Mercruiser Outdrive Hydrofoil - Pros and Cons

[Scorpion8]

My 2003 Trophy 2359 takes what I consider an inordinately long time to come up to step. You have to do it in phases. An old Bayrunner 16 that I used to own benefitted greatly from the addition of one of those Sport Marine SE Sport 300 hydrofoils to cure the same thing, but of course we're talking totally different power-to-weight ratios. Thoughts, pros and cons of adding a hydrofoil to the outdrive for the Mercruiser 350? Will it interfere with trailering while lifting the outdrive? Will it help the boat come up to plane faster? And "no", I don't bury the throttle with this boat. It's a heavy cruiser so I run it up in steps gradually.

 

[Rick Stephens]

I'd think you'd be better to run tabs of some sort. I stuck 'el cheapo' Smart Tabs on my 17 foot IO and my wife thinks I am brilliant. Not only does it plane at 13 mph, instead of 18, but it took the banging out of crossing wakes and running in waves. Like having shock absorbers.

Those hydrofoils put a lot of pressure on the drives. Seems like a lot of possible wear and tear and less surface to push the boat up on plane. Never owned hydrofoils, so that's conjecture.

Rick

 

[ondarvr]

You will be much better off with trim tabs.

 

[alldodge]

Agree trim tabs are better then hydofoils, but I have a stringray foil on my 23 footer and love it. The foil has been on it since it was about one year old, and its a 1995. I am unable to trim one side or the other but come up on plane quick and at slower speeds. If you try a foil don't get the soft flimsy ones, you will want the hard types as the stingray, not the whale tail.

This one comes up quite often and the opinions are the same (including mine). Let the show begin opcorn:

 

[mr 88]

How much pitch on the prop are you running and where are your max rpms vs whats called for ?Not familiar with the model but if its 23' with that motor it should pretty much pop out of the hole.I don't think Bayliner built a "heavy" boat compared to other mfgs. Before I went out and started buying and throwing trim tabs and hydro foils on it I would make sure I have the right prop [ and right compression numbers on the motor if the prop is corrrect ]. If you bought the boat used the previous owner may have over pitched it thinking his fuel mileage would be better,just a thought.

 

[Scorpion8]

The boat already has trim tabs, but they don't help it get up on step, just stay balanced from side-to-side when running. This hull form has very little boat below water so it's very sensitive to side-to-side weight. For almost 5,000 lbs loaded, I'd say it is a heavy boat once fuel and pax are aboard. The prop is what came on it, and is supposed to be the fuel-economy prop, not the speed prop. I do not know what the pitch is. I usually run the engine up slowly to 2,000-2,500 rpm, let it get moving and stabilized, then up to 3,000 rpm, then to 3,200-3,600 rpm depending on where I want the cruise speed set point. It cruises at about 20~21 mph depending on sea state. So it's no speedboat. The compression on the engine checked out good, and it gets serviced every spring and fall. Right now the engine is running the best it ever has, smooth and steady.

 

[HT32BSX115]

I usually run the engine up slowly to 2,000-2,500 rpm, let it get moving and stabilized, then up to 3,000 rpm, then to 3,200-3,600 rpm depending on where I want the cruise speed set point. It cruises at about 20~21 mph depending on sea state. So it's no speedboat.

Howdy,

When loaded for "normal" ops, What is your Wide-Open-Throttle (WOT) RPM and speed?

 

[Scorpion8]

When loaded for "normal" ops, What is your Wide-Open-Throttle (WOT) RPM and speed?

You know, my tach goes to 6,000 rpm and the farthest I've ever advanced the throttle is 4,800 rpm. Fuel economy goes way down at that range and I'm running out of throttle advance. That results in a cruising speed of 25 ~ 28 mph depending on sea state.

 

[alldodge]

Hitting 4800 your at max RPM with your engine.

 

[ondarvr]

To find the correct prop and do any fine tuning you need to run it at full throttle and record the speed and RPM's, with that information people can give suggestions on how you can get the best performance possible

 

[airshot]

In reading your method of advancing your throttle, you are causing your own slow to get up on plane problems. You may well have prop issues, but.....bring your throttle up to about 15-1600 rpm then open it wide open.....as you get up to your cruising speed then start pulling the throttle back until you settle in at the speed you want to run. You are using far more fuel getting up on plane the way you describe. Any I/O mfg will recommend what I have stated to get up on plane with the least effort and least fuel consumption. You will get away with your method in your car, but slow acceleration in a boat uses more fuel and takes more HP to get going. My own 22' cruises at 22-24 mph around 32-3400 rpm depending on chop, but I get it up there fairly quickly to save on fuel and wear on the engine.

 

[HT32BSX115]

You know, my tach goes to 6,000 rpm and the farthest I've ever advanced the throttle is 4,800 rpm. Fuel economy goes way down at that range and I'm running out of throttle advance. That results in a cruising speed of 25 ~ 28 mph depending on sea state.

A lot of tachometers have a range of 0-6000 RPM. That of course does not mean that you would ever operate the engine at that RPM nor would the engine actually be capable of it. (Most newer ignition systems have REV limiters that will not allow more than a preset RPM....usually no more than about 100-200RPM above max rated RPM) )

It's important to know your maximum RPM because it's the only way to determine if you're "propped" right for the engine/hull and load assuming the engine is capable of producing rated power.

To do this simply push the throttle to the "stop" when loaded where you would normally operate the boat. Do this in a no-wind condition and little or no chop. Adjust your tabs FULL-UP (I.E. out of the water) and adjust your drive trim in small amounts UP/DWN to achieve maximum speed.

Note your speed (using GPS) and use a known tachometer.

Once you have this information, you can determine if you need to use a lower or higher pitch prop.

Determine the manufacturers maximum RPM range for WOT (for example: 4400-4800 etc) and select a propeller that will give you maximum RPM (4800 in the above case) when lightly loaded.

The above process is applicable to just about ANY planing type I/O or outboard boat.

If your engine has a rated max RPM of 4800 and you are able to easily exceed it, you have the wrong prop on the drive.


Regards,


Rick

 

[JustJason]

Get the right size 4 bladed stainless prop,(or 5 if you want) make sure you are fully trimmed in, bury the throttle. That is the quickest way to get up on plane.

 

[littlerayray]

Gotta bury the throttle your not doing the boat any favors stepping up your boat like that

 

[JimS123]

My DoelFin has been on my Mercruiser outdrive for 31 years now. My son has had one on his Mercruiser for 7 years now. His also has a 4 blade prop. We both would not be without them. Not a single negative, all positives.

 

[Scorpion8]

Well, I've never "buried the throttle" to get a boat up to speed because the prop immediately starts to cavitate and the boat wallows while it's trying to match the prop speed. Much better to bring it up slowly. We're talking over the course of a minute and a half, not an hour long process. Last weekend I did grab speed-versus-RPM data and charted it out. There's two speeds displayed on my GPS/sonar, "speed" in knots and "water speed in kts" so both are plotted.

Except dang program won't let me upload a JPG or an attachment....

 

[HT32BSX115]

You have something else going on there. There should be no cavitation on hole-shot.

Not a single negative, all positives.

Some do report "negatives" though.

They do increase hydrodynamic drag. This will decrease top speed. It's not a problem for most people.............


I did a little search for "Doel Fin" and found the following.

divergent...unstable on my Yamaha 8hp 2 stroke. got rid of it right away. I replaced it with the 200 something or other which is a concaved single fin...much nicer

We have been catching some air with the new wings. sharp turns also cause some unbalance

some people swear by these things.. id you need stern lift, better hole shot.. dont buy this.. get a 4 blade prop or trim tabs instead..

The not-so-good-stuff:

Really, just two things - 1) Definitely reduced top speed. Prior to the Doel-Fin the boat would do about 31 mph at WOT in a lightly choppy sea. After the Doel-Fin, max speed at WOT seems to be about 27 or 28 mph. This represents about a 10% reduction in top speed. Not a big deal, since I don't really operate the boat at WOT much anyway. 2) The Doel-Fin really, really catches kelp! It is very tricky to get in and around the kelp to fish or dive without snagging the Doel-Fin. When this happens, the usually procedure of reversing the prop briefly doesn't always clear the snag.

The above examples are using it on outboard boats with NO power adjustable trim or tabs. That really makes it an experiment that will either "work" or not.

Since you DO have adjustable trim AND tabs, you probably do not need to add a "wing" to your drive.

You DO need to know that your engine is running correctly (producing rated power etc) and you need to know your WOT RPM so you can determine if you're "propped" correctly.

If you have a whole lotta "throttle" left at max rated RPM, you need to change your prop (assuming this drive is the correct drive ratio AND/or No one replaced the drive ?)

You might benefit greatly by installing (the correct pitch) 4 blade prop, and experimenting with trim limits and judicious use of trim & tab adjustment for holeshot.

 

[Scorpion8]

My speed-vs-RPM chart almost exactly matches Trophy's published charts. Maybe the prop isn't physically cavitating but it sure thrashes around needlessly by just burying the throttle from a standing start.

 

[ondarvr]

After what you just said my opinion may change a little.

First, while it could be cavitating it is more likely to be ventilating, they are two very different things, but get confused at times.

Depending on the exact prop, HP, hull design, load, plus other things the prop can suck are from the surface and ventilate, this can make noise and vibration but little to no forward motion. Trim tabs can't help with that, but a fin (foil) can, it's what they actually do well. They just block air from being drawn into the low pressure zone in front of the prop, just like the AV plate that's already there, only larger.

Different props can help eliminate it too, it just takes some testing to dial in the right combination.

 

[HT32BSX115]

My speed-vs-RPM chart almost exactly matches Trophy's published charts. Maybe the prop isn't physically cavitating but it sure thrashes around needlessly by just burying the throttle from a standing start.

So your WOT trimmed for max speed RPM is what??