Rudder Fin Question

[J Solinger]

There used to be an OB maker or two that bolted the rudder fin on the side of the leg. It's something I haven't seen in a long time. What is the down side or up side of doing this?

 

[Grimracer]

prather

grim

 

[Christian Lucas]

Hi,

some years ago i played with such fin on this brass soldered unit .A outrigged fin give you some advantage to counter propwalk as you set the fin with an angle against the proshaft line .Also the side you mount the fin give different way of waterflow of the incoming water acting like a pre flowforming system in jetengines and they are moore efficancy as the flow in front of the prop is slower as the flow behind a propeller .

If you do like on my unit with screws you can adjust the angle and position from the center and you can change the size and hydroprofile like a wedge or elliptic symetric or clark-y .You will have many options.

 

[moparbarn]

Mostly used on ob riggers. allowed a thinnner, deeper rudder needed on riggers. not so good on tunnels, every time i've tried it the boat wanted to hook. tunnels seem to want less skeg/rudder, not more. a version of the offset rudder for k&b 3.5 lowers was included in the #8400 mod engine kit. it was intended for use with the prather spitfire ob rigger.

 

[J Solinger]

Mostly used on ob riggers. allowed a thinnner, deeper rudder needed on riggers. not so good on tunnels, every time i've tried it the boat wanted to hook. tunnels seem to want less skeg/rudder, not more. a version of the offset rudder for k&b 3.5 lowers was included in the #8400 mod engine kit. it was intended for use with the prather spitfire ob rigger.

Thank you. Thats what I wanted to know. Im finalizing a design for an fe ob. From a manufacturing stand point it would be easier to attach to the side. But, if it doesn't work well for tunnels Im not going to do it.

Christian, Thats cool! Did you get it in the water?

 

[J Solinger]

I was looking at the Prather mod kit. It looks like it offset the skeg by alot. Im looking at an offset of 3/16". Would this still have such ill effects?

 

[moparbarn]

Not sure, jay. i've only tried the prather rudder.....

 

[Christian Lucas]

What kind of effect do you search ?

Yes ,the little drive workes well with a 540 size electricmotor ,i build two 20 inch tunnel ,the red for the outbord and the white with an beltdriven Z-drive and inboardmotor steering with a ruder.

I show you also a super simple outborder i made only with a brasstube and strutbearing ,no fin .It was my first surfacdriven boat september 1980 having only a Octura 1.6 competitionpropeller works well see the pic ,turn ok.But the turnabylity was moore on the tunnel sponson .I build them with no V angle (flat) at the front and 12° V at the transom prevent the hull doing spinouts .

 

[Paul Renna]

Here's what we used "back in the day".

This is on my outboard rigger...

It was made by a company called "XXL" I can't for the life of me remember who that was.... But it worked GREAT!

 

[moparbarn]

The xxl line was from garcia mini marine. i still have some of their ob pipes.

 

[J Solinger]

Thats a slick arrangment,