To sponson or not to sponson

Discussion in 'Boat Design' started by prof, Mar 29, 2007.

  1. prof
    Joined: Mar 2007
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    prof New Member

    I am designing a flats type, shallow water fishing boat to be constructed from welded aluminum and powered with 50-60 o/b. The LOA is 18ft and beam of 6ft with a flush deck and 4ft x7ft cockpit. This will be a non -tunnel hull with 2 degrees of deadrise at the stern and a relatively sharp v at the bow. Many contemporary designs for 17-20ft boats like this use a sponson-type of transom, which is basically a box shape cut out of the stern to move the outboard forward for extra transom flotation and I suppose, to keep the center of gravity at the midline of the hull for shallower draft.

    I do understand that the boat’s CG shifts according to how it is trimmed so let’s assume that all weight items like gas tank & batteries, etc. are balanced correctly. Since stern squat is a primary concern in this design, I am debating whether or not to use this type of transom and I’m looking for sage advice here.

    If I utilize a sponson-type of stern, wouldn’t I be sacrificing a few square feet of hull area to float the transom? Why not just use a squared transom and hang the motor on the back? What are the advantages in sponson-type transom designs? Seems to me that I simply end up with a 16ft hull that has sponsons.

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    Last edited: Mar 29, 2007
  2. ted655
    Joined: May 2003
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    ted655 Senior Member

    I don't see the advantage. Essentially it becoms a OB motor well. You also lose that much more cu. ft. of bouyancy, adding to the squat.
    Unless you need to acheive a specific length overall that includes the OB length, why add all the x-tra cutting/welding and lose all that bouncy. It looks "cool" maybe but offers less total capacity.
    I have a swamp skiff that is 19' 6", 6' wide with 17" sides. It is flat bottom, V bow with a rake. Fuel is built in in forward step that aids in stepping up to bow deck. I'm running a 50 Merk with no trim issues. It gets right up on plane. Helm is also more forward. My only issue is that it's a "wet" ride as a result of the flat rake/pointed bow. A little forward deadrise is definatly a good idea.
    Sorry shes's dirty, I'm busy with my other boat right now.

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  3. waikikin
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    waikikin Senior Member

    Prof, might have some advantages on a work boat or one that handles nets. Regards from Jeff.
  4. erik818
    Joined: Feb 2007
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    erik818 Senior Member

    I've been asking myself the same question so I read this thread with interest.

    I see an advantage with a recessed OB bay with respect to balance if you have tiller control of the OB. If you have remote control you may as well move the driver forward until you get the desired balance of the boat.

    Even though I like the looks of boats with sponson-type sterns, for me aestetics only wouldn't justify the complications in design.

  5. Colin Stone
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    Colin Stone Junior Member

    Prof, I added sponsons after I had finished the boat.The reason for this was to increase the bouyancy at the stern, as the boat has a narrow transom. With the Honda 90, live bait tank, battery and two fishing,it needed a little help.Works well.Colin
    With any luck there is attachments

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  6. Design_1
    Joined: Nov 2004
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    Location: Georgia

    Design_1 Water Logged Hack

    Prof, the sponson-type transom is debatable based on your needs. I have designed and built several Bay boats over the last twelve yrs, and some times I use them and some times I don't. I generaly decide my LOA based off of a stem to stern measurement and if I have room for more, I add sponsons for styling.
    We used them in the bassboat industry for styling as well as added floatation. We generally filled them with foam, but with the proper planning it can serve as additional storage, ie. battery, oiltank, boarding ladder..etc. The key to remember is to use them as "add on", and not to decrease your space. If you are absolute with your boat being 18ft; and you have already designed based off of your stem to stern reaching 18ft, don't notch for them. If you are still flexible at this point and you want added floatation, storage or steps, then by all means "add" them onto the transom. I would never cut into my transom to notch for the engine. This is just my opinion, and it is worth what you pay for it.
  7. messabout
    Joined: Jan 2006
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    messabout Senior Member

    There are signifigant structural advantages for a well designed notched (sponson) transom.

  8. dick stave
    Joined: Dec 2004
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    Location: MISSION B.C. CANADA

    dick stave Senior Member

    I have a 14 ft. flat bottom dory skiff with a 60" bottom. Originally had a 30hp Yamaha 2 stroke (122 lbs.) mounted on a 5" setback fixed jackplate.It worked pretty good,but, I was still honing my skills at prop repair.Bought a Yamaha 40/30 four stroke tiller jet (204 lbs.) and it really dragged its arse. I saw those "flotation pods" on one of those beavertail sites and built a set 18" long x 12" high and as wide as I could so the motor would still turn lock to lock.The jet is mounted on the same jackplate (with the foot about half exposed below the bottom)with an adjustable plate and a UHMW strip to the leading edge of the foot.I was pleasantly surprised when I discovered It would go up runs where the water is literally curling over the rocks.It exhibits a strange phenomenon whereas you can actually feel the stern lift in the ridiculously skinny stuff (I dont understand the science) but I get a s--t eating grin on my face every time I do it...
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