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Author Topic:   Predicted Speed
dogfish2 posted 05-08-2003 10:50 PM ET (US)   Profile for dogfish2   Send Email to dogfish2  
Would someone please tell me where I can located Clark Roberts' Formula on predicting speed, using weight, motor h.p., propeller, etc. Thanks much.
JBCornwell posted 05-08-2003 11:19 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Dogfish.

As I recall, Clark's formula is to estimate theoretical top speed, assuming ideal prop and rigging.

Square root of HP/weight (gross)
X bottom factor.

Bottom factor is variable, flat bottoms (at stern) get 225, High deadrise gets 175, others between.

I used 225 bottom factor for my Montauk and was stunned.. . .it was within .5 mph of my GPS top end.

Red sky at night. . .

dogfish2 posted 05-09-2003 01:13 AM ET (US)     Profile for dogfish2  Send Email to dogfish2     
Thanks for the quick reply. I have a 2000 Montauk with the console and RPS. Would you know the weight? I can figure the rest of the equipment.
JBCornwell posted 05-09-2003 07:17 AM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Dogfish.

Your boat, dry, is listed as weighing 950lb.

You need to add engine, controls, fuel, gear and passengers.

I estimated my DF70 Suzuki powered Montauk gross weight at 1800lb and used 200 for a bottom factor. The 225 factor I quoted above was an uneditable error.

Red sky at night. . .

Clark Roberts posted 05-09-2003 08:16 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
JB, I too, find that Montauk predicted speeds come out right on the mark with the 200 "hull factor" (K = constant). It is very important to get the gross weight correct as the weight is the swinger and also there is said to be a +/- 10% horsepower variation from advertized! So since weight estimate can be off and hp can vary a little results can also be skewed! Also interesting is to use the formula to predict effects of added/reduced weight, added/reduced hp etc.. Also summer hot weather can rob some hp... bah, blah, blah...
jimh posted 05-09-2003 08:51 AM ET (US)     Profile for jimh  Send Email to jimh     
See the Reference section and the article on propellers where there is a careful explaination of the formula and its elements:

We call this "Clark Robert's" formula because Clark introduced it to us, but I don't believe he was the first to come up with it. It is widely known in the boating literature under another name, which escapes me at the moment.

The formula does not use any parameters of the propeller as a variable. It simply relates speed potential as a function of horsepower thru use of a hull constant and total weight.

dogfish2 posted 05-10-2003 12:20 AM ET (US)     Profile for dogfish2  Send Email to dogfish2     
Thanks Clark and JB! The formula is amazingly close to the top speed that I can get with the 115 4s on the Montauk. It's nice to have a reality check and not be misguided by lofty expectations. The Merc 4s seem to require a lower pitch prop than a 2s or even a 4s Yamaha in order to reach the maximum rpm's. I still have some fine tuning to do. I appreciate all your comments and suggestions.
Clark Roberts posted 05-10-2003 08:05 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Jim is correct and I take no credit for the formula (not that smart). I first read of this basic formula in (if my memory serves me correctly) Popular Mechanics back in the early 60's most likely. Mercury Marine's Racing Division uses same formula and only differences are the hull factors (constants) used. Back in 60's the hp was crank rated (it's prop rated now) and there were virtually no deep V's so no constants available... My only contribution is the fine tuning of the constants to get better (more accurate) results. As the referenced article (above) states, the recommended constants are 180 for semi-V, 200 for tri-hull (classic whaler) and 220 for flat bottom hulls. Back in the 50's and 60's we had no reliable/accurate boat speedometers and we put stakes in shallow bay to mark measured mile and used a stop watch to clock speeds. This too, was prone to some human error so we had to make repeated runs. Now days we have the GPS which makes it all sooooo easy. Happy Whalin'... Clark... Spruce Creek Navy
Richard Quinlivan posted 05-10-2003 03:09 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
This formula is known as Crouch's formula. See the folowing url for the FAQ file for the recboats news group.
I have also seen it in Dave Geer's Propeller Handbook referred to as Crouch's formula.
Peter posted 05-10-2003 09:22 PM ET (US)     Profile for Peter  Send Email to Peter     

Now that I have a classic 15' hull in my fleet, I followed your example in the referenced link with quite a bit of interest.

I am a bit curious as to whether others think the hull factor constant of 200, as used in the example, is the best hull constant factor to use for the 15' hull. I think that the 15' hull form more closely approximates a moderate vee hull than a tri-hull and so the hull constant factor of 180 would seem to be the better choice.

Using 50hp, 1150lbs gross weight and 180, the formula predicts a top speed 37.5 mph. My observed top speed using GPS over multiple runs is 36.7 mph. Interestingly, if I use 48hp (the engine is a Johnson 48 SPL), the formula yields a predicted top speed of 36.8 mph.

dogfish2 posted 05-11-2003 09:06 PM ET (US)     Profile for dogfish2  Send Email to dogfish2     
Here is a report on "predicted speed" versus "actual speed." Ran my 2000 Montauk with a 115 Merc 4s. Did the calculations using the formula provided and the predicted speed came out at 47.8 mph. Actual speed was 46.8 mph (GPS). If you factor in the unknown variables such as tide, wind, surface chop, h.p. allowed difference of 10%, I would say that this formula is as close as one could possibly expect. Thanks for all your help, guys. Len

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