need prop advice

[heypawpaw]

I have a 1977 35hp Johnson on a 1448 aluminum flat. 12 gallons gas, two batteries,trolling motor, 1/2" plywood floor, 300 pounds people. With Solas aluminum prop 10.5X11 max wot 5780 rpms 25 mph hardline tach.. Bought a new Solas Saturn 10.5x11 stainless prop. With new prop max wot rpms are 6030 rpms 27.2 mph gps. Prop performs well at all ranges. Need advice to lower wot rpms . Would cupping do this or would repitch be better?

 

[alldodge]

Thinner blades, less drag and better efficiency, so rpm and speed up

To lower rpm, either increase pitch or have new prop cupped a bit. The cupping will increase bite and should increase speed and lower rpm. Cupping also will create less bite when going in reverse.

Would be nice if you can find a local prop shop to try a higher pitch prop, but you will loose some hole shot, or a different prop

 

[harringtondav]

A fairly accurate rule of thumb is a 200 rpm change for each 1" of pitch change. So if you pitched up to 12" you'd theoretically drop to 5830 - close to where you started. Good prop shops can easily change pitch 1" in either direction. But no guarantees what your new top speed would be. 'Theoretically' faster. But you may be at the power limit of your 35 hp motor.

 

[heypawpaw]

Thanks for responses. Alldodge , more pitch less hole shot, I was thinking opposite., that's why I'm here trying to learn. Wot range 5200 to 5800. The boat is commercial hull with a 30hp suggested rating. It planes easily and I only run wot for testing and cruise at 5000 rpms. I sometimes use boat for shrimping which would add another 200 to 250 pounds trawling gear, ice, and shrimp. With added weight and old aluminum prop boat performed well, slightly slower out the hole and a couple mph slower at cruising speed which is understandable. When trawl in water I pull at 1800 rpms.

 

[alldodge]

The faster the prop spins when giving it throttle the faster the boat comes out of the hole, provided the prop doesn't ventilate. Like putting the transmission in a lower gear. More pitch means like starting in a higher gear.

With only being 230 rpm over max, I wouldn't change a thing. Throttling back a tad from WOT may reduce fuel usage. When adding more weight when shrimping and you may not notice any hole shot difference.

 

[jimmbo]

The WOT range of a 1977 35 HP 5000-6000 rpm, according to the 1977 Brochure. So you are propped pretty much perfectly

 

[heypawpaw]

Thanks Jimmbo I appreceiate your clarifying this. I scoured the web looking for wot range and all I could find was an iBoats page with 5500 rpms for 1977 35 hp and other pages for 1981 to 1983 35 hp 5200 to 5800 so I was going by that.