Boat Propellor Slipage

waiting for supper so ran some statistics for the day.

It appears the boat propeller is only about 53% efficient, multiply that by 40% engine makes it only 21% fuel efficient.

But it is really fun.

I wonder how much better the twin counter-rotating ones are? Saw one recently with 2 big stainless props, it looked expensive. Recently when I was out kayaking a fishing boat was backing out, 400 hp outboard! I can remember when a 35 was a big outboard!

I am going to get some numbers from others next season and see what is more efficient. This is a 21' boat with 305 Chevy 205 HP. It is a zippy little thing, never figured that much slip in the prop.

I am waiting for wife to get ready for dinner and this popped into my head so I ran the numbers.

Slippage is not as bad a thing as it sounds.

It's gonna happen, nothing you can do about it.

Think about it as a poor excuse for a torque converter on a car transmission. What you loose in the torque converter, you make up for in the differential gearing.

As long as the engine RPM is at the top of the power band trimmed out, with a clean bottom, balanced typical load, it is where it needs to be. About 4800 is the top recommended RPM.

If the boat is under powered, or commonly overloaded, used for skiing, and you want more hole shot, drop the diameter or pitch a little, and be ready to back off the throttle once it's up on plane.

I assume you do have a good stainless prop?

The counter rotating props are not for reduced slippage, though that may be a side benefit.

They are to reduce steering torque.

Big motors cause a LOT of steering torque. When a boat is up on plane, or getting up on plane, the bottom of the propeller is getting a lot better bite than the top of the prop. The top is grabbing air, as it is right at the surface, and even above the surface when the boat comes up on plane.

Having to deal with all that torque has 2 negative effects, one is on the steering. Most have power steering which takes the load off the drivers arms, but it takes engine power to run the hydraulic pump.

The other is having to run with the engine at a slight steering correction angle (because of the torque) is terribly inefficient. If you notice, listening to the engine going straight, then enter a gentle curve, you will hear the engine slow slightly. That is not good for efficiency! The counter rotating props help eliminate that, but then it's another set of gears to turn, which takes a bit of power, so it somewhat of a trade off.

But the reduced slippage has to be compensated for by reducing the pitch, diameter, or gear reduction. The goal is to keep the engine at the top of the torque curve/HP under full throttle.

What is the efficiency of a ducted prop ?

I was reading an article when Volvo invented the twin props(Mercruiser uses them now). If you have three blades, only two of them are pushing at any one time. Four blades only three are pushing and if you have a two blades only one is pushing at anyone time. Boats are weird,I have three of them.

Years ago I had an 18' bass boat w/Merc 150 XR6, had a FIVE bladed stainless prop. It topped out at around 62MPH with a light load but had one heck of a hole shot but you traded off top speed. You might check some of the boat boards/prop companies for a prop that would be optimum for your conditions. I would guess a 4 bladed one with the right pitch would help out quite a bit...of course your "mileage" may vary;-)

Hello David,

What series prop are you running now? Diameter/pitch/blade count? Just curious. Am I correct in thinking that was the runabout you had posted a picture of awhile back?

Carl

Sounds like retirement is making you board lol

I sure don't get it. I remember when my dad would take us kids fishing. Anything above a whisper and he was yelling at us to be quiet we're scaring the fish away. I can only imagine what he would say to a 400 hp "fishing" boat screaming through the water.

I guess you can always get some oars ?

(quoted from post at 17:01:14 09/24/20) waiting for supper so ran some statistics for the day.

It appears the boat propeller is only about 53% efficient, multiply that by 40% engine makes it only 21% fuel efficient.

But it is really fun.

Click to expand...

Sometimes that's not a bad thing.

With my inboard (4 cyl OMC) turned down to 425 RPM my boat speed was 2.7 - 2.9 MPH a little too fast for trolling and the engine would cough and fart.

Finally tried putting on my spare prop, it was one I found in my parts shed, some what chewed up and the wrong pitch but good enough to get me home if need be.

That little bit of efficiency loss was exactly what the boat needed.

I can now idle along at 450 RPM doing 1.7 -1.9 MPH and the engine runs so smooth and quiet you can almost hear the fish whispering to each other, perfect for a peaceful day of fishing.

Yes, retirement is agreeing with me, time to think.

The prop is a SS avenger 19".

I'm running a Mercury Laser II SS and at just on planing speed, where I do most of my running it's 28% slip due to the "ports". At WOT when water
pressure has sealed the ports and no longer dilute the fresh water across the blades with exhaust gasses, the slip decreases to 8%.....www. go-fast.com
prop slip calculator.

Course I bought the Laser because it was ported....dazzling hole shot due to rpms getting up fast, and plenty of mph on the top end.