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Old 04 December 2007, 18:17   #11
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18% cruising at around 38-42mph was typical for me when I ran a 200hp on a 6.5 heavyish rib. It reduced a bit when going faster.
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Old 04 December 2007, 18:22   #12
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Quote:
Originally Posted by Chris View Post
I cannot figure out where I am going wrong on this.

MY RPM at WOT is 5400
Ratio on the etec is 2:1
Prop is an 18" SS

so using the calculation according to Downhilldai...

5800 engine rpm / 2 = 2700 prop rpm
2700 prop rpm x 18 inch pitch = 48600 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
48600/63360 = 0.767 miles per minute
0.767 x 60 = 46mph
(theoretical mph with your set up at 5400 RPM, assuming nil prop slip.)

Yet my speed according to GPS and Speedo = 47 mph (40.5 knots)

Even if I add a couple of inches to prop size to allow for cupped SS blades then I should only be getting without slip 51mph

So I appear to be getting negative slip or virtually no slip


What all this about then

I am getting my speed by running both with and against tide and then averaging the two readings. I am definate about the prop size I am running. the Tacho appears to be reading correctly and I have checked it against a timing gun with tacho.

C
I get the similar results using that calculation .
Its tells me my boat should be doing 34mph approx when its really doing nearer 40 on the GPS.

One thing for sure since fitting a lower pitch stainless prop iam getting the revs i needed but with less top speed .
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Old 04 December 2007, 18:26   #13
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So I might be in the ball park then with my set-up.

Trouble is I've not finished adding bits to the boat yet so still adding weight, I thought I might get 45knotts now and be reduced to 40 at normal running weight.

I spoke to Roy at Osprey today and he definatley thinks the engines under performing, he said he had 46MPH out of a 6.5 Vipermax fully kitted out with a 150 Etec bolted on the back and a 19" prop and thats a bigger boat.

I'm awaiting a call from BRP
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Old 04 December 2007, 18:43   #14
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Don't get overly hung up on slip.
The stamped pitch vs actual pitch of the prop can vary considerably, which messes up your calculations.
Everything else being equal, lower slip is more efficient, but everything else is never equal, because to reduce slip you've usually had to compromise on something else. e.g changing from a 3 to 4 blade prop of identical blade shape should reduce slip, but might also reduce top speed because of the losses from the extra blade.
FWIW, always go with the smallest possible blade area, and min no of blades if you want max speed - the compromise in this instance likely to be planing/low end pull, which for a "lugger" may not be desirable!
If anyone ever claims to be a prop guru, ignore em and ask someone else. It really is an art on anything hi-performance.
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Old 04 December 2007, 18:44   #15
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There are a few outboard dyno's kicking around - neil holmes sold a pair recently, although not sure to whom.

Quote:
Originally Posted by Hightower View Post
So I might be in the ball park then with my set-up.

Trouble is I've not finished adding bits to the boat yet so still adding weight, I thought I might get 45knotts now and be reduced to 40 at normal running weight.

I spoke to Roy at Osprey today and he definatley thinks the engines under performing, he said he had 46MPH out of a 6.5 Vipermax fully kitted out with a 150 Etec bolted on the back and a 19" prop and thats a bigger boat.

I'm awaiting a call from BRP
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Old 04 December 2007, 18:44   #16
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Quote:
Originally Posted by Hightower View Post
So I might be in the ball park then with my set-up.

Trouble is I've not finished adding bits to the boat yet so still adding weight, I thought I might get 45knotts now and be reduced to 40 at normal running weight.

I spoke to Roy at Osprey today and he definatley thinks the engines under performing, he said he had 46MPH out of a 6.5 Vipermax fully kitted out with a 150 Etec bolted on the back and a 19" prop and thats a bigger boat.

I'm awaiting a call from BRP
is it run in yet and on the expensive oil.
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Old 05 December 2007, 02:54   #17
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Quote:
Originally Posted by ian parkes View Post
I get the similar results using that calculation .
Its tells me my boat should be doing 34mph approx when its really doing nearer 40 on the GPS.

One thing for sure since fitting a lower pitch stainless prop iam getting the revs i needed but with less top speed .
I think people tend to get hung up on RPM especially if your tacho is not too accurate...

Why sacrifice top speed just so you get your ideal RPM reading on the tacho?

Most engines are designed to work within a band of about 1000 RPM at top speed anyway for instance my Etec is rated at 4500 to 5500 RPM.

I find the best results are running the 18" prop which gives me a top RPM of around 5400 on the tacho. On my previous engine (Johnson 90) I found that the best results were obtained running the engine at around 5200 RPM (on the tacho) I suspect the tacho on this engine was at least 100 RPM out though so I may have only been pulling 5100 RPM.

Its more about power band than RPM.

Chris
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Old 05 December 2007, 03:44   #18
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I'm not being hung up on top speed, however what I am trying to do is find out whether or not my engine is performing correctly, which I suspect not. Most people who have been following the build of Solent Viper will know of the problems with the high pressure oil pump etc, which has now been replaced along with the oil lifter pump and should be sorted.

So how does a Dyno or test wheel work? I would have thought the outboard shop would have used one of these when I had my concerns, but they haven't.
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Old 05 December 2007, 05:22   #19
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So how does a Dyno or test wheel work? I would have thought the outboard shop would have used one of these when I had my concerns, but they haven't.
Andy, there will be a test wheel specific for each engine. The idea is that the test wheel loads up your engine to its maximum power so when fitted and run up in a test tank your engine should pull its ideal RPM (in your case 5000). It it does not then you have problems.

Chris
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Old 05 December 2007, 05:44   #20
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To save cock ups in the maths use a website like this one: http://www.go-fast.com/Prop_Slip_Calculator.htm

FYI our record run managed 9% slip, but as Matt said the pitch of a prop isn't exactly an exact science, and for those getting 3% or even negative slip there is something wrong!
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