impellor adjusted perfect, WOT was 33.1 and it started to porpoise bad, I noticed the water coming out of the tunnel was directed to the bottom of the pump housing with the jack plate all the way up, Im sure adjusting the tunnel extension would fix this, but my plan is to move the 2 stern mounted batteries forward under the front deck, and weld the tunnel up to create a true MV/Flat bottom hull. This will give me 3 things, more fuel economy, more top end, and more draft at idle. It will take me a couple of weeks to get this done, i got a couple of boats to finish up then I can get back on mine.

What happens when you smack that PTT motor? Does your PTT have a way to disengage so it can pop up out of the water? No? That's a PRO of having a tunnel on boats over 60/40. Tunnels can bite in a turn and act like a skeg.

If you want to go faster move all your stuff to the back. Fuel in rear makes most sense since the engine is back there and it keeps unballasted weight in the stern which can hold the load better instead of being up out of the water.

redrum said:

What happens when you smack that PTT motor? Does your PTT have a way to disengage so it can pop up out of the water? No? That's a PRO of having a tunnel on boats over 60/40. Tunnels can bite in a turn and act like a skeg.

If you want to go faster move all your stuff to the back. Fuel in rear makes most sense since the engine is back there and it keeps unballasted weight in the stern which can hold the load better instead of being up out of the water.

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Really? Wow....Move all the weight to the rear of the boat to speed up and stop porpoising.....Ok.

Anyway, not intersted in fastER, but i do want this boat at or above the speed it was running before the weight was reduced at least 200 lbs, and get rid of the porpoising.

If you do weld up the tunnel, maybe make it a sealed chamber & at least get some flotation benefit out of it.

I'm guessing you wanted a more complicated answer. Looks like you put a ton of work into your boat to drop a few pounds. I'm guessing to go faster and now it has your weight distribution jacked up. It looks like your motor is too high, maybe not. You need to be trimmed out so the jet nozzle is flat and pushing your boat forward not UP.