Inboard shaft angle vs prop efficiency

[tkinak]

I have a question concerning shaft angle and prop efficiency. Would it be better to have a small tunnel/ pocket or a flat/ delta pad and reduce shaft angle? I'm looking at a 27' Farallon 18* deep V planing hull that is on the heavy side and was thinking (dreaming) of stretching it during a repower. The standard installation is a straight shaft. Reduced draft and easier trailering would also be a plus for me. The same hull comes in a 31' version that seems to hold the same speed with the same power. Thanks for your input!

[mydauphin]

Since your dreaming.... Install surface piercing drive, Like Levi, Arneson, etc.. There are many threads here on those.... Our just use an outboard on a bracket.

And yes should give you better higher end performance though acceleration may suffer.

[Guest625101138]

The efficiency does not change much with shaft angle for a heavily loaded prop. Even with a lightly loaded prop the main advantage of a flatter shaft angle is steadier loading on the blades. It will reduce vibration.

Providing tunnels and steps can cause aeration problems if not well designed.

The suggestion regarding surface drives is probably the best choice if you really want to reduce draft. Again needs to be designed properly.

This thread may give you some ideas:
Hull modifications
It shows what reasonable tunnels are like. This boat does quite well for the power and weight since it was modified. Most of the benefit came from reducing appendage drag. It has moderately loaded props. As you can see they are monsters.

The surface drives get rid of all the underwater junk that increases drag. The surface props are not particularly efficient but reducing the drag usually wins out if you want to get above 30kts or so particularly if you want to swing small submerged prop to reduce draft.

Rick W

[tkinak]

Thanks for the replies and links.

I do a lot of salmon trolling as well as trap fishing for crab and shrimp. How would a surface drive handle working gear and trolling?

Current boat performance,from what I read with a 300 Cummins is 25 knots wot and 18-20 at 2400 ish. A note I have from a shop that has repowered several Farallons that states, with a 380 cummins and a 19x26 3 blade cupped prop the boat will reach 30kts.

I sold an Arneson I had a few years back. Oh well!

Thanks, Tom

[mydauphin]

I dont know about surface drive crabing... May be make cage around propeller. Just not going to go to fast... May be water jet....

[Ad Hoc]

tkinak

What is the real question that is behind this? Since on such a small vessel which has from what you've written, has varying rolls. As such you wont get one solution that will fit all...it will be a compromise. BUT, which is the no.1 on this list that must be satisfied first: cost, weight, power, speed etc?

[tkinak]

Ad Hoc that's a very good question one that's not easily answered because who wants to compromise! Seriously, the real question is: Can the efficiency of this vessel be significantly improved by changing the shaft angle and possibly lengthening the hull. I believe these weigh 7,000 lb empty.


The first compromise was deciding on a heavy deep V boat instead of a light fast aluminum knock your fillings out rig. The ride quality of this vessel is excellent- at a cost of fuel consumption. I'm trying to figure out if this can be minimised with some hull or running gear modifications. I can get into a used Farralon from between $10,000 and $35,000.

I need to keep it simple and cheap so staying conventional shaft drive would probably be best. Yes as a recreational vessel it will see many different duties. Pot fishing, trolling and making a 170 mile round trip to a remote cabin. The trip to the cabin is why I'm asking if these modifications would increase efficiency. If there is a significant increase in mileage the modifications would offset their cost and I'd assume increase the potential cruise speed if needed.

Thanks, Tom

[Ad Hoc]

tkinak

Ok..before we all rush off, what do YOU mean by "..the efficiency of this vessel be significantly improved.." Since effiecney comes in many guises, be it cost, weight, deck area, fuel consumption, power, maintenance costs etc.

Hence the first Q above...what is the driver, you say being efficient. Ok. But in which area.?..too many to address and all with different solutions.

You really need to sit down and think what is it you ideally want...then address whether it can be done.

As a quick rough rule of thumb, if you increase the length of a vessel, (at parallel section or as close to), and the increase in displacement is the same or greater than the increase in mass, the boat will have the same speed or go a bit faster. How much depends upon many factors....still to be determined.