Prop size

[Roy Smith]

Help needed
Boat 3 up 3/4 to full tank
1st Prop standard 23" black S/S Yamaha W.O.T 6200 rpm 43 Kts
2nd Prop 22" Yamaha polished S/S pro plus as above went to rev cut out 43 knt
3rd Prop 24" Yamaha Polished S/S pro plus as above W.O.T 5500rpm 45 kts
cavitation plate sits 3" above bottom V of hull.
my last rib was 1"&1/2 above.
Dose any body has any experience i thought i should get a bit more speed
but the bigger prop at lower rpm is faster ? any thoughts please

[Bigmuz7]

Take it conditions were the same all round ref wind etc, also on same run & tide, ... and was speed taken from GPS ? since running with and against tide in some areas makes a hec of a difference. 5 Knots of tide against you for example could explain your lack of apparent speed gain (sorry Roy I only suggest without knowledge of your experience) Also how long is 8m+

[jwalker]

Originally Posted by Roy Smith

Dose any body has any experience i thought i should get a bit more speed
but the bigger prop at lower rpm is faster ? any thoughts please

I don't have any Scorp experience but your figures sound about right to me unless Scorps are built pretty light. 250hp isn't a lot of power for an 8.5mtr boat

You mention the height of the motor, is there a prop holding problem?

[Hightower]

If your prop is gripping in a corner and not ventilating at top speed then I would guess that outboard hight is OK.

Agree with above....Big RIB and 250hp sounds like 45 knots is about right

You might like to re-prop for different circumstances like i used to. ie: smaller prop for heavy running and poor sea state. Larger prop for lighter running and smooth sea state.

[qcamel]

I have an 8.5 Scorp with 275hp and get around 49 kn with a 19 inch prop, the 21 gives me 50 kn+ but seems a little sluggish at cruising speed acceleration. My 8.1 stepped hull Scorpion which was a pretty light boat and ran a 225 Opti was capable of 53 kn so looking at things I guess that you wot top speed is about right. The interesting things is that the 8.5 is so much smoother that in the real world it is a quicker boat over a distance with varying sea conditions but the way it delivers the speed is so relaxed it feels slow. For me the 8.5 is a much better boat than my 8.1 was.

However the easy way for you to hit 50 kn+ is buy one of the new 300 hp engines that are out there!!!! Oh and you could buy me one at the same time if you are feeling generous.

[ian parkes]

havin similar experiences , bigger pitch went faster at 300 rpm less untill I had the usual 3 people and gear on board and a bit of chop then it was slower .
Only positive thing is I went from a 17 to a 19 so if i can find an 18 Ill be sorted .

What I need now is quick release re usable split pin for the prop nut as Iam changing them a lot.

[Roy Smith]

Thanks for the help
Yep same conditions inside langton harbor. (very naughty i know)
Boats 8.1mt
Ollie i tried the 8.5 mt and it dose feel better but i keep my boat at home and the 8.1 is as big and i can get into the farm its also a big rig when towing and launching.
maybe i am expecting to much

[qcamel]

Hi Roy

Did not realise its a 8.1, does seem a little slow to me. Is she a heavy boat. A frame/radar etc? Have you had any other engine fitters look at the setup?

[Roy Smith]

Have asked BHG of Lymington Apparently UK biggest Yamaha engine supplier
he said that when they set up cobra ribs they put the cav plate in line with the v of the hull, Scorpion say this should make more drag = slower top end etc and they put it much further up.
i am going to try a 23" pro S/S should give a bit more but looks like i will never see 50 kts . To be fair the tests have all been done with literary a full tank of fuel 400 lts and 3 to 4 people so am i wanting to much.
I,ll post the speed with the new prop

[jwalker]

Originally Posted by Roy Smith

To be fair the tests have all been done with literary a full tank of fuel 400 lts and 3 to 4 people so am i wanting to much.

Roy, that's over half a ton of load. Depending on the weight of the folk, it may be well over.

[Paul Williams]

I have just purchased a new Scorpion 8.1 with a new Suzuki 300 engine and it just cavitates .I have been in touch with scorpion just 2 days ago to say the the cavatation plate is sitting 3 inches above the transam ,they have told me that this is normal and explained why.They have sent me today a new prop that I tested for them this afternoon ,its giving better grip but less top speed so hopefully a 24 inch prop is on its way tomorrow that should correct the top end speed.I have found Scorpion very helpful in trying to rectify the problem and they are apologetic but have said this engine is new to them as well as this is the first 300 engine they have supplied.

[codprawn]

Originally Posted by Paul Williams

I have just purchased a new Scorpion 8.1 with a new Suzuki 300 engine and it just cavitates .I have been in touch with scorpion just 2 days ago to say the the cavatation plate is sitting 3 inches above the transam ,they have told me that this is normal and explained why.They have sent me today a new prop that I tested for them this afternoon ,its giving better grip but less top speed so hopefully a 24 inch prop is on its way tomorrow that should correct the top end speed.I have found Scorpion very helpful in trying to rectify the problem and they are apologetic but have said this engine is new to them as well as this is the first 300 engine they have supplied.

What sorts of speeds are you getting with different props - interested in the 300 myself!!!

[Paul Williams]

With the 23 inch prop 3 I am acheiving 60mph but the prop is cavatating so the accelaration is poor so trying a new prop this weekend

[Arjen]

Hi Roy,
I own a 8.1 with a 250 E-Tec. My propeller is a Mercury Revolution 4 21" pitch.
The top speed that I have managed (1 person) so far is 90 km/h which is over 48 knots.
Since it is a 4 blade propeller, I do not have a power lift installed, I expect to go at least
5 knots faster when optimally tuned with faster prop and power lift.
The Scorpion handling is amazing, it drives easily at top speeds (no chinewalking whatsoever).

[Roy Smith]

Well the engine is now Cav plate 2 1/2 " above hull line and i am collecting a
23" by 14 1/4 pro plus competitions Yamaha prop.
as the other props where 14 this extra 1/4 should help a bit and it should rev a bit more = better top end so hopefully fingers crossed this should be it
I had the same problem when i had one of the first 250 they said the same to me new engine etc. but i would like to know what yamaha recommend there must be a maths calc to go by
It would be nice to know what others have had there engine set at

[Tony]

Hi Roy
I have a Scorpion 8.1m with a 250 DI, using a 23 mercury s/s high 5 prop.
I was out Monday 2 adults 4 teenagers dingy, small outboard,and about 280ltrs of fuel. I achieved about 40-42knots. On my own the previous week in calm seas with 160ltrs fuel (boat empty of rubbish..) I get 46-48 knots.
In the new Sports boat and rib mag there is a catalogue showing lots of different props, i quite fancy the new Merc Enertia..

[Roy Smith]

It makes me wonder how much we have to spend to achieve
4 / 5 kt more i bet that high 5 prop was not cheep
This pro plus thing i am collecting to day should have been £650 lucky for me they have taken the old one back in part ex
Things where much less complicated when i had a 560 Avon with a 100 Yamaha (cheaper to)

[Roy Smith]

Well out to day
new Yamaha pro plus 14 1/4 " 23 " prop
7 up 350 lt of fuel doing 44kts at 5500 rpm WOT so i am at last happy
have not checked fuel used but it cant be any worse than before.

[Bittentobuggery]

Originally Posted by Roy Smith

7 up

Is that a drink or a Snow White fantasy ?


Hi Roy, will the 250 not rev up to 6000 ?

If you're happy, that's the main thing.

ATB

Brian

[Roy Smith]

No i could not get 6000 rpm
this prop thing is weird i can only get 6000rpm with a standard 23"black S/S
or 22" pro. So i thought i will keep the standard black for bad weather and water sking and the new one for cruising.