Sailboat Keel design

[bfraga]

I need information in reference to the performance of my boat, which as I understand from the Beneteau people it was designed by Berret Racoupeau, but they didn’t answer any of my mails. The boat is a Moorings 38 from 1991 (also called first 38’5’’).

Actually I’m doing extensive cross Caribbean sailing, so she sail a lot in open waters and I’m experiencing problems with her handling that have the tendency to jump around, and go to the wind with small wind changes. I have worked reducing the sail area, making a reef, but the boat performance is dramatically reduced.

So mi question is, if that problem could be diminished by changing or modifying the small keel that the boat actually has? If it’s so I’ll really appreciate any suggestions and ways to fix it.

I realize that a shoal keel is excellent for island cruising as for anchoring in sandy shallow waters, so mi first approach will be to keep the old keel and adding wings, if that is possible or not, or you have a better solution I’ll appreciate your comments and suggestions.

I’m posting an attach file of my own wing design, that need to be finished because it’s based on proportions, I also need to define the wings section.

Sincerely

 

[tspeer]

Why do you think the problem is the keel and not the rudder? If the keel is stalling, I don't see how that would make the boat head up - it would sag to leeward but if anything would tend to turn down a little bit. But if your rudder were stalling or ventilating, that would cause the symptoms you describe.

I'd look first at having the keel and rudder accurately faired to their intended profiles.

Slapping a winglet on a keel is not a slam dunk to make a net improvement in the boat's performance. The winglet itself adds drag, so it has to be shaped and twisted so as to reduce the keel's drag more than the winglet adds.

You've shown the winglet adding depth to the keel. I suspect you'll get as much benefit by simply extending the keel to that depth. In other words, mount your "wing" straight down!

 

[dougfrolich]

I with the response above. I would add that the real problem is most likly caused by the boat rolling out too much as it heels due to the very full aft sections, I would bet that the rudder will start to "suck" at slight heel angles, since reshaping the hull is not a likely solution, a more effective rudder could be an answer, as well as lowering the CG to make the boat stiffer to limit heel angle, or both. Or you can just live with the bluewater limitations of the design, and enjoy the benefits of it as an Island hopper. Enjoy the Carrib.

 

[Ian Ward]

The description actually fits having too little directional stability and poor hull balance when heeled.

1) Most new boats these days have the centre of of lateral plane well forward. Let the helm go, even when motoring and they quickly go round in ever decreasing circles. Actually they steer much better going backwards.

Moving the lateral plane aft is a good fix..but perhaps not so easy to do as it affects rig placement as well.

2) Poor hull balance results in the boat rounding up when heeled. In strong puffs of wind this give significant weather helm. You may not feel this initially because the rudders these days are usually balanced with area ahead of the stock to neutralize this bad feeling..no-one likes to feel heavy weather helm..so they hide it!

Poor hull design results in this misbalance and weather helm. It is almost impossible to fix with minor keel alterations.

Basically these boats are not so good for the purpose you have in mind. I am sure there are a few good older cruising designs which far better suit your needs, but be careful not many are truly balanced.

I can steer you in the right direction if you need a truly balanced hull design.

 

[Guest]

Why couldn't you move the CE of the sails forward slightly. This is what happens when you reef the main and leave the head sail up. I would think that moving the forstay mount forward about a foot would reduce weather helm. I realize that it may induce some lee helm in light winds but would that really matter?

 

[Guest]

That is the way most boats are set up these days.

This is exactly the problem which means the helm changes and the boat rounds up whenever you get a gust. It also puts a lot of load on autopilots, draining the batteries etc and is not a very satisfactory situation for long voyages as you need to concentrate on the helm all of the time. It is not a nice feeling to sail that way either.

In a well balanced boat, you can leave the helm for several minutes at a time, the boat sails itself, even with gusts and at all angles of heel.

This is not a dream, I have such a yacht, it is just that most off the shelf designs these days are poorly balanced!.

 

[dougfrolich]

I agree that many of the off the shelf boats are poorly balanced. It is a shame. For sailing is a most enjoyable experience, but only truly realized when the boat properly designed.

 

[Guest]

I suspect this is because none of the designers know how to make a balanced hull which goes straight at all angles of heel!

Hardly any aof the text books cover this, and some of the new ones ignore the issue altogether.

 

[SailDesign]

Guest says (from behind a mask of anonymity....):
"I suspect this is because none of the designers know how to make a balanced hull which goes straight at all angles of heel!"

Pppppfffttttt!!!!!!! On behalf of designers everywhere...

 

[Guest]

Lystergram

Run a Lystergram. This test is quite easy, and unlike a spiral, can be done on a sailing yacht.

This would show if the boat is unstable on course or not, so you could either make the rudder larger or look into heel/yaw cross products instead.

 

[dougfrolich]

I think many boats designers are forced to fit too much into the back of thier boats by marketing pressures. This results in the unballanced hull forms, look at the Hunters and the like. There is no magic to designing a balanced yacht, but it does appear that some is needed to sell one to the entry level buyers.

 

[Ian Ward]

Hull Balance

Firstly, there is certainly no intentional anonymity, it is simply that even though I have registered, the forum did not recognise me. As a new user here, I was unaware of some pitfalls of the software. I have added my name above, so hopefully you recognise me from now on.

I am most interested in the response by Saildesign as I would be very interested to find any designer who has a method for determining hull balance. So far I have encountered none at all, including Americas cup designers I have spoken with.

By hull balance, I do not mean lead of CE over CLP, nor directional stability nor moment of the the heeled rig, each of which are easily managed and well known.

Rather, the hull balance I refer to is the intrinsic weather or lee helm generated as the hull is heeled. ie: take a hull without sails, heel it over and push it along. It should go straight at all angles of heel and all speeds if properly balanced.

I am aware of Admiral Turners metacentric shelf balance and also th etechnique of balancing wedges, but have found both wanting for short keeled configurations.

I have never heard of a Lystergram applied to yacht design and would be most interested to find out more about this and if it applies to the hull balance error I have mentioned. Could you please refer me to the information source on this. It is not mentioned in any of the standard yacht design texts I have reviewed.

 

[SailDesign]

Ian,
I was merely responding in juvenile manner to your "I suspect this is because none of the designers know how to make a balanced hull which goes straight at all angles of heel!" comment, since I happen to be one.
I would strongly suspect that there is no method currently known to do this, but there are plenty of designers out there who have developed hulls that have that kind of balance, and by selective "evolution" of the shape have managed to keep that quality from one design to the next.
I am not talking about Clorox-bottle shops like Hunter, where the boats are designed in Marketing, but real, genuine, honest-to-God designers.
Steve

 

[Guest]

This idea of determining hull balance is interesting. I would guess that a hull that is symetrical longitudinally and transversly and having a gc dead center of the immersed sections would give a balanced effect at all angles of heal. I suspect, however, that increased speed will also have and effect on balance. It seems that if the above were true it would be possible to create a furmula to predict balance..No?

 

[jonathan]

Originally posted by Guest
I would guess that a hull that is symetrical longitudinally and transversly and having a gc dead center of the immersed sections would give a balanced effect at all angles of heal.

I don't really know what you mean by that, but the underwater shape will not stay the same in these conditions. Only a revolution body has that property, as far as I know... I attach a drawing I did quickly. It is a symmetrical hull as you suggest. You can see that when heeled 15° (top image) the lines develop significant assymetry.