Ribcraft 585/150 Evinrude prop size help!

[JABS]

Hi,

I just changed the engine on our Ribcraft 585 to an Evinrude Ficht 150 two stroke. Has anyone experience of that combination and can you offer advice on prop choice? I have tried 23 and 20 pitch and am still struggling to achieve WOT. Could I have a fuel starvation issue???

Thanks.

Tony.

[Hightower]

Facts and figures is what we need. What revs and speed is she pulling at WOT.

I have a 150 FICHT too, she pulles about 5000RPM and acheives about 40knotts with a 19" prop on a 6.2m. You should be getting about 44knotts of speed at least with a 5.85.

[JABS]

First run was with a 23 inch pitch p[rop. I got 4000 RPM and 39 knots. Fairly calm conditions.

Put on a 20 inch pitch and, although it was too rough to go balls out for long, was only getting around 4300 and no more speed.

I need to do a run on the 20 inch prop in calm conditions, but I think it is still too coarse.

I have been offered a 19 to try and think that looks promising.

23 an 20 are stainless, 19 is ally. Is that a problem??

Speed as measured on GPS, on the 23 in two directions. On 20, only one way in S'hampton water, so could have been a knot or two against me.


Didn't want to get into so much detail initially.

Thanks for your help.

Tony

[Neil Harvey]

Tony, You are welcome to try my 19" stainless at some point if it helps.
I went from a 19" that had me pulling over 6000 rpm to a 21" pulling about 5800-6000, that's on a 175 DI and 7.2m rib.

[Hightower]

Don't forget that the 175 has a higher WOT throttle range than the 150 which is 4750-5250.

Are you sure there's no extra weight on the RIB like a bilge full of water?

My speed drops to about 37 knotts with 2 up and 200 litres of gas on board, revs drop to around 4800rpm trimmed right out.

When I used the 21" prop the speed was pretty good at about 42 knotts and the revs dropped to 4400.

The other thing that you can check easy is the throttle is opening fully when the throttle handle is fully forward.

Hope that helps a little more.

[JABS]

Originally Posted by Neil Harvey

Tony, You are welcome to try my 19" stainless at some point if it helps.
I went from a 19" that had me pulling over 6000 rpm to a 21" pulling about 5800-6000, that's on a 175 DI and 7.2m rib.

Neil,

Would love to try that prop.

I hope, this weekend, to try the 21 in calm conditions, even if I have to go up to the container basin. That will give me a final result on the 21. Then it could be time to go to 19. Thanks Neil.

Hopefully more on Monday.

Thanks for all the help. RibNet is a truly valuable resource.

Tony.

[JABS]

Originally Posted by Hightower

Don't forget that the 175 has a higher WOT throttle range than the 150 which is 4750-5250.

Are you sure there's no extra weight on the RIB like a bilge full of water?

My speed drops to about 37 knotts with 2 up and 200 litres of gas on board, revs drop to around 4800rpm trimmed right out.

When I used the 21" prop the speed was pretty good at about 42 knotts and the revs dropped to 4400.

The other thing that you can check easy is the throttle is opening fully when the throttle handle is fully forward.

Hope that helps a little more.

Andy,

We dry sail, so don't think we have a bilge full, but that is a valid point which I will check.

The tests so far have been done with one up and 80 litres of fuel. Nothing else of note on board, except the results of many beers over the years!

I need to have a definitive run with the 21, hopefully this weekend.

Thanks for your help, Andy.

[Hightower]

Well check the throttle, that's easy. Fully open the throttle lever on the console and see if there is anymore travel in the linkage under the cover (butterfly valves horizontal).

one easy method of discovering whether the outboard is being starved of fuel is to go WOT on a cruise and get someone to pump the primer bulb. If you get extra performance when the primer bulb is being pumped then the answer is yes, if you don't you keep looking.

One other performance area to look at is the height of the outboard relative to the hull bottom. Too much outboard leg in the water will cause drag and slow the boat, so check that too this weekend too.

[Downhilldai]

Who fitted the motor? Do they have facilities for you to test different props?

The WOT RPM sounds low to me for such a high power o/b on a light boat. Set-up may need looking at, as Hightower says above.

[JABS]

We finally got some WOT running on the 20 inch pitch ss prop on Saturday. We achieved 4400rpm but had a GPS failure so can't report the speed attained. Even at the top end of Southampton Water, in calm conditions, we didn't gtet above 4400.

The boat jumped up onto the plane easily and was very responsive.

Could the Tacho be out??

I now have the 19 pitch prop in the car, ready to try. I don't want to risk over revving the outboard.

Is the general opinion that we have another issue to address???

Thanks

Tony