Propeller pitch on quad installations VS twins

[Henry Swanepoel]

Hi everyone, I have been trying to figure something out, but it has been breaking my brain.

Q: How does the pitch of the propellers differ when having a twin installation as opposed to a quad installation. For example, let's say we have a 37ft freeman rigged with 2 x 350hp, and then rigged with quad 300hp engines?

Thanks in advance.

 

[Barry]

Hi everyone, I have been trying to figure something out, but it has been breaking my brain.

Q: How does the pitch of the propellers differ when having a twin installation as opposed to a quad installation. For example, let's say we have a 37ft freeman rigged with 2 x 350hp, and then rigged with quad 300hp engines?

Thanks in advance.

You originally had 700 hp and now would have 1200hp. At wide open throttle, obviously more speed. Therefore the relative inlet velocity to the props would be higher so would require a higher numerically pitched prop for a given diameter
I would expect that websites like Michigan Prop and others might take this into account. Also, I would check on line at Boating.com, as they are doing tests on all type of boats including those with quads, and they normally list pitches, speed etc

Or call Freeman

This is what your 37 Freeman with quad 300 might look like 106gph .6mpg


PB_FRE_37VH_quad_F300UCA_2015-02-26_OCC-CAT | Yamaha Outboards https://yamahaoutboards.com/en-us/home/outboards/350-150-hp/v6-4-2l/f300-(4-2l)/pb_fre_37vh_quad_f300uca_2015-02-26_occ-cat
Or from Yamahas website 14 3/4 x23

 

[Henry Swanepoel]

Thanx Barry will have a look.

 

[gonzo]

The pitch is directly related to maximum speed. However, the bigger engines will be heavier. The fuel tanks may be larger too if you need to keep the same range. The extra weight needs to be taken into consideration. It is not as simple as just increasing the speed. For example, the loading of the propellers may increase, which will require more blade area. On the other hand, it may decrease, so you can change from a 4 blade to a 3 blade propeller.

 

[jehardiman]

At wide open throttle, obviously more speed.

Perhaps, perhaps not. Besides the issues Gonzo brings up, there is another issues with just simply up-pitching. For every engine there is a maximum torque that can be generated just off idle. This means that there is a maximum pitch and BAR the engine can handle without lugging and burning up the engine. Doesn't matter if you have one or one hundred engines, there is a limiting pitch and BAR not driven by top speed, but rather by low speed and acceleration. See these posts.
What is the max RPM any propeller can spin before it becomes useless? https://www.boatdesign.net/threads/what-is-the-max-rpm-any-propeller-can-spin-before-it-becomes-useless.65477/#post-906498
Oversized prop compromise https://www.boatdesign.net/threads/oversized-prop-compromise.64505/#post-888107
I agree with Barry that this requires some forethought and experience. Even with quads, after up-pitching expect poorer maneuvering and less acceleration until sufficient Va is achieved.

 

[DogCavalry]

And that's one reason why the Evinrude Etecs are so popular: bottom end torque.

Terrific insight there, @jehardiman ! Of course there's an advance rate/prop pitch, beyond which an engine just can't cope. Unless it's grossly overpowered for the installation. And yet I hadn't heard that articulated so clearly before.

Sounds like a situation that could be improved by a 2 speed. Like Mercury's latest. Low gear and high gear, for the exact same reason a land vehicle has them.

I'll take the moment to point out that a higher pitch prop is intrinsically less efficient than a low pitch prop, even when geared properly for the situation.

 

[jehardiman]

I'll take the moment to point out that a higher pitch prop is intrinsically less efficient than a low pitch prop, even when geared properly for the situation.

Ummmm. I'd not go that far. Too many things tied up in the actual propeller geometry. Just up-pitching an existing propeller geometry; yes. A properly designed high pitch prop...not so much. But then high pitch, low Blade Area Ratio (BAR) props have their own issues.

 

[DogCavalry]

Well, net force on any given blade segment is a few degrees aft of normal to the local chord. So the increased pitch rotates all net forces everywhere back. More drag, less lift.

 

[johneck]

That is an interesting way of looking at the problem, unfortunately it is an oversimplification and not actually correct. If the boat were not moving, it might be right, but efficiency is defined as useful work (thrust produced) for a given input power, so simply looking at the direction of the lift vector is not enough. Higher pitched propellers will generally be more efficient, subject to the constraints of cavitation.

 

[DogCavalry]

@johneck , I'm going to have to politely disagree. While you correctly define efficiency, your conclusion is not correct. The higher pitch propellor allows a higher speed, at a given rpm, provided the power is available, but at every speed below that max, it produces less thrust for power input.

 

[DogCavalry]

High pitch props are used where rpm at the shaft is limited by the gear ratio, such that the engine reaches it's rated rpm, but the boat is not yet at top speed, for that power. In applications where performance matters more than tradition, the designer includes a multispeed gear box, and gears up at that point, rather than using a prop that is a mediocre compromise up to that point.