Blowout Help!!!(?)

[Miniton]

I am having some issues with a new "used" boat that I purchased. When under heavy acceleration and tight turns I am getting prop blowout. (or at least that's what I think it is.)

I have a 1994 Astro 20fsx fish and ski with a 1994 200hp Mercury Black Max. I get 64mph by Speedo,(no GPS) at 5300 rpm WOT with 2 people and 3/4 tank gas. Mercury Tempest SS prop # 48 825864 A45 23P

When I heavy accelerate from sitting still it starts to go a little then just high revs and spins without the boat going. I have to slowly throttle to get her up. Once running she runs great except in really hard turns when I try and accelerate. The motor is currently mounted on the second hole from the top so I have 1 more to lower it or 3 to raise it.

Is there anyone out there that can help me with this issue? I just have no holeshot what so ever.

Pics here:

Thanks in advance to all you experts,
Mini

 

[Bifflefan]

Re: Blowout Help!!!(?)

Motor is to high,, drop it as far as you have room....

 

[wca_tim]

Re: Blowout Help!!!(?)

sorry to disagree, but that's not high at all - in fact deep by many standards for a fast shallow bass boat.

Are the vent inserts in the prop the plugs or are they open?

 

[wca_tim]

Re: Blowout Help!!!(?)

an added thought. anything that disturbs the flow of water as it goes over the prop blades contributes to what you're seeing. That includes nicks on the leading edges of prop blades, impefections in the gearcase housing and skeg, etc... I notice from the pics that your skeg is chewed up a bit. how bad is it? Is it straight? How about the prop? good shape? The prop would make a difference even at the low speeds you're running - even a slightly bent prop...

the other thing that might work longer term is to go to a 4 blade, maybe something like a trohy. You might find yourself raising the motor more if you do that (and going faster), but would have to watch your water pressure....

If you had a gps (and a tach), it would be interesting to know what kind of slip numbers you're seeing...

ps. Go Here, there are some really nice people... some of the guys I see down on the river with boats that run between 90 and 130+ and a lot more like them. They live fast outbaord rigs and likely someone (or ones) with tons more experience will take the time to give you a hand. Cheers! Tim

www.screamandfly.com

 

[a70eliminator]

Re: Blowout Help!!!(?)

I just went through this my boat, lowering it one hole made the difference, even though the cav plate was level with the keel. I would lower it and see what happens you have nothing to loose, it looks to high to me.

 

[Miniton]

Re: Blowout Help!!!(?)

sorry to disagree, but that's not high at all - in fact deep by many standards for a fast shallow bass boat.

Are the vent inserts in the prop the plugs or are they open?

wca_tim

OK, maybe a dumb question but I am fairly new to boat set ups. What are vent inserts and what do you mean by them being open?


Thx again for your help,
Mini

 

[Miniton]

Re: Blowout Help!!!(?)

I think I see what you are referring to. There are 3 small holes right underneath the props. All are open.

Thank you,

 

[wca_tim]

Re: Blowout Help!!!(?)

I think I see what you are referring to. There are 3 small holes right underneath the props. All are open.

Thank you,

That's what it looked like. The holes (vents) are there so that when you hit the throttle at lower rpms (as in taking off), they dump exhaust gas out on the leading edge of the blade so the prop can slip to let engine rpms get up... vented prop can help a lot of you're having a hard time taking off because the engine doesn't have enough low rpm ooomf to get it going. by letting the prop slip it allows the engine to get up to higher rpms where it makes more power to help it jump out of the hole - sort of like having a stall converter in front of the auto transmission in your street rod, or slipping the clutch to get the rpms up out of the hole...

I wouldn't do anything with your engine height until you can cover the holes.
Can you tell it that prop was vented from the factory? Lots of merc props are. if so, then you can buy inexpensive inserts that are plugs or three different sizes of hole. You change the inserts to control the amount of exhaust gas that escapes at the leading edge of the blades to tune your holeshot. At higher speeds, the water running over the prop has to much pressure to let much if any exhaust out and so they don't (supposedly) impact higher running speeds.

I've seen a number of do it yourself vents drilled in props and they're easy to screw up and hard to fill in. some prop shops do a great job with this...

here's an example. the top two props were drilled by a prop shop. The bottom two are the merc type that use the inserts. the laser 2 on the left has medium sized vented inserts, and the mirage plus on the right has the plugs in it.

Hope this is helpful...

 

[sschefer]

Re: Blowout Help!!!(?)

I agree with the vents, you shouldn't need a vented prop with that much HP and that light of boat. Think there's a chance that the P/O pulled the good prop and put that one on. It happens a lot.

 

[rs1lollie]

Re: Blowout Help!!!(?)

check out this thread they talk about outboard motor height.
http://forums.iboats.com/showthread.php?t=347549

 

[wca_tim]

Re: Blowout Help!!!(?)

I agree with the vents, you shouldn't need a vented prop with that much HP and that light of boat. Think there's a chance that the P/O pulled the good prop and put that one on. It happens a lot.

that's kind of what I was thinking.

as a follow-up add on. if they're drilled holes as opposed to those that will take inserts, you can fill them with epoxy. something like marinetex or epoxt thickened with milled glass, etc... will work. I would cut pieces of polyethylene or polypropylene (ie visqueen, vapor barrier, the sheeting sold for painting, etc... anything you can put gasoline or acetone, laquer thinner etc on without it dissolving) to tape firm;y over the holes on the outside, then fill them from the inside. after it hardens up remove the tape, hopefully you can just touch up sand or fill any any voides missed and then run the prop and see how it goes...

 

[Randybeall]

Re: Blowout Help!!!(?)

Reading the entire post I am in agreement with both schools of thought. The vented prop is most likely not necessary with this kind of horsepower, at the most it could be very small vents. That said, it would look from the pictures that your prop and vent plate height are very close to correct. I would suggest lowering the engine one bolt hole and try it on the turn situation. Set up can be a fine line. If the former owner of this boat normally carried an extra battery or such in the rear of the boat the set up might be higher than you need with your normal loading. The turn blow out will be only very slightly affected by the vents in the prop so you need to look elsewhere for the two problems to both be fixed.

 

[Dhadley]

Re: Blowout Help!!!(?)

Motor height is greatly dependent on the technology built into the prop and how it works with a given hull design. There is no one, single measurement for all props on any given hull. That prop doesn't have a great deal of technology built into it. Once you get a prop better suited for that hull design you may find it's way too low. You'll probably also find you can gain some efficiency with a little setback (jackplate).

Given the type of prop you have the issues you're having isn't really surprising. The vent holes do indeed play an important part in acceleration and the hole size has nothing to do with horsepower. We have 90 mph boats with 300 hp that require 2 sets of PVS holes. Nothing unusual at all.

 

[sschefer]

Re: Blowout Help!!!(?)

Motor height is greatly dependent on the technology built into the prop and how it works with a given hull design. There is no one, single measurement for all props on any given hull. That prop doesn't have a great deal of technology built into it. Once you get a prop better suited for that hull design you may find it's way too low. You'll probably also find you can gain some efficiency with a little setback (jackplate).

Given the type of prop you have the issues you're having isn't really surprising. The vent holes do indeed play an important part in acceleration and the hole size has nothing to do with horsepower. We have 90 mph boats with 300 hp that require 2 sets of PVS holes. Nothing unusual at all.

I'll by that. A jack plate with the correct offset for the hull type can be an amazing add on. I have a deep V with a CMC PL65 I can jack that almost all the way up running in a straight line with great results. I usually run it about an inch above the bottom of the hull and that cleaner water gives me no problems with blowout (ventilation) in the turns.

It is also an amuzing toy. Trim it up with full lift and you can blow some wild rooster tales or soak your buddy down good. Watch out, paybacks are hell.

 

[Miniton]

Re: Blowout Help!!!(?)

Thanks for all the help with this guys. Guess I will have to locate the plugs for this prop. Any ideas?
It does certaintly look like they are professional drilled to me.

I will try and keep you updated when I locate them and give it a try.

If I went with a reasonably priced jackplate, are there any recomendations on brand or type?

Thx again,
Mini

 

[sschefer]

Re: Blowout Help!!!(?)

Thanks for all the help with this guys. Guess I will have to locate the plugs for this prop. Any ideas?
It does certaintly look like they are professional drilled to me.

I will try and keep you updated when I locate them and give it a try.

If I went with a reasonably priced jackplate, are there any recomendations on brand or type?

Thx again,
Mini

I use a CMC PL 65, Bobs has one they make themselves and there are several others of very good quality. Bobs were faster if you go hydraulic which in my opinion is worth evey dime. CMC now has a faster pump. CMC is completely self contained so there's not external tank or pump to deal with. None of them are inexpensive but then it's not a place you want to cheap out. The better unit take into consideration transom leverage issues and account for that in structural spread/distribution of weight. Unless you like paying salvage companies to pull your engine off the bottom of lake or river don't skimp.