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Author Topic:   1997 21' Outrage: Prop Venting in Rough Water
rtk posted 07-19-2004 01:38 PM ET (US)   Profile for rtk   Send Email to rtk  
This past Saturday I fished in the ocean inshore. As I made my way back into the Barnegat Inlet early afternoon I experienced prop ventilation. The inlet gets real churned up on a weekend with boat wake action. The entire ride back in the bay was no better. Between the incredible amount of boat wakes and the chop, I experienced ventilation even when the motor was trimmed all the way down. It was so bad I had to really back off the throttle (basically off plane) to get the prop to bite again. I was running at a speed that just kept her on plane, 23-MPH and 3300-RPM.

When running in conditions without boat wake activity I have no problems with propeller slip. Boat jumps up on plane at 2800-3000-RPM with no slip.

Motor is mounted one hole up from lowest mounting height. This seems to be a very common engine height after looking at couple of the same boats around.

This boat has a rather large T-top hardtop. Could this be causing me to run "bow heavy" and keeping the stern up?

Propeller is a Powertech 4 blade 15.25" X 16 pitch which I understand is has a fairly aggressive cup.

Is this performance inherent in this boat design? I imagine I could drop the motor down another hole. Does anyone else have the motor mounted as low as it can go?

With it as is, I have some real concerns about using the boat in rough conditions going in and out the inlet. It is not a place where the excessive slip is acceptable. Total control of the boat is a necessity.

I am considering a jack plate to be able to adjust the engine height for varying conditions and to provide some setback to run in some "cleaner" water and perhaps get the stern down a bit by moving the engine back.

Appreciate your input.


BW23 posted 07-19-2004 02:16 PM ET (US)     Profile for BW23  Send Email to BW23     

Just out of did your MIRAGE propeller perform? Any better than the Powertech??

I'm using a 15-1/4 X 15 on my Mercury 225 EFI with a 30-inch shaft on a 23 Conquest. I'm experiencing the same cavitation condition.

I've played with trim and tabs. The props seems to break loose and takes a sensitive throttle adjustment to get it to bite again.

I've been boating 30 years and have never experienced a boat/prop setup so sensitive.

rtk posted 07-19-2004 02:45 PM ET (US)     Profile for rtk  Send Email to rtk     
I tried the Mirage at the current engine height I am running now, one hole up. I experienced excessive ventilation when bringing the boat up on plane and some when running in relatively smooth conditions. I had to really work the throttle to get up on plane. I do not have that problem with the 4 blade Powertech, it will jump right up on plane when advancing the throttle moderately. That is why I am selling the Mirage.

The Powertech is the only propeller that I have been able to run on this boat without having a "chronic" ventilation problem, except when I encounter extreme boat wake driven chop.

I have run many boats myself but not many deep-v outboard configurations. It really is a safety concern to me. In a crowded inlet or channel with excessive boat wake I am experiencing real control problems with this boat.



Barry posted 07-19-2004 05:07 PM ET (US)     Profile for Barry  Send Email to Barry     
Where is your anti-vent plate in relation to the bottom of your hull?
LHG posted 07-19-2004 05:24 PM ET (US)     Profile for LHG    
Ventilation is almost always the result of engine mounting height or trim angle, or improper prop design for application, assuming there is no serious hull projection causing turbulence.

I would check with Boston Whaler for correct mounting height on this model.

It sounds like the engine needs to come down.

rtk posted 07-19-2004 05:31 PM ET (US)     Profile for rtk  Send Email to rtk     
I really should [check with Boston Whaler about recommended engine mounting height on the transom]. That would be helpful to know : ). When they installed the new motor it was too high. I wanted to be there when they dropped it down a hole to check the height, but they dumped the boat back in the water before I could check.

I probably should just have the marina pull it, satisfy my curiosity. Could very well be the problem.

Only one more hole or about 3/4" of adjustment down left, I would think I am pretty close to having the anti ventilation plate parallel to the bottom of the boat as it sits. Others are running some pretty high engine heights with reports of very good performance, 1 1/2-inch to 2-inch above the bottom of the keel.


rtk posted 07-19-2004 06:14 PM ET (US)     Profile for rtk  Send Email to rtk     
Thanks LHG, good suggestion.

Re your comment on a hull projection causing turbulence: do you think a transducer mounted too low would cause enough disturbance of the water to cause this?

There are two transducers mounted on the starboard side (boat came that way). One is for a fishfinder and one for a depth sounder. The depth sounder didn't work last year. I noticed that one of the transducers were mounted slightly above the plane of the hull. I dropped it down a bit to provide flow over the transducer. Maybe I went a bit too low.

Much Appreciated


rtk posted 07-20-2004 12:02 PM ET (US)     Profile for rtk  Send Email to rtk     
I spoke to Mr. Bennett at Boston Whaler this morning. He did not have any specific information for this model regarding engine height since they did not rig them at the factory at that time. He was suprised that I was having a problem with the engine mounted where it is, but did seem to think it needs to be dropped down.

I "need" a new color fishfinder anyway, I guess this is as good as an excuse as any to install one!

Appreciate the input.


BW23 posted 07-20-2004 03:34 PM ET (US)     Profile for BW23  Send Email to BW23     
Keep us informed if things change. When I was testing props, my MIRAGE 15 1/4 X 15 tested great!

After 20 hours (did not hit anything) it seems to ventilate more often. Not sure what changed.

It is quite easy to ventilate out of the hole. Top speed and RPM's are quite good.


rtk posted 07-20-2004 04:25 PM ET (US)     Profile for rtk  Send Email to rtk     
Will do.

In the absence of boat wakes I rarely experienced this problem. From June on in the summer there is a very large increase in the amount of boat traffic on the weekends. Perhaps the recent ventilation you are experiencing is due to this?

The frequency (constant) and varied direction of the wakes (1-3 foot) really keeps the boat high on the water. The amount of ventilation I experience is definitely related to boat wake action more so than sea state.



jimh posted 07-21-2004 07:48 AM ET (US)     Profile for jimh  Send Email to jimh     
To rtk: What engine are you using?
rtk posted 07-21-2004 09:08 AM ET (US)     Profile for rtk  Send Email to rtk     
It is a 2003 Mercury 250XL EFI.


handn posted 07-23-2004 11:18 AM ET (US)     Profile for handn  Send Email to handn     
Has your prop been repaired? I had fairly good luck with a 16" 13p three blade mirage on a 23 Conquest before the prop was dinged up and repaired. The repairs looked perfect but the prop broke loose climbing up 3 footers and was very annoying. I went back to my 15" 15p 3 blade mirage which is not perfect. I am surprised you are having trouble with a 4 blader unless it is out of alignment.
rtk posted 07-23-2004 01:39 PM ET (US)     Profile for rtk  Send Email to rtk     
The Powertech 4 blade was purchased brand new in May of this year and has been run about 30 hours. No repairs have been done.

The Mirage Plus was purchased brand new and installed with a brand new hub kit that came with the prop.

The Mirage Plus did ventilate a bit at speed in moderate chop and boat wake and significantly when bringing the boat on plane. The Powertech does not do this unless there is significant chop, the boat gets up on plane very nicely at about 2900-3000 rpms with no ventilation unless I really lay on the throttle.

A problem with the Powertech is certainly worth looking into. Just because it came out of the box new doesn't mean the prop is right.

Almost sounds like your prop guy took a bit of the cup out of the prop when he reconditioned it.

I am going to have the boat hauled and I am going to check the position of the anti ventilation plate with the bottom of the boat to start. Dropping the prop off at the shop to get the hub checked and put on a machine isn't a bad idea. It is a rubber hub, not the removable insert like the Mercury props.

I appreciate the recommendation with regard to the prop.


rtk posted 07-25-2004 09:30 PM ET (US)     Profile for rtk  Send Email to rtk     
Had the boat hauled today. Anti ventilation plate approximately one inch above bottom of boat. Looks as though by dropping the motor to lowest position will put the plate just about parallel with the bottom of the boat.

Would a propeller slip value give any indication of proper engine height and "traction"? This is the performance I have observed. This is only one run.

Boat: 1997 21' Outrage with Hardtop T-Top/T-Top enclosure

Engine: 2003 Mercury 250XL EFI

Load: 2 adults (320lbs), 80 gallons fuel, 10 gallons water
2 group 27 AGM batteries, safety equipment

Propeller: 15.25 X 16 pitch Powertech 4 blade OFS

Engine Height: One hole "up"

Bottom: Painted, good condition, a little dirty (slightly fouled)

Conditions: Wind 5-10 mph (abeam), very light chop, temp 80 degrees

Moderate engine trim, just enough trim tab to level boat


1000 5.4 2.3 37.63
1500 6.7 3.1 48.41
2000 7.7 5 55.53
2500 9.5 8.3 56.11
3000 17.7 9.3 31.86
3500 26.2 12.9 13.54
3800 28.2 14.3 14.29
4000 30.6 15.7 11.64
4200 31.4 17.4 13.65
4400 32.7 19.5 14.16
4600 34 20.2 14.63
4800 36.3 21.4 12.65
5000 37.7 22 12.91
5200 39.5 24.6 12.26
5400 41 25.3 12.31
5600 42.8 27 11.73
5750 43.5 27.8 11.62

Do these slip values seem to be a little high? Slip values were calculated using the propeller calculator on this site.


Peter posted 07-25-2004 10:10 PM ET (US)     Profile for Peter  Send Email to Peter     
Are you sure that the propeller isn't a 15P? I don't see a 16P listed at their web site.
rtk posted 07-26-2004 07:24 AM ET (US)     Profile for rtk  Send Email to rtk     
Model number is CLU200 OSF4R16 polish. I am pretty sure it marked as a 16P on the prop.


Whalerider posted 07-26-2004 07:48 AM ET (US)     Profile for Whalerider  Send Email to Whalerider     
They do seem high.

As a note, while I like powertech props, I had a smaller prop for a 90hp and it didnt seem right. I called them about it and was advised to return it for inspection. It turned out the pitch was WAY off. They replaced it with another prop that worked great. I would send your numbers to them and see what they think. They were very helpful and easy to deal with.

rtk posted 07-26-2004 11:53 AM ET (US)     Profile for rtk  Send Email to rtk     
Gave Powertech a call this morning. They suggested bringing the motor down so the plate is at the same level as the botttom.

If there is still a problem bring the prop to a shop and have them put a bit more cup at the tip of the blades.

I guess I'll drop it down and see how it goes. Can always raise it back up easy enough.

I have heard of instances where the pitch and other specifications of the prop were not what they should be out of the box. Definitely worth investigating. I'll have the prop checked out to see if there is a problem with it.


Peter posted 07-27-2004 01:45 PM ET (US)     Profile for Peter  Send Email to Peter     
Powertech props have a reputation for having a bit more pitch than is nominally indicated leading to very low calculated slip (2% or less).
rtk posted 07-27-2004 08:54 PM ET (US)     Profile for rtk  Send Email to rtk     
I noticed there is a lot cup by the blade tips.

I have read that there is pitch and there is "effective pitch" with regards to propellers. Cup on the edge, as I understand can increases "effective pitch". Pitch can mean different things with different styles of propeller.

Very interesting topic.

Peter, 2% slip? Are these values really acheived?



rtk posted 08-20-2004 02:01 PM ET (US)     Profile for rtk  Send Email to rtk     
Well, found out that I am underpropped. Mechanic at the marina ran the boat and found I am hitting the rev limiter at 5750rpms, not getting max rpms at 5750. The Mercury specifications for the 2003 250hp EFI state max rpms is 5800. The Guardian system will not allow the motor to exceed 5750 rpms, you can't get to 5800 rpms.

The owners manual states an audible alarm will sound when the rev limiter or Guardian system kicks in due to excessive engine rpms. I did not get an audible alarm at 5750 rpms. I have the Smartcraft multi gauge installed. Reading the manual for that, an audible alarm will not sound, a "bell" icon is displayed on the gauge to indicate the Guardian system kicked in due to over rev condition.

I never noticed it. The engine seems to run fine at 5750. It was explained to me that it is not like the old rev limiters where the engine would "shake" or "buzz" or run terrible.

Slapped on my old 19 pitch LaserII stainless and maxed rpms around 5550-5600. Ran her out in the inlet this past Saturday and she powered real well up and down the big seas and swells, no where near the ventilation I had with the other props.

So, the big block V6 EFI specs at max rpm are incorrect at 5800 rpms. Max is 5750 rpms. The Optimax big block specs list max rpms at 5750. The Optimax and EFI big blocks share the same ignition system and Smartcraft/Guardian technology.

I called Mercury to ask them what the max rpms are for the 2003 250 hp EFI. They said 5750 rpms according to the service manual. I asked them why the specifications on the web site, the brochure, the owner's manual and the sticker on the engine state it is 5800 rpms when it is impossible to run the motor at 5800 rpms. The reply I got was "I don't know, I guess I'll have to let someone know about that."

Thanks for all your input.

BW23 posted 08-23-2004 11:27 AM ET (US)     Profile for BW23  Send Email to BW23     
Thanks for the update. Glad to hear you to have things worked out.
I'm still at a loss why my Mirage ran great when new and now it is prone to ventilation. Never hit anything!!

I have not checked the bottom of the boat for "growth" as it is painted with Anti-foul.

Are you going to sell the Powertech 4 blade ? Any chance I could try before I buy?


rtk posted 08-23-2004 01:13 PM ET (US)     Profile for rtk  Send Email to rtk     

That is odd you are still having the problem. It is annoying.

I'd be happy to send you the prop to try out. If you like it I will sell it to you. Email me with your address and I'll get it out to you.


BW23 posted 08-23-2004 03:43 PM ET (US)     Profile for BW23  Send Email to BW23     
Thanks Rich,

You've got mail.

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