Maxsurf Resistance (Hullspeed)

Hi all,

I have a query regarding the methods Maxsurf uses to measure appendages such as a skeg when applying the regression analysis functions such as Holtrop. As Maxsurf requires an input as to whether the hull is round bilge or hard chine and can't accurately define the hull shape, it is my feeling that the skeg increases the wetted surface area value used in the equations without properly defining the form of the vessel, hence over-predicting the required power for the vessel. Am i right in making this assumption?

This question is in regards to a semi-displacement hull form (Fn = 0.38) I am designing for a university project so I really just need to be able to justify any assumptions that I make. I am familiar with the Holtrop method and the parameters used fall within the limits of applicability.

Any feedback would be appreciated.

Cheers

 

I understand and I share your doubts.
The Holtrop method, which is long before the computation with virtual models was generalized , applied a series of factors, different for each type of appendage, to calculate the frictional resistance due to the appendages.
It is to be assumed that Maxsurf calculates the area of each appendix, if it is included in the model that the user has ,but I do not know how it can know what type of appendix is and, therefore, what coefficient should apply in each case. I think we should compare the results of Maxsurf when it uses the Holtrop method and the results obtained by applying the method directly.
As Maxsurf does not explain at all what it does, perhaps it should be addressed to the creators of the program. I think from here, normal users of the software, we can not solve your doubts.

 

Thanks for your thoughts TANSL. I do have a spreadsheet set up with the Holtrop method so I will give that a try and compare. Only reason I posed the question was because the hull inc. skeg increased the resistance by 25% when compared with the bare hull form. Considering the vessel is 55m LOA and the skeg was relatively fine in shape I felt this was an over-prediction.

As I mentioned it's only a school project and there are a number of other matters we need to satisfy in the design brief so it's just a thought for now but any further info anyone has would be good for future reference.

 

Is your boat within these parameters?

 

Fn, L/B and B/T all good. Cp is 0.51 which is a little low

 

Maybe the type of boat is not suitable for Holtrop. In any case, since we do not know how Maxsurf calculates and the results that you get seem not correct, I would forget Maxsurf and I would go directly to calculate it with the spreadsheet. In this forum you can download spreadsheets, one of them, by the way, made by me.

 

I attach my spreadsheet which I hope will be useful.

 

Thanks TANSL. Looks good, i'll give it a try.