ported prop over weekend

[tee-boy]

Drilled 1/8" holes in ss prop on 60hp merc 4stroke and found no difference in acceleration. Increased diameter to 5/32" and still found no noticeable difference. Wonder if I should continue with increasing the hole size?/

 

[LubeDude]

Re: ported prop over weekend

Make sure you are drilling the holes in the right place and "YES" keep getting larger, My personal experience is that you will have to go up to at least 5/16-3/8 before you notice a difference. My High Five is at least 1/2 or even larger.

 

[WillyBWright]

Re: ported prop over weekend

You can keep stepping up in size until you get suitable results, but if you go too far all you'll do is spin your prop and never get a bite. At least that's true with 2-strokes. My experience is that ventilating the prop doesn't work for 4-strokes. My guess is that a 4-stroke cylinder produces exhaust every other rotation of the crankshaft. A 2-stroke cylinder produces exhaust every rotation. So I'm thinking that a 2-stroke produces twice as much exhaust as a 4-stroke. I'm sure there's something wrong with that theory, but I've never heard a logical explanation as to why venting does so much better on 2-strokes.

 

[LubeDude]

Re: ported prop over weekend

Some how I missed the 4 stroke part, I agree completly.

 

[Qld_Kev]

Re: ported prop over weekend

Silly question, but why vent/port your prop? Dont you want the opposite, more bite?

 

[LubeDude]

Re: ported prop over weekend

The Idea of venting your prop is to let a little exhaust flow over the blades that lets the prop slip a little and let the engine gain some RPM, Kinda like a high stall torgue converter. Actually works quite well if you dont over do it.

 

[Triton II]

Re: ported prop over weekend

Please shoot me down with science if you are able because the Merc 4-stroke 60 is a 4-cylinder therefore it fires once every revolution. If venting works, IMO it should work almost as well as a 2-stroke as it's flowing slightly bigger exhaust gas pulses 100 times per second at 6000rpm as against a typical 3-cylinder 2-stroke 60 which would pulse a smaller amount of exhaust gas 150 times per second. One would think therefore that a 4-stroke would benefit from slightly larger vent holes than an equivalent power 2-stroke?

 

[WillyBWright]

Re: ported prop over weekend

That's what blows my theory away. If you go too big, the prop gets too many bubbles and the prop just makes foam. Maybe it has more to do with the power curve. It's like it either grabs or makes foam ... no slipping until it grabs. All or nothing. But most of my experience in 4-strokes is with MerCruiser. I've also tried ventilating props for 4-stroke outboards but didn't do too many before giving up due to the results. If anybody has had luck in this regard, please contribute.

 

[Triton II]

Re: ported prop over weekend

It may be that the two stroke needs to slip at low rpm in order for it to carburate cleanly towards its peak rpm. This may be due to the way two strokes are ported, porting is a compromise which can cause 'fluffing' at low-mid rpms under load which clears as the porting-gasflow begins to work at higher rpm. So lowering the load at low-mid rpm helps the motor accelerate faster and begin to work in its most efficient rpm area. Two-stroke motorcycles get away with the problem by having rotating valves or chisel-shaped "powervalves" which get in the way of the exhaust ports helping the mixture to burn before it disappears down the exhaust port and these valves gradually recede as the rpm increases allowing the engine to run cleanly at higher rpm. Anyway, my 'theory' (I'm not a scientist) is that maybe the venting causes the prop blades to slip slightly at low rpm, but as the blade speed increases, the water pressure on the blade washes away the vented bubbles almost immediately making the blades bite better. Thus you get better acceleration from your two stroke but as the four stroke engines have fairly linear power characteristics, there is no need for any low rpm slippage and little reason to muck about with it. Hope this makes sense?

 

[walleyehed]

Re: ported prop over weekend

Merc 4-stroke 60 is a 4-cylinder therefore it fires once every revolution.

2 cylinders fire in one revolution... <br />

a typical 3-cylinder 2-stroke 60 which would pulse a smaller amount of exhaust gas 150 times per second.

3 cylinders fire in one revolution

 

[Triton II]

Re: ported prop over weekend

Thanks for the correction. You're right of course. And it makes the post two posts above even more logical. Anyone else any ideas... as I've said I'm not a scientist!

 

[orca]

Re: ported prop over weekend

G,day. It is noticable that when you drive identical boats , one powered by 2 stroke, the other 4 stroke, that the 4 stroke appears to have a smoother pick up, rather than the kick in the pants of a 2 stroke. I have found generally that a 4 stroke needs a prop 1 or 2 sizes in pitch lower than the equivilant 2 banger. I dont know what this has to do with the subject at hand however.

 

[Dunaruna]

Re: ported prop over weekend

Whaley, its a torque thing, the 2 stroke torque curve peaks earlier than a 4 - sometimes much earlier. <br /><br />Aldo

 

[Chris1956]

Re: ported prop over weekend

I tend to agree with Triton, however I would explain it thus: Two-cycle motors tend to generate their power at higher RPMs than four-cycle motors. That means that a four cycle motor will have a gradual torque increase as the RPMs rise, and a two-cycle motor will have lower torque until a certain RPM is reached, then it's torque increases quickly. Vent holes in the prop allow the prop to slip and get the motor to higher RPMs quickly. This allows the two-cycle motor to benefit the most from the vent holes and associated prop slippage.

 

[LubeDude]

Re: ported prop over weekend

Chris1956: I think you may have nailed it! My feelings exactly.

 

[Triton II]

Re: ported prop over weekend

Chris1956, well put, that's what I tried to say above in my usual long-winded way. Too many years as a journo (paid by the word doncha know!)

 

[walleyehed]

Re: ported prop over weekend

I think you're ALL right.<br />Consider alot of 4-strokes max at 6000RPM...usually about 500RPM MORE than 2-strokes. This makes up for some of the torque advantage of a 2 stroke on top-end.<br />A 2-stroke will never have the low-end torque of a 4-stroke though.

 

[Qld_Kev]

Re: ported prop over weekend

Where do you drill the hols in the prop, on the blades or the sides? Any pictures available.

 

[walleyehed]

Re: ported prop over weekend

SEARCH FOR "VENTING PROP" OR "VENT HOLES" on the red search tab.<br />The holes are drilled in the hub just behind the blade at the leading edge.

 

[tee-boy]

Re: ported prop over weekend

I will keep drilling bigger just to satisfy my curiosity and will keep you posted.