Post test sanity check - Max RPM

9D280 · 25 March 2008, 13:16

Finally got the "new" toy out for a test run yesterday. Managed to clock an indicated 6500 RPM at WOT (in the short length of "flat" water I could find that would allow a long enough look at the instruments without stuffing it or going airbourne!

)

Now, the only reference I can find to my generic Yamriner 2 cyl 2 stroke is on a scan of a Mariner 55 "Marathon" Microfiche. It says I should have topped out at around 5500.

So, Does anyone know if the marathon was purposefully "maxed" at 5.5K, or better still does anyone have access to paperwork that might tell me what my theoretical max should have been. ('79 vintage Yam 55 2cyl 2 stroke) It was literally screaming when it happened, and sounded "normally WOT" when throttled back to 5500, so I think I may have too low a pitch on there! Mind you, 2 - stroke Yams are always rev - lovers, so maybe 6.5K isn't that unrealistic?

Apart from that it was a good trip, only other big problem was my knee against the fast idle lever. This weeks job is going to involve moving the remotes up the side of the console by about 6".........

jwalker · 25 March 2008, 13:34

My book says 4500-5500rpm. WOT.

9D280 · 25 March 2008, 17:57

Thanks for the quick reply. I had the nagging feeling you might say that! On the plus side, I still have all the pistons I left the marina with!

So, anyone know of any 13 spline 10& a bit inch diameter 15" props going a begging?

Or anyone round this corner of the world have one I could borrow for half an hour at a suitably convenient time?

ashbypower · 25 March 2008, 18:06

Hi What Pitch are you running at the moment?

Nos4r2 · 25 March 2008, 19:49

Quote:

Originally Posted by 9D280

Thanks for the quick reply. I had the nagging feeling you might say that! On the plus side, I still have all the pistons I left the marina with!

So, anyone know of any 13 spline 10& a bit inch diameter 15" props going a begging?

Or anyone round this corner of the world have one I could borrow for half an hour at a suitably convenient time?

Is that the same a the K series prop?

9D280 · 26 March 2008, 11:45

Ashby, 12" - probably spec'ed for the 9 bods it was rated to carry by D.O.T 25 years ago. Hole shot is awesome!

Nos, Haven't a clue.......

ashbypower · 26 March 2008, 12:17

Ok 12" to 15" will loose you about 600 RPM for the same boat configuration. May be woth looking @ 17" this will drop 1000 RPM for the same load.

jwalker · 26 March 2008, 12:54

Quote:

Originally Posted by 9D280

Ashby, 12" - probably spec'ed for the 9 bods it was rated to carry by D.O.T 25 years ago. Hole shot is awesome!

Maybe. Before going off to buy props, give us some figures because with a 5.5mtr boat and just 55hp I would have thought 12" pitch wouldn't be far wrong. Revs, speed, gearbox ratio.

ashbypower · 26 March 2008, 13:31

Quote:

Originally Posted by 9D280

Managed to clock an indicated 6500 RPM at WOT (in the short length of "flat" water I could find that would allow a long enough look at the instruments without stuffing it or going airbourne!

)

I was going by the Numbers already supplied!!! about 1000 revs over max!!!

jwalker · 26 March 2008, 13:53

Quote:

Originally Posted by ashbypower

I was going by the Numbers already supplied!!! about 1000 revs over max!!!

Yeh, I know. There was no critisism of you intended. It's just that a 5.5mtr with 55hp on a 12" prop at about 5500 would give around 25mph and that doesn't seem far off to me. But I don't know the gear ratio so I'm gessing.

9D280 · 26 March 2008, 15:08

I managed a GPS 26.1 knots at that rpm. Engine was screaming so didn't hang around there for long. At 5500 it was moving at around the 20-21 Knt mark. I've run the numbers through a couple of "prop guestimators" that have come out at around the 15" mark too.

Other thing to bear in mind is this boat is "only" about 550 KG fully fuelled.

As for the gear ratio, I'll need to take the bottom end to bits to figure that out unless someone has the specs to hand......

jwalker · 26 March 2008, 16:33

Quote:

Originally Posted by 9D280

At 5500 it was moving at around the 20-21 Knt mark.

Wasn't a bad guess then, was it?

Quote:

As for the gear ratio, I'll need to take the bottom end to bits to figure that out unless someone has the specs to hand......

Make sure the kill switch is OFF, put engine into gear, take note/mark the flywheel position and turn the engine until the prop had done one complete revolution. Count the turns of the engine to get your gear ratio. I was guessing around 2:1.

9D280 · 26 March 2008, 17:39

Yep. But the fact it could then carry on up to 6.5K RPM kind of hints the prop is a touch on the low pitch side......

Good call! My mind is elsewhere this afternoon.....

I'd have the plugs out anyway, its got a huge stroke so compression is a bit much to turn the flywheel by hand against! From memory the Mariner spec I saw said something like 1.7 or 1.8:1, for what that's worth.

Will let you know either a lot later tonight or tomorrow......

9D280 · 08 April 2008, 14:55

It's amazing how much fog clears when you find a delaer that actually knows the products they sell.....

Turns out I have a "High rake" prop. (I think that's what he called it) Anyway, the bottom line is that Yam appaerently did a series of smaller diameter props to allow faster boating - I guess the theory is that by reducing the drag due to diameter you can up the pitch a bit and take out the reduced drag with higher pitch and so go forwards faster. Probably also explains why it let go with the slightest provocation!

Turns out the "standard" (I now know it's a G-series) prop is slightly over an inch greater diameter than what's on the bottom of my leg. Kind of explains why I thought it looked a touch on the small side!

So, now the experimentation really begins - I'm either looking for a 15" "High Rake" or a 13" "standard.

Pablo · 15 April 2008, 17:20

I get 34 knots out of my boat. I didn't have a rev counter but the 15" prop sounded too short. I went up to a 17" one and gained a little on top end but alot at cruising speed. The engine sounded right at 28 knots where before it was around 24 knots.

9D280 · 16 April 2008, 10:31

Pablo,

Sounds like we have similar setups (because they both go through a garage door?

)

Is yours a 2 or a 3 cyl engine? Also any idea what diameter prop you are running?

Pablo · 16 April 2008, 16:23

3 cylinders, i'll ask the prop its dimensions tonight (hopefully I won't get a slap in the face

)

Pablo · 19 April 2008, 15:47

Quote:

Originally Posted by 9D280

Pablo,

Sounds like we have similar setups (because they both go through a garage door?

)

Is yours a 2 or a 3 cyl engine? Also any idea what diameter prop you are running?

It's a Solas Myth 13.25 x 17

9D280 · 22 April 2008, 18:30

Isn't that precariously close to the underside of your cav. plate?? Mind you if you have the 3- cyl there's maybe a bit more room in there.

What do you reckon your overall weight is?

Or maybe I just need to polish my hull?

Thanks, that's useful info.

9D280 · 09 June 2008, 10:15

Well, finally managed to borrow a G- series 14" pitch prop yesterday, and went for a spin.

In summary - 4 up + 30L fuel - 4500rpm, one up + 15L = 5500rpm.

So, assuming the pitch increase of 2" takes out about 400rpm, looks like the extra inch in diameter accounted for 600 rpm (using the 1-up rpm compared to the original test).

Unfortunatley no GPS this time as it was all a bit last minute and it was left behind (d'oh!)

So, if anyone has a 14" G- series that needs a good home......... (or a 13" for those days when I have passengers!)