A request from an amateur hungarian boat builder

[kitzoli]

Hello Everyone !

I live in Hungary, and my hobby has been boat building for a couple of years. (By the way, i am in the office technology business and i am 54 year old.). I have been reading this forum for a long time and i have learned a lot. I have already built a couple of boats, and have been studied the different methods of building, materials. I had faileure and success too. Usually i am foccusing on economic boats. Now, i would like to ask for you opinion, advice in a concerete case. There will be a ride („Green Ribbon Ride”) for electric boats on lake Balaton (its an 594sqm size, 77km / 45 miles long lake). I want to take part on this ride with my friend with a catamaran. I also bought a boat designer software (Prolines Basic), and i would like to get the best of it with the following properties:

displacement of each hull: 305kgs
LW: 6.12 m
WB: 04 m
Draft: 0,26 m

The software draws a diagram, (enclosed), which i dont fully understand. The drag values (wave and friction drag) are in kgs. Does it mean that i can reach the given speed with the given kgs of thrust? If so, can it be converted to engine performance? Is there a thumb rule for that? May i draw any conclusion from the increase of the curve to the drag values at higher speeds (hull speed)? The drag values of the 2 hulls can be simply added together, or i do have to seriously take the interaction of the 2 hulls into account? The longitudal axis of the hulls will be approx 1,9 -2,1 meters far from each other. I dont have a high technical skill, sorry if my questions are dummy....

I enclosed a few pics about my previous experiments and my plans about the hull design.

thx for any comments

[gonzo]

That is empty displacement. You have to add crew and supplies to have meaningful numbers

[erik818]

If you want to go from drag to engine power you do like this:
1) convert the drag diagram from kgs to N. 1 kgs = 9.81 N.
2) make sure that the velocity is in m/s. If not, convert it.
3) required power to propell the boat is:
P[w] = velocity [m/s] * drag [N]

Next step is to guess the efficiency from the motor to the propeller. My guess is between 50% and 70%. Divide P in the equation above with the efficiency and you have the engine power as a function of velocity.

After that you have to choose a suitable propeller to transfer the engine power to the water.

Erik

[hoytedow]

Welcome, kitzoli.

[Motivator-1]

You can look at 2 threads already getting very involved in Electric Catamaran Design. Mine is "Another Attempt..." and the other is "Poorqeedo". I still wish we could combine the data from these and other threads to have one site for Catamarans only design. I am not a significate poster here to feel right about starting it, but maybe someone here will attempt to put it all together.

You already have great Boat Building skills that will make you a beneficial part of this Forum, and to have relavent input of your own. You will get great advice from guys here, like Porta, Jeremy, Ad Hoc, and a number of others. (Sorry if I forgot your names). I have gained so much in only a couple of weeks.

I hope you can get what I have already received from this great community.

Welcome again,
John

[erik818]

kitzoli,
I checked the effect of the interference between the hulls for a catamaran of similar displacement and similar length and beams as yours. As expected the interference depends on the speed. At most the interference added about 10%drag to the interference of two single hulls. If this is a significant or not for you depends on what you want to use the prediction for.

Erik

[kitzoli]

Dear All you have replied me

thx for the really usefull help, we already started buiding the boat, here a pic

[kitzoli]

A bit later, but we managed to finish the catamaran, we have already tested it too, with a 4 stroke engine and with a 3*9*9,25 prop. The max speed was 15,6 knots with 600kgs total weight. I dont know its is fast enough or not, what do you think?
I have another question too. Over 10 knots speed, the water is spashing up at the stem (boot) of the engine. (see the pics) What could i do to stop this splashing? Please give me some ideas.

[Dean Smith]

Originally Posted by kitzoli

Dear All you have replied me

thx for the really usefull help, we already started buiding the boat, here a pic

looks like wonderful timber/ what is it?
In Summer I may pass by, where exactly are you?
there was a man here Rick Willoughby, maybe you could search his posts and email him, I am sure he would be very interested

[kenJ]

It is hard to tell from the pictures but my guess is that the engine tilt or that the lower end of the outboard is too shallow.

[p_smith]

very nicely done!

I would second the guess about the outboard being run too shallow. The ventilation plate should be just below the water. Perhaps mounting the outboard on a hydraulic jack to raise and lower it as opposed to triming it?

[philSweet]

The best solution I've found to the motor leg splash on a catamaran is a plank-on-edge arrangement Immediately in front of the motor leg. At 10+ knots you don't want much more than two degrees of angle of attack, so its basically a skinny wedge. Bottom should be faired into and flush with the anti-ventilation plate on the motor. It's worth doing because I've drowned motors with saltwater thrown up from the leg into the powerhead. It doesn't have to be as wide as the leg. 3/4 width works OK.

[kenJ]

Phil,
I don't understand what you are describing. Picture or diagram would be a big help.

[kitzoli]

Sometimes its difficult to express myself. Probably these 2 pics will help. Not the prop makes it but the foot / leg of the engine. Rising the engine does not help, the symptom does not dissapear or depreciate, but in bands the engine overspins.

[kitzoli]

Originally Posted by Dean Smith

looks like wonderful timber/ what is it?
In Summer I may pass by, where exactly are you?
there was a man here Rick Willoughby, maybe you could search his posts and email him, I am sure he would be very interested

.
Its fiberglass and polyester. I am in Hungary, Vasszécseny (close to the austrian border), your are welcome of you come. Contact me whenever you come

Its a pitty that Rick quited the topic. Sometimes i visit his site.