Calculating Power for Displacement Boats

Discussion in 'Boat Design' started by Mat-C, Jun 15, 2011.

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Mat-CSenior Member

It seems that whilst there are plenty of ways of calculating the power required for planing hulls, there's not so many for displacement craft.
I have seen Gerr's Displacement Speed Formula, from his "Propellor Handbook". I have also seen criticism of the method around here - though I'm not too certain just how valid that criticism is, as it often seems to come from just one or two members who appear to have a dislike for anything that Gerr does, be it his books or Westlawn... but that's another matter for another place...
I have also had a very brief look at the rather more complex Holtrop and Van Oortmerssen’s method's. I won't pretend to have fully comprehended either, but am I correct in my assumption that they are intended primarily for larger vessels - ship's in other words? And are they valid for smaller craft.. indeed any more use than Gerr's method?
I did note that at least one of them has a factor or two in there to account for variations in hull shape, which Gerr does not. And with claims like having a submerged transom can double the required power floating around here, then surely that is a must....

So - in short - which are the method's typically used for smaller displacement boats - lets say less than 50m - to calculate the required power, and which method's ought to be left well alone?

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Matt.

All these methods have caveats and limits of acceptability (whether mathematical or empirical). Therefore, first thing you must do is check the limits, even more so if the predictions are based upon a "standard series".

No point using a method based upon hulls with an L/B of 4-6, when yours is say 10. Similarly, no point using another of the Fn range is 0.1-0.2, and you're in the 0.3 range.

Just be mindful of the limits of applicability, is all you can do.

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El SeaJunior Member

For sailboats we used a simply ROM, Rough Order of Magitude. Where as 1 hp (for diesel)per thousand pounds and for gas power add 1/3 more to the hp.

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WillallisonSenior Member

Mat,
As John says, there are some pretty clear limits for most methods. Take a look here. It gives a summary & shows the limits for a few methods.

For most recreational applications, the more simple methods are likely to sufficient - the reality is that having calculated the required hp, most designers will add a signifcant safety (read fudge) factor to account for increases in weight, sea state, dirty hull, etc, etc.

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DCockeySenior Member

Gerr had an article on power prediction using Wyman's formula and Gerr's method in the June 2008 issue of The Masthead, pp 12- 17 http://www.westlawn.edu/news/WestlawnMasthead06_June08.pdf
Gerr talks about his "Maximum S/L" ratio dependent on Displacement to Length ratio which is a concept that is somewhat unique to him.

Added: Gerr has an interesting disclaimer concerning his maximum speed on page 17 of The Masthead issue I referenced above. Basically says "the hull is the correct type and normal form for the intended use" and then provides some adjustment factors for different types of hull forms and propulsion.

Last edited: Jun 15, 2011
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AlikSenior Member

Mat, try deGroot systematic series that is simple and will cover small craft in displacement range. Holtrop is mostly for bigger ships.

As to Gerr's formulas, You already know my opinion You can play with attached spreadsheet to see what happens.

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• Gerr_SHP_Formula_compare_with_GROOT.xls
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DCockeySenior Member

Alik, what is a good references for the deGroot systematic series? Thanks.

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Mat-CSenior Member

Thanks guys...
Actually I wasn't referring to you Alik... though I guess your feelings are well known!
Can you point me towards the DeGroot method?

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AlikSenior Member

For reference, can have a look at Principles of Naval Architecture p.96 or Mercier-Savitsky paper.

Other methods available are: SSPA series, NPL series, Series 64 and 63, etc. that cover semi-planing round bilge craft and are proven for small craft in displacement mode. Of those, deGroot is my favourite method as it gives performance estimate at early stages of design and very reliable.

This year I am making presentation at IBEX on performance prediction methods; will probably publish some review.

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AlikSenior Member

To add, never use only one method for performance predictions. I use at least 2-3 methods (in range of limitations) to see what happens. In case of doubt use the most pessimistic prediction.

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Jeremy HarrisSenior Member

I can vouch for the sense in doing this. I used three different methods to determine resistance (I was being very particular, because the boat relies on solar power, so only has a tiny energy source available) and found that there were big differences between each for my hull. I ended up doing some towing tests that showed, for my hull at least, that the Kaper method in Freeship gave results that were pretty close to the true values. By way of illustration, these are the power figures at the propeller output for my hull at 4mph (it's an inland waterways 18ft LOA, 3ft 4in beam, light displacement (245kg max) day boat - the canal speed limit here is 4mph):

Gerr method = 0.26hp
Modified warship method (unknown origin) = 0.14hp
Freeship Kaper method = 0.036hp
Tow test data = 0.044hp

As you can see, for this rather extreme light displacement low speed hull the variation between methods is pretty large!

Jeremy

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AlikSenior Member

Every reliable method would have specification of its limits and hull shape it applies to. Method is not applicable if any of hull shape parameters (DLR, CP, CB, LCB, L/B, B/T, etc.) is out of range or if hull shape is too different.

This is what I don't like about those 'simple formulas': there are no scientific publications on such methods where one can see the verification, there are no limits and parameters of statistical base specified, etc.

For Your purposes of very light boat added drag of wind an waves will be critical. Say, head wind of 5m/s can stop the boat. So those should be included in calculation not as service margin (usually about 20% of power), but by direct calculations.

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WillallisonSenior Member

I don't know anything about the Freeship Kaper methods, but I would have been surprised if any of those gave you sensible results... as Alik suggests, your hull is well outside normal parameters. Michlet would surely have been a better bet...

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Jeremy HarrisSenior Member

Sure, and you can rest assured that I carefully checked all the air drag figures (my background is designing light aircraft). The air drag problem isn't so much the impact on the power requirement, per se (although a 10kt headwind pretty much doubles the power required to achieve 4mph) , but the effect on steering, stability and comfort for such a light, narrow beam craft. The craft is for limited use on sheltered waters in fine weather as a day boat, so other restrictions, like the acceptable level of comfort, effectively limit the conditions it will be used in.

Waves aren't an issue either, as this is a boat for sheltered inland waterways use only, where the canals and rivers are not wide (our canals are typically around 30ft wide), or have a long enough exposed reach, for significant waves to develop.

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Brian@BNESenior Member

There are some spreadsheet templates available from http://hawaii-marine.com/templates/, at a seemingly modest cost. Towards the bottom are one for Displacement and Semi displacement Powering and another for Planing.

Are there any views on whether or not these are worthwhile?

Last edited: Jun 16, 2011
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