sponsons

I was recently given an alumacraft 14' all weld boat, and am interested in putting sponsons on it. I put a 25 hp short shaft yamaha 4 stroke, and have a friend who is a machinist/fabricator that could build the sponsons if he knew dimensions, etc.
i'm also interested in putting a tunnel in the boat, but many tell me it won't help. not sure why it wouldn't help, as it is now, the motor is probably 9-10" below the bottom of the boat, and i'm wanting it to go in a lot shallower spots than that.

 

Hi stvc86, welcome aboard

1 - Why do you want to add sponsons? (They can be a good solution for some boats in some situations, but not all- what's the motivation for adding them?)

2 - When you say "the motor is probably 9-10" below the bottom of the boat", what distance are you referring to- propshaft to keel line?

Cutting up the back of the boat and putting in a tunnel might just make it trim more bow-up / stern-down when on plane, thus not gaining you any draught advantage.

In any case, no prop-drive outboard will do well in very shallow water, no matter how you mount it- the cooling intakes are mounted on the lower unit, and if they start sucking in sediment from the lake bed.... well, outboards' water pumps aren't fun to rebuild, and getting sediment buildup out of the cooling passages can be tedious. If you don't have sediment on the lake bed, you probably have rocks, in which case any propeller would be vulnerable.

Extreme shallow-water drives more often take the form of a "mudboat" drive (usually home-built or by a custom shop) with a special weedless prop, a jet pump, or an air propeller.

 

sorry for the delay. the desire is to get the boat to not "squat" as much when attempting to start. i'm after an ultra shallow deal, want to run in 6-8" and get up as shallow as possible. We have miles and miles of salt water flats that people use fiberglass tunnel boats on, and i'm after a less expensive alternative
9-10" refers to the boat sitting on the trailer, and that much of the skag and lower unit sticks below the bottom of the boat. so, if i only draft 2" i still need 12" to float in
Jet drives have the problem of sucking grass, and poor fuel economy. the place we're going to fish is a 20 mile boat ride, so hard to haul enough fuel for a multi day trip with a jet drive.

 

marshmat is right. i would go more for a surface drive mud motor like prodrive outboards the stuff these motors with go through is amazing www.prodriveoutboards.com. there are also the longtail motors like they talk about on the weedeater conversion thread. i am planning on building on with a 24hp horizontal shaft honda like a go-devil. www.go-devil.com the props for these motors are left handed so they are harder to find cheaply. ebay is where i got mine but you can buy the new directly from the mudmotor manufactorers. not all props attach to the shaft the same so know what you want some are threaded, some smooth with key, some smooth with key and tapered hole. hope this helps. these motors are designed specifically for shallow water and mud.

 

redneck - may I suggest a little edit to that post, the closing bracket ) ended up as part of the link.... and yes, I agree, something like the ProDrive would probably be much more suitable for stvc86's mud flats than a conventional outboard would be.

"squatting" when coming up on plane is often more easily resolved by shifting weight forward, a much cheaper and easier option than modifying the hull. I would only consider some form of planing aid if you simply can't move enough weight forward to get her up easily.

 

i was not sure what a sponson was so i just googled it on www.wing.com and know i have a question about them. the boat they have that is called a wingcat cargo cat would that be considered a planing boat? i didnt think pontoon type boats planed or am i way off? it looks like a really neat boat though.

 

that means i need 2 motors. remember, i have a long drive to get to the flats. i've been in a boat with a go-devil before, and it doesn't seem best for traveling a long way in deep water.
it's just me in the boat, so unless i go for a tiller extension, it's hard to move forward

 

redneck - from the looks of it, that "wingcat" thing planes (at least when lightly loaded), just not as efficiently as a flat or V-bottom hull. Load it up and it'll most likely behave like any other overloaded pontoon boat- plowing along at somewhere around hull speed.

stvc- A few years ago I was looking up all kinds of trim tab, hydrofoil, etc. gear to help my 15' runabout get on plane. It would have cost hundreds of dollars and taken quite a while to get right.

Someone suggested clamping a 3' length of aluminum pipe to the tiller grip. Twenty bucks later, the boat just pops right up with minimal bow rise. Problem solved. Try it- if it doesn't work, well, you can always find some use for a three-foot chunk of pipe.

 

Matt,
I think that might be one of the best ideas yet. I was about ready to scrap the whole plan, sell the motor and buy a kayak.

 

Be aware that the mud motors do not do well in sand, and they are slow compared to an equal-horsepower outboard.

I run a 26-foot tunnel boat with a 115-hp Merc outboard in a sandy river. It requires 12 inches of water to get up on plane, but will run in six inches of water, and four inches over short distances. A colleague owns the twin-engine prodrive setup and cannot run as shallow as the tunnel boat in the sand.

Tunnels can work well, but they need to be well-designed and set up properly. A poor tunnel setup is a nearly worthless boat, whereas a finely-tuned tunnel setup is a marvel to behold. Look at the tunnel design on some of the flats boats around you to get an idea. Propeller design is critical in a tunnel application - you need large swept blades and a good amount of cupping to deal with the aerated water. I'm using a 3-blade Baumann prop to get my performance.

You might check out these forums to get some ideas if you pursue a tunnel boat design http://2coolfishing.com/ttmbforum/forumdisplay.php?f=36 and http://refugeforums.com/refuge/forumdisplay.php?s=&daysprune=1&f=10

You might also consider adding a jackplate with additional setback to allow your outboard to operate at a higher elevation (and use a cupped prop). However, they will almost necessitate use of pods to get buoyancy back in the stern. Pods are not rocket science, just use a little common sense in building them. There are aftermarket versions available on ebay and from Beavertail, I believe. Biggest trick is to utilize good welding skills so that you don't warp the thin aluminum found on smaller boats. Pods should be tack-welded, then seams filled in gradually - pods should also have drain plugs built in and NO foam inside.
HTH

 

I do know that i want a tunnel in the boat. great advice on beaver tail, i ordered some of their sponsons. (performance pods).
now the question is who can advice me on the size of the tunnel. it's a 14' alumacraft all weld boat. i'm using a 4-stroke yamaha 25 and going to put a jack plate on.
i'm tentatively planning on a 3" deep, 10" wide, and start the tunnel at the back of the last seat. some of the pics i've seen show it to be slightly tapered towards the top. is this necessary, or should it be straight?
thanks

 

Interesting project, if possible why don't you post some pics of your existing hull. Shots of the bottom, transom, etc.

 

proposed tunnel/pictures




some tell me the tunnel is too short in lenght, but I thought the strength of a flatbottom boat was that it's platform was so large that it doesn't draw much water. if i make a really long tunnel, then don't i basically have a catamaran(sp?)

 

more info on what we're planning, in case it affects tunnel design. after pods and the tunnel and a jack plate, we plan to put a casting platform from the bow of the boat to the back of the middle seat. Then, i'm going to get a tiller extension, and in shallow water situations, drive while standing in the center of the boat. that's part of why i need an hydraulic jack plate, to keep water flow when i shift the weight forward.

 

In general, the tunnel should be 12 inches long for every 1 inch in height; therefore, your three-inch high tunnel would need to be three feet long. Make the tunnel too short and it will not prime properly. Make it too long and it will suck too much air at the leading end. Many boat builders recommend a tunnel length of around one-third the length of the boat.

Most of the tunnels you will see will have an extra inch or two of width beyond the prop width - I believe that this is so that the prop won't lose bite in turns. As for height, most shoot for a little less than 1/2 of prop diameter - less will work but won't let you run quite as shallow, more may kill the performance of the boat. With the three-inch height, you would have the option of going with a jet foot in the future if so desired.

I recommend a round or five-sided tunnel vs. the standard three-sided tunnel seen on cheap tunnel boats. Again, look at the flats boats in your area and check out how their tunnels are designed. Ask the boat owners if they are happy with how the tunnel performs.

Some builders at a small (1/2 - 3/4 inch) vent hole at the leading edge of the tunnel - this is to break the suction that the tunnel creates that pulls the stern of the boat down into the water. I don't think the vent makes much difference in shallow water, but it may make your boat go 3-5mph faster. If you add the vent, add in a valve so you can turn it on/off. Again, I cannot stress the importance of having a good tunnel prop to make the boat perform well, especially if you use a vented tunnel. You should also have a good water pressure gauge.