Hole Shot Blowout

[Bassmagician]

I own a Sprint 290 DC PRO, it's 18.5' long, weighs 1200 lbs. with a merc. 150 XR6 and a 10" jack plate, 1.75" to 2" below the pad. I recently broke a blade off of my 23 pitch Power Tech 3 blade 13.75 dia. I have replaced it with a Turbo TXP04 23 pitch 4 blade. It is almost an 1" larger in dia. I picked up a lot of speed on top end 5800 RPMs WOT, but I blow out on the hole shot & turn 5000 RPMs or more if I hold it to the floor. I have been having to feather the throttle arround 4000 to 4500 RPMs to get on plane. I have read that the motor setting may be too high and the prop is catching air. Would it be a good idea to lower the motor? If so how far?

[1FASTLASER]

Ok what u have been told is true. Each and every prop has a sweet spot.....or I should say each and every prop,motor and hull combination has a sweet spot. You can line up 10 boats......same make,motr and prop..and you stand a VERY good chance of each one being a little different.....Its the nature of the beast. A prop from the factory is no where true and balanced out. Yes I know it seems futile to spend 4 or 500 on a new blade and then spend another 125-200 on it to get it Balanced and blueprinted. I have seen as much as 3 different pitches on one prop. Sorry for being long winded but I beleive everyone needs a little back ground info. So to answer your question and explain why.....yes the prop is bigger and has nmore blade area.......being bigger at your present motor height is alowing you to cavitate when nailing the throttle because more of the wheel is outta the water AND with more blade area will acentuate this problem. You aint far from being where you want to be. Start to drop your plate in small increments. I wouldnt go more than 1/4" at a time till you regain your holeshot. With the numbers you gave me you should blast outta the hole and run to the topend like a jack rabbit. AGAIN you arnt far from where u need to be so small moves are the key right now.

[Bassmagician]

Thanks a bunch 1FASTLASER, I am off today and I will try moving the motor down untill I reach Bassboatopia. I'll let you know how it goes.

[Bassmagician]

I got out today had to lower the motor 1/2" and that done the trick. I have to ease into the gas but once it reaches 3000 RPMs I can punch it and it jumps out of the hole. It also does better at midrange, will stay on the pad at a little less than 4000 RPMs.Thank you very much 1fast, I'm back to grippin' and rippin' again. YEE HAA!!!!!

[bassintom]

Originally Posted by D.Buirm

I am going to buy a new 09 MIM P-bass and I really like the looks of the gold pickgaurds that are on the 57 RI, but they are 10-holes. I would like to put a gold pickgaurd on the MIM p-bass but that is a 13 hole pickgaurd. Will it still work or not?

I think you have the wrong forum.The 10 hole will work but you'll probably have to put a couple new holes in the body as the 13 hole screw holes won't line up. The old holes will be covered so go for it.

[Bob Smith]

1FASTLASER did an excellent job with his explanation. I've seen a myriad number of answers to this very question on a number of different forums, including BBC (Bass Boat Central, Setups by Todd). What you got should be the "Benchmark" answer to start with because so many times when folks get a boat with a jackplate the first thing that happens is to raise it, really high. The trouble is that most folks don't understand the logic behind the jackplate in the first place. The example that 1FASTLASER gave about the 10 boats is what makes a jackplate the tool to have, you can change the set up relatively easy, "to match your rig".
Way to go 1FAST LASER, great information.

[nofearengineer]

Yes, I love having Fastlaser here. I get quite an education from him.

I have an additional question for the original poster though. Is the new prop vented? My first thought was, since he said top speed was fine, but he was blowing out was maybe the prop vents were too big; i.e, wouldn't a prop blow out at high trim more than at low trim if it were set too high?.

I would think that lowering the outboard would help a vent problem a little bit too, and maybe that could have been the real problem?

Just playing devil's advocate and hoping to learn something.

[zooker]

No fear

Venting will not cause blowout. Blowout is caused by the prop coming to the surface and losing bite. This happens during the hole shot. When the boat actually pushes the water away from the prop.

Venting is holes drilled in the prop most are 3/8".. That use exhaust gas to losen the bite of the prop in a controled amount during the hole shot.Which lets the prop turn faster with less torque..The result is a faster hole shot with less engine strain..

If your using a ss prop I recomend spending the $100 to have it done..

zooker

[nofearengineer]

I understand, Zooker. But my point is...if the prop is blowing out at takeoff, where the motor is trimmed down, why doesn't it blow out on the top end, when it's trimmed up and sitting higher in the water? It just seems weird to me.

[1FASTLASER]

Thanks for the kind words Mr Smith.


No fear.......what you are dealing with is 2 different examples of blowout.or lets use the correct terms here.....on holeshot you have cavitation which is the introduction of air into the prop path..............on the topend is where you actually experience blowout...they are similar but result from different forces.
Blowout is where the prop looses bite for a number of reasons which may include not enough cup in the blade with to high of plate setting........it could also result from a prop that simply looses bite because of lack of cup and the rpms that the engine is actually spinning the prop...hence the problem with trying to get a prop setup for YOUR rig..........there are actually very many reasons for blowout and to set up a rig you have to eliminate them one at a time to get dead on.
Cavitation on holeshot is simply air in the prop path be it exhaust gases,turbulence from the hull or a prop that is to small allowing the engines hp to override the forward momentum that the boat is trying to acheive.

[Bob Smith]

No fear, if the exhaust ports are too large on the prop you would get the same effect and it would be obvious. If you lower the motor several times and still get the air around the prop, then plugging some holes or getting a different prop would be an option. As indicated in the follow-on post, the fellows performance improved immediately, including mid-range performance. That pretty much validates 1FASTLASER's theory, and it most often the right answer. When the boat is finally dialed in correctly and propped right the general cone to pad setting is in the vicinity of 3" (+/- 1/4"). This is not a predetermined height, but just years of experience in my own rigs and helping many other guys set up thier rigs.

[nofearengineer]

Originally Posted by Bob Smith

if the exhaust ports are too large on the prop you would get the same effect

That's exactly my point, Bob.

I'm not saying FastLaser is wrong. He probably knows about a hundred times more about boats than me. Obviously, his diagnosis fixed the problem very well. Is the discussion thus over?

I understand it was not the exhaust ports. But what would lead a guy to know it wasn't them? Obviously, he knew they weren't the problem.

I simply asked "if the prop is blowing out at takeoff, where the motor is trimmed down, why doesn't it blow out on the top end, when it's trimmed up and sitting higher in the water?"

Trust me, "I don't know why, but years of experience have taught me it is so" is a very legitimate answer. I tell people "I don't know" all the time.

[1FASTLASER]

Originally Posted by nofearengineer

I simply asked "if the prop is blowing out at takeoff, where the motor is trimmed down, why doesn't it blow out on the top end, when it's trimmed up and sitting higher in the water?"

Trust me, "I don't know why, but years of experience have taught me it is so" is a very legitimate answer. I tell people "I don't know" all the time.

Ok let me try and explain or clarify. Actually upon holeshot what you is happening is called cavitation.........cavitation is simply air that has entered the props path be it naturally or forced, thus creating high rpms with little or no forward movement. Now for a correct holeshot there HAS to be a certain amount of cavitation to allow the engine to spool up (rpms raise) so that the engine can get the hull moving more effeciently. BUT the amount of cavitation needs to be controlled.........for example mercury has the removable and different size PVS plugs for sebveral of there props.
Ok now when you are up and running the whole picture has changed. Now you are no longer dealing with the factor that exhaust has introduced air into the props path because you are "out running " the exhaust.......what happens now especially more prevalent at elevated engine heights is "blowout" .....same high rpms with no more forward boat movement but has happened for a different reason. Know before I go any further let me say this ......cavitation can cause the higher rpms at speed but it is not exhaust related now...now its air being introduced because of a hull imperfection,a skimmer transducer improperly adjusted,a flatbottom boat that has rivets,,,,so forth etc etc......This is where figuring out WHAT is the cause and fixing it comes into play. After reading magicians post and getting all the pertinent info I was able to deduce that he had changed props and gone to a larger diameter prop. His engine height was previously adjusted for the smaller prop. Now when you are up and running and have optimized your setup ...in his case with a 3 blade prop.......what is going on at that time is 2 ears are in the water while one is out of the water thus allowing the engine to comeup on the rpms more efficiently but not to much. If it is too much...."Blowout occurs"....Now maybe you can see what I am referring to. Same scenario cause by a different circumstance. After reading what he posted I was able to figure out that the larger diameter prop was actually allowing to much prop out of the water at running speed....hence the raise in pms and no more mph.