Modifying a sailing cat hull to operate as a powercat

Ok, we all know that a sailing cat hull is a displacement hull and are typically fairly low powered when it comes to auxiliary power presumably because they are simply not going to be able to motor fast so most will opt to run on one engine at a fairly slow but economical pace. However if one were to dispose of the rig and become powercat this would probably be unacceptable. I own an old Gemini 3000 cat that I bought without a rig and then bought a complete rig and sails from a gentleman who was converting his to a powercat. He had removed rig, centerboard, rudders, everything sailing related and added a pair of evinrude E Tec outboards of about 50hp each. There have been quite a few of these cats turned into powercats with various different power options but as far as I know none have modified the aft hull lines to better utilize all that extra horsepower. My understanding is that the pair of 50's would push it to about 12 knots but clearly not efficiently. So, it seems to me that if one were to modify, say the last 1/3rd of the underwater lines to better support the extra weight of the larger engines and tankage you could make better use of the extra power and maybe achieve a 10-12 knot cruise with more efficiency. So, what sort of mods would you do to achieve this? I'm thinking straighten the keel line to get a deeper immersed transom and flatten the sections. Thoughts? There were over 300 of these cats built so it seems that it might be worth building a mold to make a fiberglass part that could be bonded on with minimal effort kind of like the kit car industry used to do to turn a Fierro into a Ferrari.

Steve.

 

I don't know much about Gemini 3000's, but this one appears to have been modified to become a powered cat. The photos of the beached boat at various angles gives an idea what has been altered, if you are familiar with the original shape. I'm not. If, as I imagine, it has been given a new set of "shoes", it is a big job. I wonder what speed this one produced with 2x 50hp, you'd hope a top speed in the high teens mph at least, but a lot would depend on weight, and of course the additions add weight.
» Boats for Sale » Multihulls / Catamarans » Gemini | Sydney Boating http://www.sydneyboating.com.au/listings/listing.php?id=115977&cat=294

 

Nice find. Thank you. It sure would be interesting to know how it goes with these modifications, that's pretty much exactly what I was thinking. It is a lot of work to do those kind of mods which is why I think building a female mold makes sense if there was a small market for them. Stern extensions are not uncommon on sailing cats and that's a lot of work too but worthwhile if it achieves your goal. Its not uncommon to see these cats with a single outboard in that size range just as an auxiliary.

Steve.

 

Slow speed efficiency would be killed by those pictured alterations, it would be OK at a lively cruise speed though, probably. It allows the props to go deep enough, dampens pitching in head-seas, reduces slamming in head seas. If the weight goes up too much, it might become a slug, though.

 

Seems a cheap boat, the one in the ad, you'd wonder why.

 

I designed and built a 35' power cat starting with the plans of a Malcolm Tennant Turismo sailing cat. Basically the front third and the aft third of the hulls were modified - the front for greater waterline length and the aft sections to provide greater buoyancy (by about 300 Kg's) and to give a relatively flat run aft to the two 90 HP Suzuki outboards. Performance, fuel economy and handling are very acceptable and, although only 15' wide the boat is roomy, very stable and has lived up to my expectations. A more detailed look at my boat is in the power section of this forum under 'Weekend Picnic Power Cat.'

 

I looked at your cat on the weekend picnic power cat thread. Very nice indeed.