Bad cavitation on hole shot!!!

[brandon6.0]

My new boat is a 92 Astro 17.5ft with a 92 Mercury 150 XR6 carb. Here is the problem:

Took boat out last 3 weekends since purchase and noticed at WOT hole shot at 3k rpm it would just hit rev limit or cavitate very badly. Had to back off throttle to easily get it tp plane out. Once riding i was only seeing 5000 rpm trimming up a pretty good bit. Now i know i should be seeing 5-5500 on this outboard.

Could prop be an issue? Prop has a few signs of wear but no dints or dings. Currently has LaserII 24p. Thought about running a 23p Trophy plus. For a lil more bow lift.

Checked motor height on transom:
-Bottom most part of hull or keel (15")
-anticavitation plate (18.5")
-Prop center (10 1/4")
-transom to prop center hieght is (4 3/4")

Motor is currently in the highest setting on transom. Looks to be from factory hince no marks from bolts on other holes. My transom has a cut in at the back from the transom of about 12".

Lookin forward to yalls thoughts. Thanks guys!

 

[emoney]

Re: Bad cavitation on hole shot!!!

You should probably make sure you don't have prop slip. Take a grease pencil or something similar and make a straight light all the way from the center of the prop, across the hub, to the outer part of the prop's center, and then take the boat back out and try it again. If it's slipping, then that line will be "broken". If it's not, obviously it's not a prop issue, at least as the hub applies.

I'm pretty sure your motor has the Flo-Torq hub which consists of a plastic, sacrificial sleeve that can wear, as it's designed to take the brunt of the prop getting tangled, etc. Those complete hub kits aren't much and the sleeve itself is around $20 I think.

 

[Mi duckdown]

Re: Bad cavitation on hole shot!!!

Pics of your motor setup in relation to transom would BE Very Helpfull. Because some of your measurement don't add up

 

[steelespike]

Re: Bad cavitation on hole shot!!!

The anti vent (cavitation)plate should be about even with the bottom to slightly above.
It does appear the motor could be about 3 or more inches too low.

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

i see where u could be confused. My measurements are from a flat level floor to the bottom of boat and to the engine. Thats the measurements. So i basicly have 4 3/4" to the center of the prop to the bottom of boat. Now, i read some people run around 31/4" low. So that puts me at 1.5" lower than there setups. I must have prop slip like emoney said ^^.

 

[steelespike]

Re: Bad cavitation on hole shot!!!

It appears to me that your motor is very high. Depending on the props diameter you could have more than 2 1/4" of prop above the water. Some props can handle that but it surely could be the reason you have venting issues.
Easy enough to check for a spun hub.

 

[Texasmark]

Re: Bad cavitation on hole shot!!!

As I recall the Astro has a setback from the end of the pad to the actual transom where the engine mounts. Assuming you have no additional jack plate which would put the engine that much farther back (12" I think you specified).

Go somewhere and watch a video of a bass boat on the pad....the Ranger site is a good place to look. Notice the bow high attitude. Think about the fact that with the engine behind the end of the pad the water is coming out 12" in front of the engine. The actual mounting position of the engine vs the water coming out from under the boat would be well into the water which would create a lot of drag and not what you want if you had the engine setup with the conventional height....like where the top of the clamp bracket is sitting on the transom. So you have to jack it up.

With a Laser II you can run the prop blade tips right at the surface of the water. I had a Laser I and could run the blade tips out of the water at high trim settings course speed would drop off so I'd just tuck it in till the rpms dropped somewhat and the speed peaked.

You have to have the engine sitting higher than normal under this setback condition to keeps the prop tip right at the surface thus minimizing lower unit drag. I think the rule of thumb around here is 1" per 3". For every 3" of setback you raise the engine 1" higher than you would have it if the pad went all the way back to the transom like the hull on conventional boats.

So, if you set the boat level on the trailer and set the engine vertical, take a straight edge and align it with the pad and have it extend back under the AV plate on the engine. The plate should be roughly 4" above your ruler. I think you are there or are at least close as you said that the engine was mounted as high on the transom as the mounting holes in the transom bracket will allow.

The Laser II has square vent holes in it. Purpose of those holes is to force the engine to overrev in the hole. HP is what turns the prop. RPMs are one element of HP. If you can get your rpms up fast, you can get your HP up fast and get "outta da hole" fast. The hole (port) size determines just how much overreving you will be doing. There are rubber plugs made for that prop with different sized holes. You select the one that is a compromise between too much overrevving and too slow a hole shot. My laser allowed my Merc to spin up such that I could tell it was breaking loose, but gave me a fast hole shot and after I got on plane it didn't take but a few seconds for the holes to close, the rpms to downshift (like a car going into overdrive) and the boat take off.

On the 5000 rpm it tends to say that your 24P is too much prop. Even though you have a short boat and a lot of engine, you may need to drop a couple of inches to get the rpms where you want them. I ran my tower at 6000 when I was hooked up and alone in the boat. I had a 115 with a 19P on a 17' Ranger. I think I ran upper 40's. I just ran some numbers on your rig and mine. With your engine you have 30% more hp so you should be able to run 30% more prop give or take. That ratio puts your 24 right where you are.

But the XR6 may be running the small diameter gearbox (like was on my engine) and the gear ratio could be 1.87. If that is the case you are running too much prop and need to cut back on the pitch. I'd say a 21 or 22 if available would be your better choice in the Laser II.

My 2c,

Mark

 

[1FASTLASER]

Re: Bad cavitation on hole shot!!!

i had the sister boat to yours.the prop your running was the fastest available at the time. it sounds as though previous owner was looking for topend numbers like i was and did. in order to run your setup to max performance you will need to learn to fether the throttle just as she brakes over. noe if the prop is out of wack she will have to be worked at a prop shop. like i said i had the same boat just with procraft letters and with the motor in that bolt hole and a 24 laser ll she would run with most 200's.

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

yeah thats sumes up alot. My merc is the small gear case unit. So maybe a 21-22p would be better. I kinda thought these pitches would be better but didnt know for sure. thanks for the info.

Well, the prop looks to be factory. I mean 20 years would put a hurtin on a stock prop without being ever touched. But with my rpms where they are i would like to bring them up some. My holeshot could have been hurt alot with my timing maybe being out of whack. Just doen link and sync and set timing up without module. And it seems to idle, rev way nicer and stronger than before. I would think this would hurt power tramendously!

Who is one of the best at reworking a prop? Would the trophy with four blades kill power and speed compaired to this laserII. I love the topend just need more rpms and Want a nice hole shot. More time spent riding and cruising rather than trying to drag race from hole shot lol.

 

[1FASTLASER]

Re: Bad cavitation on hole shot!!!

you stated that yiu had timing issues....have you run the boat since you have resolved them. like i stated i had an identical setup and the 24 laser ll was the prop and would turn 5600....same motir and lower.....the trophy will scrub off 3-5 on the topend but guve yiu a good holeshot. if it where me i would make sure timing was right and you are getting full throttle travel and then have your prop cleaned up by a good prop man before you go to changing things. just my. 02

 

[Texasmark]

Re: Bad cavitation on hole shot!!!

yeah thats sumes up alot. My merc is the small gear case unit. So maybe a 21-22p would be better. I kinda thought these pitches would be better but didnt know for sure. thanks for the info.

Well, the prop looks to be factory. I mean 20 years would put a hurtin on a stock prop without being ever touched. But with my rpms where they are i would like to bring them up some. My holeshot could have been hurt alot with my timing maybe being out of whack. Just doen link and sync and set timing up without module. And it seems to idle, rev way nicer and stronger than before. I would think this would hurt power tramendously!

Who is one of the best at reworking a prop? Would the trophy with four blades kill power and speed compaired to this laserII. I love the topend just need more rpms and Want a nice hole shot. More time spent riding and cruising rather than trying to drag race from hole shot lol.

Man you have an ace in the whole on this problem. The fact that Mr. Laser is running the same rig and got the rpms where they belong with the same setup is a gift. Might listen to him.

Yes the 4 blade will not be in your best interest. As I said, you have the ports for the hole and only 3 blades for WOT; you HAVE the best of both worlds.

Timing on my Merc (different engine) is like 5 BTDC at idle and 22 at WOT. Makes a difference!

Mark

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

Yeah i have not taken it out since i have changed timing around. Was planin on it this weekend if weather lets me. When i done the link and sync i still had 1/4" throttle left before it bottomed out on carb stop. Got that fixed and butterflies are opening great. So that should make a difference. My timing is at or near 8* atdc until i adjust it in water in gear. My WOT is 24* BTDC. Should i be ok with that WOT timing setting? Thanks for all the input guys. I will try my hardest to get this thing dialed in.

 

[Texasmark]

Re: Bad cavitation on hole shot!!!

Yeah i have not taken it out since i have changed timing around. Was planin on it this weekend if weather lets me. When i done the link and sync i still had 1/4" throttle left before it bottomed out on carb stop. Got that fixed and butterflies are opening great. So that should make a difference. My timing is at or near 8* atdc until i adjust it in water in gear. My WOT is 24* BTDC. Should i be ok with that WOT timing setting? Thanks for all the input guys. I will try my hardest to get this thing dialed in.

You are close enough on timing. I wouldn't mess with it. I think you nailed in on the carb linkage. If the BFs aren't flat in the venturi, you can't expect max rpms.

Mark

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

just as i thought thanks.

 

[1FASTLASER]

Re: Bad cavitation on hole shot!!!

b sure and report back after running this weekend.....i beleive you will b surprised after doing the link and sync with the tming being resett.

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

sure thing!

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

Sorry for the hold up guys. Finally took her back out. Seems to run a lot smoother. Hole shot still sucks but I can live with it. Got around 5300 rpm at most. Maybe 55mph top speed. Still tweaking and figuring things out as I go along lol. Now with the laser 2 to help hole shot don't they make some kind of PBS plugs or something? How does his work? Thanks.

 

[1FASTLASER]

Re: Bad cavitation on hole shot!!!

If you want to tailor your hole shot you will need to purchase a set of the PVS plugs. Best bet is get all 3 sizes and try different combinations. My recommend is all 3 holes with the medium plugs and go from there. That will drop rpms down.... shouldn't drop them to much but you will have to experiment to tailor to your load. It will NOT affect topend

 

[Texasmark]

Re: Bad cavitation on hole shot!!!

Sorry for the hold up guys. Finally took her back out. Seems to run a lot smoother. Hole shot still sucks but I can live with it. Got around 5300 rpm at most. Maybe 55mph top speed. Still tweaking and figuring things out as I go along lol. Now with the laser 2 to help hole shot don't they make some kind of PBS plugs or something? How does his work? Thanks.

------------------------------------
I replied to this, left it up, went and had breakfast and returned. Upon returning your title was right in my face and I put 2 and 2 together. You had said you have a Laser II and had asked about what were the plugs and function.

Well in my response is the direct answer to your question which is simply solved as I instructed you.

Happy boating.

-------------------------------------

The hole deliberately puts exhaust gasses over the prop blades in the hole shot to reduce the density medium the prop is having to turn in. Reduced density means reduced load on the prop which means less hp to turn it. Less hp requirement on that end means that for the fuel air mix coming into the combustion chamber it can spend more of the fuel mix in developing rpms.

Well, rpms, torque, and a constant of proportionality define HP. So the faster it spins up, the faster you develop your hp and the faster the prop spins in the medium.

Once up and away, when the flow-by water pressure exceeds the output of the engine in gas pressure, all the gas goes out the rear of the hub like a prop with no ports and they seal off.

On the Laser II the plugs are available to control the engine operation in the hole. If too large a hole, the engine revs excessively and the hole shot is prolonged. Too small and the holes are essentially useless, engine lugs and hole shot is also delayed.

The right plug and the engine will come up in rpms a few hundred (over what a non ported prop of the same specs would do) in a jump and then rapidly move on up as the boat speed increases resulting in a fast hole shot and soon after getting her up and blasting away, they will seal off.

In doing so, you will hear/feel the prop bite the water hard, rpms will drop a few hundred temporarily, and the speed will take off back to where you would have been with out holes. I refer to the lock up as an overdrive transmission, like cars in the old days when you were in 3rd gear and had given engine rpms and speed. You take your foot off the accelerator, feel the down shift, engine rpms lower and putting the foot back where it was the car moves on up in mph.

Great sport and it really helped me in skiing behind my bass boat. Getting my 250# up on a slalom behind a BB was a task otherwise with a 115 tower.

Mark

 

[brandon6.0]

Re: Bad cavitation on hole shot!!!

Thanks for the replays guys. Will definitely be trying some of those Pvs plugs. Currently as none which might be he problem. I am also going to mark the hub with a grease pencil to check for hub or prop slip. Wish me luck! Thanks guys.