Theoretical MPH

[MGuckin]

Does this formula seem like it works? <br />A2/B2*C2*0.00095 = Theoretical MPH.<br />Obviously it can't calculate slippage.<br />A2 = Max. Actual RPM.<br />B2 = Drive Ratio.<br />C2 = Prop Pitch. <br />It is a streamlined version of the formula that has been mentioned here in the past.<br />Did I figure this correctly? <br /> <br />

 

[Down South]

Re: Theoretical MPH

Hi MG,<br />I'm not sure about your formula but if you need to know how fast your boat runs, borrow a GPS. It takes all the guess work out.

 

[MGuckin]

Re: Theoretical MPH

I am aware of how fast my boat goes. <br />I have GPS.<br />We seem to get alot of questions about how fast a boat should go.<br />I play with numbers and programming. <br />I am working on a simple template based program.

 

[Spidybot]

Re: Theoretical MPH

To make a formula that counts in all parameters, look at the online-book mentioned in<br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=013479

 

[Down South]

Re: Theoretical MPH

Hi MG, I sorta figured you did know your boat speed. The suggestion was for just in case. I play with numbers and programming too. You can plug both formulas into a program and do a comparison to see if the results are the same.

 

[Dhadley]

Re: Theoretical MPH

I have a speed-slip calculator we use. All you have to figure is the propeller RPM via gear ratio and it will tell you the slip %. Thats what I go by. If you reduce slip the speed will take care of itself and the GPS will show that.<br /><br />Its amazing how many people tell me their boats are going 75 MPH turning a 22" prop 5500 RPM. <br /><br />Right!!!

 

[what200]

Re: Theoretical MPH

that formula would work for theorectical speed but you may want to take the .00095 number out another tenth to be completely accurate because that would equal dividing by 1052 instead of 1056. But I guess that is just spliting hairs.

 

[JB]

Re: Theoretical MPH

There is another formula for predicting top speed if ideally propped and trimmed.<br /><br />Prop HP=A<br />Boat gross wght.=B<br />Bottom factor=C: 180 for V bottom, 200 for nearly flat, 225 for Flat.<br /><br />Take the square root of A/B and multiply by C.<br /><br />Example: My Montauk has 70hp, weighs about 1900lb. with max load. 70/1900 =.037. Sq. rt of .037 =.19,<br />Nearly flat bottom factor, 200 X .19 = 38.4mph.<br /><br />GPS WOT reads 38.5mph

 

[Cast-Away]

Re: Theoretical MPH

J.B.'s formula works for my boat. 75HP, 1500# boat nearly flat bottom. .2236x200=45 mph. I can top out at 46 mph.

 

[Dhadley]

Re: Theoretical MPH

JB - What do you use for tunnel boats?<br /><br />?

 

[JB]

Re: Theoretical MPH

Howdy DHadley.<br /><br />I think I would experiment with different hull factor numbers to try to match a known boat with a known WOT. I would start at 230 to 240 and adjust from there.<br /><br />That factor should then be applicable to other boats with similar hulls.<br /><br />Good luck.

 

[suzukidave]

Re: Theoretical MPH

not bad at all JB,<br /><br />for my boat: deepv = 180, 55hp, 1800lbs gvw, = 31.5 w-o-t -- actual speed = 32 (measured by my 1967 speedo). I guess I'm propped right.<br /><br />I still think its useful to use the theoretical formula to help find the right prop pitch to get that performance. <br /><br />Does anyone have any comments on what's the best minimum cost portable GPS system that can measure speed and show position?

 

[Jango]

Re: Theoretical MPH

Hey MG, your Formula looks OK<br />1/12 times 60 div. 5280 = .0009469

 

[JB]

Re: Theoretical MPH

Hi, SuzukiDave.<br /><br />No, "my" formula assumes ideal prop but doesn't give a clue what it is. <br /><br />The formula is not mine. I got it from a member at the Classic Whaler site. I think it was Tom Clark or Clark Roberts.<br /><br />The cheapest, simplest hand held GPS I have seen is my 5 year-old Garmin GPS38. It takes a while to initialize with only 8 channels, but after that it is pretty much on the spot. Speed readings are accurate. I paid $99 in about 1998.

 

[MGuckin]

Re: Theoretical MPH

Thanks.<br />Got more info than I thought I would.<br />Think I figured it out. I did this in a spreadsheet. Seems to work. <br />Assumes A,B,C,E & F are known.<br />Gives prop and mph.<br />

 

[Spidybot]

Re: Theoretical MPH

For those interested but not really into spreadsheets, I've put a small Excel file, based on Mike's work, on this page

 

[Dhadley]

Re: Theoretical MPH

Man! I go on the road for a couple of days and you guys dissapear! <br /><br />Leave some bread crumbs or at least a trail of oil next time you switch locations!<br /><br />LOL!

 

[JB]

Re: Theoretical MPH

Howdy, DHadley.<br /><br />I could have sworn you were on the distribution list of the Email that discussed this change.<br /><br />Glad you're back!

 

[Dhadley]

Re: Theoretical MPH

Aw.....they cant get rid of me that easy! Lol!

 

[suzukidave]

Re: Theoretical MPH

Guckin and UU<br /><br />The spreadsheet's almost perfect. I can suggest one more step to take some of the "theoretical" pitch out of it, at least for the same prop model.<br /><br />You need to create a separate set of inputs for a boat's current prop pitch, WOT rpm, gear ratio, and speed at WOT, to calculate the boat's actual slip and then apply that slip to your ideal prop spreadsheet to yield your ideal "actual" prop pitch (for your current prop model). It should be pretty accurate for props within 2 to 3 inches of pitch from your current prop before changes in prop diameter start to impact on the slip.<br /><br />Of course then you may need to introduce variables for the switch to other prop models, especially aluminium/composite to stainless.