too much back pressure??

[frank]

I need help!! I run a Sea ducer 60/80 hull with CMB 90.

It has a Prestwich muffled pipe set at 12 &3/4 inches. (I know this sounds long but it won't run any shorter?!)The boat runs great, all day. Speed is 54.6 mph on GPS. I think it should be in the 60's. The motor tends too run very hot. Needle settin from 3 1/2 turns out to 5 turns out , doesn't chang e temp but speed varies about 5 mph. I run a X467 balanced and sharpened prop. I think the pipe is creating too much backpressure causing motor to work harder!?@!? ???

 

[Altimat]

Maybe the prop is loading it too much?

 

[Mike Hughes]

That is a European pipe. Those guys have to run their boats under 80 db at 30 feet. If you haven't notice it is very quiet. To get the noise down there is a lot of restriction in the muffling section. The less you let out, the less noise. This is why the pipe has a lot of back pressure. That boat should run in the high 60's. I would suggest that you buy another pipe. It would be a lot more competitive than to try and get that one to work. There are other factors involved to get it to work. They don't have access to the high nitro that we run and they run gear trains to make up for the difference.

Mike

 

[Mike Hughes]

Hey I just checked the Seaducer site and Jerry suggest that you run a X462 for an 80 engine. You might have to much prop for the 90. I would try the X465.

Mike

 

[Altimat]

My Geraghty K&B .82 liked the X560 in a Sprint Cat.

 

[Mike Hughes]

Altimat

That is the prop that Jerry suggest for the 60 engine in the 60/80 boat.

Mike

 

[Altimat]

Altimat

That is the prop that Jerry suggest for the 60 engine in the 60/80 boat.

Mike

Too bad there is no X565 or X660. :

 

[Mike Hughes]

There are few others I wish they would build with the way engines are getting better. I have a one of a kind prop by octura. It is a 1567. I have yet to run it on something. It was so old the oxidation was pretty bad. It took a long time to get cleaned up. I found it in a bin at hobby shop on there half price stuff. It was still in the original packaging. Even if it don't work, it still is really cool looking.

Mike

 

[TomMoorehouse]

1567 what the....

does anyone elas have mysterious octura props?

 

[GTR]

Hi Guy's

The analogy about silencing and increased backpressure is not actually true. The pipes that are really quiet rely on the exhaust gas changing direction, being allowed to expand and slow down/cool off to achieve quietness. You may or may not recall John Finch actually did some back to back testing of pipes and found the Irwin and Prestwich pipes more powerful and a lot quieter than other pipes that were available (nitro pipes).

If I had to guess you either have the pipe too short to pull the prop your trying for or it is plain over propped. The large capacity motors in Europe do run no or low nitro but aren't geared - they are direct drive. The 21 & 45s are geared to let them rev but pull big props subsurface.

Keep trying and experiment with different props - try reducing the diameter of the prop you want to run, or try a prather which have more pitch.

GT

 

[nitrotoys]

I would Set the pipe back to stock, and lower the pitch of the prop, and go from there.

Can you hear the engine hitting the pipe?

in a hard turn, hit it and if it hits the pipe and falls back off, your proped to high.

What are you trying to tear that ducer in half?

that motor should move that boat alot faster than 54.

You got about 6hp there, USE IT, MAKE IT WORK.

Then let us know the speeds your getting once all corrected

NT

 

[MikeP]

One indication that you have too much back pressure is, at the end of the straight the fuel tank pressure will build up and the engine should go rich.

-MikeP

 

[[email protected]]

Frank,

two minor things to change on your boat that nmobody mentioned is:

1) CMB's love lots and lots of fuel so go to Large fuel tubing.

2) Go to Large fuel tubing to your water pick up. That will help cool; thigs down so your not blowing steam out of your water outlet.

And I agree w/Mike the X465 should be perfect for that boat.

 

[frank]

Thanks for the input guys. I forgot to mention a couple of things. I recently changed the bearings. I was running 40% then switched to 60%- it didn't really change anything even after adjusting the needle- except it was HOTTER!!! It will usually go about 4 laps then a noticeable drop in speed. It' s not blowing plugs,, plugs look good. I have the large tubing in both tanks, hopper and main with big lines to both and water coolong. I think a an x465 wouldn't do it cuz the boat has plenty of rpm not speed. Going down in props would increase rpm not speed. The pipe is water cooled I have one pick up in the rudder which tees off to cool the head. It is a Tom Grannis head with the sleeve cut by Mr. Grannis. You can clearly the the head and the exhaust peeing. Maybe a seperate pickup for each?? I really want to keep this pipe it's really nice. You can hear the water splashing over the motor!! and it was very expensive!!!! but I really think it's pipe! :'(

 

[GTR]

Hi Frank,

Don't use a T piece - run the water through the engine first in summer and the pipe first in winter, the into the other. The water doesn't have to cool the pipe much is a US set-up and all it's doing is providing some additional silencing.

Keep at it.

GT

 

[zagustin]

Hi Frank , the Prestwich tubes are simply the best. They are optimized for power and fulfill the noise limitations in Germany. We use them in Europe with success. Indeed there is no reduction of power in comparison with other tuned pipes. A typical CMB 90 EVO 2 setup is to have a length of 310 mm ~ 330 mm . As already has been mentioned by other members of our community large diameter tubes for cooling and gasoline should be used. Rich fuel and the X 465 or X467 will be driven by the CMB 90.

In one of your last posts you reported a T-piece. You should not use a manifold of flow. The different flow friction coefficient in the head and that one in the water cooled section of the Prestwich pipe will influence the ratio of cooling water in each branch of flow path. It is analogous to the behavior of electrical current in a parallel circuit of resitors. That means in the branch with a high resistor => low current (flow) occures and in the other branch with lower resistor => high current (flow) results. Another impact , as a kind of avalanche effect is, that in the head the friction of flow will encrease due to steam bubbles in the outlet of the head flow branch. A very low, often not to be seen fraction of steam can influence the flow to reduce and at least to stop . The considerations done could meet your experience that the engine becomes hot and loss of power results. My recommendation : No T- piece ! Two seperate flow paths will be helpfull and will solve your probs.

In addition , have a look to the geometry of the the water outlet inside the head. It should be smooth with a little bellmouthed shape. Very important is , that the outlet bore is totally integrated in the wall and no tube comes out of the wall. A tube comming out of the wall -a so called Borda outlet- has by the rule of thumb a six times higher flow friction coefficient.

Regards............Werner

 

[frank]

Knowledge is KING!!! I ran seperate coolings lines, NO MORE HEAT problems!!!! Thanks!!! However, I'm still down on speed. I'v cheked out everything, I think it's down to the pipe or prop. Is an x465 my only option?? What's the Prather equiv.? What about a worn piston and sleeve? It has good compression,.....not what it used to be but, still good...............

 

[zagustin]

Hi Frank , please find prop recommendations from Octura here:

http://www.pirespreto.com/octura_catalog3.htm

Dimensions of props made by Prather and Octura are provided here:

http://www.aeromarinerc.com

Comming down to prop load could be to high. The usage of a smaller prop leads to an increase in rpm. The following table from Andre Westerberg homepage is showing what the result is. The last two coloums are displaying the speed in kph reached with different levels of rpm. The table is based on theoretical boundary conditions to caclulate the distance done by the pitch for the times of revolutions in a minute. The conversion factors are to get kph or mph.

Velocity [ kph] = pitch [mm] * RPM [1/min] * 0,000060

Velocity [mph ] = pitch [inch ] * RPM [1/min] * 0,00000372

Prop diam. Inch Pitch inch diam. mm pitch mm kph 20000 rpm kph 24000 rpm

x460    2.360   3.304    59.944   83.9216    100.71    120.85

x462    2.440   3.416    61.976   86.7664    104.12    124.94

x465    2.560   3.584    65.024   91.0336    109.24    131.09

x467    2.640   3.696    67.056   93.8784    112.65    135.18

Conclusions: If you switch from a X467 to the smaller X465 you will get a certain increase of rpm which comes along with a higher speed. Keep in mind it is a theoretical evaluation and is reliable if the different in pitch is negligible.

Prather table: prop/diameter/pitch

250 2,3 3,6

255 2,4 3,8

260 2,52 3,9

265 2,61 4,0

So far so good. The equivalent of an Octura X467 should be a Prather 265.

An other recommendation is to use GRAUPNER Carbon props. The ratio pitch/diameter is similar to Prather ~ 1,4. These props have an advantage. They are not stable to withstand the forces of flow and a minor reduction of pitch is given in operation.

Graupner Carbon props are available in the diameter of 51, 54, 57, 60 , 63 and 70 mm.

To extend troubleshooting to the CMB I would appreciate to know in an email which engine edition of the .90 line you use.

Werner

 

[Ian Inverarity]

Frank,

There was a stock CMB 90 powered Seaducer in my local boat club. It ran a stock X467 and used 40% nitro with a muffled CMB pipe. It ran over 60 mph. So I dont understand why you are having trouble. Do you run at high altitude?

Jerry Crowther recommends that Prather props are NOT used.

Ian.

 

[frank]

Werner,

I bought this motor used. (maybe part of my problem??)

I believe it is an Evo 2. The head has been changed to a Tom Grannis head. Would the pipe become clogged like a catalytic would, decreasing performance? I live in S. Florida where it's always hot!!