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Author Topic:   Cavitation or Ventilation
Monnas Rock posted 11-01-2002 11:49 AM ET (US)   Profile for Monnas Rock   Send Email to Monnas Rock  
How can you tell the difference between cavitation and ventilation?
With two people in the boat, when I come onto plane, the engine, sometimes revs up near red line. If I back off the throttle a little then back on, it seems to get a bite once again and all is fine. With four aboard, this doesn't happen.
22'Revenge-Whaler Drive-220hp Merc with a large hydrofoil. Any ideas?
kingfish posted 11-01-2002 12:42 PM ET (US)     Profile for kingfish  Send Email to kingfish     
The term, "cavitation" is and has been misused a ton over the years, yours truly in the guilty contingent. Under almost all circumstances the type of phenomena you describe is "ventilation", but it's probably not as important to debate which is the accurate word as it is to fix the problem, eh?

I'm not sure why it happens when you have two on board and doesn't when you have four, may have something to do with where they are located and what the weight distribution is. I would play with the trim a little and try trimming the motor down (bring the lower unit closer to the transom) and see if that doesn't eliminate the slippage. I have found both on my Outrage 22 and my Parker 2520 that the motors (both 225's) will run without slipping (ventilating) at higher trim once the boat is up and well planed than they will when we are coming up on plane.

If that doesn't fix the problem, your motor may be set too high on the transom, and you would need to drop it one notch at a time until the condition goes away. Your hydrofoil in and of itself is not causing the problem; all it might be doing is intensifying the condition brought about by the motor needing to be trimmed in some.


Bigshot posted 11-01-2002 01:51 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Brackets are famous for that. If the bottom is a tad bit slimy it will also do that. I would have the prop cupped or souble cupped which will eleviate that problem...assuming the bottom is clean.
Monnas Rock posted 11-01-2002 02:57 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks, guys.
Right you are, Kingfish. Aside from being "proper" who cares what it"s called as long as I can fix it!
I'll start by playing with the trim and go from there. I'm not sure what having a prop cupped is. I've heard of it but don't understand the process or implications.
kingfish posted 11-01-2002 02:58 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Monnas Rock-

What Bigshot said-

(He read your question more carefully than I did; I didn't pick up on the whalerdrive aspect.)


lhg posted 11-01-2002 04:30 PM ET (US)     Profile for lhg    
For Monna's Rock's situation, first of all a single engine on a Whaler Drive will be running a little high, since as has been discussed, the WD was originally designed for 25" TWINS. This means a 25" single could be running high. I'm betting he is running a standard elephant ear prop, either alum or SS, assuming the prop is not damaged. The boat should have a Mercury performance prop on it, probably a 4 bladed "Offshore Performance" model. This will get rid of the described ventilation, as they hang on like crazy and just don't ventilate much. I would get rid of the hydrofoil on a Whaler drive boat. It can only slow you down, and trim tabs (stern lifters) are not needed on the WD 20-25 boats, because of the longer WD planing surface, which already functions in that capacity. The hydrofoil is simply an engine mounted trim tab which will kill speed and mileage dragging in/on the water.

Here is some definitive info on the subject from Mercury's prop catalog:

"CAVITATION: Even slightly chipped or bent blade edges can cause surrounding water to BOIL ("cavitate"). Gas bubbles then condense elsewhere on the blade, releasing energy that causes metal erosion or "cavitation burn".

"VENTILATION: Air from the water surface or exhaust gases from exhaust outlet being drawn into the prop blades causes ventilation. This reduces normal water load, causing the prop to over-rev and lose a lot of thrust.
Ventilation is most common with high transom mounting, extreme trim-out and sharp turns. Mercury props are designed to minimize ventilation."

So, summarizing, cavitation is from damaged blades, and ventilation is mostly from high mounting, without a prop that can handle it.
To get rid of ventilation, you generally have to either drop the engine down, or use a surface piercing performance prop, as described below. I use these on both of my Whalers.

Mercury's performance line of props, mostly for boats that will do over 40MPH, have a "Performance Vent System", which uses ventilation under controlled circumstances, as opposed to the description above. They describe it as following: (quoting)

"PVS allows you to adjust for changes in load and environment simply by changing the propeller's vent plugs for optimum acceleration. On acceleration, exhaust is drawn out of the vent hole. When the propeller blade strikes this aerated water, it pushes through it much easier than it would solid water. By controlling the amount of aerated water (by size of hole in vent plug) you can control how quickly your engine RPM rises. Once you're on plane, the water then flows over the vent holes, and seals the exhaust, allowing the propeller to again operate in solid, nonareated water."

I would think that anybody operating a 4-stroke engine could make good use of a large vented prop, to help overcome sluggish acceleration.

As for Mercury, all of their props are already cupped. Un-cupped prop haven't been made, by them at least, since about 1975.
Cupping is an upward roll on the trailing edge of the prop.

Bigshot posted 11-04-2002 10:51 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I would but my 4 stroke does not have sluggish acceleration.
Monnas Rock posted 11-04-2002 01:34 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks lhg,
Monnas Rock posted 11-04-2002 01:51 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks lhg,
I think your right about the engine being to high on the transom. I havn't yet figured out how to lower a 380 lb engine by a notch or two but I'm thinking of trying it. Your solution of changing props brings to mind another question. SS props run froim $400. up. How do you determine your exact needs before investing that kind of money in what may not be the right size or pitch? I,m getting around 30mph at 4,600 rpm. If I trim any higher I gain 400-500 rpm but no extra speed. If I go to WOP, it simply ventilates. I havn't yet figured out what optium cruise speed is but it seems like I should be able to cruise at around 30mph at 4,000-4,200 rpms. I'm not as interested in hole shot as I am cruising at best rpm/mpg ratio. Any sugestions?
Sal DiMercurio posted 11-04-2002 11:20 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
If I read the post correctly, your only getting 4,600 rpms at WOT, [ not WOP ].
If i'm not mistaken, your engine maxes out at 6,000 rpms, you have far to much pitch in that prop.
Have you ever gone to outboard races /
You will notice they all ventilate coming out of the hole, this is very common when running a prop with "lots" of pitch.
You didn't post what size prop your running, i'd say a 14.25 or 15 x 17 is going to be the correct prop for your setup.
From what your describing, i'd say you have a 14.25 or 15 x 21 or 23 prop on that engine, that would explain why your blowing out coming out of the hole, & only getting 4,600 rpms at WOT.
Better borrow a prop from a prop dealer to try before you buy.
I don't think your engine is mounted to high, just far to much pitch for your setup.
Try less prop & get back to us to let us know the results,.....I think it will cure your problem.
Got M Hooked posted 11-05-2002 07:19 PM ET (US)     Profile for Got M Hooked  Send Email to Got M Hooked     
What boat is this occuring on?
Monnas Rock posted 11-06-2002 12:21 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Got M,
It's a 1989 22' Revenge with a Whaler Drive, 220 hp Merc Laser EFI, Aluminum prop. I havn't had the prop off yet (just got the boat recently) so I don't know the size or pitch. The boat also has bottom paint but it's clean.
Lots of confusion, so far I've been told:
1. Lower the engine.
2. Get a SS prop.
3. Get a 4 blade SS prop.
4. Have the prop cupped.
5. Get a surface piercing prop.
6. Bracket mounted engines are naturally subject to ventilation.
I had the boat out last weekend for the second time and discovered that if I come up on plane slowly, it's not as inclined to ventilate. Also I can run at around 30 mph at 4,600 rpm but, if I advance the throttle much more then that, rpm increases but speed doesn't (feels like it's starting to blow out).
Monnas Rock posted 11-06-2002 12:35 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
WOP was a typo, sorry about that. I've been told there are prop shops that will let you try different props before you buy. I hear Mercury has a demo program but the two dealers I've talked to so far say they don't participate in that program. First, I need to take my prop off and determine the size and pitch, then maybe I will have a better idea of what I need to change.
Got M Hooked posted 11-06-2002 08:39 PM ET (US)     Profile for Got M Hooked  Send Email to Got M Hooked     
Monnas R.,
My buddy and I purchased 2 new 2001 Dauntless 18, we noticed some venting when running in 2 ft seas, but in 2-4 ft seas it was excessive. We solved the problem with a prop. This is the way we did it, and I only suggest that you try this first:
Run the boat on a calm day 2 ft seas. Get the boat going at a good speed so that it is on the plane. Trim the motor up some till you notice that the bow is starting to porpoise up and down, then trim it back down slowly until the porpoising just stops. The boat should then be just about it's most efficient plane. Advance to full power and note the Maximum RPM reading.
There is a company called Michigan Propeller, if you cannot find their number, I can look it up and email it to you. Call them and explain to them what is happening, Give them the boat model, length, engine size, prop type and size of what you are currently using, and your maximum RPM at wide open throttle. They suggested a slightly cupped stainless prop that solved our problems, and the boat performed alot better, with almost zero venting. We have used them on several other boats, and Michigan Prop has always sent the correct prop. Even if it does not totally solve the venting, the performance should be noticeable enhanced with the stainless prop.
You may also want to ask some of the other readers of this forum, that have a boat like yours, to note the distance between the bottom of the hull and the engine's cavitaion plate of the lower unit. Compare it with yours. This way you may eliminate the mounting height possibility.
Sal DiMercurio posted 11-06-2002 10:30 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Michigan outboard props are called Raptor.
They are about equal to Stiletto props, cost about $50 more then Stiletto, but give about the same performance.
Probably the 2 best props for the money, about 1/2 or less then a Merc prop.
The Raptor & Stiletto props are bow lifters, but actually lift the whole boat, for a totally different ride, as much less boat is in the water.
Don't let anyone tell you their no good, as they compare with Merc pretty darn well, only cost far less.
I'v run just about every make prop on my engine & the $249.00 Stiletto matches performance of the $600.00 Merc prop, & OMC props belong on electric motors in the bedroom, even though my engine is an Evinrude FICHT 200 hp.
It's very important for you to give us the max rpms at wot trimmed out, & the size & [ example 15 x 21 ] make of the prop.
As soon as you give us that info, we can calculate the right prop for your engine.
Don't do anything until you give us that info.
kingfish posted 11-07-2002 09:09 AM ET (US)     Profile for kingfish  Send Email to kingfish     

Under the category of "nit-picking", I guess, I have to take issue with your blanket statement denigrating OMC props. I have a cupped and raked stainless OMC prop on my looper Evinrude 225 that has pushed my Outrage 22 to 53 mph and it grabs and holds the water even with the motor mounted two holes up so well that it is really difficult to get it to ventilate no matter what I do.


Sal DiMercurio posted 11-07-2002 10:53 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
John, yes, cupped & raked, but certainly not the run of the mill OMC S/S prop the dealer is going to put on it.
Unless you go to the better props of omc [ viper etc ], forget it.

Monnas Rock posted 11-07-2002 11:39 AM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks Guys,
I'm going to pull the prop this weekend and find out what it is. It will be a couple weeks before I get the boat back in the water though. Next weekend we leave for a long awaited Bahamas cruise.
I really appreciate the help offers and I'll get the info back to you ASAP.
lhg posted 11-07-2002 03:25 PM ET (US)     Profile for lhg    
Your Merc aluminum prop is the problem, even though they are cupped. There are very few applications with a big Merc V-6 where such a prop choice would be correct, especially on a single engine Whaler Drive installation.

Gary, in your post above, items 2, 3 & 5 are all the same thing. There are basically three Mercury props you should be looking at, the "Laser II" (which is almost identical to the OMC "Raker" prop that Kingfish is using - I use Laser's on both of my Whalers), the 3 bladed "Mirage", and the 4 bladed "Offshore Performance". For your application I recommend the latter, and by going to "" and e-mailing them for a quote, you can get one for about $400.
For less money, more like $300, you can also get a Stilletto or Michigan Wheel Rapture that will do the trick.

Regarding pitch, here is the data on your engine from Mercury's prop catalog: (engine red line is 5800, gear ratio of 1.87:1, Revenge WD dry weight 2655#)

17" pitch prop, for load range of 2900-5000#, speed range 38-48MPH. I believe this is what you want, assuming your engine is running good compression and putting out correct HP.

Summarizing, I would order a 17" pitch Merc 4 blade Offshore prop, unless cost is prohibitive. Then try the aftermarket brands. Buy the prop first, and try it out, before lowering engine. I think this will solve your problem. With bottom paint, I would think you will get about 45 of her, max.

river98275 posted 11-13-2002 09:32 PM ET (US)     Profile for river98275  Send Email to river98275     
Hi All,

I've got a 13' Super-Sport with a 1984 40HP Johnson on it. It is currently running a 12 X 15 aluminum prop for about 35 MPH at 5000 RPM (3/4 throttle). If I open up the trottle all the way, it over-revs. I've been thinking of trying an 11-3/4 X 17 prop (Michigan Propellers) on it. Think this will cure the over-rev problem???

Also, I saw some of the prices for props in the previous posts here...Try looking at
I priced the Michigan props at $87 and the Stilleto's at about $200.

Sal DiMercurio posted 11-19-2002 11:31 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
River, I think the prices you saw were, $87.00 for a Michigan Aluminum prop, & $200.00 for a Stiletto s/s.
Yes, the extra 2" of pitch will bring your max rpms down, 200 rpms per each inch added [ 400 rpms total for 2" ]to your present configuration.
hauptjm posted 11-20-2002 04:48 PM ET (US)     Profile for hauptjm    
I would think the problem is the prop as well. Properly pitched and sized should make up 90% of the deficiency. The material or design (i.e. 4 blade vs. 3 blade, aluminum vs. stainless) will make up the other 10%. As far as your bracket, I promise it's not the problem. Brackets don't contribute to ventilation; in fact the opposite is true. The very reason for using a bracket is the fact it pushes the engine further out into "cleaner" water, allowing for the engine to operate with a shallower draft.
Monnas Rock posted 11-25-2002 01:35 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Well..The weather hasn't been cooperating to allow me to take the Revenge out and run wide open. But, I did manage to pull the aluminum prop. As best I can determine, from the numbers stamped inside(48 78120 A40 19P) and a ruler, it's a 14X19. There doesn't appear to be any damage or rough edges.
It looks like at least next weekend before I'll be able to see what top end is.
Albert Dal posted 11-27-2002 02:21 PM ET (US)     Profile for Albert Dal  Send Email to Albert Dal     
Well I can't give much of an advice as I have more or less the same problem with my BW 22 with twin merc's 115 two stroke. More over, I do not get more than 32 knots out of her whilst the bottum is clean and the engine are running fine at 5200 RPM.
Why is nobody talking about high fives?
With Laser two and high fives the engines are said to have to be mounted higher. The cavitation plate to be above water. Will that cause more ventilation or whatelse will happen? Before I spend 800 dollars plus mounting cost on high performance props I need to be sure what will happen.
Albert Dal.
andygere posted 11-27-2002 05:43 PM ET (US)     Profile for andygere  Send Email to andygere     
The Outrage 22 Cuddy I just bought has a 200 with a 19 pitch High Five. When I get some experience with the boat and motor, I'll post it on the forum. During my sea trial, I felt the hole shot was very strong, but there wasn't enough room in the lake to really get an idea of top end.
Sal DiMercurio posted 11-27-2002 09:55 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
With a 200 on that 22 revenge, a 19 pitch will be to much prop, I feel a 17 will allow the engine to reach max rpms with a decent load on the boat.
Monnas Rock posted 11-29-2002 10:14 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks Sal,
If the weather will cooperate, I'm going to take the boat out Sunday and find out exactly what top end is. I'll let you know what happens. Thanks to everyone for the adfvice, I think I'm getting close to determining the right prop.
Monnas Rock posted 01-21-2003 01:25 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
FINALLY, after several of you have offered your expertise, I have gathered all the info to figure out which prop I need:
1989 22' Revenge WT with Whaler drive.
220 Merc XRI (Fuel Injected).
Mounted in the middle holes with the cavitation plate even with the bottom of the hull
14 1/4" X 19" aluminum, 3 blade prop.
4,000 lb. (aprox.) wet with 2 people aboard.
Full Mills enclosure up.
Bottom paint.
70 degrees, Flat water.
37 mph @ 5,800 rpm.
The moment I goose it, it blows out so I have to ease it up on plane then slowly increase rpm to reach top speed.
I'm not nearly as interested in hole shot as I am getting rid of the cavatation/ventilation and cruising at best speed/fuel ratio, I'd like to cruise at around 30 mph at as low an rpm as possable. I know I need a stainless prop but, what size/pitch? 3, 4 or 5 blade?
Thanks, Gary
lhg posted 01-21-2003 06:18 PM ET (US)     Profile for lhg    
Gary - Two things immediately come to mind, since you are only getting 37 mph at 5800RPM.
Something is wrong here. My 25 Outrage will run that speed on a single 200.

Your engine is too high for the prop, and the prop is shot, probably a slipping (spun) hub.
Beg, borrow or steal another prop, any Mercury 17" pitch prop, and see what get. A single engine mounted on a Whaler Drive should be down all the way in order to use a non-performance elephant ear aluminum prop. Whaler Drive wasn't even designed for single engines, as evidenced by the structural support dead center.

Also, remove the hydrofoil. Those things are performance killers.

My # 1 bet for your problem? Simply a shot prop. Besides, it is the wrong style for the raised application.

Monnas Rock posted 01-21-2003 06:29 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
You could be right about the prop but everything on it looks fine when I remove it.
I already took the hydrofoil off and threw it away.
Are you saying the engine should be mounted so the cavatation plate is below the bottom of the vee?
Monnas Rock posted 01-21-2003 06:39 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Actually my last reply is moot because I'm going to replace the prop anyway. My question is: Which stainless prop will give me the best cruise/economy combination and the most bang for the buck?
Barry posted 01-21-2003 10:23 PM ET (US)     Profile for Barry  Send Email to Barry     
Gary, when you say that the anti-ventilation plate is level is even with the bottom of the hull, do you mean even with the bottom of the Whaler Drive? The center section of the Whaler Drive is somewhat flat (at least on mine) and as a result doesn't match up with the deeper V of the hull. I would think that the anti-ventilation plate should be level with or above the bottom edge of the WD.

Based on my experience I think I would go with a 19" Stiletto. However, lhg and Sal are recommending 17" and they both have much more experience than I do. As river mentioned, Surplus Unlimited carries both Stiletto and Rapture.

I would also save that hydrofoil. My experience, and data that I've seen, indicates that hydrofoils may reduce time to plane, allow planing at lower speeds, and eliminate porpoising with very little performance loss.

gss036 posted 01-22-2003 12:26 AM ET (US)     Profile for gss036  Send Email to gss036     
I agree with you on the hydrfoil, I use the Stingray made by Bainbridge Technologies and they work great, especially for getting up and out of the water.
I thing the top spped w/200 should be in the low 40's,something is not set up properly.
Just my 2 cents worth.
gss036 posted 01-22-2003 12:29 AM ET (US)     Profile for gss036  Send Email to gss036     
I forgot to mention it, I tried the URL for the and can't get, something missing there.
Sal DiMercurio posted 01-22-2003 12:38 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Going from an aluminum to a s/s will make all the difference in the wold, as the aluminum blades flex far more then s/s, plus theres little to no cup in an aluminum.
I still feel a lifting prop such as Raptor or Stiletto in a 14.25 x 17 s/s will cure your problem.
If you go to ......"" , you will find the Stiletto prop section & it will cost you $249.00 total.
Only thing is, amarket [ after }will not exchange, but Overtons will, but they want $20.00 more.
Theres no need to go to a Merc $600 prop, but don't get me wrong, Merc makes excellent props, just real pricy.
Barry posted 01-22-2003 08:19 AM ET (US)     Profile for Barry  Send Email to Barry
Monnas Rock posted 01-22-2003 12:17 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
I threw the hydrofoil away, not because I was convinced it was worthless but because it was broken at one of the bolt holes and it was some startup's idea of a new design. It was huge at 18" wide. All in all, I think it was junk.
Sal, I.m convinced your right about SS over aluminum. But, if I go to a 17" pitch wont I increase wot rpm and decrease cruise speed at any given rpm? just the opposite is what I'm after. I want to decrease rpm while increaseing cruise speed. Ideally, I'd like 30mph at around 3,500-3,800 rpm.
Barry, Your right, the plate is even with the bottom of the WD not the vee, although there very close, maybe 1-1 1/2 inches.
Any thoughts on 4 or 5 blade SS props over 3 blade?
Regards, Gary
Barry posted 01-22-2003 05:00 PM ET (US)     Profile for Barry  Send Email to Barry     
I always heard that it was preferable to have an engine propped so that you were at the upper end of the recommended RPM range at WOT. I have always done this even if it cost me a few mph, assuming that there was less chance of lugging the engine and as a result extending the life.

Decreasing pitch from 19" to 17" should increase RPMs by 200-400. However, going to s/s and adding more cup should offset some of that. Based on my experience a 19" s/s would be fairly close and since you could always raise the engine which should increase RPMs I think I would go with a 19.

I've read that 4 blade props can provide better hole-shot, less vibration, and less steering torque. However due to the additional cost I always went with a 3 blade.

Monnas Rock posted 01-22-2003 05:44 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Thanks Barry, I appreciate the feedback. I'm kind of leaning toward the 19" ss, three blade, unless I hear somthing convincing to the contrary.
lhg posted 01-22-2003 06:07 PM ET (US)     Profile for lhg    
Gary, my post above, dated 11-7-02, has all the Mercury prop data you need. Mercury performance props can be ordered, brand new, with a hub for any brand of engine, at, for about $400. Send an E-mail to them for a quote.

But I'm sure Stiletto, Cabela's (made by Mich Wheel/Rapture) or Mich wheel would be fine also. But make no mistake, Mercury's props are superior, and have the patented adjustable vent system & interchangeable hub, so they can be used on any brand engine. You do get what you pay for. No manufacturer has more experience with performance prop applications & design, outboard and sterndrive, then Mercury. Just look at the huge selection they offer, for every conceivable boating application. See props on their website, and order their free catalog.

You often cannot tell a spun prop just by looking at it. But your description certainly indiates a slipping rubber hub, which won't carry the torque under load. Mercury's new props do not have the rubber hub any longer, but instead use an interchangeable, and easy to replace yourself, plastic drop in spline. Mich Wheel has recently been licensed by Mercury on this design, called a Flo Torque II.

You can also go to's prop selector and see what it gives you. Be sure to choose the 200 EFI model, which is the current equivalent to your engine.

I'm still recommending the 17" pitch 4 bladed Offshore prop for your rig, because of the weight of the Revenge model and Whaler Drive. Other viable alternative would be a Mirage model.

Mercury's prop chart shows the following for your engine regarding the 19" prop recomendation:

Gross weight range = 2600-4500, 44-53 MPH speed range. Since I don't think the rig could do 53, this pitch may be a bit high.
I would think you would need twin 150's for speed like that.

Barry posted 01-22-2003 11:17 PM ET (US)     Profile for Barry  Send Email to Barry     
I'll admit that my recommendation on pitch is based primarily on the RPMs using the existing aluminum prop. If that prop is slipping, and the symptoms sure make it appear that is the case, then my guess may be wrong.

However, when I use Mercury's Prop Selector I also get 19". If I use: Length(feet)=22, Engine Type=Outboard, Year=2000, Engine Class=2-Stroke, Engine=200 EFI, Performance Goal=Overall, and Gear Ratio=1.87 it shows an Estimated Boat Weight of 3954, Estimated Speed of 46.9, and Theoretical Pitch of 18.7. If I use Weight instead of Length and set that equal to 4000 the Theoretical Pitch is 19.4. I'm assuming that Weight is total boat weight which would include the boat(2,600), engine(500?), fuel(77galx6lb/gal=462), batt(100?), persons(350?), etc.

Obviously the only way to know for sure is to test it. So if you want to go inexpensive and test then either borrow a prop, shop eBay, or purchase from someplace like Overton's so that you can return if necessary.

Bigshot posted 01-23-2003 09:31 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I will say that the mirage prop would be a great choice....they bite like hell. I traded my mirage for a standard quicksilver on my Baja so I could get a bit more slip. Amazing when you compare 2 props made by mercury. They will do 2 totally different things. Believe it or not the prop can make or break how satisfied you are performance wise. People who think not....need to think again.
lhg posted 01-23-2003 07:22 PM ET (US)     Profile for lhg    
Nick - Did you consider opening up the vents on your Mirage? You can get as much or little slippage as you want.

Barry, I think the Whaler Drive could throw a monkey wrench into the prop calculator's figures. It does offer additional drag.

I've not been able to figure out what Mecury means by "theoretical pitch". Sounds like a hedge to me, with unknown factors such as tee top drag, bottom paint drag, etc. Even the radar arch took 2 or 3 mph off my top speed.

Sometimes when boats fall between two pitches, one of each is necessary, one for light loading, and one for heavy.

Monnas Rock posted 01-23-2003 11:13 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
I just tried Mercurys prop calculator. They say I should be considering a 21" pitch. Would that increase speed and/or lower rpm?
Sal and lhg, you both reccomend a 17" pitch. I'm already turning 5800 rpm at wot with the 19" that I have. Won't that cause my engine to over rev. and decrease cruise speed? Since I'm already at 5800 rpm at wot, staying with the 19" in stainless, be it Raptor, Stilleto or Mercury sounds right. Does that make sence or am I missing something?
Thanks guys for all your help and patients with a prop idiot. Out of the six boats I've owned, I've never had to reprop one before.
lhg posted 01-23-2003 11:38 PM ET (US)     Profile for lhg    
According to Mercury's prop chart, if you're turning 5800 rpm with a 19" prop, you should be going 51-53 mph. If you're not, the prop is bad.
Bigshot posted 01-24-2003 11:11 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Larry....Mirages do not have vent holes. The new mirage plus might but not the original mirage.
Monnas Rock posted 01-24-2003 02:34 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Now thats the most convincing argument I've heard yet for the bad prop theroy! Until this thread, I wasn't aware you could have a bad prop and not know it.
lhg posted 01-24-2003 02:58 PM ET (US)     Profile for lhg    
Nick - About 5 years ago Mercury brought out a much improved "Mirage Plus", with the adjustable vent plug system and interchangeable plastic hub design.

Gary, when the older style Mercury rubber hub starts to go, it usually is not a catastrophic failure, but more likely a gradual slippage, usually under high torque loading. I had one go on my 23" pitch Laser II's, and by looking at it you could never tell it was bad. I'm still betting the old aluminum prop needs a new hub. In reality, it is a waste of money to even be running an aluminum prop on a V-6 engine, or any boat that will go over 30 mph.

where2 posted 01-24-2003 08:36 PM ET (US)     Profile for where2  Send Email to where2     
I'm amazed at the length of this thread, and yet unless I'm mistaken, NOBODY suggested taking the original prop off and having a local prop shop cup it for $40.

Got M, your buddy should drop his old prop off at a prop shop and get it cupped (like the new one is). Then keep it around for a spare...

If you're running aluminum props, it's easy to wear your cupping off on a sand bar. With stainless, it takes a little more, but not as much as you'd think to sand it off. My local prop shop puts the cup back in with a hammer for around $40, which is good considering the cost of a new SS prop. Unless the rigger was a fool, it's hard to believe any dealer would rig a boat which cavitated as much as Monna and Get M are saying. However, it's easy to expect the cupping to get worn off over time.

How much paint was worn off the skeg? How much of the lower unit looks like a re-paint?

Monnas Rock posted 01-25-2003 08:47 PM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
Where 2,
I dont think that's the problem with this prop. This boat has spent its life on the lakes in the Seattle area. No scratches on the skeg or lower unit. No ware, from beaching, on the bottom paint. I doubt this boat has ever been beached. Considering the power of a 220 hp engine vs: the performance of this boat, I think the advise to reprop with ss is the right way to go. I wouldn't consider putting any money in the original aluminum prop. I'll keep it as a get home but I think thats all its good for.
I appreciate the feedback but I think my problem has been nailed, (a spun, aluminum prop). Now my only decision is wheather to stay with the 19" pitch or go to a 21" to increase optium cruise speed.

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