Prop Blow Out on an 18' Tahiti with an OMC 175.

[Matt Mead]

We bought this boat 2 years ago and finally did a maiden. Out of the hole I could spin the prop very easily, then at top speed it seemed like it was just sucking air, hitting the over-run limit of the 2-stroke motor, and only doing 43 mph. I noticed we could only have the motor trimmed fully down. Just 1" of up trim and it shot a rooster tail off the side, blew out the prop, no drive, etc.

Obviously the motor needs to come down right? So I measure the prop to pad distance -- 7"!! SEVEN INCHES!!

I lowered it an inch and we are going testing in a week to see what happens. But damn, SEVEN inches? Needs 8? All I see is 3-1/2" below pad to to 1" above pad on all the builds I've looked at. WTF? Does this thing just need that?

Prop is a 14-1/4" x 23p. Looks like is has been worked. Possibly cupped at the rear. Does cup induce transom lift? We need 0 transom lift. I couldn't even get the bow up 1" at all, it was in Formula 1 nose-down cornering position at all times.

Anyway, this is a super budget build. The conditions were not ideal in the least. The motor wasn't even running perfect, probably only making 120hp. But all I know is I could blow the prop out at the slightest stab of throttle and it seemed like it was running in air at top speed. So I lowered it. Interesting conversation/observation even if nobody has any input.

 

[Flying_Lavey]

That is nowhere. Ear too high. As a matter of fact I'd say that is pretty buried. Go off the prop shaft to pad height. I'm not sure where you are getting 7"?

Cup itself doesn't necessarily lift the transom by itself. Cup just increases the pitch at the trailing edge of the prop.

How do you know your trim angle when you are driving? Do you have a trim gage or the ol "look over your shoulder" (my method) method? Just asking cause you may be putting too much faith in an instrument that hasn't been verified to be accurate.

 

[Bajastu]

If this was my boat I’d raise it two inches and watch the water pressure. All of my outboards were alway set at pad to 1” above the side water pickups. It’s a good starting position with room to move up. Remember, the setback with a jack plate makes the engine deeper in the water when the engine is off the transom.

It looks like you are buried in the water.

 

[MPHSystems]

Nice looking boat.

 

[HST4ME]

Knock that shitty nose cone off and go back to the stock gearcase profile. The prop is missing the convergence ring which probably isn't the problem but doesn't help. Look on ebay for a 24 raker.

 

[Matt Mead]

That is nowhere. Ear too high. As a matter of fact I'd say that is pretty buried. Go off the prop shaft to pad height. I'm not sure where you are getting 7"?

Center line of prop shaft to pad height was 7". I just set it to 8".

How do you know your trim angle when you are driving? Do you have a trim gage or the ol "look over your shoulder" (my method) method? Just asking cause you may be putting too much faith in an instrument that hasn't been verified to be accurate.

We have a trim gauge, broken. So yeah, lol we are doing the look over the shoulder method. We basically needed to bottom out the motor all the way to eliminate the side rooster off the prop and get any drive at all. Even one bump of trim up and the prop spun super easy. Weird at 7" right? Oh, I forgot to mention. When my cousin was standing in the back of the boat on a speed run it all of a sudden picked up 5 mph. Like all of a sudden the prop had bite.

 

[Matt Mead]

Knock that shitty nose cone off and go back to the stock gearcase profile. The prop is missing the convergence ring which probably isn't the problem but doesn't help. Look on ebay for a 24 raker.

Is this a welded on nose cone? It looks funny to me with marks under the paint and was making me suspicious. So you think the water isn't smooth (laminar?) flowing back to the prop for good bite? That would solve the mystery better than lowering this motor a mile under the hull!

Didn't even know convergence rings existed. From what I read, it would cause the prop to cavitate also. Maybe this is just straight up cavitation from two factors. Thanks for the input!!

 

[Crazyhippy]

That motor is WAY to low!!! And lowering it is making things worse.

Lift it up so the propshaft is 2“ below the bottom and try there. Racers bitch about having to run 1“ below.

 

[mobldj]

theres a plastic ring that goes inbetween the prop and the lower unit.my pontoon was missing that and kept blowing out,exhaust exiting to soon around the prop instead of thru it.aka,cavitation

 

[Matt Mead]

theres a plastic ring that goes inbetween the prop and the lower unit.my pontoon was missing that and kept blowing out,exhaust exiting to soon around the prop instead of thru it.aka,cavitation

Ah you know I was wondering about that. There is a tiny gap between the prop and the lower. I was wondering if exhaust was getting through there. Alright, well, with everyone chiming in I'll get it sorted one way or another. Lots of things to try. Then I can raise the motor up where it should be.

 

[Flying_Lavey]

theres a plastic ring that goes inbetween the prop and the lower unit.my pontoon was missing that and kept blowing out,exhaust exiting to soon around the prop instead of thru it.aka,cavitation

That's not a problem if the prop is ran in a surfacing position. That's how cleaners and choppers work. Also the premise for hub vent holes.

 

[Matt Mead]

Yeah I was watching a video on a guy tuning a prop with rubber stoppers in holes to allow a certain amount of exhaust to leak around the blade to get the RPM's up.

Photos of prop to lower mating area. Normal or not?

 

[Matt Mead]

Hell, while I'm at it, here's photos of the prop.

And this is the possible aftermarket nose cone HST4ME said to take off. Which makes me think "How the hell am I going to do that without damaging the lower?" Anybody around BHC that does that sort of thing? Might as well put a new lower on, lol.

 

[mobldj]

Hell, while I'm at it, here's photos of the prop.
View attachment 1101814 View attachment 1101815 View attachment 1101817


And this is the possible aftermarket nose cone HST4ME said to take off. Which makes me think "How the hell am I going to do that without damaging the lower?" Anybody around BHC that does that sort of thing? Might as well put a new lower on, lol.
View attachment 1101818

 

[Matt Mead]

Well, you have a gap in front of the prop like me, so I guess that's normal. But I don't have the convergence/divergence ring on the back. Now that I look at the prop there is a perfect lip on it for that ring. Whoopsie. Whatever, just barely got it on the water once so far after buying it. I'll get her up to speed!

 

[HST4ME]

Get rid of whats left of the torque tab on the skeg and try again also. If the stringers are rotten and the hull has a hook in the last six feet this is all a moot point anyway,

 

[Ragged Edge]

The nose cone is probably just epoxied on, sand or grind it at the seam. I would use the 3M, 3 inch discs or a Dremel tool. Careful not to remove too much aluminum off the gear case. Once you have it cleaned up around the seam TAP (no wreck it Ralph shit) at the nose cone with a hammer. If it doesn't fall off keep grinding at the nose cone until it does. Sand all the epoxy and bondo off until you have smooth aluminum then repaint. Your going the need to try some different props, start with the 24 Raker that has already been suggested and go from there.

 

[Matt Mead]

Get rid of whats left of the torque tab on the skeg and try again also. If the stringers are rotten and the hull has a hook in the last six feet this is all a moot point anyway,

I will remove it. It's bugging me anyway, all cracked. It has one long central stringer and I asked for a new one when we had the floor and transom braces done. After seeing the job he did I don't think there is a new stringer in it. And I cut straight through that stringer 2 times with a sawzall removing the floor . So yeah, moot point looking for 80 mph. I would like to see 60 though!

 

[Matt Mead]

The nose cone is probably just epoxied on, sand or grind it at the seam. I would use the 3M, 3 inch discs or a Dremel tool. Careful not to remove too much aluminum off the gear case. Once you have it cleaned up around the seam TAP (no wreck it Ralph shit) at the nose cone with a hammer. If it doesn't fall off keep grinding at the nose cone until it does. Sand all the epoxy and bondo off until you have smooth aluminum then repaint. Your going the need to try some different props, start with the 24 Raker that has already been suggested and go from there.

I'm afraid to remove it because maybe it was put on due to damage. I bought this boat from Blythe so the guy was running it in the lower river shallows. It shows, too. Damage all over the hull everywhere. I'm afraid the lower is dinged to all hell under that nose cone used as a patch, lol. Anyway, maybe a new lower one day.

I will look for a 24 raker. Thanks

 

[Flying_Lavey]

Hell, while I'm at it, here's photos of the prop.
View attachment 1101814 View attachment 1101815 View attachment 1101817


And this is the possible aftermarket nose cone HST4ME said to take off. Which makes me think "How the hell am I going to do that without damaging the lower?" Anybody around BHC that does that sort of thing? Might as well put a new lower on, lol.
View attachment 1101818

Yeah, I think that nose cone is your problem that looks like a really crappy one with that blunt nose. Not much use for it with that profile.

I actually wouldn't be surprised if you're getting blow-out. When the trim comes more neutral it allows the air bubble from that torpedo to flow right into the prop blades where if it's trimmed down there is enough kf the. Lakes below that turbulence so it continues to bite

Plus that's a crappy epoxy job on that cone too. That profile is all jacked up I bet

 

[Matt Mead]

I actually wouldn't be surprised if you're getting blow-out. When the trim comes more neutral it allows the air bubble from that torpedo to flow right into the prop blades where if it's trimmed down there is enough kf the. Lakes below that turbulence so it continues to bite

Anything besides full down trim and it was just spin city. Then my cousin stood in the back of the boat (no seats) and we picked up 5 mph. It looks weird when it spins, it shoots a rooster 5 feet to the starboard side. So yeah, I dunno, just massive cavitation until the prop is fully buried to where it's impossible anymore.

So what is the actual idea behind these aftermarket nose cone's anyway? I don't really understand it, even though I could do a quick uneducated guess. It seems almost like it changes the OEM shape to something closer to a Merc M6/M8 drive. I would think I would need twice the horsepower, different height and a different prop to actually take advantage of that new shape, right? So it's dumb, right?

 

[Bajastu]

Anything besides full down trim and it was just spin city. Then my cousin stood in the back of the boat (no seats) and we picked up 5 mph. It looks weird when it spins, it shoots a rooster 5 feet to the starboard side. So yeah, I dunno, just massive cavitation until the prop is fully buried to where it's impossible anymore.

So what is the actual idea behind these aftermarket nose cone's anyway? I don't really understand it, even though I could do a quick uneducated guess. It seems almost like it changes the OEM shape to something closer to a Merc M6/M8 drive. I would think I would need twice the horsepower, different height and a different prop to actually take advantage of that new shape, right? So it's dumb, right?

The idea is to lower the water pickup and create a smooth transition. Does the nose cone have the low water pickup built in? This was never really clarified.

 

[Matt Mead]

The idea is to lower the water pickup and create a smooth transition. Does the nose cone have the low water pickup built in? This was never really clarified.

I'm gonna say no.

 

[Outdrive1]

I would make sure the rubber hub isn’t spinning in that old prop before you go making a bunch of changes. Sometimes they only spin under load. And work fine at idle or slow speeds.

 

[Ladsm]

I would make sure the rubber hub isn’t spinning in that old prop before you go making a bunch of changes. Sometimes they only spin under load. And work fine at idle or slow speeds.

I forgot about that, I spun the center out of a prop once and you didnt notice it till it was under heavy load.

 

[Matt Mead]

Well that would suck. So the other day I was able to spin the motor over by hand while it was in gear using the prop. I was fighting against the compression of the motor somewhat hard and it didn't slip. Would that be a good test or not?

 

[HST4ME]

Pull it off and post a pic of the backside of the hub.

 

[Flying_Lavey]

Anything besides full down trim and it was just spin city. Then my cousin stood in the back of the boat (no seats) and we picked up 5 mph. It looks weird when it spins, it shoots a rooster 5 feet to the starboard side. So yeah, I dunno, just massive cavitation until the prop is fully buried to where it's impossible anymore.

So what is the actual idea behind these aftermarket nose cone's anyway? I don't really understand it, even though I could do a quick uneducated guess. It seems almost like it changes the OEM shape to something closer to a Merc M6/M8 drive. I would think I would need twice the horsepower, different height and a different prop to actually take advantage of that new shape, right? So it's dumb, right?

Yes, it's idea is for a more hydrodynamic shape particularly in a surfacing application. I have one on my 200 Merc, mostly for its low water pick-up capabilities though. They aren't much help at all performance wise until 70 to 80 mph.

All prop rooster tails come off at an angle due to the water being literally thrown off the blades.

The idea is to lower the water pickup and create a smooth transition. Does the nose cone have the low water pickup built in? This was never really clarified.

Definitely no water pick-up on that nose cone,

I would make sure the rubber hub isn’t spinning in that old prop before you go making a bunch of changes. Sometimes they only spin under load. And work fine at idle or slow speeds.

Shit, I didnt even think of that. I should have since I have spun so damn many myself.

Well that would suck. So the other day I was able to spin the motor over by hand while it was in gear using the prop. I was fighting against the compression of the motor somewhat hard and it didn't slip. Would that be a good test or not?

They can be spun and still hold together enough to push the boat to varying degrees.

 

[HST4ME]

In the close up of the cone you can see how fucked up the profile is. Take a chunk to 2x4 and a small sledge and knock it off of there, grind down the filler. Then you need to take a close look at the stringers and the bottom of the boat hooking.

 

[Matt Mead]

Well, the biggest change was dropping the motor down 1" and I picked up almost 10 mph! I had loads and loads and loads of up-trim available all of a sudden now, no prop blow out except for one little weird moment at full up-trim, and zero slip at all during hole shots. So the hub on the prop definitely wasn't spun. However, I *may* have found the limit of this old, non-performance hull. I chine walked HARD at 50-52 mph. It was scary, being my first v-hull outboard capable of chine walking. It didn't have enough power to push through the walk, so I backed off every time. And there may be a way to set it up so it does it less and we get to 60...but damn!! 52 out of a 76 wet noodle!? (GPS verified)

I'm more than happy with that. Because you have no idea what shape this hull is in. The integrity of this hull is probably less than 50% of new. The stringer, the transom, the floor, the transom braces, the top half of the hull not connected to the bottom. I mean the list goes on. To get it to 52, chine walk and *NOT* hook, rip apart and sink the boat is a miracle. A damn miracle. We had our vests on each time, lol.

Anyway. I don't know if I did my calcs wrong on the prop to pad. I know some factors were against me on that calculation. I could have totally fucked that up. Also, some people talk about cavitation plate to pad distance as a good estimation. If you go by that, I ain't far off! But whatever was happening, it needed the motor down from the previous owner's setting. Regardless of all theory.

I was only turning 4200 RPM's. With a good hull, better fuel, better compression on the motor, nobody in the boat and balls of steel to drive through the chine walk....60 for sure.

 

[Matt Mead]

Video of the day

 

[Matt Mead]

One more extra reply. There is a slight chance I got into the resonance zone on my super duper sloppy steering and steering to mid section slop. If you wiggle the motor in free air the thing wiggle wopples all over the place. Like damn, are there any rubber bushing in there at all? It's possible my "chine walk" at 52 was actually "blown bushing walk" and we could be in for some mph after that!

 

[JUSTWANNARACE]

I would say the "chine walk" is from the motor being so far down and "lifting" the transom, causing it to chine.. just a thought..

 

[Matt Mead]

I would say the "chine walk" is from the motor being so far down and "lifting" the transom, causing it to chine.. just a thought..

I am a total newbie and will take any theory at this point. That sounds like a valid one. It's possible I had a cavitation bubble on the prop from the broken trim tab on the cracked skeg...but with so much more MPH and up-trim available do I dare raise the motor now? Only 1/4" maybe?

 

[Outdrive1]

If you’re too low. Your prop will try to surface and feel like a chine walk as the blades contact the water and push the boat over. I’d try and go higher and see if the motion goes away. Water pressure however is the key. Don’t run low on water pressure or you’ll burn it down.

 

[Outdrive1]

I would say the "chine walk" is from the motor being so far down and "lifting" the transom, causing it to chine.. just a thought..

This is what I’m trying to explain.

 

[Outdrive1]

One more extra reply. There is a slight chance I got into the resonance zone on my super duper sloppy steering and steering to mid section slop. If you wiggle the motor in free air the thing wiggle wopples all over the place. Like damn, are there any rubber bushing in there at all? It's possible my "chine walk" at 52 was actually "blown bushing walk" and we could be in for some mph after that!

Torque steer is normal on an outboard. Unless you have hydraulic steering or a no feedback cable set up.

 

[Matt Mead]

If you’re too low. Your prop will try to surface and feel like a chine walk as the blades contact the water and push the boat over. I’d try and go higher and see if the motion goes away. Water pressure however is the key. Don’t run low on water pressure or you’ll burn it down.

Water pressure gauge works and had plenty of water pressure. I'll try going up 1/4" and see what happens. Side note: when trimmed up "all the way" (or what I felt like was all the way, what it could take) when I let off the gas the whole boat dropped down into the water with a big shift in steering. Kinda like watching a trophy truck get off the gas and then nose dive on the brakes into the corner. But also the butt (transom) sinks down a bunch. Felt crazy have the whole boat "sink" down that much after a speed run.

Torque steer is normal on an outboard. Unless you have hydraulic steering or a no feedback cable set up.

Alright, well, I don't know how it's supposed to feel is one of the problems. But it definitely got into butt wiggle territory that scared me and the passenger, which is freaking awesome and scary all at the same time. I probably could have ridden the butt wiggle for a while, but being it was the first time feeling it I panicked every time it started, lol. Our steering wheel is on the small side, so not much mechanical advantage and the whole system is not exactly new, smooth or in good condition, so when it happened it felt like fighting the steering of a racing kart rather than just correcting a power slide in a car with hydraulic steering. It's a single cable setup, btw. Which seems dangerous as hell.

I just felt like I was 2 seconds away from doing this and want to avoid it.

I'm only scratching my head on one other thing, not sure if I should make a new thread for it. The motor idles fine in free air, but doesn't idle very nice when lowered all the way into the water in the buoy zone. Clears up when cruising and speed runs. Then after it's really warm and we get back in the buoy zone it REALLY doesn't want to idle at all. Shudders, shakes, surges and sometimes dies, but totally fine if I raise it up so the prop and cav plate exhaust holes are above water again. I'm a pretty good mechanic on motors, but this one is going to take me some time, for sure.

 

[Matt Mead]

Alright here is another photo of where the motor was for 52 mph @ ~4200 RPM. Keep in mind this motor is probably only making 120 HP, lol. I can't even trust the tachometer either, TBH.

I raised it 1/4" from there. Next test is around Easter weekend. Just as a sanity check, currently the cavitation plate is 1" above the "pad". And to be clear, this boat doesn't have a pad, it's just the end of the Vee. I figure everyone knows that anyway and everyone just decides to call it the "pad" no matter what it's shaped like on various boats.

Motor can't be very healthy after sucking disintegrated cowl sound insulation chunks through uncovered carbs, lol!

 

[STV_Keith]

One more extra reply. There is a slight chance I got into the resonance zone on my super duper sloppy steering and steering to mid section slop. If you wiggle the motor in free air the thing wiggle wopples all over the place. Like damn, are there any rubber bushing in there at all? It's possible my "chine walk" at 52 was actually "blown bushing walk" and we could be in for some mph after that!

That's a problem and needs to be fixed. It may some some play with cable steering, but it should not flop all over the place. Having that much play will induce chine walk and other issues that you cannot fix with driving when there is that much slop in the system. Where is the slop coming from? Is this dual opposed cable steering (one steering cable on either side) or just single cable? Post a video moving the engine back and forth, showing the steering cable(s).

 

[STV_Keith]

Just saw your last post...unless it's been jetted to work without the airbox lid, you need to get that back on there.

 

[Matt Mead]

Just saw your last post...unless it's been jetted to work without the airbox lid, you need to get that back on there.

Alright, will do. I was doing so much testing and tuning I left it off for both trips. I figured given the horrible state of everything else it wouldn't make *that* big of a difference. Like for example, I still need it off because the carbs aren't synced. Top butterfly lags behind the bottom two. And I was trying to set the idle by seeing when the idle set screw just barely started cracking the butterfly's open. Previous owner(s) left me an awesome never-ending trail of happiness everywhere I look. Idle set screw was four miles out of position and idle ignition timing was flopping all over the place because of it.

That's a problem and needs to be fixed. It may some some play with cable steering, but it should not flop all over the place. Having that much play will induce chine walk and other issues that you cannot fix with driving when there is that much slop in the system. Where is the slop coming from? Is this dual opposed cable steering (one steering cable on either side) or just single cable? Post a video moving the engine back and forth, showing the steering cable(s).

Since the video shows it--I am fully aware of the state of this transom and the risks involved with it. Weird thing is, after working on this boat for ~100 hours, I see signs that they actually did the transom at one point. There may be "new" wood in there. Although at this point it's definitely wet. And I realize that pretty much kills any hopes of going *really* fast. How about 55 mph tho? Good thing I have medical insurance, eh

 

[Outdrive1]

Alright here is another photo of where the motor was for 52 mph @ ~4200 RPM. Keep in mind this motor is probably only making 120 HP, lol. I can't even trust the tachometer either, TBH.
View attachment 1104041

I raised it 1/4" from there. Next test is around Easter weekend. Just as a sanity check, currently the cavitation plate is 1" above the "pad". And to be clear, this boat doesn't have a pad, it's just the end of the Vee. I figure everyone knows that anyway and everyone just decides to call it the "pad" no matter what it's shaped like on various boats.

Motor can't be very healthy after sucking disintegrated cowl sound insulation chunks through uncovered carbs, lol!
View attachment 1104042 View attachment 1104043

Wow that’s so low. I would start at 1” below the bottom and start from there. That seems way too low imo

 

[Matt Mead]

Wow that’s so low. I would start at 1” below the bottom and start from there. That seems way too low imo

I think you may have missed the OP and history of this thread, lol. 1" up from there was prop blowout city and absolutely zero up-trim ability. And that was at 7" below the pad. At this point in time I would have to raise the motor 6.75" to bring it to where you are saying, lol.

Although I'm starting to believe maybe it was just a cav bubble on the prop from the torque tab and rough skeg that I ground smooth, plus the stupid nose cone ain't helping either.

 

[Crazyhippy]

You have half a dozen people telling you that you are 6+" too low...

You can either listen and learn from everyone else's trial and error, or diy. Choice is yours

 

[Matt Mead]

You have half a dozen people telling you that you are 6+" too low...

You can either listen and learn from everyone else's trial and error, or diy. Choice is yours

I realize that. Also, I picked up 10 mph dropping the motor an inch.

So I am torn. Do I follow thousands of Google results for putting this motor 6" up from where it currently sits or scoot it 1/4" at a time until it blows the prop out again? Maybe there is one other guy out there with a 1976 18' Tahiti that found out he needed his outboard way lower than what most people need on other hulls?

 

[JUSTWANNARACE]

I realize that. Also, I picked up 10 mph dropping the motor an inch.

So I am torn. Do I follow thousands of Google results for putting this motor 6" up from where it currently sits or scoot it 1/4" at a time until it blows the prop out again? Maybe there is one other guy out there with a 1976 18' Tahiti that found out he needed his outboard way lower than what most people need on other hulls?

Does this one count

 

[Matt Mead]

Dang, that's vintage for sure. What year is that?

 

[JUSTWANNARACE]

Dang, that's vintage for sure. What year is that?

1974 or 76 I cant remember. Splash of a Tahiti. He has owned it since new. We are dealing with exactly what you are explaining since he put the new(to him) motor on. I finally convinced him to order a jackplate. It starts to lift the transom at about 55-60mph and starts to chime walk. Motor needs to come up about 4"-5" and it will be a 90mph boat(if your willing to drive it).

 

[JUSTWANNARACE]

I realize that. Also, I picked up 10 mph dropping the motor an inch.

So I am torn. Do I follow thousands of Google results for putting this motor 6" up from where it currently sits or scoot it 1/4" at a time until it blows the prop out again? Maybe there is one other guy out there with a 1976 18' Tahiti that found out he needed his outboard way lower than what most people need on other hulls?

And just to let you know.. you are not getting the "thousands of google" responses on here. You are getting "real world" advice from some of the "best in the industry"(besides me, I'm still a sponge with these guys)!! And most are talking from experience. And most of us will eat crow when we are wrong(probably even buy you a beer). But if you are not willing to absorb the info and atleast try it, you will find you will be talking to the wind shortly(only hearing your own advice blowing back at you) and future advice and tips will stop.

All in all you have a wealth of knowledge here(which is what you are asking for) at your disposal, and guys that are willing to help you through this issue(and they may even learn something in the process) but dont disregard info as you are, because you are obviously here asking for help but rejecting it at the same time! Why ask for "help" if you are going to contradict the options!?

Just some food for thought..

Water pick up(engine) will be your biggest issue as the motor gets moved up.