Hello to all,
I am opening this theme because me and my friend Kapetanios have a problem with a catamaran boat. To be more specific i am presenting you some photographs from the boat.




First of all, in the first, the second and in the third photograph i show you that the boat is polished before getting it in the sea. Secodly, in the last photograph i show you where i have put the helm and i would like to know if i am wright. Moreover, i tried many props and at last i put octura 670/2 which i think is the best for my boat. The problem is that the engine(RCMK S254evo) cannot reach the highest rpm and i believe that the exhaust system is responsible for this fault. I would like to inform me which is the best exhaust system for this engine. I must tell you that now i have set an 105 degree header as suppose to the header that is shown in the photographs, but the results are the same. Except from this, my exhaust is 2''. Thank you very much...

Sorry but something happen with the photographs. For this reason i am presenting them here:


[img/] img831.imageshack.us/img831/3171/53412503.jpg
[img/] img835.imageshack.us/img835/6398/65699955.jpg
[img/] img838.imageshack.us/img838/7431/49612185.jpg
[img/] img837.imageshack.us/img837/8556/1111qch.jpg

Right from the first photograph I knew exhaust system was FAR TOO LONG.
Using a WTC ( Wrap To Center) header even a zero band pipe pushed on short as possible would be too long by @ 25mm. Using a WTC and a 50mm banded pipe has the pipe near 75mm too long in getting to a correct length for peak rpm.

Sadly you will need to use a 100* or 105* header with that pipe in order to get a stock engine to wake up and turn rpm near 16K which it will if pipe length and prop load is correct.

On a STOCK RCMK Evo if you measure with a piece of string/cord down the center line of exhausts path from the manifold connection at cylinder to END of the band would want a length of @ 330 to 350mm approximately.

im not shur what you r saying,im no cat gruru but there r some thing you can check.i dont know what props you have tryed but for cats the 6717/3 prop works pretty good a 7016/2 works i guess there r a few others i guess someone can tell ya. you say you got a 105 header on there right now check the lenth , start at 12in from the plug to the center on the pipe(stright back)also check the strut start from the ride pads to the bottom of the strut about 3/8of a in.if you keep having problems there r a few guys on here that can help ya but i think you r going to need better pics.

My 2 cents is:

Does the engine get enough air?
Have you made openings in the air intakes? They are not just for the good looks.
If the air only can get in through the area around the exhaust, it is actually sucked away from the engine. By opening the air intakes, you will get a small extra pressure inside the hull, which supports the engine to run easier.

Cheers!

Woodie.

Woodie wrote:
Woodie...i have made two openings in the hull!!!Because i thought that might be a problem, i run the boat without lid but i have the same results...nothing change!!!
Thank you very much

Scott wrote:
Mr Scott thank you very much for your reply. I will measure the length that you told me and i will set an 105 header with the pipe. I will inform you for the results.

boatking wrote:
Boatking...thank you very much...if it is necessary i will upload better photographs from my boat!!!

I have looked at the pictures and the suggestions made, and agree they are all good suggestions.

- get plenty of air to the engine is a must. If you tried with cowl off, that eliminates that for now, but you will still need to make provisions for plenty of air in AND OUT when running with that cowl on.

- set the pipe with the 100 degree header as Scott suggested. Don't leave too much header inside the pipe. Try to limit the amount sticking in past the second Oring to about 1" for testing...when you find the sweet spot, set it to have about 1/2" sticking past the second oring.

- make absolutely sure there is not leakage at the 2 Orings in the pipe flange (the part that seals on the header). Water leakage in here will allow water into the tuned pipe and act as though the pipe is much longer, in turn never giving max RPM.

- I don't like the look of the 4th picture (the drawing)...I hope that's not really how the transom looks! Are the sponsons offset like that for real? With the port side being deeper than the starboard side? Maybe so, I've never seen one like that. In any case, try setting the strut up so that the center-line of the propshaft is about 1/2" up from the bottom of the sponsons and tune up or down from that point. This should allow the prop to free up enough. It looks like the strut has no skeg. You may have to get a skegged strut to stop the propwalk when running the prop high. Most cats prefer this. I also typically mount my strut anywhere from 1/8" to 1/4" to the right of center on cats to help with propwalk. Keep the strut flat (no negative or positive angle) for initial testing. Even when tweaking, there should never be any more than 1-2 degrees one way or the other depending on boats.

- if you have a 6717/3 stock or in 400-410 range that would be good for setting the boat up. if not, tell us what you have and we can probably suggest one out of the bunch to use as a setup prop. Maybe you have something in your box that would be better than the 670 if the setup is correct.

- Also, tell us what carburetor is on your engine and the needle settings you have it set to right now.

BTW, What kind of cat is that? PPB?

Marc, that is a Falcon cat.. or one of the few copies that are sold under a different name.


You say you are not reaching max RPM...but i do not see a Tach in the pictures, so how do you know what RPM you are turning? If you have been using a tach (just isnt in the pictures).. what RPM has it been turning? You arent going to see 17,000-18,000-19,000+rpm like we turn with modified motors. High 15,000's to 16,000 is about what you are gonna be limited to with stock ports. With everything in sync, there is no reason you should not be able to turn up near 16,000rpm with a Octura X670. At 70mm diameter & 4.41" pitch, it is a pretty mild prop that is easily spun. It is a easier spun prop then a stock 6717/3 is.