Props for OMC 15

[thatone123]

My passion is running 12 ft Jon boats as fast and as shallow as possible while keeping the weight to an absolute minimum. I have used the standard prop, also a 2 blade 9x10...no longer manufactured to my knowledge, 4 blade 9.25x9 and a 3 blade 9.5x12.5. I run on narrow sloughs with lots of turns and rpm readings are hard to take as RPM is constantly up and down. Boat has 3 inch jack plate and weight can go from just me at 190lb. to 2 big dudes loaded to capacity. The old 2 blade (weedless) seems to give lots of power in a light boat, the high pitch 12.5 seems to give good power for loads without needing to be a high RPM. The blade ventilates when hitting the throttle and seems to slip a bit to get on plane/step. I am wondering if this slippage is detrimental to engine life? I have various models of this OMC motor and like the modular late 80's models the best with the front shifter. With the different blades in number and pitch and etc.. they all seems to give performance that is close to one another? The one is using 2 blades with larger diameter, the other is 4 blades with smaller diameter and the other uses pitch to get power. Just Wondering? I have read a lot about props and know that high pitch usually means speed, and lower pitch power and that 2 blades are usually speed props and 4 blades power, most things considered. (in general)

 

[jimmbo]

You are comparing apple to oranges and oranges to turnips, 2 blade weedless, 3 blade, 4 blades. each is designed to excel in different areas. The prop losing grip/bite being detrimental to the engine longevity? Depends on how much the engine overrevs, and by how quickly the rev changes occur.
If you really want to boat on water thin enough to be a damp rag, get an outboard jet or an air boat

 

[thatone123]

No Title

Impossible for the jet or air boat as at times the Jon boat must be portaged and pulled up to 40 yards into other waterways. Ounces matter! Here is a modified 15 OMC taken down to under 65 pounds and the HP bumped up a little. Also I don't like Jets or Air boats. Both are inefficient, loud and heavy. This is how outboards should be, light, powerful, 2 stroke.

 

[Scott Danforth]

He was talking 2-stroke outboard jet.

Lower unit is replaced with jet pump

 

[thatone123]

I know, I have run and worked on many of these. Weakness is HEAVY- will not run good in weeds-20% loss in power. In Alaska there are more jets than props and the attendant burnt out engines from overstrain and over revs. If you see outboards advertised here used, with blown head, rods through block, etc, you can be sure it is from jet use, which in reality, were never designed for outboards. Want a jet? Get a inboard with 500 hp.

 

[thatone123]

Nobody has yet to answer my prop questions???

 

[jimmbo]

Nobody has yet to answer my prop questions???

You mean

Somebody has yet to answer my prop question??

Reading your first post, a lot of you questions appear to be rhetorical.

 

[thatone123]

No, I mean that NO-body must be well versed in small light motors, boats and props. Seems some subscribe to the "wet rag" theory instead of real world experience. I might have to log in to Australia and the dingy racers for some valid prop info.

 

[flyingscott]

A higher pitch prop does not automatically mean higher speed. An over propped motor can be slower because it pulls the powerhead out if the recomended RPM Range. A prop of less pitch can be faster because the motor is turning it faster. To much can also cause excessive carbon build up from the lugging it can cause. Your prop is ventilating probably because the motor is to high to begin with. When the prop loses it's bite and rpms start jumping around it is the same as a jet over revving things will break. A prop full of weeds will lose over 1/2 of it's thrust and it sounds like your motor may be to high regardless. You absolutely need to know your rpms and with your loads will need 2 different props. The 93 and later 15 cid motors had the front shift not the 13 cid block from 74-92

 

[thatone123]

F.S. OK, thanks, some good info here, I will take it under consideration. After these options I have tried, I think next spring I will go back to a standard prop and see if I can tell any difference. Yes, Your right on the 93...great motor, wish they were still made for sale in the US.

 

[jimmbo]

He was talking 2-stroke outboard jet.

Lower unit is replaced with jet pump

That doesn't look like any jet drive I've ever seen for an outboard, this thing, whatever it is, retains the stock lower unit and adds an enclosed impeller with a lot of added drag, with none of the shallow water benefit of a jet drive

This what every jet conversion pump I've ever seen looks generally like this(this one is for a merc), with the water pickup grill about an inch below the bottom of the hull

 

[Stumpalump]

Scotts pic was one of those military style safety jets. Imagine dropping off and picking up navy seals with a prop. That "jet" fixed the problem but no it's not the regular jet we often see like the orange one.