Formula to calculate theoretical speed

[Capt. Bob]

Found this on the web and it looks like fun as well as informative. I want to calculate my predicted/theoretical speed and compare it to different prop sizes now that I have a tach installed and know that I need to correct the pitch. What follows is a copy of the web page.....................................What do you think?<br /><br />Here is a handy little formula to calculate your theoretical speed for a given RPM. Us Mechanical Engineers have nothing better to do than to come up with this stuff. How close is your actual speed to the calculated? For my boat it was off by 1 MPH! <br /><br />(Prop Pitch x RPM)<br />------------------- = Theoretical Speed in MPH<br />(Gear Ratio x 1056)<br /><br />Example:<br /><br />Prop size = 15" x 17"<br />Gear Ratio 1.68<br />WOT = 4,000 RPM<br /><br />(17 x 4,000)<br />------------- = 38.33 MPH<br />(1.68 x 1056)<br /><br />My actual was about 37 MPH

 

[smalloutboardsarecool]

Re: Formula to calculate theoretical speed

WHy did you Post that here?

 

[Capt. Bob]

Re: Formula to calculate theoretical speed

From one who posted "I almost lost a finger today; pushing that little thing that opens the Butterfly valve thats about 1/8" away from the fly wheel!!!!!! " I don't think it is so off topic.<br />

 

[MGuckin]

Re: Formula to calculate theoretical speed

Capt. Bob,<br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=25;t=000002 <br />Agreed, sometimes too much time on my hands.

 

[JB]

Re: Formula to calculate theoretical speed

Howdy, Capt. Bob.<br /><br />An interesting and useful post for the Non-Repair Forum. It certainly isn't an Outboard Engine Trouble.<br /><br />If you were 15 I wouldn't comment.<br /><br />Cheers.

 

[Capt. Bob]

Re: Formula to calculate theoretical speed

In defense of posting here:<br />I bought a boat and was told that it would run 45 mph and pull two skiers.....standard presale lingo. I doubted that my motor and hull combination would produce that speed but if it would, I wanted to find out if I had an engine problem, prop, trim, etc. or not. I started with installing a tach to diagnose my engine to prop size and be able to track corrections. Now I am investigating a theoritical speed to see if I have and engine problem with loss of power from the original specs. I know I am "backing" into a hoursepower loss senerio....but I don't have access to a dyno. Sorry to have posted, what seems to be a gray area post for this forum, but I thought sharing the formula would be helpful to others diagnosing their engine output.<br /><br />As for the comment, "If you were 15 I wouldn't comment."....cheap shot.

 

[JB]

Re: Formula to calculate theoretical speed

Well, Bob, I was offended by your cheap shot at Sailboaty, who is 15. So now we're even.<br /><br />If you have an Outboard Engine Trouble, this is the place to ask a question about it. Your post is very helpful to folks who don't know if they have a trouble or not.<br /><br />What is your trouble?

 

[Capt. Bob]

Re: Formula to calculate theoretical speed

JB, since you asked....<br />My trouble seems to be from one who has assumed command of this forum and protection of selected members who post superfluous and multiple non related posts to this forum. Looks as though you have a groupie "bumper" to the "Suggestions for new posters" post you originated. Appears to be a great way to boost you status. <br /><br />It would be helpful to know, if or who the moderator is, so that I won't step on anyone's toes next time. Also, rather that replying to posts of questionable nature, I will ignore them as has been suggested. I would hope that you and others will follow this suggestion also. <br />Respectfully,

 

[JB]

Re: Formula to calculate theoretical speed

Hi, Capt. Bob.<br /><br />I am not "in charge" here, Admin5 is.<br /><br />When a member ridicules another I am going to speak up, as just another member, which is what I am and all I am.<br /><br />I try to advise posters when they have posted in the wrong forum. I try to be courteous, but I am direct. To some folks that seems bossy.<br />It comes from 50 years of teaching.<br /><br />When I asked you what your trouble is, I was talking about your outboard engine, not looking for a quarrel.<br /><br />How can I help you with your engine trouble?

 

[Capt. Bob]

Re: Formula to calculate theoretical speed

JB,<br />I am also direct and from Oklahoma...maybe that explains it?<br />As it turns out I don't have trouble this time only concerned that I might......but next time I think I have trouble.....I think, I will ask you.