To install kind of leeboards at a solar electric katamaran

I built MARTA (motor katamaran, solar electric) for nearly 8 years now. LOA 6.3 m (20'), Disp. 1 metric ton, Draft 0.25 m (10"). 2 electric trolling motors (Haswing, modified) 1.44 kW each at the transoms of the half hulls. After launching her last week, I found her nearly not steerable. No way to go a course ahead - instead yawing all the time. Because of a cabin with standing height there is a lot of windage, the lateral plan is tiny.

So I will test to go with kind of leebords, (provisional in a fixed position). I'll give them a bit of an symmetrical airfoil profile (horizontal cross section) to lessen the added drag. Their position should be at the cabin front bulkhead.

Marta's is determined to go slow and silent at sheltered (and mostly small) inland waterways around Berlin, Germany. She is a kind of swimming coffetable for pensioners.

I've a poor knowledge about hydrodynamics (and nearly no experience with boating also). Comments very appreciated.

 

Hard to tell from the photo, but it looks like your bows are sharp and deep, and your sterns are a shallow vee and not very deep. I can imagine how this would cause instability. Maybe consider adding a fin or rudder in the back? Also, shifting some weight towards the back may help.

Also, you have a lot of windage. Is it unstable all the time or just in the wind?

Cool little boat!

 

Just wanted to clarify that I think leeboards in the front might make it worse, not better. Your boat is basically pivoting off the front end, and once it gets an angle to the flow the "lift" from the deep bow will push it off course, further increasing the angle, further increasing the lift, and so on. Imagine an airplane with the rudder on the front instead of the back.

 

Thank you @dustman
I'm very busy in my shop now, will answer later.

 

Get a 10' board and mount the front of it on a pivot bolt near the middle on the inside of one of the demihulls. Let the back stick out a couple inches behind the transom. Screw a horn timber onto the board sticking up behind the transom so you can control the height and keep the board from twisting and floating about. Locate the horn timber with a couple u-bolts on the transom. If you have a scrap board and a piece of 2" pvc pipe for the horn timber, I'm thinking it's about $10 for the u-bolts and two hours for a prototype. Might want a handful of SS washers too.

 

Lovely little boat. My vote is to install some jet drives and an adjustable foil between the hulls. You could be screaming along the inland waterways while drinking your coffee.

 

I was thinking a little 60's boxer. Tell us you've got an appropriate tow vehicle.

 

Spot on! Just missing the headlights and badging.

 

You are right, the bows and the sterns are shaped as you say. As I understood your thinking about the planned position of the boards I tried to temporarily mount them near the cabin back side for a test. But I failed because the planned fixing point was far too weak for the needed sturdiness.

Independent of the leeboards I fittet the circular vertical shaft of the trolling motors with airfoil profiled fins (glass and epoxy). A first short test was really encouraging.
In spite of the fact the fins are (at now) less than half of their height immersed.

 

Thank you - but I've to admit that I'm not able to understand your suggestion. My poor knowledge of English in general and especially of nautical terms is not sufficient. Sorry.

 

Jet drives are just the opposite of what I'm aiming for.

 

https://www.boote-forum.de/showthread.php?p=5302440&#post5302440

 

Nicely done. A board would have worked for testing, but you went all out. Let us know how it goes with further testing.

 

Today (nearly no wind going) a nice test result. After fitting a simple plywood plate (6 mm * 20 cm * 60 cm / 1/4" * 8" * 1') at each stern, Marta were staying on her course. Completely different behaviour to what was until now. Also doing fine after I had to stop the port motor because electric failure so it turns from thrust to drag because I couldn't tilt it out of the water. To be driven only by the steerbord motor was no problem for her.

That is the basis to work on. Clearly the challenge will come in different conditions with wind.

 

Thanks for the update, glad it is working out!

Might be good to experiment with different size boards to see how much or how little you need to balance the boat. If you put too much it might be a little slow to turn.

Nice work on the motor mounts by the way.