Greetings all, In my intro a few days ago I stated that I'm working on the hull design for a 45 foot powercat (personal use, home built - quite shaking your head and grinning Rod Gibbons). I was leaning heavily towards controllable pitch props but have recently started to consider stern drives - an idea that I never really thought too much of before. Here are my thoughts: 1. The boat will have a relatively fast cruising speed of maybe 20 kts, top speed 25 kts or higher. Those kinds of operating speeds are in the optimal design range for stern drives, which typically are targeted towards planning hulls. 2. I want to be able to beach the boat on occasion - difficult to do with shafts & props & rudders hanging down there, though it is possible to design for that. 3. I'm paranoid about damage to the running gear - lots of logs and even ice up here in SE Alaska. I feel a sterndrive is better protected. 4. I'm also paranoid about fouling my gear on the many crab lines and fishing nets here - taking a swim in these waters to free a prop just does not appeal at all, any time of year. 5. My understanding is that a sterndrive has about 15% greater efficiency/lower drag due to no appendages from shafts & bearings. 6. I can raise one outdrive and operate on a single engine with lower drag (though slower obviously). 7. I have the ability to adjust the boat's trim underway. 8. Design and installation are considerably simpler in my mind. A transom is pretty easy to design and carries the load vs worrying about thrust blocks, shaft alignment, stuffing boxes etc. 9. I can inspect and even repair things without necessarily having to haul the boat. 10. And finally, the new Volvo sterndrive packages are really nicely integrated from prop to steering wheel. Everything's pretty much plug n' play which simplifies my life and speeds things up. I hear good things about their new generation common rail diesel engine. So obviously there must be some fatal flaws in my logic or everyone else would offer sterndrive models. What are they? A few of the possible negatives that come to my mind are: 1. Sterndrives are expensive. True story, though that's offset somewhat by not having to install rudders, shafts, etc. Initial cost is not the only consideration here. 2. Sterndrives require more maintenance and aren't as reliable. True, and I'm still grappling with that. 3. The weight of the drive and engine sit far astern which is difficult to compensate for in a catamaran and may lead to pitching issues. Yes, I'm learning that as I play with my longitudinal COG and COF. The jury's still out whether I can compensate for it, but I think so. 4. In big water the stern of the boat may rise clear of the water enough that the props will ventilate. That is a big concern, though I'm not sure how valid. This is a coastal cruiser - I'm not motoring to Hawaii where I would routinely be in big swell. Typically the biggest water I'd be running in is a 5-6 ft chop. This is a displacement hull with pretty narrow hull beams (L/B ratio is 16) so it should ride with well submerged hulls. I'm minimizing rocker. Am I looking for trouble here? 5. Prop diameter on sterndrives is too small. Maybe, but at higher running speeds smaller diameters become more appropriate. Using duoprops adds about 75% more prop area to help mitigate that too. So if any of you can provide any insights on this I would be most grateful. Thank you! Bob Deering

>Bob Deering wrote: >I was leaning heavily towards controllable pitch props From what I have heard about CPPs from Dennis Raedeke of Wild Wind IV, they work well at trawler speeds but are not that well suited to higher speeds attained by power catamarans. Perhaps Dennis will chime in with comments based on his experience with CPPs over tens of thousands of nautical miles. For those interested in controllable pitch props, Michael Kasten has written an excellent introduction to the subject: http://www.kastenmarine.com/CPprops.htm --Georgs -- Georgs Kolesnikovs Power Catamaran World http://www.powercatamaranworld.com