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 Who Knows Anything About Sponsons? 
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Joined: Mon Mar 24, 2014 8:13 pm
Posts: 24
Post Who Knows Anything About Sponsons?
I am in the early stages of thinking through my next build. I currently own a 20 HP GD long tail that weighs about 200#. My next boat 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, but I need some help with sponsons.

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 hull 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? The prop is behind the sponsons and gap does not have to accommodate a lower unit. I was thinking of limiting the gap's width to that of a pocket tunnel (let's assume around 12" at the bottom for now) 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 4" radius 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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Thu Nov 16, 2017 8:16 pm
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