Will a prop change make a difference?

[nola mike]

Props are one of these black art topics that I can't ever figure out. I currently have a 23p aluminum prop on my 3.7L alpha one. It's propped correctly, hits 45-4600 rpm with a 47-48 mph top speed (used to hit 49-50, old boat now). It performs pretty well. The only thing that I need is an improved hole shot for deep water slalom starts. I can do it, but it's a struggle. I don't know if a new prop would actually help anything and be worth the money. I'd hate to throw money at something that didn't work. I don't care if I lose a bit of top end, but don't want to underprop since most of my time is spent cruising. Thoughts?

 

[Scott Danforth]

yes! its like changing rear end ratio in a race car

drop down in pitch for motor sports and trolling.

I would suggest a 17P 4-blade for slalom skiing

I kept 2 different props in the boat when I had a 3.0. 3 total on the boat depending on what I was doing.

 

[nola mike]

Yeah, I know I can swap a lower pitch for skiing (which is a huge PITA I think). Didn't know if there was some magic prop (SS, 4 blade, whatever) that would give me a better hole shot without sacrificing my cruising efficiency. Ultimate solution is still going to be my 4.3 upgrade...

 

[Scott Danforth]

Such a prop doesnt exist. And changing props only takes a few minutes

 

[nola mike]

so why are these people spending all this money on different props rather than just sticking with cheap aluminum? I wouldn't see any improvements with SS or 4 blade?

 

[dingbat]

so why are these people spending all this money on different props rather than just sticking with cheap aluminum? I wouldn't see any improvements with SS or 4 blade?

Yes, large improvements can be made. Problem is..with so many prop designs on the market, choosing the right prop comes down to working with a pro, experience with a particular boat and motor combo or experimenting with the various props until you find the right one.

The other issue is cost. Well designed props are not cheap. Not unusual to spend $500-$700 on a quality prop. Not a practical solution if your needs require a different prop for each day of the week.

in my case I worked with a pro with experience with my boat/motor combo. My prop wasn’t cheap, but it achieved the primary goal of better hole shot and top end speed at the expense of my cruise that went up 150 or so rpm.

Overall, a better performing prop and I’m happy with the change

 

[nola mike]

Would be nice if someone had a service where you could try a few props in exchange for a credit to buy one. I know some marinas do it, but I don't think anywhere around me.

 

[achris]

Would be nice if someone had a service where you could try a few props in exchange for a credit to buy one. I know some marinas do it, but I don't think anywhere around me.

In Australia Mercury dealers do exactly that. I did it when I had my dealership.

Another option is to use a vented prop, like a Laser II. You don't need to swap props all the time, just put the vent plugs in and out. Plugs in for cruising, plugs out for 'Polaris missile' type holeshot. Use the same pitch you currently run, 23"

Chris.........

 

[Sea Rider]

What's the min-max wot rpm range factory stated for that engine ? It's all up as to how loaded you like skiing. Usually need to prop a boat to achieve full wot rpm range with just the driver, spotter and skiier. That way will have a much better and faster hole shot than doing same with added passengers load.

If wanting skiing with added passengers onboard, repitch current prop to a 22 pitch, the extra lower pitch will definitely better the hole shot. As we don't know max wot rpm range, if engine over revs while at full hammer down as loaded when not skiing, just throttle less to max wot rpm range. That way won't need to change props for cruising and skiing.

Happy Boating

 

[Chris1956]

Gee, I think you are already getting exceptional performance (if speed is accurate), with that motor/prop combo. You can drop down in pitch to help hole shot, but I do not think venting the current prop on your motor, will do much if anything, to help hole shot.

There were MerCruiser props that shifted up, when boat was on plane. I never heard anything about them, however.

 

[porscheguy]

Sent you a pm.

 

[jimmbo]

There were MerCruiser props that shifted up, when boat was on plane. I never heard anything about them, however.

There were a couple of shifting variable pitch props. Mercury had one, but the mechanism pretty much blocked of the thru hub exhaust. Land and Sea marketed one during the 70s and 80s, but I believe it is no longer available. I think it had wear issues and a few blades came off.

 

[fishrdan]

Another option is to use a vented prop, like a Laser II. You don't need to swap props all the time, just put the vent plugs in and out. Plugs in for cruising, plugs out for 'Polaris missile' type holeshot. Use the same pitch you currently run, 23"

Chris.........

So what's the downside to running a vented prop, and why would you need to seal the vents for cruising, or when you don't need fantastic holeshot? (Please explain like you're talking to a noob, as I've never run a vented prop.) As I understand, the vents get sealed off at planing speed, so top end speed isn't sacrificed. Is it because low speed handling is diminished, with the vents opened? Asking as I have a prop I've been considering venting, if possible, my heavy load/high altitude prop, 16P Alpha4.

Big downside I see with "garage" venting a prop, opposed to one vented from the manufacturer with the interchangeable plugs, is once the prop is drilled, it's drilled. Go too big or mess up the placement and your hosed.

That said, couldn't nola mike vent his current prop to get better hole shot? Guessing you may answer this question in my noob request...

 

[achris]

Any prop can be vented, yes.... Many years ago Merc produced a service bulletin, or it may have been in one of the training manuals, on exactly where to drill, and what size.

As you alluded, the vents 'self-seal' at about 3,000rpm, and it feels much like it does with an un-vented prop.But below that, the prop slips heavily, and trying to cruise is a nightmare. Especially is that seal point is right about where you want to run. When I tested a 21" Laser II on my boat, 2950rpm was about 6knots, 3100 was 22knots. Trying to sit anywhere in between was just not possible. Hence the want for the vent plugs.

HTH,

Chris.........

 

[fishrdan]

Good explanation achris!

So if you're running in heavy seas at relatively low speeds (RPM) with a vented prop like you had, it's probably going to totally mess up boat handling while fighting through gnarly conditions.... Not sure i like that. I also troll off the main engine, and it sounds like that would be a disaster with a vented prop.

I guess it depend on how aggressively the prop is vented, and the reason for different sized plugs, so you can modify the venting.

 

[fishrdan]

duplicate post

 

[jimmbo]

There is more slippage at non planing speeds. Outboards generally respond better to venting than I/Os, might be due to different Torque curves. That might change with the new smaller displacement I/Os recently introduced

 

[achris]

There is more slippage at non planing speeds. Outboards generally respond better to venting than I/Os, might be due to different Torque curves. That might change with the new smaller displacement I/Os recently introduced

The response in my boat, with a 4.3L engine, was nothing short of spectacular... Even without a vented prop my boat is no slouch. If I'm JUST on the plane and put the hammer down, it puts you back in your seat! With the vents out on that Laser II, it was neck-snapping.... You'd have to be a bit delicate with skiers, lest you dislocate shoulders.

The whole idea behind the venting is that engines don't produce a whole lot of torque at low speeds, so getting the engine revving a bit higher will help with holeshot. The vents allow exhaust gas into the front of the blades, the low pressure side, and that 'unloads' the prop. Without a high load on the prop the engine will rev a lot freer, and will accelerate up without a great deal of forward motion. Once the engine reaches about 3,000rpm, the vents 'seal'. It's not actually a seal, it's more that the holes are spinning so fast that a lot less gas can escape into the blades. Once the vents 'seal', the prop is now biting on water, not exhaust gases. As the engine is in the power band, it is producing enough torque such that it's not inclined to slow down much, so the net result of a prop biting, and an engine that doesn't want to slow down is a boat with enough acceleration to pull the enamel off your teeth!

Chris............

 

[jimmbo]

My props are vented and you can see how the engine revs then the prop bites by watching the tach and the speedo

https://www.youtube.com/watch?v=VepbXm5RFDI

 

[nola mike]

That actually does sound like magic. Thinking that that would be perfect in my boat for what I'm looking for.

EDIT: Looking on the merc website, says the laser II is for bass boats/outboards, and does not have the performance vent system?