# Tinny Hydrodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by the_big_ragoo, Jan 21, 2015.

1. Joined: Jan 2015
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### the_big_ragooNew Member

G'day all,

I have been assigned with building a pulse jet for a fishing boat. I am fine with most of the physics exept for the hydrodynamics.

The key problem is I dont know bugger all about this boat and have little way of finding out. What I do know is that it's a fairly standard tinny.
Can anybody help me figure out what it's drag will be, ballpark figure?

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### Mr EfficiencySenior Member

A pulse jet, is what, exactly ? Sounds like the V1 "buzz bomb" of 1944.

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### the_big_ragooNew Member

Thats about the size of it. Yup.
Without the bomb though...I hope. :/

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### Mr EfficiencySenior Member

So what is the object of the exercise, and what are the details of this propulsive device ?

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### the_big_ragooNew Member

The objective of the exercise is to strap a pulsjet to a fishing boat.

I have to design the pulsejet around the fishing boat. So I need to figure out what drag the fishing boat will have in the water. I can then detremine the acceleration needed, which tells me how much thrust I need.

I can then design my pulsejet around providing that amount of thrust.

6. Joined: Oct 2008
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Welcome to the forum t_b_r

Well, unlike most who seem to think design can be plucked out of thin air, we need some facts and figures. Just asking for drag values with no vessel is religion based not engineering based design

At the bare minimum, you'll need to provide:

1) General Arrangement or Lines plan, along with Principal dimensions of the hull
2) Displacement of the vessel at the given water line
3) Target speed

This would give a start. But it still would require many if's and butt's that'll it only just be a ball park figure.

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### Mr EfficiencySenior Member

I'd say you should look at the normal hp requirements of the boat you have chosen to host your exotic drive device, and work backwards from that. You may find a thread somewhere on the forum that dealt with the thrust figure ( pounds) of various hp outboards. Use the search function. Small electric outboards often are rated to pounds of thrust, but of course they are not big enough to power the type of boat you mention, to the cruising speeds they are designed for. I'd say the required figure would be in the area of 300 pounds, give or take, to have a 4 metre tinny skating around happily.

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### Mr EfficiencySenior Member

The victim, I mean skipper, of the Pulse boat will need to submit a chiropractor's report about the effects on the human body of the intermittent thrust.

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### Mr EfficiencySenior Member

I hope the pulse jet hasn't flat-lined.

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