Prop Maximization.-

[Sea Rider]

Escenario :




3.80 Mtr Sib
Short shaft, 2 strokes 5 HP, one cylinder 105 CC OB, fully operational.
Tach installed
Fully optimized OB/transom height.
Wot range 4500-5500 rpm

Data :
Total weight is 250 KG, with 1 up and a OEM factory delivered 3 blade 8 pitch cupped prop was running at just "4700" wot revs at fast displacement speed on a flat, non windy rowing court.

Question :
While maintaining same diam, which pitch would OB needs to rev towards 5500 max wot rpm range to plane combo while carrying same weight ?

Happy Boating

 

[Sea Rider]

Are all the Prop Gurus on vacation ? LOL!! This one will fly the perspective you have about rpm gain going one pitch down powering a light combo with a very under powered portable OB.

Happy Boating

 

[Maclin]

Your post sounded like a trick to me.

 

[Sea Rider]

Nope, the post is not a trick, the pic is a real boating example. It's an essay that will show the difference between prop theory and water practice. Think less, have swapped an 8 prop for a 7 one, both alum 3 blade OEM props..



Shoot your best answer on how much wot rpm OB has gained and if combo finally planed out. ?

Happy Boating

 

[Silvertip]

I'll bite. Since the engine revs were 4700 off-plane the boat was very close to planing. Therefore, a one inch difference in pitch increases revs about 200 rpm one can reason that one inch now planes the boat and likely frees the engine from "lugging". So my guess is the engine now revs at 5400 rpm.

 

[Sea Rider]

Silvertip,

+2 on your tech appreciation, With driver weight distributed max forward sitting in middle deck and throttling with finger nails at 4700 wot rpm the boat did nothing, simply stayed there at slow displacement speed at just 10 Km/ with slight bow up.

With 4700 wot revs and a thoretical gain of + 200 more rpm with 1 less pitch, how did you ended up with 5400 wot rpm from theoretical gained wot 4900 rpm ?

With 7 pitch drived same way OB revved happily to its max wot range of 5520 rpm, that's 820 wot rpm increase over an 8 pitch. So the "Rule of Thumb" of 200 wot revs per 1 +,- pitch doesn't apply on very light combos with under powered OB's. Was expecting max + 500 wot revs gain. Now the combo fully planes with one up, achieves 19 Km/H, a 9 Km/H nice improvement. Can maintain plane at 3/4 throttle on flat water cond.



This 5 HP powers a shorter lenght 3.20 Mtr Sib used with max 2 up, being shorter and with more hull drag, definitely will not plane at all, but at least will achieve a better throttle response at faster displacement speed if in need of more power to overcome wind, water currents.

Happy Boating

 

[Silvertip]

The one inch pitch change put the engine "over the hump" so to speak. In other words the engine unloaded and since it was no longer lugging it was able to rev higher. It can be compared to going up hill in your car vs down hill. Unloaded the engine can rev higher.

 

[Texasmark]

The one inch pitch change put the engine "over the hump" so to speak. In other words the engine unloaded and since it was no longer lugging it was able to rev higher. It can be compared to going up hill in your car vs down hill. Unloaded the engine can rev higher.

+1. Getting "over the hump" doesn't factor in on the 150-200 rpm per inch general rule of thumb. ST is right on with his assessment. On your Q and A session Sea Rider, unless you are there, doing that, any guess is a crap shoot. Since you are there, doing that you get the right answer! Besides you show the picture of a 5 hp on a RIB type boat and have in your hands props for 100 hp general area props.........your fun for today??????

 

[Sea Rider]

Will plainly agree that this "formula" is a crap shoot. On other over the hump 18-30-40 HP prop maximizations powering different light size boats which all planned out at 3/4 throttle, none of those OB's achieved + 200 wot revs per pitch down, all gained around + 500 wot revs with OEM cupped props with 2 adults up. Load, lenght, HP & pitch really matters so prop right your combo.

BTW, posted "close-up" props pic are 5 HP ones, not 100 HP ones LOL!!

Happy Boating

 

[Texasmark]

Nope, the post is not a trick, the pic is a real boating example. It's an essay that will show the difference between prop theory and water practice. Think less, have swapped an 8 prop for a 7 one, both alum 3 blade OEM props..



Shoot your best answer on how much wot rpm OB has gained and if combo finally planed out. ?

Happy Boating

These are 5 hp props? Big Foot on the 5 hp? Didn't know they made them for 5 hp engines? How tall is the person holding them? Must have mighty small hands. Course you are there and we are where we are. You have the throttle.........

 

[Sea Rider]

2 strokes 5 HP-OB is a standard short shaft. 3 blade props ara available from 9 to 6 pitches, including 4 blade ones in 6 pitch. Find the 9 pitch prop to be useless, lugs the OB badly, doesn't matter what OB is pushing. The lady holding both props is 5.5 Ft. Close shot pics can look very tricky to the eye LOL!!

Happy Boating

 

[Maclin]

Still think it was a trick :lol: