Home-made twin .21 catamaran

[BB Mods's]

Yes if you make some shims that go under the neck of the sleeve, you will change the location of the ports in relation to the crankcase, you need to do a few things if your serouis about building a hot cat, If you dont want to go any further then shiming the sleeve up a little thats fine but wont be the best these motors could do... If you want the easy way out.. Buy 2 x CMB 21RS motors, or 2 Novarossi Plus 21-5's..

But these motors look to have some potential, could you post pics of the internals? like internal shots of the crankcase where the sleeve goes, Con rod, crankshaft, carb measurements and current port timing numbers and squish height, we might even get into bowl shape and volume seeing as how you have access to the tools to do some of this work, and would further your education into how these motors work. PM me if your interested.

BTW, Id try it anyways with .010 worth of shim and shorten up the pipe a little more

Umm.. What pipes are you running??

 

[BB Mods's]

Ohh, and after your done with the boat and settled down a little, id pay your your asking price for a set of your struts, and 1 rudder.

 

[Jim Caldwell]

BB Mods, Remember they don't run high nitro and he will have two props in the water for thurst, Given that, won't the stock set up be the place to start?

 

[gratte-tout]

BB Mods, Remember they don't run high nitro and he will have two props in the water for thurst, Given that, won't the stock set up be the place to start?

You're right, aver here we actually use 25-30% nitro, not more.. I'm going to use MAcs tuned pipe, with home made muffler.

Some picts about the inside of the engine I use. They are great machined, very serious, not like a Sh one, it's asian engine but weel done, and they are supposed to take high rpms.

Mufflers :

BB Mods's, not problem for a pair of struts and a rudder, but I will use metric screw and metric bushings, don't you think it's could be a problem for you?

 

[anthony_marquart]

If you do shim the sleeve up in the cylinder, you will want to take the same amount off of the mating surface on the head button. This will place the head button back in correct place relative to the piston. This put the compression back where it should be.

BB Mods, Remember they don't run high nitro and he will have two props in the water for thurst, Given that, won't the stock set up be the place to start?

You're right, aver here we actually use 25-30% nitro, not more.. I'm going to use MAcs tuned pipe, with home made muffler.

Some picts about the inside of the engine I use. They are great machined, very serious, not like a Sh one, it's asian engine but weel done, and they are supposed to take high rpms.

Mufflers :

BB Mods's, not problem for a pair of struts and a rudder, but I will use metric screw and metric bushings, don't you think it's could be a problem for you?

 

[BB Mods's]

Props that are spun out of the hydro dynamically efficient range become so inefficient there worthless. Its the same reason us FE guys don't try to spin our props 60k rpm's. I think you should try the motors stock, I'm not deterring that. My comments where to you if you found the motors to just not be enough.

 

[gratte-tout]

Next news : gluying the tunnel, and starting the radio box.

To be continued : gluying the deck (after putting some foam into the closed area), and starting to put all the hardware. As I done it on CAD, it will be fast I think.

I thought about what you tell me about the engines, at first I think I will have to deal with the engines settings (carb almost). I won't add now difficulties to set them.. To help me, I will put a datalogger system (eagletree) to give me precious informations about rpms and t°. And it will also help me to know how much rpm can take the engines, and decide (or not) to do what you tell me.

 

[BB Mods's]

Just trying to help.

 

[gratte-tout]

You were right tell me that, I didn't know I could do such mods, don't worry, it's marked in some place in my head, I will remember it if necessary, and I think I will contact you by pm to have much details about theses mods.

You also talk about cranckcase mods, may I contact you to give me more information if necessary??

Thanks again!

The eagle tree system will give me precious information, about the engine rpms. Wich could be the upper limit for the rpms? You talk about 25.000 rpms.. I thought the more rpms the engine take, the best it is, but you seems to told me it's not really true.

So do you think it's better to run a X445 @ 25.000 rpm than a X442 @ 28.000?

 

[topfuel443]

Very cool buildup Fabrice, nice work!

 

[BB Mods's]

No, Id spin the 442's for testing, since no one knows where the best rpm range for them motors is, we can only try and guess, then set up form there. Test many prop's and carb settings.

 

[gratte-tout]

That's what I guess : I bought many differents props, I have 1740/3, 1742/3 (these two may have too many lift effect), 1.6/3, X442 and K45 for break-in. All in pairs

 

[gratte-tout]

Last news :

Installing the hardaware on the hull. : strut and drives.

Detail of the remote controlled angle of the drives :

The engines are on place :

And then, before : a M6 screw, after, a stainless steel water outlet!

 

[dschigoda]

I really enjoy seeing your project photos and progress. About 20 years ago, I designed a similar boat, but never built it. One reason I decided not to build it was that I was not sure how the boat would steer if the engines were not turning the same RPMs. Please keep us posted as to how the boat runs. I recall years ago that there was an "engine syncronizing" device on the market, made for multi-engine r/c airplanes. The device would monitor the RPMs of each engine. It would slightly retard the throttle of the engine that was running too fast, to match the slower engine, thus keeping them in sync.

Dave

 

[gratte-tout]

In fact, I bought the Eagle tree system to monitor the engine's rpms. It would help me to synchronize them, even if I thought it's better to have a little difference in the rpm, to avoid the resonnance (same frequenty) of the engines.

I hope, if there's different rpms (and I'm sure it will be the case), the rudder will correct it, and the prop may also have little different sliding.

I hope also it would not be too hard to set them, bigger engines would have been easier to set, but would not have screamed @ 28.000 rpm like .21 do!!

 

[gratte-tout]

Last news : decks are glued, one or two pins, few clamps and it's ok

The front of the hull is plenty of foam.

Fabrice.

 

[Nigtmare]

hey Fabrice. You should have the exhaust exit from the bottom of the hull. That will make the boat alot quieter and maybe easier to clean.... You are doing a excellent job on that boat.

Nick

 

[gratte-tout]

In fact, there will be mufflers on the tuned pipes, and the exhaust exit will be on the bottom I think.

At first, I wanted to put them on the top, just to see the smoke of the engines, but then I thought like you, it may be quiter.

For the first tests I think I'm going to put two silicon exits, on the side of the hull, just on the top of the deck, then I may choose a perfect place for them.

Thank you for your compliment.

 

[gratte-tout]

Hi all,

The hull is finished, just have to add two pads under it..

 

[topfuel443]

Very cool boat Fabrice, and first class work! When does it get wet?

~James