FlexiTab drive

[masalai]

Hi Frosty and welcome back see you in Drivel as soon as I catch up - only 4 posts since I left to go to church

[Frosty]

Hi Marsh,--I very exited about it because I have a bit to go yet in so much as there was a tendancy on my part to be conservative with the cutting and I can cut some more off to get the parasitic drag to zero on the unused leg.

I also increased the aspect ratio of the rudder blade and made it removable. I did this on both sides, now that the cut was successful that part can be totally removed. I also went to a lot of trouble to make a wedge rudder as perfect as possible.

This brings in your point. My rotation is as normal, ie- right is right or to be correct STB is right. I was hoping for a little discussion with Brunello on exactly this point.

I agree with you about the rotation being better towards the rudder but for a different reason,--I was thinking that the pressure front from the rotating prop would be better managed if the rudder was there to deflect thrust rear. But Arneson and Flexi series 2 and 3 don't have such tunnels along with many others.
It is just a gut feeling that I have.

When I first fitted the drives I fitted the props the other way round and have to admit to feeling no difference in any way.

Before cutting the rudders I tested the boat with no rudders at all. This resulted in not so surprisingly being unable to steer the boat at all meaning I could not hold throttles open long enough to get onto the plane. I was expecting to be able to steer with throttles,--not so.

I am now unsure if the boat could not get on the plane because of the throttles not being held open long enough or the lack of the tunnel rudders giving some effect I am unaware of.

[masalai]

Is your top speed still the same or where?

[Frosty]

Well I assume it is much more. If it was 17 at 3400 and is now 19.8 at 3400 and the engines will pull 4200 I suppose it could be as much as 24-25 ish.

Im getting a bit out of my depth here but it appears to have brought the prop slip from 28% down to 16%. I would imaging to have increased economy also.

[masalai]

Speed freak - I think my current option will do less than 10knots and more? with all sails in a gale? -

[Frosty]

Damn I was hoping some one would have wanted a conversation on tunnel rudders and propeller rotation. Personally I think It very interesting, their must be something in it with a surface prop running close in a tunnel.

Would there be some advantage to the rudder being on the entry side of the propeller blade or the exit?

[masalai]

Well, what is your take - I was looking forward to something upon which to comment? - - seeing as how I am still a learner in this matter.... I feel both views expressed so far on location of remaining half rudder have merit and end with the same result....

How about taking your boat out fro a run and post a list of engine speed and speed through the water.... and gut feel on turning and handling response ?

[Frosty]

Oh cripes mas --take the boat out!!! Ive got to take the satelite dish off first and then all the plants on the dock. The missus will go bannanas.

Ille have to dive on it and clean it up,--maybe next year.

Once the good weather comes in (thats not today) I intend to beach it again and go the whole nine yards and cut some more having just the one rudder in the water.

At the moment I can unbolt the rudder blade and simple bolt it onto the other side of the tunnel. This is achieved with 8 countersunk high tensile 6 mm bolts. So Ide better make my mind up wich leg gets the rudder blade.

As it seems, the experiment although very labour intensive is a success it is unknown what the effect would be other wise.

You are aware of course that changing the rudder blade side would mean dismantling the hydraulics to accommodate for the hydraulic tie bar to be in compression between the rudder rams. It can be done --ive done it but it means all the pipes off from upper and lower helms and re bleeding. Not a nice job.

[masalai]

Why not make another rudder that has both sides removed to just above the water flow when you are sort of planing so it still gets some resistance from the upper half of the prop travel wash but not the drag? would that work?

[marshmat]

Oh cripes mas --take the boat out!!! Ive got to take the satelite dish off first and then all the plants on the dock. The missus will go bannanas.

Good point Frosty

It would be interesting to discuss the issue of tunnel rudders and prop rotation in more detail. Exam season is about to hit and so it'll be a couple of weeks before I can really take a good look at this though.

[Frosty]

No problem,-- Ill wait.