Ran Ashanti with a powerboat instructor onboard today.Out of curiosity we got max 4,200 rpm for 11knots out of my Yamaha 50hp on a 13" prop.
At 3,200 rpm she settled on 8 knots .
Any idea what range my 50 litres might achieve .
Is that good,bad or normal.
Hi Gary,
sounds similar to me and my M. I was getting around 11 to 12 flat out depending upon wind and waves. It chews through fuel tho!! I burned a tank between Marchwood and Ryde (without looking it up I'd guess at 15 miles and an hour and a half run). I opted to fill up before the return. I now have 4 tanks...thanks to Roly's research.
If you have the four stroke Yam 50 I think it produces most power around 5500k rpm so optimally would be using full rev range.
I have now fitted a different prop (displaces less water - I'll have to come back to you with size maybe 14x11) so I should see revs in the 5000s. Time will tell if this makes a speed difference.
Hopefully fixed my overheat problem so will get out to play in the Solent Thurs Friday next week. If so do will report back..
I also am fitting a new 11 inch prop soon and will report back .
On our X with a 15" prop we can get 14 knots if we dump the ballast (Yamaha 50). The boat is full of all our gear though so would expect it to be a bit faster if it were empty. We rarely run this fast for any length of time because as Matthew says it drinks fuel. I would be interested to hear what improvement you get with an 11" prop.
Regarding range we reckon about a litre per mile when motoring at around 4-5 knots.
Hi Gary,
When I owned ASHANTI I did some trials to work out the fuel consumption
At 5-6 knots( in displacement) approx 2900rpm fuel 5L per hour giving a range per 25L tank of about 25M
At 10-12 knots (on the plane) approx 4000 rpm fuel 8Lper hour giving a range per 25L tank of about 30 M
The official Yamaha 50 figures are
RPM L/Hour
1000 1.14
1500 2.28
2000 3.40
2500 4.6
3000 6.50
3500 7.20
4000 8.70
4500 9.90
5000 12.10
The best speed I got was 15-16 Kn in flat calm but with any waves 12/13Kn
I hope this helps,
Good to see Ashanti is getting used,
Regards,
John
Thanks John. She is being used presently from swinging moorings at North Fambridge Yacht Station.
I don't wish to boast but I averaged 20mph in Sowenna with her Yamaha 50 on a run from the QE Bridge to Gallions Reach near the Woolwich Ferry.
Slack Water like glass, no breeze, everything was still and very quiet with a slight haze coming off the water. The Marina manager was waiting for us to get back, so he could lock up and said we just seemed to appear silently out of the haze on a plane. My crew raced Ribs and trimmed while I steered.
I hope this wasn't a 'one- off', (well it was for me)
Simon
11" Prop.Calm water.Average up/down stream
3 persons on board ,full fuel.11" prop calm water
RPM KNOTS
1000 1.7
1500 2.8
2000 3.9
2500 4.5
3000 5.6
3500 6.3
4000 6.9
4500 7.5
5000 8.0
Well the 11" has allowed the engine to increase revs up to 5,200 max from the 13" 4,200 max .This pleases the Yamaha engineer who says it should operate between 4,500 -5,500.He also suggests the 11" will be easier on manoeuvres in a marina.
From my point of view it seems I have lost speed at all rev ranges.13" gave 8 knots for 3,200 rpm and 11" gives 5.8 for 3,200 rpm.
The engineer explains the 13" was holding back the engine from achieving the correct revs range ,like driving slow in a high gear .
However the max of 4,200 on the 13" ,ignoring fuel burn,was returning better SOG .
I guess whilst pottering in the river Crouch is ok it would be better to change back to the 13" on a passage.The four fuel tank idea also seems the way to go.
That's really interested Gary, thanks for sharing that data. I can understand that the engine might be more responsive with the lower pitch propeller but the sacrifice in speed for any given rpm is significant. Like you say, on passage do you really want the engine screaming away at 3-4000 rpm to achieve 6 knots?
I bought our engine second hand and it came with a 3 spare props 19" 17" and 15". I had wondered about using the 17" or 19" on rivers and canals. The idea being that at low engine speed (so lower noise) the boat would make faster progress. Admittedly the engine will have to work harder to do this but can't see why that would be a problem at 1000rpm.
The optimum range 4500 to 5500 is all very well but thats high fuel burn also.If 5,500 is the max according to Yamaha then 50% will be 2,750 & 3,300 60%.More sensible fuel burn .
Yamaha suggest 7 litres h/r at 3,300 rpm giving 8knots on a 13" prop.
What speed does 15" give at 3,300 ?
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