Hulls & Fins

Hi everybody,
does any one knows the correct position of the Fin compared to the Hull ?
Is the Fin a “disturbing” element for the water flow along the Hull ?
How to compensate ?
Is the Fin longitudinal position specific for each Hull form ?
Someone suggest to use the top of the “curves of areas” diagramme as the reference point.
Just Hull and Fin alone, any think to do with hydrodinamics.

Probably to many questions, let get simple :
“where to put the fin ?”

Will be nice to hear your opinions on that subject.

Set the boat in water. Push on side of boat with finger, gently. When you find the place on the side of the hull to push so that the boat moves evenly through the water sideways, you’ve found where to attach the keel (fore/aft! Keel goes on the bottom of the boat, naturally - lol).
How heavy should the keel be? Just enough so that the hull sits at it’s ‘natural’ water line (and doesn’t imediately sink to the bottom). As much as practical, all the weight should be consentrated at the bottom of the keel. How long should the keel be? Something like 1/3 or less of the hull length (VERY rough approximation! If it ‘looks’ right, it probably is.). How ‘thick’ should the keel be? As think as humanly possible while still being strong enough to not bend, warp, or otherwise deform from the forces applied in normal use. Are any of these dimensions exact or ‘set in stone’? Not even close! Depends a lot on what the keel is made from, the ‘style’ of boat it’s on, how nice it ‘looks’, and your preferences. All the above should get you into the ‘ball-park’, the home runs are up to you…

  • 'Doc

Thanks Doc .
my interest is actually more concentrated on " hull bottom" water flow with and without the fin presence.
I’m assuming that in order to obtain an optimised water flow around the Hull, there should be an optimised Fin position, not only related to the lateral forces but also to the frontal ones. Your suggestion, is the one I use to determine the CLR , in this case all is integrated, Fin + Bulb + Rudder.

It’s not exactly like I know what I’m talking about! The above is just a general way of doing things, meaning that there are always exceptions. I’m sure there is a ‘perfect’ way of doing all this but so far no one has found it (including those ‘full sized’ guys), so the ‘best’ is still what works ‘best’ for you. (The one exception is my sailboat, which is ‘perfect’! If you believe any part of that, I have this gold brick for sale.).

  • 'Doc

Hi Claudio

The only similar issue I can think of is the problem of where to place a wing on an aircraft, and I think two comments might be helpful.

For low speeds, I don’t think it matters where to put the fin or wing, there is no ‘optimum’ position. Instead, the wing goes where it needs to go relative to the plane’s CG, or the fin goes where it needs to go relative to the rig’s centre of effort. In this sense, the fin+mast can slide up on down the boat centre line depending upon whether you want your sail power relatively forward (some think this is good for manoeuverability and beating) or relatively aft (some think this is good when running or when over-pressed). Then, having put the fin where it needs to go, you then spend some effort in fairing the fin into the hull. Now this is where little effort goes currently into a boat, but it does pay. You want some fairing, and you want some fillets (leading and trailing edge root extentions, LERX in aerodynamics).

For higher speeds, certainly on high sub-sonic, trans-sonic, and supersonic aircraft, and on some canting-keel boats, there is an advantage to ‘waisting’ the hull and especially the bulb. That is, at the place where the fin or wing joins the fuselage or hull, or where the fin joins the bulb, you slim down the frontal area of the fuselage, hull, or bulb in proportion to the amount of frontal area of the wing or fin being introduced into the flow. This is called the ‘area rule’ in aerodynamics. The idea is that, as you force a volume through a fluid, if that volume suddenly grows because of an attached appendage, you take away some volume in the region of the root of the appendage attachment in order to give the fluid somewhere to flow to, to give it some ‘breathing room’. That is, you avoid compressing the fluid more than you have to (if it is compressible, like air), or avoid forcing it to speed up more than you have to (if it is incompressible like water).

thank you, very nice answer, but it trigger another question, what it is “high speed”
for our models as such to render suitable a “digging” or " wasting" ?
Any sketch to shows the principle ?