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Author Topic:   Performance predictions and some rules-of-thumb!
Clark Roberts posted 03-27-2001 04:04 PM ET (US)   Profile for Clark Roberts   Send Email to Clark Roberts  
I'd like to share some performance prediction formulas! To get a fairly good(usually dead-on) estimate of a boats max. top speed, use the following formula: divide the horsepower by the gross vessel weight! Take the square root of this quotent and multiply by one of the following constants (the constant that fits your hull type!). Constants: 180 for moderate Vee bottom hulls; 200 for tri-hulls (I use 200 for all Whalers with sponsons),; 220 for flat bottom hulls. Bing, you will know what your estimated top speed is and can change weights and power on paper to see the effects! I have used this formula for years and never had it fail to be an accurate indicator. Merc Racing Division uses the same formula but different constants.. I have developed the above constants by trial and error over the years and they are very satisfactory! Most predictions will be within one mph of actual tested top speeds. The formula works only if motor actually produces rated power, weight is accurate, correct prop and set-up are present, hull is clean and straight (no hooks, rockers or defects...).. Try it, you'll like it! If predicted speed is not realized then the motor is not putting advertised power, weight is incorrect, hull is dirty or defective, incorrect prop or set-up or some combination of these factors. Some rules of thumb: 1. A change of prop pitch will net about 200 rpm either up or down 2. Raise engine as high as possible (must maintain cooling water pick-up 3. To determine max displacement speed (the speed above which a hull will try to climb out onto plane), take the square root of the waterline lengh and multiply by 1.25 (example> a boat with a 16' waterline will start making a huge wake and try to plane out at a speed of 4 X 1.25 or 5mph.. ) Speeds at or below this speed will be extremely economical as boat is passing through the water at "hull speed" (displacement)or below. This formula works best for true displacement hulls like huge cargo ships and sail boats but it's interesting stuff. Just thought you guys might benefit from some of this stuff.
Happy Whalin'.. Clark.. The Old Man and the Sea

blackdog posted 03-27-2001 05:14 PM ET (US)     Profile for blackdog  Send Email to blackdog     
It Works! I just ran the #ís real quick on 16 Dauntless
115hp/ 2000 = .05750 sq root of that = .23979 x 180 = 43mph. That about right. Unbelievable! Clark , did you say you used to work for NASA.


Clark Roberts posted 03-27-2001 06:51 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Blackdog, over the years I have held onto those things that work for me and am always interested in new ideas... as you have seen, this formula is a keeper! I have never revealed it to anyone (except of course on this forum)! Locally, I get calls from dealers and they ask me to tell them what hp is needed to achieve a certain mph on one of thier boats or to tell them how fast a certain combination will go... you see if you know any two of the three variables (weight, power, and speed) you solve for the unknown!
I don't tell any of these guys how to do it and want to keep it that way.. you have learned well, Grasshopper.... happy Whalin'.. Clark.. Spruce Creek Navy
Dick posted 03-27-2001 08:22 PM ET (US)     Profile for Dick  Send Email to Dick     
Great formula.
Looks like I can pick up a couple more mph with some tweaking.
Clark Roberts posted 03-28-2001 07:13 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Re: a question from "Post-Classics"... Gross weight is everything but the trailer (boat, fuel, batteries, tools, passengers, gear ..). It's important to estimate the full gross weight or calculations will be off! Of course it's best to actually weigh! Try taking off 100lbs. and re-run the figures... rule of thumb is that 100lbs is one mph (up or down).. not exact , mind you, just a rule of thumb. You can determine the exact effect of adding or removing weight by simply "running the numbers". More "rules of thumb"... It takes one hp per 40lbs of gross weight to bring a planing hull to minimul plane! It takes one hp per 30lbs of gross weight to achieve 30mph. Happy Whalin'.. Clark... The Old Man and the Sea
Peter posted 03-28-2001 08:52 AM ET (US)     Profile for Peter  Send Email to Peter     
Thanks for sharing that Clark, it really is a great estimator. For the second generation Outrage (Revenge) hulls (1978 - 199?), the 180 constant appears to be most accurate for its moderate vee. My 18 Outrage with 150hp Johnson with no tweaks standard set up usually achieves about 47mph. Formula predicts 47mph. My 22 Revenge with 225hp Yamaha with no tweaks standard setup usually achieves about 45mph. Formula predicts 45mph.
compounder posted 03-28-2001 11:16 AM ET (US)     Profile for compounder  Send Email to compounder     
Thanks for the formula Clark. Very interesting!

My first calculations were with an estimate of course for weight. Speed seemed very high,so going back I found I forgot to include my own body weight.

This now gives me an estimate of 46mph for my Montauk with 90hp. Seems a little high to me, but I've never had a speedometer and just recently purchased my first gps unit.

There's a landfill just down the road with a commercial drive-on scale. Of course, it will take two trips, one for the boat & trailer and one with just the trailer, but I'm going to get an accurate weight, then check out the actual speed with the gps. It'll be interesting to see how the actual compares with the calculated.

By the way, care to share how you came up with this? Read about it somewhere? Or,experimented on your own?


Clark Roberts posted 03-28-2001 12:31 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Joe, I'm not smart enough to come up with this stuff on my own. I was however, back in the late 50's able to determine what the question was, and with the help of an article in Popular Mechanics (or some other mag) the basic relationships of speed, power and weight were presented but the constants were not applicable to the boats we fiddled with (plywood skiffs with anything from a 10hp Merc Hurricane or a 22hp Jonhson opposed twin... etc..etc..) . The constant is the key and is derived emperically (by testing and/or observing)! Many tests (stakes and stopwatches) later the basic constants solidified. As newer hulls and hp rating methods evolved the constants had to be adjusted! I am at present trying to get enough data to determine the constant for the new twin hull (catamaran)style hulls(wish Whaler made one). As you can see, once the corrent constant is determined for a particular hull, then the effect of changing power and weight can be estimated with uncanny accuracy (see Peter's results above)! I don't know when Merc racing started using the formula but I think the may have been using it for many years...

caddis posted 03-28-2001 05:51 PM ET (US)     Profile for caddis  Send Email to caddis     
This post could not have come at a better time! I am about to take my 1981 Revenge V-22 in to the shop to get some fiberglass work and new power this weekend. I was vacillating on which motor to get. From the formula, I would not realize much difference in top-end performance between a 200 hp and a 225 hp. So, the question comes to this. Which would give me better fuel economy? I'm looking for brand and hp. I understand this could be subjective, but any help before I plunk down $11,000 or so would be a real help!
JimU posted 03-29-2001 12:24 PM ET (US)     Profile for JimU  Send Email to JimU     
My calcs show speed of 49 mph with boat and load estimate at 1500 lbs. As per our previous thread I'm getting 42 which you said was consistent with your experience with a 90 Yam and 16-7. What am I missing?.
sorcerer posted 03-29-2001 01:48 PM ET (US)     Profile for sorcerer    
7 mph !
jameso posted 03-29-2001 03:22 PM ET (US)     Profile for jameso  Send Email to jameso     
Clark, excellent post. This is a variation of formulas the Air Force uses to compute safe take off and landing distances for cargo aircraft. If you know horsepower or torque, gross weight ect. these things can be computed. We also use ambient temp and field elevation, both are variables concerning horsepower. MY 15 runs a bit slow according to your formula but matches the figures given by you and jimh in the reference section. I am only guessing weight. Also your caveat of "only if motor produces actual horsepower proper prop ect" is good. Parasite drag of hulls can be a real deterent to performance. In the days of the wooden navy a large ship would drag chains under the hull while underway to dislodge all the seaweed and other crud that would grow on a long (years!) voyage.
Thanks again for the great post. Jim Armstrong
Clark Roberts posted 04-01-2001 11:29 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Jim, I think some Yam 90's may not actually put out advertised hp.. this could be the answer (manufacturers will tell you that hp can vary plus or minus 10% of advertised hp). Now, it's a good engine and has very good low end torque and good hole shot with correct prop but all my 90 Yams ran low 40's with 15" props... wouldn't swing a 17" pitch... you can plug your speed into the formula and solve for hp! I have had 17's with 90 OMC's and Mercs and OMC 88's and 90's run mid 40's and a 90 merc will hit close to 50mph.. It's all a matter of actual power produced...etc..etc..etc.. Others may have had different experiences... Keep tweaking, who knows?! Happy Whalin'.. Clark .. Spruce Creek Navy

PS> interesting note: I know a fellow with a 15' Sport who got the following results (if my memory serves me correctly): with a 40hp Yam - 36mph; with a pro 50 Yam - 40mph; with a 70hp Yam - 46mph; with a 90 Yam - 47mph! He had better overall performance with the 70hp! I never drove the rig when it had the 90 on it, but the performance with the 70hp was awsome!

lhg posted 04-01-2001 08:54 PM ET (US)     Profile for lhg    
Regarding the 2 stroke 90's, the Yamaha is a little short on cubes at 70. The Merc 3 cylinder 90 has 85, and the older in-line 6 90 HP had 99, as do all of the OMC V-4's.

The Merc 90 is a real sleeper, and even by Mercury's own propeller charts, will stay with their 4 cylinder 115! I'll bet it's putting out more than 90HP. It is one of their most popular engines, and they sell tons of them. A lot of Montauks come out with this engine installed. Weight is 305 lbs, so it's a lot heavier than the smaller Yamaha. But it's likely the Yamaha gets better gas economy.

Lil Whaler Lover posted 04-03-2001 12:51 AM ET (US)     Profile for Lil Whaler Lover  Send Email to Lil Whaler Lover     
Clark etal:
The magazine you recollect may have been a product of Popular Science and it was called "Outboard Boating Handbook". I just found it (after many years) and I think it may be the same resource you refer to. I have a few other formulas etc that may be considered a "rule of thumb." Some caution may be needed in applying them as they were developed when horsepower was crankshaft rated and boats were somewhat less sophisticated in design. I have to say that while biting my tongue because the early 13 and 16 Whalers were very sophisticated designs and the Penn Yan Swifts from the 1950's may have the most complex hull ever designed. That said, here are some things to ponder.

Pounds/HP required to achieve a specified speed: 45-1 = 22 mph, 40-1 = 25 mph, 35-1 = 28 mph, 30-1 = 31 mph, 25-1 = 35 mph, 20-1 = 38 mph, 15-1 = 42 mph, and 10-1= 45+ mph. I actually these to be quite close on the 13 and 16 Whalers that I previously owned. This is from a graph that specifically refers to average boats. They also graphed a line for "fast" boats that shows about 4 mph faster at each ratio, and a graph for racing hulls that reads about 12 mph faster at each ratio.

They also published a formula to estimate the change in boat speed from a change in horsepower. This may be helpful is a repowering situation. I have not yet compared it to Clark's formula which could be used for the same purpose. The formula is as follows: divide the new horsepower by the current horsepower and then subtract 1 from the result. Multiply this by 40% and then add 1 back to that result. Multiply that by the current mph and the result is the new mph. A simple example would be to calculate changing a New Sport 13 from a 25hp to a 40 hp. 40 divided by 25 = 1.6, then 1.6 - 1 = .6, then .6 x .4 = .24, then add .24 + 1 + 1.24. This result means that your speed would increase by 24%. Perhaps some members of the forum could compare this to real world experience.

This is getting too long so I will stop for now. Dave

rwest posted 04-09-2001 01:54 PM ET (US)     Profile for rwest  Send Email to rwest     
I've been searching the classic whaler site to see what I couold expect with a new 50 HP 2 stroke Merc on my classic 30 years old 16 foot Nauset. Using the formula shown with square root I come up with mid 30's(mph) for speed with a total weight of 1500 lbs. Does this sound about right? I had an identical boat back 30 years ago with a 4 cyl. 2 stroke Merc 65 and it could only get 27 kts. (31 mph). I'd be interested in hearing from folks with a similiar rig. Engine is being mounted in the moring so I am going to get a chance soon to see how she does.
Clark Roberts posted 04-09-2001 03:51 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Ron,35-36 is about right. I had a 1985 Montauk with a 1999 50 hp Merc 2 stroke and it would do 36 with one person aboard. The new engines are rated at the prop and the old 65 was rated at the crank and if yours didn't have trip and tilt that alone would make a big difference... let us know how she goes! Clark
lhg posted 04-19-2001 05:18 PM ET (US)     Profile for lhg    
Clark - Is there a factor to use in your equation regarding single vs twin HP. I. E. twin 100's vs a single 200? Most seem to think that smaller twins would be less efficient than a big single. Thanks
Clark Roberts posted 04-19-2001 09:51 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Larry, the following may not hold exactly true (again a "rule of thumb") but try the following (all things being held constant like same boat and same set-ups): Twin engines will use one third more fuel and loose one third of performance (accelleration and top speed) compared to a single of same total hp. This is due to extra weight and lower unit drag of the twins! I have never checked this out and don't know the source... others may some better comparison.... If you know of two identical boats , one with twins and another with a single of same total power, it would be interesting to find out how well the above holds true! Clark
lhg posted 04-19-2001 10:16 PM ET (US)     Profile for lhg    
Acually, Clark, the formula, if I'm doing it right, is not working for my 25 Outrage, even considering the 400HP as a single. I used 5000 lb loaded weight, which would include some fuel and 2 persons. Using the 180 V hull constant, I get 52 mph. I have DGPS clocked it under these weight conditions at 58-60.

Admittedly, I'm set up with bracketed, lifted engines and 23" vented Merc Laser props, which could change things. Maybe either a higher factor is necessary, or the Merc EFI's are putting out more than 200HP each (I have suspected this). I even have the radar arch, which I remember when it went on, hurt top speed.

Clark Roberts posted 05-19-2001 11:33 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
keeping this thread alive for some recent perfoamance and power questions!
compounder posted 05-19-2001 01:25 PM ET (US)     Profile for compounder  Send Email to compounder     
Clark, I finally got my rig weighed at the landfill scales. I had to go back with the empty trailer to find out my boat weight, and I was somewhat shocked to find out that the rig with me in it hovered around 2000#!!

I carry a lot of stuff, including kicker,extra fuel, dual batteries,tools, radios, etc. so this may be higher than some other Montauks.

Using your formula with the new weight, it comes out to 42.4mph. At 5400rpm WOT I gps'd at 39.7mph. Real close, especially considering some other factors.......Yam 90 not "fully" broken in (approx. 25 hrs), large custom bimini top deployed, fairly rough hull surface of anti-fouling paint, & not quite sure where the best trim position is yet.

I'd say this makes your formula look pretty darned accurate!


jameso posted 10-10-2001 09:26 AM ET (US)     Profile for jameso  Send Email to jameso     
This is too good not to review occasionally.
JFM posted 10-10-2001 11:12 AM ET (US)     Profile for JFM  Send Email to JFM     
Clark, I thought I was having problems with my new Montauk. I took it to a yamaha dealer and he checked it out and told me everything was OK. When I applied this formula with the added weight I had on the boat it came up dead on. Thanks for bringing this thread back Jamseo and thank you Clark for sharing it with us. Jay

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