# Blow out!



## firecat1981 (Nov 27, 2007)

Well I put a jackplate on and now I think I'm experiencing prop blow out (cavitation) 

ok heres the deal, the whole reason I built my jack plate was because I didn't feel I was running shallow enough and even when on plane the cavitation plate was still a few inches below the surface even without a loaded boat. With the plate installed at it's lowest setting I am sitting about 2 inches above where I was and set back is about 4.5 inches. I also have a SE200 hydrofoil on my motor (2009 yamaha 20hp 4-stroke)
When I was testing it in the lake I first started with the second to lowest setting which gave me about 3 inches of lift. As I got on plane and speed increased it started feeling like the motor was choking, or someone was tapping the brakes, I tried adjusting the trim setting and it was best when the motor was at a negative angle (hole closest to the boat) but still not perfect. I went back with the JP now adjusted to the lowest setting, 2 or so, and it was a little better, but still very much there. 
I decided to take the SE200 off and see what happened. The blow out seemed slightly less, but it was harder to get on plane and stay there and I had some porposing. When I look back as I'm on plane now I can see the top of the SE200 slightly out of the water.
So any suggestions? My thoughts are first trying to redrill the JP, I think I can get maybe another .75" lower. The only other thing I can think of is maybe a new prop. Any suggestions there?
I think if I were running loaded down with buddies, ice, and full livewell then this would be less of an issue, but I fish alone alot too, so.......


----------



## Brett (Jul 16, 2008)

So much fun, tweaking a set up for the absolute best performance.
Type of prop used? Stock? Cupped? Stainless or aluminum?


----------



## firecat1981 (Nov 27, 2007)

prop is the stock aluminum piece, not sure of the size. Hey Brett maybe it's a design flaw  ;D


----------



## Brett (Jul 16, 2008)

Stock aluminum props are designed to be run with the
cavitation plate 1/4" to 1/2" below the surface of the water.
In order to run in the turbulent, aerated water found at the wake surface,
you're going to need a cupped, stainless, performance prop.


----------



## Tom_C (Dec 10, 2006)

First if your motor is to high, you are ventilating, not cavitating. I don’t think you are ventilating, because taking the SE200 (Anti-ventilation Plate) off would make it worst not better.

_Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving._

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Do you know what your RPMs are? With everything you are saying it sound to me that you are hitting your motor over revving limit setting. When you hit the over revving limit your motor will cut out.

Lowering your jack plate make help out or a larger prop.


----------



## firecat1981 (Nov 27, 2007)

I don't know what my RPM's are, I was thinking of getting a tiny tach so I can find out. The over reving would make sense, but it starts choking at about 2/3 to 3/4 throttle. Is there a local shop to the bay area where I can buy a tach?
I hope I don't need a SS prop cause thats a bit out of my price range right now.


----------



## firecat1981 (Nov 27, 2007)

Also how does the motor cut out when it hits the rev limiter? is it a studder, or does it just lose power, cause this is more like a studder.


----------



## Tom_C (Dec 10, 2006)

> Also how does the motor cut out when it hits the rev limiter? is it a studder, or does it just lose power, cause this is more like a studder.


 My johnson will studder.


----------



## mark_gardner (Mar 18, 2009)

most motor will stutter or act like they are kinda misfiring when hitting the rev limiter. sounds like a ss 4 blade prop might be in your future  but you might try getting the motor back down a little more too in the mean time


----------



## Tom_C (Dec 10, 2006)

> I hope I don't need a SS prop cause thats a bit out of my price range right now.



An aluminum prop with more pitch will reduce your RMPs. No need to go to a SS. I have gone from a 10.5 X 10 to a 10.5 X 12 aluminum prop.


----------



## mark_gardner (Mar 18, 2009)

> > I hope I don't need a SS prop cause thats a bit out of my price range right now.
> 
> 
> 
> An aluminum prop with more pitch will reduce your RMPs. No need to go to a SS. I have gone from a 10.5 X 10 to a 10.5 X 12 aluminum prop.


true that he doesnt actually need a ss prop but if he's hitting the rev limiter from blowing out how would more pitch keep that from happening? sounds like he needs to get it back down in the water a bit more or a 4 blade to give it more grip and a little less slip  ;D


----------



## firecat1981 (Nov 27, 2007)

ok before you guys get me all freaked out about SS props and such, where can I go buy a tiny tach today around tampa/lakeland?


----------



## firecat1981 (Nov 27, 2007)

after calling around all morning I have a line on a tiny tach locally so I may go pick it up. But it's gonna cost me 
That's just the price you pay for instant gratification I guess.


----------



## Gramps (Nov 1, 2007)

It should only take 3 days or so to arrive if you order online.


----------



## firecat1981 (Nov 27, 2007)

too late, picked it up a few minutes ago while running around, I'll wire it up after lunch and see what I get on the lake before continuing my height adjustments.


----------



## firecat1981 (Nov 27, 2007)

ok it's definately not over reving. I bought the tiny tach this morning and have spent about 3 hours figuring it out and running the lake. First I wrapped my upper cylinder and it was showing an idle of 1050-1100 RPMs, it seemed to read better the more wraps I put on it. Then I tried the lower cylinder and it read an idle of 600-650. 
Anyway the whole time I was running around the lake the motor whould never get above 3000 RPMs no matter how I set up the tiny tach. Can this be? The range of the motor is 5000-6000 RPM's how can I only be able to reach 3000 at full throttle?
I'm not exactly impressed with the tiny tach, I called to make sure I have the right one and the tech said I did. I don't get why everyone raves about these, there has to be a better product out there to read RMP's, The thing that annoyed me most was the updates every 1/2 second, which really is more like 1 second. I like to be able to see my RPM fluctuations in real time.


----------



## topnative2 (Feb 22, 2009)

DITTO!
I can not get past 3600rpm on a 20hp 4str 10p--called em and they advised maybe more raps --kinda hard to do after cutting the wire accord. to instructions

go lower or cup it


----------



## firecat1981 (Nov 27, 2007)

Well I think I may just scrap the whole jack plate and return to my original setup. I'll set it to the side for the next project maybe. I'm thinking that I may have the prop set as high as I could have with the pocket drive and higher transom already. Building the Jack plate was fun and all, but it's causing me more trouble then it is worth.


----------



## Brett (Jul 16, 2008)

The purpose of a jack plate is to get the last bit of performance from an outboard.
Trial and error to find the point where the propeller starts to ventilate,
then adjust back down a hair to where it functions properly.
Or, if you're like me, you get it close enough and go fishing.


----------



## mark_gardner (Mar 18, 2009)

well i was talking to capn' ron today about my situation and with my ss prop i'm hitting 7000 as per my "new" tiny tach, he's says thats crazy as it should be bouncing off the rev limiter  so i switch props to a steeper pitch and i'm down in the low 5000's and the motor sounds a little healthier  so we're chatting about the accuracy of the tiny tach and he says he's seen to many hits and misses with them and his going a different route, i'm with you though as i wasnt overly impressed when i got mine, gonna try a few different set up to see what happens


----------



## firecat1981 (Nov 27, 2007)

> if you're like me, you get it close enough and go fishing.


Ahh Brett if it was only that easy, but I have a tournament coming up and 2 other guys depending on my rig, so it needs to work as good as it did before I started tweaking it so I know I can make the run to the weigh-in.

I think I'll be returning the tiny tach once I test the boat back at the stock height, it really is a POS in my opinion. I'll be on the lookout for more options later on.


----------



## Tom_C (Dec 10, 2006)

> > if you're like me, you get it close enough and go fishing.
> 
> 
> *Ahh Brett if it was only that easy*, but I have a tournament coming up and 2 other guys depending on my rig, so it needs to work as good as it did before I started tweaking it so I know I can make the run to the weigh-in.


The first JP I made was a similar to this. It was a Pain In The ***, to adjust. That's why I designed my JPs to adjust on the water.


----------



## Tom_C (Dec 10, 2006)

> > > I hope I don't need a SS prop cause thats a bit out of my price range right now.
> >
> >
> >
> ...


I don't think he's blowing out, if he was blowing out his RMP would sudenly rev high. He may be close,but with more pitch and his Anti-ventilation Plate back on he may be just right.


----------



## firecat1981 (Nov 27, 2007)

Well I was out today fishing with a buddy and loaded down heavy it ran much better but still not great. I'm planning one last ditch effort where I'm going to redrill the main plates so I can lower it 1 more inch, if that doesn't do it I'm taking it off and going back to stock.
i took some pictures the last time I was out and a video, I doubt anything can really be told from them especially since my digital camera doesn't have sound, but here you go.
Also if I were to go with a new prop, any suggestions? my prop now is 9.25x10.


----------



## JC (Feb 28, 2010)

were in lakeland do you live im from lakeland too i got a 25hp yam with a 4blade ss maybe i can meet you some were and let you try it to see how it does with i think my prop will work on you motor


----------



## DuckNut (Apr 3, 2009)

> it needs to work as good as it did before I started tweaking it


Go back to the original configuration and take Brett's advice...go fishing


----------



## firecat1981 (Nov 27, 2007)

thanks curr sent you a pm.



> Go back to the original configuration and take Brett's advice...go fishing


I'm still fishing, just not as quickly as I want to. Anyway Before I go through the trouble of filling the holes and repainting the transom I'm giving it one last shot, I redrilled the JP so I can lower it 1 more inch. I'll test run it monday morning when I get off work. I think it might work out this time. Fishing yesterday with a buddy, loaded cooler and livewell the boat actually ran ok up until 3/4 throttle. After I return the tiny tach I may see about a new prop, does anyone have a suggestion which way to go? I have 9.25x10 now,should I go up to 11 or 12?

Here you can see the holes in the outer plate I drilled to lower it an inch, and you can see how I had to regrind a new radius on top so I could crank down the motor clamps.









With this last adjustment I am now sitting just about 1 inch above stock, which the cavitaion plate just about equal with the top of the pocket drive. Plus I have about 5 inches of set back.









I'll let you all know how she runs on monday.


----------



## firecat1981 (Nov 27, 2007)

Tested it again, and while it it much better it is still there. Plan now is to remove the jack plate and go back to stock setup. I'll set it aside for my next boat build in a few years and enjoy this boat until I cut it upand trash it.

Now lets say I want to replace my prop anyway, can anyone suggest one? When I was out with the tiny tach :, it would never get over 3200rpm, and really I think that may not be far from the truth, at full throttle the motor is not even close to screaming, it really is a nice smooth cruise, but I want some more speed out of it if I can. I'm looking for an affordable aluminum prop, 3 balde, 4 blade, pitch? someone give me a heads up.


----------



## flyfshrmn82 (Oct 12, 2007)

Due to the low RPM's and weird sounds, here's a dumb question....What is the condtion of the gas you are running in it? K.I.S.S. first.  Is one cylinder getting noticably hotter than the other? Temp gauge or touch test (but be careful as the head above each cyl. will be pretty hot). 

My 15 yami will turn a *heavy* solas 9.25 X 10p SS prop at 4800 RPM's... It labored the engine a little too much, so I will have it tweaked soon. For now I run a 9p aluminum with a pretty good cup in it. Man does that make a huge diff. I haven't had any issues with the Tinytach. When I am running WOT it reads ~5500 and when I blow out it will read ~6400.


----------



## mark_gardner (Mar 18, 2009)

> thanks curr sent you a pm.
> 
> 
> 
> ...


 in the pic where your holding the straight edge.... stick it further up the pocket to get a more accurate assesment of the direction in which the waters gonna exit said pocket drive...know what i'm saying  :-? :-/ 

p.s. i just got a stainless power tech today from the people at get-a-prop.com so you might try them if your going to re-prop, they carry aluminum as well


----------



## firecat1981 (Nov 27, 2007)

I wish there was a plug in tach I could use as opposed to some POS that requires me to adjust it based on wraps :

The gas is about 50% 6 weeks old and 50% 1 week old, plus it was treated with startron. Inside of the tank is clean plus I'm using the same gas in my lawnmower and have no issues running it. The engine wasn't running hot at all on either cylinder, I know cause I had my hands all over the motor rerouting and adjusting the tiny tach. It runs super under 2/3 throttle, and at full throttle doing a holeshot. The studder only happens when on plane. 
The cavitation plate is equal with the tunnel top, I've tried every trim adjustment the motor has and while some are better then others it still blows out with all of them. 
Thanks for the needaprop.com suggestions, there stuff it dirt cheap and I may pick one up just to see what happens.


----------



## Gramps (Nov 1, 2007)

Odd ball question but have you hit anything? My boat will seem to blow out/cavitate at odd times without reason, this started down in the 'glades. And now I'm pretty sure I bent a tip on my prop after bumping pretty hard off a sandbar, it runs fine on flat water without turning, but in chop or turning quickly it blows...


----------



## firecat1981 (Nov 27, 2007)

Thats a good question, I have a good amount of sand burn on my prop but I can't tell if it is bent or anything. I think my best bet is to pick up a cheap prop once someone suggests one and see what heppens.


----------



## B.Lee (Apr 29, 2008)

You can check the runout of your prop blades by holding a fixed pointer, like a ruler, against the skeg, and the tip just off the closest point of one blade. turn the prop and each one should be the same distance.

My initial thoughts were that once you got the engine up and out of the water, that it was running much more efficiently and freely, and over-revving. If you lower it back down and it runs fine, I would stick to that thought.


----------



## firecat1981 (Nov 27, 2007)

Well I checked the blade, and it looks slightly off to me on one of the fins. I'm gonna order a 4-blade 11 pitch and see how it runs before I rip the JP off.


----------



## firecat1981 (Nov 27, 2007)

Well I got the 4-blade, and I'm still having problems, so it's good bye jack plate, she will sit on the shelf until I need her. I'll redo everything so it is back to stock, and then try the 4-blade again as it does seem to lift the stern pretty quickly. If it doesn't work out I'll order a replacement 3-blade.


----------

