Ribtec 535 Prop

[HMJB]

Hi

I wandered if anyone could give me some advice - I have a Ribtec 535 with a 2006 Yamaha 2 stroke 75hp.

When I bought the boat it came with two props.

One which was on the engine and is marked SSI 13x19-K and another marked 13 1/2 x 15-K.

The current one on the boat seems to have quite a lot of holeshot (gets on the plane relatively quickly and easily with 6 adults on board) and is giving a top speed of just over 30 knots.

Last time I used it (with two adults and three small children onboard) we covered around 30 miles at around 20 / 25 knots and about 8 miles up and down the Beaulieu river at around 5 knots and used around 45 litres of fuel.

I was wanting to see whether I can get better economy with the other prop. Am thinking not; but am not sure?

From what I understand the spare prop is going to give more holeshot as a lower pitch and lower top speed. But as is a larger diameter I may be wrong? I don't know whether another 1/2 inch diameter is going to make much difference?

Any views greatly appreciated.

Cheers

Hartley

[Pikey Dave]

Originally Posted by HMJB

Hi



I wandered if anyone could give me some advice - I have a Ribtec 535 with a 2006 Yamaha 2 stroke 75hp.



When I bought the boat it came with two props.



One which was on the engine and is marked SSI 13x19-K and another marked 13 1/2 x 15-K.



The current one on the boat seems to have quite a lot of holeshot (gets on the plane relatively quickly and easily with 6 adults on board) and is giving a top speed of just over 30 knots.



Last time I used it (with two adults and three small children onboard) we covered around 30 miles at around 20 / 25 knots and about 8 miles up and down the Beaulieu river at around 5 knots and used around 45 litres of fuel.



I was wanting to see whether I can get better economy with the other prop. Am thinking not; but am not sure?



From what I understand the spare prop is going to give more holeshot as a lower pitch and lower top speed. But as is a larger diameter I may be wrong? I don't know whether another 1/2 inch diameter is going to make much difference?



Any views greatly appreciated.



Cheers



Hartley

What’s your WOT rpm with the current prop?

[HMJB]

Was showing just under 5k on an old analogue tacho - so not sure if reading correct, as seems pretty low.

[Pikey Dave]

Originally Posted by HMJB

Was showing just under 5k on an old analogue tacho - so not sure if reading correct, as seems pretty low.

Yup, you need to get some proper RPM figures as a baseline before you start prop juggling. That said, swing as you already have the second prop, there’s no harm in giving it a try. Dropping 4” in pitch is going to make a much bigger difference than going up 1/2” in dia. I’m guessing you’ll gain around 1000rpm. As it’s a 2 stroke, & presumably carbed, they perform better when free revving & not lugging.

[HMJB]

Thanks Pikey Dave - would make sense the second prop would then get to around the upper limit of RPM. Without knowing if the tacho is accurate - sure you are right, may as well have a try with the second and see.

Theres also a small ding in one of the blades on the 19 and the 15 is undamaged, which I suspect will make some difference as well

[Limecc]

1/2" smaller dia is exactly the difference for four pitches bigger and 75hp. Props have power curves like engines have power curves and ideally both are matched. In general terms if the blade design and number of blades is the same, then a larger diameter has more area and can transfer more power with less slip. Slippage is wasted effort and a waste of fuel with no gain. However because there's quite a difference lower pitch you may find the engine revving more highly negates any low slip advantage. Slip reduces at WOT because water behaves more as a solid at speed.

The number one rule before making changes is to make sure your engine height is optimal being as high as possible without ventilation. All tests should be made with the boat loaded how you will most often use it, so you could substitute people weight for sandbags.

Rather than relying on one figure - whether or not it will pull max rpm, you can take the GPS speed and rpm at 1000rpm intervals to plot a chart of your slip for the midrange rpms and compare props. The 15" pitch is middle of the road. Purely looking at the propeller tables, 19" looks to be too high and you confirm this in real life.

Running at displacement speed will have a hugely negative effect on fuel economy because of the wall of water you are pushing. A Doel fin reduces effort to plane and size of the boat wake so this might be an option even if not permanently fitted.

[HMJB]

Thanks Limecc. Thats really helpful.

In short and with my limited understanding it sounds like the 15 is probably going to be the better bet for all round use, albeit that it's probably pretty marginal.

My main use is cruising around the 20 / 25 knot mark (and achieving a good fuel economy as I can) and pulling the children on an inflatable - they're not very old, so that's not at great speeds either.

I'm not overly interested in trying to achieve the absolute optimum prop, just as I had these two thought I may as well use the one which best suits the engine and my use. Am also not going to mess around with engine height or buy other props. Interesting to understand that if I was though that need to look at performance across the rev range.

As the 19 has a ding in one of the blades and the 15 is pretty much perfect then sounds like it may be the best all rounder.

One other questions - when changing the prop, am I likely to be able to straighten and use the same cotter pin, or will I need a new one?

[Limecc]

You should get over 30 knots and better holeshot with the 15" prop, cruising speed under 5000rpm with less throttle opening and better economy?

P.S. Steel Developments can repair the old prop so it still has value if you decided to sell it. Probably not worth anything if aluminium.

[HMJB]

Thanks - will give it a go