Need for rear sponsons on outriggers

[mitaccio]

???I am building my first outrigger and want to get it right. I see some outriggers that have no rear sponsons, and others that have them under the tub, yet others are outside the tub. What is the rule of thumb and how do I choose?

 

[Climate Boatworks]

There is no "rule of thumb" a lot depends on the design of the rigger, and the power in it.

Rear sponsons keep the back of the tub out of the water. It is a simple as that. When the boat is running they provide a bit of lift and some stability.

Also a lot depends on the power available. A high powered rigger will not need them. A low powered one will. A lifting prop will sometimes eliminate the need for them.

Basic rule is that if the boat needs them to run, use them. If it runs fine without them, dont even bother putting them on.

Peter

 

[Hammerhead]

I put mine under the tub because I believe you get the staights characteristics of a 3 point rigger and almost the cornering characteristics. I run my rear sponsons anywhere from 1.8-2.5 degrees AOA(angle of attack).

Maybe Tom or someone else could help you out with the side mounted type.

As far as no rear sponsons. I think this is a SAW application.

Some boats run three points where there is only on rear sponson.

These are hot in the straights, but you have to counter steer them when you come out of corners to get back into the straights. Makes for dangerous heat racing.

Hammer

 

[MikeP]

I have run mine(.12 .21 .84) with and without rear sponsons. I have found on mine that the boat is faster all the way around without. But as soon as the water gets rough they all get unstable when the front sponsons catch some air, the hull will flop side to side and becomes unpredictable. I found that rear sponsons keep the fronts flat. The wider apart the rears are the more stable it should be. Rear sponsons under the hull are stable enough for me. They are much eaiser to build too.

-MikeP

 

[TomMoorehouse]

The way I see it rear sponsons do lots of good things for you and only 1 bad. They can slow you down from drag. What can they do good for you? Well they can make you faster. A lot depends on your goals. If your into saw then no rears for sure. If your into fun or heat racing rears will likely be a good choice. Basically they improve the boats reliability. They will give better control of the boats ride attitude, they can help unload the prop, they will improve both straight line and cornering stability. They will improve your rough water running most of the time.

what type? well i have tried single center dual under the tub and on the sides. I prefer on the sides. my experience is that the single center can cause the boat to stuff. a lot of the eagle sg's would do it. I have never tried the "V" bottom center sponson to see if it is better. The dual under the tub rears are better for aerodynamic drag and are easier to make but for the most part non adjustable. I like the on the side rears because of the ease of adjustmant. I hold mine on with the foam servo tape. If it is not running the way you like you can easily at the pond change them.

 

[bubba]

well with my exsperience on outboard riggers all the back sponsons are on the side of the boat. They stick out [behind the transom] about 2 to 3 inches. If you want a very nice boat that flies........the phenix outrigger is for you. with a nice 3.5cc engine that sucker will burn it on the strait aways... ;D ;D

matt

 

[Ian Inverarity]

I like the idea of a centre sponson for low drag (it shrouds the stuffing tube and part of the strut) but all the ones I have seen have been very long, not like normal rear sponsons at all. It is my GUESS that it is the size of these sponsons that cause the stuffing problem. Has anyone tried a centre sponson which is more like the length of a normal under-the-tub type rear sponson? It requires an S bend type stuffing tube, but that is no big deal. I actually designed and partially built a boat with this type of sponson a few years ago, but didn't like other things about how the boat was turning out, and I never finished it.

Considering what others have said about two rear sponsons providing stability in situations wher the front sponsons are out of the water, I guess that the single sponson may not be ideal.

I agree with Tom's suggestion of two taped on side mounted rear sponsons. ;D

Ian.

 

[Don Templeton]

Well Guys,

Check out page 2 Project Riptide R-12 on this forum. I'm building one with a center sponson and "S" bend shaft. I guess, just because this is the way we used to run the old Crapshooters with a solid shaft and brass tube full of bearings in a wooden strut/sponson. However, we usually ran adjustable outside rears also and I have a pair just in case ;D

 

[Ian Inverarity]

Don,

I shall be interested to hear what your experimentation turns up! 8)

Here is a pic of the design I did for the single rear sponson 90 rigger. Note the S bend shaft and short rear sponson.

Nitrocrazed racing: Active variable geometry aerodynamics is legal for outriggers.

 

[Don Templeton]

"COOL" 8)

 

[Saugling]

Hey Nitrocrazed, what exactly do you mean with "Active variable geometry aerodynamics" ??? ??? ???

 

[Propjockey]

That means the faster it goes, the more the hull flexes! ;D

 

[Ian Inverarity]

Saugling,

Hehe ;D. It has nothing to do with what I drew there!

There are no restrictions on aerodynamics for outriggers. It has occured to me that for the faster boats it may be possible to use aerodynamics to counter the roll moment generated in corners. That is a rigger will want to roll out of a corner, but this is generally counteracted by the hook in a turnfin which tends to pull the fin down during side-slip in corners. By mounting the fin on the inside sponson, in corners it holds the inside sponson down on the water.

There are a couple of problems with this tho. If you try to turn right the turn fin starts to lift instead. Not much left turn can be maintained before the boat starts to hop or maybe spin. The other issue is the drag of the fin is on the far right of the boat, which has the effect of pulling the boat into the course, much like prop walk. A lot of what we often think is prop walk is actually turn fin drag! To counteract this people sharpen the turn fin on the outside only, shim the fin, run more toe in on the RH sponson etc.

If the fin could be purely vertical and mounted on the tub instead of the sponson it should reduce this offset drag. But a method of counteracting the roll moment is needed otherwise the outside sponson will dig in, trip and the boat will roll.

So I had the idea of replacing the sponson booms with a two element wing structure. The first element is the structural one to support the sponsons and would provide a small amount of down force. The second element would be like ailerons on an aircraft, coupled to the rudder.

Just an idea!

Nitrocrazed racing: More ideas than I have time to build and test.

 

[_laxmasta2039]

ian,

kind of like those curved metal plates on OB riggers, right? [i think i've got a pic of mine in the "OB hydro pics" post in OB board if you havent seen them before]

i think if it was done right it could replace the turn fin. that way there wouldn't be any drag, but the extra pushrod setup might not be worth it.......one of those ideas that has to be tried out!

now that you've mentioned it im tempted to try it on my OB rigger, the only problem is it already has them on both sides to keep the nose down b/c the CG is so far back.

Joe

 

[Ian Inverarity]

Joe,

Kinda like that in principle, but a bit more sophisticated. A full wing section for the first element, and the second element would be able to move with control. The winglets on your OB hydro are to keep the front sponsons on the water at high speed. My idea is to counteract the roll moment in cornering.

The principle is not to replace the turn fin. I am not sure that this can be done. The boat would still need the lateral force generated by the turn fin (although I have ideas how to generate the lateral force without side slipping the hull) to hold it in corners. My idea is to remove the hook from the fin and mount it close to the centre to reduce the offset drag. 8)

Nitrocrazed Racing: Shirley you cant be serious.....

 

[Propjockey]

<--- (how many?)

I'm very serious...and don't call me Shirley.

 

[Ian Inverarity]

PJ!

You are a God now! Congrats!!!! ;D

Nitrocrazed racing: He's not the Messiah, he's just a very naughty boy!

 

[Don Templeton]

" OH NO"

If Propjockeys a God I think I'll stop going to church! ;D ;D ;D

 

[TomMoorehouse]

hey another god.....

what ya think..... should i raise the god bar? how about 1000 posts till god

 

[Propjockey]

Hey! Come on now...I just made it and you want to up the antie?

Besides, I can't be a God -- I'm not perfect (Just really damned good! ;D )