Keel vibration

[Guest]

Hi, my father and i own a sport boat with a fixed T keel, we are still new to this boat so still trying to come to grips with it. My concern is if we are under genneker and start going above say 10 knots there comes a vibration through what feels like the keel, is this normal as i am used to Farr 1020s. When racing i engage the engine throttle in reverse.

Cheers Paul.

[Guest]

I hazard a guess here but seems unlikely it would be the keel. Are you sure its not the rudder?

[Battleship 100]

we have this on our sr26 rudder the vibration comes at about 10 knots but goes away again if/when speed increases, some people recommend putting a slight bevel on the trailing edge to git rid of the issue but it nether really bothered us.

[wheels 543]

Vibration can be common for any boat that starts to go faster than it's displacement speed. It depends on if the Keel is really the problem. Could be rudder, could be P Bracket, or whatever else is down in the water and it depends on why it is vibrating.

Any Trailing edge should have a flat edge. The width of that edge can be found in the Table of Foil Dimensions. The flat edge acts as a "spoiler" or more accurate description is that it breaks the drag by creating turbulence behind the trailing edge.

[Guest]

Vibration or cavitation?

 

Keel vibration is very rare.

 

Rudder vibration/cavitation will be felt thru the tiller.

 

Rudder vibration can come from loose bearing/s. cavitation from shape or tailing edge issues

 

Do you have any skin fitting or outboard well in front of the rudder that is sending disturbed (AIR FILLED) water past one side of the rudder?

 

If cavitation of either foil and you suspect the tailing edge or last third of the foils is the cause. try bevelling the training edge @45 degrees. this is a solution that can have great results if water flow is disturbed on one side.

 

The other issue, which is a major, is keel flex/ twist. If you have a t keel with a fin that is not centred or aligned correctly or constructed poorly and has lateral movement then this could vibrate.

 

is the fin a composite (steel?/foam/glass)?.

[John B 106]

I've a nuisance low speed rudder shudder on my boat that I haven't been able to kill yet, so I've researched some of the issues myself. I still don't know whether its from buffeting ( turbulence from something else) or something wrong with the rudder shape and or bearings.

There's good data out there on fluid dynamics and vortex shedding. ' karman vortex street' will get you some useful information on that ( if that is the cause) and the J105 site also had a write up on the same issue.ie high speed singing or vibration.( which is what your issue is sounding like to me.) The first fix for that being as said above, to shave the trailing edge at an angle as said above.There's pictures out there .

 

here ya go, something to start with.

 

http://www.teamakula.com/fluid_dynamics.htm

[Island Time 1,246]

If it's foils, then it can be caused by non symmetry. Does not take much. You could try a contour gauge

like this to measure the foils. It takes a while, and requires careful measurement, but the results can be interesting...

[waikiore 405]

Yep what John B says, we used to experience it regularly on my Pied Piper steel keel, at about ten knots, go a lot faster and it changes or goes away. this I feel was due to the dreadful profile of the keel.

There was a TV Doco in the seventies called the hum which was recently put up on this sight referring to just that noise on Geoff Staggs spencer 40 Whispers 11 .

I was interested in Hydrodynamics of foils back then and in fact modified my Murray Ross piedy rudder ( which was very good), to a shape suggested to me by an Aero / Hydro man working on shapes for Bert Rutan who in his spare time liked foiling cats, and in fact designed a great deal of the cat that beat our Big Boat challenge. With recent developments in the AC everything that he predicted has come to pass!

So don't worry about the noise, but you can change it by improving the keel shape and paying particular attention to the keel hull interface.

[waikiore 405]

It would be very worth while trying IT's suggestion first, cheap and likely to surprise you.

[John B 106]

Yes, thats what I have planned for my rudder issue over winter. Get it off and in the workshop.

[Guest]

yes profiling and fairing is best.

 

Use a grid line system to work to to ensure replication from side to side.

 

Generally the last1/3 of a foil causes most of the issues.

 

Hull / keel interface, as suggested, is a big area of concern and can actually effect the rudder if disturbance is big enough.

[Adrianp 120]

How fixed is your "fixed keel" - Do you mean fixed like in a keel boat or fixed as in its not a canter?

 

Are we talking a hum, a shudder or a wiggle? Some sporties are known to get a keel waggle that ain't much fun.

 

My Sportboat only got a hum when she was getting near 20kts but she had a fin on the end of the bulb which might have been there to stop such things.

[Crocket 12]

Probably totally useless info and old school thoughts but here goes.

When boats evolved out of the displacement era and into the lighter go fast boats, the idea for the trailing edge was to be squared off with a flat face about 3mm wide on the back edge. This was apparently the best way for the water to leave the foil without inducing drag. As the speeds increased through the years, this would create a hum at certain speeds. The belief at the time was that with the water from each side leaving the foil at exactly the same point and supposedly meeting some distance behind, any deflection, or lateral force would mean one side had a slightly higher flow than the other. This would mean water from that side would get to the meeting point first and push the small vortex to one side. The other side would then try to correct the problem and push back, creating a small vibration, or hum through the foil.

I remember with Gaucho, the hum started at about 11 or 12 knots. Some bright spark suggested making the square edge on the back at a slight angle (about 5 degrees I think), which would make one side the dominant side all the time.

Hum gone.

As I've noted, this is probably old school thoughts now, but maybe worth a look.

[Guest]

Probably totally useless info and old school thoughts but here goes.

When boats evolved out of the displacement era and into the lighter go fast boats, the idea for the trailing edge was to be squared off with a flat face about 3mm wide on the back edge. This was apparently the best way for the water to leave the foil without inducing drag. As the speeds increased through the years, this would create a hum at certain speeds. The belief at the time was that with the water from each side leaving the foil at exactly the same point and supposedly meeting some distance behind, any deflection, or lateral force would mean one side had a slightly higher flow than the other. This would mean water from that side would get to the meeting point first and push the small vortex to one side. The other side would then try to correct the problem and push back, creating a small vibration, or hum through the foil.

I remember with Gaucho, the hum started at about 11 or 12 knots. Some bright spark suggested making the square edge on the back at a slight angle (about 5 degrees I think), which would make one side the dominant side all the time.

Hum gone.

As I've noted, this is probably old school thoughts now, but maybe worth a look.

 

Spot on and my belief too!

[John B 106]

I really could have used an 'and' in that sentence couldn't I.

[Guest]

And get it off in the workshop?

 

I miss a bit of fur on the bottom .

 

 

say no more .

[Guest]

Last time we put the boat in the water, i was quite suprised by how much the keel swung from side to side we just transporting in the travel lift, even if you are moored in the marina you can feel what seems like the boat is pivoting on the keel.The boat we have is the Ross 8m called Cu.On Sunday i some what felt it hindered us from unsticking to begin to plane,will my concerns hinder our speed?

[Guest]

I sailed on CU for several years, the builders/owners and I were all apprentices together.

 

the keel was held into a tapered socket in the hull, and back then was stiff as all buggery. It may well be past time to remove it and reseat it in epoxy with new bolts.

we never had a hum issue at any speeds even after the weed cutter was fitted. the trailing edge back then was around 1/1.5mm and flat.

 

I would suggest that there has been some profiling lost along the way between sands and antifouling. and the section profile may well be asymmetric in the critical zone

 

personally if you don't have a weed issue I would remove the glass tube from the front of the keel fin and take the section back to its original profile

[Guest]

I should add - you should consider your foils as the underwater sails of the boat.

 

I consider them as, if not more important than a new set of sails, this also includes antifoul application/finish

[Guest]

A couple of years ago i remember reading an article about this boat hitting the bottom in Wellington ( over at Seaview?)

 

I think it then lost its Carbon rig about a year later.