Anti-cavitation deflector for a cat.

Hi, i'm building the below cat lengthened to 30' plus 29" for the engine to mount and also as a swim platform. It has a 10' beam. Due to tight finances I only want to mount a single centered outboard. I have no problem beefing up the transom for a centered OB engine and the engine cavitation plate will lie flush or just below the surface of the water.
Do I need to design a anti-cavitation deflector into the underside of the cross beams to direct water to the engine? Will be a 90 hp engine. Havn't had much luck searching for a clue. Thanks very much, Stan
https://www.boatdesigns.com/28-Bear-Cat-Cuddy-power-catamaran/products/779/

 

Yes you do!!!

see my Skoota 20 hp trailable powercat with central outboard on my Latest News page

I fitted a deflector from Day 1, but took it off just to see what happened. Awful! Speed had to be kept below 5 knots or the engine went underwater

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

How to make a power cat run satisfactorily on a single engine has been the bug-bear of the genre, and judging by the almost universal fitment of twins, not really solved. Suffice to say you need the cav plate well under the aerated water generated in the tunnel, so you require either an extended outboard leg, which is gonna generate considerable drag, or some kind of skeg/deflector ahead of a conventional outboard leg. The only ones that seem to be successful in Australia in boats your size range have a diesel sterndrive with a modified, extended drive leg. They go OK but are pretty slow, probably due to the deep leg drag and the not inconsiderable power needed to shift these craft. Google SharkCat or NoosaCat and you will find examples. The only ones I've seen with single OB set up are Webster Twinfisher, a small cat about 16 feet. Sold a few so must get away with it. I heard people were using 4 and 5 bladed stainless props on them to beat loss of prop traction.

Also, I think 1x 90 hp sounds very low for your boat, more like twin 90's would be a starting point you'd think.

 

from a magazine review of the Twinfisher...........

Weird Science
So why would a boatbuilder settle on a twin-hull design for a single-engine fishing boat? Trailer Boat spoke to Ron Webster about the method to his madness.

"I knew catamarans were more stable, but they can make you terribly wet from splashing and are expensive because they need two engines," he said. "With a single engine, there had always been too much cavitation.

"I was determined to defy the experts and work out how to build a twin hull that operated with a single engine in the middle".

After five years of tub testing and four or five full-sized prototypes, he finally came up with a hull that worked. He then left his job of 22 years as a welder with Email to set up shop in a shed after many requests "for a boat the same as yours".

Ron was the principal until April this year, when he was forced to sell the business due to health reasons. Husband and wife Daniel and Kelly Schofield took over the business with all the same employees. Ron is still on hand for advice and new designs.

"Our number-one goal was to have a website so customers from all over can see our range and hopefully get a much clearer view of these boats and how great they really are," said Kelly.

"There is absolutely no change in the design or layouts of the boats. We work very closely with our dealers to give them the best boat at the end of the day, and we are happy to work together to improve or continue the way the boats are designed and fitted out."

There are now seven employees constructing over 200 boats per year.


Sounds a bit daunting, but obviously can be done.

 

http://www.boatlocator.com.au/boats/14210
..........plenty of pics there. And some practical discussion of another single rig cat here.........

http://www.ausfish.com.au/vforum/showthread.php?t=141680

 

Just to clarify, my Skoota is not a planing cat, cruising speed is 15 knots with a single 25hp

I agree that for high speeds you need twin engines, one behind each hull. So if you only fit one engine don't expect speeds much over 12 knots, however big the engine.

I did some testing on a 35ft sailing cat about 20 years ago with a variety of engines. With a 4hp it did 4 knots, with a 9.9 it did 6 knots, but with a central 55hp the boat would only do 11 knots, because of aeration. However when we put the engine on one hull it did 16 knots, and did 22 knots towing a waterskier with twin 90's. Of course that was a sailing hull so was inefficient as a powerboat. And the drag from the rig must have slowed it down significantly. You can see a photo of it towing a waterskier on my website

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks every one. I appreciate the comments. Stan

 

Years ago I worked for a Powercat dealer and it wasn't uncommon for someone to want a single engine. The solution was a factory made pod the attached to the center of the tunnel and was slightly higher then the bottom of the main hulls. This prevented the confused water in the tunnel area from creating a shower as it hit the leg of the outboard.
This pod only needs to be a couple feet long and about a foot wide. Good luck with your project and happy boating.

 

Anti-ventilation foil on a powered Tornado