View Full Version : speed sanding bottom.

04-17-2004, 03:00 PM
I am interested in thoughts,opinions and info on speed sanding the bottom of a boat. Eg. does it work? what grit of paper? any info is appreciated thanks.

04-17-2004, 03:15 PM
what is speed sanding?

04-19-2004, 09:56 AM
this is the sanding of the aft 3-4 feet of the bottom of the hull. It is done to help reduce the tension of the water to the bottom. this in turn can give more speed. Eg; less drag=more speed and quicker acceleration. this trick was told to me by and old racer. I am looking for the info on the propper grit of paper to use. he said that at one time mercury had/did a bulletin on this procedure.

04-19-2004, 11:44 AM
Speed record boats built by Darrel Sorenson are finished as smooth as glass. Darrel's boats hold a lot of APBA speed records. You can listen to whoever you choose, I choose the man who builds the boats that set speed records.

Winger Ed.
04-19-2004, 03:39 PM
I'd tend to defer to Sam on this one.

I have no idea how effective it is, but I used to know a guy who ran (mud sucking) jet drag boat that did it. He used 600 grit 3M wet or dry. It was sanded just enough to break the gloss.

Being sanded that lightly, if it doesn't do ya any good, you could just re-polish the gloss back up.

Gordie Miller
04-19-2004, 04:38 PM

04-19-2004, 05:18 PM
There is such a thing as laminar adhesion, but the boundry layer is so thin it is insignificant. Flat and smooth is the way to go. If the laminar adhesion boundry layer were significant props and lower units would be dimpled.

When a significant number of speed records are held by rough surface equipment I'll be convinced. Until then I say that theory belongs with snake oil.

Gordie Miller
04-19-2004, 10:13 PM
a golf ball has dimples that are clearly visible to the naked eye. Take note of the finish on the Sportmaster gearcase, not only is it NOT a mirror finish, it is quite rough by gearcase traditional standards. I certainly don't think a golfball type surface is what we are after but a mirror finish is not the answer (IMHO) either. The bottom steps in most HP boats of the last ten years or so would tend to support the laminar adhesion principle. Bear in mind, these are basically theories as we (to the best of my knowledge) do not possess data acquisition equipment capable of telling us for sure what is best.

04-19-2004, 10:47 PM
I must agree with Gordie on this one.

Dave S
04-21-2004, 07:29 AM
How about Tunnas???:p

04-21-2004, 09:24 AM
thanks for the info on the 600 grit w/d. I saw r. summerford sanding the starboard sponson and waxing the port sponson on his stv prior to a race. he said it helped it turn left. Yes this sanding has to do with laminar adheasion,as per.the finish on the sportmaster and speedmaster cases. also the speed coatings have a satin finish. It could be snake oil but sand paper is cheep and getting more HP is not. So I will try it at worst I will buff back to glass finish. Thanks again and if any body has any more info or test this them selves please post .

Shaun Torrente
04-21-2004, 12:59 PM
I am with gordie also... on my boat i sand with 600 then do 1200 and a final of 2000 grit.....


Dave S
04-22-2004, 05:45 PM
That sanding is rite. A old Smart boat racer,who worked at interlux told me to take a wet rag and push it on both surfaces to see which is better. A easy test.

04-22-2004, 06:14 PM
Next time I'm racing across a wet rag I'll remember that:)

05-01-2004, 10:48 PM
Have you ever tried drag racing belt sanders? Its a lot of fun. You need a good belt sander, 100 ft of drop cord, 100 ft. of hard surface and a power outlet. Tip-- 40 grit to get off the line fast, 100 grit for better top end. Practise pushing the plug in the socket at the green light, for good reaction time, use lite weight drop cord.

Travis Fulton
05-02-2004, 07:52 AM
try doing some searches??? i have read many posts from racers about this very process,

Gordie Miller
05-02-2004, 08:03 AM
did you get my e:mail? Sent it a couple days ago about 15" mid with jet.

Travis Fulton
05-02-2004, 08:06 AM
nope??? fultonconst@hotmail.com

Travis Fulton
05-02-2004, 08:08 AM

05-02-2004, 08:14 AM
The 'golf-ball' dimpling is not effective at our speeds at all. Most important factor (by a long shot) is the 'flatness' of the surface. ABSOLUTELY, 100% DEAD FLAT is what is fastest. Once you have this, then 600 grit sand will slightly improve the laminar flow boundary layer, particularly at the leading edge. (This will be better than mirror or waxed finish). But 'flatness' will still be most important!

The whole issue of laminar flow is a very difficult one on powerboats. In displacement hulls, the surfaces exposed to drag are very predictable - the leading edge (most important part for drag generation) is well defined. In a powerboat, as speed changes, so does the 'wetted' surface area, and so does the location of the 'leading edge'. Since the configuration and surface condition of the leading edge is the most important, it's tough on a powerboat, since the leading edge is a 'moving target'.

So even though you can improve the drag at the leading edge of your planing surface by 'slightly roughening', it only helps at that one location - and you don't know where this leading edge location will be at any given speed. That is why the benefit is quite limited.

That brings another point...the trailing edge condition (of your planing surface - ie: sponson or pad) is also very key to induced drag. Should make as clean and sharp at trailing edge as possible.

Mini Max
05-02-2004, 08:42 AM
Buffing heats the surface and can cause the sustrate to postcure and shrink here and there. You do not want that.

Mini Max
05-02-2004, 09:02 AM
Wax is Slow. How slow?

Take a piece of something, maybe 2 feet square that replicates the surface finish of the bottom of your boat, whether gel coat or paint,. Sand the whole surface uniformly with 220. Then tape off a 2” by 24” stripe down one edge. Sand the remaining surface with 320. Tape that off. Sand next with 4 something. I get tired by the time I get to 600 but I am old. Buff, wax, polymerize, and goose grease, whatever, the remaining strips.

Now for the test. With the sheet flat, place a drop of water on each surface treatment. Slowly raise the panel. The drop that transits its surface treatment and that reaches the bottom of the panel first should identify what to do your boat bottom to go fast.

Where do you think the name “Turtle Wax” came from??

05-03-2004, 09:35 AM
Jim, how about explainin' how the old Allison hook, which was an almost undetectable turndown (less than 1/8" on some boats), right at the back of the pad figures into your "ABSOLUTELY, 100% DEAD FLAT is what is fastest" theory. There are other hooked pad V bottoms around too that have measurable speed advantages over their straight pad counterparts.

Mini Max
05-03-2004, 03:13 PM
I am a Jim also, so I guess that means I can give my opinion.

Rocker at the transom lifts the Bow up.
Hook at the transom lifts the stern up.

Trial and error defines how much of either works best for the speed/handling compromise for any paticular hull design.

You can't go wrong "starting" with "ABSOLUTELY, 100% DEAD FLAT".

Ted Stryker
05-03-2004, 04:15 PM
I know Surfers wax the bottom of their board for more "traction" on the wave..

05-03-2004, 04:37 PM
If memory serves Hydrostream tried sanded bottoms vs. smooth, and even shark-skin like bottms (sprayed gelcoat). The results and differences were mixed. A good driver and power set up is the best way to win

05-03-2004, 06:07 PM
Raceman - As MiniMax says...Rocker at the transom lifts the Bow up. Actually, Hook at the transom "pushes the bow down". Rocker is a "fix" that is usually added after the "straight" hull's performance issues are identified. It will provide the "lift" that is needed when you're balanced on the 'rocker' but is somewhat unpredictable when you 'fall off' the rocker point. The Hook will ALWAYS add unecessary drag. It may well solve a problem of "too much bow lift" - but I suggest that there are other, more efficient design solutions to such problems.

The most efficient lift/drag configuration for a planing surface is "ABSOLUTELY, 100% DEAD FLAT".

Still...after saying all this...I will still say that "there is no substitute for seat-time".

05-03-2004, 11:31 PM
Jim, my experience with fast V bottoms has proven that flat isn't always the fastest. I think the best way to describe some of the hooked bottoms is like a spoiler turned upside down that literally lifts the rear of the boat, unwetting bottom, while the boat remains relatively flat by comparison, thus blowing a smaller hole through the air.

Paul Allison (Darris' Father) drew me his little hook on a table napkin about 30 years ago. Sufficient power against a properly designed hook can litterally lift the boat off the water rather than planting the bow.

05-03-2004, 11:55 PM
If you wanna go faster by sanding, put on a bigger motor and sand down the cowl! Guaranteed to work!

05-04-2004, 12:27 AM
I don't know......my "equipment" sure works best on a well wetted surface.....that really gets the "laminher flow" going:D

05-04-2004, 12:36 AM
you PIG!!

05-04-2004, 12:41 AM
Wildman....from the looks of your picture off to the side there....the condition of your hulls "bottom" should not be an issue!!:D

05-04-2004, 12:55 AM
No, it's that SPECIAL hook I put in the bottom! It aerates the nose at higher speeds. Also, the heat from the flames under the front tend to lighten the nose!

05-04-2004, 12:58 AM
Enjoyed visiting your site....you've got some awesome looking boats there! ;)