Converting sailing cat designs to motor power: What are the prime considerations?

[Chotu]

Originally Posted by valhalla360

A high speed version would be very different.

A low speed displacement shouldn't be substantially different. Just a few items really:
- Fixed props.
- You could go with smaller rudders.
- You could go with smaller keels (but still keep some keel)
- Maybe increase fuel tank size if the original was very small.
- If you wanted to, you might de-strengthen some of the areas where the rig attaches as they no longer need to be as strong...but not really necessary.

Exactly. That’s the punch list

[contrail]

For what it is worth, here is some actual experience: In 2010, I took my 1999 Leopard 45 sailing cat down the ICW, Chesapeake to Ft. Lauderdale. I trucked the rig down, separately, as I was going to renew the rigging in Ft. Lauderdale, and I wanted the riggers to start working before I got there. My boat was the perfect ICW cruiser! Without flybridge, we never had to wait for a single bridge other than the two that float....don't know if they have been replaced. But even the low ones were no challenge...we had about a foot or two above the arch and only had to quickly swing down the SSB antenna, on the fly. The engines are two Yanmar 4JH3E'S, rated at 50 HP sustained and 56 HP max, each. I am accustomed to motoring on one engine, anyway, and I averaged a bit over 7 knots at about 1.25 NM per hour. So the mileage was about 5.6 MPG, actual. If I used both engines, about 8.8 knots was the max cruising speed. The fuel tanks I had then were 80 gallons each, with about 75 usable, so the total was about 150 gallons, maybe a gallon or two under (I never ran out!) Anyway, if you do the math, range was almost 850 NM. She maneuvered perfectly, and drew about 4'5". The only issue was that I had to mount a temporary VHF antenna, and steaming light, and the radar went on the truck with the mast. I always said that if I wanted a power cat, I knew just what mods to do, which would include removing the mast and the arch, and replacing the latter with something lighter out of stainless steel, as it would not need to take the mainsheet loads. Don't know if this example is of any help, but it was about a thousand mile trip, and being able to just charge along ignoring all the bridges was a huge advantage!

[Chotu]

Contrail: That’s the experience we are all speaking from I think. I don’t think anyone here is just imagining what it would be like.

[Sand crab]

Lagoon and just about everyone else makes a power version of their cats. Does anyone know if the use the same hulls (and keels) or if it's a unique design? I think most have much larger engines than the sailing versions so maybe they are planing hulls.

[Chotu]

Originally Posted by Sand crab

Lagoon and just about everyone else makes a power version of their cats. Does anyone know if the use the same hulls (and keels) or if it's a unique design? I think most have much larger engines than the sailing versions so maybe they are planing hulls.

It’s definitely unique. They use a flat rocker in the back. They tend to use semi displacement type hull shapes. Much larger engines. They aren’t as fuel-efficient as what the original poster is looking to do. But they are a lot faster.

I can’t seem to find a good picture of one on the hard, but you can even see in this picture of one underway. The Stern is flat. Like I was posting earlier in the thread. No rocker aft. This gives it a semi displacement type of hull.

[valhalla360]

Originally Posted by Chotu

It’s definitely unique. They use a flat rocker in the back. They tend to use semi displacement type hull shapes. Much larger engines. They aren’t as fuel-efficient as what the original poster is looking to do. But they are a lot faster.

I can’t seem to find a good picture of one on the hard, but you can even see in this picture of one underway. The Stern is flat. Like I was posting earlier in the thread. No rocker aft. This gives it a semi displacement type of hull.

That's a newer one with a design goal of running in the teens. Yes, if you want high speeds, you need lots of power (hundreds of HP) and a hull that can exceed normal hull speed considerations.

When they first started doing trawler cats, I'm almost positive they just used standard sail hulls and upped the hp from something like 50hp to 75hp per hull. Speeds were more trawlerish (9-12kts).

My guess is just like regular trawlers, the folks buying them were more interested in speed than efficiency. A lot of the monohull trawlers have gone to large twin diesels and semidisplacement hulls because they have a goal of speed not efficiency.

Some more dispalcement focused designs:
https://www.yachtworld.com/boats/201...yacht-3746841/

https://www.yachtworld.com/boats/202...ce-my-3648068/

[Sand crab]

Comparing the Lagoon 40 MV to the Lagoon 39 SV of the same vintage one can see many similarities. The L40 has a stern extension so it is a little longer but the beams are just about identical. Exteriors look pretty close too, note the hatches in the hulls. I think Lagoon used the 39 hulls but modified the design for flatter sterns like Chotu said and probably modified the keels too. That's what the stern extension does. I see one L40 with 55 hp engines so probably not a speed demon but cruise at 15 knots or so. I'll bet a beer they are basically the same boat.

[Sand crab]

Here's the L40:

[mcarthur]

Originally Posted by valhalla360

When they first started doing trawler cats, I'm almost positive they just used standard sail hulls and upped the hp from something like 50hp to 75hp per hull. Speeds were more trawlerish (9-12kts).

My guess is just like regular trawlers, the folks buying them were more interested in speed than efficiency.

There's been power cats for quite a while, but FP sorta started things with the Greenland 34 as a production powercat that was designed to cruise (as distinct from other purposes). They even named it a "trawlercat". As far as I know, this is the first production cruising powercat and it definitely had a SD hull shape. 44 were built I believe.
This was around the time Malcolm Tennant in NZ was creating his hull shape, Brady in Australia was experimenting, and others around the world were seeing what could work in a power-specific cataraman. I've a little list on my blog about what makes a long distance power catamaran, and another one on cruising powercat fuel efficiency that needs to be brought uptodate.

I agree with the speed comment too - I'd say there was two groups: one being owners of existing monohull "trawlers" and sailing boat that wanted more space and a little more speed. The second being sportfishing and faster boats that wanted longer range and more space, but not at the expense of speed. The powercat provides for both: run then at 10-12kn and consumption is much better than most mono trawlers at speed (I'll ignore Dashew ), or run then at 20kn and get much better consumption than the sports boats.