What prop with my AQ170A 270

"Hi all,
I used my restored b

"Hi all,
I used my restored boat for the first time this weekend and I found that I need a new prop.

I have with me 2 used props. One is a 15X17 the other a 14X19.

I started off with the 15X17. The engine reved to 5000 RPM with very little throttle opening and the boat was very slow.

Then I used the 14X19. There was some improvement but still this is completely the wrong prop for me.

The boat is an old Yamaha STR-18 (18 foot) and I use it on the lake which has flat calm water.

Even with the 14X19, the engine revs to 5000 RPM very easily. I still have a LOT more throttle to open. The speed of the boat at 5000 RPM is only 30 MPH. I just opened the throttle past this to see how much more there is and it reved VERY QUICKLY past 5500 and 6000 RPM... with still more to go. I quickly shut the throttle back.

What size of prop should I go for ?
15X21 ?

If there were 2 props which give a boat the correct WOT rpm, one is a small diameter with high pitch and the other large diameter with less pitch.... what difference in top speed, acceleration and fuel economy would the user feel between these 2 props ? Boat in the water"">in the water

"Nice looking boat, you did a

"Nice looking boat, you did a good job restoring it.

Regarding your props, something does not sound right. I would make first sure that they are not spinning in the hub and that the clutch is not slipping; and that the splined sleeve (part 45) is not ruined:

http://www.volvopentastore.com/LOWER_GEAR_UNIT_AQ_DRIVE_UNIT_270_270T_850788/dm/ store_id.366--session_id.863367404--cart_id.933579902--category_id.332331--list_ time.1200421729--view_id.317309

Also, make sure that the gear ratio in the outdrive is 1.89:1 and that the prop's pitch has not been modified (lowered).

For fuel economy and thrust, I would use a 15" diameter prop rather than a 14".

Regarding top speed, I am using a 15 x 17 " on a 280 outdrive and get around 32 knots at just over 4000 RPM (could 4200 or 4400 RPM, I don't remember now), but my boat is 22' and heavy, and the power plant is around 300HP. Since the gear ratio in my outdrive is 1.61:1, you should be able to obtain the above speed either at around 5000 RPM if your outdrive's gear ratio is 1.89:1; or at 5600 RPM if the gear ratio is 2.15:1 (which is for 4 cylinder engines). Since by the sound of your post you don't seem to get that speed, you may want to start checking what I suggested above."

"Hi,

Thanks for the complim

"Hi,

Thanks for the compliment on the boat !

During the restoration, I had to dis assemble the drive into its 3 basic components (upper gear unit, intermediate unit and lower gear unit)
The splined sleeve and the splines on the shafts are fine.
Since this drive was imported from the USA, I wanted to check that I had been supplied a drive with the correct ratio. I marked the universal joint and prop shaft and turned the "U" joint. The prop shaft goes one full turn just before the "U" joint goes 2 full turns so I figure the 1.89:1 ratio is correct on this drive.

The prop hub does not seem to have turned. I can check this by marking the hub and prop body with 2 lines and checking after running if the lines are still together. I will do this check.

How do I check if the brass cone is slipping ?
If I shift the drive into gear and try turning the prop with a lot of force it should stay without turning (in one direction) right ? if it turns, then the clutch (or something else) is slipping correct ?
Is that an acceptable way to check if there is an internal slippage in the drive ?"

"In my opinion, the best way o

"In my opinion, the best way of knowing that the clutch is slipping is when the math does not add-up. With a 17" propeller and the engine at 5000 RPM with a 1.89:1 reduction gear, the boat should do 37 knots if there was zero propeller slip. Since zero slip is virtually impossible to attain (other than with wind and currents in favour), you can apply a guesstimate of 10% to 15% slip for a rough calculation, which would give you a speed of 31-33 knots. If the speed is much lower and the prop is not slipping, the next suspect would be the clutch cone. I would first make sure that the shift cable is properly adjusted; and if it is, you may have to dismantle the upper gear assembly and get the clutch cones lapped."

"Hi,
I went this week end to

"Hi,
I went this week end to my boat and checked everything as adviced.
This is what I found....

The Drive ratio is 1.89:1
The prop hub does not slip on the prop.
The gear selector cable WAS out of adjustment.
When the gear lever is shifted into forward, the selector on the drive did not lock in the fully forward position.
I rectified some free play on the linkage and shaft and made sure the selector on the drive detents into the fully forward position when the gear lever is pushed into forward.

The new VDO rev counter on the boat needed some fine adjustment to make it read the correct RPM.

Then I tested the boat again with the 14X19 prop and measured speed using GPS. This is what I got.

21 Knots @ 3500 RPM
24 to 26 Knots @ 4500 RPM
31 Knots @ 6000 RPM (WOT)
The cruising speed for my engine is 4500 RPM

So... do I still have a slipping clutch cone ?"

"Oh.. and by the way ... I&#39

"Oh.. and by the way ... I'm using Monograde, Mineral SAE 40 oil in the drive (and engine) at the moment."

"More than a slipping clutch I

"More than a slipping clutch I think you may have a wrongly set tachometer or a faulty tachometer connection. The maximum speed for your engine is 5000 RPM, so if your engine was indeed running at 6000 RPM at WOT I suspect you would have blown it by now.

Most tachometers have a switch where you can select a 4-6-8 cylinder engine, and another switch to selet 2 or 4 stroke. Make sure you have the right combination.

31 knots at around 5000 RPM sound about alright, but not at 6000 RPM . You may want to check with an external tachometer (many timing lights have one) what your real RPM at WOT are.

The oil viscosity is, in my opinion, on the high side; but it should be OK for year-round warm climate and warm water temperatures."

"HI EL,
Thanks for all your a

"HI EL,
Thanks for all your advice. I am doing all these checks and adjustments as per your advice.

The tach on my boat is a new VDO unit. It only has 3 switches to adjust for the number of cylinders and mine is correctly set up for 6 cylinders.

After I read in the instructions that it needs to be manually calibrated for final accuracy, I took with me another tach and calibrated the one on the boat. It is reading correct now... only issue with it is, the pointer does not stay steady at any given RPM. The pointer has the shivers. Not by a lot but does go up and down about 500 RPM.
I will contact the vendor and ask his advice on how to rectify this.
I n the mean time, I will run my boat again with another tach connected and re-check the rpm/speed relationship and get back to you.
I have an automotive multimeter which has the capability to read RPM off the contact breaker pulse.
What you r saying is that with my 14X19 prop I should get 31 Knots at 5000 RPM ? ... I have a nasty feeling that I will have to end up pulling the upper gear unit out.
Lets wait and see what happens."

"Chanaka, for an outdrive with

"Chanaka, for an outdrive with a 1.89:1 gear ratio like yours, you can use the following formula to calculate the theoretical speed of your boat:

knots = 0.000435393 x RPM x prop pitch in inches

This means that with a 19" prop, your boat would do a theoretical speed of 41.35 knots if the propeller slip was zero. Back inthe real worls, with a propeller slip of 10%-25%, you could expect a top end speed between 31 and 37 knots. If much lower than the 31 knots, then something is not quite right: either the RPM reading is incorrect, or the prop pitch is incorrect, or your boat is not planing correctly, or something (prop hub or clutch cone) is slipping (or could be something else that we haven't quite figured out yet ). This is why I suggested that first you check how accurate is the tach reading.

With the 15" diameter propeller you can expect a significantly lower propeller slip than with the 14", especially if the propeller has a wide blades and the blade tips are cupped."

Thanks for that.. let's ke

Thanks for that.. let's keep working at it and get it perfect.
I will run the boat again with the tach on the automotive multi meter and see what it comes up with.
I have another baot.. an old 15 foot Glastron with a 150HP out board. I have a 21 pitch prop on that and at WOT I get 5500 RPM and 57 MPH. is that any good as far as calculations go ?

"Difficult to say without know

"Difficult to say without knowing the gear ratio. Once you know it, multiply the results in the above formula by 1.89, then divide the result by the outboard gear ratio."

"Chanaka, EL, just another t

"Chanaka, EL, just another thought here.... could be speed measurements be off? You guys have pretty well hashed out the possibilities of errors on the RPM side of the equation... could it be that RPM measurement is OK and speed measurement is wrong? (maybe both a little wrong?) I am not saying speed portion IS wrong, just prompting a look in that direction to hopefully help out with the problem. GPS units can be tricky to set up with correct datum, coord. system, and units of measure and such. Just a thought.....

Good Luck!

Matt"

"Hi,

I ran my boat again to

"Hi,

I ran my boat again today....
First did an oil change.
The installed the diode supplied by VDO to get rid of the pulsing tacho pointer.
Then re-calibrated tacho using external hand held pulse tach.

I noticed that the trim adjusting pin was in the trim down position. I changed it to the middle position.

Finaly tested on water.

The engine noise is MUCH less after the oil change.

Now at WOT I get 33 Knotts and just below 5000 RPM. Around 4700-4800 RPM. This is with a normal load.

Now the figures add up correct right ? I don't have any slip now do I ?

Since the cruising speed for my AQ170 motor is 4500 RPM, I'd like to change the prop to attain this at WOT instead of the slightly higher 4800 which I have now.

my current prop is 14X19.

What should I change to ?
15X21 ? 16X19 ?"

"Chanaka,

"Propeller sl

"Chanaka,

"Propeller slip" is a mathematical concept based on the difference between the theoretical and real boat speed, and it always exists in one grade or another. So the anser is yes, you still have slip (currently around 20 %) and always will.

Regarding the prop, I would leave it the way it is, as 5000 RPM are the design RPM for your engine at WOT. However, if you want to experiment, you can try a 14x21 (if you expect a bit more speed); or a 15x19 to obtain around the same or a bit less speed but more thrust. In either case, make sure you don't overload the engine."

"HI EL,

Thanks for that.

"HI EL,

Thanks for that.

What I really meant by slip was not prop slip but slipping clutch.

You did say earlier that this is about the speed I should expect with my current prop at 5000 RPM and now the boat is attaining it.
I suspect the trim pin being in the wrong position was the cause of the reduced speed earlier.
According to the workshop manual and user manual, the reccomended cruising speed is 4500 RPM.
I would like to prop the boat so that it attains no more than this with normal load.
I'm always watching the water temp and oil pressure gauges so I'll know if the engine is being over loaded.
The max prop diameter for this drive is 16" right ?!
You said use a 15X19 to obtain around the same or a bit less speed but more thrust.
Since more thrust is a good thing... what if I go for a 16" diameter prop and the chose the pitch which will give me 4500 RPM @ WOT ?
will I be sacrificing thrust (acceleration ?) or speed with this choice ?
How do I decide if I should stick with a smaller diameter prop (14") or go for a larger diameter (15 or 16") ?"

"Chanaka, I would not recommen

"Chanaka, I would not recommend a 16" diameter prop. I have tried some in the past with very poor results, which proves that bigger is not always better. If you go to the following link and check page 36 (52 of 90) you will see which are the most common props for the size of boat and engine you have.

http://www.miwheel.com/MIWheel/pdf/99021208.pdf

As a rule of thumb: low diameter + high pitch = high speed but low thrust. The prop that gives you the highest speed will not give yo the highest trust (and vice-versa), so you will have to find the happy medium term that suits your boating preferences."