Skin fitting hydrodynamics

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scrambledegg

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My boat is in a yard about to have all its skin fittings and seacocks replaced. It currently has flush fittings and I could replace like with like, but it is cheaper to replace with the slightly protruding type with external flange. There's going to be increased drag with these protruding types and I am reluctant to go that route but the economics can't be ignored.
Does anyone have hard data about that drag increase?
My limited understanding is that such a skin fitting on a part of the hull that has laminar flow could increase drag considerably , as it would cause the flow to transition from laminar to turbulent. On a part of the hull that is normally turbulent perhaps it would have very little effect on the drag? Unless, that is, it provokes flow separation. Where on typical hulls is the flow laminar and turbulent?
As you can tell I am guessing quite a bit. Is there anybody out there who's studied this, or knows of a study and would be prepared to share?
Yours hopefully.
 
As far as i know the flow will be turblant after about 18 inches at 6 knots, so most skin fittings are after transition. That said flush or no fittings are better than proud fittings, how much better is difficult to calculate depending on where on the hull they are located.
 
The hole in the skin fitting is going to cause turbulence too whether it's flush or not. I'm of the opinion that unless every 100th of a knot counts then it probably makes very little difference. Do you longboard the bottom after you antifoul?
 
Thanks guys for the responses. I had to google "long-boarding", so I have at least learnt something.
It seems that hard evidence may be lacking, but that the drag increase may be relatively small. With 8 of them in my boat, I wonder if it adds up to something significant.
 
You could fit the type with an external flange and then fair that in with some underwater filler to reduce the drag of the flange. Arguably that adds weight but it a compromise that may work. Pretty sure I have seen this done on a few production/racing boats.
 
SailSouth said:
You could fit the type with an external flange and then fair that in with some underwater filler to reduce the drag of the flange. Arguably that adds weight but it a compromise that may work. Pretty sure I have seen this done on a few production/racing boats.
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Yep it's pretty common to fair them in. I thought about it but didn't bother (there are 3 on mine). A bad tack or two would cost you more imo.
 
scrambledegg said:
Thanks guys for the responses. I had to google "long-boarding", so I have at least learnt something.
It seems that hard evidence may be lacking, but that the drag increase may be relatively small. With 8 of them in my boat, I wonder if it adds up to something significant.
Click to expand...

I have Elanco seacocks Landermores supplied & fitted them
 
scrambledegg said:
Fantastic!
If I can find a local measured mile and a way to reproduce exact engine revs, it would even be do-able.
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Having a fair hull (as achieved by long-boarding) is generally considered to have the most impact at slower speeds. Within that the for'd part of the hull is considered the most important (apart from the foils) if you have to cut corners.

So a lot of it depends upon your attitude to low-speed racing. I find it the most challenging but you do have to pay an awful lot of attention to detail. If you're doing that throughout then you'll want faired skin fittings. If you let your crew clump about like yetis on a beach holiday when the wind gets light then it's not worth the bother.
 
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