posted 01-28-2008 04:48 PM ET (US)         

Bottom paint factor
Engine mounting height factor
10% horsepower rule factor, usually unknown (engine brand factor?)
Propeller factor (considerable)
Gear case factor
jack plate factor
Transom bracket factor (WD being a negative influence)

It seems to me that putting all this together and calling it a "factor" (constant) makes no mathematical sense, and makes the formula not relevant. The so called "hull factor" is really nothing but a fourth variable to be solved for when the other's are known.
Actually, all four figures in the formula are variable, with no real constant, which makes it useless. So the end result is solving for the "factor" (constant) instead. This is not the way it's supposed to be! The only way the formula comes even close to working would be by switching engines, of KNOWN HP OUTPUT, on the same boat, for your own re-powering estimates.

By example, lets take the hated Bass and Walleye Boats mag shootout of the 225HP engines. On the same hull (same Crouch hull factor), the Merc 225 Opti ran 90 MPH. The Evinrude HO ran 84 MPH. Explain that. You can't, except to say the two engines have vastly different HP output, which is probably not true. So the other "factors" I mentioned above must be the reason.

posted 01-29-2008 12:27 PM ET (US)            

Crouch's formula does an excellent job of predicting speed for a given horsepower and weight once you have determined the appropriate constant for your boat. Crouch has never provided guidance for how to develop a "constant" for a boat by analysis of the many items Larry mentions (such as bottom paint, engine mounting height, propeller, gearcase, jackplate, or bracket). Crouch simply gave some guidelines for very general categories of boats.

If a method or formula exists for predicting boat speed by using the factors Larry mentions, it is unknown to me. Crouch has never made a claim that he has provided this type of analysis, so to find fault with Crouch on that basis seems inappropriate.

posted 01-29-2008 02:41 PM ET (US)         

But so is Crouch's use of the word "constant". He may be a Naval Architect, but from my "Building" Architect and Structural Engineering perspective, he clearly doesn't know what a mathematical constant is. When you want to know the area or circumference of a circle you use a true "constant". His is no such thing.

In his formula, for correct results, you have to SOLVE for the constant. What? Then it's not a constant. It's a 4th variable in his formula. If you know speed, weight and published HP, you can solve for a GIVEN, RIGGED, BOAT'S individual constant. That's not my idea of a mathematical constant. Nor is the bare hull configuration the only force acting on the correct prediction of speed, as I have mentioned above.

posted 01-29-2008 08:17 PM ET (US)            

Crouch has given us a simple approximation for predicting boat speed based on two variables: weight and power. These are the variables and the other factors involved in this calculation are aggregated into a constant.

A constant is just that, a constant value. In Crouch's formula there is no variable for influences other than horsepower and weight. Everything else which effects boat speed has been turned into a simple constant.

The usefulness of Crouch's formula may not extend to boats operating at very high speeds, say over 60-MPH, or in the case of boats which undergo a dramatic transition from planing mode to very high speed mode where barely any hull is in the water. In those situations the influence of the hull changes, as at very high speeds not much of the hull will be left in the water. So it is natural that a "hull factor" or constant based on much greater immersion of the hull will not remain accurate when no hull is left in the water.

Before I can accept Larry's conclusion that this formula "is of marginal value and accuracy" it would be necessary for me to have something better to consider. In the absence of anything else which can predict boat performance, it is hard for me to discard Crouch's work. If you want to throw Crouch overboard, you need to have something to replace it.

If someone has developed a speed prediction formula which will take into consideration multiple factors, such as:

--bottom paint
--engine mounting height
--engine brand
--propeller
--gear case
--jack plate
--transom bracket

please be so kind as to contribute this information.

posted 01-30-2008 01:04 PM ET (US)            

Sight unseen, I would be hard pressed to come up with a model of a boat's performance without knowing EVERYTHING - not only the most obvious parameters like weight, horsepower, et al. - but also, the wetted perimeter, the frontal area, the surface roughness, the wind velocity and direction (relative to the boat's travel), the angle of attack, prop thrust axis, water conditions, et al. In short, not knowing all of those parameters, I could not produce a viable prediction model.

Larry addreses a point regarding a constant - which is apparently back calculated from a trial run. Such a constant will be applicable for ONLY that given condition - that boat, that loading, that speed, that engine attitude and speed, that water condition, that wind (velocity and relative direcction), et al. Perhaps a better word for that "constant" wwould be "specific form factor".

And I would bet you a steak dinner that no-one will fullfill your request:

If someone has developed a speed prediction formula which will take into consideration multiple factors, such as:

--bottom paint
--engine mounting height
--engine brand
--propeller
--gear case
--jack plate
--transom bracket

please be so kind as to contribute this information.

And then there is one thing that many engineers will never do - predict the future (performance, economy, et al.) from previous data. Use previous data to help develop a model incorporating all specific parameters - yes, but otherwise no.

But, Crouch is apparently very knowledgable - and if his equation works for you, use it, but realize it's limitations. ----- Jerry/Idaho

posted 02-19-2008 08:53 PM ET (US)            

Boston Whaler has excellent boat test data. I think there is no argument with the accuracy of the data--it's gathered by Boston Whaler engineers. The engines in these tests are Mercury Verado. Who could ever argue that the horsepower ratings of a Mercury Verado are not exactly as indicated on the cowling?

The boat test data comes from this document:

http://www.whaler.com/rec/pdfs/performance/20.pdf

a test of a 225-HP and 250-HP Verado engine on a 235 CONQUEST.

First we take the data from the 225-HP test:

Weight = 5,245
HP = 225-HP
Speed = 41.9 MPH

We enter this into Crouch's Calculator to derive a hull factor constant:

Hull Factor = 202.3

Now we use this data, but change the horsepower to 250, and let the calculator predict the speed.

Weight = 5,245
HP = 250
Hull Factor = 202.3

The predicted speed is 44.2-MPH. Now we compare that with the actual test data from Boston Whaler. Their test with a 250-HP motor produced a speed of 43.9-MPH. That is a difference of 0.3-MPH , which if you figure the percentage difference, is only off by 0.6-percent. I don't know about you, but being able to predict performance with an accuracy of 0.6-percent does not exactly sound to me like something I would call a "fudge factor."

posted 02-19-2008 09:24 PM ET (US)            

For the 250-HP test at 43.9-MPH we get 201.1.

For the 225-HP test at 41.9-MPH we get 202.3.

posted 02-20-2008 02:01 PM ET (US)            

But, in the end, doesnt it all boil down to OEM transom specs? Even tho these OEM transom specs may be calculated using Crouch and other Marine Engineering methods you guys are debating, the bottom line...

Will the lightest engine that meets max OEM HP rating result in the most efficient performance (top speed) without regards to the hull factor(s)?

mk

posted 08-14-2008 04:49 PM ET (US)            

Mercury Marine still uses Crouch's Formula to this day! They have modified (read "updated") it to reflect two things. The first is a conversion from knots to MPH and the second takes into consideration today's more efficient hull designs. If you are using it straight up, multiply the "Maximum Speed" number by 1.15 to get MPH. Next be aware that today's hull coefficient are a little better (read higher) due to the advancement of technology and design that has taken place since his heyday designing "Gold Cup" racers, and winning, in the 20's and 30's for Dodge, (Yeah, they boats too!), and Chris Craft.

But just because you have a newer boat that is still basically a runabout, doesn't mean that you plug in a "Stepped Hydro's" coefficient. Remember there is also a drive train loss of 5-10% between the flywheel and the prop as well.

Then do a speed calculation for both "Theoretical Maximum" and another one for "Prop Slip". A 300 HP engine will not spin a 28" prop at 5500 RPM with a 1.5 drive ratio unless it is in on a surf board, and you know that won't work!

The idea is to use Crouch's formula along with prop formulas to zero in on where your boat fits. If Crouch's formula says your boat won't go faster than "X" MPH, it won't. If your top speed by using a prop calculator, says you can go faster than Crouch's formula says, it won't. You have entered something wrong.

If you have weighed your boat accurately, and accounted for fuel, anchors, coolers, fishing gear and people, you are off to a good start. Check the numbers you get from actually running your boat with whatever engine HP, drive ratio and prop on it and study them.

If your numbers are higher than calculated, you have missed something. If they are lower, which I think that they are, it points to things that need to be improved on your boat. Maybe your bottom paint or fouling is causing it to slow down. Maybe you need a tune up or a prop is not up to spec. Could be you are over or under propped. I have run calculation tests with a spreadsheet program that i made, on all kinds of boats that are in my class, and it is invariably correct. Th Max from Crouch's should always be just a few MPH faster than actual Top Speed and then you know that you have entered the data correctly.

Good Luck.