Prop Speed Calculation
 

Does anyone know how to calculate prop speed.Now a stupid question.Why when using a 1.27 ratio lower compared to a 1.5 ratio why do we have to drop prop pitch.With the 1.27 ratio the prop shaft is actually turning slower so i would think a bigger pitch prop would be required.

The gear ratio refers to the number of engine revolutions for each prop shaft revolution.

If the engine turns 5000 rpm with a 1.5 gear ratio the prop shaft rpm is 3,333.

With a 1.27 ratio at 5000 engine rpm the prop shaft is turning 3,937 rpm.

So in theory you will go faster with a higher (numerically lower) gear ratio with the same prop at the same engine rpm.

Explained perfectly.Thanks

If you go to a higher gear ratio (lower numbers), it will take more power to turn the same pitch as previously used at the same rpm as previously turned with the lower gears (higher number).

If you can do these things, turn the same rpm with the same pitch with the taller gears (lower number), you will go faster.

Just changing the gears dose not do this. You change the gears because you ran out of prop pitch and you are on the rev limiter or over reving the engine. (or other reasons we won't go into)

In theory which will go faster if you even out the numbers say. A 1.5 with a 28p or a 1.36 with a 26p. I was always told a 1.5 with a 28 would be faster than a 1.36 with a 26.

V = 0.000947 * Engine RPM * (Pitch / Ratio) * Efficiency

V = Speed in MPH
Pitch = Prop pitch in Inches
Ratio is Drive Ratio
Efficiency = ( 1 - Prop Slip)

In Theory, it would be best to turn the largest prop you can as slow as you can. You engine speed's going to be a given, so the faster your transmission is spinning, the more heat and frictional losses you will incur. Same with a prop, the faster you're spinning it, the lower your efficiency will be because the thrust to drag(torque) ratio starts to decline.

The negatives: The bigger the prop, the more torque it will generate, and acceleration suffers.

Finding that perfect delicate balance is Throttle Up's Specialty!
:)

When you get into it it's a delicate balance.How many people would be out there that are searching for every little bit but are missing out due to a simple factor like gear ratio or pitch.Take a reasonable size vee,mid weight.It would need torque more so than a comparably size cat i guess.Remember the post on here about Fountain using 1:65 ratio Bravo's.Were they achieving something by turning a big prop slower?

Agreed Larger pitch at slower rpm is faster. I guess thats why the racers years ago favored the 1.68. It should accelerate faster.

Z Zuperboat , in your case it seems a ratio change is in order 35 pitch props tend to look like paddle wheels, I would calculate ratio to end up near a 29.
Jim

Ok if my rpm now is 5000 with the 1.68 going down to a 1.5 should bring my rpm down to say 4600 ? Using the same prop. I dont want to do gears and a prop change. The boat has run some good numbers now. But theirs always room for improvement !!

Z what are you running 100 102?
Jim

If you are running a #6 drive or other drive that uses a diameter of 16", or larger, 35 pitch props are fine. Pitch is limited by diameter. A larger diameter prop can have a larger pitch.

Diameter is the limiting factor for pitch.

So is a 36 pitch prop inefficient with a 1:27 ratio?Because i am nearly there.

Haulin the mail. Twin engines often benefit from a tall pitch. Single engines on the other hand seems to me like less in their set up.
Jim

That depends on what prop you are running.

A Bravo 1 36 is quite a bit smaller than marked. It's not uncommon to replace a 36 Bravo 1 prop with one of our 32 or 33 5 blade props.

81 or 82 could be possible just by running some good gas and swapping some pullies. I really dont want to do that. My prop slip is over 20% so my best is 74. Its a very controlable ride with lots of slip, the boats not really flying, very level and stable. Im hunting for a prop that will make it feel like a potatoe chip again. In the speedmaster world theirs a big learning curve. Looks like im getting an education in it but havent graduated yet, actually I think I mite fail the corse , LOL The boat had 575 hp went 85 and was fun to drive but a bravo ! Call seatow !!

Z So you added a couple hundred hp's and a few hundred lb's to the boat? Has Jon offered any suggestions? Dave MDG has been working with them. As far props and ratio go My train of thought goes toward working around a 29 inch prop. Reason being since the prop is acting alone, a high pitch like a 35 slaps the at a sharp angle and as they boat leaves and enters the water it side steps the stern alot. Also less pitch will direct your thrust closer to parallel to direction of travel. I know there are many varying opinions and it is expensive to change stuff, But for what its worth here are my 2 cents. I see 5% better slip is 6 mph. It probably wont come easy.
Good luck
Jim

John was on the boat this weekend at the race. He looked great lost some weight from his neck inury. Said 94 was fast enough and leave it alone already! I would say most of john experiance is with bravo boats. Dave at MDG runs a conrad using bravo type props, totaly different animal. Ive been working with brett from bblades on props and hes been very insightfull. Also Skip from MRD has been great, its just their has been nothing around to demo ! The only boat around close to my setup is owned by Mr Zul. Everybody has their own opinion and their all different. All agree testing is key. Where can I go to test a bunch of props all at the same time. mercurys lake X ?

Z sounds like your talking with the right people, To some extent John is right. There are some many combinations of heights, ratios and props. Don't get so caught up in they hunt that it stops being fun. The only other guy I know that might offer some insight is Craig Collabella in St cloud he worked at the Lake for years. Again Good luck,
Jim