# To sponson, or not to sponson, that is the question,,,



## firecat1981 (Nov 27, 2007)

I've been thinking about design tweaks I will be making shortly before building. and while I was dead set on sponsons before now I'm not so sure.
I see some pro's in them, but I'm wondering how many cons? Basically even though this will be a skinny water boat it may occasionally see open water while crossing areas. I'm also thinking that with no sponsons I may be able to get a few more inches of hull length......
interested in hearing some thoughts.


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## Brett (Jul 16, 2008)

You asked at the right time...
I've been out in the garage eyeballing the Next.
Got to wondering what the USCG uses as the definition of length
so I wandered back to the computer and looked it up.
Here's the picture


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## Brett (Jul 16, 2008)

So length is the overall distance from the bow of the hull structure
to the farthest point aft of the hull structure.
That means the rear of the sponsons determines overall length.
Given that, you tell me which shape has more bouyancy
considering they have the same outlines and overall length,
except for the transom indent...


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## firecat1981 (Nov 27, 2007)

Brett I had looked that up as well, which is why I'm wondering now, and your drawing of the 2 boats is my exact delema. Either way I need her to be at or slightly below 16ft. Now the big question really is will moving the motor up 10-12 inches offset the missing hull cutout? Man this is gonna boggle my mind for a bit


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## Brett (Jul 16, 2008)

Which shape adds to the materials bill and hull weight?
More hull surface to build and glass...


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## firecat1981 (Nov 27, 2007)

The sponsons would add more to the BOM. More wood needed for the added shape, more tape for all the seams........Really I'm kind of thinking I would love my old boat back, just with no tunnel and a flat deck, so......


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## Brett (Jul 16, 2008)

12 inch indent, 24 inches wide, 6 inches draft

6x12x24=1728= 1 cubic foot = about 62.5 lbs bouyancy


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## firecat1981 (Nov 27, 2007)

you know I'm really interested in knowing how it will effect the ride characteristics.

Think of it this way, if the back of the sponsons are straight up and down instead of a 14-15 degree slope like the transom then even though the bottoms will extend 12 inches the top will only be adding 6 inches or so......does this make sense. It's hard to see but you can get an idea of what I mean on the model I made.


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## copperhead (May 30, 2008)

Hate to say this but you're asking a question that has no answer.  Obviously I build both style of boats.  Both have their performance attributes and as you noted, a sponson style transom complicates the build.  As Brett noted, the loss of bouyancy isn't really all that much and the loss is mitigated by moving the engine weight forward.  The sponsons act kinda like trim tabs too so keep that in mind; you aren't going to 'air-out' a sponson hull at high speed.  But you probably weren't looking to build a 50 mph hull anyway


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## Brett (Jul 16, 2008)

That's the reason for building models first.
Finding the problems and solving them,
before you get down to the real deal.


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## firecat1981 (Nov 27, 2007)

> Hate to say this but you're asking a question that has no answer. Obviously I build both style of boats. Both have their performance attributes and as you noted, a sponson style transom complicates the build. As Brett noted, the loss of bouyancy isn't really all that much and the loss is mitigated by moving the engine weight forward. The sponsons act kinda like trim tabs too so keep that in mind; you aren't going to 'air-out' a sponson hull at high speed. But you probably weren't looking to build a 50 mph hull anyway


Damned it! why do the experts always need to inject logic into my ramblings? 

Well now I'm thinking about leaving them out of the build, making the boat a full 15.5ft, and buying some smart tabs


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## copperhead (May 30, 2008)

Ok, overall 'boat package' length is a design element you are constrained by. A sponson style design allows maximum waterline length since the engine isn't adding that much to the overall length now. And in this size skiff, a little more waterline it a good thing IMO. food for thought.....


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## Brett (Jul 16, 2008)

Buoyant trim tabs? :-?

Not considered part of the hull structure,
and lets you adjust your ride to the sea conditions.


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## firecat1981 (Nov 27, 2007)

hmmm decisions decisions? Mel you bring up a great point, but after being tormented with this cooler project I think I'm leaning back towards simplicity in design, heck if the SUV doesn't need em then niether do I . I can always add sponsons later on, or try to make Brett's floaty tabs . I'll still need to do some thinking on this.

Lets say it gets rough out there, I mean that never happens but lets pretend . Would having sponsons hurt?


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## lemaymiami (Feb 9, 2007)

One other small twist to add to your considerations about sponsons.... Years ago it was common to add on bait wells designed just like sponsons on the transom.  You could run them wet or dry (when dry you got the added floatation).  When wet you had the ability to fill them with bait (filling is misleading here - they'll only fill to the hull's draft, no matter how tall they are...) without using any kind of pump at all...

The best transom wells were configured exactly like the hull bottom, with the appropriate hatches.  Adding trim tabs when you had bait boxes was another challenge since you had to reinforce the are on the transom well (or bait box) to be able to handle the load the trimmers would generate....

Here's a pic of my old SeaCraft 18 that we tore down to the pieces and completely restored. The transom wells you see are what I'm talking about. You could run them wet or dry ( using both you could really load them up with whitebaits). 









Wish I'd never sold that boat....


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## copperhead (May 30, 2008)

Sponsons won't hurt you in rougher water. Probably help due to the longer waterline.


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## Frank_Sebastian (Oct 15, 2007)

There are a lot of proven boat designs on the internet. Anytime you design your own it is like saying I can do it better for me only. Later if you need a different boat for a different type of fishing, family growth or what ever, you now have a very specialized boat that isn't so good for you?

When you say loss of 62# bouyancy isn't significant think of placing a very large battery a foot in front of the engine. It seems the advantage of a sponson is like an afterthought to offset an existing problem. I tend to think of jack plates (a dynamic and reversible solution) tunnels, sponsons, trim tabs and the whale tail engine elevators as assessories for special uses.

They act to redirect thrust by using engine power to correct an undesirable situation.

Just my free 2 cents and worth it,
Frank_S


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## firecat1981 (Nov 27, 2007)

> Adding trim tabs when you had bait boxes was another challenge since you had to reinforce the are on the transom well (or bait box) to be able to handle the load the trimmers would generate....


You know this was one of the reasons I'm considering not using sponsons. I was trying to figure out how to make an area rear of the full thickness transom strong enough not for tabs, but for securing it to the trailer (those of you who know me know why I'm so concerned about this part )

Really at this point and time I'm seeing more downsides then up, yes it will make me float 1/2" shallower in the rear and add a little more waterline, but it will add alot of cost, time, and complication to my build. Without the pocket drive I'll probably gain that 1/2" anyway so is it worth it in the end? Then comes issues of less area to mount accessories and no access to run wires for stuff like depthfinders and under water LED lights......  I think my mind set right now is I can always add them later on if I find I'm not drafting shallow enough.
Does this make me lazy? :-/


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## swampfox (May 9, 2007)

I have been pondering this for sometime on my build. The first time I built the plug I did not use sponsons. Now on build 2 I was seriously considering them-not so much now. But here is my thought -anyone feel free to correct me. One small outboards such as my build it is not really a advantage. Unless you move the tower in front with the motor. If you leave your platform in the same location as many builders do there isnt much if any gain in draft. To keep it simple lets devide the rear of the boat into thirds. Lets remove 33.333% of surface area. Moving that 95# motor 12 inches forward does it make up for the loss of surface area? Now if you move the engine forward and move the platform the 220# man forward there should be a gain. But if you have a 250# motor and no platform there would be a significant gain. Anybody see my point?

Side note Firecat if the tiedown method is your concern do what I have done for years. I run a strap left to right across the deck and tiedown to the trailer on each side.


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## firecat1981 (Nov 27, 2007)

> Side note Firecat if the tiedown method is your concern do what I have done for years. I run a strap left to right across the deck and tiedown to the trailer on each side.


I did, that's what snapped. Even if it didn't there was no way to fully hold it in place and the strap just from an inch or 2 of movement was eating into the rub rails. It did the same on my gheenoe. I'd rather have 2 straps coming off the rear like most better boats have, it's twice as strong and doesn't allow any movement.

As far as the poling platform is concerned if left in the same position and the sponsons extend beyong it then that would do the same thing as offsetting the weight of the motor and draft less. It is an advantage, but how much is the question, I would guess about an inch max, and right now an inch doesn't seem worth it to me.


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## Brett (Jul 16, 2008)

My thoughts on sponsons (floatation pods)...

A way to balance load on a hull by adding buoyancy to the stern.

A way to provide lift at a point that supports one of the heaviest mechanical parts of a boat.

A way to compensate for loss of displacement in a tunnel design.

Also directs the water flow to the lower unit in a tunnel design, by continuing the tunnel sides.

May or may not add to hull weight and costs, depends on build procedure.
I can see a way in plywood/epoxy/fiberglass construction
for both hull types (with/without) to work out almost the same.

Provides bump protection to the outboard from side impacts above the waterline.

Adds to the overall hull length, may or may not be a good thing depending on your needs.
Since the outboard extends beyond the transom, the sponsons don't really affect garage storage
as long as they don't extend past the rear of the outboard.

If added after the boat is titled, they are a way to bypass the USCG regs regarding hull length.


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