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Author Topic:   REFERENCE Article: Propeller Basics
jimh posted 03-30-2001 07:04 AM ET (US)   Profile for jimh   Send Email to jimh  
A new article in the REFERENCE section describes how to calculate the maximum speed potential for a particular propeller and engine combination. See Propellor Basics

Please use this thread for comments or follow-up questions.

jimh posted 03-30-2001 07:16 AM ET (US)     Profile for jimh  Send Email to jimh     
Although this article is quite basic, I hope to add some additional material in follow-on articles.

I though that demonstrating the process of calculating the maximum potential speed might be of interest to readers who were new to the topic.


triblet posted 03-30-2001 10:28 AM ET (US)     Profile for triblet  Send Email to triblet     
The "Determining Boat Speed" section should
discuss the necessity of two-way runs.


jimh posted 03-31-2001 08:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Chuck makes a good point: when measuring boat speed there can be external influences which will affect the measurement.

Wind and current are the greatest influences. By making two runs over the same course in opposite directions, the effects of wind or current should be diminished.

Since the measurements in the article were made on an inland lake with no current and no wind present, they were not compensated for wind or current effects by being repeated in the opposite direction.


jimh posted 05-25-2001 10:53 PM ET (US)     Profile for jimh  Send Email to jimh     
The article on Propeller Basics has been expanded. There is now a second part which discusses:

--propellor materials
--propeller theory and slip
--speed prediction.

jimh posted 05-25-2001 10:56 PM ET (US)     Profile for jimh  Send Email to jimh     
Perhaps in another month or two, we'll be ready to discuss "bow lift" and how it is actually accomplished! :-)


jimh posted 05-29-2001 09:10 PM ET (US)     Profile for jimh  Send Email to jimh     
I extracted some performance data for the Whaler 26-Conquest and added this to the propeller article.

This is a test using twin 200-HP Optimax engines and MIRAGE 3-blade props.

Jem Wrenolds posted 06-05-2001 11:47 PM ET (US)     Profile for Jem Wrenolds  Send Email to Jem Wrenolds     
Greetings: Reference: Propeller basics;

I'd like to share a simple "english" formla to calculate theoritical hull speed and slip factor;. With this formula, you can calculate with a pencil, or a calculator or a computer spred sheet.

Eng RPM divided by transmission ratio equals..........Propeller RPM.

Propeller RPM times propeller pitch (in inches) equals inches of fwd travel per minute. Divided this by twelve to get feet per minute

Feet per minute times 60 equals feet per hour.

Feet per hour divided by 5280 equal the theoritical speed in statute miles per hour.
Feet per hour divided by 6072 equal the theoritical speed in nautical miles per hour.

With the above, one can build a spred sheet that, with the insertion of three data; RPM, gear ratio, pitch: can in an eye blink calculate any combination.
Such a spred sheet can also add numerous formulas to calculate slip factor, fuel consumption at various rpm, legnth/beam ratios, hp/weight ratios, and much more.

If ContinuousWave can support an email attachment of lotus 123 I will be glad to share

Jem Wrenolds

jimh posted 06-06-2001 01:47 PM ET (US)     Profile for jimh  Send Email to jimh     

Welcome to the forum. The technique for calculating the various parameters you mention in your posting is pretty well described--at least I thought it was pretty well described--in the PART 1 section of the article.

As for Spreadsheets, well, in fact I have one that I use to calculate the numbers.

By the conclusion of this article series I hope to have a working on-line version of the spreadsheet, so people could plug in their numbers and see the results, and also to be able to accumulate these results so others could see them, too.

Now, can you explain how bow lift if accomplished?


jimh posted 02-03-2002 11:37 AM ET (US)     Profile for jimh  Send Email to jimh     
A separate article whaler/reference/performancePrediction.html now presents the table of performance predictions calculated by Peter Elke.
13sport posted 02-05-2002 11:19 PM ET (US)     Profile for 13sport    
I believe that the performance predictions are very unrealistic. From my lurking in the background of this forum I would say that the majority of contributors do not come close to weighing 170 lbs. That assumption could easily underestimate the weight of the boat by 60 –80 lbs (2 people). Thus the estimated performance is overstated.

OK guys, Fess up.

I am in at 200 lbs.


peteinsf posted 02-06-2002 01:06 AM ET (US)     Profile for peteinsf    
Hmm, I took that from FAA guidelines for average pilots weights. Maybe the Coast Guard has official average boater weight. I know my local cops seem bigger then 170, maybe everyone is heaver then 170? I’ll have to write the FAA (grin)
Smallfrye posted 02-07-2002 05:34 PM ET (US)     Profile for Smallfrye  Send Email to Smallfrye     
Who owns the boat listed as Calc test ?
I would like to compare notes with one I have now in the shop ?
Thanks, Jim
lhg posted 02-07-2002 07:06 PM ET (US)     Profile for lhg    
Pete: The calculations on "Whale Lure" don't quite work out. First of all, each 200 Merc EFI weighs 415#. I assume you're doing all of these calculations with a full tank of gas, but even so, I calculate total rig weight, full, as about 5500#. But I do know that with a 1/2 tank of gas, and 2 persons on board, the boat will turn 21" pitch Laser II props up to 6000RPM, which translates to 60mph. The boat is not a Whaler Drive, but instead, a full transom with Armstrong bracket. This system results in higher engine mounting heights (less lower unit drag) and no increased bottom drag from the Whaler drive. On plane, the bracket runs clear of the water. There is also about 100# less hull weight.

My general reaction is that these calculations are pretty good, maybe a little on the slow side. There are just so many variable factors, including use of bottom paint, engine brand (rated HP can be within a + or - range), props used, wind resistance items such as arches, tee tops or canvas, engine mounting height, etc.

Thanks for doing a nice, thorough job on this.

Hank posted 02-11-2002 10:00 PM ET (US)     Profile for Hank  Send Email to Hank     

I'm a bit unclear,in your discussion, about the effect of boat speed on "angle of attack". It is my understanding that angle of attack of a propellor blade is the angle the blade makes with the direction of motion of the boat. I don't see what that has to do with boat speed.

Am I missing something ?


jimh posted 02-12-2002 12:00 AM ET (US)     Profile for jimh  Send Email to jimh     
I believe the effect is related to the speed of the water that is flowing into the propeller.

When the boat is not moving, the water around the propeller is stationary. As the blade begins to to rotate the effective distance between the leading edge of the blade and the trailing edge of the blade is the the same as the physical separation between them.

As the boat builds up speed, the water is moving into and past the propeller very rapidly. The result of this is to effectively reduce the separation of the leading edge of the blade with the trailing edge of the blade, in effect changing its angle of attack.

A good illustration would help. [Help!]

jimh posted 02-12-2002 12:20 AM ET (US)     Profile for jimh  Send Email to jimh     
Quoting William G. Van Doren from Oceanography and Seamanship ( an excellent book ):

"Contrary to popular opinion, a propeller does not screw itself through the water, but, rather, flies through it. Like an airplane wing, the propeller blades are shaped hydrofoils, and are designed to have a maximum lift (thrust) to drag ratio when operated at the proper angle of incidence (THETA) to the relative flow. This angle... is shown in [illustration not reproduced here].

"When rotating in a real fluid, owing to flow advance in developing thrust, the propeller does not advance by (PITCH feet per revolution) but by some smaller distance....

"The which a working propeller lags behind the advance it would have if delivering no thrust is called slip.

"...the working propeller's effective incidence angle is...[much math omitted] in plain English,

angle of incidence = pitch angle - ARCTAN(inflow_velocity/angular_velocity).

For the complete math and illustrations, see p.299-303.

It is really quite complicated, so I just take it as gospel.

jimh posted 02-12-2002 12:29 AM ET (US)     Profile for jimh  Send Email to jimh     
By the way, I bought that Van Dorn book about 20 years ago at an electronics flea market, and I have treasured it ever since. It is amazing what you can find in there, presented in mathematical terms with well written analysis. It covers everything to do with ships and seamanship. I believe there is a newer edition out now, but rather more expensive than the $3 I paid for my copy.

Back to the forumula, it shows that as inflow velocity increases the effective incidence angle decreases, QED.

After this point, I defer to others to pick up the discussion.

I also think there is a reason most propellers are painted black, and that's because of the magic they contain.


jimh posted 02-13-2002 08:40 AM ET (US)     Profile for jimh  Send Email to jimh     
Looking at the predictions and comparing two boats I've been onboard, my own and WHALE LURE, I think there is more difference between them than the prediction would imply.

I re-calcuated the prediction for WHALE LURE based on the twin 200 Mercs having a bit more horsepower than the decal on the cowling says they have, the hull weight being lighter because it is not a Whaler Drive, and the hull factor being slightly higher due to the bracket mounting of the engines. This results in a predicted speed of 50.2 MPH, which is closer to observed results but still below numbers that I have actually seen on the speedo onboard.

In making this table of calculations, Pete used 8-lbs./gal. as the fuel weight, which is slightly higher than the actual weight.

Various sources cited the density of gasoline to be between 0.75 and 0.85 grams/ml, which converts to 6.25 to 7.0 lbs/gal. by my calculation.

There are also other factors which can affect the performance prediction's accuracy. Hulls that have bottom paint perhaps should use a slightly lower hull factor to account for the rougher surface.

Twin engine boats may run slightly slower than a single engine boat of the same horsepower due to additional drag from the second lower unit, although perhaps greater propeller efficiency may help equalize this.

Interesting to see is how much more horsepower it takes to push the speed up, since the predicted speed increases only in proportion to the square root of the horsepower. To double your speed you must increase the horsepower four fold.

Hypothetically spending about $15,000 to repower my boat to 115% of the maximum horsepower rating only bumped the speed up about 8 MPH. Those are expensive miles-per-hour to pay for!


peteinsf posted 02-16-2002 11:03 PM ET (US)     Profile for peteinsf    

Thanks for the details, I specifically estimated Whale Lure because I recalled a report of 60+ and thought it might spur discussion. One thought I had; Once a boat is airborne that its drag factor might change.

1st=hull-speed 2nd=on a plane 3rd=75% out of the water.

Not all boats have the raw power for the 3rd level.

I will update the fuel weight and remove Lure’s "whaler drive" that might get it closer. I suspect the real problem is that your 200 EFI's are real rockets. Did your older carb 200's run as quickly?


PS, I will also add the Post Classics now that I have the specs.

lhg posted 02-18-2002 09:21 PM ET (US)     Profile for lhg    
The difference between my previous carb 2.4 liter Merc 200's and the new 2.5 liter EFI 200's is about 300 higher allowable RPMs (5500 Vs 5800), adding about 2 or 3 mph in speed. The older 200's were pretty fast, but not as quiet, with the EFI eliminating the carb induction noise, which faces the cockpit.

But there are other things that can goof up the formula. Engine mounting height, thereby reducing lower unit drag, is a factor. The bracket allows this to be about 4" higher than transom mounted engines, believe it or not, and still keep the water pickups submerged and not inducing slip. The engines are running in the water "swell" coming off the boat. Then this is combined with the surface piercing Laser II props, adding bow lift, which also reduces bottom drag. Here are some very interesting figures, which I got from the factory back in 1989:

Outrage 25, Whaler Drive, 400 HP: 50mph
Outrage 25, notched transom, 400HP 55mph
(admittedly 100HP over rating)

I didn't know what I would get with my bracket mounted installation, but it turned out to be 60mph, maybe a little higher. It's the reduced engine drag, no bracket drag (like the Whaler Drive has), and performance props, that do it.

The radar arch took a couple of MPH off the top end when added. A real top end killer is a Tee Top, or running with the full Mills canvas up. I try not to go over 40 when canvas is set.

peteinsf posted 02-19-2002 08:32 AM ET (US)     Profile for peteinsf    
Very cool Larry, I wouldn't expect a 10MPH range of difference with the same motors.

I don’t think I am going to rip out my Whaler Drive right away, maybe after I buy new motors (with my next lottery ticket proceeds)…

Her is the motor mounting instruction I received from Chuck. Bennett.

“Engine properly secured to hull: Washers provided by engine manufacturer must be used for installation. All thru-bolts securely tightened.

Marine sealant is to be applied to the shank of bolts before being run through the transom. Sealant is to run out from under the washers when bolts are tightened.

Bolt length: At least one full thread is to be sticking through the nut.

Engine height: Ventilation plate is to be in line with bottom of boat.

Toe in: Measure the distance between prop centers and forward edge of the gearcase is to be 1/2" to 3/4" less than the prop center measurement.”

Can I make effective use of Laser-II props if I mount the motors higher then they recommend? Clearly not a high has Whale Lure’s with her bracket.

In the past I had cavitation problems when I mounted a V4 too high on my Montauk so I would like to get it right the first time. I assume I can drill the “transom” in such a way that I can adjust a bit up and down. I recall having to re-drill for non-adjustable motor bolt in the Montauk to move the motor back down…


JoeyP posted 02-24-2002 06:31 PM ET (US)     Profile for JoeyP  Send Email to JoeyP     
26 Conquest Test w/ twin 200 Merc Opti's: "3000RPM 27.6 9.7 64.9 (Wake must be huge!)" Would a change in pitch,diameter or both help alleviate this?
jimh posted 02-25-2002 12:50 AM ET (US)     Profile for jimh  Send Email to jimh     
Re CONQUEST 26 Test Results: This seems to be a "feature" of some of the newer large Accu-Track hulls: they don't have much of a slow-speed plane. They are either plowing along with the stern squatted down (9 MPH) or jump to 27-MPH plane, all with just a 500 RPM difference in throttle setting.

One thing that might help is trim tab adjustment, setting them to raise the stern.

Often once you are on plane you can slow to a speed below where you currently are and maintain plane with some trim adjustments.

The problem can also be caused by the engine(s) not being able to produce enough power at the lower crankcase speed to keep the prop turning at the slow-planing speed, with the result the engine loses speed and the boat drops off plane. It is easier to maintain a slow planing speed if you have large engines that develop good horsepower at the lower RPM range.

All hulls have this tendency, but on some it is more pronounced.

lhg posted 02-25-2002 05:49 PM ET (US)     Profile for lhg    
Pete - I would install the engines with the anti cavitation plates about 3/4" above surface of water flowing under them. Laser II props will run fine under those conditions. On most brands of engines, each bolt hole is worth 3/4".

Those toe-in instructions you mentioned intrigue me. If I read it correctly, that would amount to toe-out. Can you check your writing there? I agree that toe-in is what is wanted, but I always used about 1/2" less than engine centerline mounting dimension. The dual engine tie bar actually has to be adjusted longer to accomplish this.

peteinsf posted 02-27-2002 04:11 PM ET (US)     Profile for peteinsf    
Thanks Larry,

I’ll start looking for the Laser II’s. I suspect my props will have to be 1” or 2” less pitch (then Lures) based on Whaler’s speed predictions with the Whaler Drive. As for the toe-in, if the front of the gear case is the “toe” Chuck’s instructions would have them pointing “in” to the center of the boat, “…the gearcase is to be 1/2" to 3/4" less than the prop center measurement.” Why do they need to be out of alignment anyway?


baracuda posted 06-15-2005 12:58 PM ET (US)     Profile for baracuda  Send Email to baracuda     
Hi there I hope someone can help me on this subject without thinking Im too thick.I hava a 2 6 cylinder Ford Engines in my boat 128 bhp,2800 do I work out what prop I need for it.I tried to use the calculator but Im afraid not very good at it.
I would apriciate if any one can help me.further info....the boat is 16m long, 4.50m wide,1.5meters water line and waighs 20 the moment I have 3 blade props on them but the engines sims to be running without an afford.and not doing a good speed.(9 knots)Im sure the props are to flat.
thankyou guys.
jimh posted 06-16-2005 12:16 AM ET (US)     Profile for jimh  Send Email to jimh     
Obtain a copy of PROPELLER HANDBOOK by Dave Gerr. He has over 100 pages of text, formulas, charts, and graphs which will assist you.

The Propeller Calculator is not an oracle, it just computes one variable based on the input of four others.

You may also benefit by reading my article PROPELLER TESTS. Therein I describe in detail the process of estimating propeller pitch for a particular boat and horsepower combination. See

You can apply this same methodology to your situation.

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