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Lot's of interesting stuff here. My apologies for being absent from the debate - I recently had another (rather large and uncomfortable) hernia repaired, and have been knocked for six a bit for the past week.
First off, some of the boats were running at surprisingly high Froude numbers. The general rule of thumb is that the Froude number for a displacement boat is around 1.34 (hence the "1.34 x sq rt of the waterline length in feet" method for estimating max displacement speed).
If you assume that all the boats were 16ft long, with about a 15ft waterline length, then they should have only been able to reach around 5kts. Dennis knows that he was doing about 6kts, I was perhaps 1kt slower at most, given the time it took for him to overtake me. Joe was probably doing around 8kts or so at a guess, perhaps a little more. Dennis was probably running at a Froude number of around 1.55, Joe at a Froude number of around 2.00. Joe's boat was clearly well up its own bow wave and would almost certainly have planed had it had flatter sections aft, IMHO. Despite me getting my weight as far forward as I could in the boat (well forward of the C of B) I couldn't keep the bow down, so clearly my boat must have been right at the point where it was trying to climb over the hump, too, despite the lower speed.
In general terms, catamarans have a higher hull resistance for a given displacement than a monohull or stabilised tri. The reason is that they tend to have both a higher wetted area and suffer from interference drag from the waves between the two hulls. The HPB people have pretty much proved this by experiment, too, with most fast HPBs now being similar in configuration to Dennis' boat.
At this relatively low weight and speed, hydrofoils need about the same power, perhaps slightly more, than a planing hull. Although the lift drag ratio of the foils themselves is significantly better than that for a hull, the added drag from the surface piercing supports and the added weight reduces any advantage to near zero. One problem is that it is hard to accurately predict the power needed for a planing hull at these low weights and speeds, as the normal methods used (like Savitsky's method) aren't truly valid in this operating region.
My gut feeling is that a very light planing hull probably needs less power than Savitsky's would predict, based on the very limited power that is developed by a dinghy or windsurfer rig. A 10m² sail on a board sailing at 10kts on a broad reach, with a relative wind speed of 10kts (i.e. alpha = 45 deg) will be generating about 180W of propulsive power. This is comparable to that from a single cordless drill and significantly less than the power predicted by any planing boat power prediction method I've seen.
Clearly it should be quite possible to make a planing hull for this competition, but the secret is going to be dealing with the point Dennis has raised, how to overcome the narrow efficient speed range of the propeller. Dennis and I were excellent examples of the problem, albeit for slightly different reasons. Dennis had a propeller that was very well matched to the cruise speed of his boat, but which struggled to provide enough low speed thrust to accelerate the relatively high weight of his hull. I had the advantage of a much lighter hull, but I was overly optimistic when choosing my propeller pitch, and ended up running with the blades far too heavily loaded for the true speeds the hull was capable of. The light weight still allowed good acceleration though, even with the mismatched prop.
To work well, a planing boat of this light a weight and power almost needs two propulsion systems if it is going to work over a wide speed range with limited power; a low speed thrust set up to give good acceleration and a high speed thrust set up to then continue to accelerate the boat up and on to the plane and beyond. This is similar to the systems used on some warships. Years ago I did some sea trials on the T23 when it first entered service. That used big diesel electric systems to get to around 15 to 18kts, then the Spey gas turbines kicked in and gave the boat a real shove as it accelerated towards it's maximum of over 30kts (we took Norfolk to around 33kts during acceptance trials, and joked that we could have water skied behind her!).
I suspect that a very light hull, with enough planing area to keep the surface loading down, might just be able to get away with a big prop mismatch at low speeds. Certainly Quackers seemed to accelerate fairly well, even though the prop was running a long way from optimum at low speeds (it would have given of it's best at around 9 kts, much faster than the boat could go in practice). I've since run Quackers with a lower pitch prop of slightly smaller diameter, and she is slightly faster (I managed 5.8kts on a single drill), with much quicker acceleration (she takes about 3.5 seconds to go from a standstill to maximum speed). There's no way that she will plane though, as the stern just buries itself in the water as the speed increases.
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