?? "Hydro Foils" on Outdrives on Scepters

BigLew · #1 08-13-2007, 08:31 AM

I am looking for opinions on using hydrofoils, a.k.a. Doelfins, on outdrive units on 23'Scepters.

My thinking is this. 900-1200 lbs. of engine and drive at the stern coupled with about 1100 lbs. of fuel in the 140 +/= gal. belly tank. If we can get a few hundred pounds of lift from such fins, are they worth the effort and what are the trade offs. Trim tabs then become more of a tool to balance the load port/starbord and since they needed to be extended less- reduced drag.

I would be interested in the thoughts of those members that have considered/investigated such things.

Thanks in advance for your contributions.

JohnB · #2 08-13-2007, 10:17 AM

If your talking about an outboard setup, a 4 blade powertech prop give you quite a bit of lift, but you give up just a little on the top end. Trim tabs also can help, so can moving everything that can moved forward. I found between the old battery mounting pads in the back bilge, wet wood in the transom and fuel platform, etc, I was able to shave off a couple hundred pounds. I have used Doelfins on smaller boats, was never a fan of them on big boats.

Fr. Frank · #3 08-14-2007, 08:02 AM

First of all, Doelfins will work on sterndrives, just as they do on outboards. They only create an appreciable drag when the drives are trimmed all the way down.

Second, in a chop, they are very good at keeping the bow down cutting into the chop for a smoother ride, and they cost less than trim tabs.

However, unlike trim tabs, they cannot be used independantly of the drive angle. This means that underway and planing, you cannot adjust the boat's attitude while maintaining the best thrust angle on your outdrives. Remember that the best outdrive thrust angle while underway on a plane is as close as possible to parallel to your direction of travel, given the detractions of ride quality and trim angle. For thhis purpose, trim tabs are much better.

Given cost considerations, I'd probably go with the Doelfin or Stingray hyrdofoil stabilizers. You WILL get on plane faster, and be able to plane at a slower speed.

I have a Merc 150 OB on a Seafari 20, and I have a Stingray XPRIII mounted to my cavitation plate, AND a stabilizer mounted to my skeg.
I plane in under 3 seconds, and have a top speed of nearly 50mph. It looks funky, but works well.

BigLew · #4 08-14-2007, 10:17 AM

Fr. Frank,
I was hoping you'd chime in!! Understand I have a V/P 260 I/O on my 23' Scepter.
I actually have both a Doelfin and tabs. You make a point about maintaining the cutting angle in a chop and I do appreciate that benefit as well.

I'd like your thoughts on the following point. Does the Doelfin, etc. generate any lift like a hydro-foil and if they do, would that "lift" (I'm thinking maybe a few hundred pounds worth) reduce the amount of trim tab extension needed to maintain optimal trim of the boat. My thinking is that the lift possibly generated by the Doelfin comes at a lower cost, in terms of drag, than the same amount of "stern" lift generated by trim tabs?

Any thoughts? I look forward to hearing back from you and anyone else with thoughts on the subject.

Fr. Frank · #5 08-14-2007, 12:06 PM

The overall drag to achieve a given effect will be about the same regardless of where you create that drag - at the lower unit with a hydrofoil stabilizer or at the transom with trim tabs. The difference is in effective thrust based upon the angle of that thrust to the direction of travel. the closer to parallel to your intended direction of travel the more effective thrust.

This is why outboards and sterndrives with power-adjustable trim on the drives are so much more efficient than most straight inboards. They can produce thrust very nearly parallel to travel, where an inboard is usually pushing at 8 to 15 degrees out of parallel in a downward vector.

The most efficient thrust system available is a trim adjustable surface-piercing drive, because you don't have the power loss from going through 2 90 degree directions changes and a u-joint. All you have is the u-joint.

That being said, there are 2 times you want that stern-lifting thrust at the rear of your boat: getting on plane, and keeping the bow down in rougher water.

1bayouboy · #6 08-14-2007, 04:25 PM

Fr Frank......what kind of stabilizer is on your skeg...
I'd be interested in seeing a shot of that setup.

BigLew · #7 08-14-2007, 07:10 PM

I spoke with Fr. Frank this morning. Boy, that guy has forgotten more than we all know put togther!!Well, maybe not that much, but he's just as nice a guy as you could ever imagine.

I'LL SAVE HIM THE TROUBLE. I looked up the website and here it is:

http://www.hydro-shield.com/

76Red18 · #8 03-20-2008, 03:00 PM

Quote:

First of all, Doelfins will work on sterndrives, just as they do on outboards. They only create an appreciable drag when the drives are trimmed all the way down.

Second, in a chop, they are very good at keeping the bow down cutting into the chop for a smoother ride, and they cost less than trim tabs.

However, unlike trim tabs, they cannot be used independantly of the drive angle. This means that underway and planing, you cannot adjust the boat's attitude while maintaining the best thrust angle on your outdrives. Remember that the best outdrive thrust angle while underway on a plane is as close as possible to parallel to your direction of travel, given the detractions of ride quality and trim angle. For thhis purpose, trim tabs are much better.

Given cost considerations, I'd probably go with the Doelfin or Stingray hyrdofoil stabilizers. You WILL get on plane faster, and be able to plane at a slower speed.

I have a Merc 150 OB on a Seafari 20, and I have a Stingray XPRIII mounted to my cavitation plate, AND a stabilizer mounted to my skeg.
I plane in under 3 seconds, and have a top speed of nearly 50mph. It looks funky, but works well.

Fr. Frank, I can only find a XRIII no XPRIII. Is the XPIII what you have? It's the no drill to mount model. If so did you choose it for that reason or better performance? Richard...