# Working Out Putt Putt Displacement/Weight

Discussion in 'Boat Design' started by Swami, Jul 14, 2014.

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### SwamiJunior Member

Esteemed Boaties.

I am trying to calculate the displacement of my 19ft putt putt boat to thence calculate the best prop diameter and pitch after changing engines from a 4hp diesel (at maximum prop revs of 750) to a 10hp petrol (with maximum prop revs of 1200).

The following I know:
LWL is 18.8 ft; Beam at WL is 5.6 ft; mean draft is 1.1ft. Therefore displacement speed is 5.8knots. Whilst the new motor weight is known (300kgs), hull weight is unknown to me.

I believe I could figure out the weight it if I knew the mysterious "block coefficient" for such vessels. From that I can figure out the correct prop sizing. I've been playing with Simpson's Rule but it's now given me a headache.

Is there an available listing of likely block coefficients somewhere online or can someone here make a call on it for me?

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### gonzoSenior Member

The displacement and weight are the same thing. You don't need block coefficient or any other. The displacement is calculated by adding all the weights of everything, including crew, of a vessel. A small boat is easy, just get it on a scale.

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### SwamiJunior Member

Huh? But I DON'T KNOW the weight of the boat. Yes, the weight of people, plus engine, plus beer I know, but the hull timber weight I don't. But can't I figure that out by (somehow) figuring the displacement of the water below the waterline using the boat dimensions I know?

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### SwamiJunior Member

PS ... this is not a trailer-able boat so I can't take it over a weighbridge. It's now upside side down being stripped for epoxy glassing, new engine beds, prop shaft, prop and then (one day) a half cabin built on top. All add-on weights will be included in the overall. But I don't want to buy a new prop, only to find it ain't the right one when the project is finished.

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### TANSLSenior Member

Swami,
If, as you say, you've been playing with the Simpson Rule is, I suppose, because you have a series of cross sections. Then the displacement can be calculated easily. Only you would have to supply me the cross sections, longitudinal profile, if you have it. I will calculate the hydrostatic values for various drafts

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### SwamiJunior Member

Thanks for your offer TANSL. Yeah, I played with cross sections and mathematics for hours and gave up, believing there would be some easier method. I got the block size(LxWxD= 18.8 x 5.6 x 1.1) then, I was told by someone, to just multiply that number by the Cb for "this style of boat". He said "maybe, around 0.45". But that gave me 1.8 tonnes (3858 lbs) which does NOT SOUND RIGHT! Is block size x Cb too rough a method? Maybe I'll have to get more accurate cross sections and send you them for expert work!

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Unless you have a close known comparable the block coefficient method is likely to be highly inaccurate. Commonly for round bottom powerboats block coefficient might be somewhere between .3 and .42. That's a big range. Taking 3" of draft off for the keel that gives 1677 pounds (salt water) at .3. If the boat is lightly built with little inside it could be lighter than that. If it's heavy built of heavy material it could be over 2000 pounds.

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### SwamiJunior Member

Yeah, get your points. The hull is cedar, so lightish. Very sharp bow. Waterline is only 6" above the prop. Sits quite high in the water. So .. you're saying ... go with the cross sectional method and the expert calcs as TANSL invited!?!?

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### PARYacht Designer/Builder

You can get very close approximations of displacement by multiply the LWL by it's beam WL, by her hull depth, then by the percentage of her midship coefficient, which you can assume to be about 65% for your hull (assuming typical displacement shapes). This will get you to about 95% accuracy, if done with some care.

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### TANSLSenior Member

The definition of the coefficient block (Cb) is : Cb = Displacement / (L x B x Draft).
Therefore : Displ = (L x B x D) x Cb
The problem is that, surely, you do not know, when they tell you Cb = 0.3 or Cb = 0.xxxd, where they measure the "L" and "B".
There are other factors, like the prismatic coefficient, but needless to complicate the issue.
If you have cross sections, even in .pdf or .tif format, upload them here and I'll make the calculations.

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### Mr EfficiencySenior Member

As a boat limited by it's waterline length, is weight really that critical in selecting a propellor ?

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### SwamiJunior Member

To Par.

When a boat's draft is mentioned for calculations, is this maximum draft at lowest point of belly (I know to discount the keel depth) or mean draft all along profile?

To TANSL
Thanks for offer. I will have to remeasure my cross sections again as I suspect they are not quite right, and also so as not to waste your time!

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### SwamiJunior Member

To Mr Efficiency.

I'm only a newbie at this stuff but, from what I have read and via the various online calculators I have been exploring, yes, weight of a boat is definitely a substantial factor in prop calculations due to the simple fact that as my boat will now sit lower in the water (due to the increased motor weight and therefore a longer LWL) and more horsepower at different revs, the diameter and pitch of the prop, as matched to the maximum revs of the motor's power and torque, will vary in proportion to the weight of the boat ... of which I have no bloody idea at this stage!

As well there seems to be several factors at play when selecting the correct prop: (i) the theoretical displacement speed, based on LWL and (ii) the maximum desired speed one might desire to travel at. Some people (and maybe myself at times) like to push water a bit more than hull speed, and hence you need to know your motor can comfortably handle that. Au contraire, if you want your motor not to be up in the noisy rev range and/or wish to travel somewhat slower than hull speed, you can "over-prop" a bit for the same speed but with less revs. I am just beginning to discover the complexities of it all.

And these links request boat weight: ... They must think it important ...
http://www.vicprop.com/displacement_size.php
Freeware Propeller Calculator - The Alberg 30 Site

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### YobarnacleSenior Member

Estimate the maximum and minimums your boat might weigh. Run both weights through prop calculator. In your 19 ft boat size, a couple hundred pounds difference in displacement estimates, is offset by when you have an extra guest on board. Use the prop estimated for heaviest load condition.
Program might recommend exactly the same prop for BOTH displacements, since props are not infinite in variations.
Wouldn't THAT be an unsolvable conundrum?

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### abcdefgJunior Member

Get hold of a load cell or scale. Can be hired in most places.

Have used them many times for boats up to about 50' Just need to get them under the hull or on a hoist. a 19'er should be easy.

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