Posted by Don Dando on November 25, 2002 at 09:56:39:
In Reply to: How can you estimate speed by horsepower? posted by steve on November 24, 2002 at 08:44:01:
It would be my OPINION that such speed quotations are for general comparisons and not intended to be a scientifically accurate statement. I think the ESTIMATES are based on a stock lower unit (Not a racing unit) and a utility prop as supplied by the engine maunfacturer on the engine when sold new. Also, we found when racing that barometric pressure, temp, humidity, wind and water conditions effected top speed.
We had about 40 props some for the same boat/engine combination and we maintained performance charts for each of them based on the weather and water condition. Eventually we collected enough data that we could fairly accurately predict the best prop for the conditions for a given heet of racing. We had a recording barometer that we kept in the motor coach for reference.
The point? There are so many variables, many of which I have not even covered in this reply that I beleive quoted speed is just a generalized statement.
Don Dando
: Good day.
: I wonder how people come to say a baor will do XX MPH with a certain motor size.
: I have researched about props and have come to find that HP is not really the factor for speed.
: For instance, My 6hp at 4500 rpm with a 7 pitch prop can theoreticaly go 14 MPH
: My 10HP at 4200 rpm with a 5 pitch prop would only go 12 MPH.
: That is why i ask , How can someone say how fast a boat will go without the Details of the engine?
: If anyone knows of a link that i may learn more about props, i would be greatly appreciative.
: Steve