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[quote=spfx]I have used a 1.5 hp two stroke and a 5 hp four stroke on my P19.  The standard prop pitch on the 1.5 was too aggressive and mostly just stirred the water with little forward motion.  Its mount was nothing more than a couple of custom cut delrin blocks allowing the transom clamps to pinch the rear crossbar traveler.  When not in use I had planned on simply strapping it to the boom or on the tramp behind the mast.  My guess is, with a properly tuned prop it may have reached 3-4 knots.  
The 5 hp motor was way too much power and weight (67 lbs).  For its mount I used SS tangs, SS turnbuckles, rectangular aluminum tube ~14", SS hardware and SS rivets to fab something off the rear crossbar.  The mount was pretty quick to fab and worked well, but that much weight aft was too much.  Freeze this at around 24 seconds for a look. 
 https://www.youtube.com/watch?v=uaviVb5abSQ 
 
I think it did around 8 knots or so.

My reasons for a motor were based on making a 25 mile channel crossing to Santa Cruz Island and sailing during the winter months where winds around here can get fickle.  Definitely could have used one for the Catalina trip last month with a 12 hr crossing going there and a 10 hr crossing back.  Right now I'm searching for a 2-3 hp motor no more than 35 lbs and figure a speed of 4-5 knots is all I need.  The mount will be similar to what Carl made for the H18 he and Geno took to Catalina.  Since the H18 hull has a lip on its deck, he spanned a 2x6 across the hulls behind the rear crossbar and fab'd a clamping mechanism to catch its underside.  Worked out great, cheap, quick, simple, easy to install/remove and clean installation.  For day sailing I think a mount like this would be ok as any requirement to shift CG can be accommodated.  On a boat traveling with camping gear, it may be best to move the mount in front of the mast crossbar.  There's so little hull volume aft, I noticed the H18 transoms inches above the waterline and Carl told me the motor ingested water a couple of times (maybe just mounted too low).  This type of mount is so easy to build It could be easily modified to fit forward or aft.  At the very least something like this could be used to test where the motor fits best before investing in a more involved mount.

I took a look at electric propulsion and its advantages and disadvantages.  For me the numbers didn't work out.  If I'm half way across the channel 10 miles or so out, how much battery would be needed and what speed could be expected.  Even at say 3 knots the weight in lead acid batteries exceeds that of an outboard with way more hp.  If you intend on traveling short, in harbor, distances this is a great solution.  With the use of LIpo batts you may be able to shrink the package enough to fit down a cats hull hatch.

If you want to travel at low speeds under 3 knots, you may need one hp.  1 hp= 745 Watts.  On a 24V system an electric motor will draw about 30 Amps (745W/24V= 31A).  30 Amps X 1 hr = 30 amp hrs @ 24V.  If you used two standard blue top optima batts (38 lbs each) that would give you a bit over an hour of runtime.  A couple of motorcycle sized batts maybe 15-20 min.  For Lipo batts you may need three 5.8 AH seven cell packs at $80 each for 20 min.
Want to go faster, hp required increases by the square of the speed.  6 knots will cost you more than 4 times the batt capacity.  Electric propulsion quickly gets out of control with speed.[/quote]

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