Tomcat. First off, I must say you sure are the Rokit Sientist of the OSO with out a doubt. But on a techinal note (or better yet question). The other night on the SV Dragboat competitions they were mentioning water temp and the density of water affecting the speed due to prop lock-up. What are your thoughts on that with respect to the calculations? Would the prop slippage be less (if you would) with colder water which I guess would be more dense? Ie. Would the boat speed in 70degree be better than in 85degree water?

Bob

It sounds like what i need.

Hi Tom,I was waiting for someone to drop the water density curve ball into your equations All in all,I think it is a great idea to make it run web based.Come on Jeffery,let him have at it.The TOMOSOCALCULATOR sounds like a great addition to the site.

Actually, I figured the density question would come up with respect to sea water/fresh water. But here goes; first let me put on my propellerhead hat.

I checked my books and a drop from 86 to 68 degrees F would only increase water density by 0.35%. Is this small increase going to make a noticeable difference? I don't know. Considering all the sources of variability in a drag boat race, I wonder if it is possible to prove that a speed increase was due to reduced prop slippage due to such a small increase in water density. Since air and water temperature tend to go down at the same time, a corresponding reduction in air temperature is going to have a much bigger effect on engine horsepower, which seems a more likely explanation for extra speed.

However, the density of sea water is 1.025 g/cm3 at 15 degrees C, which is an increase in density of 2.6% at that temperature. This is a much bigger effect, and it is common to hear people claiming slightly higher speeds in salt water. Is this due to less prop slippage or increased buoyancy/reduced wetted surface? I don't know, but I can believe that this effect is noticeable and repeatable.

I don't plan to try and allow for either of these effects in the computer program, because there are other sources of greater variability. But it was a good excuse to put on the hat.

I think the saltwater thing is not valid for a planing hull. Once on plane I don't think buoyancy is much of an issue. On a displacement hull it's another story, I think the difference would be significant. I personally have never seen a difference in speed.

Sounds like salt water is another pointless incremental improvement! One thing that reinforces the "salt water is faster" theory is when you buy a "true" 80 mph boat in Florida, and it only goes 75 mph when you get it home on the lake.

cobra marty - Check your email.