Displacement hulls, twin screw more econimical than twin screw?

[JohnJewel]

Hi,

There appears to be a lot of solid expertise on this forum and i would appreciate some opinions on the following.

I often here that single engine and screw displacment boats are more economical that twin engine and screw boats. I also here that slow turning props are more economical that fast turning props.

Making a few assumptions, 50ft displacment hull that is in single engine and screw version is fitted a 150hp engine and a 30" prop and in twin engine and screw version is fitted with two 75hp engines and two 30" props.

The prop size of 30" is the max feasible due to draft and hull clearance issues. The gearbox gear ratio, prop pitch and no of blades can be changed and should be the optimal prossible is both cases.

Given these assumptions, what would be the prop effeciency for the single engine and screw boat and what would be the prop effeciency for the twin engine and screw boat?


Many Thanks
John

[YankeeDelta]

What is the boat used for?

[JohnJewel]

Originally Posted by YankeeDelta

What is the boat used for?

Long distance (passagemaking) pleasure boat

[FAST FRED]

I figgure the single screw would cross oceans at the same speed using 15% less fuel.

It would have 50% less maintance and up keep , and the engine room would be 100% easier to work in and ventilate at sea.

At slow disp speeds a tunnel might be able to get you to a 34 inch prop, for more efficency , and perhaps to pull a big 3 blade rather than a less efficent 4 blade propellor.

The hull shape will be as important as the drive system, the most efficent will look just like a sailboat , sans balasted keel.

FAST FRED

[marshmat]

Fred's estimate of a 15% difference in overall fuel efficiency sounds about right to me. In general, twin screw configurations are used for redundancy, manoeuverability and shallower draft, while single screw tends to be superior for efficiency and cruising range. Yes, big slow screws do tend to be better than small fast screws. Looking at other long-range passagemakers, you'll notice that boats designed to cross oceans are more often single-screw, while those designed for inshore runs tend to have twins. Each option has its advantages, it's a matter of weighing trade-offs for the desired application.

[JohnJewel]

Originally Posted by marshmat

Fred's estimate of a 15% difference in overall fuel efficiency sounds about right to me. In general, twin screw configurations are used for redundancy, manoeuverability and shallower draft, while single screw tends to be superior for efficiency and cruising range. Yes, big slow screws do tend to be better than small fast screws. Looking at other long-range passagemakers, you'll notice that boats designed to cross oceans are more often single-screw, while those designed for inshore runs tend to have twins. Each option has its advantages, it's a matter of weighing trade-offs for the desired application.

Mashmat,

yes this is the common perception, I am looking for some theory to back it up.

Thanks
John

[JohnJewel]

Originally Posted by FAST FRED

I figgure the single screw would cross oceans at the same speed using 15% less fuel.

It would have 50% less maintance and up keep , and the engine room would be 100% easier to work in and ventilate at sea.

At slow disp speeds a tunnel might be able to get you to a 34 inch prop, for more efficency , and perhaps to pull a big 3 blade rather than a less efficent 4 blade propellor.

The hull shape will be as important as the drive system, the most efficent will look just like a sailboat , sans balasted keel.

FAST FRED

Fast Fred,

i wanted to keep the debate free of the broader issues and see how the theory applies based on a fixed set of asumptions.

With regards to maintanence most long distance single engine boats have a bow thruster and wing engine so the maintanence issue is not so clear cut.

Also unless you have a full standing room engine room the twin engine configution allows you to stand in the keel and work on the engines. On the current boat the saloon floor is 6 inches above the top of the engines but i have about 5 foot of standing room.

Cheers
John

[Willallison]

There's still much more to this than efficiency. For instance, you are suggesting 30" props for both single and twin installations. Assuming that the total hp remains the same for both boats, the twin will require props of much lower pitch as they will be unable to swing the bigger ones.
Also, draft comes into the equation - a single prop will require more draft than twins. OTH they will not benefit from the protection of a keel. And so the compromise goes on - and on - and on.
If you really want to get a grasp on what works best for you, I'd suggest getting a copy of Dave Gerr's book - The Propeller Handbook. You'll get all the info you need - and more

[JohnJewel]

Originally Posted by Willallison

1. There's still much more to this than efficiency.

2. For instance, you are suggesting 30" props for both single and twin installations. Assuming that the total hp remains the same for both boats, the twin will require props of much lower pitch as they will be unable to swing the bigger ones.

3. Also, draft comes into the equation - a single prop will require more draft than twins. OTH they will not benefit from the protection of a keel. And so the compromise goes on - and on - and on.
4. If you really want to get a grasp on what works best for you, I'd suggest getting a copy of Dave Gerr's book - The Propeller Handbook. You'll get all the info you need - and more

Hi,

1. Effeciency is the main reason why single engine long distance power boats are considered more effecient. I would like to see if theory supports this.

2. Agreed. Could also use higher gearing meaning the prop turns slower for the same engine rpm.

3. Yes the compromise and possibilties goes on and on but this should cloud the basic question, hence is have listed some assumptions in my orginal post. BTW no problem to build suitable protection for props and rudders on a twin screw displacment boat which would have the added advantage on allowing the boat to sit on it own bottom.

4. True but i would need to order it from the US (im in singapore) and there is some solid expertise on this forum and i thought it was an interesting topic for group discussion.

Cheers

[Willallison]

You can always get a copy through Amazon.com if you can't find it locally, but I understand what you're saying... I'll dig thru my copy and see what I can find. I suspect that there is little hard data - such as a formula or similar - as so many variables exist. Still, there have been studies into almost everything under the sun, so you'd think that someone would have done some testing...

[JohnJewel]

Thanks Mate... I am also an Ozzie, from Perth.

[Willallison]

Nice part of the world.. I spent a few months over there when the cup was on - bloody good fun...
What you bring up is a valid question - and not just for displacement craft. There is rarely any account taken of the number of screws in most of performance prediction formula's....