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Old 19 June 2019, 10:17   #21
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>>>I'd noticed the outboard end is a Quicksilver fitting as used on Mercury/Mariner 2-strokes. It seems to seal on the Yamaha spigot OK but just in case that mismatch has potential to pull in air I'll get a proper Yamaha one.

I made that comment in post #11 above and the new Yamaha connector (part: 6E5-24305-06) arrived today (on the far side in image).

Despite the Quicksilver type the OB arrived with appearing to seal on all the test tub runs it did have a lot of play even when the clip was snapped home. In contrast the Yamaha one takes a bit more effort to push home but once you hear the click that it's on the joint is totally rigid.
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Old 19 June 2019, 10:27   #22
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Beautiful.
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Old 24 June 2019, 11:31   #23
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Well first sea trials undertaken in an excellent long weekend at the coast.

The Yamaha 15 has addressed the too big (bulk/weight wise) and too small (HP wise) of out last two OBs as demonstrated by the fact we are back to a neat if full pack where everything goes in the estate load space with both rear seats usable.

A first time start once floating at the slip and settled to a strong idle when the choke was pushed in after 30sec. Turned the boat around to face the estuary and with 6" of water under the skeg jumped in, put in gear and it went away smoothly with no fluffing or stall threat pushing the boat at idle. After a few yards when I judged I'd have at least two feet of water under the skeg the motor started to labour and I opened the throttle thinking damn what now. Over the next five yards or so the engine was doing less and less to move us and bogging down.

Then I saw a blue rope attached to an old fishing smack rise out of the water in a taught line between my transom and the stern mooring rope on the smack. Luckily it must have caught a bit above the skeg as it had risen right above the AV plate and stopped at the splash plate and kept away from the prop.

Thankfully I could reverse off and slightly alter my route away from the slip... rope cutting embarrassment right in front of the RNLI shed avoided.

Image attached at low tide where the rope is clear to see.. stretching across a third of the slipway and the following day I discovered much grumbled about by other dinghy and smallcraft users.

Motoring around 5kts to pick up Mrs F at the pontoon it felt OK but not totally right but put that down to it perhaps not having had a run for a while.

Motored about the yachts for 30mins up to 7kts to warm it properly and did a test hot restart OK then off to the derestricted waterski area.

Managed 16.6kts on a two way run which was lower than expected after my experience with the Mercury 15 2-stroke on the same boat with heavier load.

Also more splash than expected given I have a 45mm rise. None other than the odd spot in the boat but a good fountain behind.

Returned to the pontoon to reflect and realised I'd left the trim pin trimmed right in when I'd cleaned and greased at the service. So I moved it out and off for another test. This time a two way speed of 17.1kts and far less splash. Thankfully no need to raise the transom any more. Also the engine now felt right in the 3kt-7kt displacement speed range. Tach still fitted under hood so no idea of revs.
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Old 24 June 2019, 11:49   #24
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Next day I temp moved the tach out on the tiller for testing.

At 17kts we were only doing 5020 rpm (of a makers max revs 5500) which rather confirms my thoughts the prop it came with at a 10.5" pitch is too much given my experience with the Mercury/Mariners which have virtually the same gear ratio yet run perfectly on a 9" pitch.

In addition looking at it I can see a degree of cupping which I understand can pull the revs down.

I always carry a spare prop on trips to Scotland so happy to get another daily use one and keep this as a spare but the options are rather wide ranging...

A 9" 9.75" or 10.5" not cupped Yamaha OE at 85-50 inc post.... or a 9" or 10" cupped (both these aftermarket at 38.95 plus post).

As an aside there were guys there with an orange RIB where they'd launched but the engine turned over and just wouldn't fire up. I do carry a stupidly comprehensive tool kit for a smallcraft so offered help if they needed anything. But the guy asked if I had Eazi-Start which is one tool I don't carry.
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Old 24 June 2019, 12:36   #25
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Just worked out the fuel used. It equates to 2.2Nm per lit. I can't find any figures for my Mercury 15 2-stroke but Max said he was getting 1.9Nm per lit from his on an Aerotec.

So about a 36 Nm safe trip using the rule of thirds on this 25l tank.

The other comparisons I have is my 4-stroke Suzuki 20 Efi with managed 2.85 Nm per lit and the Mariner 9.9 4-stroke which was excellent at 4Nm per lit but of course at lower speeds.
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Old 24 June 2019, 13:12   #26
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Looks a nice set up now David the engine looks mint o you think your far off top speed around 18 knots ish
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Old 24 June 2019, 13:50   #27
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I think you're right... 18kts or so is near enough but I'd like the engine to be "breathing" a little more freely perhaps around 5350rpm or so.

I was a bit thrown to find that our prop is cupped as it's not something I've had on any of my sub 25hp motors so don't know how it effects the down pitching vs revs.
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Old 24 June 2019, 16:01   #28
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This explains it David cupping reduces RPM 150-300

https://www.mercuryracing.com/prop-s...t-4-blade-cup/
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Old 25 June 2019, 04:03   #29
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Quote:
Originally Posted by Fenlander View Post
Just worked out the fuel used. It equates to 2.2Nm per lit. I can't find any figures for my Mercury 15 2-stroke but Max said he was getting 1.9Nm per lit from his on an Aerotec.

So about a 36 Nm safe trip using the rule of thirds on this 25l tank.

The other comparisons I have is my 4-stroke Suzuki 20 Efi with managed 2.85 Nm per lit and the Mariner 9.9 4-stroke which was excellent at 4Nm per lit but of course at lower speeds.
Mine varied a fair bit from around 1.7 to 2.6 but close on 2 I guess is fair for mixed use.
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Old 28 June 2019, 09:05   #30
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Well the test can begin soon... somewhat over the top for a 15hp on a rubber boat I know.

Left to right...

10.5" pitch it came with looking to have a degree of cupping (aftermarket possibly)

9" pitch standard type (aftermarket)

9.75" pitch OE Yamaha standard type

9" pitch higher thrust type (aftermarket)

Just need tide and spare time to coincide and we'll be to and fro the slip with prop nut spanner in hand.
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Old 02 July 2019, 11:58   #31
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Because the small tach will be used to judge the best prop choice I wondered how accurate they are so just out of interest tested against my "reference" laser tach which picks up the spinning flywheel revs.

Quite a close comparison really at idle... and that replicated up the rev range.
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Old 21 July 2019, 18:59   #32
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Prop test day today. Speed vs rev tests carried out at West Mersea in the waterski area of the Strood Channel... then general handling performance over towards Bradwell in a nice choppy sea OK for around 15kts.

There's a handy large mid-channel bathing platform/tower thing at the start of the Strood Channel de-restricted area which we could tie to when changing props. Still a bit of a worry when spanner, pliers, nut, washer & split pin could all have ended up in the water for a slight slip of the fingers.
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Old 21 July 2019, 19:11   #33
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The prop results were interesting. Left to right in photo below...

The left most one was the 10.5" pitch it came with which I reported was a bit low on revs at WOT and just didn't feel right at displacement speeds.

2nd from left a standard 9" pitch but aftermarket brand. This was disappointing in that it felt and behaved almost as the 10.5"... low on revs at WOT and just not a good feel.

3rd from left a Yamaha OE 9.75" with a very fine blade design. This immediately felt right at displacement speeds... upped the revs to 5250 at WOT and produced the maximum speed of 17.9kts.

Right most one an aftermarket 9" pitch regarded as a higher thrust type. This was a surprise turning in almost as good a performance both measured and feel as the Yamaha 9.75".... making 17.2kts at 5300.

So I'll be keeping the Yamaha OE 9.75" pitch and as a spare the aftermarket higher thrust 9" pitch.
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Old 22 July 2019, 04:25   #34
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just goes to show with 1&2, design is the unknown where you know dia & pitch and still return unexpected results
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Old 24 July 2019, 11:28   #35
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Yep I didn't expect that result Jeff. And it really shows if someone has a starting position with a known size/pitch prop of a certain brand and wants to pitch up or down... then only keeping to the same brand can be sure of a certain result.

It may just work out going from say a 10" Solas to a 9" Mercury if you want to pitch down but the result could be unpredictable.

Glad that's sorted anyway, like to think the OB is running at its best. And with further proof on this outing that the 45mm transom lift suits the Yamaha 15 2-stroke... i.e. no splash and no ventilation on the tightest turn... I've committed to drilling holes to bolt the OB on. Bit of fettling was needed on the alloy plate and now it all looks quite neat.
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Old 24 July 2019, 14:56   #36
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So do you get a crisp response on and off the plane in the rough with the OE 9.75 prop too David
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Old 24 July 2019, 16:43   #37
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Yes really good. But there are other factors at work too.

When we first had the Aerotec we were self, Mrs F and one light teen girl plus 22lb dog. A 15hp was fine and then it was a Mercury 2-stroke on 9" prop. Then second daughter came out with us more which blunted performance so I went to the 20hp Suzuki which was best on a 10" pitch.

Then it went back to just self and Mrs F with neither girl and lost the dog too at 15yrs hence I gave that 10hp a go but that was a bit too flat in the midrange.

Now just self and Mrs F plus she's lost 4 stone. So compared with when we were running the Mercury 15 with a 9" pitch we are lighter by at least 12 stone which means the 9.75" pitch on the Yamaha gives more grunt midrange than the Mercury with the 9" and heavier load.

So weight reduction has played a large part.

As a matter of interest these recent prop tests were without daughters or dog but we did add one 8.5 stone daughter later in the day and she took a little of the mid range grunt away plus lost us 1kt on top speed. So if she was always with us I'd probably drop back to a 9" pitch.
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Old 05 August 2019, 10:17   #38
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Very final prop test. A standard design OE Yamaha with 9" pitch.

Reduced top speed of 16.3kts and revs up to 5600... felt under pitched. Bit surprised as this was the standard pitch on the Mercury/Mariners that served us well in the past.

Anyway kept it for if we have a holiday where we are 4-up with both daughters where it will probably be ideal.... but the Yamaha OE 9.75" pitch is going back on for our normal use as the best all round performer.
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Old 05 August 2019, 14:26   #39
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Prop Maximization.-

Before venture playing with prop pitches need to have a spot on transom/lower leg heigt optimization to achieve no prop aeration or water splahses side or over transom whatsoever, deck load even distribution, correct trim setting and correct tube/keel inflation if being a Sib.

An Induction tach is more suitable to be installed at lower front motor or side of the throttle grip than a laser tach, very uncomfy to aim at with removed cowl while driving at full throttle, are intended for static shop use when testing motors with special test props on proper test tanks.

Personally like to play with OEM props when needing a prop maximization to pull wot revs up, if you test two same X pitch props from 2 different prop manufacturers, both will dial different rpm readings.

Ideal is to maximize a prop for motor to run at least midle rpm if wanting sligh faster top end speed or max wot rpm range if waning faster hole shot as factory stated for each HP motor brand, if with a fixed constant load much better, varying load is a rpm serial killer. Most factory delivered motors with a prop are being factory delivered over pitched and some way over. The factory wants to play safe as won't know wich HP motor will be powering which type of boat.

All my Sibs and Ribs have been prop maximized for my Tohatsu 18 HP motor to run to max 5800 wot rpm range with max 2 up as like cruising at speed to far away distant places. A tiller extender is a must install if need to shift driver's weight forward or when using a driver seat for a much better comfy throttling/driving position along better forward deck load distribution.

Happy Boating
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Old 05 August 2019, 14:41   #40
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Sometime Loco I wonder at your posts... do you read what's gone before?

In 11 posts above I have just been through a test of 5 different props using a small tach and GPS after getting the outboard height spot on with a raised transom, boat properly inflated, consistent well balanced load and suitable water conditions for test.

I have considered a balance of max revs, max speed, grip when opening the throttle from lower speed, displacement speed behaviour, ventilation in tight turn and splash... all to end up with the best result.

And then reported back to perhaps help someone in the future who might have similar prop choices.

Yet you post a "mini lecture" on the aims and methods of prop choice as if I hadn't a clue how to go about it. Odd indeed.

And who the hell would use a laser tach on the move at sea?
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