Converting a flat bottom pontoon boat to a tunnel hull

[mark herbert]

okay, so you built cat hulls, but why?

There are a couple solutions that sort of pop.

Build the hulls out as a jon boat like monohull. This would eliminate wave interference and allow for green water at the prop. It would give you more reserve buoyancy for heavier loading. The cost is drag; the boat will be slower, but slower than what.

You could try lifting strakes or you could try lifting strakes and a jack plate both. A little lift from the strakes and a little lower with the prop might be enough. The strakes get set so they are under the at rest waterline at the stern and at or above it at the bow. It is a little beyond me, but a lot of the flats skiffs on bateau.com use strakes for shallow water work.

You could widen the toons spacing so the waves meet behind the motor. The amount of width can be calculated. It seems a little odd to widen them, but if it is easy; it is also a solution.

A few ideas to ponder. I don't have any specifics. Your post title is rather confusing because pontoons don't convert to tunnels or at least I've not seen it.

We are actively modifying this boat so widening the space between the pontoons is not out of the question. You said the amount of width can be calculated. What formula? We have built this type of boat for ease and low cost. It has no welds or sealed pontoons. These boats work perfect as a slow slug with low hp

 

[mark herbert]

What kind of prop are you running, alloy or stainless, number of blades ? This is crucial.

been running alloy tried 3 and 4 blade props. 4 blade seemed to help a small amount and i am sure stainless will improve performance a small amount. The key to this problem is getting a good stable flow of water to the prop

 

[fallguy]

My answer will be crude, but I come up with about 5' between the hulls to avoid the hull wake. I did not account for placement of the engine forward of the stern and used 20(10 each) degrees and 14' hull length. This might be unattainable and is why I suggested jon boating the hulls instead.

Another solution or part of an overall solution would be to move the engine mount forward into midships; well two or three feet. You would need to run a camera to see how far the current waves meet or adjusted width waves meet.

You may actually end up doing both, widening the boat say two feet and moving the engine location forward to undisturbed water.

A cheap video camera mounted under the boat will show the issue well.

The fact you are varying 6" is also problematic which is why I suggested jack plate and the jon boat idea.

The jon boat seems easiest to me. Weld in a patch right over the other two toons inboard edges. You could even just tack it for testing.

 

[kapnD]

Extending the length of the hulls would probably reduce the squat that increases draft at the stern.
This would also offer more leeway in fore-aftmotor position.
I don’t know how to do the calcs either, but running the numbers at various lengths would be an interesting exercise.

 

[fallguy]

Extending the length of the hulls would probably reduce the squat that increases draft at the stern.
This would also offer more leeway in fore-aftmotor position.
I don’t know how to do the calcs either, but running the numbers at various lengths would be an interesting exercise.

I am not an NA, but I am pretty sure you take half the Kelvin wake angle for each hull. Lengthening the hull would not change the bow wave interference point.

 

[Mr Efficiency]

been running alloy tried 3 and 4 blade props. 4 blade seemed to help a small amount and i am sure stainless will improve performance a small amount. The key to this problem is getting a good stable flow of water to the prop

You must go to stainless, and ideally, five-bladed, if you are running in aerated water, to have the best chance, but I would be guided by the contributor here who mentioned specialist props for the purpose. Alloy props are far from tolerant of skinny water.

 

[dreamingbarrierreef]

water wheel (like those on ancient steamboats) might actually work well with the kind of thin water situation too

 

[Village_Idiot]

Try these in order (powered jackplate is a given, momentary foot switches are great for operating on the fly):

1) use a SS prop, four-blade with large blade area and double cupping, check with Baumann Props in Houston as they specialize in tunnel props designed for operating in aerated water. If Baumann is not an option, try PowerTech Props. Mike Hood on the TX gulf coast is also good.

2) add an inverted hump, mid-hull, between the pontoons. It can extend as low as the bottom of the pontoons. This will help purge air from water flowing between the pontoon hulls, and make for greener water at the prop. It also has the effect of shooting the water back upward to the prop. Look at good pocket tunnel designs and you will see this feature, a bulbous hump of sorts at the leading edge of the tunnel.

3) add extensions, or pods, to the aft end of the pontoons. Make these even with the hull bottoms on the pontoons, assuming you won't be using this in rough waters. This will effectively extend the length of the hull and prevent the transom from squatting during holeshot, as well as giving lift to the aft end of the boat.

For reference, i run a 26' aluminum flat-bottom with mod-v bow and an 8-foot pocket tunnel in the transom. Engine is 115hp Mercury four-stroke with four-blade SS Baumann Blaster prop with 13" pitch to get motor into 5800-6400 operating range. Holeshot is within a boatlength, noat runs in less than six inches of water over hard sand, three inches of water in soft bottom when on plane. Off-plane, boat needs 10" water to putt around, 12-14" over sand for holeshot, 8-10" over soft bottom for holeshot. With that size of boat (26'), it will carry a lot of weight/cargo and lose very little draft.