High Thrust Prop

[centerline]

By the way Centerline.. Im not at all saying who is wrong.. could be me. Im just not enjoying arguing, its causing my panties to wad (and probably my BP to rise).... and you have a lot of good input in the thread..

I didnt know there was an argument... I must have missed that part... but its been a nice discussion and a lot of information shared each way that keeps the mind active in trying to follow and understand it.... prop science is difficult and confusing at best, due to the many variables and very few constants.... and its a subject sail boaters should never have to think about.
I have enjoyed the interchange and I feel bad that your Panties started to wad, 'cuz its an unpleasant feeling. but in my opinion, its better you than me... and take care of that BP, and remember, in the long run, it just dont matter!...

 

[Brian S]

Brian, what RPM numbers did you measure vs what prop pitch (if you dont mind sharing).

I looked up my numbers. Nissan 5hp 4 stroke, used to turn 2160 rpm at WOT with stock 7.7" diameter, 8" pitch 3 blade propeller. Actually, I'm assuming that's what the prop is - it's listed as stock for a Tohatsu 5 hp 4 stroke, but the only marking on the prop is an "8" in a square, which matches the stock 8" pitch as per the chart.

I replaced with a Tohatsu high thrust prop as per the Tohatsu prop chart, which is listed at 8.375" diameter, 6" pitch 3 blade elephant ear prop. With the new prop, I am turning 2340 rpm at WOT, making 6.3 knots on GPS with a bit of breeze. I am followed by a significantly large wave train, further indicating I'm moving at hull speed. It does 1.7 knots at 750 rpm, the lowest I can get it to run under load.

 

[kloudie1]

To further muddy some waters, consider that a diesel engine (if not running at max RPM) can make more horsepower at a fixed speed.. In a discussion earlier, there was a note of boat speed dropping to generate more thrust as a result of added dink drag.. Yes that requires more horsepower.. and what happens is that the governor adds fuel to hold the RPM setpoint that happens to be selected by the "throttle" (which is not a throttle in the sense of a gasoline engine but rather tells the governor what RPM the pilot has selected.. the governor will add or subtract fuel to attempt to hold the RPM setpoint. Example.. if a big headwind begins while you are motoring, the RPM will stay pretty much constant; the boat will slow a bit because of the additional drag which must be offset by more thrust which asks for more horsepower which is fulfilled by the governor adding fuel.. It all comes to equilibrium again pretty quickly.. to regain previous boat speed, more RPM will have to be added by changing the RPM setpoint ..

 

[walt]

Brian, thanks!!!

I have the Tohatsu 4 hp (similar to your 5 hp) and it list the peak hp rpm range as 4500 to 5500. I think you are at lower elevation and getting about half that with either prop.. Im surprised. Edit, I checked the 5 hp, it also list 4500 to 5500 rpm for peak hp.

I am about to order to one of those small outboard rpm meters to take some measurements on my 9.8 hp Nissan. This is the one you have (copied from the link earlier) http://www.amazon.com/gp/product/B00ACDP6YU?keywords=tachometer&qid=1448377150

Hard to believe the tach wouldn't be accurate and if your getting 6.3 knots seems the motor is running fine but ???

I cant find a plot of hp vs rpm for the small outboards but there is a plot in this article for a larger outboard http://www.dynomitedynamometer.com/articles/powerboat/powerboat_on-the-curve_article.htm (figure 2). This outboard has peak hp at about 6000 hp but less than half peak hp at 3000 rpm/. If we "scale" that and assume the small outboard has a similar curve, it would say you are only operating at about half your peak hp or about 2.5 hp.

The one you ordered sure looks like it would work fine and not expensive.. but seems a little odd to measure the low rpm

 

[walt]

I was just reading the reviews on the tach and it brought up that a four stroke will have the spark plug fire every other revolution. Ie, for a single cylinder 4 stroke, the rpm is twice the number of times the spark plug fires. That tach measures the rate the spark plug is firing. Since you have a four stroke, just checking to see if the measurement took this into account. If we double your numbers it would make more sense.. just checking..

I guess for my 9.8 two cylinder 4 stroke, I would measure one of the spark plugs and the rate the spark plug was firing would exactly match the rpm.

 

[walt]

jumping the gun a little bit...

For Brian's numbers, if we assume that the rpm numbers are likely 1/2 what they actually are and that that the second prop used got the rpm up where the outboard did reach peak hp (2x 2340 = 4680 rpm which is in the range of what the outboard lists for peak

If we scale Brian's numbers for the two props to compare to figure 2 in the link at the bottom, the second lower pitch prop gets scaled to 6000 rpm to match the peak rpm in figure 2. The original prop that came with the outboard would then scale to 5538 rpm.
Looking at figure 2 of the link I posted earlier, 6000 rpm gives 220 hp and 5500 give 218 hp. This scaling would say that Brian only gained a tiny bit of hp going to the lower pitch prop with the higher rpm (4.95 hp vs 5 hp).

Or scale it a different way using a linear relationship between hp and rpm:

If 2340 rpm (or actually 4680) gives 5 hp
Then 2160 rpm (or actually 4320) would give 5*(4320/4680) = 4.61 hp

So Ill speculate that for the worst case scaling, the first prop with the lower rpm delivered 4.6 hp and the second prop with the higher rpm delivered 5 hp. Using the scaling in figure 2, you would be getting almost the same hp for either prop.

The link again with figure 2
http://www.dynomitedynamometer.com/articles/powerboat/powerboat_on-the-curve_article.htm

 

[Brian S]

Walt, so sorry, I started to read your post (#46 in this thread) and I kinda glazed over. Mainly because a) I set the tach for 4 stroke as opposed to 2 stroke, and b) because the tach read 700 rpm at idle under load, and one would expect a motor to run about that slow at idle under load, and then finally c) the motor just does not sound as fast at WOT as all the other outboards I've ever dealt with running on planing hulls. So this all leads me to believe that the motor is really doing its best.

Let's give further examples of my 2hp 2 stroke Suzuki running fast on a 15' boat. Not enough juice to get it on plane, but it moves along and sounds like the motor is running close to WOT RPM spec.

Also have seen a Precision 185 running along with a Honda 2hp 4 stroke, running quite well.

So, given all of that, my conclusion is that my 18.5' O'day 192 is actually probably overpowered with the 5hp 4 stroke, unless, of course, I'm running into a headwind. (Current wouldn't affect this, because a 17.5' displacement waterline is going to make roughly 6 knots through the water, either in still water, or if the boat were to make 4 knots over ground into a 2 knot current. 4+2=6. It's all limited by wavelength. Wavelength is determined by LWL, and the speed of the wave is determined by its wavelength. That's why longer displacement hulls have a faster theoretical hull speed than shorter....) But a headwind will increase the windage resistance, and more power can overcome windage.

Further to this, I see boats like J70s, which are longer than my boat, using a 2hp motor.

On the flip side, I wouldn't re-power with a Honda or Tohatsu 2.3hp, because they don't seem to make high thrust props for them, and I think the high thrust prop makes a difference.



I may go back and try to read your post again.

Ok, fine, I've read your post now. I see how you are trying to demonstrate a change in HP between my props. Which I don't care about.

What I tried to do with a re-prop was to get my motor working closer to a higher RPM range (which it did increase, but not by all that much, really...) and to have a prop that just plain throws more water to move the boat better, particularly in reverse. I dunno, did it help? I guess a bit. Then again, I also get the satisfaction of feeling like I'm properly running a high thrust prop for a displacement hull, again circumventing the stock setup for a planing hull.

jumping the gun a little bit...

For Brian's numbers, if we assume that the rpm numbers are likely 1/2 what they actually are and that that the second prop used got the rpm up where the outboard did reach peak hp (2x 2340 = 4680 rpm which is in the range of what the outboard lists for peak

If we scale Brian's numbers for the two props to compare to figure 2 in the link at the bottom, the second lower pitch prop gets scaled to 6000 rpm to match the peak rpm in figure 2. The original prop that came with the outboard would then scale to 5538 rpm.
Looking at figure 2 of the link I posted earlier, 6000 rpm gives 220 hp and 5500 give 218 hp. This scaling would say that Brian only gained a tiny bit of hp going to the lower pitch prop with the higher rpm.

Or scale it a different way using a linear relationship between hp and rpm:

If 2340 rpm (or actually 4680) gives 5 hp
Then 2160 rpm (or actually 4320) would give 5*(4320/4680) = 4.61 hp

So Ill speculate that for the worst case scaling, the first prop with the lower rpm delivered 4.6 hp and the second prop with the higher rpm delivered 5 hp. Using the scaling in figure 2, you would be getting almost the same hp for either prop.

The link again with figure 2
http://www.dynomitedynamometer.com/articles/powerboat/powerboat_on-the-curve_article.htm

 

[walt]

thanks again for posting those numbers, things would be a lot more clear if that was done more often with props on these small outboards.. The one other number I would have liked to see is your WOT speed with the old prop to compare with the 6.3 knots you got with "high thrust" prop. I would guess its just a tiny bit slower. If you tach is set up right, your outboard likely does have some issue only getting to about 1/2 the rated rpm.. I think even with the higher rpm you got with the high thrust prop, you are only getting about 1/2 the hp you should be - assuming the numbers are correct.

I am going to go ahead and order that tach. The one thing I dont like is that it looks like you cant replace the battery.. I have two props that I am going to measure the rpm vs boat speed on with a 1990 Mac 26S and a 9.8 hp Nissan outboard. I will have all the info I need (pitch, gear ratio, rpm, actual speed) to use a calculator like this one http://www.mercuryracing.com/prop-slip-calculator/ to tell me the slip Im getting for speeds of say three different speeds (maybe 4.5, 5.5 and 6.5 knots) plus the peak speed.

I will do this for these two props that I own. Should be interesting. I expect that prop slip will go up with each increase in boat speed and the interesting thing will be to see for a given boat speed, which prop has the highest slip. I am guessing that the numbers for the two props will be similar (with the high thrust prop always operating at a higher rpm) for the same boat speed.

 

[Brian S]

Walt, the way that tach works, it has one wire. You wrap the end of the wire around a spark plug wire about 4-6 turns, and it picks up the ignition charge via inductance. It's a simple LCD display - I doubt it would wear out a battery anytime soon. I just used it for mucking around temporarily, because I didn't figure there was a good place to mount the thing, though I guess some do mount it, given it also has a built in hour meter.

As for my max speed with the bigger pitch prop, I never actually measured that. I vaguely remember seeing WOT speeds similar, indicating I was hitting hull speed, even with the bigger pitch stock prop.

Given that I once had a planing hull boat, which should have supported the weight of the Nissan, I'm still annoyed with myself that I never put the Nissan on the boat to see what it would do before I sold the planing hull. Then again, it would have been a PITA to get the Nissan off the bigger boat, which was kept in a slip, just to put on the dry sailed little boat... Still, would have been fun to see if it would push the little boat up onto a plane. Planing is fun...

Meh, I suppose I should take the Nissan in to a mechanic to check over and see what's what... Maybe he'd tell me there was something wonky preventing higher RPM at WOT. On the other hand, I also know that pushing the boat at 6.3 knots is not allowing fast enough speed to get the motor up to higher speed through water, and thus higher RPM. Is spending the money worth it just to satisfy my curiosity? Or if there is a problem, just let it go until there's an obvious major problem, like it stops running and strands me?

 

[walt]

Sounds like you get at least several years out of that battery. I will almost for sure lose or at least lose interest in the tach by then..Yep.. that 9.8 hp is also way more than I need with that 26S (probably more than double).