Help with Keel placement when increasing chord lenth.

I have a boat with the “OEM” keel and bulb, that is fitted to a boat.
Being that it is OEM, the placement is in the correct position for the boat, and it sails wonderfully.
The boat I have is a Viper, designed by Bob Sterne. A photo shows the boat with the standard keel attached. Again, nothing wrong at all with this setup. I consider this my baseline for measurements.

My thread is respect to installing an “aftermarket” keel and bulb into the boat. The aftermarket keel has the same profile, but a larger chord lenth. While the photo shows that the lengh of the black carbon keel is longer, it will be cut to the same length as the “OEM” keel. My pond has depth restrictions, so installing a longer keel is essentially not an option.

My goal is to install an aftermarket keel in the boat. with higher chord legth (therefore increasing “lift” from the keel) while maintaining the same sailing properties.

The dimensions of the “OEM” keel are as follows:
16 3/8 fin lengh from boat to bulb
4" Chord at the root (under the boat)
2" Chord at bulb

The dimensions of the “aftermarket” keel are as follows;
16 3/8 fin lengh from boat to bulb (will be after it is cut)
5" Chord at root (under the boat)
3" Chord at bulb.

So essentially. The final question is, Where should the keel be placed, assuming a 1" increase in chord length?

A longer chord does not mean that you are going to get better lift, the profile together with thickness that ideally should be in the area of 6% of its chord, is what is going to provide the best lift.

Anyway, the question is how to find the right position for your new keel.

One non-scientific way is to mount the original keel and rudder on the boat and put it in the water without the sails on, ideally with no wind.

Then with a straight ruler or similar object, position it on the side of the hull near the rail & push the boat away from you evenly until the bow & stern move evenly away from you.

You will need move the point of pressure fore or aft along the hull until you find even side-ways balance point, this is the CLR line of the boat.

Another way is to fix a tight line along the boat centre line and have a small ring over the line with another line tied to the ring so that you can pull the boat toward you, either way it works the same.

Pop your new fin into the hull and then “move the fin” fore or aft until you have the boat balanced on this same point as your original fin. I have done is and works fine, but I’m sure there will be a more scientific way of doing it.

Being shorter fin you aware that the righting moment of the boat will become worse, if you using the same bulb weight that is ?

Cheers Alan

The “aftermarket keel” is 6% TC, The “OEM” keel is also 6% TC

As stated, the longer “aftermarket keel” will be cut to match the same length as the “OEM” keel. The only difference between the two will be a 1 inch chord length difference.

The bulbs are interchangeable and therefore identical.

In your first post you list the old and new lengths as 16 3/8 and 13 and 3/8. A typo??

Are you replacing the keel trunk or are you going to re-shape the new fin to fit the old trunk?
If it was me I would re-shape the fin(unless your old trunk leaks or something) lining up the 25% line of each fin, if that makes any sense.

There is a lot of “wiggle room” in a fin as regards to strength. I have cut away up to half of the part that goes into the boat with no sign of weakness.

YES! that is absolutley a Typo. I’m sorry. Both keels will be identical in terms of what will show under the water, except for chord length. I’ve edited the first post. Thanks!

Yes, I will be cutting away the fin at the top to fit into the keel trunk inside the boat. Where I make that fitting is dependent on where the keel needs to go fore & aft (To account for the increase in chord length)
The keel trunk in the boat is very strong, and not an issue. I’m not worried about the keel/keelbox strength after the new-larger chord lengh keel goes into the box.

With this new fin 1 inch wider will be helping to reduce lateral drift but you have to pay in added wet area !

I think the KIWI is right on the money. To make the boat react the same way, you should make sure you do not move the CLR. But, the changes are small here, so it should not be an issue. It is the relationship between the CLR and the CE of the engine that is of concern. Raking and jib pivot point can all be adjusted to compensate for small changes like this.


I’m in complete and total agreement here, and I’m not concerned with it at all.
I understand I’ll be paying in increased wetted area, and I’ll live with it. I’m looking for a the benefit of reduced lateral drift.

When this keel is done, this boat will have a total of 3 perfectly placed and balanced keels that accomplish different things. This keel will hopefully be the 4th of the set, and will accomplish something that the others do not. In my original post I only mention the one “OEM” keel as this is the correct setup if the boat only had one keel to rule them all.

This will be an interesting experiment. I would put a new bulb on the new fin so you can swap back and forth and see if there has been an improvement or not. Bob Sterne posts here occaisionally. It would be interesting to here his opinion. Bob???

Right. Now you are on the right track.

But increasing Chord length will change the CLR. How so?
To match the CLR of the “OEM keel”, which is the boat’s known good position… due to the above cause & effect relationship, does the keel need to be moved forward? aft?
Stay right in the same place, and let the overhangs go 50% on each-side?

Granted you can make the above mentioned adjustments in the rig to fine-tune the balance.
However, this is a fundamental change in balance on 1/2 of the CLR/CE equasion, and is permanent. I can’t move the keel around on a daily basis.

I need to find what will happen to the CLR given the only change is an increase in chord length.
Given that change, does the keel need to move fore or aft in order to match the existing CLR on the “OEM” keel setup.

I’ve ruled out different bulbs.
I am working with 1 single bulb, and the quesion of, “What does an increase in chord lenght do?”

Granted I have other bulbs that will be used at a later time, but I am working only with the “OEM” bulb as to avoid any confusion or complication.
The only changes that will be made is an increase in chord lengh

Since the CLR is just the center of lateral area then overhanging 50% on each side will keep it in the same spot. That said, I have read that the actual “CLR” moves forward when the boat is moving that is why I suggested matching up the 25% line. Maybe a compromise and a test. Moving a fin 1/8" or so is not a big deal. Just cut a bit off one end and glue some on the other. I have done this and it works fine. I’m sure there is a more accurate way of doing this but sometimes it’s just cut and try. Thats the “art” part of boat building :slight_smile: .

My initial un-sceintific thought is the same as yours… That it should essentialy go in the same place, but just be longer in chord length.
But, it’s only a thought. not backed by anything substantial.
Draw the art, and hope it comes out right.

Using sails as an example, If you increase the height, the CE will move higher.
There has to be a similar cause-and-effect relationship here. If you increase chord length. the CLR moves … X
Give that the CLR moved to X, the keel needs to move fore or aft to counter, and accomplish the same CLR as the OEM keel.

This is a nice keel and I refuse to cut & paste on it. So once it goes in, it’s staying. Unless it’s a disaster. But it’ll only be a disaster if I guess at it.
I know that the difference may not seem like a big deal, but If I can find a logical reason to place this keel I will.

To me the difference is position is important. Considering that on one of my “Classic” marbleheads, the difference between the boat porposing and submerging down-wind, and staying afloat can be a 1/4 inch adjustment in the mast foot.

Any inacuracies in the keel placement can be countered with rig adjustments, But if the boat performs with a certain known CLR, I want to try and match that rather than guessing at it if possible.

And, although I am the one mounting the keel… This holds true for any boat and is more conceptual than subjective to my boat…
On anything, if you want to increase the chord lenght, while keeping all other paramaters equal, what will happen?

Thanks all so far.

I must be a sandwich short of a picinic today sorry :confused:but why fix something that ain’t broke ?

Of course, you could try this to goof with clr…

Claudio and his carton trick…


Yo KIWI… “why fix something that ain’t broke?” This from the man who rewired his working boat on the way to a regatta? :slight_smile:

Because I have a nice keel that was going to go into another boat, but is collecting dust instead.
And, furthermore this keel that I’m discussing is going into a second viper I own… so I’ll be sailing with the same boats, at the same time. with the same bulbs, same rigs, and same everything except different chord length keels.

Don’t take my words as any sort of insultive please, anyone. I just realize that there are many, many, many factors involved with this… and I’ve eliminated them all down to 1 basic question so we don’t have to get off-topic.
The only factor that for all intents and purposes is changing between boats and keels is the chord length.

While this may be an interesting experiment, I am sure that you will find that the boat will sail better with the original fin. A wider chord will increase the wetted surface, obviously, but I highly doubt that it will reduce leeway.

The higher aspect ration of the current fin is more efficient at generating lift than the new wider one. The wider one may produce more lift at the same angle of attack, but at the expense of higher drag. The only time that the greater area may be of benefit is when you have no flow over the foils and are getting blown sideways, when area is the only thing that matters. However, that would hopefully be a miniscule part of your sailing time and it would be at the expense of speed when you do have good flow over the foil.

Bob Stern put a great deal of time and experience into designing that boat. He could have put any fin he wanted on it and he chose that one. That was during a time when everyone was going to narrower cord lengths. I would bet that he would have gone even narrower if he could use a material that had enough structural support to hold the bulb without bending or twisting. Water is very thick at our Reynolds numbers, lift and attachment are not our problems.

My vote (if I had one) would be to stay with the narrower foil that is on it.

Actually, if they are both 6% the thickness is changing too.

Yes, you are right, the thickness is different, with the longer chord length fin being narrower width, but both having the same 6%TC