The top speed is determined by the prop pitch. Whether a smaller ai or a ti the top speed from a 2.5 single engine will be around 7mph maybe a little more on flat water.
I did a lot of testing on the bow wave, and it hits the back of the boat at around 8mph, going faster than 8mph the horsepower need to go up exponentially, ( because of hull type and physics).
On my TI I had twin 2.3 hp hondas. I re-propped the engines with several different pitches.
With a 8” prop the top speed goes up to near 12mph, however because of the bow wave, the back of the boat is submerged at that speed, ( because it’s a non planing hull)
We converted our hull to a planing type hull, and got up to around 15mph top speed, ( which was max speed at that pitch, ( which you can calculate).
Our highest recorded speed with the stock motors with the 8” pitch props, wot plus all 3 sails out was a little over 20mph, ( on an Egmont key fl run, ( but we were dragging the props, ( going faster than the props were spinning). But the winds were around 20mph.
I did make a set of 10” pitch props up, ( for use with the hydrofoils), that topped out close to 25 mph, but I had to add nitrox injection to the motors to get enough horsepower to run those props, ( to get up on the foils). With all the sails out in really good wind, if you hit just the right angle to the wind, I could stay up on the foils. But it was so dangerous I retired all the souping up, and foils in around 2012, after some pretty nasty crashes and pitchpoles, ( just not worth the risk).
The trick with these boats is to use both the sails, and the outboards together, ( but you have to have higher pitch props)
Wing sails work better. Adding a hull mod to convert the hull to a planing type hull, ( similar to a weta hull shape), all give you the best bang for the buck.
We had a blast with ours
FE
Explaining the video, ( added later). The boat in the video is pointed around 10 -15 degrees off the wind, ( best performance window). The forward motion of the boat creates it’s own wind across the wing sail, which is added to the natural wind, so if the natural wind is 7mph, plus the forward motion of the boat, ( say 10mph), then the usable wind over the sails and into your face is 17 mph. The now directed and accelerated wind is forced over the main sail in an organized fashion which creates at least half of the required propulsive power to propel the boat. The engines are always set at around 1/4 throttle producing around 1hp each, ( around 2300 rpm), but because of the high pitch props, even at lower rpm the boat is propelled to cruising speed, by both the props and the sails, ( thus greatly reducing the hp requirements of the motors). We pretty consistently acheived 60-80 mpg fuel economy, or around 3 hrs runtime on each of the 1 liter fuel tanks. We typically carried 100-200 miles worth of fuel on board, ( just in case). Our daily range was a little over 100 miles, ( keep in mind the keys are huge), and we were mostly offshore, sometimes great distances out.
I always pedal for added power, ( and exercise). The mirage drives were retrofitted with eclipse fins, which provide useful propulsion up to around 20mph, ( standard fins are only good to maybe 10-12 mph before they become useless). However those fins require tremendous leg strength at lower speeds.
We seldom ever took the boat out in winds over 7mph, and flat water.
Hopefully this helps in calculations of speed and horsepower requirements, we ran this setup for around 7 yrs with few changes on three different TI’s we owned. We had to quit because of health reasons.
https://youtu.be/a1OjgyqBsXk?si=U_SSdMn7CT3XDpdk