finding bulb position


Ok got my new IOM, a wooden Goth XP built by a friend - it is very rigid and light - I have built the top and medium rigs and installed the electrics so all the weights are now in position. I now need to find the right bulb position on the keel fin.

I have been struggling to find time to bring my boat to the sea to do a flotation test. So I am looking for an alternative solution to find out the right position of the bulb so the boat floats on its line.

using an arrow shaft I have found the CG of the bulb. Now what are my options?

Should I remove the keel and roll the boat fully rigged up on the same arrow to find it’s CG and attach the bulb so the 2 CG align?

any other solution?

thank you for your help.

I am not sure that aligning the two CGs will automatically cause the boat to float on its lines.

I did my last scratch build by floatation in the bath tub. With the radio gear and battery in place, and a weight equal to the rig sitting on the mast step, and the fin taped lengthways under the hull, I floated the hull with the lead slung in a bridle under the hull. The bridle aligned with the marked cg of the bulb. I moved the bridle fore and aft until the boat floated on its lines, and marked the hull at the bridle. Out of the water, I set the boat in a jig with the bow and stern waterlines exactly level, and dropped a plumb line. With the fin in place, I moved the lead until its mark aligned with the plumb and transferred a mark to the bottom of the fin for use during assembly. You could probably ignore taping the fin under the boat, and just use the adjusted weights later to compensate for the absence of the fin’s floatation component. Same comment for ignoring the rudder.

The following article was published in the Canadian Radio Yachting (CRYA) magazine by Barry Fox, based on a jig designed by Lawrie Neish. I made one of these and use it for assembling boats, as it allow the boat to be held in position with the fin aligned and set the depth of the lead and the tilt angle and hold everything in place for glue up.

Hi Gilbert,
If this may help ! see sketch.
In principle he bulb positioning is done in the water with the full rigged boat ‘ready to sail’ except the bulb fixation.
The bulb is sustained as in the drawing below.
The static alignment shall take into account that the transom shall be lowered by few mm while the bow raise of similar amount. In principle, dynamically once sailing, the wind force will acts on sails and will re-establish the Waterline as per design.

With this method no need to know where are the LCB and CG since only the waterline is the guiding parameter !

It happen to me in the past to use this method with a fin blade simulation of same weight, see sketch

Gilbert, sorry if this is too late to help.

Claudio’s method is fine, but since you can’t get to water, aligning the CG of the boat and bulb is what you want to do. If your friend still has the plans, the CG will be indicated there. Carefully measure the CG dimension on your boat and line that up with the CG you found for the bulb.

If your friend can’t give you the dimension, check here:

I’m sure Frank would be glad to tell you the proper position.


thank you all and thank you for your contribution they each helped me with my problem.
we finally had a break in the weather and went last Sunday when it was low tides in the shelter of the club house to put the boat in the water and following Claudio’s advice the transom is a little bit lower than the lines and the bow equally higher.
I was talking to Frank alright - he’s a sound guy indeed.
The problem now is to drill the bulb. It’s a Dave creed one with low profile which I like. But obviously I need to drill vertically so the bolt epoxied in the fin keel go through the bulb and I can screw on the but at the other end. Gosh easier said than done. I marked the bulb for the hole position started to drill slowly but then the problems started when I think I hit the centre rod of the bulb. 2 broken drill bit later and a joly mess on either end of the hole.

Thinking back on another boat where I assembled the bulb to the fin I drilled 2 holes across the width of the bulb and fin and slided 2 rods across. Easier and it never moved. but here to get the most of the 42cm of depth I need to position the fin on top of the bulb with a little recess forward of the LE to allow the bulb to cant at -2 degrees and I might drill a small hole towards the back edge of the fin to epoxy a pin which will go in the bulb to stop the bulb twisting on the fin.

Not an easy thing to do though… rather messy and I don’t talk about lead or dust here…

find it difficult to do a neat job which it requires…

Hi Gilbert,
according to my understanding should not be a problem at the condition that you can slowly drill the existing hole with a larger mesh, let say 8mm.
Once the broken mesh is extracted, you can restore the bulb by melting some lead and pour it inside the hole. Hole may be larger if you prefers.
See my suggestion as always with some sketch, my preferred international language…

In principle I personally fix the bulb retaing rod and nut at about 35% of the fin chord where the profile is larger and secondly I insert a pin behing the main rod to avoid rotation unless your fin is already profiting of some housing inside the bulb.
Of course the chirurgical intervention could be larger !!! if you can melt more lead !!!


Merci Claudio,

I just managed to push the drill bit that snapped out by hamering a rod in the hole to push the broken drill bit at the other end and it worked.

But with all that messing my hole is not that accurate anymore.

don’t worry I love your drawings they do explain greatly the concepts.

yes will put a small rod at the back end to ensure bulb alignment.

Merci Claudio

Been at it for few hours now - much complex than I though at the start

Glad to hear !
I can imagine, but not very drammatic since you managed in a simpler way.

quick update
I have finally managed to attach the bulb to the fin and I started “digging” in the front of the bolt to sink in the LE of the fin in the bulb to get the 88 degrees cant but the back end is sticking out a bit so more work for tomorrow evening.

Getting there but it is messy and not easy to be accurate. I think I will build a support that will present the bulb at 88 degrees to the drill tower so I’ll know the hole will be straight and at the right angle for the next one.

@Celtic Spirit:

I find lead is very tricky to drill accurately.
The most successful method I have found is to use a very sharp drill running at a very low speed - drill only a couple of mm, then back the drill out and clean the flutes of the drill, repeat until through.

If you try to rush it, the drill fills with lead and will cause all sorts of problems, much like the ones you describe…

I’ve found it helps to get lead very cold by putting it in the freezer overnight, and to use large quantities of mineral spirits as a lubricant. Plus going slow.



An assist in the drilling of lead is also to use a tiny bit of turpentine as a lubricant. It seems to give better control over the material removed by the flutes maybe because one needs to provide a bit more pressure to overcome the slipperiness of the “turps” and then adjust by feeling how the bit is going. Personally I found the smell of turps a pleasant addition to the cedar shavings and other traditional flavors that used to abound in the workshop. (Not so much modern epoxies and such.)

even better if using two different drill diameters !