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Old 24 January 2022, 10:05   #1
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Yamaha 30 Prop size vs rpm

Morning all, while itís not fitted to a rib it is engine and prop related so thought Iíd sneak in under the radar

Outboard is a Yam F30 four stroke (F30AET built 2008) and prop is 11 3/8 x 12

Outboard is attached to an Orkney grp boat 16í unladen 250kg

Yamaha digital tach is showing 44 (4400) WOT with one adult approx 90kg and maybe same again with gear on boat.

Variables -
ē Faulty tach or not set up correctly
ē Weight of the boat not allowing higher rpm (waterlogged hull)
ē Throttle not opening fully
ē Wrong prop size

Generally the engine runs well, no issues starting and idling fuel is premium.

Iíve no issues with boat or outboard just think that I should be seeing 5000-5500 rpm from the outboard/tach. If Iím not right then this will be a short thread
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Old 24 January 2022, 12:28   #2
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Edit: this outboard only revs to 5000 rpm

Canít insert image from phone showing manual spec. 22.1kw@5000rpm
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Old 24 January 2022, 15:52   #3
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Remind me, is that a Strikeliner? What's the speed at your 4400rpm max? Just picking some guestimate figures out of the air I'd have thought you'd need to prop down to 9.5" or 10" to see 18-20kts or so at correct max revs??

Hopefully not water in hull. Is hull clean if a moored boat?
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Old 24 January 2022, 16:20   #4
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Prop choice: Hole shot vs top end speed. You have to decide which is more important. Otherwise you go for the middle of the 2. Top end speed also is determined by RPM. Sounds like one of 2 things. Prop is designed for hole shot, or you have a tach or engine problem. Take a handheld GPS or look at your onboard GPS and get your top speed and then report back here.

Also, lok at your prop and get the pitch # from it and then look up what is recommended for hole shot or speed.
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Old 24 January 2022, 16:44   #5
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Think you need to address & eliminate variables 1,2 and 3 to get an accurate picture & go from there.
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Old 24 January 2022, 16:57   #6
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Quote:
Originally Posted by Yabadabadoo View Post
Morning all, while itís not fitted to a rib it is engine and prop related so thought Iíd sneak in under the radar

Outboard is a Yam F30 four stroke (F30AET built 2008) and prop is 11 3/8 x 12

Outboard is attached to an Orkney grp boat 16í unladen 250kg

Yamaha digital tach is showing 44 (4400) WOT with one adult approx 90kg and maybe same again with gear on boat.

Variables -
ē Faulty tach or not set up correctly
ē Weight of the boat not allowing higher rpm (waterlogged hull)
ē Throttle not opening fully
ē Wrong prop size

Generally the engine runs well, no issues starting and idling fuel is premium.

Iíve no issues with boat or outboard just think that I should be seeing 5000-5500 rpm from the outboard/tach. If Iím not right then this will be a short thread
If you have an accurate GPS speed, because we know the pitch can work out the rpm for you.
Don't forget about engine height as another variable.
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Old 24 January 2022, 18:57   #7
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All good points.

Think I can only only answer a couple with certainty. Yes itís a Strikeliner and on very flat water yesterday I was seeing 16.1 knots on the plotter

Im less bothered about seeing the 4400 revs on the tach after reading only 5000 of them existed at the factory. Would be good to know itís propped right.

Hull is anti fouled to not a great standard, bit lumpy so Iíd say there was some drag there, no visible growth from top side looking down.

Hole shot Iím not bothered about. Be good to know Iím getting the best economy, speed while being kind to engine.

Didnít get exact mpg but couple of weeks back it was doing roughly 15L for 15 nautical miles in mixed use, wot, 3/4 throttle cruise and 6knot estuary etc

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Photo of prop stamp attached. There is a Facebook group member (worked at Orkney) seems to think they suit a prop 2 ins less than standard prop
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Old 24 January 2022, 19:09   #8
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Yamaha 30 Prop size vs rpm

Thereís better experts on here butÖ
I would expect 1ltr/nm from a 150hp plus. I would say your either water logged and pushing a ton of weight. Over propped although your rpm suggests not by by much or your figures are way off. Important that the the data you use toMake any changes is reliable !
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Old 24 January 2022, 19:48   #9
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Something is amiss, I would think you should be getting 22-24 knts with a 30 on the back of a Strikeliner
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Old 25 January 2022, 04:11   #10
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Quote:
Originally Posted by Yabadabadoo View Post
Edit: this outboard only revs to 5000 rpm

Can’t insert image from phone showing manual spec. 22.1kw@5000rpm
Here it is.
Gear Ratio is 2:1
WoT range 4500-5500
12p prop is middle of the range

Using 16kn, 4400rpm gives 26% slip
If the slip comes down to 15%, a 10p will return 19kn @ 5500rpm.
On a light boat, 12" pitch returns 24kn at 10% slip.

26% slip is poor, indicates heavy boat or drag. You are not reaching WoT range. Reduce slip by fitting larger dia prop or 4 blades, reduce pitch to compensate for increased blade area.
Make sure engine height is optimised before messing with props.

It's a restricted/rev limited version of the F40. Maybe a used ECU would uprate it and you can keep the 12p? Before anything, set the engine height. It's free.
F40 WoT range 5000-6000rpm

I've yet to try my Hustler, it was only £95 inc hub kit & vat. Highly recommended. Notice how the dia increases when the pitch decreases.
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Old 25 January 2022, 08:55   #11
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Wow, was not expecting that. Next level info.

These orkneys have a rep to hold water between the skins. Worst case keel guard screws letting in water, usual suspects are the anchor well through hull drain and general rainwater getting in around cuddy or deck. Boat will come out in next month or so and Iíll go over it and clean up hull.

This F30 is carbed would an ecu change things? I read a while back difference between 30 & 40 was a plate on the back of the carbs. https://www.thehulltruth.com/boating...rade-40hp.html quite a bit of chat there on the way Mercury did it using the Yam block. Not sure where to find part numbers to compare.

My hunch is weight, smooth hull and prop. Outboard is bolted through the hull so it had better be lined up right!
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Old 25 January 2022, 09:13   #12
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Quote:
Originally Posted by Yabadabadoo View Post
Wow, was not expecting that. Next level info.

These orkneys have a rep to hold water between the skins. Worst case keel guard screws letting in water, usual suspects are the anchor well through hull drain and general rainwater getting in around cuddy or deck. Boat will come out in next month or so and Iíll go over it and clean up hull.

This F30 is carbed would an ecu change things? I read a while back difference between 30 & 40 was a plate on the back of the carbs. https://www.thehulltruth.com/boating...rade-40hp.html quite a bit of chat there on the way Mercury did it using the Yam block. Not sure where to find part numbers to compare.

My hunch is weight, smooth hull and prop. Outboard is bolted through the hull so it had better be lined up right!


If itís on a trailer take it to a weighbridge find out what weight your pushing
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Old 25 January 2022, 10:13   #13
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Good call on the weighbridge.
Weighed my boat a few years ago at one of my local 'Civic Amenity' sites - aka local tip.
IIRC cost £5 & got a nice printout of the weight which I carry in the car.
Ring around to find one with a weighbridge.
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Old 25 January 2022, 13:14   #14
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This F30 is carbed would an ecu change things? I read a while back difference between 30 & 40 was a plate on the back of the carbs. https://www.thehulltruth.com/boating...rade-40hp.html quite a bit of chat there on the way Mercury did it using the Yam block. Not sure where to find part numbers to compare.

My hunch is weight, smooth hull and prop. Outboard is bolted through the hull so it had better be lined up right!
Thanks.
There may well be a restrictor plate, maybe different carbs or jetting, I assumed it was fuel injection because I've a two year older Suzuki 50 that's EFI. Most of those extra 10 horses will be produced at the higher revs and it will have a rev limit set in the ECU to prevent them being liberated. These guys can modify your existing ECU even if just to change the ignition advance or rev limiter:
westwalestuning.co.uk/yamaha-outboard-remapping/

The parts list to compare models is here:
F30
F40


They normally are bolted through in my (limited) experience. I'd even bolt for 20hp. If the sealant's failed this could be a source of water ingress.

The AV plate should be set level with the bottom of the hull, but usually 1 hole up (19mm) will still be fine, and less often 2 holes up (38mm) can be set without the prop venting. Do measure. It can be worth a pitch or two to get this right. All for free! Otherwise you are wasting energy. You shouldn't have to re-drill the transom.

A few years ago I 'uprated' a well set up Merc 30 with stainless prop for a new Merc 40 engine on my RIB and was underwhelmed to say the least. The Marina's workshop guy set the new engine below the keel.
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Old 26 January 2022, 23:23   #15
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All good intel, Iíll come back to this in a few weeks.

Boat will come out of water at some point in Feb, see about draining it, (!) sealing all potential sources of leaks, clean hull, check height of OB vs keel.

Spoke with Yam dealer who thought the block and camshaft are the same for 30ís & 40ís and it might be the jets that are different.

Iíll have a look at those parts numbers
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Old 27 March 2022, 11:17   #16
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Just back from a day scraping anti foul upside down from boat. Took a couple of photos to further this thread.

Cav plate is maybe 2-3cm higher than the ĎVí of the transom.. donít know if thatís good or bad - it doesnít suck air in a straight line or hard turn. Itís bolted through transom so adjustment is possible but only to move the outboard higher.

ĎKeelí of boat is maybe 12cm lower than the transom ĎVí (see second photo)

Anti foul (the hell) was a bit rough and pitted so I elected to scrape it off. In hindsight a wet sand and another coat of paint would probably bring the same result as a complete strip down. How much drag can a pitted hull create?

Did get some water out of the hull but trailer wonít tip back far enough to get it all out. Not a huge amount at roughly a 30 degree angle, maybe a litre or so.
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Old 27 March 2022, 11:39   #17
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Edit am I having a brain fade - is the cav plate the lower plate? Facepalm
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Old 27 March 2022, 14:11   #18
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Quote:
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Edit am I having a brain fade - is the cav plate the lower plate? Facepalm
Yes av plate is the lower plate, the fact you have a keel further forward youll probably need to experiment to get the best out of it. Normally I'd start with the av plate slightly above the hull and keep raising one hole at a time until its ventilates excessively then drop back one hole & your about spot on. With yours as its already fitted I'd probably start moving it up & see what happens youll know when you hit the sweet spot

Sent from my SM-G950F using RIB Net mobile app
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Old 28 March 2022, 09:49   #19
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Agree with Ken re the keel... I'm not entirely sure about setting up an OB on these hulls but the results will be different to a SIB. My gut feeling is still to take the OB up an inch or two and drop pitch by a similar amount.
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