Skeg vs. Fin

Outboard designs up to 14'

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Moeregaard
Posts: 347
Joined: Mon Dec 03, 2007 11:31 pm
Location: Thousand Oaks, California

Skeg vs. Fin

Postby Moeregaard » Thu Sep 11, 2014 11:31 am

While our Zip was under construction, we debated whether a skeg or fin would work best. The argument in favor of the skeg was that launching and retrieving would be easier, and it would allow beaching of the boat. While the hull was still inverted, I installed a 54"-long mahogany skeg, that tapered from a one-inch depth forward to a maximum depth of two inches at the aft end, which terminated 24" forward of the transom.

After running the boat for several years, we determined that the skeg was great for longitudinal stability, but really didn't do much for maneuverability, i.e., the boat just didn't turn well. So, I spent a long afternoon, lying on a creeper, removing the skeg and wishing I hadn't used so many counterbored screws (with plugs) to install it in the first place.

I made up a fin from 1/4" aluminum plate, rounding the leading edge while leaving the trailing edge square. Its depth is six inches. This was TIG-welded to an aluminum base I had machined. I spent another couple of hours under the boat, drilling the necessary holes and bedding this thing in epoxy/filler. I placed it midway along the waterline--about 72" forward of the transom on our boat.

Here's the good part. The boat still has excellent longitudinal stability, but maneuverability is far better. You can feel a little more P-effect through the helm, but this thing will literally turn in its own length. Obviously, launching and retrieving require a little more care, and beaching is now out of the question, but these are minor complaints. The boat just feels more lively.

-Mark Shipley
A boat is just a wooden box with no right angles.

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jenko
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Re: Skeg vs. Fin

Postby jenko » Thu Sep 11, 2014 12:47 pm

good to hear a positive back to back comparison :)

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travis24
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Re: Skeg vs. Fin

Postby travis24 » Sat Sep 13, 2014 4:20 am

Mark

I also thought of going with a fin but decided against. I'll have to take measurements of my current skeg setup to compare. My boat currently handles well under full power with no cavitation when making tight turns and is responsive on gradual turns also,you need to hang on for those high speed turns. The boat has around 40 hrs on it so far. I'm still in testing but from what I have noticed the boat performs well. For speeds I'm hitting 31 mph on my Mark 58 w/ an 11" pitch 2 blade and at that speed is what I believe is the hull design max speed. Yes I would like to go faster and could do that but I don't think that hull can take Much more speed and be safe at the same time. I have to laugh and I'm sure your the same these boats attract a lot of attention where ever they go, some times I have to "cover up the power plant" due to causing an accident trailering the boat. :lol:

Travis
It's like anything you Build. You have to be Patient.

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Roberta
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Re: Skeg vs. Fin

Postby Roberta » Sat Sep 13, 2014 6:31 am

I placed my skeg (60" long) 36" ahead of the transom. Mine turns very well with no skidding. It was tapered from 1" to 2-1/2" for about 48" and tapered to 1-1/2" at the aft end. Your skeg may have been too far aft and not enough depth.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

Moeregaard
Posts: 347
Joined: Mon Dec 03, 2007 11:31 pm
Location: Thousand Oaks, California

Re: Skeg vs. Fin

Postby Moeregaard » Sat Sep 13, 2014 11:19 am

travis24 wrote:Mark

I also thought of going with a fin but decided against. I'll have to take measurements of my current skeg setup to compare. My boat currently handles well under full power with no cavitation when making tight turns and is responsive on gradual turns also,you need to hang on for those high speed turns. The boat has around 40 hrs on it so far. I'm still in testing but from what I have noticed the boat performs well. For speeds I'm hitting 31 mph on my Mark 58 w/ an 11" pitch 2 blade and at that speed is what I believe is the hull design max speed. Yes I would like to go faster and could do that but I don't think that hull can take Much more speed and be safe at the same time. I have to laugh and I'm sure your the same these boats attract a lot of attention where ever they go, some times I have to "cover up the power plant" due to causing an accident trailering the boat. :lol:

Travis


Travis, your performance figures are in the same ballpark as ours. We're currently running a tired '56 Mark 55 and get 29-30 MPH with a 10"-pitch two-blade aluminum prop. A little history here: In 1956, my dad built a Zip and powered it with a brand-new Mark 55. The boat was initially run without a fin or skeg, and after he spun it out (with Mom and her toy poodle on board), he decided to install a fin. When we launched the current Zip in 2008, one of his complaints was that it didn't turn as well as the first one. I thought it turned OK, but am always game for an experiment. My first test with the fin two weeks ago was with a three-blade ski prop, so top speed was a little lower. Next time out I'll reinstall the 10" prop and see what happens. I don't expect much (if any) increase in top speed, but with the fin the boat was much more responsive to steering inputs. Regarding safe speeds for the hull, I think 40 MPH on good water is a good maximum. These boats are at their best in protected waters anyway.

Yeah, I get all kinds of attention when towing and at the lake. I even had one guy with a huge picklefork--with two equally huge Mercs on the back--ask what brand of motor my Mark 55 was!

-Mark Shipley
A boat is just a wooden box with no right angles.

gdcarpenter
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Re: Skeg vs. Fin

Postby gdcarpenter » Sat Sep 13, 2014 1:11 pm

That's what makes each of our boats unique.

My skeg is one piece with my keel, and runs the full length of the keel. It is 3/4" wide and 3/4" deep with a 'bullnose' bottom edge, and tapers from half round profile to nothing at both ends over about a 12" distance. My thought was to have as long and gradual a taper at both ends to create less disturbance/turbulence cutting through the water. Even with ending my tapered skeg very close to the prop I get no cavitation.

I now run a steady 30 MPH @ 5,600 RPM with my 10" diameter, 10" pitch 3 blade brass prop with my 1959 35 HP Mark35A. With a cupped 10" diameter, 9" pitch I will run 28 MPH @ 5,800 RPM but have an amazing hole shot/acceleration.

I just personally like the feel of sliding out a bit in fast turns!

I would guess that 40 MPH would be nearing the 'comfort' zone for a ZIP.
This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com


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