I've had gas motors on my TI's for about 3 1/2 yrs now. I'm kind of a destination sailer, and we mostly use the boat for Island hopping, scuba diving, and basically our TI replaced our Sea Ray powerboat so pretty much everything we did with the sea ray we do the same with the TI. We have several powerboating friends that we typically meet out at some remote sand bar and just hang out once in a while (like tailgating), pretty much all the standard stuff. Our TI is our SUV and we often have 3-4 people on board (I've had as many as 6). When we have a large group or a lot of scuba equipment (6 or 8 tanks), we typically deploy our dingy which is a 4 person inflatable ( 650 lbs additional capacity that I store in the front hatch when not being used) and tow it behind the TI. Many times we also tow groups of kayaks when we have too many people. In the keys most of the coral reefs are 5-10 miles from launch, a typical day for us can be as far as 50 miles travel as we go from one place to another (obviously not 50 miles out into nothingness, that would be nuts).
I bought the first outboard because the first night out on our new TI back in spring 2010 my wife and I got caught in an outgoing tide (about 6mph) in one of the big passes into Sarasota bay trying to come in from offshore in late afternoon when the wind died completely (pretty stupid and dangerous thing to do, but we were 'who knew' LOL). Needless to say it took us about 2 hrs of really hard pedaling to get back in so I got a free ticket to buy a motor from wife.
(if your married and your wife is giving you issues about spending the money, just check the tides first and take her out in the late afternoon when you know the wind will quit, 'it works' LOL ). I'm just sayin I deny any knowledge of this.
If you go to the topic Ultimate Tandem Island ( viewtopic.php?f=69&t=33720&start=135
) in the later pages you can see some of my escapades starting to use the motor for more than an emergency safety backup.
In south West Florida the typical winds are just not there 10 months out of the year (typically 4-6 mph (if that much)) and in the hot sun it's like sitting in a frying pan out on the water unless your moving at least 6-9 mph. In the low typical winds around here with a stock TI (with just a mainsail) you can pretty much expect to go 3-4 mph while pedaling, and literally fry out there in the sun.
If I were you I would look into the tri-power capabilities of your TI ( wind, pedal, and auxiliary power (hybrid gas or solar/electric). Here is a GPS track from a typical day for me in around 5mph winds using the tri-power capabilities of my TI.
Wind direction simply doesn't matter since you are creating your own apparent wind to propel the boat. Actually the best performance is almost directly up wind because the wind coming at you increases the apparent wind going over your sails. And obviously downwind is the worst since i'm sailing faster than the wind I run over my huge spinnaker (135 sq ft) so it becomes useless unless the winds are faster than 15 mph or so, then it's like driving a rocket downwind.
Any more I only powersail, I can literally go out for the day and maintain speeds 6.5 to 10mph all day and it only costs me a buck or two in fuel. I always carry enough fuel on board (2.3 gallons) to go between 150 and 250 miles if needed (economy is dependent on how much wind of course).
Yea you could probably mount a Honda 5hp motor on your TI, but keep in mind the TI has a displacement hull and peaks out at around 10mph, and the horsepower requirement goes up exponentially. If you go to this web site you can calculate your boatspeed from the prop ( http://propellerhub.com/boat-speed-calculator.html
) (the Honda 5hp has a 7" pitch prop and tops out around 5500 rpm). However because the TI has a displacement hull (can't plane) you will never see the 15 mph on a TI, it would require more than 50 hp to push the TI over 15 mph according to my calculations.
If you plug 500 lbs, 5hp, and hull factor 100 into this calculator it comes out to about 10mph ( http://continuouswave.com/cgi-bin/crouchcalc.pl
), about what you can expect as your top speed with a Honda 5hp motor with propeller drive only (not using sails or pedals at all). I'm a former powerboat racer myself, and have designed and built several boats over the years. We still have friends that we hang out with who race in the superboat races in both Sarasota, and Key West annual superboat races (we have houses in both Sarasota and Key West).
If you don't believe me about hull speed and horsepower requirements you can go to this web site and calculate it all your self ( http://www.psychosnail.com/boatspeedcalculator.aspx
). The math is just not there (that's why jetski's have 120 hp). Of course there is some planing going on even with a TI at higher speed especially if you can get the bow out of the water. I've had my TI up to 18-20 mph of a few occasions (in strong 22mph plus downwind runs (with all 265 sq ft of sail up), but have also had the rudder snap off and AMA's break at those speeds so it's not safe, and hitting a large wake at that speed would destroy the boat (the front hull would fold in the area around the front hatch without proper re-enforcement), and any side gust that would bury your AMA would instead snap them off (I'm talking about the bars themselves, not the little nylon pin).
I Highly recommend the 27 lb Honda 2.3 hp outboard, it is extremely well built, reliable, and lightweight and will give you years of trouble free service. If you use it in conjunction with your sails (tri-power sailing) you can go out all day for just pennies in fuel regardless of the wind direction and speed.
A word of caution if you have the motor in the water and running (with clutch engaged) at higher speeds (over 12 mph) there is a great risk of over revving the engine and it will explode, so if going fast you must raise the motor. FYI