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Mostly Harmless
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Discussion Starter #1 (Edited)
I am in the early stages of thinking through my next build. It will be a mudboat designed for a 20-23 HP longtail mudmotor. It will be a ~16'x44" flat bottom, round chined hull. I am very comfortable with how to get the basic hull design drawn up.

The sticky part is that I'd like to add enough displacement behind the transom to float the hull reasonably level when empty except for the motor on the stern and a lawn mower battery and 6 gallons of gas in the bow. I absolutely must be able to pole or push this thing backwards from the bow as shallow as it can float while dragging its bottom in pluff mud. Experience has taught me that this is miserable or impossible if the hull squats in the stern. I know the performance costs of sponsons, but this is not a high performance application. Given the weight of the motor and the intended use, I think sponsons' benefits could outweigh their costs if designed properly. However, I don't know how to design them to limit their negative effects, so any advice is welcome.

How wide should the gap between the sponsons be to feed water to the prop in turns? The mudmotor has a 72" shaft, so the prop is behind the sponsons and gap does not have to accommodate a lower unit and the tilt of the hull drives the prop down a bit in turns. I was thinking of limiting the gap's width to that of a pocket tunnel to maximize the width of the sponsons. The gap would be open on top, so it will create no suction like a true pocket would.

I pretty much plan on making the sponsons as long and wide as the motor will reasonably allow to support this anvil of a motor. Until I decide on the width of the gap, mock it up and take some measurements, I don't know what that is, but let's say about 6" deep x 12" long x 16" wide. That gets me a bit under 55# of displacement behind the transom at 4" of draft once I subtract the displacement lost by the soft chimes and 7 degree sponson bottom angle (if used). That seems be enough to be worth the trouble to me.

Should the bottom of the sponsons run straight back off the bottom of the hull or angle up 7 degrees to facilitate the expected angle of plane?

I planned on angling the front and both sides of the gap 45 degrees compress the water to the center of the gap on plane and make it easier to slide backwards in mud. Straight sides push mud like a dozer blade and a straight sided gap would collect mud like a bucket.

While the hull will have soft chimes, I think I need to keep the edges of the bottom of the gap sharp to encourage clear separation of the water from the hull.

If you know something about sponsons, please critique my plan.

Nate
 

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Zephyr Cove is on FIRE!
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I believe if you want to limit stern squat you might look at something called a “hunt deck”. It is basically one sponson across the entire transom that is not much taller than the draft of the boat and has a raised mounting plate for the motor. I don’t know much about mud motors but these hunt decks are usually fabbed for aluminum duck boats. I guess you might as well just make the boat that much longer and it would be basically the same thing as far as draft reduction. The only thing having two sponsons does that really helps is they get the weight of the motor further forward rather than at the very rear of the boat.
How far back is the prop from the transom? If it’s too far back having two sponsons will not really channel water to the prop much better.
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Zephyr Cove is on FIRE!
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I want displacement behind the motor. I can balance the load to some extent, but I want to start reasonably level with minimal load. That is hard to do with a mudmotor.

Nate
Some longer than normal pods are going to be the ticket BUT not so long that they get in the way of the swing of the motor. Keeping them as low as possible to the actual draft of the boat will keel them out of the way. 8” tall or so?
 

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Mostly Harmless
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Discussion Starter #6 (Edited)
But you will be poling from the bow, correct? In that case, won't your body weight on the bow help lift the stern? Or does this motor outweigh you? The GoDevil 23HP weights in at 178lbs according to their website.
Mine is the 20 HP Honda and weighs 198#. Yes, when I pole from the bow, I counter balance the motor, but when it really gets ugly, I am in the mud pushing the boat backwards off whatever bar I ran up on. Plus, when I am in the stern with only my gear and a dog in the bow, the stern really squats as is. If I start with a reasonably level floating hull, it makes it easier to balance the load.

Nate
 

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Brandon, FL
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Mine is the 20 HP Honda and weighs 198#. Yes, when I pole from the bow, I counter balance the motor, but when it really gets ugly, I am in the mud pushing the boat backwards off whatever bar I ran up on. Plus, when I am in the stern with only my gear and a dog in the bow, the stern really squats as is. If I start with a reasonably level floating hull, it makes it easier to balance the load.

Nate
Nate,
You are going to battling an almost unsurmountable task. 44" is just too narrow.

Thinking outside the hull so to speak - what if you were to do the opposite of what smackdaddy posted. By that I mean what if you were make an engine mount inside the hull and the motor extend backwards over the transom. You might even be able to trim the transom down to mount the motor a fair distance from the stern. It would take up space inside but when not driving you could spin it around to a 45 degree angle to get it out of the way.

upload_2017-11-17_7-45-5.png
 

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Mostly Harmless
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Discussion Starter #10
You don't want to go down that rabbit hole. This beast is a fossil that has been resurrected from several graves and Frankensteined until it was functional. It is an '86 Go-Devil that started out with a 23 HP engine on it. Some prior owner replaced the 23 HP with a 20 HP Honda. Another prior owner had a new "lower unit" welded on by some redneck who thought he was a welder. From what I can tell, all Go-Devils 16 HP and larger share the same frame.
 

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Mostly Harmless
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Discussion Starter #11
Ducknut,

Yeah, I know I am asking gravity to give me a pass.

I have thought about what you describe. I worry because that 72" shaft make the prop run in the "hump" when the motor is mounted on the transom. If I move the motor mount forward from the transom, I will move the prop forward in relation to the hump and force it to run a bit deeper.

I also considered a pocket drive under a hunt deck with a forward mounted motor mount, but I don't think that would be a good thing given the speeds this rig will run. Pocket tunnels need speed to create the suction necessary to work properly.

I think I am stuck with the gap between sponsons to break that suction.

My current hull is 30" across the bottom of the transom and I manage to limp along, so I think a 44" transom with sponsons would make a hell of a difference. My goal is reasonably level, not "on the bubble" level at rest while empty. The closer I get it, the easier it is to balance my load. My current rig cannot be balanced level unless I were to bring unnecessary weight.

Nate
 

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Zephyr Cove is on FIRE!
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The issue I see with two sponsons on a 44” wide hull is that once you get them far enough apart to be clear of the motor you won’t have added much wetted area and your $ and weight might not be worth the little extra bit of floatation. If you can build them out of 1/8” and keel them no taller than the draft of the boat you might be ok and get a couple of cubic feet of displacement which would give you roughly 120# extra floatation and bring that stern up at rest. Boyd’s Welding in Florida took my hand drawn plans and fabbed step pods for my aluminum skiff and I welded them on.
 

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Brandon, FL
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make the prop run in the "hump" when ... forward in relation to the hump and force it to run a bit deeper.
I do not believe this would be the case. The depth of running is determined by many factors and none of which calculate where thrust is coming from. Whether it comes before, on top, or behind the hump, it should have zero effect on your draft. What it might do it cause your prop to be 2" lower in the water column but your prop is designed to hit bottom.

Why not give it a try on your boat now. Make a stand and set it in your boat and go for a ride. I think you are on to something that you may very well find nobody has ever tried it before because it is easier to buy a different boat. If you look at all of the mud boats out there they all look nearly identical.
 
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