RPM, slip, gears etc..?
 

Can someone tell me if I spun a motor at 7500 RPM with a 1.5 gear ratio, how much slip a 15 inch Hyd Q4 or B1 30P prop would have?

What I'm after is how much I can spin the motor before the prop cavitates....:D

Thanks,

Richard

It depends on your set up. Drive height, gearcase, hull design(cat or V) and speed. I assume you are running a Bravo out drive. I guess my question is why spin the motor so hard. Their are some people who do run the motors up to 7000 rpm, but not for any distance, some race boats will for short bursts on a race course, its just hard on equipment. But as far as the prop cavitating due to high rpm, maybe Brett, or Throttle up can sa if their is a point that that happens.

Hi Beak,

Thanks...

To answer your question.... Why spin so hard? Well, I am planning on 8000 RPM as a reserve, and the motor will be built to live up to 7500. I am not worried about the motor breaking, as it is being "built right."

I am more concerned about the drives breaking. If the drive can only take 900HP, and I can have 900 HP from 6000RPM, to 8000 RPM, it means I can get a higher top speed without propping a much higher pitch prop, and having a tug boat motor that is limited to 5500 RPM, and break drives all the time as all the torque is in by 3500 RPM. I can effectivelly control the motor's HP and TQ, and increase the powerband to a point that the drives are rated, and keep it in that range with a reserve in the RPM Limit versus the drive limit. I know I should just buy the Arneson conversion, but I don't want to spend that much extra right now.

I am running an IMCO extreme SC, 1 inch shorter than stock, and it has a 1.5 ratio, that will supposedly take 900HP through it. I have a 24ft V-Bottom, no step hull.. I just wanted to know if there is a point at which the prop slip is so much that the RPM's need to come down as the prop loses grip in the water..

I'll ask the prop guys as well...

Richard

Bravo style drives will not hold up very long over 6500. We have spun ours 6800 but they wont last long over that.I Would call imco I believe they told me 6500 max. Bearings get to hot tolerances change ect...

I called IMCO, and Dave said that the drive should live past the 6500 in my setup, as I have an oil cooler plumbed into the drive, and additional cooling for it. So i think the 7500 RPM will be ok at the upper end. He also said that TQ hills drives, as the shafts, bearings, etc. get toasted, but this varies by application, as the weight, drive ratio, TQ band, and the like all play a part in the drive breaking...

I think I should be fine with my setup after chatting with IMCO.

Thanks,

R

I don't think anyone can tell you the amount of slip you will will have with that apllication, you are going to have to run the 30 up to 7500 and then run the number to see the slip amount, and trial and error form there. I don't know how fast your looking to go but at 7500 rpm 30p prop and 1.5 gears using 100 mph as a base speed comes out to about 30% slip

I would also be concerned with the prop throwing a blade at those rpms.

Why build an 8000rpm 900hp engine when you can build a 6000rpm 900hp engine???? The lower rpm engine will have a flatter torque band and work better for a boat.

Hi Hooz,

Thanks, I am looking into it further, and the base you indicated gives me some ideas. I'm not sure how fast the boat will go, and I am still trying to figure if the hull design can be stable at those speeds.. I am still in discussion with Formula.

Griff,

The high torque at low RPM will kill the drive. It is easier for me to build the motor to perform at a certain level, and keep the drives alive, than to buy new drives all the time. Unless I do an Arneson conversion, I have the drive as a limitation. I can manipulate the torque peak, and the entire band to where I want it, and keep the drives alive. The 8,000 RPM is a design function, and a safety measure. The motor will not live there, but it can..

Depending on the power to weight ratio, and total drag coefficient, in principle, a higher RPM motor within the drive's limitation will have the drive living longer than a low RPM high torque motor. Just science, no more or less..

Torque kills drives, nothing else...... IMCO confirmed this when I spoke with them, and so did my shop math. HP is a function of TQ..

Anyway, I'm building it because of the design parameters I am setting, and the limitations of the equipment that I have as well as my budget.

In an ideal world, I would do a more powerful motor same 8,000 RPM limit, and an Arneson....:cool-smiley-011:

R

check out this web site and play with some numbers.
http://www.rbbi.com/folders/prop/propcalc.htm

Slip is determined by all the things BB stated. You can't assume you will have 10% or 15% untill you get a base line and then work on improving from there. Often higher slip numbers can be beneficial for top speed on V bottoms.

In the old days we said under 15% was good and under 10% was for race set ups. Today, everyone thinks their boat should run under 10%.