Ribcraft 5.85 Prop sizes

[jools]

I know prop sizes are talked to death, but I know there are a number of people with simular boat setups to mine......

Ribcraft 5.85
Mercury 4 stroke 90 hp

(I think clarky's is identical)

Anyway I currently have a 15" Pitch Aluminium prop. When I was out on the solent the other week (nice and calm)

3/4 throttle = 5000-5500 RPM - about 25-30 Knots
WOT = 6000 RPM - Max speeed 35 knots

The engine never seems to sound as if it is under any real load

I suspect that at 6000 the engine is being limited by a rev limiter rather than running out of "power"

So from this I assume I am underpropped and would like to upgrade to a stainless prop, and I was wondering what other people with the same boat have fitted.

The very nice chap at Race & Marine in Taunton where I have my Rib serviced is going to lend me a 20" stainless prop to have a play with on my Scillies trip - but this seems like a big jump. (He doesn't have anything in stock inbetween) - So I'll give this a try and see how I get on.

It would just be interesting to know what others have - Geoff, Clarky, Mstatcy etc.

Cheers
Jools

[LURCHER]

I have 115 Evinrude Ficht on a Ribcraft 5.85 . At present it is fitted with a standard OMC prop part no. 176214 13.1/4 x 17 this produces 41mph at 6000 rpm which is causing overev warning as WOT is 5750 . I tried a 19'' OMC prop but this does not pull more than 5000 and acceleration is poor . I think an 18'' would do the job but none are made .I am told Steel Dev can alter mine to 18'' Anyone have an opiion .

[mstacey]

Jools

Whilst I have the Ribcraft 585 my setup, weight , engine etc is quite different.

I have the 140 Suzuki with 3x14x21" prop that hits 6200 rpm with ease and gives a very respecatble 40+ Knots depending on Sea conditions.

[stuart hedges]

It's not a Ribcraft but a Ribtec 5.85 with a 135 Optimax, I tried loads of props on that and eventually ended up with a 19'' stainless and a 4 bladed plastic as a spare. They both work well. I have a spare 19'' stainless that you can borrow if you want.

stuart

[jace]

Hi Jools

I would first of all check the engine height; we tend to fit the majority of engines one hole up on the transom. Bit more cavitation in tight turns but generally better running.

Reckon 20” will be over propping her, If its a S/S Prop your after, would look at an either a 16" if the engines at the right height or maybe an 18” if it can be jacked a hole.

Normally a 17” allu prop would be fitted.

Jason

[Dave Clarkson]

Hi Jools,
Yes, my set up is identical to yours. However I have to admit I have no idea what prop pitch/diameter is fitted!! Does that bar me from the auspicious pages of Ribnet??
I can confirm tho' that my performance is exactly as you quoted above. When you try the SS props, I'd be very interested to know the results.

Regards,
Dave

[JCW]

Jools,
Bern Hanreck & I have a 19" aluminium prop on our 5.85 Ribcraft + Yam F100.

We pulled 35 knots this weekend with 5 people and approx 30 gallons of fuel @5500RPM. I believe the Yamaha F100 should be propped to pull 5500 - 6000RPM at WOT. I am interested if it pulls to 6000 with a lighter load.

Another question,

According to our Floscan we used 105 litres of fuel last weekend and covered 80 (statute) miles. Admittedly this started with a full 40 gallons of fuel and 3 - 5 people on board at all times. This seems high to me although when we next fill up we will verify the accuracy of the gauge.

What fuel consumption do all you 5.8m rib owners average?

Does a floscan require calibration?

Thanks in advance

[benjamin]

Jools,
I have an identical set up to yours - from the same supplier indeed, although I have not manged to hit the magic 35 kts yet. As with the previous response, I am very interested to hear how you get on with a s/s prop, as I found 2 huge nicks in the leading edges of mine today (looks like floating debris as I am certain I haven't grounded it) My plan is to invest in a s/s one and keep the existing as a spare.

[jools]

My fuel consumption for a "Normal" run out is almost spot on 1l per nauticle mile.

This is running most of the time at about 5500 - 25-30 Knots

It will be interesting to see what a change in prop makes.

Thanks for all your replies guys - and I will keep you posted!

cheers
Jools

[Alan]

Jools I'd say 20" is going to be too bigger jump. 17" sounds like a winner though. Stainless props are not always better - I got better performance on Yellow Tang from a 17" Alu rather than a 17" S/S. (When running the old Johnson VRO 150). I do run a S?S prop on Blue Ice though.

Jason - That does sound a bit Thirsty. I'd better get you that floscan info I promised!

Alan

[JCW]

Cheers Alan,
We plan to have another trip out on Sunday so when we fill up we will see if the gauge is out. I hope it is.
How accurate is you floscan Alan? Have you ever calibrated your unit?

Jools, Our current prop is a Stainless 19", not an aluminium. I thought it was ali as it has a painted finish!!

[jools]

OK - findings from my trip to Scillys

Fitted with 20" Stainless prop

The engine sounded much happier - not racing away like it does with a 15" ali, a little more under pressure! - but this time a little too far the other way (as suspected)

At WOT only managed 4900 RPM, so as we are aiming for 5500 I thing we will try a 17" SS prop (200 RPM per " and that should be about it)

Comments?

Jools

[batfalcon]

What was your speed at WOT with 4.900 RPM ?

[jools]

just about 30 knots - although conditions not ideal!!

[JCW]

Jools,
a 17" sounds about right for you. The Yam F100 has a 2.3:1 gear ratio which would be why we have a 19" prop and achieve approx the same performance.

[wavelength]

Jools
Isnt the merc 90 four stroke rated at 6,000 rpm (with the 75 at 5,500)