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Swellmonster posted 03-26-2002 09:56 PM ET (US)   Profile for Swellmonster   Send Email to Swellmonster  
I have been reading some info on props...
Pitch and diameter are mostly what they talk about. Lets talk about blades, how much, how big the blades are. We might have a blade with 17 pitch, going down will bring it down approx 200 rpm's. No one talks about the size or meat of the blade. Who can help me?
Bigshot posted 03-27-2002 10:23 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
The size of the blades usually correlates to diameter. The bigger the diameter the more fanlike and larger the blades are. Hence why larger diameter props are usually slower due to drag. Larger diameter props give better cruise due to surface area moving water.

This does not always work, depends on efficiency and slip.

My Montauk had a 14x17 and top end was 35mph @ 5800(redline). 4000 rpm yielded like 22mph.

I went to a smaller diameter but bigger pitch which you would think would give better top end but marginal cruise....nope. 39+WOT @5500rpms and at 3400 I am doing 22mph....go figure.

Clark Roberts posted 03-27-2002 04:14 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Easy to figure! The smaller diameter jet of water can be moved faster by a given hp than a larger jet of water! Action/reaction sends the boat forward... hp is like voltage and speed of jet is like amperage... so holding the hp constant the only way to speed up the water jet is to increase prop pitch and/or reduce diameter. If pitch is held constant then the only way to increase jet speed is to reduce prop diameter. There may be a better analogy but it's the only one my feeble brain can come up with! Happy Whalin'/// Clark ;;; Spruce Creek Navy
Swellmonster posted 03-28-2002 09:42 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
Your the guy, and I am glad you replied, that answered another forum about weight, speed and HP. If you have two, you can determine the other.
If I remember, it got all high tech and I think "bigshot said just get a GPS.
When buying a prop, what measurments do they give you?
Swellmonster posted 03-28-2002 09:44 PM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
I typed skn, it should have been sck. (spruce creek navy) Pardon my typo...
Clark Roberts posted 03-29-2002 09:37 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Swell, usually the diameter and pitch are are stamped on prop somewhere... usually inside hub.. Quicksilver props are also stamped on the outside. More info would have to be gotten from prop tech sheets etc.. there's still a lot of "witchcraft" in selecting the right prop and testing is a must in my book... any formulas leading to prop selection only get you in the "ball park" and you have wing it from there. Happy Whalin'... Clark.. Spruce Creek Navy
B Bear posted 03-31-2002 11:22 PM ET (US)     Profile for B Bear  Send Email to B Bear     
I have a question- I thought that one of the major factors in determining propeller pitch and diameter was the rated WOT of the engine.
That increasing the pitch would reduce RPM by about 200 per inch of pitch and the opposite from increasing. Also that as the diameter of the propeller would effect RPM around 50 per inch, lower RPM as the diameter increases.

I know SS is heavier with less distortion and that too will effect RPM, up when I went to SS increasing the pitch by 2 inches, 15 to 17, and reducing the diameter from 13 to 13 put me at 5450 RPM which was still in my WOT range 5000 to 6000 RPM. The aluminum propeller I replaced ran about 6100 at WOT, this meant there was a drop of almost 650 RPM.
All I can figure is that the aluminum must have distorted or flattened at the higher RPM lowering the pitch. I am repitching the SS to a 16 now.
I thought I would get a higher top end, but that did not change either.
Am I thinking right about what happened? Any thoughts would be helpful.

Swellmonster posted 04-02-2002 11:43 AM ET (US)     Profile for Swellmonster  Send Email to Swellmonster     
Now I am getting geeky...
In the April Boating magazine they talk about Blade Area "the forgotten factor"

"When sizing a prop, we always start with blade and pitch and number of blades. Rarely do we ever hear about "BLADE AREA"
Blade area is what pushes your boat forward. Pitch and diameter simply determine the rate of push.

Blade area equals the surface area of each blade in square inches multiplied times the number of blades. Thats the total area that the prop uses to deliver the thrust.

You can think of blade area as a percentage of an imaginary flat disc that has the same diameter as the propeller. That area is known as "disk area ratio". (DAR)

The standard DAR for a 3 blade is 50%, 70% when dealing with wide blades.
Four blades deliver 70%, 95% with wide blades.
Five blades is 95%, 106% with wide blades.

If the DAR is too small, it can reduce thrust, while increasing cavitation and vibration. Its safer to err in favor of too much.

Bigshot posted 04-02-2002 11:49 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Guys...props are the biggest performance factor on the boat. Problem is there is no correct answer. You have to decide what prop suits your needs the majority of the time.

I can take a 17 Montauk with a 90 Merc and run 4 different manufacturers but exactly the same size and probably get 4 different figures. I like to run OEM props for that reason. If I am getting 5800 out of a 13x17 alum oMC prop, I betting that I will get 5500+ out of the SS OMC prop. You put a Balistic or Rapture on there and may only get 5200 or 5700 but too much slip, etc.

Summary: Trial & error.

B Bear posted 04-02-2002 12:17 PM ET (US)     Profile for B Bear  Send Email to B Bear     
Thanks Bigshot,
I'll have tweeked in this SS by changing the pitch on it. The whole process seems pretty expensive for trial and error. I guess the idea is to figure out what you want it to do and to get into the ballpark first, then adjust.
Dr T posted 04-02-2002 12:53 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Trial and error--and a little black magic.

I am still looking for better prop for my old 35 hp OMC. Not all 10x13s are created equally as Nick points out.

OBouy posted 04-02-2002 08:54 PM ET (US)     Profile for OBouy  Send Email to OBouy     
Dr. T,

My new OMC 10.5 x 11 SS prop pushes my 84 13.5' with 82 35 Evinrude electric start, my wife and 2 little kids 5650 RPM at WOT (28 mph). I was informed by Bigshot to look into an 11 x 13 alum. to drop the RPM to(5400?)and pick up speed. It's in the OMC tach. pulse posting. Maybe this'll help...

Dr T posted 04-03-2002 09:59 AM ET (US)     Profile for Dr T  Send Email to Dr T     

Those numbers are virtually identical to the ones I have gotten solo at 5000+ ft altitude with the same boat and engine.

I have an old OEM OMC SS 10x13 that won't spin much faster than 4700 at WOT. My initial thought is that it is at a critical point in the torque curve coupled with an inferior prop design. Interestingly, the boat tends to porpoise more with the 10x13 than the 10.5x11. BTW, I have DoelFins on the boat.

So, I suspect that IF I can locate a decent 10x13, I just might be able to get a little more performance out of it.

Let me know if you find something. I am still waiting for ice out.

Bigshot posted 04-04-2002 01:32 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Like I said on the other post, have that 11 double cupped into a 12"(cheap). The 10x13 is a great aluminum size. I would therefore think a 12" SS is good. But......
simonmeridew posted 04-06-2002 06:17 AM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
Are you saying that the ss Rapture has too much slip at all RPM's or just at slower speeds?
I noticed when I put a 14 x 17 ss Rapture on my 70 omc Montauk to replace same aluminum, WOT dropped from 5600 to about 5200-53. Seems to go onto plane easier--no jump up, just a nice transition.
Bigshot posted 04-08-2002 11:38 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
No I was rambling. I meant you may get 5200 or might go to 5700 which would be slipping more, etc. props with vent holes have more slip to get on plane faster. They act like a slipping clutch or a stall converter on a car. On heavy boats they work great. On lighter boats I do not like. I tried one on my Yamaha and all though the RPM at WOT was ok. It sucked at cruise and would fall off plane, etc. High Perf my eye...OEM SS yamaha was tons better. 5200-5300 on a 70 is kinda low. They max at 6000. I would try a different SS. Maybe a 13x17.

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