# Thinking of adding sponsons



## Capt. Hamp (Oct 5, 2016)

I recently purchased a 15' Johnsen type skiff that I am starting to rebuild. I want it to float as shallow as possible, but I do not want to power the boat with only a 10 or 15 HP. The build itself is going to add more weight than I would like, so I was thinking about adding sponsons. Has anyone checked their draft before and after adding sponsons? If so, how much difference did they make? Any feedback is appreciated.


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## yobata (Jul 14, 2015)

There was a build thread where someone added sponsons to a Johnsen skiff, but they have not updated the thread with any feedback yet:

http://www.microskiff.com/threads/johnsen-skiff-rebuild-wont-even-look-the-same.17443/


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## Capt. Hamp (Oct 5, 2016)

yobata said:


> There was a build thread where someone added sponsons to a Johnsen skiff, but they have not updated the thread with any feedback yet:
> 
> http://www.microskiff.com/threads/johnsen-skiff-rebuild-wont-even-look-the-same.17443/


Thanks. I will follow that build


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## CurtisWright (May 9, 2012)

Not much. Depending on the size they will offset 50lbs or so.


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## DuckNut (Apr 3, 2009)

Get your calculator out and calculate the added surface area and translate that into draft numbers.

As Curtis said...not much


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## Vertigo (Jun 3, 2012)

As noted above, sponsons are not going to reduce draft that much when floating level. What they will do is float the stern a little higher when getting out of the hole and allow getting onto a plane quicker and in shallower water.


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## WhiteDog70810 (May 6, 2008)

The problem with narrow hulls is that it is tough to add sponsons with enough displacement to support any significant weight without making them obnoxiously long. If you make them long enough to support the weight of that big of a motor, it will steer like a train.

Nate


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## DuckNut (Apr 3, 2009)

In addition if you are off 1/4" when you put them on you could make the bow of your boat plow which causes terrible bow steer.


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## jonrconner (May 20, 2015)

I don't know the chine width of your boat, but, if it were 4' and you need 20" between the sponsons which can be 16" long if you want steering, each is 14" wide, at 4" draft you get about 1 cubic ft of displacement, 64lb, at 6" 96lb, so it it is not inconsiderable. The challenge is to make them in a fair straight run off the the existing bottom.
JC


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## firecat1981 (Nov 27, 2007)

Don't forget to add in the 20-30lbs of material it will add to make them structurally sound. You have to take that away from the total positive effects. You will also need more room in between then as you go up in HP so 20" may not be enough. As said they work well on wider boats, but not great on smaller skiffs, you wold get a better effect by balancing the load.


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## Rollbar (Oct 20, 2016)

Vertigo said:


> What they will do is float the stern a little higher when getting out of the hole and allow getting onto a plane quicker and in shallower water.


Question, if the sponsons help lift or float the stern, wouldn't bigger trim tabs fixed, or, do the same thing when getting out of the hole/pushing down, thus lowering the bow to get on plane faster or am I missing something?
Thanks,


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## firecat1981 (Nov 27, 2007)

You are correct, the sponsons act like a fixed trim tab when running and getting up on plane, and yes in that respect the trim tabs can have the same effect. The real reason for sponsons is to offset motor/push poler wieght when at rest. I'm sure you have seen pictures of guys standing on the platform where it looks like the rear is about to swamp, and if alone the nose is up in the air. A heavy outboard combined with you standing over it is a lot for a smaller boat. When running or trying to jump up on plane, tabs combined with a well balanced boat are better imo.


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## Rollbar (Oct 20, 2016)

Ok got it thanks.


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## Vertigo (Jun 3, 2012)

If sponsons are correctly installed they will have little or no effect when running. Trim tabs, OTOH, will have little or no effect when at rest or moving slowly. When getting out of the hole at slower speed, the boat is trying to climb over it's own wake, thus the stern tends to submerge. Sponsons will float the stern higher, and thus have an effect similar to trim tabs. As noted above, sponsons also keep the stern from sinking when there's lots of weight aft at rest.


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## Smackdaddy53 (Dec 31, 2012)

I built an aluminum skiff that had sponsons AND Lenco trim tabs. They worked great and I could still pole 8" with a 90 on her and I'm 220#.


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