bad blowout right before plane

[procraftguy]

Ok i put a 1976 1500 elpt on my procraft with tilt and trim with a 4 inch jackplate rasied it up alittle to far as it works great at troll pops up too plane very quick but dosnt get there at 3500 rpms it blows out its almost there just wanna know how far down as to not get to far i was level with were the transom insets im thinking 3 more inches down maybe

 

[Sea Rider]

The best tengine/transom height for best performance on overall water conditions is at a position where water flow at speed passes right under upper water deflector plate and the sharp angle that extends to AV plate. Will achieve good hole shot, immaculate close turns at speed, nice top speed if counting with at least 70% of the max HP rated for that boat. The only way to check if engine sits at sweet transom height spot is having a rear engine visual inspection when boat rides on plane.

If this is too much hassle, lower one hole at a time and trial each one, stay with the hole that gives the best water performance for that combo.

Happy Boating

 

[jestor68]

You're not experiencing "blow out"; you are experiencing excessive ventilation due to the motor being set too high.

Try setting the prop shaft center line 3 inches below the pad and go from there.

A quick way to do it is to place the motor level, then place a straight edge on the pad extended out to the front of the "bullet" on the gear case. Set the center of the gear case "bullet" 3 inches below the top of the straight edge.

The actual height will depend on what kind of prop you're running, but 3 inches is a good place to start.

 

[Mi duckdown]

Got PICS?

 

[procraftguy]

Ty for all the help im taking it out today too test i will post pics if its still too high i found lots of info on transom motor hieght but none with inset at bottom at pad . I have alot of time in this boat and motor just wanna get it perfict

 

[Sea Rider]

Ty for all the help im taking it out today too test i will post pics if its still too high i found lots of info on transom motor hieght but none with inset at bottom at pad . I have alot of time in this boat and motor just wanna get it perfict

Post a good pic of your current transom/lower unit installation to have a look, as to check bottom pad.

Happy Boating

 

[Texasmark]

If you are running a jackplate you have to deal with "setback". Look up the subject in the archives. For every so many inches of setback, there is an inch of height required above the normal line of the hull. Reason is the boat rides in the water in a bow up attitude. This puts the engine lower in the water and it gets worse as the engine moves out farther from the transom with things like offset/stepped hulls like on a lot of bass boats, and jack plates. Having the engine too low in the water creates unnecessary drag and is self defeating. So on a bass boat with 4" of step in the hull with a 4" jack plate could have it's AV plate several inches above the line of the hull.

Go somewhere where these types of mountings are on boats locally and check out for yourself where the AV plate is located vs the line of the hull.

I run a conventional alum hull, transom mounted engine, one hole up on engine mounting bracket. I run a fairly high performance prop; SS, high rake, cupped, ported. Depending upon my trim setting, I can make my boat blow out numerous times in the process of getting on plane. I find that between the ports in the prop and trimming out beyond vertical, even with the blowouts, which allow the engine to rev before the prop catches again, I get on plane much faster than the conventional method of trimming the engine all the way down......reason is the blow outs develop rpms and rpms are part of the hp equation so the faster I can get them up the faster I have the ponies to twist the prop and get going.

Mark

 

[procraftguy]

Ok i got perfict ride hieght but i am not getting passed 4900/5100 rpms and about 40/42 mph gps iwas thinking i should be in the 5500 rpm range and 50u plus any ideas on pith and prop im running a aluminum now

 

[Sea Rider]

Could try same diam prop with at least 2 less pitch sizes to rev your engine near 5500 rpm if that's the max wot rpm factory stated for that engine. Was the trial done with you alone, lightly loaded boat on flat calm no wind water cond ?

Happy Boating

 

[Texasmark]

Is that the 18' ProCraft combo bowrider fish and ski? My son had one with a late '80's Merc 150 and it ran right at 55 with 2 of us in the boat. Didn't have a jack plate. I was only in the boat for a test ride when he bought it used. Don't remember the prop nor rpms.

Cutting pitch will increase rpms, but seldom have I found it to increase speed also. You can run the numbers yourself and see. If your slip is constant between your current prop and a potential new prop of the same geometry and all but less pitch, go to the BAM prop slip calculator www and fill in the blanks. Gear ratio is just the number like 2.0 or 1.78 or whatever your's is. Plug in your current WOT rpms, current prop pitch. It will solve for theoretical speed which is with no slip but there is always slip. Skip the slip block and plug your WOT speed in the last block. Solve for slip....number like 15% more or less. Slip is the difference between running a screw through a solid and running one through the water....and your prop is a screw.

Then go back to pitch and tweak it moving back out to the right but this time don't solve for slip.....save current number, solve for WOT actual speed. A SS prop with high rake for bow lift to help in minimizing wetted area of the hull and thus drag, thin blades which SS are known for, and cupping of the trailing edge can work wonders for your rig. With the right prop, you can take the current pitch and increase rpms and mph because of the things mentioned.

That hull will respond nicely to a high rake prop and usually you don't get that in aluminum. If you ever ran a high rake on a boat you would know what I'm talking about in getting the hull up and out of the water. Has a profound impact on efficiency, aka top speed with a given rig. Also if you go back to the calculator when finished running said prop, your slip will drop significantly meaning you have reduced your drag, the prop can bite the water better and act more like it was turning in a solid.

HTH,
Mark

 

[procraftguy]

Yes its a 1988 fish and ski 1780v it seems my issue was my fault when going thru this motor after sitting for 9 years and rewireing mounting i forgot my new plugs 2 were barely screwed in so i was running about 4 1/2 cylnders after new plugs my idle rpms moved up 2-300 so i will be retesting but with ur help i think this 19 pitch will be just fine for speed and ill get a 21 for tubing if im not confused

 

[Texasmark]

Running 2 concurrent posts can be confusing. I just answered your other one.

That hull responds to a moderately high performance prop (most alum. props are not) that first of all has a good rake. Rake is essentially grabbing the blades and pulling straight back on them. The tip of the prop blade is well behind where the blades emanates from the barrel at the root and may be as far back as the end of the barrel . This provides bow lift I mentioned and it is necessary to reduce the wetted area of the hull, reducing drag and increasing rpms and mph.

A guy like Jester68 can put you into the name of a good SS prop in current production with porting (improves hole shot with a prop pitched for WOT performance)....he's up on current production props; plus high rake, plus cupping to help in holding at high trim angles and tight turns, and and naturally thin blades are part of it for reduced prop drag.

The problem you have is that you are looking at a completely different prop so expect completely different results. For a ball park first guesstimate I'd go back to the slip calculator and plug in your engine's WOT max rated rpms and use 12-15% for slip. Tweak pitch till you get your number over 50mph. That ought to put you pretty close.

The bad part is this setup is for your 50+ mph. You will have rpm problems with this prop towing water toys as there will be too much drag. So your answer there is a prop for toys and a prop for the thrill.

I am going to post this and go and look for the picture of my prop which has the things I am talking about. Will edit and add it when I find it.

Good Luck,
Mark

 

[Texasmark]

Running 2 concurrent posts can be confusing. I just answered your other one.

That hull responds to a moderately high performance prop (most alum. props are not) that first of all has a good rake. Rake is essentially grabbing the blades and pulling straight back on them. The tip of the prop blade is well behind where the blades emanates from the barrel at the root and may be as far back as the end of the barrel . This provides bow lift I mentioned and it is necessary to reduce the wetted area of the hull, reducing drag and increasing rpms and mph.

A guy like Jester68 can put you into the name of a good SS prop in current production with porting (improves hole shot with a prop pitched for WOT performance)....he's up on current production props; plus high rake, plus cupping to help in holding at high trim angles and tight turns, and and naturally thin blades are part of it for reduced prop drag.

The problem you have is that you are looking at a completely different prop so expect completely different results. For a ball park first guesstimate I'd go back to the slip calculator and plug in your engine's WOT max rated rpms and use 12-15% for slip. Tweak pitch till you get your number over 50mph. That ought to put you pretty close.

The bad part is this setup is for your 50+ mph. You will have rpm problems with this prop towing water toys as there will be too much drag. So your answer there is a prop for toys and a prop for the thrill.

I am going to post this and go and look for the picture of my prop which has the things I am talking about. Will edit and add it when I find it.

Good Luck,
Mark

 

[procraftguy]

Sry about that i was trying to update my? As the hieght problame is fixed and i ty for all ur help this prop stuff is all new my last motor was a 50e hp force and i was greatfull to get the boat to 20 on same boat most days it was 15 but with new transom and motor i was maybe excpecting too much

 

[Texasmark]

my last motor was a 50e hp force and i was greatfull to get the boat to 20

Gotta laugh at this. Apples and oranges and rotten apples. (Opinion)

Mark