Prop Blow Out

[28Eliminator]

Need some help from the Guru's.

I put a B1 28p on my boat, and it now blows the prop out when trying to get on plane. at about 2500 - 3000rpms it looses traction suddenly and revs up. Once it gets up on plane it seems fine.

Question; The prop has the PVS holes. Now I know these are primarily for exhaust going through the prop hub to ventilate a prop, but I have transom exhaust so should be irrelevant, Right? Can the PVS holes still have an effect on the prop blowing out trying to get on plane?

Prior to the new engine, this boat popped out of the water fast with a 26p prop.

Admin/Mods.. please leave in here long enough to get some traction before kicking me to the Tech section

 

[BHC Vic]

That happens with the labbed 26p that I have. It blows out getting on plane if I get on it too hard. After that it runs great. I had the holes filled.

 

[ONE-A-DAY]

outboard or I/o?

 

[Ragged Edge]

My instinct is yes, but without the exhaust it's hard to say how much effect the holes are having. I would plug them and run the prop again to find out what difference it makes.

 

[Bigbore500r]

Maybe this is a candidate for that Max Machine Works fin?

 

[Bigbore500r]

outboard or I/o?

He's got a 28' eagle with a B1 / 650hp BBC

 

[Uncle Dave]

This is the nature of the beast.

When you have a high (performance) prop shaft depth, pushing big single with a steep pitch 4 blade you get that.

You can play with holes and such, but you won't stop this completely unless you pitch away down and add another blade.

Its on you to on manage an over rev and keep the shock off the drive getting out of the box.

UD

 

[240Hallett]

Have Steve detune it for you...

 

[28Eliminator]

outboard or I/o?

This is the nature of the beast.

When you have a high (performance) prop shaft depth, pushing big single with a steep pitch 4 blade you get that.

You can play with holes and such, but you won't stop this completely unless you pitch away down and add another blade.

Its on you to on manage an over rev and keep the shock off the drive getting out of the box.

UD

So you think the PVS holes really have no impact? perhaps a 5 blade is in my future...

 

[Singleton]

So you think the PVS holes really have no impact? perhaps a 5 blade is in my future...

5 blade harder on drive then 4.

 

[Uncle Dave]

So you think the PVS holes really have no impact? perhaps a 5 blade is in my future...

on a rig like yours? Not much. On a lightweight tunnel - a little more.

As singleton says - 5 blade is harder one the drive, which you actually need to worry about.

I would only run a 5 blade on lakes vs ocean.

You got steering on that rig?

 

[Ziggy]

5 blade harder on drive then 4.

on a rig like yours? Not much. On a lightweight tunnel - a little more.

As singleton says - 5 blade is harder one the drive, which you actually need to worry about.

I would only run a 5 blade on lakes vs ocean.

You got steering on that rig?

He waited all summer for his new IO internals..........should be good for a few years of abuse
.
.
He's got an awesome tiller for steering

 

[Uncle Dave]

He waited all summer for his new IO internals..........should be good for a few years of abuse

Hope they aren't stock stuff, but more like an sc, or a platinum.

28ft big block single with high propshafts shred drives easy - ask me how I know...

 

[Flying_Lavey]

So you think the PVS holes really have no impact? perhaps a 5 blade is in my future...

Either way, the plugs are cheap and should be pretty easy to come by. Grab some, throw them in and give it a shot. It MIGHT be just enough to keep it from losing bite like that.

 

[Ziggy]

Hope they aren't stock stuff, but more like an sc, or a platinum.

28ft big block single with high propshafts shred drives easy - ask me how I know...

His last setup lasted several hundred hours and an extremely reputable builder did his outdrive knowing what power he would be producing. It was his engine that finally got tired so he refreshed everything.

 

[Uncle Dave]

His last setup lasted several hundred hours and an extremely reputable builder did his outdrive knowing what power he was be producing. It was his engine that finally got tired so he refreshed everything.

sounds like he's set for a while - on a 4 blade anyway

does he have steering ?

 

[ONE-A-DAY]

Prop answers don’t usually come from asking others, only advice or suggestions, just keep testing, out the plugs in and see what happens

 

[stoker2001]

cant hurt to plug holes on that application,another option isto have a custom Prop shop roll the leading edges of blades abit

 

[Jonas Grumby]

So, are you used to just flooring it and getting on plane? And it doesn’t break loose? All my boats, its ease into the throttle as you can, and at the break loose point you pull back, it’s around 3000 rpm, then feather it with a bit of prop slip until she rolls over. Keeping around 3000 rpm.

 

[Ziggy]

sounds like he's set for a while - on a 4 blade anyway

does he have steering ?

Yeah, think he added hydro quite a while back.....the old wooden tiller gave him splinters, that was hell on his sex life

 

[BUDMAN]

I can’t believe nobody has suggested “ have you tried a different prop”

 

[Uncle Dave]

Yeah, think he added hydro quite a while back.....the old wooden tiller gave him splinters, that was hell on his sex life

Good man.

For sure he has to see and a 3rd party can only tell you what they've experienced.

Its seems the OP' hasn't really experienced much blowout getting on plane, and its always good to know if others are having the same issue - what he's describing is totally normal to me.

UD

 

[DarkHorseRacing]

FWIW, I have a 26 Eagle with a TCM 900 and an Imco SCX with the big shaft (not #6). My speed prop is a B1 28p (15-1/4"D) which wasnt very good coming out of the hole. So I switch to a Maximus 28p (now 15"D) and that was the sweet spot for getting on plane and good midrange, top end I wasnt really concerned with.

However at the moment for the lake I'm currently on I dropped to a 26p Maximus and that really helped the holeshot and cruising but the RPMs are a little higher than with the 28p so gas mileage went down.

If you've got a stock B1 drive on a 28 I would probably be careful with 5 blade options unless its a TCM Platinum or better like UD said. I assume its the 1-1/4" shaft?

 

[Ziggy]

Good man.

For sure he has to see and a 3rd party can only tell you what they've experienced.

Its seems the OP' hasn't really experienced much blowout getting on plane, and its always good to know if others are having the same issue - what he's describing is totally normal to me.

UD

new prop for him so I'm sure he's just gotta dial it in

 

[Singleton]

He waited all summer for his new IO internals..........should be good for a few years of abuse
.
.
He's got an awesome tiller for steering

true, don’t want him blowing drive before 2021

 

[ToMorrow44]

FWIW, I have a 26 Eagle with a TCM 900 and an Imco SCX with the big shaft (not #6). My speed prop is a B1 28p (15-1/4"D) which wasnt very good coming out of the hole. So I switch to a Maximus 28p (now 15"D) and that was the sweet spot for getting on plane and good midrange, top end I wasnt really concerned with.

However at the moment for the lake I'm currently on I dropped to a 26p Maximus and that really helped the holeshot and cruising but the RPMs are a little higher than with the 28p so gas mileage went down.

If you've got a stock B1 drive on a 28 I would probably be careful with 5 blade options unless its a TCM Platinum or better like UD said. I assume its the 1-1/4" shaft?

Do the Eagles tend to run a pretty high propshaft height (to use RDs tech terms)? My Advantage is ~4.5” below the keel which is kinda deep but rolls on plane at 3000rpm, never any blowout. Labbed 4 blade props. Just curious if Eliminator set those up a little more aggressively.

 

[DarkHorseRacing]

My SCX I believe has a spacer to run it a little deeper because we do a lot of skiing and boarding with the boat and it rolls out of the hole well, but I have big tabs to help that as well. I don't know what size spacer.

I think Eliminator, as mentioned before, probably has a decent X dimension where its more performance oriented. Like that article says, easier to space the propshaft down if needed than it is to raise the drive (as a unit) up. So if you set them high you can play with spacers. If you don't, it gets the top speed number.

 

[28Eliminator]

sounds like he's set for a while - on a 4 blade anyway

does he have steering ?

Dual ram assist, not full hyd, not yet anyways. I’m very gentle on the drive, getting on plane and off.

 

[twocents]

Look for an RDP artticle very soon explaining the difference between "cavitation", "ventilation" and "blow-out" -- each is a very different situation.

 

[dezrtracer]

I can’t believe nobody has suggested “ have you tried a different prop”

Damn it !! You beat me to it ..

 

[28Eliminator]

Prop answers don’t usually come from asking others, only advice or suggestions, just keep testing, out the plugs in and see what happens

It’s the advice and suggestions I’m after. At least it’s a place to start. I’ll get a set of plugs and see what happens.

In the meantime, I’ll call a couple prop shops and see what they suggest. I really don’t want to go to a 5 blade if I don’t have to.

 

[guest hs]

Try having a prop shop put some additional cup in the trailer edge

 

[bocco]

I switched from a 26 Bravo to 28 Signature. It does slip on acceleration but it grabs pretty smoothly as soon as the boat starts to plane.

 

[28Eliminator]

So, are you used to just flooring it and getting on plane? And it doesn’t break loose? All my boats, its ease into the throttle as you can, and at the break loose point you pull back, it’s around 3000 rpm, then feather it with a bit of prop slip until she rolls over. Keeping around 3000 rpm.

Oh lord no.. lol. Ease in, and ease out. But never had any blow out before. The new engine makes a lot more HP at much lower RPMs, and a shitload more torque. Maybe it’s just something I need to get used too...

 

[28Eliminator]

I switched from a 26 Bravo to 28 Signature. It does slip on acceleration but it grabs pretty smoothly as soon as the boat starts to plane.

what are differences between the bravo and signature.. besides the obvious pitch difference?

 

[28Eliminator]

Another question . What effect does the diffuser have on blow out?

 

[DarkHorseRacing]

Another question . What effect does the diffuser have on blow out?

None, its on the wrong side of the prop for it to affect the water stream to it.

 

[bocco]

what are differences between the bravo and signature.. besides the obvious pitch difference?

Not much that I could see. It is cheaper but I don't remember how much. I'm happy with it. With my Bravo 26 I was almost hitting the rev limiter. With the 28 I'm at about 4800 RPMs at 78 mph. Two people in the boat. I'm happy right now keeping the RPMs down a little.

 

[28Eliminator]

UPDATE;

I installed the PVS plugs and it cured it 100%. Problem solved!!

 

[getreal]

How PVS works:

As the boat accelerates an amount of the exhaust gases exiting the propeller’s tube «escapes» through the vent holes and is driven between its blades. The direct consequence is the aeration of the water between the propeller blades, which simply means that the water becomes «lighter» and therefore the load applied to the propeller is reduced. Thus, the propeller is able to slip notably allowing the engine rpm to increase rapidly, until the propeller bites, leading to a quicker acceleration and shorter time to plane.

So the role of PVS is only to allow the engine to raise faster its rpm. In fact, it reduces the time it takes for the engine to reach that level of rpm, where the torque is as high as required so as to overcome the big resistances making the boat «get out of the hole», and so therefore the total time we need to get on plane is reduced.

At this point it should be made clear that the venting system is effective only during the start , and once the boat gets on plane and running, it makes no difference at all on the rpm, performance and, of course, on top speed.

The explanation is very simple:

When the boat is running, the water that externally is flowing over the propeller vent holes prevents the exhaust gasses from coming out the holes. In a way, it blocks the holes, as the flowing water pressure becomes clearly much higher than the one of the exhaust gases and so the prop blades operate in solid, homogeneous and non-aerated water.



Vented or non-Vented, and if Vented how much?
When is the venting of the propeller’s blades useful?

When our propeller is not lagging during the planing, then obviously there is no reason of propeller’s venting. On the other hand, when it is difficult to get the boat on plane, for example in the case of heavy loads, or of using longer pitch props - often choosing so as to achieve higher top speed - or even when our engine doesn’t make plenty of power at lowest rpm, then will we see a significant improvement in the planing time by leading the exhaust gases between the propeller's blades, allowing it to operate in a «lighter» environment, thereby reducing its load and causing an instantaneous engine overrevving.

But how can we know the percentage of ventilation needed for our propeller so that it is not excessive and won’t cause over-ventilation leading to a complete loss of its bite?

There is a simple way to determine the certain amount of venting that is most proper, which of course requires some time of sea trial.
Place the Mercury’s solid plugs in the holes. Drill a 7 mm hole in the plug’s center and go out for testing. If you don’t have a good holeshot or your start looks like as having 3rd gear in your car, you need additional venting. Widen the holes further at 9mm and retry to see the effect you get. If you find a slow holeshot again, re-widen the holes with a 12 mm drill. On the other hand, if you see that the engine is overrevving and the prop doesn’t «hook-up», which means the water is getting too much aerated you need to choose the plugs of smaller diameter.

In this way you can find the «sweet spot» in which you have the best results for your use and you are ready order the proper diameter of plugs from Mercury.

 

[Shlbyntro]

So you think the PVS holes really have no impact? perhaps a 5 blade is in my future...

I still believe there is a siphon effect going on that draws in air from the top of the outdrive and allows it to bleed out these holes. I'd spend the $10 on a plug kit before I'd buy a new $500+ prop

 

[02HoWaRd26]

My Max5 has those holes. Just for fun i popped the plugs out, it gave me a bit more rpm when getting on plane, but otherwise ran the same.
Put the plugs in, won’t change the world but i believe it’ll make a little difference.
Also if your looking for a MMW whale tail I’ll sell mine, it’s perfect, just has the hole for the drive shower and my shower is now from the side of the drive. I’ll do for 40$ off new seems fair to me and saves you a drill bit as that stainless is a bitch to drill.

 

[Good Stuff]

My Max5 has those holes. Just for fun i popped the plugs out, it gave me a bit more rpm when getting on plane, but otherwise ran the same.
Put the plugs in, won’t change the world but i believe it’ll make a little difference.
Also if your looking for a MMW whale tail I’ll sell mine, it’s perfect, just has the hole for the drive shower and my shower is now from the side of the drive. I’ll do for 40$ off new seems fair to me and saves you a drill bit as that stainless is a bitch to drill.
View attachment 929361

Who tapped that for you? I am not really satisfied with mine. The prop is at least 1/4 to 1/2 out of the water while I’m running fast and my drive isn’t staying as cool as I think it should.

 

[02HoWaRd26]

Who tapped that for you? I am not really satisfied with mine. The prop is at least 1/4 to 1/2 out of the water while I’m running fast and my drive isn’t staying as cool as I think it should.

I did, MMW didn’t have the time so i bought the bit and tap as I’d already built my drive stand.

 

[Singleton]

My Max5 has those holes. Just for fun i popped the plugs out, it gave me a bit more rpm when getting on plane, but otherwise ran the same.
Put the plugs in, won’t change the world but i believe it’ll make a little difference.
Also if your looking for a MMW whale tail I’ll sell mine, it’s perfect, just has the hole for the drive shower and my shower is now from the side of the drive. I’ll do for 40$ off new seems fair to me and saves you a drill bit as that stainless is a bitch to drill.
View attachment 929361

what drive shower is that and how is it picking up water

 

[02HoWaRd26]

what drive shower is that and how is it picking up water

The new boat has a fresh water pickup in the bottom of the hull, to a strainer ladida....
So the shower is fed by the lwp on the lower unit, there’s a plate inside the hull that blocks off what would normally be the drive water pickup, so drill (45/64) hole then tap -8 (1/2-14 npt) then put the stainless-8 90* in there.

At no wake going up river it floods the drive top. It’s a MMW drive shower spider or whatever it’s called.

 

[RiverDave]

UPDATE;

I installed the PVS plugs and it cured it 100%. Problem solved!!

I just saw this thread right now, and was going to say "plug the holes". Even though they are meant for exhaust to blow through them and ventilate / cvaitate the prop.. They still agitate the water enough to cause cavitation..

I have run into this a few times on different boats, and plugging the holes works.

RD

 

[RiverDave]

How PVS works:

As the boat accelerates an amount of the exhaust gases exiting the propeller’s tube «escapes» through the vent holes and is driven between its blades. The direct consequence is the aeration of the water between the propeller blades, which simply means that the water becomes «lighter» and therefore the load applied to the propeller is reduced. Thus, the propeller is able to slip notably allowing the engine rpm to increase rapidly, until the propeller bites, leading to a quicker acceleration and shorter time to plane.

So the role of PVS is only to allow the engine to raise faster its rpm. In fact, it reduces the time it takes for the engine to reach that level of rpm, where the torque is as high as required so as to overcome the big resistances making the boat «get out of the hole», and so therefore the total time we need to get on plane is reduced.

At this point it should be made clear that the venting system is effective only during the start , and once the boat gets on plane and running, it makes no difference at all on the rpm, performance and, of course, on top speed.

The explanation is very simple:

When the boat is running, the water that externally is flowing over the propeller vent holes prevents the exhaust gasses from coming out the holes. In a way, it blocks the holes, as the flowing water pressure becomes clearly much higher than the one of the exhaust gases and so the prop blades operate in solid, homogeneous and non-aerated water.



Vented or non-Vented, and if Vented how much?
When is the venting of the propeller’s blades useful?

When our propeller is not lagging during the planing, then obviously there is no reason of propeller’s venting. On the other hand, when it is difficult to get the boat on plane, for example in the case of heavy loads, or of using longer pitch props - often choosing so as to achieve higher top speed - or even when our engine doesn’t make plenty of power at lowest rpm, then will we see a significant improvement in the planing time by leading the exhaust gases between the propeller's blades, allowing it to operate in a «lighter» environment, thereby reducing its load and causing an instantaneous engine overrevving.

But how can we know the percentage of ventilation needed for our propeller so that it is not excessive and won’t cause over-ventilation leading to a complete loss of its bite?

There is a simple way to determine the certain amount of venting that is most proper, which of course requires some time of sea trial.
Place the Mercury’s solid plugs in the holes. Drill a 7 mm hole in the plug’s center and go out for testing. If you don’t have a good holeshot or your start looks like as having 3rd gear in your car, you need additional venting. Widen the holes further at 9mm and retry to see the effect you get. If you find a slow holeshot again, re-widen the holes with a 12 mm drill. On the other hand, if you see that the engine is overrevving and the prop doesn’t «hook-up», which means the water is getting too much aerated you need to choose the plugs of smaller diameter.

In this way you can find the «sweet spot» in which you have the best results for your use and you are ready order the proper diameter of plugs from Mercury.

I tried a series of "low tech" experiments with the plugs and a through hub exhaust boat.. At least with a 454, if you leg that thing getting out of the hole, it will actually pull negative pressure on the inside of the hub once it gets up and running.. Meaning once it starts to roll over the cavity behind the prop is actually "negative" void (and around the hub) I think the only time the exhaust is actually being pushed out the holes is on initial throttle up.