Converting from Sail to Power

[Dave Chapman]

I have been thinking about changing a sail cat to be a power cat..... has anyone done this or have any thoughts about it??? thanks

[Hudson Force]

Give us some more information about the plan and the purpose. When you lower the sails and motor the sailboat, it is a "power cat", although with far smaller engines than I suspect you're desiring. Are you looking for a change to accomodate fixed bridge clearance? ....more speed? ...is the sailing option a "complexity"?

[Dave Chapman]

well for 1 thing I have heart probs so working sails might be a problem ... I live on east coast so will be using icw and have a lead on a dufour 82 reasonably prices ... thought of removing a couple of beds and installing fuel tanks

[GordMay]

Greetings and welcome aboard the CF, Dave Chapman.

[Hudson Force]

Yes, the mast height of an 82' catamaran would not manage to transit the ICW. I looked at the specifications for a Dufour 82 and saw an example powered by a pair of 130hp diesels and a 4'10" draft. This sounds very suitable for the Alantic Coast ICW. The question that comes to mind would be the need for such a huge vessel. With physical limitations it might not be the best choice unless there were ample crew. Also, if you would be cruising within the areas of the ICW diesel fuel sources are available within distances that might not require a major tank refit, but I can see the advantage in holding more deisel at a good price. Some might consider a vessel that size as a burden rather than an advantage.

[Dave Chapman]

ty gord and hello to you as well ... the dufour is just a thought running thru my head ... I just wanted to get some input about it ..... thought it would be a nice ride from here to the Caribbean

[Sand crab]

Sailboats have keels or boards to counter the sideslip. You won't need these at all with power and they will slow you down. A daggerboard boat would probably be preferable because you could just put those somewhere else off the boat. But then it would be an Outremer or a Catana. That would be like converting a Ferrari to a motorhome. The engines on sailboats are usually fairly wimpy with most not even able to attain 10 knots. Plenty of powercats out there. Just buy one of those.
That big Nautitech 82 was only built as a charter version. You have to leave the salon and go outside to enter the cabins and go down the steep stairs to each one. So, you have to go outside to go inside! You do not want this as your personal boat.
Someone recently bought a really big Nautitech (bigger than 82) in the northeast US that was already undergoing a major refit. It was only a few $100k which is cheap for a 100' boat. But ya better have about $1 mill at least to finish her. Is this the one? It's the same go outside to go inside deal.

[double exposure]

Yes, it can be done, here is a youtube video of my boat. Mine is the opposite of what you are talking about though, it is a powercat (planning hull powered by twin Evinrude E-Tec 115's) and has a sail rig.

Sailing Catamaran Double Exposure, 0 to 20 knots in less than 25 seconds on a 40' cruising cat - YouTube

[Cotemar]

Sail Catamaran to Power catamaran is an easy conversion.
Someone has already done a Dufour 82 Sail to power.

Fountaine Pajot will sell you any of their sail cats in a power cat format at a cheaper price.

[Sand crab]

DoubleX. What do your hulls look like? I mean the actual configuration? Are they the exact same as the power cat version? I like your boat btw.

[Lagoonboy]

Chapman,
The main factor when using same hull is the hull rocker, which causes the squatting under power at higher speeds. Depending on the size of boat and design of hull this may be a small or medium issue. If your trying to get to 15-18 knot range it usually is ok, but try to push sailing hull faster than 18-20 and you will experience squat. John shuttleworth actuall had trim tabs on his earlier power cats for that very reason.
To give an example the Lagoon 52 cat powers at just over 10 knots with 75 hp Yanmar w sail drives & fold props, take off the rig and throw in 110hp yanmars with standard prop and shaft and she "should" hit 15knots. She may have a little squat if trying to push faster than that speed. Not bad for a 56,000lb boat. So if you want swift trawler type speeds go for it, if you want power cat 20+ speeds get proper hull design or modify the existing hull rocker.

We developed a 52ft cat design that can be configured as Trawler cat 10-12 knot cruise or performance sailing cat. It's a lighter weight version of the Lagoon 52 at 18,000lbs. Power will be 75-120hp for trawler version. And will top out at 15-18 depending on power.
More info at Maltese Catamarans MP52-trawler.

Side note: have friend who plans on buying older sail charter cats for good price and do exactly as you plan.

Reality is that is many places most of these condomorans are used as just big power cats anyways. Maybe actually really only sailed 5% of the time.

[Sand crab]

FWIW here is that Nautitch 98 in Rhode Island that I was posting about.
Nautitech 98' Charter Cat For Sale at CreepyCrabs | Buy Multihull For Sale

[gosstyla]

Sail cats are also much wider overall than optimal for a power cat. On a sail cat you want distance between the hulls for stability. On a power cat you want the hulls just far enough apart so their bow waves don't interfere under the bridge deck.

[DaveOnCudjoe]

Perhaps the obstacle is more easily understood if approached from a different perspective. The force driving a sailboat is applied at the center of effort of the sail plan, the hulls are designed to efficiently transfer that force into forward motion. When driven by power the force is applied to that same hull at a point below the waterline. Since these forces are applied at opposite sides of the center of bouancy it's difficult to design a hull to respond well in both situations. It squats because the bow is too full and the prop digs a hole that the stern settles into because it is too fine. Dave

[Lagoonboy]

Gostyla, The width factor you mention is not really due to wave interaction but rather the need for sail cats to be wider for stability reasons. Power cats can cheat slightly on width because they don't have sail rig trying to blow them over..

Dave, The squat issue you speak of does occur when you are trying to push a certain (narrow stern) hull design way past its actual hull speed or due to lack of buoyancy in aft part of hull from over heavy motors. As long as you don't try and push past the 18-20 knot barrier most hulls will not squat. But throw in way too much HP and try and break that speed barrier and you are better served with proper power cat hull or modifying the existing transom design.

Example below: A colleague took Viva sailing Cat and modified to Power Cat with lightweight Pod and reverse bow design. With twin 15HP he can cruise the 30ft? cat at 15knots and tops out at 18knots with no squatting issues. Notice the hull design. He claims to get 8-10MPG at cruise speed.

Moral of story is if you are ok with sub 20knot top speeds a performance sailing cat hull will do fine under increased power. If its a slow sailing cat to start with then maybe that may be a issue..