SPONSONS=Porpoising??? input please!!!!

[seaARK03]

does any one know if sponsons on an aluminum boat create violent porpoising, i am 19 years old and have almost completed my drem skiff, it is 16 ft factory but now it is roughly 17'2'' with the sponsons on the rear. my problem is my skiff radically porpoises @ 27mph and it is very frightning, i can eleminate it with the motor completely trimmed down but then my engine dosent run well at all and then stalls, i have abit of weight aft and am considering adding weight 2 the front, but i dont know how much can someone please save me from this bopating nightmare???!!!

[nevd]

Hello,

In themselves sponsons don't cause porpoising.

Please therefore provide more details of boat and motor eg
Boat weight and beam and transom deadrise
Motor weight HP and type
Propellor type and pitch
Sponson width and shape
Distance sponson is below hull
Confirmation this boat did not porpoise prior to the sponsons being added
Trim angle of sponsons compared with trim angle of keel and chine
Confirmation the front curved part of the sponson is well above water level
when the porpoising starts
Trim angle the boat runs at just prior to the porpoising starting

Regards,

nevd

[seaARK03]

The boat is an aluminum flatbottom 'jonboat' style 16ft seaArk factory (roughly 17'2" w/ sponsons)and the hull beam of 48", the width from freeboard to freeboard would be 64" @ the top. The boat is rated for a 30hp motor, i run a 40hp suzuki dt40 with stock prop, i would guess the weight of the motor roughly @ 200lbs, but that is a sheer guess, i have reinforced the transom w/ a custom built jackplate (utilizing much thicker aluminum weighing rougly 10LBS) plus i figured the displacement from the sponsons would ease the depth of the stern sitting in the water due to the increase in weight, that is why i added them. the sponsons are 18" long and 12" wide they r shaped so when the boat gets on plane they are supposed 2 ride out of the water. the sponsons dont go below the bottom of the transom (i hope they arent supposed to). the boat never porpoised prior to the sponsons being added, i would like to remention that there is ALOT of weight aft and not much at all fore. just before the boat begins to porpise the motor is trimmed slighly lowere than level & once i break 27mph the porpiosing rythm begins to begin. Im not sure of the deadrise degree on the boat but it sits real nice in the water with no one in it. another imnportant peice of information is: the entire boat has had new decks installed so it now has the same floor plan as your modern day skinny water skiffs i.e. hewes, pathfinder, egret etc. so there is a very large initial front fishing deck, gunnels with a side console, and a small rear deck wich holds 11gal fueltank, 6gal livewell and two batteries plus the operator. that is all the information that i know of this boat. if some of my terminology is wrong i will do my best to clarify- look forward to anyones input thank you, nate

[nevd]

Originally Posted by seaARK03

The boat is an aluminum flatbottom 'jonboat' style 16ft seaArk factory (roughly 17'2" w/ sponsons)and the hull beam of 48", the width from freeboard to freeboard would be 64" @ the top. The boat is rated for a 30hp motor, i run a 40hp suzuki dt40 with stock prop, i would guess the weight of the motor roughly @ 200lbs, but that is a sheer guess, i have reinforced the transom w/ a custom built jackplate (utilizing much thicker aluminum weighing rougly 10LBS) plus i figured the displacement from the sponsons would ease the depth of the stern sitting in the water due to the increase in weight, that is why i added them. the sponsons are 18" long and 12" wide they r shaped so when the boat gets on plane they are supposed 2 ride out of the water. the sponsons dont go below the bottom of the transom (i hope they arent supposed to). the boat never porpoised prior to the sponsons being added, i would like to remention that there is ALOT of weight aft and not much at all fore. just before the boat begins to porpise the motor is trimmed slighly lowere than level & once i break 27mph the porpiosing rythm begins to begin. Im not sure of the deadrise degree on the boat but it sits real nice in the water with no one in it. another imnportant peice of information is: the entire boat has had new decks installed so it now has the same floor plan as your modern day skinny water skiffs i.e. hewes, pathfinder, egret etc. so there is a very large initial front fishing deck, gunnels with a side console, and a small rear deck wich holds 11gal fueltank, 6gal livewell and two batteries plus the operator. that is all the information that i know of this boat. if some of my terminology is wrong i will do my best to clarify- look forward to anyones input thank you, nate

Hello,

I am confused about the sponsons. How have they extended the boat length and how do they come out of the water at speed - are they on the sides of the boat and not the bottom? If they are on the sides, what part of the sponsons touch water when the porpoising starts?

Can you please provide a sketch of the boat both in plan and section?

Regards,

nevd

[seaARK03]

Sketch Attached.

[nevd]

Originally Posted by seaARK03

Sketch Attached.

I would have called your additions afterplanes and not sponsons and that is what has caused the confusion.

The porpoising is most likely caused by the bottom shape you have put on the afterplanes. You should have either continued the planing surface back or alternatively had the bottom surface of the afterplanes well above the planing surface so it did not touch the water at high speed.

There are some problems with both approaches and if you want to discuss more, suggest you send me a private email where we can discuss actual dimensions and what you are trying to achieve.

Regards,

nevd

The boat design can be evaluated for it's porpoising stability index, but not with the information you've given. However, a couple of things that you've said stand out as being likely issues. To avoid porpoising, a boat's wetted length (on plane) must be sufficient to provide the boat with enough longitudinal stability, and the boat's center of gravity must be far enough forward. In addition, the shape of the boat's aft sections must be such that it encourages a clean disconnect of the water flow off the stern and chines. In other words, straight flat aft sections. Technically: straight and parallel buttock lines in the wetted length when on plane.

You mentioned that the boat has A LOT (your words) of weight aft and that the boat is a relatively lightweight aluminum boat. It is likely that your center of gravity is too far aft. If you can load up the boat as though it were going to be used and you have a hydraulic jack and a friend or two, you can likely find the center of gravity by trial and error (lift the boat off the trailer at various points, with people holding onto the boat, to try to find the balance point). The center of gravity should be at least one waterline beam forward of the transom, preferably more like 1-1/4 waterline beams forward of the transom. If the waterline beam is 4', then the center of gravity should be 4' to 6' ahead of the transom. You can move the center of gravity forward by reducing weight in or on the stern, e.g. lighter weight motor, move batteries or fuel forward, move your steering station forward, etcetera. Another way to move the "effective center of gravity" forward is through the use of straight trim tabs, preferably adjustable while underway (hydraulic or some of the new spring loaded types work). Trim tabs force the bow down, increase the wetted length, and otherwise make the boat behave as though the center of gravity were moved forward.

Next, be careful with those sponsons ...if they curve up near the transom as shown, watch the water flow off the stern when near your porpoising speeds. If the water flow is being lifted up rather than sliding straight aft behind the boat, then those sponsons are pulling the stern down. Opposite of the aforementioned trim tabs, this moves the effective center of gravity aft and will enhance the porpoising, and every time your bow goes up, the stern will go down and increase the effect. I can see how it could become violent. Left to me, I'd leave those puppies behind like a bad dream. If you can take these off the boat and give the center of gravity issue some thought as mentioned above, then you can try the boat out to see what kind of improvement you got. I'm betting that you can eliminate the porpoising or greatly reduce it. If it is not eliminated, then put the straight trim tabs on as recommended ...they should be skimming on the water when on place, not lifted out and not submerged (follow the installation instructions). Wedges or 'shingles' glued on the boat's bottom near the transom are to be avoided ...while they may help like the trim tabs do, they'll be less effective and only be optimal over a small range of speeds. Adjustable trim tabs are a much better solution.

Good luck,
Brian