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To crab or not to crab :)

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Old 08-31-2011, 06:01 PM
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Default To crab or not to crab :)

Hi,

I have been testing various props on my ol Phantom 25 with 350 sci. During these tests I have experienced a sort of blow out with the gearcase caused of what I think is a combination of high transom height, gearcase crabbing and too much positive trim. It was like being hit by a small wave from starboard.

I tested the positiv trim on flat water the other day with the Rev4. Normally the rpm increase if you overtrim, but here you got the starboard hit..

I also experienced that the Mirage Plus prop did not like to semisurf. If too high, it lifted the transom and the boat moved little sidewise.... I did not experience this with the torquemaster gearcase, but the speed was lower then.

Maybe the bigger 5,44 is more sensitive to high transom height than the torquemaster 4,88 case. And it start acting weird. As we now a bigger gearcase will give more transomlift.

Thats the only down side I can find for the moment about the 350 sci. I still learning about the new setup, so I am sure I will hit the sweet spot in the end.

Any experiences with the new 5,44 diameter gearcase?

Cheeers and thanks, Toffen
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Old 08-31-2011, 06:26 PM
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Toffen how much wheel input are you dialing in @ WFO?
I did a test on a tunnel boat that was doing a lot of crabbing at speed. W/the boat on the trailer I set the motor straight and then put a wire tie at TDC on the steering wheel then made a pass.
I found that I was correcting for crabbing w/the wheel by turning it approx. 90* or to the 3:00 position!
It was an OMC case w/the tiny skeg so I sketched out a pattern and had a fab shop add a plate to the skeg simulating what's on a Sportmaster. Approx. 50% more surace area than OEM.
Re-tested and cut the crabbing my 50% and was much easier to drive.
The plate added to the depth of the skeg as wells as length which allowed me to run the longer torque tamer that was too long for the OEM case.

If it truly is "blowing out" the bow will drop and the steering wheel becomes a handle

Last edited by Twin O/B Sonic; 08-31-2011 at 06:28 PM.
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Old 08-31-2011, 07:16 PM
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Hi!

Thanks I will put a mark when straight and see. I do not think its so bad. When I had a Promax 300, it was delivered with a Torquemaster gearcase. This case has a built in tourque tab in the skeg to offset the crabbing. The Poseidon gearcase has none, but has a big solid skeg.

I read an article in Powerboat Mag about Laveycraft 28 EVO with a NTX drive. They modified the skeg to offset crabbing. They gained 3 mph. You can order a sensor which will show you how much off center you are. But a tape will do the same

Will test soon.

Cheeers, Toffen
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Old 08-31-2011, 08:53 PM
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Hi Toffen;
glad to hear your running the big motor.

There are two things people call blow-out. One is a prop torque issue with over steering and trim like you described, the other is when the gearcase switches from going through the water to being and little boat and pops up dropping the bow. The second one is the more spooky one.

I suspect the aspect ratio of the fat case is most of the problem. The water is banging off the blunt front and causing trouble. Have you heard of the guys putting on a nose cone on CLE case and improving everything, speed and handling. You may have to do something like that or get a skinnier case.
We have often welded on more skeg like Sonic said and that always helps.

What numbers are you at now?
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Old 09-01-2011, 04:00 PM
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Hi,

Thanks I am testing props at the moment, but best speed is 80 mph at calm water with a Rev4 25 blade spinning 6,400 rpms. Gear ratio 1:1.75. Rev4 25 is reworked with more cup, but still stock tube. The tube will be shortened this week, which should give less transomlift / more bowlift. Normally you can add 1 mph doing this.

The boat do 79 mph at glassy water. As the boat is a classic v hull design without steps and notch, glassy water steal speed. When I had a Promax 300 SVS (Brucato intake and CDI ECU), top speed under these conditions was 73 mph. So the 350 sci is a strong engine. And the boat fly higher too with it.

The gearcase limit in a single configuation is around 82 mph. So we are close already. If I can get there without nosecone, it will be the best. I have a warranty I have to take into consideration here. Adding nosecone creates more drag if I can not rise the engine more and still maintain bowlift. I doubt that since its a straight V.

I will check out how much I steer off centre. Cross my fingers is not so bad.

Cheeers, Toffen







Quote:
Originally Posted by TUFFboat View Post
Hi Toffen;
glad to hear your running the big motor.

There are two things people call blow-out. One is a prop torque issue with over steering and trim like you described, the other is when the gearcase switches from going through the water to being and little boat and pops up dropping the bow. The second one is the more spooky one.

I suspect the aspect ratio of the fat case is most of the problem. The water is banging off the blunt front and causing trouble. Have you heard of the guys putting on a nose cone on CLE case and improving everything, speed and handling. You may have to do something like that or get a skinnier case.
We have often welded on more skeg like Sonic said and that always helps.

What numbers are you at now?
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