Introducing MalwareBytes 3.0 (affiliate link)
Kennneee

Sharpening Chine and Transom edges?

11 posts in this topic

Hi Guys,

How much effort is it worth to take the 3/8" radius on my chine and transom edges and sharpen them?  I rounded them prior to glassing and know that sharper edges are more efficient but also more prone to damage. I don't have a sense of how much difference I will notice in terms of fuel efficiency, etc.  Any wisdom appreciated. Thanks!

Ken

Share this post


Link to post
Share on other sites

If you are class racing against the same boat design, then make the chines and transom edges crisp, otherwise, you'll never notice any differences.

Share this post


Link to post
Share on other sites

PAR- I am talking about an Outer Banks 26. Doubt I will be racing it:). Just dont want to be racing to the gas pump. 

Cheers,

ken

Share this post


Link to post
Share on other sites

Wondering what some of you Bluejacket Builders did? Any guidance from Tom on spending time sharpening the edges?

Thanks again.

Ken

Share this post


Link to post
Share on other sites

Round or sharp edges in relation to performance is beyond my skill.  I rounded pretty much all edges to facilitate the glassing.  I was also told that paint doesn't work well on sharp edges.  In my eyes a rounded edge looks better on a boat but these would be under water anyhow.  To save at the gas pump make the boat as light as possible that does make a difference. 

 

Egbert

1 person likes this

Share this post


Link to post
Share on other sites

I think Egbert got it right.  I think you'll probably save more fuel by using less ice in the cooler than by sharpening the transom edges.  I didn't do the math on that, though, so if someone else knows better I hope they'll add to the conversation.

"Simplicate and add more lightness" (William Stout or one of his employees)

Share this post


Link to post
Share on other sites

Efficiency differences with crisp over reasonably radiused edges, would be measured in small fractions of a single knot. On a sailboat, actively raced with others of the same type, this can mean being more competitive. Speeds are such that even minor improvements over the fleet can help. On faster craft, not so much, until you start pushing past say 60 MPH and even with this, still fractional improvements. Weight is the enemy of all small craft performance envelops. On racers every effort is made to reduce weight, as these savings can be placed where it's more beneficial, such as adding it to a ballast casting or increasing tankage size. Many flow bench and tank tests have proven these things out dozens of times.

   Basically, radius the edges so fabrics lay down and paint doesn't tend to crack, on too tight a curve. This is especially true of powerboats that will bash and pound their flanks and edges a great deal, testing the flexibility of the sheathing and coatings in these areas. Lastly, maintaining really crisp edges is a difficult, hardly justifiable chore. You'll constantly be touching up dings, nicks, chipped and cracked paint, etc. I trick I used on a powerboat racer a few years ago (100+ MPH) was to inlay aluminum bar stock along the chine and transom edges. It was "let" into the surface and bonded down with epoxy, then machined flush with both surfaces. A hand file was used to fix dings and insure a truly crisp edge, during the racing season. These edges were also back cut slightly a few degrees, offering a chiseled edge to the flow, so separation was ever so slightly improved. These guys were racing and every little trick in the book was employed to get a better top speed.

   I designed a special raw water intake for this boat to replace the typical mushroom style of strainer usually used. The team showed up at the ramp with the secret intake covered with a plastic bag and duct tape and the boat was hauled out with the same intention, to hide their new found advantage. Near the end of the first season, a competitor was caught breaking into their storage shed, with a tape measure and camera, under the boat, getting intel on the raw water intake. The special strainer wasn't something new, just something they hadn't thought of yet and it was just an appropriately sized "sea chest" style of strainer.

 

86180F-p.jpg

86193F-p.jpg

   These are what was common in the fleet and you can see these are really draggy at high speed. My design was flush, let into the planking, bonded down to remove the seam and made a 3 MPH difference on the top end. This is how anal these guys get, for a .02% speed improvement, that cost about $500 for a local welder to cut up and assembly from stainless. For your boat, the fraction of a knot or pint of fuel you might save over an afternoon's run, needs to be justified.

 

Share this post


Link to post
Share on other sites

My speed is defined by SWMBO's comfort level. Whether the boat exceeds that level, I have never given a thought about that one.  As it relates to efficiency, well the key is how on top of the water your boat runs. This is where I may differ from a lot of people. I am a big fan of external chine strakes, which does help with increasing speeds for the same setup and power, working with the reverse strakes more than just sharp chine edges .

Share this post


Link to post
Share on other sites

I very much doubt that anyone could detect a difference in drag/speed in slight rounding of the transom edge on a Bluejacket.  On a very small boat the percentage of drag increase would be greater than on a boat, of say 16, feet but still not measurable at normal speed for our boats. 

 

On a racing sailboat, I might sharpen the transom bottom edge and foil trailing edge but not anywhere else.  Some of the difference that might be seen is probably due more to perception than reality.  Feeling good about the potential of your boat may be as important as any actual advantage. 

 

On a Bluejacket, I would go for the small radius edge that will hold whatever surface you put on it.  It is far more rugged and you will never see any difference in fuel use, speed or anything else.  All liquids hate a sharp edge and will pull away from it.  The technical term for this tendency is called "surface free energy" or surface tension by the layman.  This is what makes it so difficult to cover a small nail hole or pinprick hole from a bubble in paint. 

Share this post


Link to post
Share on other sites

Hi Guys- thanks for the insights!  Since my own personal edge isn't as sharp as it once was I might has well keep the edges a bit soft on my chines... Glad to save some work and be able to get back to the Torture Board.

Cheers,

Ken

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now