# Help with prop calcs: using a planing hull as a tug

Discussion in 'Props' started by Sketchy, May 7, 2013.

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### SketchyJunior Member

I am trying to do some propeller calculations for a 11.8m planing hull. The main problem is one of the requirements is a 2.8t static bollard pull. Due to the boat never being designed for bollard pull, the props are limited to 24" diameter.

The boat is twin shaft and will be powered by 2x300hp engines. I have come to this engine power by calculation in Dave Gerr's Propeller Handbook, formula 5-9, page 61. Giving 4.1t of static thrust but once discounted by 70% (as per his recommendations for planing hulls with small fast spinning props) gives 2.9t of static thrust

My main problem is trying to choose a gearbox for this set up, nothing is said for sizing for bollard pull. As the speed of the boat isn't of importance, would you go for the highest reduction available or size it for say 10kts? Reduction ratios available are 1.61 to 3.46.

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### gonzoSenior Member

Well, it is like using a sports car as a tow truck. Are you allowed to do any modifications to the shafts? You could have a propeller with many blades that gives you a BAR of over 100%. It will be overall very inefficient though.

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### SketchyJunior Member

The shafts are currently 1 3/4" and sized for 450hp engines. Yes I understand that its an extremely inefficient solution but its the most cost effective solution. The boat will only be used 2-3 times a year so costing far outweighs efficiency.

Do you have any recommendations on how to approach this problem calculation wise?

I am having a hard time getting my head around the problem, how would you determine what gear box ratio to use? Too low of a ratio and there will only be a short instant of high thrust at start up before a stream tube develops. Too high of a ratio and it may not be able to absorb all of the engines power.

What happens to a planing hull with gearboxes and propellers sized for say 20kts top speed when tied down and engines put at full throttle? is the engine unable to reach max rpm or does the prop cavitate and max rpm reached?

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### gonzoSenior Member

I think you are thinking that the hull is going to influence your propeller choice more than it will. At the low speed of towing it won't make that much of a difference. The problem is that with small shafts like you have, if you put a high ratio gearbox, the torque will snap them. You need to calculate the torque necessary and then size the shaft accordingly.

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### SketchyJunior Member

When referring to planing hulls it was mainly due to the fact that they have smaller fast spinning props which aren't suited to towing. Changing the shafts is not a problem. How would you go about sizing the gearbox? higher the ratio the better?

thanks for the help gonzo.

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### gonzoSenior Member

Not really. It depends on many variables. For example, the rated RPM of the engine. Also, the maximum diameter of the propeller. If you are going to tow, 4 Kt may be enough as top speed. If you are going to the trouble of changing shafts, repowering, etc. it may be cheaper to find an appropriate vessel or build a new one. The structure of the hull is not likely to take the stress of towing.

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### SketchyJunior Member

The hull is an aluminium work boat hull, 6mm bottom thickness, 4mm sides, 6mm frames and stringers. It isn't a repower as such, the old engines blew up and the boat has since been sitting in the yard for quite a few years.

The main limitation we have is the maximum diameter of the propeller, 24" is the maximum whilst still giving acceptable clearance. The current engine we are looking at is a cummins 6CTA8.3-M 300BHP @ 2500RPM.

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### jonrSenior Member

You might find a low pitch, many bladed, 24" prop that you could consider using and then calculate what torque and rpm your engines and shafts could deliver to it.

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### gonzoSenior Member

Unless you need twin engines, cut a tunnel in the middle of the hull and install a single with a Kort nozzle. It will increase your bollard pull with the same diameter propeller.

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### SketchyJunior Member

gonzo, twin engines is a requirement from the customer.

so does 2.8t bollard pull from 2x300hp engines sound doable to you guys? If its not even in the ball park then i'd rather know now, once the customer is serious we will consult a prop specialist.

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### johneckSenior Member

Yes, it is possible. I think that you should select a gear to set the shaft RPM at about 1100 -1200. That makes for a fairly high tip speed but help keep blade loading manageable. The DAR will need to be about 1.0. I would also take a close look at the tip clearance. If it was 20% with a 24" diameter (~5") you may want to go to 26", every bit of diameter will help.
I suspect that Gonzo is right that the shaft diameter at that design point is very marginal, depending on material.

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### gonzoSenior Member

You could use Kort nozzles for twin engine installation anyway. In that case tip clearance won't matter, and you could get another couple of inches diameter. Also, with a nozzle the tips of the blades are cut to form a circle instead , which increases the BAR.

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### SketchyJunior Member

Shaft material is ss2205, with a 2:1 reduction ratio shaft RPMs would be 1250 and the shafts will still be able to take the torque comfortably. Right now with a 24" prop the tip clearance is already down to 12%. Kort nozzles will be used if we run into trouble but at least we can take comfort in knowing that we aren't miles off for now.

Thanks for the help gonzo & johneck!

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### michaeljcSenior Member

Is there any software or tables out there that deals with props for high bollard thrust? ie diameter, RPM. pitch, torque etc? In my case diameter is not a constraint.

Michael

Floatainer Systems

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### DCockeySenior Member

The KT-KQ-J coefficient system works at zero advance ratio. It is described in Ship Resistance and Propulsion by Molland, Turnock and Hudson which also has charts of the coefficients and tables for calculating the coefficients for several propeller families.

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