Jump to content

New top speed....


Recommended Posts

Terrapin hit a new top speed of 18.5 knots today.  I see 20 on the horizon with some professionally built sails.   The leeward hull was planing nicely, with no hint of diving.

 

I will say that she gets a bit squirrely up above 18.  The rudders don't seem to be ventilating, but the tiller is very light and I feel as though any laspe in concentration might bring a sudden turn downwind into a jibe.  

 

Can I improve feel in the rudder at high speed by imparting a bit of toe-in?  Or is that too draggy? 

 

I was running one dagger board in the starboard hull.

 

Here is a bit of video:

 

https://youtu.be/7vBKJrrUD8w

Link to post
Share on other sites

Looking good! Nice and tidy without too much to trip over, exactly what I'm aiming toward! ;-)

 

Seems I have something to look forward to - So far I haven't left single figures.

 

Next weekend things should be all go. It's the last big karate event for the summer on Sunday, then I'm free to go sailing!  :thumbup:

Link to post
Share on other sites

I was spending time at the other end of the speed spectrum today -- had problems reaching five knots. This was possibly the last sail of this season. The temperature had dropped to 3 C when I got back to my apartment.

 

Very nice, calm and quiet as I was all but alone on the water.

 

Tiller gets very light above 18 knots? That's odd I think. Do you know what wing section is used for the rudders? Are they balanced in any way?

 

/Martin

Link to post
Share on other sites

MartinRF,  When you say balanced, are you talking about how far forward they are raked?  

 

The rudders are supposed to be Naca0013 according to the drawings, but the leading edge does not match that profile.   So I'm not really sure WHAT the builder built.   I suspect he built a profile that SEEMED workable, but was easier to build than that 0013 with it's curved leading edge.  At about the 30 second mark, you can see the profile of the rudder in this video:

 

 

Martin - I agree it is odd.  I was getting a bit nervous, because I thought the boat was gonna spin out or crash jibe.

Link to post
Share on other sites

I'm no expert here, especially with bicycles, but it could be possible that is not the rudder at all. It might be worth having a look at how the water leaves the board. If thats creating a vortex, it could be that it is travelling back as far as the rudder at 18 knots.

Only ever experienced it once (on a mono) and we all had massive smiles on our faces hauling along on dead flat water then the helmsman let out an expletive. End result about 2 seconds later was gybing at 20+ knots.

Link to post
Share on other sites

Crocket,

 

It is entirely possible.  We were pretty far off the True wind as you can see in the top video clips.   I can see how turbulence coming off the dagger might be fouling the flow over the rudder.

 

I had only one board down and had both rudders down...

 

If that's the answer, maybe I'm pushing the limits with this boat and it is trying to tell me something?  I certainly felt like she has more to give, once I break down and get her a new suit of sails.  She felt solid in all other respects...

Link to post
Share on other sites

Yes - One reef.   True wind 18 gusting well maybe 23 knots ( I don't have instruments, but that's my guess.)

 

Freedom - Have you had your boat speed higher than 18 knots?   Does this condition worsen?

 

I've had a similar issue on the Hobie 16 up around 18 knots, but I resolved it with a bit of toe-in.  The F28-R I crew on has a rudder problem at 18 to 19 knots as well, though I believe that to be a VENTILATION problem on the stock rudder.

 

I might ask around bit more, and try to adjust the toe in this weekend (if the wind is up!)

 

All this could be just the vagaries of a home built boat.  It has never been professionally tuned or anything.  I know for sure I don't want to crash jibe and injure someone....

Link to post
Share on other sites

MartinRF,  When you say balanced, are you talking about how far forward they are raked?  

 

The rudders are supposed to be Naca0013 according to the drawings, but the leading edge does not match that profile.   So I'm not really sure WHAT the builder built.   I suspect he built a profile that SEEMED workable, but was easier to build than that 0013 with it's curved leading edge.  At about the 30 second mark, you can see the profile of the rudder in this video:

 

 

Martin - I agree it is odd.  I was getting a bit nervous, because I thought the boat was gonna spin out or crash jibe.

 

OK, here is what I think may be going on after having looked at the video above. I see you use the same 'geometry trick' I do for balancing transom hung dagger-board rudders. There is really only one mostly aestetic difference: In my case the rudder blade is vertical and the rudder axle is raked.

 

Watch the attached drawing: The  left part shows the original design from Tennant. The right-hand side shows what I have had since 25 years or so. I have reduced the distance between the centre of effort and the rudder axle. Your geometry does the exact same thing.

 

The right-hand side also shows a potential problem with this design: If only the lower part of the rudder blade is active (disturbed flow around upper part, transom starting to lift out or whatever...) the centre of effort moves down and may eventually cross the rudder axle.

 

You can shape the rudder blade plan form to avoid this by raking the lower part of the leading edge backwards. New, deeper rudder blades designed like this are in  their planning stages -- since several years :-)

 

/Martin

Rudder_axle_variations2.png

Link to post
Share on other sites

We had momentary speeds of around 21  ( mainly involuntary). 

 

Force of velocity is exponential. At top speed, Freedoms nose comes back out of the water, rudders are actually deep in water most of the time. You get the feeling you are on a raft which  starts vibrating and bouncing over the back of waves. Steerage comes and goes. Water  squirts out of the rudder and centre casing and the hulls flex a lot .  A long trail of white water behind the sterns and lots of spray everywhere. 

 

Consistently sitting on around 17 knots is a lot happier for Freedom. Some of the 8.5s that  have less rocker probably have a higher top speed. 

Link to post
Share on other sites

Another factor I think is rudder blade twist. The leading edge start to 'gybe' through the water so it loads one way then the next. I have had this same thing on many smaller boats at high speeds but mostly SR26 and my GBE both capable of high speeds..

I broke 2 rudders on my GBE before building 2 new and very strong somes which helped a lot. the first set of rudders used to twist really bad and then eventually self destructed. I also found it while getting towed at about 17kts one day with no effective helm load.

The centreboard was also very critical to the helm load.

 

I agree with you Adrian, above about 18kts boatspeed I used to start feeling like the old girl was going to self destruct. 

 

Here is a video of us taking the youth training program out for a blast.

 

Link to post
Share on other sites

Thanks to everybody - Lots to think about there.   Martin - I hope you build your new rudders, and I am very interested in the outcome!   I will be doing some experimentation also.

 

Freedom, I hear ya.  The steerage "comes and gos" is definitely something I'd like to fix.  

 

We felt the rocker in action. I really does plane nicely.     My wife, sitting on the downwind hull said she had the sensation of flipping over "backwards" when we were really moving.   The windward hull was generally hitting the water about 1/3 of the way back from the stem.   And the leeward maybe 1/4 of the way back.

 

I wish now I'd had a GoPro watching the rudders.....

 

B00B00 - First of all that video was the thing that pushed me over the edge last year.   (Part \of the reason I own this boat today!)  Anyway, food for thought on the rudder twist.  Mine are built to Tennant's plans (internally anyway.)   Central I beam etc.   Hollow.   I know this because I broke one earlier in the year, at maybe 14 knots in a jibe (the repair gave me a look at the internals.)

Link to post
Share on other sites

Another factor I think is rudder blade twist. The leading edge start to 'gybe' through the water so it loads one way then the next. I have had this same thing on many smaller boats at high speeds but mostly SR26 and my GBE both capable of high speeds..

I broke 2 rudders on my GBE before building 2 new and very strong somes which helped a lot. the first set of rudders used to twist really bad and then eventually self destructed. I also found it while getting towed at about 17kts one day with no effective helm load.

Yes, bending and tosional stiffness are important, especially if the blades are raked forward: Bending will induce angle-of-attack increasing twist...

 

/Martin

Link to post
Share on other sites
Guest Ketchup

Food for thought.

 

If you Can still steer the boat when the helm goes light, and its not wanting to steer itself off to windward/leeward, then all is ok.

 

In other words thae rudders are working well with flow over both sides and no hydrodynamic seperation.

 

If the helm is loading up" prior to the "light feeling" (when you have no steerage) then take note what it Is doing. If it is wanting to go one direction prior to this lightness then this suggests separation of laminar Flow on one side of the rudder/s.

 

Rake of rudders is worth investigation. To far forward can make this lightness/control issue worse.

 

While the forwrard rake may feel nice upwind and downwind up to certain speeds it can result in loss of controll (bu no physical loading) of the rudder/s

 

Looking at the chord/shape/length of those rudders tells me that the most likely cause is they are not the correct shape/size (in relation to the boards) to have influence on control at higher speeds. But they would be great up to those higher speed.

 

If you think about rudders (or Even plane wings) logically you will see that. more "width" allows water (air) to say in contact longer and be influenced by the "shape" better. But the draw back to this better control is drag and load.

 

Just look at planes with wide (chord) fat wings versus thin sections. They are all designed to control lift and weight at different speeds.

 

Theoretically The faster you go the smaller the wing / rudder you need to maintain control... to a limit (where shape is important)

 

Another thing to consider is the balance between rudder and dagger boards and the influence they have on each other.

 

At certain speeds the centreboards (if they are down in this situation and depending on size /shape) can have more influence on the Boats control and direction than the ruddes. Just think of a plane wing in relation to the tail rudder when landing. The wings shape (flaps/flaperon... placement can determine speed/direction/controll / and balance a lot more than the little rudder on the back (which at low speeds does bugger all)

 

I would suggest theat big dagger boards would control a boats direction as much if not more that the small rudders on the back.... at high speeds.

 

It would have been good to see what happens at high speed without boards down.

 

Also worthy of consideration is the dagger boards shape and influence (airation/alignment/shape A) on the rudders. Rememberwith the boards down the give the boat a direction and very little leeway. with them up itS all on the rudder for control and direction. Maybe halfway down is a perfect balance?

 

Do you have anything In front of rudders that could "airate the water prior to the rudders? (Venturis, outboard wells?...)

 

Ne last thing is the rudder length. If your boat, at this speed, goes Nose down. Then it could be simply a lack of rudder in the water. Imagine how light your rudders would be with half the blade in the water.

Link to post
Share on other sites

Thanks for putting thought into it Ketchup.  Definitely the bows rode higher as speed increased.    They are pretty high once on plane, partly due to the rocker I think.    The rudder blades are fully in the water at speed.  

 

There are no thru hulls or anything ahead of the daggers or the rudder.

 

I Intend to raise those daggers at my next high speed opportunity.

 

wildfire7.jpg

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...