Bravo I - Any ideas on this?

[Shallow Minded]

Wondering what anybody's ideas might be on this. Running a 26 ft Wellcraft Nova with a Bravo I, mildly blown 454, and a 23 pitch 3-blade. When I hammer it out of the hole, it goes for a short period and then feels like I have a cavitation issue. If I baby it up until it's on plane, no problems. My trim must have slight leak as the outdrive requires fairly regular adjustments and the trim will "sag" if it sits on the trailer for a few days.
My question, is this a cavitation issue with the prop or is my trim pump not strong enough to keep the outdrive down in the water? I think it's the latter because when this "blow-out" occurs, I've noticed a little rooster tail from the prop angling upward.
Has anyone experienced something like this? Would a new trim pump fix it? Thanks in advance.
SM

[phebus]

Your going to have to fix the trim issue, so repair it and see if your problem goes away before dealing with the prop.

[Shallow Minded]

I really don't think it's the prop and I'd just as soon narrow it down before I start throwing new parts at it. I've read those pumps can get debris in them that prevents the valves from sealing completely. Would that explain both the sag and problems when we launch?

[Bob Hostetter]

Normally if you have a problem with the trim pump or cylinders you can spot them by trying to move the outdrive by hand when the boat is on the trailer. Trim the o/d down and then try to move the o/d, try hard.........If you can't then probably the trim system is okay.
On the other hand if the o/d sags on the trailer or the boat constantly needs to be retrimed while under way then you may have a leaky check valve in the pump.

[Havasu Hangin']

My question, is this a cavitation issue with the prop or is my trim pump not strong enough to keep the outdrive down in the water? I think it's the latter because when this "blow-out" occurs, I've noticed a little rooster tail from the prop angling upward.
In forward, the force of the prop will push the outdrive down, not up. Usually, the drive tucked in will give you a better holeshot...so I'm not sure that's it.
I agree- get the trim fixed, so you can try different trim angles when coming out of the hole.
If it's not leaking anywhere (check for sheen in the water when sitting), then fluid is getting past the valves in the pump or seals in the cylinders.

[Shallow Minded]

I've looked several times for a sheen on the water, but have never noticed anything. I don't seem to be losing fluid, either. Guess I hadn't thought of the prop pushing itself down during take-off.
I'll give the deadlift-the-outdrive-test a shot tonight. This should be interesting...
Thanks for the ideas, guys. Anything else I should consider?

[75spectra]

It could be leaking down and you still will not be able to move it by hand! The stall pressure in these pumps is a terrific amount. BEFORE you start buying trim pumps you better have the system pressure checked, it could be the checkvalves in the pump or it could be o-rings in the cylinders letting it bleed past. Pumps are not cheap but o-ring kit are inexpensive and easy to install. Just my .02

[75spectra]

Hey after going back over your original post... blown 454 i'm assuming thruhull exhaust??? Make sure that the exhaust bellows is removed! If that bellows is there with thruhull exhaust you will create a massive vaccum inside the drive and prop untill the boat breaks over on plane this could be causing your "blowout" Just a thought cause this was the case on my Eliminator.

[Shallow Minded]

Hey after going back over your original post... blown 454 i'm assuming thruhull exhaust??? Make sure that the exhaust bellows is removed! If that bellows is there with thruhull exhaust you will create a massive vaccum inside the drive and prop untill the boat breaks over on plane this could be causing your "blowout" Just a thought cause this was the case on my Eliminator.
This sounds exactly like what is happening to me. I'll get a nice hole-shot and just as soon as the bow starts to bow down, the "blowout" occurs. I do have thru-hull exhaust.
Dumb question, but what exactly are the exhaust bellows? Those butterfly-looking valves that redirect the exhaust on my captain's call? We might be getting somewhere here.... :) Thanks for the idea'rs, guys...

[BADBLOWN572]

I don't think that the trim has anything to do with your cavitation. HH is right, when you are going forward, the force in pushing the drive down and down is where you should be getting the best hole shot. Cavitation usually has to do with the prop itself. Try another prop and see if it does it. If it doesn't, you have found your problem.
I really don't think that you could have gotten debris into your system unless you poured it into the trim pump and only then could small particles get into the pump. It really wouldn't cause a problem. I think that it could be more of the "O" rings that are in the trim cylinders or pump. If the O rings have any slight problem with them, they would be letting fluid pass and causing the trim to drop. Same with the valve. If the trim valve doesn't close fully it could cause that problem as well. Good luck.

[Havasu47]

Sorry if this sounds blunt, but...... a 26' Wellcraft, blown BBC, 3 BLADE PROP
Put a 4 blade Bravo 1 prop and the cavitation will go away. The blower creates large amounts of torque at low RPM'S. Just when the boat rolls over is when the torque curve comes on strong. If your trim system isn't working properly get it fixed of course, but it will still blow the prop away with a 3 blade.
just my .02 of course :D

[75spectra]

This sounds exactly like what is happening to me. I'll get a nice hole-shot and just as soon as the bow starts to bow down, the "blowout" occurs. I do have thru-hull exhaust.
Dumb question, but what exactly are the exhaust bellows? Those butterfly-looking valves that redirect the exhaust on my captain's call? We might be getting somewhere here.... :) Thanks for the idea'rs, guys...
Ok, if you have "captains call" exhaust then you need to replace the bellows with a exhaust tube. This is called for in Mercruisers service manual. The tube might be there if the captains call was installed at the factory. Tilt your drive up and look underneith of it, you should see a large bellows thats the u-joint bellows and the lower one is the exhaust. The small one on the right side has the shift cable going through it. If you see a "tube" that is open and seals off when the drive is down then you need to look at a diffrent prop that prevents blow-out?

[Speedin' Ian]

Sorry to hijack the thread but 75 Spectra has me thinking about my own boat. On my 25' Commander I have thru hull exhaust and I think my drive still has exhaust bellows installed on it. Is this bad? I haven't notices any problems getting on plane, and it has been set-up like this for almost ten years. How does having belows create a vaccum?

[75spectra]

Sorry to hijack the thread but 75 Spectra has me thinking about my own boat. On my 25' Commander I have thru hull exhaust and I think my drive still has exhaust bellows installed on it. Is this bad? I haven't notices any problems getting on plane, and it has been set-up like this for almost ten years. How does having belows create a vaccum?
If the "U"-pipe has been removed and blocked off then the prop will create a low pressure area inside the drive when you start moving water away from the prop. The Bravo drive has pressure reliefs just above the cavitation plate, feel for them they are there they go up into the drive. When these reliefs finaly break the vaccum some props will instantly "blow out".... NOT ALL BOATS HAVE THIS CONDITION! This info is in Mercruiser's service manual. I remove all exhaust bellows from any boat we put thru hull exhaust on. Silent choice is diffrent, for this you remove the bellows and replace it with a "exhaust tube" this tube is a Merc part.

[Rev. Williams]

Sorry if this sounds blunt, but...... a 26' Wellcraft, blown BBC, 3 BLADE PROP
Put a 4 blade Bravo 1 prop and the cavitation will go away. The blower creates large amounts of torque at low RPM'S. Just when the boat rolls over is when the torque curve comes on strong. If your trim system isn't working properly get it fixed of course, but it will still blow the prop away with a 3 blade.
just my .02 of course :D
I'd have to agree with this comment......... I run a 26p 4 blade B1 and I can blow it out when I come over on plane. It catches up quick once the boat lays down, but for that brief few seconds I get nothing from the prop.
In ***boat West there is a thread going about 28p props and within that thread someone posted a number to a place that lets you test/rent different props to see what works best. I'm gonna test the 28 on mine, I'd suggest a 24-4 or a 26-4 for what you have.
Good luck and have fun with it.....
BTW: when I was growing up we had a Wellcraft 21Nova. Talk about a smooth riding boat!!

[Shallow Minded]

Sorry if this sounds blunt, but...... a 26' Wellcraft, blown BBC, 3 BLADE PROP
Bluntness is ENCOURAGED. I just want to figure out how I can get a little better take-off and optimize the power I'm making. I've spoken to another guy who runs a very similar set-up and he's running a 24 pitch, 4-blade. My boat weighs about 6500 lbs which might have something to do a very tame pitch. Considering I'm only running 4800 RPM's at WOT, I'm a little gunshy about stepping up to a 24.
Also, as for the bellows, I'll take a good look at it tonight. I'm sure nothing has ever been altered here, just a standard Silent Choice set-up. I really appreciate the info.
And yes, these Nova's ride like a Caddy...

[Shallow Minded]

From the looks of things, the exhaust bellow has been removed. I compared my outdrive to the B1 on my brother's boat and I did NOT have the flexible, connecting tube from the transom to the actual outdrive. I'm assuming that is the "bellow."
Also, sounds like I need to put a 4-blade prop on my X-mas list. Anybody have a 4-blade, 22 or 23 pitch laying around? Thanks.

[superV]

I have owned thre Nova's 26,27 and a 23. With that set up you should be running a 26p clever.The nova's need the setern lift! And I would also look into your motor 4800 with a 23? That where mine would run wioth a 26P Just my .02

[spectras only]

I'm surprised you're only turning 4800 with 23p and a blown engine. I've got 6.2's , n/a [320HP at the prop] and turning 5000 with Bravo1 4 blade, 15X26p . My boat is ass heavy with the twins ,but no problem getting on plane with the K-planes.

[Havasu Hangin']

I'm surprised you're only turning 4800 with 23p and a blown engine. I've got 6.2's , n/a [320HP at the prop] and turning 5000 with Bravo1 4 blade, 15X26p . My boat is ass heavy with the twins ,but no problem getting on plane with the K-planes.
Off the subject...
...are you gonna change your screenname to "spectras sometimes"?
:D

[1stepcloser]

Handle it.

[spectras only]

Off the subject...
...are you gonna change your screenname to "spectras sometimes"?
:D , HH , can't do that, I still have 2 spectras left :cool: . Only the 20' jet in my avatar gone to a friend :cry: .

[Raisin Wake]

Ok, if you have "captains call" exhaust then you need to replace the bellows with a exhaust tube. This is called for in Mercruisers service manual. The tube might be there if the captains call was installed at the factory. Tilt your drive up and look underneith of it, you should see a large bellows thats the u-joint bellows and the lower one is the exhaust. The small one on the right side has the shift cable going through it. If you see a "tube" that is open and seals off when the drive is down then you need to look at a diffrent prop that prevents blow-out?
Will the exhaust tube help limit this blow out? How is it different from the bellows when the drive is down? I'm assuming that this (http://www.go2marine.com/frameset.jsp?servletPath=/g2m/action/GoBPage/id/92913F/hiLiteSku/92913/categoryId/13618/) is what your talking about.
Thanks for the info.... :)
Details: I'm running thruhull exhaust, i have a sound selector, but, I don't need it or use it. My exhaust ports are below the water line. I currently have the bellows installed and get the same blow out that has been talked about in this thread.

[mike37]

what was the question

[Raisin Wake]

What benefit does the Exhaust Tube give you over the Exhaust Bellows?

[mike37]

What benefit does the Exhaust Tube give you over the Exhaust Bellows?
I dont think you have a choice the newer drives have the tube
and its only connected on one side you dont have the problem of the bellows cracking with the tube
performance wise there is no difference
if you have transom exhaust you dont need ether the exhaust port will be capt

[Havasu Hangin']

What benefit does the Exhaust Tube give you over the Exhaust Bellows?
Probably never rots. If you have through-hull tips, I'd just put a cap and clamp where the bellows were. Cheap enough.
However, the bottom line is I think you need 4 blade prop. A band-aid may be a bigger bolt-on cavitation plate...but a 4 blade will give you more bite.

[Raisin Wake]

Is there a specific part you can buy to cap the exhaust line?

[Havasu Hangin']

Is there a specific part you can buy to cap the exhaust line?
I'm sure there is a specific cap out there...but you probably could just measure the diameter of the exhaust (probably 3" or so) and look for a plastic pipe cap at Home Depot. A stainless clamp will hold it on.
However, I wouldn't cap the exhaust before getting rid of the Captain's Call- it could spell trouble.
I want you to say these words with me:
"FOUR"..."BLADE"

[Raisin Wake]

Four Blade........Four Blade........Four Blade.......:)

[mike37]

I'm sure there is a specific cap out there...but you probably could just measure the diameter of the exhaust (probably 3" or so) and look for a plastic pipe cap at Home Depot. A stainless clamp will hold it on.
However, I wouldn't cap the exhaust before getting rid of the Captain's Call- it could spell trouble.
I want you to say these words with me:
"FOUR"..."BLADE"
I second that

[Havasu Hangin']

On my last 23' boat, I used to cavitate a three blade prop pretty good. I could also break it loose in a hard, slow turn (like picking up a skier).
A four blade solved it.
The chant starts outta the crowd...
"FOUR BLADE...FOUR BLADE...FOUR BLADE...FOUR BLADE..."

[spectras only]

If you still going to have cavitation with a 4 blade , just add another engine like on this picture :wink: :D http://www3.telus.net/spectrasonly/mirage%20transom.bmp