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

[Matt Mead]

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).

Well isn't that interesting. Hmm.

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 that's exactly why I came here. Listen I'm not trying to ignore everyone and say everyone is wrong. I raised the motor another 1/4" but I have to wait another 2 weeks to test. I would love to test it every day, and update the thread more often, but I can't. And I would love to all of a sudden end up 6" higher than I am now and be at 70 mph. It's going to take me months, though. I'm working though it, baby steps.

Why ask for "help" if you are going to contradict the options!?

I figured maybe I had a special case that went against all the common rules of thumb for this sort of stuff and if I documented it, tested, tuned, documented and ended up with a solution in the end maybe I could learn and share my findings with others, too. Listen, if I raise this motor 6" and it goes 20 mph, what did we learn?

In my OP "Interesting conversation/observation even if nobody has any input."

My original plan when I bought the boat was the RAISE the motor. I was like "damn, that looks pretty low." Then I pick up 10 mph lowering it, so it's a major WTF moment. It doesn't make sense to me either.

 

[Matt Mead]

I feel like in the midst of all my rambling nonsense we need a recap of what has happened so far.

At 7" from prop to pad I did 43 mph and could not trim up even a millimeter from full down position.

At 8" from prop to pad I did 52 mph and could trim up for miles until the boat got super duper sketchy and loose and I let off.

7.75" prop to pad test is Easter weekend.

 

[Matt Mead]

I literally lost some sleep over this, so I'm going to brain dump to try and feel better.

I think it's the stupid nose cone. Here's why. First speed run ever on the first test run ever was trimmed up halfway and we did 38 mph. Then my cousin wanted to look at the motor so he moved to the back of the boat and we all of a sudden in 2 seconds did 43 mph. So I trimmed all the way down and repeated the 43 mph with him in the front seat. Keep in mind this boat is gutted. No interior whatsoever besides two plastic framed bucket seats in the front. There's currently 100-200 lbs "missing" from the back of the boat. And the symptom was massive cavitation on hole shot and top speed. You could feel it and hear it. You barely stab the throttle and it spun the prop way too easy.

So I lowered the motor an inch and bam...all of a sudden zero cavitation and 10 mph more on the top end. Then all of a sudden I could trim up for days. It was finally acting like a lightweight boat with an outboard that handled exactly like I would imagine an outboard would handle (I've driven jet boats and PWC my whole life). However, when you look backwards at full up trim and top speed the motor now shows signs of being buried. I mean, finally! No rooster tail, very flat boring action going on there, plus no weird starboard rooster like before, and of course no RPM. Well at least I have a baseline now!! That's actually awesome!

So my goal is to raise the motor back up 1/4" at a time until cavitation occurs and that would be the limit of....the nose cone...right? I mentioned this is a super budget build. So knocking the nose cone off can potentially put me in a $$ situation that I would like to avoid. This boat is a shared ownership situation and unfortunately with my disability I ran out of cash to throw at it, so now it's the other two owners throwing money at it. And guess what, they don't care about top speed, lol. All they care about is an interior and 20 mph. So trying to get to ~60 mph needs to be done 1 mph and $20 at a time, if you catch my drift.

So I guess what I'm trying to say is with that silly nose cone on there, no convergence/divergence cone on the back of the prop, a worked prop that's not stock, a light boat with no weight at the transom, two fat dudes in the front, and about 19 other unique factors, it ain't going 70 mph, yet. If I raise the motor 6" from where it currently sits without nocking the nose cone off and changing a handful of other items, it's just gonna be cavitation city, population 2.

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.

So I basically started doing exactly that. I am a massive introvert, nearly a hermit, with anxiety disorder that has nearly ruined my life. So this is par for the course dude!

 

[STV_Keith]

If you're going to run the gearcase low, you want to get rid of that nosecone. Given you don't know what's underneath it, I'd do that first, just because. Want less transom lift, go to a chopper style prop with more rake. Only make one change at a time, so you know what each change does.

 

[Matt Mead]

If you're going to run the gearcase low, you want to get rid of that nosecone. Given you don't know what's underneath it, I'd do that first, just because. Want transom lift, go to a chopper style prop with more rake. Only make one change at a time, so you know what each change does.

I forgot to say it above, despite my long monologue into the wind, but I think there is way too MUCH transom lift. And I think the nose cone is inducing it also. I could be wrong, but this boat is so freaking light it's hilarious. I can lift the front of the trailer 4 feet in the air and move the whole boat around the garage by myself. It's gutted with zero weight in the back besides one battery. The oil tank is gone, premix now, no carpet, no back seat, no hatch, no cooler, nothing. A guy who was testing and tuning his boat next to me on the lake was like "Damn, that thing doesn't weigh shit eh? Probably like 1000 lbs" and I was like "You might be right! Because yeah it really doesn't weigh shit!" So I feel like I need to reduce transom lift as much as possible. At least until we get the interior put in in June.

I agree, one change at a time, so I know what does what. But yeah, I'm scared of taking off the nose cone. Given the rest of the damage on every other part of the boat I can only imagine what amazing horrors lie underneath that cone. There's literal holes and patch jobs from the previous owner hitting every part of the bottom of the river in Blythe he could, and I gotta run the bilge every 10 mins until I finally patch up all the holes. I'm banking on the fact that when I remove it I will probably just want to put a whole new lower on anyway. So basically I'm trying to slap this thing into shape...with the nose cone, for now, until next year at least.

Thanks.

 

[HST4ME]

Motor needs to come up about 4"-5" and it will be a 90mph boat(if your willing to drive it).

 

[STV_Keith]

Sorry Matt Mead, looks like I originally missed the "less" in my post...I meant the chopper will give less transom lift with more rake (generates bow lift). I've edited my post.

 

[Flying_Lavey]

One thing you'll notice is that if you want to go faster and you have a friend with you, move them to the back. That way the boat isn't using some of the engine perpolsion power to carry that weight. It will allow the boat to free up more and pick up speed. That normal on just about all boats but MUCH more pronounced on outboards.

@JUSTWANNARACE your buddy and Matt Mead, pay attention to what the spray off the hull does as speed increases. If the spray moves forward with more speed, that could be a solid indicator of hook in the hull. Hook will produce transom lift and push the bow down with more speed.

 

[boblins]

Have never had any success running a stock type prop much higher than what the pic shows with the many small outboard boats we used to run and sell at our dealership back in the days the posters boat was built. With a surfacing type prop, sure. The trade off was that the boats became hard to get on plane as the props have a tough time finding bite when sitting so high on normal V bottoms (which almost always had a built in hook), but good speed and lots of fun once they get going. Probably wrong again, but personally think you are on the right track. If we sold that boat the cavitation plate would have been set up level with the bottom of the boat or just slightly higher ( an inch or two ) if using stock props.

 

[Matt Mead]

With a surfacing type prop, sure.

Which this is not. So that would make sense. I have been wondering that the whole time.

If we sold that boat the cavitation plate would have been set up level with the bottom of the boat or just slightly higher ( an inch or two ) if using stock props.

It's essentially level with the bottom of the boat right now. I figure with this setup much higher and I would need more set back on the jack plate to let the water curl up behind the transom and then bite the prop again. I believe I am within 1/2" of where it wants to be without making major changes.

Unfortunately today's test day was cut short when I couldn't get the motor to idle. Our luck has run out on the fuel system. I am going to rip it all apart and go through everything. I have a new VRO delete pump to install already, just need lots of fresh new fuel line, new filters, fittings, clamps, primer bulb, possibly new pulse line and pulse limiter, clean the carbs out again, etc, etc. Primer bulb was collapsing, fuel selector switch was not working properly, ran high idle in free air then died immediately when lowered under water, so I believe it's as lean as it could be and I sucked up years of junk and redistributed it all over the system. No amount of idle timing adjustment or "link and sync" could fix the problem. Got away with 2 test days, now it's going to be essentially an entire fuel system rebuild, but that's what I signed up for! I'm surprised I even got it to do 2 speed runs so far, lol.

 

[Bajastu]

Don’t dump the VRO. Johnson invested serious money to make that work right. No motor ever blew up due to a VRO. It’s engineered to make your engine run with the right oil mixture. Low at an idle and 50:1 at the high end. I’d rebuild or just buy a new one.

 

[Matt Mead]

I disabled the VRO a long time ago. I was supposed to install the non-VRO pump, but got lazy. I have no idea what condition it is in or if it's sucking air anywhere even though I capped off the oil line well. I did basically the same thing on my Yamaha R5. I understand the systems, and I don't want them. I understand they work 99.99999% of the time, but I am a minimalist. I don't mind premixing, and I don't mind smoke at cold start. I went back and forth between both camps for the longest time, keep it or not, and I decided to go premix finally. My Yamaha actually injected too much oil, lol.

When I was diagnosing the engine if I pumped the primer bulb I could get it to idle longer under water. Last thing I want to do is rebuild/replace a VRO pump when I already bought a nice non-VRO. https://maxrules.com/VRO_06-200.php

 

[STV_Keith]

No more primer bulb, run a Holley red pump. Gave plenty to my old Monty Racing V8's at 380hp.

 

[Ultra...Good]

That amount of slop you have in the steering, or whatever it is needs to be addressed. If that breaks free when you are going fast, you are going to want to have impact jackets and helmets on. Don't find out the hard way.

If you put a new and or have an old filter inline, the fuel filter goes after the fuel pump. Filters are made to be pushed through, not drawn through.

If you are having trouble idling, are your carbs spitting fuel back out through them? You might need a new set of reeds if that is happening. They are a wear item, not to hard to replace, and they made my motor run a lot better at all rpm ranges.

This advice is only worth what you paid for it.

 

[HST4ME]

Don’t dump the VRO. Johnson invested serious money to make that work right. No motor ever blew up due to a VRO. It’s engineered to make your engine run with the right oil mixture. Low at an idle and 50:1 at the high end. I’d rebuild or just buy a new one.

 

[Matt Mead]

No more primer bulb, run a Holley red pump.

I am kinda getting tired of pulse pumps on two strokes after having so many of them over the years. One of these days I will run a low pressure electric, but I already bought a really nice pulse pump 2 years ago and it's just been sitting on the shelf. So that's what it gets for now.

That amount of slop you have in the steering, or whatever it is needs to be addressed. If that breaks free when you are going fast, you are going to want to have impact jackets and helmets on. Don't find out the hard way.

Yeah, I know

are your carbs spitting fuel back out through them? You might need a new set of reeds if that is happening.

Yes and yes that's what I am figuring I will need. I was planning on rebuilding this whole motor, but the first speed run was so promising I figured hey let's go a summer before we do that. But there are too many signs the reeds are shot and I'm trying to fool myself into not touching them yet. I know, how the hell will you get it running right without doing them? The age old question of working on any old motor basically. If I had $10k in my back pocket absolutely everything would be rebuilt with shiny new parts bolted everywhere, but I don't...so it gets the shade tree mechanic treatment unfortunately. I know they aren't hard to do, the carbs are all coming off tomorrow anyway.

 

[Matt Mead]

I don't want to post pics, but basically I removed the entire fuel system today and it was a disaster. Cracked, loose, plugged, you name a problem it had it. The pulse fitting to the VRO pump was hanging on by a thread. I'm glad I finally got in there and ripped it all out.

 

[HST4ME]

That motor will still have the boss for the old style fuel pump, just pick one up and put it on.

 

[Ultra912]

FOLLOWING
Interesting thread. I too have a walking issue. Reading through some mentioned transom lift will cause walking. I have manual trim tabs and they are all the way down. This would also create lift on the transom? I have a chopper I need to try but a 4 blade 24 Tempest Plus setup walked at around 45 mph. I'm gonna try a 24 chopper with the tabs all the way up and see what I get. Also, I think I can lift my motor a bit. I wanna see 6K at WOT,,,,,,,but I gotta get rid of walking first. Photo is a 22 chopper

 

[Ragged Edge]

FOLLOWING
Interesting thread. I too have a walking issue. Reading through some mentioned transom lift will cause walking. I have manual trim tabs and they are all the way down. This would also create lift on the transom? I have a chopper I need to try but a 4 blade 24 Tempest Plus setup walked at around 45 mph. I'm gonna try a 24 chopper with the tabs all the way up and see what I get. Also, I think I can lift my motor a bit. I wanna see 6K at WOT,,,,,,,but I gotta get rid of walking first. Photo is a 22 chopper

Yes, having the tabs buried will make the boat run nose down, transom up. Make one change at a time. Raise the tabs all the way and go run it with the Tempest. Then try different props, then raise the engine up or, keep running the same prop and raise the engine no more that 1/2 inch at a time, then start with the different props. Make notes as you go.

 

[Luv2gofast]

Ultra912: Set the tabs flat with a straight edge. They just help you get on plane but for sure should not be down from flat with the bottom of the boat. Both Matt's boat and the Ultra will need a low water pickup to run a chopper correctly. The numbers you refer to of even or one inch above are with low water pickups and choppers which want to run with just the bottom half of the prop in the water at speed. You can't do that without a low water pickup. That prop needs to be in the water and should not have a roost with the trim down. There is a lot of good information from others but I have a new question for you. When you are trimmed down and hold a straight edge on the bottom, what is the angle of the prop shaft. Is it really trimmed down? It's very strange that you have a roost "off the side" with the motor trimmed down.

 

[Ultra912]

Yes, having the tabs buried will make the boat run nose down, transom up. Make one change at a time. Raise the tabs all the way and go run it with the Tempest. Then try different props, then raise the engine up or, keep running the same prop and raise the engine no more that 1/2 inch at a time, then start with the different props. Make notes as you go.

My jack plate is a fixed bracket so moving half inch intervals isn't possible for me. The motor can be moved up but the distance looks to be a couple inches. I'll have to check it out closer

 

[Ragged Edge]

My jack plate is a fixed bracket so moving half inch intervals isn't possible for me. The motor can be moved up but the distance looks to be a couple inches. I'll have to check it out closer

If you're having to move the motor up on the plate try to go one hole at a time. I know, its a pain in the ass this way but it will give you a better understanding of what each change does to the handling of your boat. Be mindful of your water pressure when raising the engine.

 

[Matt Mead]

So I replaced the entire fuel system including a new fuel pump and completely rebuilt carburetors. Ran absolutely perfect on the trailer, so took it out for test #4.

The motor starts, idles, and runs absolutely perfect now, so it was a good test. I lost track of where I am prop to pad, but it's up 1/4" from the 52 mph run. Was able to hit 55 mph and it was stable as a rock. Plenty of variables between all the test runs, but I'm super happy with everything now.

Trim was working as expected. Every bump of the trim resulted in a nice RPM boost, a little more bow rise, and a little more roosty-ness from the prop. I figure everything is where it needs to be as I was at 5100 RPM, which IIRC is the exact HP peak of this motor. More (too much) uptrim resulted in nothing but spray, and it's not chine walking now. So I figure we would be fighting for 1 or 2 more mph and we honestly don't care. 55 mph on this thing is flying to us. It feels great, super smooth.

One cylinder has significantly lower compression than the rest, so I'm pretty sure we are just out of power more than anything. Any higher on the jack plate and I feel like we would be cavitating again, because it did it just a tiny little bit at this current height. Any taller pitch prop and it wouldn't have the RPM. Sure, change the nose cone, put the divergent/convergent cone on the back of the prop, and tweak a handful of other tiny things, but whatever. Honestly, it would probably go 2 mph faster without ~450 pounds of ballast in the front seats in the form of two dudes! Me and my cousin aren't exactly skinny guys, hahahaha!

Note: In the video my cousin is trimmed all the way down and cruising at like half throttle. Wasn't a speed run.

 

[MK1MOD0]

Kick ass ! Great to see ya get everything all ironed out. The boat looks and sounds great.

 

[Matt Mead]

Small update with some more theory/learning.

We put a new interior in the boat so now it weighs more, especially in the back. It still had a great holeshot, but I lost over 1,000 RPM and 10 mph especially with people on the back bench (no surprise). I raised the motor 1" for the next test session. Now it's getting slightly closer to where most other performance boats have their motor mounted. We will see where we are at since I just guessed where the height should be.

This leads me to believe with a completely gutted interior AND two big guys up front that the balance was way off from ideal, and that caused a lot of head scratching. The prop we have isn't made to lift the transom, but there probably is some transom lift. The nose cone wasn't helping. Then couple that with ~400 lbs of ballast in the front seats and a single wet noodle stringer meant up-trim didn't really do much but lift the prop out of the water. Kinda makes sense now. I caught a glimpse at the factory specs in a brochure, the boat only weighed 900 lbs less motor with a full factory interior. So without interior we were probably under 800 hull only, which seems pretty light to me! Just wasn't balanced. Never drove it solo either.

Also, bit of a duh moment a few months ago. I leveled the trailer and boat, trimmed the motor all the way down and looked at the relationship between the anti-ventilation plate and the hull. Turns out full down trim is actually neutral or slightly up-trimmed because the transom doesn't have much angle on it. That explained a lot all of a sudden and I started trimming the boat differently the next few trips and things made way more sense. On my first tests I eyeballed the motor angle and it turns out I was trimmed WAY too high all the time, thus the "cavitation". On our particular setup I cannot make the nose fly sky high. It's just not possible. So that started lighting up light bulbs in my head. Now when I holeshot I have it trimmed all the way down and when I go for a speed run I only give it a few burst of trim instead of a lot, because it won't take it. Sort of a big "Ah, OKAY!" moment, lol.

Burned about 15 gallons of gas a few days ago, so we are definitely having fun! This is still the goal, though:

 

[STV_Keith]

Camp Mack is a cool place to hang out and run. The red/black STV at 3:26 is my little brother's rig.

 

[Markus]

Don’t dump the VRO. Johnson invested serious money to make that work right. No motor ever blew up due to a VRO. It’s engineered to make your engine run with the right oil mixture. Low at an idle and 50:1 at the high end. I’d rebuild or just buy a new one.

Thousands of motors blew up due to VRO malfunctioning.

That’s why there were so many versions.

The later versions did not have a lower mixture at idle, btw.

 

[Markus]

I raised the motor 1" for the next test session. Now it's getting slightly closer to where most other performance boats have their motor mounted. We will see where we are at since I just guessed where the height should be.

You are making far too small changes given how far off the setup seems to be. Raise the engine up to where it should be. 7 inches up or whatever it was. If that does not work, move it down 3.5 inches. Then divide the interval by 2 again.

 

[HST4ME]

It's a 55 to 60 mph boat. And that is all.

 

[coolchange]

What’s up with that tilt pin?

 

[Berdes]

And post the same questions on Scream&Fly. They are outboard guy's too.

 

[stingray11]

I believe the prop is your problem you need to get a better prop that will run near the Surface. A 4 blade trophy plus 24 pitch will probably work good on that boat Then you can raise the motor.

 

[Matt Mead]

Tested the motor at the new height. I think I'm done messing with it. With a full or half load of people it holeshots perfectly and is fast enough to scare everybody except me, lol. I lost most of my up trim now, but it doesn't really need it. I can only bump the trim up 3 or 4 bursts before it just spins the prop. It feels like hull isn't strong enough to lift the bow after the initial bump up in trim. And after cutting the floor out and looking at the single, weak and rotten stringer (that never got replaced by the guys we paid) it's pretty obvious why. I mean, I can sail across medium chop at 20 mph and you can't feel a thing because it's flexing so much, lol. It's got like trophy truck suspension, which is the exact opposite of what you would want!

It's a 55 to 60 mph boat. And that is all.

Yeah, I agree. It's pointless to even keep trying at this point. The hull design and integrity are not up to any higher speeds. It already started "chine" walking on me again at low speeds. Exacerbated by completely worn out mid section bushings. It was never designed to be a performance boat in the 70's anyway. It also doesn't have a proper pad at the back. Just a big radius round keel. Meh.

I believe the prop is your problem you need to get a better prop that will run near the Surface. A 4 blade trophy plus 24 pitch will probably work good on that boat Then you can raise the motor.

I think you are right, but we are done looking for speed. Once I get this prop a little out of the water it loses all bite and needs a full reset. I figure I'm on the wrong prop to try and surface it. Maybe next boat I'll mess with a nice prop!

It hauls 4 people around at 30-40 mph all day on the lake. That's all they wanted anyway.

 

[Matt Mead]

What’s up with that tilt pin?

It bottoms out on another part of the casting before it hits the pin in that forward most hole. Would need a wedge on the jack plate to get any more down trim.

 

[WavetoWave]

Looks like a neat boat Matt. Key thing is make sure it's safe, double check the steering, get those motor mounts addressed, either with new ones or solid ones, that will make a world of difference. If the steering is not smooth, new cables are somewhat economical and make a world of difference.

If you can remove the cone, do it. Or, keep your eyes peeled for a replacement lower unit, you might find a clean used lower unit you can swap out. Nose cones are for semi surfacing 80+ MPH and won't help your setup. With a stock case your prop will perform much better and you'll be able to run it at the right height, with the ability to use some positive trim. Small diameter props like Rakers, Laser 2 etc will work well on your boat. It's not a rocket, you want all around performance.

That said, the elephant in the room is the integrity of the boat. The transom, stringers and floor are the super structure, they need to be 100%. Then, the hull has to to be really straight. It's most likely in need of some work. But, just by making the structure solid, and if you're happy with the performance, the key is being safe first.

Integrity of the boat, Steering, motor mounts. My 2 cents.