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

[Ifitsworthdoing]

As the future commissioning owner of one of these (a Prout-derived Broadblue 346) I wonder about historical precedents, user experience and general principles. I am aware that the Canadian PDQs and Floridian Endeavourcats were derived from prior sailing versions, but this seems to be much less evident a trend in Europe.

Please note I am focussed on cruising/trawler models with displacement hulls not semi-planing high-HP fuel guzzlers, and in particular I am looking forward to the specific demands of the coming electric propulsion era. Of note here are pioneers such as the Sunroof Eco line where largely identical sailing and motor versions are available. I would welcome comments especially from those with nautical design and/or engineering backgrounds.

[Dave_S]

If your just looking to travel at low speeds, most sail boats already have a suitable motor for that installed, ready to go. Just take down the rigging.

If you were starting from scratch to build one or the other, you would make the hulls differently but how much that matters at low speed on a catamaran I'm not sure. The sails provide some stability and I would think a pure motoring cat may have some extra stability in the hull design and weighting but again, I'm not sure it's really much of an issue.

[Sand crab]

I'd probably cut off the keels.

[valhalla360]

Originally Posted by Sand crab

I'd probably cut off the keels.

You might modify the keels but you do still want them.

We had lifting boards on our Gemini. If you kept them fully retracted, it was a total pig getting into a slip, sliding sideways and getting blown about by the wind. A foot or so of board down it was night and day difference close quarters maneuvering.

To the OP: Basically, remove the rig and you are generally good to go. Most modern boats already have sufficient power for displacement operation. Unlike a monohull sailboat that derives dynamic stability thru the mast, a catamaran hull is inherently much more stable.

[Chotu]

Adding to the above, that’s all correct information. Especially about the keels. Do not cut them off completely. You will be moving sideways all the time when the wind pics up. When I have my dagger boards up, my boat does not handle well in close quarter power situations. You have to have a little bit of something down there. To keep it from going sideways. Literally.

The mast does not contribute to stability on a catamaran in any way, a Catamaran is form stable. That means the shape of the boat itself is what gives you the stability. If you think of it like a car and a motorcycle, that’s a good comparison. A motorcycle is like a mono hull, a car is like a catamaran. It is the shape/form of the car that keeps it stable around the corners. The motorcycle leans. If anything, a mast subtracts some stability from a Catamaran in the form of weight too high up and forces acting to tip the cat over. A power catamaran is basically the most stable configuration of a boat possible. Including all monohulls. In the doomsday storm, you’re in the best position in one of those.

If you wanted to get a little bit more speed out of the thing, I believe getting rid of some of the rocker aft is a good idea. That contributes to a better top end speed. You’ll notice more efficient power boats do not have rocker aft..

Depending on the specific engines, you may like to upgrade to larger engines and different larger diameter props. Maybe. Depends on your ultimate goals. But it will operate like a good trawler out of the box. With my Catamaran, with a single outboard at 30 hp, I pass lots of trawlers.

Then there is the main point of my post.. Tankage. You’re going to want more fuel. No matter what, a sailing catamaran is not designed with the level of fuel that a trawler has. So you want to find a way to get more tanks in the boat. That will increase your range and let you compete with the other trawlers.

[Ifitsworthdoing]

Originally Posted by Sand crab

I'd probably cut off the keels.

Not an option as the clean water tanks are inside the stub keels ...

[Ifitsworthdoing]

Originally Posted by Chotu

Then there is the main point of my post.. Tankage. You’re going to want more fuel. No matter what, a sailing catamaran is not designed with the level of fuel that a trawler has. So you want to find a way to get more tanks in the boat. That will increase your range and let you compete with the other trawlers.

I realise that even with a large solar panel estate and battery banks, electric propulsion will only allow for day hop excursions of 30-40nm. For actual passages (in the Med never more than 250nm) a diesel hybrid set up is unavoidable, and my default 200 litres tank is just about sufficient for that using around 12kWh of generator capacity.

[valhalla360]

Tankage is a consideration but displacement vessels are pretty good on fuel economy.

Our 34ft catamaran with 25hp gasoline outboard, we estimated 6MPG at 6kts. We had about a 300-350mile range on full tanks. For coastal cruising, that will typically be more than adequate.

Diesel versions of the boat got a bit better and if we slowed down to say 5kts, there was a big jump in MPG.

[MIRELOS]

but really, WHY?......you can use it as a motor sailer and if the engines fail...raise the sails..voila!

[valhalla360]

Originally Posted by MIRELOS

but really, WHY?......you can use it as a motor sailer and if the engines fail...raise the sails..voila!

For coastal cruising, mast height is often a limiting factor. Particularly with bridges.

Even with draw bridges, they often open on a schedule. I recall areas of Florida where you either race to get to the next bridge or you have to lollygag for an extra 30-60min to slow down for the following lift. Then you are bunched up with other boats who were waiting when you do take off.

[Sand crab]

I guess I was wrong about the keels. Does anybody have pics of the motor and sail versions hauled out for comparison?

[jmschmidt]

Most charter sailing cats are motored 90% of the time anyway such that the charter companies have started to charge for fuel where it was once included in the charter fee for sailing cats.

A 47ft. sailing cat that I part-owned had long rudders, a short skeg keel, folding props and two 54HP engines. The 47' PowerCat that I now own has short rudders, no external keel, fixed pitch props and two 150HP engines. Why don't you just buy one of those.

[Searles]

A true motor only cat would have verry different designed and build than a vessel designed as a sail vessel,they may both be cats but are totally different beasts ,if done properly.⛵️⚓️

[valhalla360]

Originally Posted by Searles

A true motor only cat would have verry different designed and build than a vessel designed as a sail vessel,they may both be cats but are totally different beasts ,if done properly.⛵️⚓️

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.

[valhalla360]

Originally Posted by jmschmidt

The 47' PowerCat that I now own has short rudders, no external keel, fixed pitch props and two 150HP engines. Why don't you just buy one of those.

What type of hulls?

I know in the power boat world, standard V-hulls are pigs around the dock. We had one with a 6" keel on our flybridge and it was much better handling at slow speeds compared to similar boats without.

Maybe with 2 long narrow hulls, your power cat has decent directional stability