Propeller pockets or partial tunnels

I am designing and building an aluminum hard chine 29' planing hull with a 16 degree midship deadrise with a 14 degree transom deadrise. Beam is 9' 6", The power is a 315 HSP Yanmar diesel inboard with a 20" diameter propeller. I would like to start my propeller pocket approx 10' forward of the transom where the propeller will be located. I need the pocket to reduce draft, improve shaft angle for improved thrust forward and hopefully to increase propeller efficiency with close hull clearances between blade tips and tunnel. I will have a half circle at the transom with a 21" diameter. I am using a v-drive to move the shaft entry into the tunnel as far forward as the design will allow. If my pocket starts at 0 and gradually progresses to a 21" half circle diameter what could I expect regarding smooth unobstructed water entry into the propeller?? I know some designs with propeller pockets have a real problem with the bow not wanting to come down due to the loss of lift aft resulting from the propeller pocket. However, the propeller pockets I have looked at do not facilitate smooth entry of water into the propeller and they are large and grotesque looking. Would a 21" half circle cause that much loss of lift? Any guidance would be appreciated.

Thomas

 

I think Penn Yan built something like what you are talking about in the seventies.
I knew a guy who had one but I never asked him what its virtues and drawbacks were so I can't be much help there. Maybe you could locate someone who has one. Or snoop around and see if you can find some information on the web.
My gut feeling is that it might run alright but that it might not maneuver very well.
I think the boat was about that size too.

 

just add air

Some of the folks that run shallow-water recreational and fishing boats down south have found that injecting air into the tunnel significantly improves performance. They add a thru-hull pipe, usually 1" to 1.5" diameter at the leading edge of the tunnel, then run a pipe or hose from that to the stern and up over the transom, so that if any water pukes back up through the hose it will be piped overboard. Many of these folks add a PVC ball valve to adjust the amount of air being sucked into the tunnel.

With throttle application, the prop starts sucking water through the tunnel and creates a vacuum which in turn sucks the stern down into the water. By allowing air into the tunnel, the stern jumps up on plane much quicker. A few folks have added a PVC ball valve inline with the pipe so that they can adjust the amount of air being sucked in, but most say that it isn't needed. Those who have done the modification report letting air into the tunnel is like pulling up an anchor that you'd been dragging around - they report about a 5mph increase in top-end speed as well as the ability to jump on plane much quicker.

Of course, this is in flat-bottom jon boats in the range of 14-20 feet with 20-115hp outboards, so may not work as well in your application, but may be worth investigating. If you want to test it, you can glue a 2" i.d. PVC half-pipe to the inside roof of the tunnel and run it up the transom to where it can suck air - this way you won't have to drill a hole in the hull for testing.

One item that may be critical to this modification is the use of a highly-cupped propeller. Those who have run tunnels for years insist on large blades, large blade area (e.g. - skewed blades), double-cupping, stainless steel and typically more blades (e.g. - four on these rec boats that would normally use three blade props). These prop attributes are able to handle ventilated water much better than a standard prop.

Also, I believe that many tunnels (or pockets, rather) are designed backwards, starting small at the leading edge and getting larger toward the stern - I think that this just serves to introduce turbulence. I would suggest starting large (think wide and shallow) at the leading edge and reducing the overall cross-sectional area as you move aft. This would serve to force water up into the tunnel as well as raise the stern of the hull as water is forced out the sides. With this setup, air injection may not be needed at all. For an example, look at the operation principles of Bill Allison's flatscat www.flatscat.com

Btw, we recently had a 26' landing craft built with a 12' beam. Powertrain is a 200hp Yanmar diesel with Hamilton jet drive (no tunnel or pocket). Unladen, she drafts 14 inches at the stern and five inches at the bow. With 11000lbs (5000kilo) payload, she drafts 18 inches at the stern and 9 inches at the bow.

 

Tunnels either full or partial can be tricky, and with 1" prop clearance in a metal boat you will most probably get propeller induced sturcture borne noise.

There are a few companies experinenting with a trim tab in the tunnel for the trim problem, and I have seen lots of tunnels with a little hook in the end.

Like I said, tricky. If you are going to do this you may want to think about modifications you may have to make after the fact to get things right, or you may get lucky the 1st time. I would reconsider the prop clearance though.

Also, a 10 ft tunnel sounds extreme, you may actually induce more problems by having such a long tunnel.

just my $.02

Steve

 

You need a minimum of 3" tip clearance, 4" would be better. I would also question your need for a V-Drive, unless you need it to achieve something layout wise. I'm familiar with a 29' production boat that has a centerline pocket and is actually powered by the Yanmar 315 with a 20" prop - it's a Back Cove 29 built in Maine. That boat has a 30" draft and displaces 10,500 lbs, and uses a straight driveline. There's actually a small picture of the pocket on their website if you look hard enough. I don't think a 10' tunnel length is unreasonable as you are transitioning from the concave section of the centerline V forward to the convex section of the tunnel over that length. Good luck!

 

Well, as you can tell, the above recommendations above all over the map.

Probably the best expert on this topic is naval architect Don Blount, Don Blount & Associates, Chesapeake, VA. Tel: 757-545-3700. Don wrote a paper back in the mid-late 1990s called "Design of Propeller Tunnels for High-Speed Craft". Call and ask if you can buy a copy of the paper which gives very good practical guidelines on how to design a proper tunnel. I am sure he won't charge too much, he's a good guy.

There was also a good article on this topic in Professional Boatbuilder magazine, issue #44, Dec/Jan 1997, pg. 38, "Faster, Farther, and More Fuel-efficient", by Dudley Dawson. It is the cover article. In this article is a sidebar called "Guidelines for Propeller Tunnels on Small Craft" which repeats the recommendations in Blount's paper.

That will give you everything you need to know about tunnels.

Eric

 

Propeller tunnels

Thank you Eric and others for the info. I will follow up on these references. It sure seems that the pros far out weigh the cons with this type of design..Why has the pleasure boating industry not progressed further with this concept.. especially if fuel efficiency through increased propeller diameter which equals a slower turning screw is acheived, better attack angle on prop shaft/ thrust forward vs down and forward, reduced draft/wetted surface/drag, better propeller protection?

Thomas

 

At least one factor is this. How much trouble and complication are you willing to deal with to get the last one percent of propulsion efficiency?
Here is another. What percentage of boat owners need or even want to run their boats in one foot (insert whatever small number you like) of water? In case you are a little fuzzy on the answer to this one it is a very small number. And those who really need to operate in shallow water have boats that have features that make it possible. So it's not likely you have stumbled onto an idea that will revolutionize the entire boating industry even though it may be a good idea.

 

The whole propeller tunnel/shallow draft has been well worked over and the real time construction very small. William Atkin did much work in small pleasure boats by combining his sea bright skiffs with a tunnel stern. Did quite a number of designs - some of which will run in less than 10" of water. But not many have been built. Dave Gerr has done a couple of designs like this. Unfortunately the demand is not great at this time. I keep hoping that will change. I think you can get a good speed at a moderate cost in engine and fuel.

 

I think there's a bit of ambiguity surrounding the concept of a "prop pocket" VS that of a hull tunnel or a "tunnel drive". I think that what Thomas is after here is really more of the former. Eric's advice advocating further research into this is really the most prudent post here, however I don't recall how much Blount addresses pocket design (particularly entry design considerations) in single engine installations like we're discussing.

The bottom line is that there is legitimate incentive to recess a single inboard prop - when it comes to draft especially. And we're not talking about shallow water operation here, we're talking about the potential of reducing prop draft (on Thomas' boat) from about 40" to about 30" - significant when it comes to small craft like this which are generally operated near shore or in shoal water. While the demand for tunnel hulls is indeed small, a 40" draft for a 29' planing power boat would be unacceptable to most potential operators/buyers. The industry as a whole does build boats on a large scale which incorporate pockets in one form or another, so the concept is well proven and used every day.

There are downsides to consider - the additional costs in both labor and material, the dynamic complexities in the design and execution, and the risk inherent in "getting it wrong". I suggested looking at the Back Cove 29 as a way to avoid re-inventing the wheel. That is obviously a successful hull form in that they've built hundreds of these things in the last few years. It's the same size with the same power, and has a centerline pocket as Thomas originally described.

 

Everything I mentioned in my previous post dealt with the "prop pocket" concept, save for the flatscat reference which could be considered a highly-modified tunnel, pocket tunnel, or catamaran depending on your point of view. However, it is designed to function similarly to pocket tunnels.

Again, a great deal of work has been done in the southern U.S. on pocket tunnels, except that it has been primarily for jon boats and flats boats in the 14-22' range.

For fiberglass boats, general recommendations are longish pockets, up to 2/3rds boat length, that are half-pipe in design. Double-cupped props highly recommended.

For aluminum boats, general recommendations are somewhat shorter pockets, although too short (under three feet length) will lead to priming problems. Some folks recommend four-sided pockets, with five-sides being better, don't know of anyone that has built a half-pipe aluminum pocket.

Two biggest issues with these pockets are 1) loss of top speed and 2) poorer handling characteristics. Both of these are consequences of turbulent water flow in the pocket. Adding an air vent near the front of the pocket alleviates much of the problems, but a highly-cupped prop with large blade area (for better 'bite' of the water) is necessary for best performance in these ventilated conditions.

Many manufacturers of pocket hulls are now offering the option of adding extra flotation pods on the transom of their hulls, forming a key slot in the stern. The extra flotation makes up for the loss of flotation created by designing in the pocket. See http://www.fisherbeavertail.com/floatationpodsintro.html or http://www.g3boats.com/GatorToughSeries/Tunnel_Jons/?m=1860CCT#model for examples. Be aware that these transom pods can limit bow lift, so may not be feasible in you're running in large seas. Trim tabs would be the answer there.