posted 03-30-2007 09:01 AM ET (US)        

Many thanks to Tom Clark, Sal, and others who gave of their time and expertise to help me. Sal, I have to tell you, four blades are sometimes better than three. When time permits I'll provide a more detailed report to include speed data. My engine uses so little fuel that it's a complete non-issue and I won't be calculating fuel usage.

posted 03-31-2007 07:51 AM ET (US)            

Thank you.

I was concerned that a four inch drop may have been too drastic but it is just about dead on.

Tom Clark and other participants have an amazing appreciation for just what propeller adjustments can improve the performance of Boston Whaler boats.

posted 03-31-2007 08:52 AM ET (US)            

Although 6000 RPM is the absolute maximum recommended WOT RPM on the 115 FICHT on my boat I would never run at that speed for more than a few minutes. I've owned the boat and engine for eight years. During this first outing with the new propeller the boat was loaded at the highest weight it has ever seen or is likely to see.

It remains to be seen how the engine will perform with the four blade propeller at a light or moderate load. I suspect it will come much closer to the 6000 RPM redline lightly loaded and perhaps around 5700 to 5800 RPM with a moderate load. Time will tell.

Would you agree that in order to make 6000 WOT RPM with a light, moderate, and heavy load a variable pitch propeller would be required, or alternatively three different propellers?

Eight years ago the three blade 19 inch pitch propeller seemed to work well except for hole shot which has always been disappointing before changing to the four blade propeller. Adding a hydrofoil improved hole shot but very little compared to the new propeller.

The hydrofoil remains on the engine. I'm considering removing it before further testing as it may now be a liability. I certainly wont know until I try it.

posted 03-31-2007 06:18 PM ET (US)            

Thanks again for the perspective. I'm not sure I completely understand everything you've said but I certainly believe you are correct. As earlier stated, the engine manufacturer recommends WOT RPM of 5500 to 6000. As the engine is now making 5500 RPM is it really lugging, or were you refering to the situation with the 19 inch propeller?

I'll try to clarify a little bit then I'll reserve comment until I can report minimum RPM and speed to maintain plane, speed at WOT RPM, and, if I can time it, how many seconds it takes to get on plane. I'll try to have the data under three loading conditions light, moderate, and heavy. This may take some time but I'm determined to obtain the data.

Finally, as you have correctly recalled I was unable to get over approximately 5000 RPM with the 19 inch three blade Stiletto with a heavy load. A heavy load for the boat is 25 gallons of fuel, three adult male fishermen with a lot of tackle, food and beverages, half full ice chest, a full five gallon live well, and all the electronics and safety equipment a prudent boater would carry.

Moderate load would be two fishermen with a little less bait and ice.

Light load would be just me running around without bait, tackle, food, etc.

Unfortunately I'm unable to borrow a 15 inch three blade Stiletto propeller for testing or I would have done so before buying the four blade.

posted 04-01-2007 08:16 AM ET (US)            

If you look at a propeller curve, you will see that HP required by the prop at any given rpm is less that the engine is capable of producing until you get to the point where the prop HP and the engine HP curves cross. So, if you only idle around the harbor and never try to get up on plane, I would not consider that over propped for that use, even if you can’t get on plane because you might only be using 5 HP when the engine can produce 20 hp at that idling rpm. Just an example, not a realistic situation. If you want to use all of the HP that the engine is capable of producing, yes, you have to prop it so that the prop HP and the engine HP curves cross at the maximum engine rated rpm.
__________________________________________________________
If he could only turn 4500 rpms yet his throttle was a wide open, he's using the absolute maximum fuel the engine can pull, yet he's going slower than the boat is capeable of.
__________________________________________________________

Fuel consumption is not dependent on throttle position, but HP output. HP at 4500 rpm is usually less than at 5500-6000 rpm, therefore fuel usage is less. Look at it this way, with the throttle wide open, at 4500 rpm the engine is sucking less air that at 6000 rpm. An engine needs the correct air fuel ratio to run, does not matter what rpm or loading, the mixture ratio stays the same. So, at 4500 rpm, less air is being used therefore less fuel than at 6000 rpm. Thus, you are not using the maximum fuel the engine can pull, but you are using the maximum HP the engine can produce at that 4500 rpm, and you are using the maximum fuel the engine can use at that 4500 rpm, which is less that it can use at 6000 rpm.

Just some finer points to consider.

posted 04-01-2007 10:48 AM ET (US)            

Butch,

The recommended WOT range for your motor is 5500-6000 RPM. You have reached 5500 RPM with your new propeller. Strictly speaking, you are fine. If you were egregiously over propped, the manufacturer would NOT say it is OK to have a WOT engine speed of 5500 RPM.

However, I have found that most outboards will simply perform better if they are propped to reach the upper end of the RPM range. In your case, the range is small. If indeed you were more heavily loaded than ever before, then by all means go back out and test again under more normal conditions.

The other thing we have not talked about is motor mounting height. You will find that the Stiletto props will tolerate much higher engine mounting heights. Depending on where your motor is now, you might consider raising one set of holes and re-testing.

posted 04-01-2007 11:25 AM ET (US)            

If I may, let me restate what he said about horsepower and boat speed.

We have seen (from Crouch's forumula) that as boat speed is increased, the amount of horsepower needed to produce this increase goes up exponentially by the power of 2 (or "squared").

We have also seen from manufacturer's charts of horsepower as a function of engine speed that the engine power output tends to increase as a linear function (i.e., not an exponential rate of increase). As engine speed increases, horsepower increases in a linear fashion. (Note that in some engines horsepower will actually decrease above a certain engine speed.)

From these two observations we can see that as the boat speed decreases from the maximum, the rate at which the actual engine horsepower output decreases will be less than the rate at which the propeller shaft horsepower needed decreases. In other words, the two curves meet at maximum boat speed, but below that the engine should always be able to produce more horsepower than is needed by the propeller shaft for a given engine and propeller shaft speed. This situation is the definition of being properly "prop-ed".

posted 04-01-2007 12:58 PM ET (US)            

Nowhere in my post did I say that over propping would give you better fuel economy.

I apologize if my post was not clear.

What I said is that fuel usage is related to hp produced and usually an engine can produce more hp at higher rpm, therefore it can use more fuel at higher rpm, but that is only because it can produce more hp at the higher rpm.

But, the amount of hp an engine CAN produce at any rpm is not necessarily the hp being used, this is two different things. Jim discussed this above.

Fuel usage is directly related to power produced to move the desired load at a certain rate. If your WOT is 4500 rpm at 30 mph you will be using less fuel per mile than WOT at 6000 rpm and 45 mph, does that make sense? Theoretically, ignoring prop and engine efficiency at different rpms, moving your boat at 30 mph uses the same amount of fuel, whether it is at 3000 or 6000 rpm, 100 or 200 hp engine.

This has gotten off topic, but to continue the thought train, I posted here.

posted 04-29-2007 07:59 PM ET (US)            

The work was both more difficult than I imagined and easier. The reasons the job was difficult were:

1. I had never done this job.

2. I had no help.

3. My drive is not level so the boat was not level.

The reason the job was easier was that I had good advice from forum members. One issue that could have been a problem was that the engine was low on one side while attempting to line up the holes. This was solved by using a piece of wood and a mallet to move the wood under the skeg to the side resulting in perfect hole alignment.

I've applied liberal amounts of Boat Life Caulk as recommended.

Now, I'm looking forward to trying the boat once more.

Is this fun, or what?

posted 04-30-2007 01:26 PM ET (US)            

Jimh, your observation that hp increases "linearly" with engine speed (RPM) is half right. It is true only under full throttle. In actuality when RPM is decreased it is usually by the throttle. Fuel consumption varies more or less linearly with RPM and also linearly with manifold pressure (throttle).

Sal is wrong I believe in stating categorically that there can be no improvement in fuel economy with overpropping. The automotive analogy supports this, where very tall final gears ("overdrive") are most economical, yet they do not yield highest top speed (and nowhere near maximum RPM at highest speed). My BMW 540 6sp is actually fastest in 4th gear, but quieter and more economical in 6th.

Why more economical? Less engine friction is the main reason. Plus there may be slight improvements in combustion efficiency in running at slightly higher manifold pressure and slightly lower RPM.

posted 05-29-2007 07:25 PM ET (US)            

Finally got on the water today to test the result of raising the engine one hole.

WOT was about 5950 and speed was about 42 MPH GPS. I believe I can get to 6000 RPM WOT with a little bit more work on engine trim adjustment. Conditions today were not great. A lot of wind and 1 to 3 footers of chop. The readings were into the wind, BTW. Not sure of wind speed but estimate about 10 - 20 MPH.

The propeller wants to ventilate in sharpish turns with the trim at or near maximum. My idea of maximum trim is the point just before porposing begins. I believe a better result aill be observed in better conditions.

I would have chosen a better day for testing but this was the day the planets permitted a fishing trip for me and one of my regular partners. I have to admit my spouse does not allow me to be on the water by myself and she prefers not to be on the boat.

I will try to get out on a good testing day and record some numbers while taking the time to get the best result possible by adjusting trim.

One very beneficial result of the propeller change and engine height adjustment is that I'm now able to remain on plane at 3000 RPM. Previously, 3500 RPM was about the minimum. Time to get on plane is about three seconds and I have to start trimming the engine out about the time I give it full throttle to get on plane. Performance is now so good that I often leave the engine trimmed when it's time to move and the hole shot is quite good with the engine fully trimmed.

I'm very happy with the preliminary results.

A big thank you to all who helped with special thanks to Tom W Clark.

posted 05-31-2007 02:29 PM ET (US)            

1) Get you on plane faster

2) Allow you to stay on plane at a lower speed.

posted 06-01-2007 09:51 AM ET (US)            

I just plugged in the numbers from the latest WOT result on my engine. The resulting slip calculation was 0.6 which seems much too good to be true. Do you think it's possible the HP rating on my engine is slightly understated at 115? I've heard that many FICHT engines produce slightly more HP than indicated on the data plate.

posted 06-01-2007 09:54 AM ET (US)            

Regarding your slip numbers, what you report is typical of the Stiletto propellers. They seem to perform like a propeller with two more inches of pitch than indicated. This is why I usually recommend folks drop to the next lower pitch model when switching to a Stiletto from some other brand of propeller.

posted 06-02-2007 10:16 AM ET (US)            

Thank you. I recalculated the slip based on a 17 inch pitch. The result was 13 percent.

Is slip influenced by conditions such as load, sea state, and wind speed and direction? I assume it is but admit to being a complete ignoramus on boat performance issues.

I'm guessing that if speed is higher than the 42 MPH used in the calculation that slip would be lower and that therefore conditions have everything to do with slip. But, if more favorable conditions simply allow the RPM to increase along with speed then slip will be more constant.

Does this make sense and is 13 percent slip reasonable? I'd guess less slip equals more efficiency and therfore one would seek to minimize slip. True?

posted 06-02-2007 10:31 AM ET (US)            

13 percent slip is perfectly reasonable. Understand that you reached that conclusion simply be inputting an artificially raised pitch, but yes, 13 percent is fine under most circumstances.

Conditions will affect the propeller slip to some small degree but certainly does not "have everything to do with slip."

No, lower propeller slip is *not* necessarily better. I think there is a bit too much attention paid to propeller slip. Ultimately you need to look at the performance of the boat, speed, fuel economy and handling.

posted 11-17-2007 10:13 AM ET (US)            

But, the reason for this post is to report on WOT performance.

It was a cool morning with air temp of about 50F degrees at 0730. Chop was light at about 8 inches. Wind was around 7 MPH. The tide was falling. Two aboard with the usual live bait, ice, lunch, and gear.

Running upstream with the wind on the forward quarter WOT was 6000 RPM, trim was at maximum tilt and GPS ground speed was 44 - 45 MPH. This was in salt water at 60F degrees. I found that to be quite surprising but welcome news. Due to the current and head wind I was very surprised at speed over the ground.

In the afternoon the air temp was 72F degrees and the tide was slack with virtually no current. The water was mirror flat aith a temp of 69F degrees. Wind was from behing at about 6 MPH. Weight had increaed by about 25 pounds. At this time I was able to make only about 5900 - 5950 RPM WOT with speed over the ground of 42 - 43 MPH.

The only thing I can attribute to the difference in speed over the ground and WOT RPM is the chop earlier in the AM.

I believe my boat, engine, propeller combination is about as well tuned in for optimum performance as possible without custom propeller tuning or professional adjustments in which I have no interest.

I hope this thread will be of some use to other Dauntless 16 owners.

Tom, thanks for the perfect propeller for my needs.

posted 11-17-2007 11:02 AM ET (US)            

Tom He recommended a Stiletto three-blade 19-inch pitch propeller and the numbers were excellent for our 115-ETEC. This prop actually holds the boat in the water better at faster speed and produces a safer ride. After a small modification of the intake manifold by the dealer this pro hits close to 48 MPH at 5700 RPM. BRP is actually recommending to the dealers a to shoot for a range of 5600-5800 RPM for the 115-ETEC motors.

posted 11-17-2007 11:47 PM ET (US)            

A 20 degree F rise in temperature is reduces power to 95% (convert both temps to absolute by adding 459, then divide). If boat speed varies as the square root of the power (a fair assumption) you get 97% of your former speed, which is about what you got.

posted 11-18-2007 10:06 AM ET (US)            

As this is a year round boating area perhaps we should have a seasonal selection of three peopellers; a Spring/Fall, Winter, and Summer propeller.

posted 11-18-2007 11:14 AM ET (US)            

While the air temperature (and hence air density) certainly does affect the power output of an outboard, the speed difference you experienced was mostly attributable to the short steep chop you were running in that morning.