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Author Topic:   Fins & Stabilizers - Why aren't the AntCav plates larger
prm1177 posted 10-19-2003 12:32 PM ET (US)   Profile for prm1177   Send Email to prm1177  
I've read with interest all of the postings concerning Doel Fins and Turbo-Lifts. If these items provide such a performance benefit, why don't outboard manufacturers make their anti-cavitation plates wider? Just curious.
jimh posted 10-19-2003 01:02 PM ET (US)     Profile for jimh  Send Email to jimh     
I have been asking the same question. If the addition of a $25 piece of plastic (which could be added to the engine at time of manufacture for about fifty cents) can work such miracles, why don't outboard manufacturers include them?

It seem improbable that the entire outboard engine industry and all of its designers and engineers have failed to see the light, yet small specialty suppliers working out their garages have achieved this break through in technology.

RJM posted 10-19-2003 04:32 PM ET (US)     Profile for RJM  Send Email to RJM     
Very good question guys! Maybe they limit top speed or maybe they are not needed on most boats. I look at this addition something like Short Take off and Landing (STOL) applications on aircraft. One could probably achieve the same results by manipulating pitch and diameter on propellers, but of course at a much higher price. This leads me to ask a parallel question; Has anyone used the pitch adjustable "PROPULSE" propellers? If so, what do you think of them? Do they do the same think as an after market fin? Do they work better? (I am thinking of purchasing one because I pull my kids around on a wake board and I use my boat both in the ocean and at Lake Crowley, California, which is a high elevation lake).
Hobie1981 posted 10-19-2003 09:19 PM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     
Hi All: I have both the Turbo-Lift and the ProPulse prop on my Nauset (1988 Yamaha 70 HP). Big difference on reaching planing speed with the Turbo-Lift. I paid $160.00 which included shipping last spring. Am going to move motor up two holes next spring, to see how she behaves. It's my understanding that some folks have gone to the top hole.

I trailer the boat all over the place, and consequently travel in waters that I have limited knowledge of. Although I use CMAP NT+, as well as a Raymarine L365 fishfinder. I decided to remove the SS prop and replace it with the ProPulse. Am very happy with it, only lost 2 MPH with the change, at WOT.

Was messing where I should not have been last weekend and had to replace 3 of the blades. Swapout was really easy, and cost was $60.00.

Much cheaper than replacing a bent prop shaft, and repairing the prop!!!!!!!!


Sal DiMercurio posted 10-19-2003 10:18 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
The reason the manufacters don't put larger anti vent plates is, the dealers would have to mount the engines right, instead just ploping them on the transom all the way down.
If you mount the engine to low with a large antivent plate, you would never be able to trim the bow up.
Even if the antivent plate is mounted right, the larger antivent plate would force the bow dopwn to much in sloppy water causing a very rough ride.
I'v driven boats with the after market large antivent plate & couldn't get the bow to break loose & had a very rough "slow" ride because of to much wetted surface.
Great for out of the hole, but detrimental on the top end.
JohnJ80 posted 10-21-2003 11:49 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I would pretty much agree with what Sal says with one exception. Certainly in agreement that most dealers don't have a clue how to set the motor to the boat.

What I have come to find is that the motor and the boat operate as a system. The whole thing is interrelated in terms of trim and performance. You can't change one thing on a boat and have it have only one effect. Not being a guru in fluid dynamics, but what I do understand is that when you start messing in the interface between fluids, it gets interesting - in our case that is the interface between water and air.

So, I don't think that it is possible to make a motor with the perfect size AV plate unless you know the boat that it is going to be mounted on and how it will be used. It varies by applications and you have to experiment. I don't know of any other way around it.


lhg posted 10-22-2003 12:17 AM ET (US)     Profile for lhg    
Engine manufacturers go to great lengths to make lower units as drag-free as possible. This aids in acceleration, top speed and overall economy. Why in the world would they want to GREATLY increase DRAG on their slick gearcase designs by adding one of those "stern lifters".

The main reason a given boat won't plane off quickly (the only reason for these things in the first place) is poor hull design and weight distribution. Why would a manufacturer want to give up the efficiencies of a hydro dynamic lower unit on all boats to accomodate a few poorly designed hulls? These things should only be used on boats using elevated engine heights, so that once on plane, the fin rides clear of the water flow off the hull bottom. Acceleration and hole shot will still be hurt, however, unless boat is stern heavy, underpowered (or over propped)
for the load.

JohnJ80 posted 10-24-2003 03:56 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
LHG - if this was the case, there would be NO AV plate at all. There would be NO skeg if you look at this from only a drag perspective. They would specify the transom angle and fix the motor at a particular trim point.

But there is more to it than that. Each boat and motor is a system. Ideal weight distribution all depends on what you going to use it for. While the mfg may feel that an ideal boat may have a Center of Gravity x feet in front of the lower unit, that may not jive with what you want to do with the boat when you take your two kids, two of their friends and their dads out with you fishing.

If your are bass fishing in TX and are never in deeper water than 3 feet, then a fast hole shot is more important to you than max top end. Maybe you are like me where you boat in very wake choppy waters and your spouse thinks the ride softness is more important than the top end speed (which she doesn't like anyhow).

In EACH of these cases, the CG is different and is totally unpredictable from the engine mfg's point of view.

So, adding fins is a function of what you want to do with the boat and how you intend to use it. The motor guys get that.

So, they provide a minimum drag configuration and leave the 'system integration' up to you and the way that you use your boat.

There is no 'correct' way to set up your boat. It all depends on what you want to do with it, how you load it, and where you boat. If you don't believe me, go ask Whaler tech support. They will tell you the same thing - "here is the configuration that we recommend is best for most users, but that you should fiddle with it if it doesn't seem to meet your needs."


RJM posted 10-25-2003 12:12 AM ET (US)     Profile for RJM  Send Email to RJM     
Hobie, Thanks for the reply on the Propulse propeller. I went over to the coast to do so boating and saw the Propulse at West Marine. I will probably get one next time I find the bottom with my current prop.
JohnJ80 posted 10-25-2003 08:14 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Hey Hobie,

I think you will be able to go up those two hole with no trouble. The guy at GI marine told me I would be able to do just that when I mounted my Turbo Lift. I was more cautious than his recommendation.

After running it for a while (performance greatly improved), I picked her all the way up. Performance was yet better and no cooling or blowout problems. The more you can get your lower unit up, you dramatically improve the drag configuration.


Hobie1981 posted 10-25-2003 11:29 PM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     
Hi JohnJ80,

As I repowered my Nauset with the Yamaha 70, I was able to utilize the OMC mounting holes on the transom. The lower two mounting bolts use the "blind holes" on the engine mounting bracket.

When I go to lift the engine up, I'm going to have to fill the lower holes in the transom, and then drill two new ones, as I will be using the slot(s) on the mounting bracket.

Before I go to all this trouble, anybody know for sure if this will really be worth my trouble ????

Boat is standard "Nauset" configuration. Battery located in stern. Gas tanks(2-6 gallon units) are located under console. Ocassionally I replace these with 2-13 gallon tanks when I go offshore.

Boat is primarily used for:
20 % Waterskiing
60 % Crusing/fishing
20 % Misc

Boat is used primarily in the ocean about 70% of the time,(lots of chop). The other 30% is spent on the local lakes in the area.

Hole shot is more important to me than top end speed.

Wife prefers a softer ride when possible.

Anyone have a similar set-up using the Turbo-Lift ???



Sal DiMercurio posted 10-25-2003 11:59 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
The Turbo lift will give you an excellent hole shot, "BUT' ...unless you mount your engine high enough where that big fin is "ABOVE" the water when on plane, it will force your bow down & give you the roughest ride you ever had in your boat in your life, & every ripple will feel like a 2 ft wave at speed.
Hobie1981 posted 10-26-2003 11:45 AM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     
Hi Sal:
The Nauset gets up on plane really quick, but the ride is really rough. I find that I seem to be trimmed out quite a bit to keep the bow from plowing. Is this what you mean Sal ?

Looks like I'll move the motor up over the winter.


Richard Quinlivan posted 10-26-2003 12:19 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
I have a '95 dauntless dual console (like a Ventura) with a '95 Merc 125 two stroke. I have always had a problem with mid speed porpois1ng with any trim at all.I raised my motor as high as it will go and that helped some to give me some trim range. I had tried a Bob's Machine planing aid with limited success. This summer I installed a Turbolift on the basis of what I have read here.

It has made a significant difference. The boat comes smothely to plane with no abroubt change going onto or coming off plane.I have significantly increased trim range which makes it a whole new boat. I have not had an opportunity to check it for effects at max speed but I think it will be out of the water totally by then so I do not anticipate any effect. I would hartily encourage any one with a porpoising problem to raise your motor up to get the anti ventilation plate up a couple of inches and add a turbo lift.

BTW I run with both a High 5 and a Laser II and found that choice of prop had essentially no effect on the porpoising issue on its own.


JohnJ80 posted 10-26-2003 03:01 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     

I guess my experience differs from Sal's with the bow attitude issue.

I found that getting my bow down a bit more really helped the ride in choppy waters since I am taking the waves more with the fine entry of the bow instead of the flat aft part of the boat.

I added trim tabs, because I got obsessed with this whole trim issue on boats, (now very glad I did). As I push the bow down, my boat (Dauntless 15) handles waves much better than if I let it rise up. There is no doubt that this slows the boat a bit (1-3mph at top end) but there is also no doubt that my speed through chop comfortably has increased maybe 20%. Planing speed is also much lower (dropped 2-4 mph depending on conditions).

When I repowered, as they always seem to do, my dealer mounted my motor all the way down. When I ran the boat, I was so disappointed since it seemed as if my boat had lost all its seakeeping qualities, the ride was incredibly rough when on a plane. No one in my family wanted to ride in it (me included).

Just picking it up one hole made a huge difference in the ride. Adding the fin helped again. Then going up all the way made it even better.

The primary benefit for me was that the fin really knocked down the porpoising and made my hole shot much, much better. The ride quality was mostly fixed by raising the motor up.

I'm not that familar with your boat. But, if you move it all the way up and you don't get blowout in the ocean (I don't think you will), and you still have cooling water peeing out the back, then you will probably be just fine and much better off than you are today. If it does blow out or doesn't cool, then drop it down a click. My advice would be to go all the way up first though.

If I understand your mounting situation correctly, now with your new motor, and once you raise it and fill your old holes you will be then mounted to industry standard hole configuration. From there, moving the motor up and down is pretty easy if you are on a trailer. So, I would do it anyhow.


SpeedBump posted 10-28-2003 07:31 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
I have a similar situation like Hobie, 1970 Nauset w/ 70 Nissan. I am mounted down in the first hole and use the blind lower bolt pattern. there is about 3/4 of an inch space from motor bracket to top of transom. Much of my boating is like hobie's but also is done in a large bay that gets very choppy and is a rough ride on most afternoons.

My Av plate is at the top edge of the aft drain hole/scupper or @ 1 3/4" above the hull bottom, I think It is in the proper range from all I have read here in various threads.

While looking at the AV Plate when on plane it appears the AV plate is a fraction below the water surface while I lean over the transom to take a look. With out my 250+ lb weight in the stern looking over it may well be at or above the water surface.

The boat rides nicely now and I can get lots of trim angle to play with motor settings and hull attitude but it still needs to be tweeked. From my positive experience with my 13 that had a dolfin on both a 25HP evenrude and now a 40HP Yamaha, I'll be adding a turbo lift this coming spring to soften the ride.

If that isn't enough tweeking then I'll consider raising the engine and drilling new holes and filling old ones. given it's current set up engine raising will be a last resort.

Hobie1981 posted 10-28-2003 08:19 PM ET (US)     Profile for Hobie1981  Send Email to Hobie1981     

I knew I had a twin somewhere. (LOL) From your post, it seems like our boats are set up the same.

I was thinking of rather than filling the lower bolt holes in the transom and redrilling new ones, to going to a jack plate, most likely some model from either CNC or Bobs Machine Shop.

Did you have any thoughts along this line????


SpeedBump posted 10-30-2003 12:05 PM ET (US)     Profile for SpeedBump  Send Email to SpeedBump     
Hobie- yes I have considered a jack plate and have looked at the manual plates from CMC. They seem to be well built and a reasonable price, the hydraulics are to large, overkill and very pricy. The static plate looks like a pain if you don't get it correct the first time out, so I am looking at the manually adjustable one. have seen the ones from Bob's Machine shop and will have to investigate.

Two reasons I haven't taken the plung on the jack plate are:

A) the set back may be to large and moving the engine back 4-5" will move the CG aft and make the stern squat more, then requiring more trim angle and perhaps more instability

B. I had such great success improving the ride of the 13 using a dol fin on a short shaft evenrude 25 and later on a custom jack plated 40 yamaha I just feel the fin should come first.

Keep in mind that even if you raise the engine a hole or 2 or 3 or 4 you will still experience harsh bumping in a chop and possible porposing when on plane. Granted you are reducing the resistance/friction of the lower unit in the water by raising the engine and you'll get a bit more top end, but it does nothing to stabilize the hull dynamics like a fin will.

The turbo-lift is expensive at $160.00 but appears to be much better than a $45 piece of injection molded plastic that will flex on a 70HP motor. Even with the engine raised people are using the fin. I don't really need any more top end speed so I prefer to focus on the ride comfort, and hole shot that the fin will hopefully produce.

JohnJ80 posted 11-03-2003 06:29 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Just to answer's Sal's statement. With the Turbo Lift, and the engine mounted all the way up, the fin is riding right at the water/air interface. This is different than most fins.

So, as the bow tries to rise, the fin enters the water and pushes it back down again - helping to control any propoising tendency. When in the water and the boat at rest is is well below the surface. As you go to a plane, it then can provide a much faster hole shot until it is at a plane and riding at the surface in a much lower drag environment.

I do agree with Sal that a fin in the water would be pretty bad from a drag perspective. If you look at the configuration of the T-L you can see that it is a lower drag unit in general.

Using the T-L I think you HAVE to do it in conjunction with adjusting your motor up vertically. There is an optimum point in getting the fin up high enough and reducing drag.


AQUANUT posted 11-05-2003 11:02 PM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
PROBLEM:my boat doesn't plane fast enough

SOLUTION:re-prop and/or repower with more horsepower.

FYI; in an racing environment where fast planing is imperative to positive racing results. mercury racing type gear cases acutually have as little metal in the water as possible...its all about weight verses horsepower..

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