Tunnel skiff design

This is a preliminary design of a 16' 6" river skiff for hunting and fishing on one of our local river systems. It's anticipated uses will include driftnetting, and moose hunting in a highly braided and seasonally low water river system. It's based on Jim Michalaks' Pickup Squared which was then scaled up and modified to include a jet tunnel.

Comments or suggestions on the design are welcome. I'm especially interested in input on the flow characteristics of the tunnel, technics to improve fairing of the hull, as well as improving the plate developability. I've included the skiffs fbm file to allow closer analysis of the design.

MM

 

I don't know much about jet tunnels, other than to keep them small (under 3-foot length, under 3-inch height). Make it just big enough to cover the jet-foot. Can't help on width, but I think you'll have turning issues (cavitation) whichever way you choose.

If you can, go a little longer on the hull. It will give you better load-carrying capacity for hauling moose around. OTOH, a longer hull will be a little less maneuverable in a narrow rocky river.

 

Years ago I helped a friend design a tunnel hull for his river skiff, and for the most part I'd say it was a success. The modified Vee up forward helps to displace the air generated when running in rough water away from the tunnel section. All the flat bottom skiffs I've looked around here (when talking to the owners) don't, so cavatation of the jet unit even at reduced speeds, and in calmer water is a common complaint.

Tunnel length seems to be one area where designs vary the most. Short steep tunnels appear to generate more turbulance in the flow pattern causing cavatation of the jet unit even in relatively calm waters. Lengthening the tunnel may help smooth out the flow, but at some point you start picking up too much air from up forward. There must be a happy medium where you can maintain smooth laminar flow, keep induced drag and floatation loss to a minimum, while keeping air induction to an acceptable level. Compromise must certainly be the order of the day when considering all these factors, and neither I nor my skills with Freeship are currently up to the challenge.

I've talked to a number guys concerning hull length and 16' seems to be the preference even by those with bigger boats. Running the river here is all about reading the minute changes in the water as it flows over and around sandbars, and being able to change course quickly to stay in deeper water. The manuverabilty to initiate those quick course changes is key to staying afloat, and a longer hull sometimes can't turn quite fast enough.

By going with a 60" bottom and higher sides I thought it possible to increase safety, load capacity, and yet still maintain the draft and manuverability I was looking for.

This design isn't about looking good or trying to impress your friends at the boat ramp...it's all about getting the job done weither that's packing gear into a hunting camp or hauling out a 1000 lbs of moose meat.

MM

 

Marathon Marine used to have a nice jet boat tutorial on their website, but I don't know if it's still available. Their conclusion was that a little deadrise definitely created the best performance with the jet tunnel (for the same reasons you mentioned), as opposed to a flat-bottom. See if you can find some Marathon or Wooldridge boats in your area and take a look at them - I think they put a lot of R&D into their designs. Also, look for other popular, custom-built boats in your area and ask the owners what they think of them.

If you normally run in deeper water and concerned about hitting the occasional rock or boulder, tunnel jet on a v-hull seems like the way to go. If your only concern is sandbars, then a tunnel prop on a flat-bottom would be better, IMHO (but you will need a heavily-cupped prop). I've personally run a 25-foot flat-bottom tunnel hull over a sandbar in 3-4" of water for some distance (would need six inches for sustained running with that size boat and design).

I mentioned length mainly due to cargo-carrying capabilities. Long narrow hulls seem to carry cargo better than short wide hulls of the same buoyancy. However, as mentioned, they can be tricky to maneuver in tight quarters. I remember reading elsewhere on this board about how Alaskan natives used long oversized canoes for efficient cargo transport with very little power.

 

Hi VI,
Yep it's still there...I just finished reading it.

That's exactly what I'm doing...taking the time to stop and look at every design configuration that I can find as well as talking to the owners, when available, to see what it is they like or dislike about each boat. I'd say that the custom built models are probably the worst designs due to the lack of engineering skills on the part of the owner or builder. Just because you think you have a good idea for a boat doesn't neccessarily mean you're going to be successful in executing that concept.

If I can come up with a decent design I'd like to fashion a female mold and build these in fiberglass. With that in mind I have a couple of concerns

1) how do I shape the gunnel for ease of fabricating and ease of use?

Since cost is going to be important point I need to keep my labor costs under control. A strong simple design is going to a better selling point then one that looks good but isn't practical in the real world. Like every boat on the river they will at some point be stuck on a sand bar and need to be manhandled into deeper water. That means they should have hand holds built in from bow to stern...that's not really practical but it is something I have to keep in mind. A rolled gunnel would be both practical and functional in this regard.

2) How do I form the bottom strakes?

They could be molded in, which I don't like from a fabrication point of view, but it would add stiffness to an otherwise featureless bottom panel.

Or added on externally.
I kind of like this idea as it allows me to customize the handling chacteristics of my design as it's being developed. This also allows for repair or replacement of a vunerable part of the bottom without affecting the integrity of the hull.

I suppose you can debate the hull length issue adinfinitum. Longer is better in almost every aspect, but what I'm looking for in this design is a highly manuverable skiff with a lower weight factor and that means limiting the hull length.

I was hoping that by posting my Freeship file I'd get a little more feedback on the dynamics of the hull and especially the tunnel. I'm a little disappointed in the lack of response from the design gurus around here. I can only surmise the design is so uninteresting as to not warrant further discussion.

MM

 

Mike, Svendsen Marine down in Wrangell builds aluminum jet skiffs that look pretty similar to yours for running the Stikine. I know they've refined their tunnel designs over the years. If you know anyone down in Wrangell, perhaps they could give you some details on how Svendsen is doing it.

 

Deering,
Thanks for the tip. I checked the web site and didn't see any examples of tunnel boats, but that doesn't mean anything...lol. If I get down to Wrangell I'll have to cruise the harbors and backyards looking for one.

MM

 

I hope to get down there sometime this summer. If I do I'll take a few pics for you. I might even talk to Svendsen when I'm there.

 

Having a tape measure in your pocket wouldn't be a bad idea either..

I think the Taku and Stikine rivers offer the boater different challenges then the Chilkat. Where both of these southern rivers have higher flow rates, narrower channels, and deeper water the Chilkat is wider, slower, and shallower.

Take a look at these two shots of the Chilkat and I think you'll agree we have different circumstances to deal with.

MM

 

Will do on the tape.

I'm not a river rat, Mike, but I've been up both a few times. As soon as you get above the tidal areas, you're into reading braided channels. I think you'll find the Svendsen design to be quite applicable to the Chilkat.

 

How about a spot of inspiration?

Whangani River jet boats.

http://www.youtube.com/watch?v=STBVnfRSHL8

 

Impressive handling, but I wonder how it does in shallow water?

I think that jets weren't really designed for shallow-water operation, but for operating in clean water that has the occasional shallow boulder or rock that takes out lower units. We tried the outboard jets in a sandy river here and they were a maintenance nightmare with excessive impeller/housing wear, etc. That's why I went to a tunnel prop - runs just as shallow as an outboard jet, with none of the maintenance or performance issues. Poor handling, though, since there is little below the boat hull to turn it - you need to plan your turns in advance and slide through them.

To the OP, as far as bottom strakes go, that is a tricky one. If you make them too deep, it can be almost impossible to unstick a boat if you run aground. If you make them too shallow, you slide to much in the corners. I've considered retractable strakes (or ice runners), would work great but be a pain to build into a hull - you could retract them with hydraulics (like a jackplate or trim tabs) or even a simple mechanical turnbuckle arrangement at the transom. Keep them deployed until you run aground, then retract them to unstick the hull.

 

Pericles,
Impressive performance and boat handling on display in that video. I'm not sure it's applicable to the thread, but it was fun to watch.

VI,
Not being an avid river runner myself I can only assume that the overwhelming preponderance of outboard jet units that one finds strapped to every river boat I've seen so far is a pretty strong indication of the local preference. The alternatives seem to run more toward airboats, and hovercraft then props. Because of the expense of outfitting these type of boats one can only assume that they (props) have been tried before and found lacking.

MM

 

Tunnel props aren't widely used because they can be difficult to set up properly. First and foremost, you need a good tunnel design, which many factory setups don't have. Then you need a good tunnel prop (lots of cupping) to deal with the disturbed water. A hydraulic jackplate is good to adjust the motor to running conditions, but you don't want too much setback with a tunnel. Then you might want to vent the tunnel if you need an extra 5mph top end. Add floatation pods to make up for the lost hull buoyancy and improve shallow-water holeshot. Once you get all of the nuances figured out, then you need to learn how to properly operate a boat with a propped tunnel. For holeshot, jackplate down, trimmed down; for getting up shallow, might want jackplate up with motor cranked hard left; get water moving in tunnel before you hit WOT, stay well below WOT when running through the skinny stuff so you don't blow out the tunnel, vent closed in shallows, vent open in deep water, etc., etc.

There is a guy over on the 2coolfishing forums that runs a Majek RFL with a custom prop on it. He occasionally posts a video of himself running through muddy sloughs with a half-inch to an inch of water at most on top of the mud. He quickly takes the video down though, because the environmentalists use it for fodder to try to get grassy bays shut down to boating (even though he doesn't run the boat through any grass).

The boat I run is a pretty big one, so five inches is about my limit before I start dragging the skeg in the sand.

Properly set up, a tunnel prop is a beautiful thing to behold. Improperly set up, they are a royal PITA. Most people won't bother with the effort.

 

Hi VI,
The main reason I posted my design was to illicit responses concerning what constitutes "good" tunnel design. Although I've tried to inspect as many factory and home made designs as possible I couldn't help but wonder if they were the best that one could do? It's easy to envision that even small changes in entry width, side plate angles, tunnel width and depth might have a profound effect in the flow characteristics. Since jet outboards are extremely sensitive to even the most minute disturbances in water flow it stands to reason that the last thing you want to do is build that into your hull with a poorly designed tunnel.

It seems as though props and jet units share a lot of the same misfortunes. If they're properly set-up they work fine...if not then they can indeed be a PITA. Jack plates both manual and hydraulic appear to be a common solution to some performance issues found while running in shallow water. I've also found some interesting home remedies to the setback issue... everything from welded on tunnel roof extensions to tire inner tubes all designed to help maintain water flow to the jet unit.

If there is any significant difference between the jet vs prop tunnel is has to be the depth and width of the tunnel itself. Here the jets win out simply because they're shallower and therefore have less impact on hull performance. And I've yet to see a jet boat with sponsons either as a factory or after market add-on. Not that they wouldn't benefit from the extra floatation only that so far it doesn't appear to be a customary fix.

After rereading your description of trimming a prop unit I realize this is the exact opposite of how jets are trimmed using jack plates. With the boat at rest the unit is raised not lowered because in the shallow water environment we're dealing with you're more likely to be grounded out at this point. You may even have to pole or pull yourself back into deep water so you can take off. Once you're up and running the engine can be trimmed down in order to avoid cavatation caused by waves aerating the tunnel.

Another possible benefit of the jet vs prop tunnel is the elimination of the tunnel vent and it's constant adjustment as you move from deep to shallow water. Here that might require more attention then you can muster as you move from one depth to another in just a few boat lengths.

For my purposes I have to consider the user who is most likely concentrating on trying to get from point A to point B without getting stuck on a sand bar. He's already occupied trying to remember where the deep water channel was last week or even yesterday. He's already keeping an eye out for snags, sweepers, stumps or the occasional tree all while trying to read a river that can be pretty unforgiving if you make a mistake. I seems like this is a perfect example of why one should consider employing the KISS principle.

MM