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In past issues I have explained some of the methods used by outboard hot-rodders to increase the horsepower of their production powerplants. Often, however, even after modifications have been carefully planned and correctly executed, the engine's performance when reinstalled on the boat is quite disappointing. All too frequently after several attempts at tuning or trying to diagnose a non‑existent internal engine problem, the owner quits in disgust. He sells or scraps his high dollar powerhead, convinced that modified engines have no low speed power and not much more top speed than a near stock engine.
Fortunately, neither of those conclusions is correct! Assuming that an engine has been properly modified, it will be quite capable of delivering both a substantial increase in top speed and acceleration over its stock form. The “secret” to achieving maximum performance lies in accurately placing the operating power/rpm curve of the modified engine in synch with the power requirements of the hull! Of course, this is not a problem exclusive to modified outboards, it's a concern for anyone trying to get optimum performance from a boat powered by an internal combustion engine.
To illustrate the importance of getting the engine operating on its power curve, let’s look at a typical example. We can take an average combination like a stock V‑6 outboard installed on an 18' V‑bottom. We’ll assume the owner (I’ll call him Joe) has already done extensive testing of his stock rig and has attained best performance driving a 14 x 28 Chopper prop at 5,800 rpm with a 1.78:1 ratio lower unit. Top speed from Joe’s combination is now 84 mph with good acceleration. Still, Joe wants more, so he decides to go the route on the powerhead by getting it modified.
Before we go on, let’s examine Joe’s present horsepower vs boat speed curve. First, we need a wide open throttle boat speed vs rpm curve (Fig. 1). Joe could get this with his present tachometer and speedometer.
Next, we need to examine the stock engine’s horsepower curve. (Fig. 2) Most boaters won't have access to a powerhead dynamometer, but conveniently the power curves of identical brand, model, and year stock engines are typically very close. It is only necessary to find someone who has already done a powerhead dyno test on a stock engine identical to your own. If you can not get a power curve from the engine’s manufacturer, try one of the outboard race engine builders. They should be happy to supply you with dyno curves for their modified engines vs stock engines in the interest of getting your business.
Next, using the information from the two previous tests, we can plot the horsepower available vs boat speed in 10 mph increments (Fig. 3).
Now Joe sends his motor off for porting, installs bigger carbs, high compression heads, etc. He also has the engine dyno tested to generate a new power curve (Fig. 4).
The results look impressive and Joe can hardly wait to get his boat on the water. Unfortunately, when Joe installs the modified powerhead he, like many hot-rodders, gets a rather shocking disappointment! The boat is slower accelerating and has less top speed (only 70 mph) then when the engine was stock. Why!? Take a look at the speed vs horsepower curve for the modified engine (Fig. 5) compared to the curve for the stock engine in Figure 3.
At every speed increment the available power is less than it was with the stock engine! At 5,000 rpm and 70 mph the boat’s load exceeds the power available from the modified engine. So Joe has a loss of 14 mph compared to the stock powerhead. Even acceleration. is reduced because the engine has less power surplus at each speed level. All that high rpm power waiting and Joe can’t even use it. Sad Joe, he needs to find a long down hill section of a lake to get his motor up to speed!
Well, Joe may not find any navigable “down hill areas” on the water, but he could regear the lower unit and realize the full performance potential of the modified powerhead. A change of the gear ratio from 1.78:1 to 2.2:1 would raise the engine rpm by about 24 percent at any boat speed. Take another look at the horsepower vs boat speed curve for the stock motor (Fig. 3) compared to the regeared modified engine (Fig. 6).
Now Joe's happy! Not only is there increased power available at every boat speed for considerably stronger acceleration, but top speed has moved up as well. Actually, if top speed were Joe’s only concern he could use a bit less gear to make the 270 hp peak intersect with the boat’s speed/power requirements at about 95 mph. (Power requirements increase approximately as the square of the speed increases.)
Many boaters will wonder why the same effect couldn’t have been accomplished with a simple prop pitch reduction, saving the hassle of changing lower unit gears. First, realize that the most efficient performance props (in the types and sizes typically found in 60‑100+ mph outboards) all have a pitch/diameter ratio close to 2:1. Go too far up or down from this “optimum” 2:1 pitch/diameter ratio and you will need more horsepower just to make up for the prop's reduced efficiency.
Actually if the operating rpm range of Joe’s engine had only been slightly off its power curve, then a prop change would have worked fine. But a prop change equivalent to the gear ratio change used in his case would have required dropping to a 22" pitch. Joe’s original prop was a 14 x 28. The only 22" pitch prop available in the same style also has a 14" diameter. So, while the original prop had a pitch/ diameter ratio of exactly 2:1, a 14 x 22 prop has, a pitch/diameter ratio of only 1.5:1 and therefore would waste valuable power due to its lower efficiency.
Even if Joe was able to obtain an 11 x 22 prop of the same type, to retain the 2:1 pitch diameter relationship, he would still have problems. The blade area of such an 11 " diameter prop would not displace the required volume of water (at prop shaft speeds in the 1,500‑4,000 rpm range) to efficiently handle Joe’s power. Which is why outboard manufacturers use different gear ratios on various models rather than attempting to cover all applications with different pitch props only!
Try to follow this procedure whenever replacing a powerhead with one having a different power curve. First, insist that your engine builder give you a power curve for your modified motor. Pass this information, together with your stock combinations speed, power and rpm data, along to your prop man. Have him help you select a gear ratio to put your new engine on its power curve with a prop in the 2;1 pitch diameter ratio range. Then you can do your engine and propeller fine tuning from that point. Warning: if your prop guy isn't concerned with such “details” maybe it's time to find someone who works with a calculator as well as a hammer.