# Sponson Design Parameters



## DuckNut (Apr 3, 2009)

I wouldn't spend anymore time on it than you spent typing your question.

Don't you use your boat with a mud motor as well? That will play into the equation.


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## WhiteDog70810 (May 6, 2008)

DuckNut said:


> I wouldn't spend anymore time on it than you spent typing your question.
> 
> Don't you use your boat with a mud motor as well? That will play into the equation.


I think I am just gonna sell the mudmotor and put the money toward an outboard. This boat ain't made for it and I can't get enough displacement behind the transom to help with that beast. It is actually over-rated in the mud anyway since I can't pole for crap with it on the boat. I'll just have to pole further if it is too shallow for an outboard.

My next build will be a mudboat, but I want a 23 hp SWOMP mudmotor by then.

Nate


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## DuckNut (Apr 3, 2009)

What does 50#'s of displacement translate to in terms of depth of the hull?

If you moved all of the stuff in the boat to the front you would probably achieve the same thing.

Again, I personally would not spend the energy.


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## WhiteDog70810 (May 6, 2008)

I don't really know how much the sponsons would actually help. I don't know how to do the math to figure the effect on draft. Right now when alone, between me, the mudmotor and gear (~450# total) in the stern, I think I draft about 9-10". I'd get a lot more displacement from the sponsons with that asymmetrical load due to the greater draft, but I don't know where it would balance out.

Right now, the stern squats so much under the mudmotor that poling backwards is a miserable experience, but it will squat so much less with an 85# outboard I am not sure that is a valid concern. When poling distances, I'd really like to be able to pole from the stern even when alone, but that is probably just a dream. I weigh around 225# right now, so even with a lighter motor I am not sure a mere 50# of displacement would really matter a significant amount. 

My gas and tackle is already forward. My 12# mushroom anchor in the stern because that is where I use it. My starter battery is also in the stern, but it is a little lawn mower battery. My next motor will probably be pull start and I will skip the battery completely. The cooler sets in the middle and when full of ice, it will provide a lot of ballast that can be shifted to balance the hull

Right now it may seem I am trying to mess with two variables at once, but I am just thinking. I will not add sponsons until after I have an outboard and have spent a lot of time running it.

Nate


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## jmrodandgun (Sep 20, 2013)

Wouldn't that translate to fractions of an inch in terms of draft gained? Or is there something I'm missing?


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## yobata (Jul 14, 2015)

i think that it is probably a bigger effect on a smaller boat - on a 14' boat it might make a big difference to offset 50# off of the transom (especially if all of your other weight is already moved up to the front - gas tank, batt, etc.)


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## DuckNut (Apr 3, 2009)

I also wonder if the 50# calculation is for the entire sponson, not just the couple inches that will be underwater. If they are 12w x 12d x 12h and only 8 inches in the water then the calc has to be for 12x12x8

I actually think you would get increased desired results when under power rather than when poling.


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## pt448 (Mar 22, 2014)

When I was reading over this thread and your dilemma, CurtisWright's Jealous of Shalla build popped into my head. He made a cutaway in the back of the boat similar to how a solo skiff is set up to solve some of the same problems you're having. Could you make your sponsons similar to the sides of that cutaway? Would likely give similar results with an outboard on your boat.


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## WhiteDog70810 (May 6, 2008)

Each sponson would be 10"x16". There are two of them. That is a 320 sq in footprint behind the transom. With each inch of draft, the sponsons would displace a combined 320 cubic in of water divided by 231 cubic in/gal. That is ~1.4 gal or 11.6 lbs displacement/inch, correct?

These are just horribly simplified numbers I puked up for discussions sake. I only put them out there so someone could reply "no, idiot, you need twice that displacement/inch to bother messing with sponsons".

Nate


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## WhiteDog70810 (May 6, 2008)

jmrodandgun said:


> Wouldn't that translate to fractions of an inch in terms of draft gained? Or is there something I'm missing?


I suspect you are correct and, if so, this is just a mental exercise for me.

Nate


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## firecat1981 (Nov 27, 2007)

I haven't done the math, and to be honest I'm not going to right now, lol. However I think you forget to add in the 20lbs or so of material you will need to achieve the build. I agree with Duck, I doubt you will get the effect you are really looking for. 

A better option would be to design a new boat, with sponsons incorporated into the design from the start.


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## DuckNut (Apr 3, 2009)

No you idiot your numbers are all wrong.

Stop calculating such a small foot print.

Let me help - 18 feet long at the center line, 58 inches at the waterline, 5 degree dead rise, modified vee.

You following?


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## WhiteDog70810 (May 6, 2008)

DuckNut said:


> No you idiot your numbers are all wrong.
> 
> Stop calculating such a small foot print.
> 
> ...


You want the displacement for the entire hull? Good heavens, I've never done that math. It has to displace 37.4 gallons empty because it weighs 312#. I have no idea what it drafts empty because I never floated it empty after completions, so I can't backwards engineer the bottom area.

Nate


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## DuckNut (Apr 3, 2009)

WhiteDog70810 said:


> You want the displacement for the entire hull? Good heavens, I've never done that math. It has to displace 37.4 gallons empty because it weighs 312#. I have no idea what it drafts empty because I never floated it empty after completions, so I can't backwards engineer the bottom area.
> 
> Nate


No!

Recheck the dimensions (hehe). It was a hint to start a new build and fill the lifeless board with boat porn again.


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## WhiteDog70810 (May 6, 2008)

Oh... ...my sarcasm detector is limping today.

Nate


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## DuckNut (Apr 3, 2009)

WhiteDog70810 said:


> Oh... ...my sarcasm detector is limping today.
> 
> Nate


They have a pill for that.


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## WhiteDog70810 (May 6, 2008)

DuckNut said:


> They have a pill for that.


How well does it pair with bourbon?

Nate


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## DuckNut (Apr 3, 2009)

WhiteDog70810 said:


> How well does it pair with bourbon?
> 
> Nate


Might just straighten you up.


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