21 foot very slow hole shot, prop to big?

[fabrimacator21]

Re: 21 foot very slow hole shot, prop to big?

Is it possible he's getting more prop slip due to a different prop design (cup, etc.)?

 

[hwsiii]

Re: 21 foot very slow hole shot, prop to big?

Fab, paul said they are both aluminum so that can't be the problem but possibly a blade on the prop is messed up and that could do it.

H

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

no both props are in excellent condition.

And it is not impossible as I am looking at it and I went to the lake and recreated it, exactly.

I have been a professional marine mechanic for a good part of my life, although on larger vessels.

I have all the diagnostic tools to do this with a high amount of precision.

Now, lets think this through a bit, and not just can't happen, sorry it did. There are many more interactions going on here.

I was looking a the torque curves for the motor vs hp. In the case of this motor I was behind both curves at 3000 rpm. The boat was just starting to hit the curves when it ran out of power due to load with the 21 prop. With the 17 the engine easily got on to and well into both the torque and hp curves.

This engine specs out like new with compression etc. However, and I did state this before I am at 4300 feet, which is a whole different ball game than sea level. It was hot out which makes the air less dense as well.

as the boat transitions from lets call it resting or semi displacement to planing the bow on this boat comes quite high in attitude, exposing the maximum drag profile. With the 21 prop it took 15 or more seconds to come to plane. With the 17 the attitude of the boat is fatter making the step transition much more efficient and much faster IE 2-3 seconds.

If the leg had a high altitude kit in it this would be less problematic.

So you see the observed conditions can happen, and can exist.

I think that a better statement would be, under normal conditions , at standard temperatures and pressures, we would not expect to see this type of result, however, there are other dynamic forces at work that can change the situation.

just for thought

 

[hwsiii]

Re: 21 foot very slow hole shot, prop to big?

Higgy, let me say I try to NEVER be confrontational here, I am here to learn more and to fine tune my programs for picking the correct pitch props for boats and doing analysis for prop and horsepower curve analysis as well as prop slip percentages and propeller comparitive analysis charts and a few other analytical charts as well.
That being said, I will have to say that the normal average expected RPM change in the same make and model of prop on a 21" pitch prop to a 17" pitch prop at a 1.51:1 gear ratio is right at 800 RPM, which is about 200 RPM per inch of pitch, and the gear ratio of the lower unit is directly proportional to the change in RPM expected. And that is not taking into consideration Torque and HP curves, because as a normal course of events that is a very secondary matter and does not have any great bearing on the final results. But at higher elevations I have found that to not be the case, as your results indicate as well. The manufacturers recommend a high altitude kit for their motors after you reach a certain altitude, and all that does is increase the gear ratio to create more torque at lower RPM, but it does work. But then you have to change props to a much higher pitch to use the new gears. In my opinion it would be better to rejet the carburetor and just lower the prop pitch you are currently using. As the real problem is lack of air pressure at higher altitudes and not anything to do with the drive ratio, but by increasing the Torque output when you raise the gear ratio it helps with the problem, although it is very expensive and not the best solution.
I did pull a Torque and HP curve for a GM 350 Vortec engine, as that is all I could find, and it apppears that the Torque and HP crosses at about 2,800 RPM and that is very close to where your problem was. If you would like to give me the RPM and speed numbers for your 21" prop starting at 2,500 RPM and in increments of 250 RPM thereafter I would like to do a Torque/HP output and prop required power analysis to see what exactly is happening to that prop and motor combination and prepare a chart to show the analytical results from that test. But I do understand that is a lot of work and you might have other obligations or prefer no to waste the time. Or if you already have them would you please post them as I am very interested in this.

I did do an analysis of your boat and prop combination and came up wth an 18" pitch prop as the prop for your uses, it will not have quite as much hole shot and speed advance as you have with the 17" but if you bought a 4 blade prop it should increase your hole shot and acceleration over the 17" as well as lower your prop slip at WOT.

Boat and Motor

Prop Picker

Prop Slip

H

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

so the outcome of this is that it was hot out and the air was less dense. i run my boat in 96 degrees and at 36 degrees and the out come is very very similar. so at 4300 feet 1" in pitch change results in 400 rpm change? any body else run at high alltitudes and see this sort of change. very interested

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

HWSIII;

I did not take the posts as being confrontational, sorry if my wording sounded like I did, I as well, am looking for information and knowledge, I hope that I did not offend you or anyone else.

I have already ordered the jets for the carb. I would like to switch it over to EFI if I can find the parts at the correct price.

I do not want to put in a high alt kit either.

I like you proposal, however, it will be a few weeks before I can do this. We should draft a testing protocol to achieve this.

Junior, I did not state that it was a just hot and the air was less dense. If you read the post again you will see that I think that the temperature had some to do with it. If you look at the normal atmospheric laps rate on a hot day the engine thinks it is much higher, yesterday the density altitude was about 6000 in the morning and almost 7000 in the heat of the day, at altitude this become quite significant.

What I did hypothesize is that with the 21 prop the engine is not able to make the horse power needed when not running efficiently to get onto the area of the hp and torque curves to be able to make the boat run/plane efficiently. I also tried to relate that back to running resistance do to the attitude of the hull in the water as the boat transitions between sitting or moving slowly, I am going to call that semi-displacement mode to being totally on plane which has very little hull actually in the water.

When the nose of the boat is high in the case of slow to plane the maximal cross section of the hull is exposed to the water and drag is at the highest, with out the hp or torque needed it is very difficult to get out of this attitude.

With the new prop the boat get though this very quickly, the bow comes down and the boat is at a less degree of resistance very fast, allowing the engine to accelerate into the hp and torque ranges faster etc. Once the engine is running optimally it may be able to handle a bit more prop.

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

no im saying alltitude and temp. any boat that will turn a 17 @5000 will spin a 21 at 4000-4300 which is enough to hit torque and hp needed to plane. 10,000 feet or 0 feet. as far as running on a plane at 2500 rpms there is some thing going on other than alltitude and tempature. so you reran the 21 and had same results. i can see slow hole shot but once on plane torque and hp are plenty to spin 4000+ becuase your already on a plane. no hard feelings i just find this thread very hard to believe. so wondering if a 19 would spin 4200 like you would say 400 rpms per inch or if it would spin 4600 like the real world #'s

 

[hwsiii]

Re: 21 foot very slow hole shot, prop to big?

Paul, even cooking times change at altitude. Until you have seen the differences yourself it is hard to understand, I know. Why do you think they make high altitude gear sets for boats, there is a reason. You, just like me don't know everything there is to know about motors at higher altitudes, so please try to be more understanding and let's try to do a study that can help explain what is actually going on here. I believe it is more than 1 reason that has caused this to happen, but until we have more data I am keeping an open mind and trying to learn from this and see what the end results determine the problem or problems were. Higgy has already replicated the same conditions and had the same results, so I don't believe it was any of the reasons you gave as the problem, because once the 17" prop ran good the problem shouldn't have happened again just because he changed to the 21" prop again. So let's go forward with some more testing and see if we can analytically find the problem.


H

 

[junior1113]

Re: 21 foot very slow hole shot, prop to big?

ok really looking ito this situation. learning more about alltitudes. there is a thread if you search 'prop pitch high altitude'. where a guy with a 5.7 volvo on a 20'5" glastron. had a 21p at sea level was good. at 5500 it was only pushing 3900. he went to 17 pitch and achieved 4850. this sounds very realistic 950 rpm change with 4 " pitch change. which is expected with altitude and prop change. so he got 237 rpms per inch.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

At what elevation and density altitude?

We get some huge changes in density altitude here. I check AWOS to day and the density altitude this morning at 5 am was 4500 feet, at 5.30 pm it was 8200 feet. This amount of change has the possibility to significantly change the way a engine performs.

 

[hwsiii]

Re: 21 foot very slow hole shot, prop to big?

Perhaps this will help explain some of the things that happen at altitude and the effects it can create.

Aircraft safety


Air density is perhaps the single most important factor affecting airplane performance. It has a direct bearing on:

? The lift generated by the wings ? reduction in air density reduces the wing's lift.
? The efficiency of the propeller ? which for a propeller (effectively an airfoil) behaves similarly to lift on wings.
? The power output of the engine ? power output depends on oxygen intake, so the engine output is reduced as the equivalent "dry air" density decreases and produces even less power as moisture displaces oxygen in more humid conditions.

As a result of a density altitude that is higher than the actual physical altitude, the following effects are observed:

? The aircraft will accelerate more slowly down the runway as a result of reduced power production.
? The aircraft will need to achieve a higher true airspeed to attain the same lift - this implies both a longer roll down the runway before liftoff and a higher true airspeed which must be maintained when airborne to avoid stalling.
? The aircraft will climb more slowly as the result of reduced power production and lift.
? The aircraft service ceiling (maximum altitude which can be attained) will be lower due to both lower power output and reduced lift, reducing the ability to fly above obstacles such as mountains


H

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

H,

The answer that you gave goes farther into density alt than I was talking about, the relevant parts are on engine power and related issues.

I am out to the lake tonight I hope.

 

[Utahboatnut]

Re: 21 foot very slow hole shot, prop to big?

Well I live in SLC Utah, the city floor is right around 4400' and the mountian lakes can be upwards of 6500-7500'. I have the same size boat and engine/drive set up as higgy. Yes the prop is different but with a 21p hi five I can yank two big slalom skiers out of the water as fast as ANY inboard out there. My problem is at 4400 rpms I'm only doing about 42 mph. I can't imagine dropping down to a 17 or 19. I think if I had a 23p hi five I would be perfect at 7000' to get close to 50mph and still have a wicked hole shot. With that prop I can easily plane at 2500 rpm. The difference between a 5 blade stainless and a three blade alum is night and day for me. Just a side note I would not be running a 20+ year old boat at 5000-4800 very long if you value your boat too much. Fine for an outboard, but 350's don't like it so much.

 

[tkmichael11]

Re: 21 foot very slow hole shot, prop to big?

And you need to be propped to run at your WOT RPM range, or you will damage the intake and exhaust valve, due to "lugging" the engine.

Does this mean you should be running full throttle out of every hole shot?

 

[fabrimacator21]

Re: 21 foot very slow hole shot, prop to big?

Does this mean you should be running full throttle out of every hole shot?

No.... Half throttle doesn't mean lugging. Lugging is referring to a motor struggling to gain any more rpm when the motor is already at a fairly low rpm. Really has nothing to do with how much throttle you give it... has more to do with gearing and power.

 

[soaringhiggy]

Re: 21 foot very slow hole shot, prop to big?

OK a bit more info,

Water was calm today. The boat pulling two in a tube was out of the hole and on plane in 3 seconds. The rpm came to instantly to 3.5K and the rapidly built to a WOT of 4600 on glassy surface with a top speed of approx 46 mph. I think the prop is the correct pitch for this elevation.

I did notice a very small and quick bog when the secondary plates in the carb opened for full throttle.

I was pulling at 25 mph at 2700 rpm. The boat does not want to drop off step when turning like it did before.

I am going to re jet and put on Bennett trim tabs next.

OK what next??

 

[dazedandconfused]

Re: 21 foot very slow hole shot, prop to big?

Did you make anymore changes?

 

[sho3boater]

Re: 21 foot very slow hole shot, prop to big?

An engine can 'fall off a cliff' when loaded (high pitch) to much. I've had this with various boats, they get on plane but then they never really get going. It is the load rising faster than the HP/TQ of the engine or some factor involving that. I think they will slip some at low speeds and when you get on plane the slip lowers and drops rpm too low to make the power.

A while back I took a 19p off an outboard running near the top of the rpm range, it had a little added cup too. I put on a 20p bow lifting prop, both props are SS. RPM went from 5300 to 3600 and mph did the same. One issue was the motor was too low for the bow lifting prop, but still that is an ugly drop. The second prop may have been a little more diameter not much, and the first prop is a standard SS and manufacturer selected for that motor/boat. The rpm range was 4600-5500. I trimmed it up and down, it would not do anything. The motor is lifted .5" above stock.

Now given an I/O auto engine often has sufficient torque to overcome this, but not always. Another factor is how the hull lifts, it may be on plane but wetting a lot of the bottom so lot of drag. As you gain more speed it unwets. Maybe that also is a factor.