walt wrote:
Retired guy (me) must be a little bored this morning ... but I played with some numbers given my peak speed with a 2.5 hp outboard and Husse0416's peak speed with a 5 hp outboard.
Listening to H's video, I think the 5 hp outboard was running up into the proper RPM range for peak HP and not rev limiting. So Im assuming that both he and I were getting in the range of the rated HP. He did not have the mast on (I did) and I dont know if the pedal drive was in place (mine was) and his boat is a little heavier from the add on.. but its a close enough comparison. In both cases, one person in the boat sitting in the back seat.
H didnt give the exact peak speed (a little over 9 knots) and for this little exercise, I assumed H's peak speed was 9.3 knots. My peak speed was close to 8 mph or 6.95 knots.
We know that hull drag follows a square law with speed at low speeds and then the exponent increases at higher speeds for a variety or reasons. Anyhow, we have two data points and in this case, assume an exponential equation for curve fitting the two data points. For sure a linear equation does not work for this case.
Details available if someone wants but the exponential equation that fits those two data points is
Speed (knots) = 6.956 * ((hp/2.5)**.4188 )
Note that a square law would have an exponent of .5 and a cube law would have an exponent of .333. So on the TI, the speed vs hp required for that speed is somewhere in between square and cube.
Using this equation, I plugged in some different HP numbers to see what speed would be predicted. Note that the numbers for 2.5 and 5 exactly match the real data.
HP / Knots
-------------
.625 / 3.89
1.25 / 5.203
2.5 / 6.95
5 / 9.3
10 / 12.43
That equation only is accurate near the two exact data points at 2.5 and 5 hp but its somewhat interesting to look outside the range. Defy here
https://www.hobie.com/forums/viewtopic. ... 8&start=45 was getting 5.1 mph with the 400 watt Torqueedo at peak power (didnt post what power the control said was being used, that would certainly be interesting). Note that the equation based on the 2.5 and 5 hp data points predicted that 466 watts (.625 hp) would only get up to 4.47 mph or 3.89 knots and Defy was going faster than that. I take that to mean that at the lower speeds, the exponent goes back closer to square law of the typical parasitic drag vs speed equations.
Also.. its likely that the equation over estimates the speed for 10 hp as the exponent is probably lower for 10 vs 5 or 2.5. But I think its fair to say that you will not get speeds over 12.43 knots or 14.29 mph if for some crazy reason you did put 10 hp on a TI.
Ok.. back to doing something more useful (or maybe defending all that junk.. which could be incorrect).
LOL!! LOVE IT!
Thanks for clearing this up... I didnt understand half of it
but it sure looks very convincing.
The speed on my TI with the 5Hp, mast and all, is about 9knots at about 60% trottle. Pedals in the rear cockpit. Speed only increases marginally to just under 11 knots on full throttle but it probably is doubling the fuel consumtion.
About half throttle is perfect crusing speed if the water is calm. To make it go faster I guess i could try to add some more hydrofoil fins on the hull but that is a new separate project far ahead in the future.
It will be interesting to see how the 5Hp handles the extra weight that I will soon be adding with all the batteries and the electric motor (~50kgs extra). All of that weight is in the front. The drag will for sure increase but I hope the kayak will get better balance and "plane" better.
(Well it actually never will fully "plane" off course but I hope you all get my meaning)
Range is more important for me then speed so I hope it handles well even when fully equiped and loaded.
So far I am very happy with the setup.
I am the only guy around with a Hobie Island here in South of Sweden... it sure would be nice to meet up with some Islanders but you are all to far away.
Thanks again for the scientific analysis!