Jeppe

Hi,
According to the BAM speed calculator we should have between 10 and 13% slip to get maximum speed. Right now we have 5% slip with Mirage 23 at 74mph. The boat is a Magnum 27 sport with two 383 Strokers with about 420 hp a side @ 5400 and TRS drives.

If I was to follow the BAM recommendations I need more pitch and more slip. Any idea what I should aim for?

hotjava66

I get a 6% slip on my 28 Saber, was told by prop guys that was a great number and changing props wasnt worthwhile.

firehawkcat

Yea 5 percent sounds pretty good dialed in nice

Jeppe

Here is the BAM page I'm reffering to http://www.go-fast.com/boat_speed_predictions.htm

The Magnum would fall into the category Heavy V with its weight and deep TRS drives and according to the table 10-13% would be optimal. So the question remains, what prop would give us more slip so we can increase the pitch?

hotjava66

I think you are reading too much into that table. Most V hull setups are unable to get a better slip number than 10-12%, the lower the slip number, the more effiecient and faster the boat. Your hull is similar to mine, and with your design and drive depth you are probably right on with your current props, or could do a little better through labbing. I would talk to BBlades, Throttle Up, or other prop guys, they know their stuff and will get you headed in the right direction. Generally deep drive V's in our size run the fastest on 3 blade props.

Jeppe

I definitely think a 3 blade is right in this case but I don't believe low slip is equal to high speed or efficiency. You can have a prop with low slip producing low speeds and you can have propellers with high slip giving you high speeds. Its all about thrust, I want to try and find a prop that produces more thrust than the Mirage at the same RPM and HP.

It seems that the majority is running Mirage or Mirage + on their old school boats with TRS. Is this from experience or because it is good enough?

Someone must have tested running something else than Mirage and Bravo 1 on TRS.

bbladesprops

Your right on a few levels.
3 blade props are 99% of the time faster than 4 blades do to drag (if they stay hooked up). Also, 3 blade props create less stern lift than 4 blades and therefore more bow lift which is desirable with deep V boats.
Thrust direction is a very important overlooked part of the equation.
4 or 5 blade props may give you more thrust on your boat but only under the correct trim levels. Getting to the trim level you require may change the thrust level and reap negative returns.
I calculate 5.62% slip.
I have never seen that low of slip with any Magnum. You will eventually break something if the numbers are true.

Brett

39squadron80

I have a 27 Starfire . Original 454 / 330 Trs boat . Now with 468s - about 500hp otherwise unchanged . I get 80 with 25 Mirage Plus at about 5000 rpm . Can't pull a 27 but gets the same speed . Also have 25 open hub Merc 3 blade cleavers . The most fun - 5500 rpm but a mile or two slower . I'm thinking larger cleavers - Merc never made them to the best of my knowledge - or a lab finished MiragePplus - 25 would probably be easier on the hardware .

Powerquest230

Jeppe,

Sounds like your dialed in pretty close- before making any changes are you confident in your speed and rpms numbers? ie- GPS and accurate tachs.. Once your numbers are confirmed I would rely on the prop experts such as BBlades or Throttle-up for recommendations.. Good luck, RB

39squadron80

positive on the equipment - GPS and solid tachs