Reducing hull friction

[nordvindcrew]

as usual, lots of conflicting opinions, not to knock any idea, but does any one have any test results to back up a particular theory? I don't need a make believe mental boost, I need something that will actually reduce drag on my rowing boat to help win races at 5/6 knots

[jehardiman]

The 3M mirco-riblet film mentioned above will reduce skin friction in most smooth water conditions without large motions if they are aligned with the flow AND selected and sized for the Re. The problem occurs when cross flow conditions occurs and higher/lower Re's occur, such as most open water situations and definetely in whitewater. When the riblets are no longer aligned with the flow, or sized for the Re, then the skin friction is increased. Now remember that we are not talking much here...maybe a 7% reduction and a 15% increase in Cf.

Google "3M riblet film" for a whole bunch of work on the subject.

[PI Design]

Originally Posted by nordvindcrew

I recently had a conversation with a person that had done design work on 12 meter keels and he said his research indicated that sanding with 1200 grit wet paper to get a finish where water disperses rather than beads is the way to go. His explanation is that then the shear is water to water rather than water to solid finish surface. Can't back it up, just an idea I heard from a pretty smart guy

This is an idea that has been tried many times before, but it is proven to be wrong. Keels and rudders should be absolutely as smooth as possible. Even the difference between a reasonably smooth and very smooth foil is significant, especially if using a laminar flow section with a low drag 'bucket'. Bethwaite's book certainly has details of experiments which show this, and I suspect Marchaj's books do as well.

For canoe bodies the flow will become turbulent at some point, no matter how smooth, so there may possibly (but I don't know) be some advantage to shark skin textures. However, whatever you use will need to comply with the rules and regulations you race under - which will probably prevent certain substances which are harmful to the environment, especially those which release substances and need regular reapplication. Teflon polishes are quite common.

[nordvindcrew]

Thanks to Jahardmam and PI designer, now we're getting closer to the nub of the issue. I'm not familiar with either of the books referenced by PI Designer,but his comments are to the point. My boat is sort of canoe-like in section and general shape so there has to be turbulence involved. also as all our races are in open water with waves coming in all directions, it seems like the shark skin is not an option. Sounds like good design is still the most rewarding way to go. I analyzed my small boat (16' 8" ) by filling the mold with water to see what the real waterlines are and was horrified to see how bad they are as you go lower on the hull. Bad design mistake on my part. Modifying the mold now seems to be in order. 1/10 of a knot is so hard to get and returns so much in a race

[bewylde]

Unless specified, most of the boats i have refinished have been in Endura.(my product of choice) It is virtually bulletproof-with a very slick surface that nothing sticks to. In the water you can feel how slick it is compared with other finishes, This is also why it is the product that is used for painting planes and helicoptors-there is virtually no drag in the air, raining or not. I've been using this product for the better part of 25 yrs, and i have had guys with drag boats tell me they gained speed because of it.(maybe their imagination-who knows) It is more expensive than anything else, but worth it. If you need any info you can email me at b_wylde@hotmail.com