motors on beach cats

I've got a tired old Nacra 18 square that I'm planning on bagging a hard deck up for, center console seats and an engine well.

Question is should the motor be mounted more amidships and continue using rudders or push it rearward and use the motor for steering? I'm assuming partial daggerboard use to keep it from skidding.

 

I should add that motor size is going to be between 8 and 12 horsepower and I'm planning to build a surface piercing skeg just in front of the motor like the Livingston cats that were built around here.

 

I would mount the engine forward and use the rudders for steering. Why would you keep the daggerboard if it's not going to be sailed?

 

You dont want daggerboards for a powered boat over 6 knots. nasty things can happen. You would be better of doing a 6" keg from 2/3rds back on the hull running up to the stern.
In my experience with outboards on narrow hulls, build it like one of the early Kelsall catamarans with a substantial pod down the middle, and the outboard on the transom. This stops the wash from flooding the motor, and makes for the best steering. If you put the motor amidships, the steering goes all strange.

 

Maybe there's no problem with the engine installed further forward ...

A couple years ago I saw online pictures of a power catamaran with hulls like a beach cat. It had an outboard forward (not amidship) that provided only thrust, no steering. The boat was steered with twin rudders like a beach cat, but they were connected to a center console so the captain could stand up in the middle of the boat.

The pictures made it look like a great little catamaran for playing in the surf and waves. I think it was used as a surf rescue boat if I'm not mistaken. Unfortunately I've looked for the pictures again many times and have never been able to find them.

The outboard forward was claimed to work great. It ran in clean water of course, so it avoided the turbulence and aeration that sometimes occurs in between catamaran hulls abaft.

 

It's really prone to strong afternoon winds around here and I thought the daggerboards could assist in giving ''directional bite'' during trolling and docking maneuvers.

 

I can pull the daggerboards up at speed, mostly I'm trying to anticipate maneuvering an extremely light boat in gusty winds in a tight marina and thought the boards would give me decent bite at slow speeds.

I've had the pleasure of steering big cats with twin motors and what a slick situation that made of docking and turning so maybe that's the way to go with a couple of little clamshells on each hull and small twins for push.

 

If I was you I'd go for twin motors, mounted aft. A second motor on the sea is always a good idea. Outboards like to push, so mounting them foreward in my opinion is not such a good idea. When you sail you trim the motors up, when you motor you trim the rudders up, as with the daggerboards.

A single motor in the centre is used, I've seen that, personally I don't like it. In choppy waters you could experience some wave slap at speed on the motor, while the ones behind the hulls won't have that problem and the water level is kept at the right height for better effieciencey for the motors by the hulls displacing the water at speed. It is also going to be much easier to make a strong enough transom for the motor mounts on the hulls than to make one amidst deck.

 

It would be terrible playing in the surf! For a start you lose all the manouverbility, you swamp the motor when you drive through waves as well.
And *what* turbulence and aeration between cat hulls abaft???? Thats a strange cat!

If you could have short daggerboards for directional manouvering, but you wouldnt want them while under way.

Twin outboards would make a lot of sense, but I didnt suggest them because these seemed to be a budget exercise.
Fanies angle on safety is a very good point for smaller outboards.

 

Obviously you never saw what I saw.

Not at all, you're theorizing about something without seeing or understanding it first.

Nope, not on the boat I'm talking about.

Turbulence from wave interference based on the existing waves and the angle at which the hulls interact with them, in conjunction with the waves generated by the hulls themselves.

 

The rudders and daggerboards require some forward velocity to develop lift; you won't have very much steering authority below a half knot. An outboard steers fine at any speed; rudders would be redundant. Long shallow skegs would serve better than daggerboards because they would be more tolerant of transient side loads. They would still provide some lateral surfaces, but the daggerboards would be too effective, especially in small waves and crossing wakes. The hulls themselves will provide a lot of 'railroad track' stability at slow speeds in gusty crosswinds, primarily because they have little windage. If you're going to surround this vessel with a white picket fence [spray shield] and a badminton net, [center cockpit widshield and rod holder bar] the windage is of your creation.

One precaution. The slippery smooth design of the sailing hulls will work against you; water clinging to that smooth wetted surface will suck the boat down when it could be rising up to plane if it had some short flat surfaces. Remember the lessons of early seaplanes. It took a stepped hull to get up to flying speed.