Keel Placement

[Alixander Beck]

When designing a boat how is the position of the keel determined?

By position I mean to say how is it known where to place the keel on the length of the boat?

Is there a LWL ratio used ? or is it something relavent to the centre of gravity of the hull? is there a ratio to determine how far aft the rudder goes from the Keel?

Thanks

Alix

[usa2]

i dont know the explicit specifics of this, and there are others on this forum who do, but my simplified version of what others know more about than I would be that keel placement would be determined by the center of the lifting force produced by the keel, and the center of weight on the hull so it isnt bow-up/down. The rudder placement is usually determined by when you can get sufficient leverage to turn the boat and be able to fine tune the course but not so far aft so that the boat is slow to react to it.


Im sure others will chime in with a better more informative version of this.

[jehardiman]

You can put the keel anywhere on the underbody (including aft of the rudder). All you have to do is make sure you get a proper balance/relationship with all the other players (i.e. CG, CB, curves of form, instant center of laterial resistance, instant center of effort, yaw inertia, roll inertia, pitch inerta, stability indices, etc.).

[Skippy]

Interesting point about a forward keel nosing the bow up. Only when heeled of course. I think keel placement is often constrained by the position of the rig, which can't be moved very much since it's so big.

[Wynand N]

I usually balance my hulls with the keel and ballast when doing weight calculations. The last weight to be entered is the keel and I move this forward/aft until the LCG of total mass is in alignment with the hull's static LCB - simple as that.

[Skippy]

A lot of designs seem to have ballast located forward in the keel. That means it can't be quite as low as if it could be distributed through the entire length of the keel bottom. That has me interested in bowsprits as a way to line up the keel and ballast better.

[Wynand N]

Skippy,

The only difference a bowsprit will make, is to lengthen the lead with will correct excess weatherhelm.

For the uninformed; "lead" is the amount the centriod (CE) of the sail plan leads the centiod (CLR) of the lateral plane.
CE of the sail plan is the total of main and 100% fore triangle. CLR of the underwater profile includes keel and usually excludes the rudder (although some designers with their particular designs include the rudder)

Lead is expressed as a percentage of the waterline.

[jehardiman]

Skippy, with unstayed rigs and wingsails, rig placement is basically irrelavent.

Historically, boat/ship design is very conserative. You copy what works (hulls and rigs) and only make minor changes. Out of this we have developed "rules", including one for lead of the CE from the CLR. On the other hand, one must not slaveishly follow lead rules. Lead rules are based upon the fact that most boats are "boat" shaped and most rigs are similiar in shape and design. Modern hull and rig forms greatly change what some of the old rules would say is the proper lead. Applying a lead without understanding why that lead is used is poor practice and may cause a cranky hull (i.e. some of the extreme ULDBs). It would be better, Alixander, to get a good book on the subject (I recommend The Aerohydrodynamics of Sailing by C. A. Marchaj but there are others) and be fully versed in first principles. With this grounding, you can make an informed decision about the relationship between CLR, CE, and instant center.

[Skippy]

Wynand N: The only difference a bowsprit will make, is to lengthen the lead with will correct excess weatherhelm.

Hi Wynand. That's my point, if the lead is already enough, then you can scoot the keel forward a little to fit the ballast better. But now I see the comment about the boat nosing up too much with a forward keel, so maybe that explains why it's not done more often.

jh, I don't understand how you can calculate helm balance without including rig position, or what difference it makes whether it's stayed or a rigid wing. Oh wait, I see. You mean the lack of stays allows more room for adjustment.

[safewalrus]

I rekons the best place for the keel is on the bottom of the boat! if 'tis at the top you has got a problem -----you is upsidedown!

Totally irrelavant I know but it lightens the thread up a bit! And with all the grim news over the past few months anything that does that is to be applauded!

[Tim B]

In order to balance a yacht you have a number of factors to consider, among these are the center of pressure of the sails (ie. the "point the lift acts through") and the similar center of pressure for the hull. You realise that the hull is lifting too, of course. The small problem here, is to get perfect balance at every condition we would need a moving rig, some damn clever computer software and enough computers to sink the boat. Generally, this isn't a viable option. so we have a rule that says that the GEOMETRIC lead should be 8% to 12% of the LWL and guess what, it works pretty well.

Now, jh, if the boat isn't "boat-shaped", then we need to run a full cfd analysis on it (or a panel-method at the very least) and work out exactly where the centers of pressure are. This is a long job to set up and would take a long time to run, even in parallel. Most people (and some designers) don't have the skills or software to do it, so that's why we go on basic rules. Incidentally, as the lead changes, the hydrostatic conditions will change too, so it is a linked problem. What you can't say is that the rig placement of any rig, on any hull is irrelevant. It is very relevant and the boat will handle like a pig if you get it wrong.

Back onto keel placement... I use a similar system to Wynand. Use the keel to help get the weights right I also try to put the CG of the lead in line with the torsional center of the fin. Sometimes it's not possible, but you can usually get close and that reduces the loads a bit (and makes the stressing analytic for the keel which is much quicker). What you have to remember is to keep plenty of volume forward to "carry" the keel at a level waterline. ie, put the keel CG under the CB of the hull if you can.

Tim B.

[Karsten]

I'm not an expert in this field but I would put the keel somewhere at the bottom of the boat so that the centre of gravity is right. You can move the effective CLR anyway by changing the angle of the rudder. I don't think that you will have problems if the rudder is big and well balanced. Besides that you can move the CE by raking the mast or by trimming the sails.

[Alixander Beck]

Thanks for all the input and ideas.
I have wondered for a while why keels are not move moveable on a hull. Aside from Canting keels, they pretty much seem to be built in their place permanently.
I am working on a design that would allow a high aspect keel to be moved forward and aft on the hull and as I have not seen this design before I wonder why?
I can only conclude that it is because it is a redundant design feature for a boat. However for my personal applications I find that it will be useful.

[Tim B]

Basically, it's the first place the boat will leak. being as it is, pretty much at the deepest point.

Karsten, what you end up with then is an un-balanced boat which will require either weather or lee helm. this is very slow indeed, because of the drag produced by the rudder. It can also make the boat difficult and dangerous to handle as well as introducing cavitation problems much earlier.

Tim B.

[Karsten]

I crunched some numbers. I assumed that the keel area is 70% and the rudder area is 30%. The keel is placed at 50% of the waterline length and the rudder is placed at 0% of the waterline length. The drift angle is 5 degrees.

If the rudder is midships the effective CLR is at 35% of the waterline length. 2 degrees of weather helm would move the CLR to 31% of the waterline length and 2 degrees lee helm to 40% of the waterline length. A rudder angle of +/- 2 degrees would move the CLR by 9% of the waterline length. If the size of the rudder is increased in comparison to the size of the keel the effect would be larger. I neglected the effect the keel "wake" has on the rudder. The wake will reduce the lift on the rudder. Therefore you need a greater rudder angle to produce the same amount of lift which increases the drag of the rudder.

Having some weather helm could actually reduce the overall drag if the aspect ratio of the rudder is larger than the aspect ratio of the keel. The rudder would be more effective in generating lift and therefore the total drag decreases if the lift on the rudder is increased. Obviously there is a limit because if the rudder angle is too big the lift points increasingly backwards which increases the drag. My guess is that the optimum angle is somewhere between 0 to 5 degrees weather helm. If the rudder is badly shaped it should be better to have some lee helm. If the lee helm is equal to the drift angle the lift on the rudder is zero. That is not good because you just drag the rudder through the water.

Cavitation should not be a problem. We are talking about sailing upwind. The rudder will STALL if the drift angle plus rudder angle approaches 10 degrees.

My point is: If you placement of the keel in relation to the rig looks right you shouldn't have problems with the CLR - CE placement. It's not going to be out by 9% of the waterline length and you can adjust the CE with some sail and rig trim anyway.