Propeller Issue

I'm designing a monohull ferry with the following data:

LOA = 18m
B = 5.3m
D = 2m
d = 0.8m
Displ = 38t (approx)
Propeller size = 0.8m
Vs = 11.5knots (approx)

The tip of propeller is only 100mm below loaded waterline. What would be the propeller issue and what is the best solution?

Thanks.

 

Depending on hull shape it could aerate and just lose grip. If not shrouded in some manner it will aerate as waves pass. It will be ineffective if above the waterline when unloaded.

Seems odd that you have a 2m draft but the 0.8m prop is at the surface? Why not set it deeper?

Rick W.

 

Thanks for a quick reply.

Owner limits propeller within 800mm draft. This boat is a twin screw. I guess 660mm diameter propeller is enough. So there's 140mm allowance before the props pop-out of the water.

Is it ok if the tip of the propeller is at the water level in lightship condition?

 

Better if Prop is always submerged. Posible ways to improve condition.

1. Reduce Prop Diameter - increase D.A.R. (more or wider Blades)

2. Reduce Prop RPM - Increase Pitch

3. Provide Anti cavatation plate above Props

What is the Total HP?

Jango

 

810 HP total

 

Max Speed with Disp. of 38 ton and 810 HP is 9.9 kts or 11.4 mph.

Recommended Props are as follows:

at 1000 shaft rpm - 32.5 (.82m)Dia x 21.5 Pitch. Min D.A.R. 55%
or
at 1500 shaft rpm - 25.6 (.65m)Dia x 14.5 Pitch Min D.A.R. 55%

Increased rpm will tend to Increase Prop airation, but may be the way to go, do to smaller Prop Dia.

 

Whos designing this boat you, or the owner?

In rough water it will slip and you may not even achieve head way. Possibly a dangerous situation.

props don't do shallow draft, think about jets!!


How far in from the transom is the props?

 

Hi, Jango,

Thanks for the inputs. I will re-check my calculation.

Hi, Frosty,

I'm doing the design. 800mm is the distance between propeller and transom. Do you have suggestions? Thanks.

 

If transom remains submerged it should not aerate. Could be induced if the boat is given heavy throttle at low speed or in sloppy conditions.

Still seems an odd condition that the prop shaft is 1.5m above the keel line? Is it for drive through RORO operation?

Rick W.

 

Shaft end is 330mm above the keel with 2degrees angle. Boat to carry passengers.

 

Does not gel with 2m draft.

Have you got a profile view or GA you can post.

Unless a prop is effectively shrouded by the hull you need the blade to be at least its chord length below the surface to avoid the tip vortex causing an air spiral into the blade. If the blade aerates, the thrust will drop to at least one third. The engine load will fluctuate dramatically as a result. Not good for props, shafts and motors.

Rick W.

 

attached is the profile in way of propeller.

 

If it doesnt work to epectations you may be able to find different props that can survive those conditions.

 

A picture is worth a thousand words - for some reason I was taking the draft at 2m.

From previous post lightship draft is 800mm. This means the transom will be just submerged in calm water. I expect the boat will squat and drag the transom a little deeper when underway. This will prevent air from getting at the prop in all but rough sea conditions.

I don't think there is any serious risk of the prop getting air in normal operation. With the transom you will get attached flow once moving and water will follow the curve even if the transom is higher than normal water level. However it would be risky to design for the prop exposed when light. The prop will suck air initially and if you have a strong headwind may not make way.

One option with the transom is to continue the racked keel line up to the fully loaded water level rather than flattening it out. This would reduce the transom drag when loaded.

I have attached a quick rendering of the hull - am I close?

If this is near the mark then I get loaded draft at 0.96m. Under this condition there is significant transom drag so I can understand the desire to lift the transom. As long as the prop is not exposed I cannot see any problem with this. It would help loaded performance.

I get a different result for power to Jango but I am using Michlet and it has limitations when applied to a hull like this. I would not think it is grossly in error though. I am showing you could do 11.5kts with around 220HP on each prop (say 450HP total and 500HP with auxiliaries if the props are 32 X 25 spinning at 1000rpm. I get a wave height over 2m at this speed so you might find some complaints from this angle if you are operating in a congested waterway.

If you reduce prop size to 24 X 17 spinning at 1500rpm then power goes up to 240HP per prop. I get the best result with a 4-bladed prop with this diameter.

I am showing that 10kts will require about 35% of the power required to 11.5kts. If the owner is serious about doing the latter speed why not make the vessel longer?

Rick W.

 

It's close except that breadth from midship to aft remain constant, and shape on fwd is a bit rounded.

I'm using 800mm as the full load draft.

The keel is raked except for the tunnel 1m off CL.

Actually, the Owner wants 20knots speed. But, I'm only getting 19 knots for 810HP (2 engines 405 HP each) using 55% overall efficiency.

(Sorry, for my previous post for saying 11.5 knots, I only used 405 HP in my calcs, that's why).