Propeller choice and slip {Yamaha Stainless vs Revolution4}

[antaris]

Hello all,
I am in the process of fine-tuning my recently acquired 208. After reading a lot about props and slip and WOT and RPMs, I had the chance to test a 17' Merc Rev4 - and compared it to the stock "17M" Yamaha (Stainless, 3-bladed) that was on the boat when I bought it.

Here are the test results (obviously this is not scientific, but I guess a relative approach should make some sense):

CLICK HERE FOR FULL SIZE IMAGE http://i.imgur.com/WxPQ4.jpg

The grid on the right is the GW test results - middle is Yammy 17M - right is Rev4.

NOTES:
- ok, we got an average of 1mph less with the Rev4, although the rest of the characteristics (control, agility and lift) were better.
- What is the story with prop slip? Why so high? I know that the results are not 100% scientific, but 12-17% slip for a 4-bladed prop is quite surprising!
- Another weird thing. I got WOT of 6000RPMs with both props. I thought that going to a 4-blader, I should have a drop of 100-300 RPM at WOT.

Are Gradys so heavy that slippage of that magnitude is normal? I read in this forum that slippage over 10% is a sign of something going wrong.
The numbers prove that I should stick to the Yammy prop (should the sea be always calm), although the boat felt a lot more responsive and agile with the Rev4. So my gut reaction is to get the Rev4... the difference in rough seas was a lot more obvious - to the Rev4's favour.

By the looks of it, I could move up to a 19'? From the info that I gathered, everybody props their 208 with 17' props.....
I don't think I'll get the chance, just trying to determine whether I should buy that Rev4.

Your thoughts? I'd appreciate any feedback, since I am now more confused than before!!! :hmm

 

[seasick]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

I can't see the entire test. Something in the posting.
Note that the original factory test was most likely run on non ethanol fuel and if you new test were, you need to note it. E10 is less efficient than non E10 and can lower mileage by several percent.
Since I can't see the right of the chart, I don't know if the hardtop was included. The hardtop will also affect performance due to weight and added drag. It's equivalent to added at least another person in weight. Original Grady performance tests probably run with unpainted bottoms. The paint adds drag too.
Bottom line as you mention, is that it is hard to compare.
One thing that can be compared is the 'savings' in fuel versus the cost to replace a prop. If you get a 5% mileage improvement, that works out to about 26 cents per gallon counting gas(@4.50), oil($25 gal) and ring free($1 an oz). If you but a $300 prop, you won't start seeing return until you burn through 1150 gallons of fuel. That would be several seasons in my 208 with my usage. Just some things to consider

 

[antaris]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

Here is a direct link to see the full image: http://i.imgur.com/WxPQ4.jpg

The main point to observe is a comparison between the two props on the same boat - the manufacturer's test was years ago and I was not there. I've included it for reference only! Plus, I got bottom paint and hard-top and all safety and some fishing gear, as you mention.

Cheers Seasick, your point is taken - although it is the other way around! If I buy the Rev4 prop (450USD) and I get a reduction in speed / increase in fuel burn... Seems like a bad idea - but I loved the handling! I am based in Europe and Ethanol has not seeped into our fuel (yet)...

So the mystery remains... That much slip?

 

[seasick]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

Here is a direct link to see the full image: http://i.imgur.com/WxPQ4.jpg

The main point to observe is a comparison between the two props on the same boat - the manufacturer's test was years ago and I was not there. I've included it for reference only! Plus, I got bottom paint and hard-top and all safety and some fishing gear, as you mention.

Cheers Seasick, your point is taken - although it is the other way around! If I buy the Rev4 prop (450USD) and I get a reduction in speed / increase in fuel burn... Seems like a bad idea - but I loved the handling! I am based in Europe and Ethanol has not seeped into our fuel (yet)...

So the mystery remains... That much slip?

The slip is not all that surprising. Up to 10% is sort of typical. A 4 blade prop at low speed pushes more water and adds thrust and therefore better low end and hole shot performance. On the top end though, the wash of the 4 blades can create more turbulence(aerated water). That means more slippage. As you point out, you may have to decide what parameters are best for your boating, low end, high end, hole shot (and towing), or fuel economy. I guess you can't have it all.

Thanks for the data and tables. It's interesting stuff.
Cheers,
Seasick

 

[antaris]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

I see your points Seasick...

Interesting - there are very many fast RIBs in the Med (of the 40-60mph variety) and I've noticed that many of them sport 4-bladed props. But then again, at closer examination, I notice a tendency for slippage to go down as you increase RPM beyond 4500... So, quite possibly, for a 50mph boat, the rpms would be in that range (coupled with a 21-23-24' prop)...

 

[seasick]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

I see your points Seasick...

Interesting - there are very many fast RIBs in the Med (of the 40-60mph variety) and I've noticed that many of them sport 4-bladed props. But then again, at closer examination, I notice a tendency for slippage to go down as you increase RPM beyond 4500... So, quite possibly, for a 50mph boat, the rpms would be in that range (coupled with a 21-23-24' prop)...

The science of prop design is not all that simple. You cant make blanket statements about 4 blade or 3 blade as the design and shape of the blades as well as the hub can make big differences. You need to also look at the load presented to the prob and that is affected by vessel weight and design. You will see 5 blade props on some speedsters. Ther are no absolute rules. That's why there are prop experts, a group to which I do not belong

 

[antaris]

Re: Propeller choice and slip {Yamaha Stainless vs Revolutio

Well, me neither!

I'm just thinking out loud...