Basic planing hull design

[RJGoodhead]

Hi,
I'm a final year student working on the hull design for a wing in ground effect aircraft.

So far I've done a fair amount of reading on hull design and the different shapes and properties. Now I'm looking to predict the resistance of the hull during the take-off phase.

The basic design so far is:
Constant Deadrise = 10deg
L ~ 8m
beam ~ 1.5m
Displacement ~ 730kg

I've created a spreadsheet that works through a method based on a paper (Wetted Area and Centre of Pressure of Planing Surfaces; Kroukovsky, Savitsky and Lehman) presented in the fourth edition of the book "High-Speed Small Craft" by Peter Du Cane.

In order to estimate the effect of lift from the hull I have used a formula found later in the book for estimating CL and using this to find lift and augmenting the displacement of the spreadsheet, also taking values for lwl and lcb taken from a model in Delftship.

I then repeated this process until the trim gained from the spreadsheet and the CL converged and took the resistance values at this condition. I based this on the assumption that the decrease in wetted are was not accounted for in the calculation.

Is this a reasonable method?

thanks in advance for your help

[peterraymond]

You would probably be best off looking here:
Savitsky Power Prediction

[RJGoodhead]

I've spent some time working with this sheet and it appears the empirical data on which it is based does not apply to the craft I am designing. The spreadsheet is unable to return results for the CG of my vehicle.

It is not possible to alter the CG to a position where this spreadsheet would return values as priority has been given to the aerodynamic issues,

Can anyone suggest a suitable method for predicting resistance and whether the craft might be planing?

Also, is anyone able to confirm whether my previous method has any validity.

thanks,
Richard

[hoytedow]

Any reason a GEA can't have foils under the hull?

[yades]

have a look at this paper I've attached time ago...and see if of any help.....


Power And Prop Selection For Hard-chine High Speed Planing Crafts

[yades]

Originally Posted by yades

have a look at this paper I've attached time ago...and see if of any help.....


Power And Prop Selection For Hard-chine High Speed Planing Crafts

http://www.boatdesign.net/forums/att...l-1-master.pdf

[RJGoodhead]

Thanks Yades, the paper looks very helpful. It seems to tie together a lot of information I've found in books and papers as well as introducing a substantial amount of new material and clarification.

hoytedow, we originally considered hydrofoils but due to the risk of the foil meeting the water at a negative incidence in flight and the additional complexity of retractable foils they were ruled out.
There was an example of a vehicle with hydrofoils that met the surface in flight and flipped the craft, though I can't recall which one

[hoytedow]

Originally Posted by RJGoodhead

Thanks Yades, the paper looks very helpful. It seems to tie together a lot of information I've found in books and papers as well as introducing a substantial amount of new material and clarification.

hoytedow, we originally considered hydrofoils but due to the risk of the foil meeting the water at a negative incidence in flight and the additional complexity of retractable foils they were ruled out.
There was an example of a vehicle with hydrofoils that met the surface in flight and flipped the craft, though I can't recall which one

Thanks. That is an excellent reason not to use them for this application.

[RJGoodhead]

I've been working through a resistance estimation for my craft and before I go much further I just wanted to clarify some bits.

On page 1: Speed to length ratio is quoted as Vs*LWL^0.5 as it's a ratio should this be Vs/LWL^0.5?

Are all the calculation applicable in either metric or imperial units? Also, are the non-dimensionals 'proper' and not a mix of knots and feet? I'm hoping to avoid converting from my metric values if possible.

On page 10: it states ranges of hull parameters for which the data is applicable there are two symbols I'm not familiar with and cannot find elsewhere in the paper. these are a small case ' o ' with an ^ above it and an ' e ' with two dots above (I don't know how to insert these characters or the appropriate names). The o^ seems to denote some angle though I am unsure what.

I'd be grateful to anyone who can offer assistance on these points.

thanks,

Richard

[baeckmo]

Originally Posted by RJGoodhead

I've spent some time working with this sheet and it appears the empirical data on which it is based does not apply to the craft I am designing. The spreadsheet is unable to return results for the CG of my vehicle.

It is not possible to alter the CG to a position where this spreadsheet would return values as priority has been given to the aerodynamic issues,

Can anyone suggest a suitable method for predicting resistance and whether the craft might be planing?

Also, is anyone able to confirm whether my previous method has any validity.

thanks,
Richard

When you use the Savitsky algorithms in any of the applications available, the Center of Mass is required as an input, measured from the transom step. For each speed, the calculation will give trim, drag and often keel length. The keel length is used to check if the bow is wetted, then the basic algorithms fail (or an empirical correction is applied).

I can see no reason why you should have trouble with your hull as long as you keep within the application envelope. The only additional calc necessary would be to stepwise compensate for the aerodynamic forces from the wing, causing a change in hull load and possibly introducing a trim moment, depending on physical arrangement.