No need for a published spec, hull speed is easy. It’s just a brute force calculation, although you need to know the water line length of the boat (not overall length).
See Wikipedia, for instance - as you can see, it is NOT an exact value, hence the variation in coefficient. The truth is, it’s a one-significant-figure parameter, so a pretty coarse measure:
I believe what you are interested in is the same thing touched on in other posts.
Namely, effort to maintain a cruising speed, rather than all out top speed.
Here is what I believe, for what it’s worth.
Narrower is almost always faster.
There is a point of diminishing return for length, and it’s probably shorter than most people think.
The slower the speed, the less volume you want in the ends. And my understanding is that it doesn’t just mean width, it can also mean depth.
The slower the speed, the less asymmetrical you want the boat. There may even be a point where fish form requires less effort.
interesting, I guess I thought hull design would make more of a difference
Hull design absolutely makes a difference. However, the theoretical parameter called hull speed does not take hull design into account - that’s why it’s not particularly useful as a hull descriptor or as a quantitative measure to compare boats. In fact, it’s no more useful than just stating the water line length of a boat.
Guillemot article is generally correct, but misses one important point in his explanation: for a given displacement, a longer hull not only has a more gradual entry (allowing for slower displacement of the water ahead), it also has lower frontal surface area presented to the water (resulting in less water displaced per foot travelled). An 8 foot boat 3 feet wide displacing 4 cubic feet and a simple diamond profile with no rocker has a frontal area of 4x12x12x12/96x2 square inches, or 144 square inches. At one foot per second forward motion it must accelerate one cubic foot per second to a velocity of 4.5 inches per second. Stretching the same profile to a 16 foot x18" boat we have 4x12x12x12/168x2 or 72 square inches. At one foot per second forward 0.5 cubic foot per second must be accelerated laterally to a velocity of 1.125 inches per second. This gives a 32 fold difference in energy expenditure for displacement between short and fat versus long and narrow at any speed. For simplicity I have ignored varying the draft of the hull, making the short boat the same width and sit deeper in the water would mitigate this to some extent. Specifically, the 8’x18" boat would accelerate 1 cubic foot per second to 2.25 inches per second, for an 8 fold difference in energy. I don’t know whether his drag curves reflect this difference or not.
Oops, the second equation should be 4x12x12x12/192x2.
I’m having no luck understanding these calculations. Boats do not push water directly forward or directly sideways, and certainly not at a constant bulk velocity. Is this just spit-balling, or is it based on a method you’ve seen used before?
The Guillemot article is well written and scientifically accurate, the conclusions are well-established. The resistance plots were generated using software that includes water displacement by the hull - it is included in the form drag.
It is a simplification to conceptualize the impact of frontal surface area on resistance to forward motion. I should have labelled it as such. Simplification because, as you state, the water displaced is not all moving in a single direction at a single speed. With regard to the article from Guillemot, just noting that frontal surface area is a significant factor which he does not explicitly mention but has a large impact on the difference in resistance between longer and shorter boats. I agree overall the best article on boat length and paddling resistance I have seen.
With regard to the plots, I have not seen the math in the program, and probably couldn’t verify that it was correct (or incorrect) if I had. Presumably it is sound and empirically verified across a range of shapes, but I have seen all kinds of things calculated by programmers using the wrong inputs as starting points.
Fair enough - the graphs were generated with a software package that’s been around for a while and verified experimentally on some level, IIRC.
Interesting topic. I can only add that glide deteriorates quickly as you get to higher speed. One curious thing I noticed tracking a 120 pungo. The paddler stopped at intervals to wipe his forehead, and the boat lost very little speed in the glide as I continued to paddle in a 145 Tsunami.
