RG 65 bulb shape/profile


I am in the start of building rg-65 viperfish sailboat.
And i wonder what is a good shape/aeroprofile for bulb might anyone have sketch or smth familiar?

Here you go:

If i understant this graph.The 2nd shape of bulb is the best?

Looks like naca0010 and more is suitable…


The graph is from the AMYA site and shows the ‘maximum lift, minimum drag’ theory of bulb design:


Here the second one shows the cross over point of minimum induced and parasitic drag.
However the first one is closer to the second approach:

The spreadsheet (from I believe Footy Angus) shows the ‘minimum lift, minimum drag’ MIT approach as used in the design of slender bodies like aircraft fuselages. It has a spherical nose, cylindrical body (of any length to get mass), and parabolic tail.

As the bulb has a changing angle of attack, lift is not advantageous.

The empirical difference is minimal…

What is a good target weight for RG65 bulb?

MGeo - The answer is: it depends.

As the RG65 is a design class there is no one “target weight”. It is desirable to concentrate as much weight in the bulb as possible. How much of the overall weight can be in the bulb depends on the design displacement of the hull you’re building, the finished weight after the construction of the hull and appendages, the weight of the r/c components and batteries, and lastly the weight of the rig. What is left is the amount of weight that is available for the bulb.

These can be estimates, but the more accurate these estimates are the more likely you can spare yourself hours of shaving lead off a too heavy casting. You should, however, be a bit generous with the casting because you can always remove lead if the bulb is too hefty but you can’t add any to a casting that is already poured.

For one-off castings, or when you are unsure that you have the correct volume, I would recommend you give sand casting a try. This method is tricky at first but it will provide for multiple castings of different experimental shapes and weights without going to the extent of a cement or Hydrostone mold. Once you’ve come up with a shape that you are happy with then you can go with a more permanent mold. Casting sand, and a thorough explanation of the process can be obtained from sculpture and jewelry suppliers , or some small foundries.

Alternatively, for one-off home “guesstimates” - you can build your hull and weigh radio gear/rig/sails/keel and rudder PLUS batteries. Adding this all up, you can weigh stones, gravel, nuts/washers,e etc. until you reach this weight. Place it in the center of the hull near/next to your keel trunk and then add more weight until boat floats on it’s designed waterline. (You can use tape to temporarily mark that location) Once you are to your designed waterline, just weight the ADDED weight after rig, etc. has been added to boat and it should get you very close to your bulb weight.

Depending on your design, the designer of the plans may have provided a bulb weight - or you can start at about 1 lb. (500 grams) and place it in the hull to see if it is enough or if too much.

According to the Viperfish Plans, the designer has listed bulb weight of 550 gram - plus weight of rudder/keel, etc.

the Viperfish was designed by Arne Semken from Luebeck in Germany. Here the “standard” keel bulb has weight of 550g which fits in most cases. Total weight of this boat is should be about 950g, which leaves 400g for hull, rigg, sails, fin, rudder and r/c-equipment. However, you can use heavier bulbs in a breeze

In the moment two ways of development can be seen

1- make the hull lighter and put more weight in the keel. Some guy are using up to 800g, especially in a breeze. Henning Dresel has won a race with strong winds in The Netherlands with his very narrow design HIP, using 800g for the bulb mounted as far aft as possible. The HIP was a rocket downwind.

2- others are preferring lightweight designs going the M/2-way. Scaling a M-Boat down by a factor of two results in a displacment of about 750g with is a bit low for an RG, but 800 - 850g can be found on the water. With this boat, the keel bulb is usually around 500g.