Scorpion Owners - Propeller Question

[Chris Murray]

I am struggling a bit here ...

Most of my propeller tests are unreliable, on account of the hull fouling problem over the last couple of months.

I am told that my boat - an 8.5 metre Scorpion with an Etec 250 (and heavy arch + radar and an 80 gallon fuel tank) attained 44kts at 5800 RPM when "tested" before I took delivery. Even pre-fouling, owing to a problem with the initial propeller being faulty (slipping against its rubber hub) I have never been able to reproduce this.

The prop is a 25" pitch Renegade (4-blades). Before the growth set in (about a week!) this prop gripped the water well and had strong acceleration.

Talking to Scorpion, it seems that 19" pitch props have been fitted to some Scorpions and that these might give better performance. Everyone says that selecting props is a "black art" but it would be really helpful to know the following about your boats (ideally 8.1 metre and 8.5 metre)

Size of boat and fuel tank
Propeller fitted (make, name, pitch, number of blades)
Top speed attained (in kts)
Max RPM at which top speed was attained
Your view of how well it grips the water

I am looking for top speed (ideally 45kts plus), economical cruising (at around 35 kts) and good grip. Is this attainable on my boat?

I really don't understand how reducing the pitch so dramatically can improve top speed.

I look forward to hearing about your experiences.

Thanks for your help.

Chris.

[Fjordrafting]

Hi! We used to have a Scorpion 8.75 with twin 250 Rudes. It is not to compaire, but I know the boat.
We sold it and bought a Revenger 29, same length, but with stepped hull. It also has twin 250`s. What we have learened tha could help you, is that we have tried to drive it with one engiene out of the water, so the other was off center, very little gasoline and one person on board. We did 52 knots going with the wind and waves! The steps may give 3 extra knots.
The propellers on this one is Bravo 1, a four blade propeller. The pitch is very different as the gearratio on the Suzuki is 1:2.29 vs the Rude has 1: 1.85. But the pitch is 32".

We have an other Revenger witch we have tested with Bavo1 and Revolutin IV. The Rev IV gave us 5 more knots without load, and 2 knots less with load.

The latter means that you can build for: light boat and high speed, but it will not work with load. So there you go!

Also, lift the engine four holes, and you gain a lot of speed when empty boat.
Have a nice day at sea!

[codprawn]

My boat is of a similar size (revenger type hull) and with a similar sort of engine and gearing - with a clean bottom she will do 41kts with a 19" 4 blade prop. I have a 24" I am about to try but I don't know if my engine will swing it.

From what I have been told a 21" is pretty ideal - better top speed and not much worse acceleration.

I am suprised at you running a 25" on a big heavy boat and still being able to hit the rev limit - if they told you the truth that is - what revs are you actually getting? IF a 25" prop is doing 5800 rpm what the hell would a 19" hit???

As you say it is hard to get proper figures if the hull is fouled - I have lost a load of speed.

[Roy Smith]

Hi Chris
Mines a 8.1 With a 250 Yamaha F/Stroke 80 gal tank, i think that as the engine is a heavy lump, You have a E-Tec (lighter) the differance with the A frame should be about the same.
I went out with Graham and a man from BHG
using a 23 inch Yamaha pro plus prop had 3/4 tank GPS read 49.5 kts
I desided to go for the stanard 23 inch yamaha S/S prop best speed 47 kts
but as we do a lot of water sking/wake boarding, the comprmise is good
as we get up on the plane quicker. We did the round the I.O.W cruise on Sunday and fuel used worked out at just a smig over 1.20 lts per mile and that is with 5 people and two hours of wake boarding at the end of the day.
I dont keep mine in the water so hull is always clean ?.
Must be honest the more i use it the more i love it, They do build a great boat. They are a few ribs going to Alderney on Sat ?
Regards
Roy

[jwalker]

Originally Posted by codprawn

....I am suprised at you running a 25" on a big heavy boat and still being able to hit the rev limit - if they told you the truth that is - what revs are you actually getting? IF a 25" prop is doing 5800 rpm what the hell would a 19" hit???

And that's a 25" 4 blade prop. I'm with Codprawn on this one.

Something fishy in the figures, methinks.

Unless the boat is lighter than we imagine.

Could it be possible the the prop hub was slipping a bit when they were getting 5800rpm?

[Jimbob]

Hi

I saw your boat at Northney we were next door to you, remember, we have a 200 Fitch (evinrude) 7.5 , 19 inch * 14 and a bit i think, we have just put a 19 inch slightly different dia. both work well, but i am not a speed man, we do about 24- 30 knots generally and both seem fine

is the engine mounted correctly and what did powertech saw while you were at Northney

[MarkM]

Have a Scorpion 7.5, LOA 7,75m, with a wrap around console and closed deck.
The latest model Evinrude 250DI from 2004 and a 3 blade 23" prop.
At 5800rpm I do 53 kn with 2 people onboard and a 100 gallon tank.

[gtflash]

Originally Posted by MarkM

Have a Scorpion 7.5, LOA 7,75m, with a wrap around console and closed deck.
The latest model Evinrude 250DI from 2004 and a 3 blade 23" prop.
At 5800rpm I do 53 kn with 2 people onboard and a 100 gallon tank.

i love the foredeck on that boat its ace! hows she comin on? anymore pics?

[Chris Murray]

Thank you for all your replies so far - much appreciated.

Hi Fjordrafting - Interesting to note that the Revolution IV performed well with a light boat but not quite as well with a loaded boat.

Hi Codprawn & JW - I currently have another 25" Renegade (on loan from Evinrude, I think) but it has scale on it, so not a perfect example. My top speed dropped to 29kts before lifting. This increased to 36kts after removing the slime but the barnacles are still there, so I guess that something like 42-44kts is feasible.

It achieved 5600 RPM (assuming that it was not slipping at the time) when I got 36 kts. A week later, it was down to 32kts and revs had dropped a bit. Fouling should become a thing of the past, once the hull has been properly cleaned and antifouled. It should be all done by early August.

Hi Roy - 47kts? Now that's more like it! But 23" is a long way from 19" that has been suggested. This does seem to justify my confusion!

I am afraid that I can't come out to play on Saturday! I am taking the boat in for the hull work this weekend. I will certainly be game for this next season ... but the petrol is MUCH cheaper in Guernsey! Have a good trip!

Thank you for the fuel consumption info, I doubt that my E-tec will be much different to this, which is most helpful.

I think there may be a limit on post size, so will continue ...

[Chris Murray]

... continued

Hi Jimbob,

I wondered if you were the same "Merlin" that posted on here!

The real value of my visit to Hayling Island was to get the engine checked out. At least two of the spark plug gaps were set incorrectly when first installed (way out), which Keith put right. I was also coming out the other side of the extra oiling period (the first ten hours), which really seems to screw things up performance-wise and doesn't help when testing different propellers.

Keith also kindly offered to try a 23" Viper (3-blade), which achieved 41.7kts The boat was about 3 weeks old (so already some fouling) but with a fair amount of slippage - so not an ideal prop.

At that time, I had a 26" Raker (not sure how many blades). This slipped to the point of being completely useless! Keith was not impressed!

I later tried a 21" Revolution IV, which I think was a good prop but the hull was pretty fouled by then. It did give 36kts at 5400 -5500 RPM though, so might be worth revisiting and perhaps comparing with a 23" prop.

Hi Mark,

53kts - if only...!

I guess that being a metre shorter, there will be less boat in the water, so you can get speeds that I can only dream of!

Thank you all for your comments. Further comments and statistics would be most welcome from all you Scorpion owners out there.

Happy and safe boating!

Regards,

Chris.

[MarkM]

@gtflash: I'm so happy with the Scorpion and the engine. For more pictures:

http://www.rib.net/forum/showthread....948#post138948

and: