Speed/efficiency -poly vs fiberglass, composit...

@Guideboatguy said:

@Mountainpaddler said:

Okay, so if the speed is the same (7 mph), which would you say would hold it’s glide better: a boat that is 21 X 17 or a boat that is 17 X 17–assuming for the sake of argument that the hulls are shaped identically?

I’m going to have to read between the lines here since you didn’t provide any units of dimension. Assume that in this case you are trying to compare two boats of the same length? Both being 17 feet but one being 21 inches wide and the other being 17 inches wide? In that case, I’d expect the narrower boat to have better glide at high speed. Off the top of my head, I’m not sure which boat would have more wetted surface area and thus more skin friction, but at high speed, wave-making is far and away the biggest source of resistance, and I’d expect that to control which boat has better glide. With a narrower boat, approaching hull speed is less like “hitting a wall” and more like “pushing against the side of a balloon”. Both of these boats would be more like the second case, with the narrower boat being more so. The speed you list is well over hull speed for this length of hull, so the boat with a more-forgiving entry into speeds of that range will glide better. To put it in a way that explains why, the narrower boat requires less propulsive force to achieve the given speed, which means that there must be less resisting force pushing the other way, which in turn means that once “the power is turned off”, there’s less force present slowing the narrower boat down.

I thought those were units of dimension?
The first number is traditionally the length and the second is width.
So 21 feet long by 17 inches wide
and 17 feet long by X 17 inches wide.
Otherwise the hulls are shaped the same.

This illustrates what factors become important when applying high amounts of power and needing all the speed you can get in return, so it’s very different from the comparisons that were pointed out earlier in this thread regarding long boats versus short boats and how they perform at relaxed paddling speeds. The example here is more applicable to the comparisons that you yourself are more interested in, though.

Yeah, we’re way off topic from the OP (my apologies), but it sounded for a minute that you were saying that at a good speed the boat would be slower if longer. I was looking for clarification.
I guess the definition of “cruising speed” is important here.
I would definitely say that a longer boat will be slower at a low speed due to the wetted surface.

William Froude was actually the first to demonstrate conclusively that there is no one hull shape that is maximally efficient at all speeds, which had been the previous belief. Froude also broke resistance of a hull moving through water into two components: frictional resistance and residual resistance. The major component of residual resistance is typically due to wave making, although in some cases other sources of resistance, such as wind resistance, become important. This graph shows the relative contributions of frictional resistance and residual resistance at different speeds. The difference between total resistance and frictional resistance is, of course, residual resistance:

At lower speeds, residual resistance contributes little to total resistance. As speed increases toward maximum theoretical hull speed, frictional resistance continues to increase, but becomes near linear. On the other hand, residual resistance becomes more important at an ever-increasing rate.

Froude determined that the speed of waves in knots was equal to the square root of the wavelength (in feet) x 1.34. This is where the hull speed formula comes from. As a hull moves through the water, transverse waves form as a result as a result of the energy lost pushing water out of the way. At low speeds, a number of transverse waves form along the length of the hull. When hull speed is reached, there is one transverse wave at the bow and one at the stern, and the wavelength is therefore equal to the waterline length of the hull. At this point, the hull is nestled between the waves crests. To surpass hull speed it is therefore necessary to “climb over” the crest of the bow wave, which usually takes a considerable increase in propulsive power.

America Cup yachts and racing shells are somewhat special cases because they are designed to allow quite small increases in speed even at the expense of greater frictional resistance and lessor efficiency at lower speeds. In competition even a very small potential increase in maximum speed is important, providing of course that there is enough propulsive power to take advantage of it. Displacement hulls with very fine ends and narrow beams are “wave piercing” designs that enhance the ability of the boat to exceed maximum theoretical hull speed. It is said that some racing kayaks have the potential to exceed maximum theoretical hull speed by 100%.

Theoreticals aside, when it actually comes to paddling different boats, some will give you better results than others. Try a Kayak Pro 14 footer for example–they seem to defy the length rule. One might assume that a Swede design would almost always be a little speedier than a Fish design–that may, or may not be true. Some boats I have paddled are quite quick off the line and seem to have a fair bit of speed, but hit the wall prematurely–the Rockpool Alaw Bach comes to mind.

For myself, a boat that tracks well is easiest to paddle at a higher pace when you aren’t having to make subtle adjustments in stokes and with limited edging.

@Mountainpaddler said:

I thought those were units of dimension?
The first number is traditionally the length and the second is width.

What I meant by that comment was that there wasn’t any inclusion of the units, as in “feet”, “inches”, “meters” or whatever. I settled on the same units that you intended because your question made the most sense that way, and only then did I even figure out what you were getting at. I had no idea there was some convention among boat enthusiasts or builders or whatever that specifying units when describing dimensions is unnecessary. What I’m used to is that naming the units is critical (especially in calculations).

Anyway, drifting off-topic aside, it seems that you and I agree on what’s going on here. At the very least, all of this contributes to the understanding that there is no ideal boat for multiple purposes.

@pblanc said:
When hull speed is reached, there is one transverse wave at the bow and one at the stern, and the wavelength is therefore equal to the waterline length of the hull. At this point, the hull is nestled between the waves crests. To surpass hull speed it is therefore necessary to “climb over” the crest of the bow wave, which usually takes a considerable increase in propulsive power.

In my previous posts regarding “how it feels” to drive a boat up to a speed that is near hull speed, this situation was what I described as “hitting the wall”, because in general-purpose boats, the amount of effort needed to provide any increase in speed at this point is enormous, so very little can be done to make the boat go faster.

Displacement hulls with very fine ends and narrow beams are “wave piercing” designs that enhance the ability of the boat to exceed maximum theoretical hull speed.

In describing boats like these, this I what I think I might have called a “less abrupt” affect as hull speed is reached, and I might also also have called it a “softer” situation, like “pushing against the side of a balloon instead of hitting a wall”. This is the effect I was trying to describe in words for boats for which the necessary increase in propulsive power is not as enormous when trying to go faster than hull speed as it is for most boats.

I’m not trying to clarify Pete’s remarks at all. I’m just trying to put my own previous descriptions into the same context as that which Pete has provided, since maybe the aspect of “how it feels” to push different kinds of boats up to those speeds will be more clear now to some who’ve been reading this but didn’t make sense of my vernacular wording.

Good reading thanks guys