Skeg length / shape question.

[Mr. Demeanor]

I am thinking about adding to my skeg.
For those not familiar I run a transom water pickup allowing me to run extremely high engine heights. My boat / motor love the height. It porpoises less before packing air, launches flatter, and is simply faster.
I have about 3/4 of the top of the skeg out of the water at speed.

The downside is excessive crabbing and in some conditions like crossing a wake Im running alongside when one sponson goes up and over the wake the stern lifts and she wants to walk out to the right.

I have two ways to do this.
#1. Add to the length of the skeg. I could easily add an inch to the bottom of the skeg but this would increase the leverage/stress on the top of the skeg where cracks have been an issue for me.

#2. Add to the front of the skeg. I dont see where this would add as much stress to the upper rear where skegs tend to crack. In fact, I think it might actually help by moving some of the stress forward to the stronger thicker area of attachment to the bullet.

In my mind I "think" adding to the length would create more drag than adding to the front.
I also "think" that adding to the length would help handling better though.

Thoughts?

[Mr. Demeanor]

Better pic

[Greg G]

I am thinking about adding to my skeg. In my mind I "think" adding to the length would create more drag than adding to the front.
I also "think" that adding to the length would help handling better though. Thoughts?

You have put an incredible amount of time chasing top speed. The numbers you have achieved are amazing. I may be missing something but why would you hunt for a bit better handling at 60-70mph with more skeg while affecting the top speeds that are the prize? I'm sure your handling is good. Just my thoughts. Focus on profiling the skeg for 100.

[Mr. Demeanor]

You have put an incredible amount of time chasing top speed. The numbers you have achieved are amazing. I may be missing something but why would you hunt for a bit better handling at 60-70mph with more skeg while affecting the top speeds that are the prize? I'm sure your handling is good. Just my thoughts. Focus on profiling the skeg for 100.

The skeg is blueprinted.
It can get a little sketchy in a 13' boat when it starts crabbing and when I put the BIG wheel on it crabs more. It may actually go faster with more skeg if it goes straighter.

At the Suwanee run I was running my drag4 and I had never run it light on fuel. That prop has more stern lift but also more blade so it crabs more. When I got to about 1/4 tank of fuel I couldnt stay in the pedal. At about 5k rpm when it starts making power the back end would slide out to the right pretty bad. The boat is plenty fast and I would give up a mph to be safer.

I havent had a chance to run my big wheel yet but I will before I make a change to the skeg. The prop is on the boat now just waiting for the wind to let up :-) Ill be very suprised if it doesnt go 100+.

[sammythebull]

my thought on this is to look at what mercury has already done to the A6 notice the changes from standard drives to race drives big difference in skeg design

[olboatman]

If the boat is going to go straighter with more skeg I can't see where adding to the leading edge( or trailing for that matter) would add any drag. If it still crabs with the enlarged skeg then I could see more drag. JMO... seems like basic physics... but I'm not an engineer either. Gary

[pyro]

I'll bet the crabbing causes more drag than a larger skeg would.

The challenge is welding onto a cast skeg without creating weak spots and uneven tension in the metal, which usually results in cracking after a few runs.

[Greg G]

Ahhh .......... I did not realize that your handling concern is actually crabbing. If the boat has 1/4 tank of gas at what speed does the crabbing start? 85? You might want to pay Jim Russell to do a little consulting for you on skeg options. He has an incredible amount of knowledge and sometimes can save you time and money in the long run. Also, a great guy in general.

[phillnjack]

Adding to the front of the gearbox is much better than adding at the bottom or the back.
Less stress at the front as its nearer the centre of the steering point.
as long as you dont take the blade forward of the steering point it might help.


But your riding so high and going so fast, you are at a point where the boat is going to crab and prop walk no matter what you do.
Every surface propeller is going to give some degree of prop walk unless it has an opposite to counteract (twin engines and counter rotation).

Stupid question, but have you got razor sharp edges on the prop ? and I do mean razor sharp.t
This can help in prop walk if they are mega sharp as will cut down on the blade slap as it enters the water.
ive done this at under half the speeds your getting and found a huge improvement against prop walk.

just a thought

[baja200merk]

We always add to the front of the OMC skegs. Basically profile them like a sporty but the end up a bit shorter with the same shape. Makes a big difference with big props running high.

[Mr. Demeanor]

We made that skeg like a sporty when we first did the lower unit. Its an oddball lower unit and had a VERY strange skeg that went all the way up to the point of the bullet. We cut that back and had to add some to the length.
This is what it started as and what we did:


This is what I would do if I had to do it over but make sure not to go in front of the steering pivot (which it did in stock form).

[Mr. Demeanor]

I'll bet the crabbing causes more drag than a larger skeg would.

The challenge is welding onto a cast skeg without creating weak spots and uneven tension in the metal, which usually results in cracking after a few runs.

Pyro I am working on the welding issue. One of the things Ive noticed is they usually crack in the same spot at the top rear edge. This is also where most welded on torque tabs have a horizontal weld across the top of the tab. This is a problem in two ways.
1. the torque tab stiffens the skeg and moves any flex to a point right above it.
2. the point right above it is the thinnest part of the skeg and now weakened by the weld itself which runs horizontal.

What we just did with mine is run the torque tab all the way up and welded the top of it to the bottom of the actual bullet. This eliminates the horizontal weakness in the cast aluminum that caused this. It cracked and broke right at the top of the torque tab cruising at about 75mph and turned left pretty hard. Luckily enough skeg remained to keep control. This lower unit was retired.

If the new one lasts with the full length tab I will start thinking about adding to the front.

[scawd the dog]

Chris your set up is insanely good , look at what you have accomplished so far. Have you tried a smaller diameter wheel with big pitch ?

[Mr. Demeanor]

I need the 15" diameter to stay hooked up. Thats why i have been running the Performance Propeller props.

[Greg G]

I need the 15" diameter to stay hooked up. Thats why i have been running the Performance Propeller props.

Have you tried all the 15" or bigger wheels just to see how/if it helps with the crabbing? I'm sure you have but just throwing it out there as it is a hell of a lot cheaper to use the $25 loaner prop programs before messing with the skeg.