Hydrofoil to reduce ventilation?

vissertrades

Petty Officer 3rd Class
Joined
Aug 16, 2003
Messages
79
I think I'm getting some slipping from ventilation at full trim out. Prop breaks loose and motor revs up a bit with my new prop. Also in corners prop breaks loose easier than the old prop. New prop is a OMC 11-1/2x19, old prop was a OMC 11-3/4x17, both cupped aluminum. 19 pitch yields 5500 rpm with average load so I think it's proped correct. Anti-vent plate is a 1/2" or so above boat bottom. Should I be looking at Hydrofoils to increase anti-vent plate size and reduced air suction? I did some checking and I'm leaning towards the Bobs Machine stabilizer plate:<br /><br /> www.bobsmachine.com<br /><br />I've looked at the others as well, Doel-fin, Stingray, Edge etc...<br /><br />Any advice?<br /><br />Thanks!<br /><br />P.S. Boat is a '84 Smokercraft "Big Fisherman" aluminum and motor is a '95 Johnson 50.
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Hydrofoil to reduce ventilation?

Hi, Piet.<br /><br />I don't think a hydrofoil is going to reduce ventilation. You are simply trimmed out too far.<br /><br />You might be a bit overpropped if you must trim out that far to get 5500 at WOT.<br /><br />You don't mention what rpm you were getting with the 17" prop. It sounds about right for a 50 on your Smokercraft. I would also try dropping your AV plate that 1/2".<br /><br />Good luck. :)
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Hydrofoil to reduce ventilation?

Your scenario sounds like what we would expect when going to a smaller diameter. A smaller diameter is the same as raising the X dimension as far as the tips of the blades are concerned. <br /><br />Bob makes a great plate for what its designed for. We have found that the secret to cutting down on venting is that the fin must cover the area above the prop. Unfortunately the Bob's starts in back of the prop.<br /><br />Good luck!
 

vissertrades

Petty Officer 3rd Class
Joined
Aug 16, 2003
Messages
79
Re: Hydrofoil to reduce ventilation?

Thanks guys, with the 17" prop I was turning 5800 rpm with the same load. I really think I need a SS 17", should I stick with OMC stuff? Also check out this plate made by Allison. It covers just over and before the prop!<br /><br />Link:<br /><br />www.allisonowners.com/hydro.html
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Hydrofoil to reduce ventilation?

Like we've talked about before -- I still think the motor will live longer at 5800 than at 5500. OMC props were always decent props as the Bomb props. Bomb just switched vendors so the newest props will be a question mark at best. We'll just have to wait and see. For consistancy I would look at Stiletto. <br /><br />That plate on the Allisons looks better but the corners are cut off. We found that cutting the corners like that reduced the effeceincy in some cases. Keep in mind that when you talk Allison youre talking 100+ mph, nose cones, low water pick ups and over the hub props.
 

AUGIDAWG

Petty Officer 2nd Class
Joined
Oct 31, 2002
Messages
189
Re: Hydrofoil to reduce ventilation?

I have heard that the OMC props were made in Mexico from inferior stainless. That is why they paint the low end model ss prop to keep it from rusting.
 

Jdeagro

iboats.com Partner
Joined
Jul 30, 2003
Messages
1,682
Re: Hydrofoil to reduce ventilation?

Piet;<br /><br />I think Dhadley is correct with regard to the prop. I assume that you were tring to get more top end by changing the prop? <br /><br />I do not agree with the use of a hydrofoil of any type do to potential handling problems, and they would likely reduce the top end. If your boat doesn't go more than 25 MPH then the handling issues are probably not going to show up much. But the top end speed is likely to be less. Hole shot would improve slightly. Check out www.Nauticusinc.com
 

vissertrades

Petty Officer 3rd Class
Joined
Aug 16, 2003
Messages
79
Re: Hydrofoil to reduce ventilation?

Thanks, I'm really just trying to prop this boat per Johnson's WOT spec. With the 17" aluminum OMC prop the motor spins at 5800 rpm with me, my wife and three kids! 550 Lbs + gear! With Just me, add another 100 rpm. With the 19" aluminum OMC prop the motor spins at 5500 rpm, again, with me, my wife and three kids! With just me, add another 100 rpm. Based on Johnsons spec of 4500-5500 rpm WOT I'm there, right? I need to get out with a GPS as the 19" feels slower all around and I get this slipping (venting?) on occasion. I have the trim level spot marked and I do tend to trim up a bit more with the 19" prop. I started out on this boat with a 12-1/4x15 OMC prop and WOT was 6100-6200 rpm! FWIW, The 17" feels the best all around.
 
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