Tunnel Questions

I have been rebuilding this 27 foot express cruiser fishing boat for a long time.





The boat ended up upside down, banging on the rocks...




I bought it and rebuilt the transom, cab and flybridge. Boat was previously setup for twin Volvo penta stern drives. To "save money" (ignoring the fact that time is money), I got a good deal on a nice used 210HP Cummins 6BT and a ZF-63IV 1.56 v-drive. All I want to do is troll and have GOOD fuel economy at 10 knots. I estimated the completed boat's dry weight to be around 6,200 lbs and a fully loaded MAX weight of 9,200 lbs. We used 10,000 lbs in the prop calculator so I bought a 22x30 3B michigan prop with a 1.75" bore. This is our starting point. I have not bought a shaft yet.

Because of the of the large prop and the requirement to trailer the boat, we decided to go with an 8-inch deep tunnel. Most of the tunnels I have seen have a "propeller region" that is straight. This straight region accommodates the rudder and prop and extends forward of the prop for what usually seems to be about the same distance as the diameter of the prop. So, in designing my tunnel's propeller region, I calculated:

rudder length requirement + prop width + prop diameter = propeller region length

Propeller region:

http://www.fiberglassin.com/wp-cont...de-view-with-pipe-for-shaft-alignment-min.jpg

http://www.fiberglassin.com/wp-content/uploads/2015/12/tunnel-transom-view-min.jpg

http://www.fiberglassin.com/pics/stern-view.jpg

I currently have the last 8 feet of the boat gutted. Put in new inner and outer stringers. Going to install another fuel tank in the center of the boat, a bait tank, and some water tanks that can help with ballast weight and or actually be useful as a fresh water washdown. The boat had "sealed" compartments before that had water in them anyway. I would like the boat to have as much "useable space" as possible even if I don't choose to use it all the time.

I bought a huge roll of brown paper from Home Depot and practiced drawing the curvature for the the front half of the tunnel. I'm not an artist, but after a lot of practice, I was finally able to draw a nice curve that matched the existing hole in the hull and the specifications required for the tunnel entrance. The propeller region of the tunnel is straight and parallel. I calculated the length of the propeller region to be 28 inches long in order to accommodate removing the prop without removing the rudder.

In order to have a 15-degree or less tunnel entrance (as suggested in Blount's paper), the tunnel entrance had to be at least 35 inches long.

After a lot of measuring, I made a paper template that lined up with the existing hole in the hull and allowed me to draw the curve for the front half of the tunnel on the hull. I used the same template for both sides of the tunnel (based off the centerline of the boat) so the tunnel is as perfectly symmetrical as I can get it. After an hour of cutting carefully with the Sawzall, the hole is now complete.

I bought the cheapest Ryobi band saw I could find at Home Depot and cut a template semi circle with a 27-7/8 inch diameter, about 8 inches high. (1/8" accomodates for masonite thickness). I used this template to cut 8 identical pieces out of half inch plywood. I clamped them all together and sanded them as smooth a possible, trying to make all 8 pieces identical. I found the exact centerline of each semi circle (not as easy as it sounds) and marked it with a Bic pen. Four of the semi circles are identical in height. These four pieces are for the propeller region which is symmetrical. I ran the other four pieces across the table saw and removed 1-inch segments from each piece. One is 1 inch shorter, one is 2 inches shorter, one is 3 inches shorter and one is 4 inches shorter. This gives me the slope I need for the tunnel entrance by aligning the semi-circles with the hole I cut in the hull.

I needed a nice solid platform to design the tunnel mold on so that it doesn't change shape throughout the building process. So, I screwed a piece of 3/4in plywood to a couple 4x4's with deck screws. This gives me a nice solid table to build the tunnel mold on. I placed the platform directly under the boat, shimmed to exactly match the same angle that the boat is sitting (using my digital level). Then, I drilled a hole through the centerline of all 8 semicircles using my drill press. I put a 1/2 bolt through all the holes and with nuts and washers, this gave me a way to adjust the spacing of the semicircles so that I can perfectly match them with the hole I cut in the hull. Once I got the semicircles to match the length of the hole I cut, I tightened all the nuts down and the mold got pretty solid.

http://www.fiberglassin.com/pics/2016-01-26/20160124_092153.jpg

I tried to align everything as accurate as the thickness of a Bic pen. Not too worried about the shape of the front tip of the tunnel since this will be all glassed in with a shaft log later.

http://fiberglassin.com/pics/2016-01-26/20160124_123254.jpg

http://fiberglassin.com/pics/2016-01-26/20160124_123318.jpg

My original plan was to build the tunnel insert on a work table 1 inch thick before I glass it into the boat. This was just so that I could "tuck" the tunnel UNDER the transom, rather than just glassing the tunnel to the inside of the transom. I think my plans have changed.

I cut the arc on the transom with a 30in diameter, while the diameter of the tunnel is 28 inches. This gave me a nice 1-inch gap:

http://fiberglassin.com/pics/2016-01-26/20160124_123204.jpg

After looking at the 1-inch gap I have around the mold, I think I will be able to lay glass and have it "tuck" under the transom without making a whole tunnel insert on the table and trying to get it to match the hull later on. This will allow me to build the entire tunnel on the inside of the boat and exactly match the inner shape of the hole in the hull. It will give the tunnel an overall better bond to the hull and match the hole I cut much nicer, so I am going to go that route since I will still be able to lock part of the tunnel under the transom. I get the best of everything by building the entire tunnel in the boat. If the boat ever hits the rudder or prop on something, this is going to be a much stronger arrangement than just glassing the tunnel to the inner wall of the transom.

Once I got it all lined up, the spacing correct and the semi circles bolted down tight, I tacked everything down with my trim nailer and pulled the mold out from under the boat and put it on my work table. From there, I used the trim nailer to secure the semicircles to the platform and nail 1-inch strips 1/8* thick masonite around the the semicircles.

http://fiberglassin.com/pics/2016-01-26/20160124_141926.jpg

Sanded and faired it all. I have the mold ready to be coated with wax and PVA.

http://www.fiberglassin.com/pics/2016-02-06/20160206_134409.jpg

http://fiberglassin.com/pics/2016-02-03/20160131_180025.jpg

I still need to finish building some stuff on the inside of the hull. For now, it's nice to get work done via the ability to climb and in and out of the hole.

My main questions are:

What kind of tabbing schedule should I use on the tunnel? How thick should the tunnel be? I will be using 18 oz woven roving and 1.5oz mat. I have about 6 inches from the tunnel hole to the stringers. The stringers are 1.75" microlam LVL.

The original hull looks to be a little less than an inch thick:

http://fiberglassin.com/pics/hull-thickness.jpg

How do you guys radius the actual EDGE of the tunnel entrance (where you cut the hole)? I was thinking of just using an air file or a grinder and doing my best. I don't want there to be a sharp edge right where the new tunnel glass meets the old hull glass.

Anyone have any pics of good working tunnels in this size range?

 

Calculate the total volume of the tunnel you intend to make. Unless the tunnel starts at the bow there will always be a certain shortage of water, no matter how much fairing you do at the entrance.
To replace the volume expelled by the prop, water flows in from all directions, not just from the entrance. So if you keep the entrance angle under say 12 degrees to avoid turbulence, the whirlpool starts just after the entrance when water from the left and right gets in and encounters a much sharper edge.
With increasing speed some water even tries to get in at the stern.

My experiences with twin tunnels can be found on this forum when you search for tunnel drives.

 

What kind of tabbing schedule should I use with 18oz roving / 1.5oz mat?

How thick should the tunnel be?

Is there a better way to radius the hull edge of the tunnel than with a grinder and straight line sander?

 

Have you spoken to a Naval Architect about the wisdom of this diesel/tunnel business ? I wouldn't be overly optimistic it will work that well unless it is beamier than it appears. And you can forget 10 knots being an efficient speed, you will need to be going quite a bit faster, or slower, to enjoy the best fuel economy.

 

No. I don't personally know any naval architects.

The primary objective of the tunnel is to reduce the draft, not increase efficiency. It's the larger prop that is going to give me better efficiency at 10 knots, but the caveat is I needed a tunnel to swing that larger prop.

V-drive tunnel boats are built all the time and are great if done right. As the tunnel gets larger, not only do you lose displacement, you lose dynamic lift. I don't want the tunnel any larger than it needs to be since the objective is to decrease the draft and I have already accomplished that goal.

I realize 10 knots is right in between planing and displacement speeds and it will not be the ideal speed for efficiency. But, the fish around here like to bite around 10 knots.

There are definitely right choices to be made as far as tunnel design, prop choice, trim, etc in order to optimize the boat for the best efficiency possible at 10 knots.

 

Wahoo (Ono) ? What is the waterline beam of your boat ?

 

Tons of Ono but also mahi at 10 knots. Tuna a bit slower, around 3 to 8 knots.

The waterline beam is 87 inches.

The waterline length is about 25 feet.

 

A bit marginal for diesel/tunnel would be my guess, but others may not agree.

 

Thanks for the input. Here is recent pic:



My questions still remain though...

#1 - What kind of tabbing schedule should I use on the tunnel? How thick should the tunnel finished tunnel fiberglass be be? I will be using 18 oz woven roving and 1.5oz mat. I have about 6 inches from the tunnel hole to the stringers. The stringers are 1.75" microlam LVL.

The original hull looks to be a little less than an inch thick:

http://fiberglassin.com/pics/hull-thickness.jpg

How do you guys radius the EDGE of the tunnel entrance (where you cut the hole)? I was thinking of just using a grinder and a straight line sander and doing my best to make both sides look equal. I don't want there to be a sharp edge where the new tunnel glass meets the old hull glass.

Anyone have any pics of good working tunnels in this size range?

 

Your tunnel ought not intersect the bottom with a sharp transition, but rather be well radiused, with a view to avoiding cavitation. Or at least, that is what is generally advised.

 

I would consider building a hump onto the keel just ahead of the tunnel (assuming it won't interfere with your trailer). The hump will allow a much greater radius at the leading edge of the tunnel and, I believe, help to draw (or direct) water into the tunnel. Think of the hump (or bulb) as displacing the water, then the water rushes around the hump and up into the tunnel. The hump will sort of act as a primer.

Sorry, I don't have any pics, references or proofs. Just theory in my head.

Another theory I'd like to try is a small hydrofoil just below the leading edge of the tunnel that would direct water up into the tunnel. I realize it would increase drag, but that may be more than offset by getting cleaner water into the tunnel.

Make sure you have extra cupping on your prop blades. The cupping will give you a better hold on the aerated water in the tunnel, and should increase efficiency. A good propeller shop should be able to do this for you. You might check out Baumannprops.com in Houston.

 

Anyone with experience like to chime in?

Par?

How thick should the tunnel be? The same thickness as the original hull? Thicker?

I was going to start building the tunnel, then every few layers I do on the tunnel, I will overlap those layers with new layers on the stringers which overlap the tunnel layers. I was then going to do more layers that overlap the stringer layers out towards the sides of the hull.

Basically, overlapping the tunnel layers every few layers, with new layers that extend out towards the sides of the hull.

You know what I'm saying?

Please only people with successful tunnel experience chime in....

 

The tunnel laminate will be at least as thick as the hull shell on the bottom. It's the shape of the tunnel, that's pretty critical and edge treatments, as previously mentioned, that you should be concerned with. You can take a guess and the boat probably will not sink, but you can run into a host of other issues.

 

Thank you Par. To radius the bottom edge, where it connects to the hull, how much should it be radiused?

Should I remake the mold and have it flare out on the bottom edge? I'd have to re-cut the hole wider for that and not sure how I'd transfer a line onto the hull...

Or should I use a grinder and air file to curve it off after I glass the tunnel in place?

The second choice is sounding a lot better

Is there a good way to radius the bottom edge of the hull?

(see pic)

 

You can buy a length of fiberglass pipe to make the tunnel, cut it to shape then glass it in, easy and quick.

You can either mold the radius or grind it to the correct shape.

PVC pipe can be cut lengthwise to be used as a mold surface for the radius.

You may want to use a differnt glass, 18oz isn't the best stuff to use in this application.