Less slippage=more load?

[donnymac]

4.3 volvo penta, sx leg 1.60. 225hp, 14.25X19 AL prop, cupped. 4600 rpm 45mph. Campion 552, 2800lbs-3200lbs
Now to the question, do S/S props have have less slippage? And if so, would the same prop in S/S put more load on the engine? My thing this year will be
26 to 30 mph cruising(due to fuel costs) But I still have to pull the kids on the 3 man tube. If I could get away with it I would go to a 4blade S/S with a 20 on the pitch. I'm thinking no.

 

[Silvertip]

Re: Less slippage=more load?

There are a couple of opinions regarding aluminum to stainless changes: 1) keep the same diameter and pitch or 2) if you have a 14 x 19P aluminum, go to a 14 x 18 stainless. To answer your question, you would likely not want to do as you proposed. If you go to a four blade, you obviously add a blade that will pull off some revs. You increased pitch by one inch so you pull off another 150 - 200 rpm, and lastly, SS is "generally" more efficient so you pull off a little more rpm. Loss of RPM means the engine is loaded, you actually need to open the throttle farther, especially during hole shot, and if you tow water toys, you will have killed performance. So your assumption is correct. Your existing prop is pretty well spot on. Reducing rpm with the thought of saving gas with a higher pitched, ss prop is not valid (unless of course your setup was way wrong to begin with, which it isn't).

 

[donnymac]

Re: Less slippage=more load?

As usual Silvertip, me thinks you are spot on. I must stop beating my dead horse and be satisfied with my stock Volvo-Penta aluminum. Is there a more performance orientated aluminum?

 

[steelespike]

Re: Less slippage=more load?

Silvertip only said no to your proposal as stated.I believe he suggests an 18"
stainless.That should put you at the same rpm but move the boat better.
You also may want to consider something like smart tabs to get the boat on plane easier and keep planing at lower rpm.They also help no wake speed wander.

 

[Silvertip]

Re: Less slippage=more load?

Agreed! A more efficient prop than the one you have can provide better performance, but how much improvement is always the unknown and unfortunately trial and error is the only way to find out. One prop may provide great hole shot but fall short on top end. Another prop with identical specs may have great top end but fall on its face for hole shot. To complicate matters more, the performance characteristics of both props may reverse, get better, or worse when installed on a different boat. For the average boater, unless there are some really pressing issues to resolve, I feel throwing big bucks at a stainless prop to save fuel or in hopes that it turns an average performer into a rocket is simply without merit. Of course there are those with really deep pockets holding thick wallets so perhaps buying three or four props is not an issue. If you can find a prop shop that has a try before you buy program, I would recommend going that route to save time and money. You seem happy with your current setup so I would continue to run what you have. If you cruise these forums (outboards as well) you will find you are not alone in your concern about fuel economy but the answer is always the same. You won't save enough changing props to pay for the prop. A better way to save fuel is to make sure the engine is properly tuned, lighten the load, go on a diet, take one six pack of adult beverages rather than a case, and drive like you have an egg between your hand and the throttle.

 

[donnymac]

Re: Less slippage=more load?

Wait a minute Silvertip, now your talking crazy. Only a six pack, diet
Oh and I installed Bennet trimtabs last month. Yep, work just like the forums said they would. Thanks for the help.