Prop diameter and gear ratio

I've noticed that almost invariably, as power goes up in a sterndrive application, the reduction gear ratio goes down - even though a four cylinder (eg 2.15:1) and V8 (eg 1.6:1) may have the same peak rpm. So if both the 4 and the 8 peaked out at 5200 rpm, the V8 would spin the prop faster.

That said, I've also read in several places that bigger, slower moving props are more efficient. With this in mind, if cruising efficiently at around 30 knts was your goal (say in a 24 foot 300 HP V8 cuddy cabin) - Would putting a properly pitched 16 inch diameter prop on the 2.15:1 sterndrive run more efficient (m/knt) than a properly pitched 14 inch diameter ( as is usually suggested) prop on a 1.6:1 drive?

 

In theory Yes. It all has to do with Prop Slip. The 16 in dia Prop will have less slip than the 14 which translates to Higher effeciency. However, the 16 Dia will require less pitch to maintain engine loading requiring Higher engine speed to maintain cruising speed. It's a tradeoff between prop slip change and Higher engine speed.

Also, I forgot to mention, Prop Dia requirements are a Function of BOTH weight and Speed. As HP increases (V8,s w. more speed), Prop Dia can Decrease for a given application.

 

Without any real information on the hull I have made an assumption that the 300HP will just achieve the 30kts. To do this, I get the hull drag will be somewhere between 9,000N and 10,000N. Seems high for a 24ft boat so maybe you can go faster than 30kts at full throttle.

Irrespective of this I have looked at two loading conditions with the different size props. See the attached JavaProp screen dumps. The bigger diameter prop would lower fuel consumption by about 1%.

Rick W.

 

Assuming your Boat weighs abt 3600 lb with min load and fuel. Max speed @ 5200 rpm w.300 hp, equals 53.2 mph or 46.2 K. (approx 30 Knts @ 3500 rpm)

Recommended Prop with 1.6 gear:

15D X 18P, 3blade, .55 D.A.R. 10% Slip at max rpm.

I would suggest that 10% slip would be the lowest posible with your HP and speed and slip will increase as speed decreases.

Jango

 

Thanks for your replies Jango and Rick. I was trying to ask a general question; but that doesn't really work with all the variables in boats. I used the 300 hp @ 5200 rpm example because they seem typical small block Chevy numbers and they match closely with many 4 cylinder rpm ranges.

I actually do have two lower gearcases for a Volvo 280 drive; a 2.15 and a 1.61 (current). My boat's about 3500 - 4000 lbs; but it has a Ford that makes about 250 hp @4400 rpm. On a good day it could run 45 mph with a 14x21; however I never drive that fast. Most of the time I'm cruising @ 33-3500 rpm and rest of the time I'm @ an idle (for trolling) or near it. I posted a picture of a later model of the same hull I have - though mine has a flat front deck and no sink.

Jango, Do you mean that theoretically (in this application) a bigger prop should be more efficient, but in practice it's not. A bigger prop has less slip (efficiency gain), but not enough to travel the same distance as a smaller, higher pitched prop - even though it is has more slip. Is this it?

I have to be honest Rick, I'm not too sure how to interpret the graphs you posted. I'll look at them again. 1% gain in efficiency hardly seems to justify the troubles of swapping my lower gearcases.

 

A second look clarified matters a bit, Rick. Would those numbers still hold close to true with the above numbers. I'm guessing my power would be around 215-220 hp @ 3300 rpm and speed is about 28 mph.

 

Simple answer - no.

The prop is lighter loaded using the actual numbers from your boat. It now ties in well with the predicted drag for a 3800lb boat. I have attached the Savitsky estimate for the drag on an approximation of your hull. It indicates drag around 6900N at 40kts. This ties in well with your engine rating, prop and rpm. The prop is getting an efficiency around 82% at full throttle. This is good.

When you throttle back to 3500rpm, speed should be down to 30kts (say15.2m/s) and prop efficiency is still good. Drag should be around 4700N.

I looked at reducing rpm with the bigger diameter prop and it does not improve.

I also looked at how you could reduce fuel consumption at cruise speed. In this case I went to a 16 X 17 prop by spinning it faster - 2500rpm. This gives almost a 2% fuel saving. The reason is that the blades are higher aspect (reduced DAR) and therefore more efficient. The downside with this is that you will probably reduce top speed if the motor is developing full power at 4400rpm. Also I am only looking at the prop not looking at the engine efficiency at higher revs.

The message here is that you should be able to get a small reduction in fuel consumption at cruising speed by going to a larger diameter prop and spinning it faster but there is likely to be a reduction in top speed. There is no benefit in using the leg with lower gearing as far as I can determine.

The potential saving in changing the prop is no more than if you were to take out any useless odds and ends to reduce weight.

The efficiency figures calculated by JavaProp may be different to what you are actually getting but their relativity will be correct. I do not know the exact shape of your blades and that will change matters a bit. JavaProp determines the optimum blade shape for the X-section I selected.

Rick W.

 

Tauntom, The Ideal Prop for your Boat: 3800 lbs with 250 hp @ 4400 rpm and a 1.61 gear

16 x 19 with .55 D.A.R., which will provide 10% slip @ 41 knts ( 47.2 mph )

10% slip as previously stated is the Best posible w. your Hull and HP (regardless of gear ratio)

Cruising RPM of 3500 ( 170 hp) will produce 35.6 mph (31 knts) @ 15% slip. In order to achive Best posible slip of 12%@ 3500 RPM, or 36.8 mph (32 knts) a 16.6 x 19 prop is required, reducing max speed by 1 - 2 knts.

Bottom Line, You WILL increase Efficiency with a Larger Prop (16 + Dia)

Jango

 

Thanks for all the input!
I usually trust the manufacturers have good reasons for what they do, though sometimes they're geared to general rather than specific applications - seems they were right on here.

I have a 15x19 prop I picked up recently that I haven't tried yet, I'll give that a try in the spring. Although Volvo states it will accept up to a 16 inch prop, I don't see too many of them around.

 

I believe you will be pleased. At full throttle, the motor may over rev a bit, unless the output is less than 250HP or the 19P has a heavy Cup (acts like a 21P)

Good luck, let us know it does.

Jango

 

I think what you are missing is pretty clear.

You not mention the power of the 4 cylinder rather just the rpm and reduction ratio. Same as v8 of 5200 rpm. A small engine will need to multipy the torque to propeller the boat more than a bigger engine will. The bigger motor has more torque then the smaller motor. So big need less multiply.

 

I'd say that goes without saying. The question was about the efficiency of a slower turning large propeller versus a faster spinning small propeller @ 3300 rpms.

 

Yes but the reason the to ratios is torque not efficency also the outdrive will not hold much power with biger motor so they have to use small ratio