Placement of single rudder on catamaran

Hey Dan.

I think you're right that this might not work as well for larger cats. My opinion is that much larger than 20-24 feet, there's not a lot of benefit in making cats trailerable. The tunnel-hull syndrome becomes pretty hard to take.

Right now my hulls have a max beam of 44 inches, which I think will leave enough width out of 8.5 feet to have a small outboard well in the central spine.

 

Does that mean you are planning a spine at 20" That's more than big enough for a motor. What is your fineness ratio on those hulls at waterline? Not as easily trailerable, but I like the look of the K680 which you may be able to emulate with your cabin.

Dan

 

Dan, my arithmtic comes out a little different than that. two hulls make 88"which I think leaves 14" for the spine. I know some small outboards are no more than 11 or 12 inches wide, so I think it will be possible.

At the waterline, my hulls will be a little finer than 10 to 1.

The cabins are quite low, only 18" above the decks.

Here's a profile sketch, though without the central spine extended aft of the transoms.

 

Right you are--unless your central spine goes under the interior projection allowing interior room.

Dan

 

Ray,
go here, http://www.woodworkforums.com/f33/boating-progression-39544/index3.html And scroll to bottom of page 3. Some good pics of the Kelsall centre pod set up on a trail 27.
Cheers, RR.

 

Reuben, thanks-- those are some excellent pictures. Mik was very encouraging when he offered some comments on another forum. That pic of the boat motoring, with the motor and tank in the central pod... it looked highly practical, didn't it?

It looks like the motor is set a little aft of where I might have put it, at the pitch center of the boat. I wonder why?

 

"It looks like the motor is set a little aft of where I might have put it, at the pitch centre of the boat. I wonder why?"

Leverage for manoeuvrability, difficult to steer with thrust towards the centre ?
RR

 

Maybe so. It's also occurred to me that maybe moving the motor aft a bit improves the prop wash effect on the rudder.

I went out to the nearest Bass Pro Shop yesterday and looked at their motors. Unfortunately, all they had in the suitable range was a fat little 3 horse Mercury, and it was at least 14 inches wide, maybe a bit more, so I don't see how I can fit one like that in a spine well, at least not without redrawing my cabins to be even narrower at the gunwales.

While I was there, I took a look at the trolling motors. Made me wonder about a couple of things.

Where I live, winds are pretty reliable. In fact I've never been completely becalmed in Slider, which I've still never put an engine on So for the most part, I could get away with using an engine.just for shooting bridges, and getting through narrow upwind canals. You usually have to have a house battery for a cruising boat anyway, to run stuff like lights and electronics, so why not a trolling motor for emergency motivation? According to the KAPER drag calculation module on Freeship, my little 20 footer has about 45 lbs of drag at 5.5 knots. Of course, chop would add quite a bit to that, but still, you'd think a 45 lb thrust trolling motor would push the boat at 3 or 4 knots.

What do you think?

 

i think not enough reservation needs to boat with this mass. smallest motor than you need is not good and will makew trouble. maybe think more on motor.

 

My biggest complaint about electric motors is not being able to know how much range/time/charge is left without more gear or guesstimating. To get the range requires more batteries and weight. In turn, this requires more time to re-charge.

I'd say go for the gas outboard but don't insist on having it centered. It's a pain for sure but it will get you home. I might consider an electric for the opposite/second motor for maneuvering purposes. Admittedly, you're not going to want to but two motors on a little trailerable but I haven't got a better idea at the moment.

 

I have, two motors and a generator !

 

You know, if I were only going to sail the boat locally, I think a trolling motor would do it just fine. But after I wrote the above, I remembered that the point of the design is to build the smallest cat that can make the crossing to the Bahamas and on to the Caribbean safely and expeditiously.

Since that's the case, I don't think a trolling motor will do the job.

It's occurred to me that I need not follow Kelsall's approach in every detail. I mean to make the center decks removable (I like hard decks) so why not hang an outboard bracket on the side of the central spine and make it removable too? That way I don't have to cut down the beam of the cabins as much. The only downside is that the rudder won't be in the middle of the prop wash, but it will get some of it. The upside is that the central spine won't have a hole in the middle, so might be stronger and certainly lighter.