# Jet drive for 14,9 m katamaran

Discussion in 'Jet Drives' started by Harald SM, Apr 25, 2018.

1. Joined: Apr 2018
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Location: Oslo

### Harald SMJunior Member

Hi!

Im a 3rd year master student studying naval architecture. We are having a project and have decided to design a katamaran to be used in the removal of thrash along the coast. Anyhow, due to the geography, we want the depth to be as little as possible and are therefore considering waterjet. The catamaran is as follows

totalt width: 9, 95 m
beam = 2,45m
T = 1,5

How does the hull need to be shaped to be suitable for a water jet? A model has been made in delftship when the thought was to have propellers installed, however, the resistance and depth makes this difficult..

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### gonzoSenior Member

You can get shallow draft with a tunnel or a surface piercing propeller too. The drawback of jets is that they can get clogged with the trash you are collecting. Propellers are generally easier to clean.

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### Harald SMJunior Member

The thrash is on land, so that wont be a problem. According to our calculations the propeller will cavitate if the vessel is to make a speed of 9,5 knots. (An estimation of the resistance was done by researchers at our university. The propeller calculations were confirmed by one of our professers in hydrodynamics.) Therefore we have to change our propulsion system to waterjet. Does anybody have good sources regarding this subject?

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### gonzoSenior Member

If the propeller is cavitating, that means that it is the wrong type. If you size the diameter, pitch and blade area correctly, it will not cavitate. Propeller Handbook, by Dave Gerr is one of the best books for an introduction to propellers. There are propellers that are designed to operate with cavitation. They are called supercavitating and fully cavitating propellers.

5. Joined: Apr 2018
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### Harald SMJunior Member

We have examined this pretty thorougly, and could not find a solution.
Supercavitating propellers are for high speed vessels, and not a phenomenon which will occur for our ship going 9,5 knots.
Since we want as little depth as possible, it is not possible to increase the diameter of the propeller. Also the blade area is maxed.
We do not want a propeller that breaks the watersurface.
The propeller cavitates, and we therefore have decided to look further into waterjet. Does anyone have information on how the hull should be formed or the requirments for this kind of propultion system?

thanks

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### baeckmoHydrodynamics

Naaah, there's something wrong in your assumptions here. What depth, power, shaft speed and propeller diameter have you based your calculus upon? What is the resistance at 9.5 knots? And what makes you believe the jet will not suffer from cavitation?

You are referring to calc's done by uni researchers and a professor and then you ask us for a solution without submitting any relevant info other than:
"totalt width: 9, 95 m
beam = 2,45m
T = 1,5";
That is far from the info needed to come to a professional conclusion; if this is a student project, make your homework and come back with a decent SOR, and I guarantee that you will get qualifyed advice here!

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### Harald SMJunior Member

The following resistance study was conducted, allthough at a T=1,2. Changes occur all the time and this is the most recent rapport.
The computations regarding propeller were done using BP-delta chart with 3- and 4-bladed propeller. We tried with n ranging from 8-15 revs/sec and the diameter of the propeller as large as 75cm.

A model has also been made in delftship, i am happy to share the file if it might help

I am that little information was given initially, I am reaching out on multiple platforms in order to obtain information regarding the matter. Alle answers are happily received.

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8. Joined: Aug 2002
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### gonzoSenior Member

It doesn't seem to make sense. With a propeller of that diameter and four blades there should be no cavitation. What are the shaft RPMs?

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### Harald SMJunior Member

The rpm was tried in the interval 8-15 revs/sec, which gives 480-900 rpm

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### gonzoSenior Member

Sorry, I edited the post after I saw the diameter you are using.

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### baeckmoHydrodynamics

It seems you are calculating for a single engine propulsion?? Even if that is not optimum, it would be technically possible here, provided suitable installation measures.

Better to split the power for, say 14 knots (total thrust ~31 kN). It would require 2 x 250 hp and roughly 710 mm propellers (4 blade, BAR 0,75). They are rather highly loaded, but operating with less than 10 % cavitation on the blades, which is OK.

Trying to accomplish the same operating point with jets would require more than 50 % extra power, still with the drawbacks Gonzo has already mentioned; jets are very inefficient at low speeds.

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