More prismatic coefficient

[Don Case]

I am trying to design an IOM. I'm trying to get the CP up to about .60 or thereabouts. I'm using Hullform software. Hullform splits the CP into foreward and stern. Does it make a difference if the total CP is arrived at with a high foreward CP and a low stern CP or vice-versa. My gut says that a larger foreward CP would make the most sense but my gut has been wrong many times.
Thanks
Don

[MalSmith]

I haven't used Hullform. Out of curiosity, does it calculate the CP for each end from midships, or from the maximum cross sectional area position. If the former, does it use the midship section area or the maximum cross sectional area the Cp calculation?

In answer to your question, I tend to want to have a balanced Cp at each end, but I'm sure that there are different opinions. From what you are suggesting, a bigger Cp forward would tend to shift the LCB forward of midships. The usual preference is to shift the LCB aft, if anything.

Cp is a bit of a blunt instrument for analysing a hull form without reference to any other variables. For instance, an immersed transom will increase the Cp, yet the actual hull may simply be a truncated version of a low Cp hullform.

[Don Case]

Originally Posted by MalSmith

I haven't used Hullform. Out of curiosity, does it calculate the CP for each end from midships, or from the maximum cross sectional area position. If the former, does it use the midship section area or the maximum cross sectional area the Cp calculation?

I don't know, they haven't been defined anywhere I've looked.

In answer to your question, I tend to want to have a balanced Cp at each end, but I'm sure that there are different opinions. From what you are suggesting, a bigger Cp forward would tend to shift the LCB forward of midships. The usual preference is to shift the LCB aft, if anything.

Cp is a bit of a blunt instrument for analysing a hull form without reference to any other variables. For instance, an immersed transom will increase the Cp, yet the actual hull may simply be a truncated version of a low Cp hullform.

Thanks, I had noticed that as I was fiddling with this the LCB was sneaking forward. My quest is to get a hull with a reasonable high CP and to have the LCB and LCF move as little as possible. I was just finding it difficult to raise the CP without having the boat look funny. Apperances are important.
Thanks
Don

[MalSmith]

What I usually do these days is to choose a mathematically derived low wave drag (theoretically) volume distribution (curve of areas) first, such as a Sears Haack distribution or some modification thereof. Then, trying to stay close to that, I will play with the sections to arrive at the best compromise between form stability and wetted surface and whatever other features I need to incorporate. Using this method, the Cp is a result of the choice of volume distribution curve, rather than being a driver.

Note, to get a higher Cp than the Sears Hacck gives, I usually morph it with a parabolic distribution.

[Oldsalt31]

Hi fellas, I thought that I was the sole user of Hullform. In Glossary p120 Hullform uses "the largest immersed area of any section of the hull"
Using criteria kindly supplied by generous NA's in this forum over years, I design to a displacementand a LCG target achieved with the lowest drag. This leads me into all manner of shapes and hundreds of designs. A great lesson in the art of compromise. Is anybody skilled in surfaces and merging hulls. I would appreciate help in these subjects . Thanks John

[messabout]

I take it that your reference to IOM is to describe an International one meter RC boat.. If that be the case you will need a lot more than mere luck and our advice to design a competitive boat. The IOM class has become a fiercely competitive class that has attracted very talented designers and the existing boats, that are contenders, have profited from extensive experience not to mention the tons and tons of money that has been dedicated to those experiments.

If you want a good competitive boat, then get some plans from a proven design. No use reinventing the wheel.

[MalSmith]

While it's a good idea to be made aware of our chances of success, I'm not sure it's such a good idea to dissuade people from having a go at designing thier own boat. Reinventing the wheel can be a good educational process, we spend most of our school careers doing just that. Futhermore, creativity is in part a chance process, so there is always a chance that Don Case will come up with a winner.

[messabout]

Don and Mal; No offense was intended by my comments. I am all for experimentation and new ways of thinking. On the other hand if one wishes to build a boat and then go racing, satisfaction will likely arrive sooner by selecting an established design. That does not prevent additional design experimentation, which I cheerfully encourage.

[Don Case]

Sorry for the late post. I have always made my own boats, sails, fins,rudders and well, everything. I have never done really well racing but that's the skipper not necessarily the boat. I recently biult a Ska. The thing is a rocket. I went from middle of the pack to the front(in speed,I still can't sail). The problem is that while I'm sailing the Ska there is a little voice saying,"It's not your boat". There is no joy like sailing your own boat, even if you don't do that well.
Don