Designing Planing Hulls...

Hi everyone,

As a freshly graduated Naval Architect, I don't have real-life experience of building high speed boats. However, I had a longstanding question regarding speed estimation of planing hulls, since I was in college.
As I could only design hulls in softwares and estimate speeds on them, my speed estimation can only be as correct as within the limits of the softwares I would use. I often hear from experienced builders that such estimations are not very reliable. Unfortunately, I cannot compare the accuracy of such software results with real-life results, as I would not be able to design myself and build a boat in the near future yet. I cannot afford towing tank and cfd calculations as well.

- One resistance/ speed estimation method I learnt during my courses is to interpolate results from basic ships, the ships existing in service and of actual speeds which we know. So, how much is it accurate to estimate the speed of the boat I would be designing from an in-service boat with similar LWL, Beam, Draft and Displacement?

- And, how much does hydrodynamics contribute to planing hulls' speed generally? Is it possible to see very different speeds in two hulls with same LWL,B,T and Displacement only due to slightly different hull shape (2~3 degrees' variation in deadrise angle, for example)?

- I also want to hear about accuracy of (analytical) estimation methods like savitsky method from experienced builders in this group.

Thanks in advance for your discussion.

 

You mean top speed ? For practical purposes, perhaps not the most important metric. It is possible for a boat with more deadrise, with the other parameters more or less equal, to have a higher top speed, with the same power, largely down to it getting wetted area down lower.

 

Many researchers and students have published on these subjects; I'd think you could get a good grip on the accuracy of methods without building anything yourself, just by googling through the many freely available scientific papers focussing on the validation of CFD or classic regression formula like Savitsky against tank testing.

 

I was going to suggest Savitsky but CocoonCruisers beat me to it. Just do a search on this forum on Savitsky and you should find what you need. Unfortunately Dan is no longer with us, but his papers and research will live on.

 

also check out Renato (Sonny) Levi

 

Savitsky's method is the standard analysis tool for analyzing planing boats. It has limitations however. Constant deadrise and constant beam of the portion of the bottom in contact with water are assumed while many boats have twisted bottoms with varying deadrise. A few years ago Savitsky developed an emperical method to calculate an equivalent deadrise angle for analyzing warped bottoms. This equivalent deadrise angle method was based on a single set of test data. I don't know if any studies have be done to verify it using other data.

 

Thanks for your inputs. I admit I haven't spent enough time to understand Savitsky papers thoroughly. So, I vow myself to look into them more.

However, as David points out above, I think it would have a few limitations for power/ speed estimation of boats with various underwater shapes in practice.
I often hear that tiny appendages such as bolts or zinc anodes on the underwater surface contribute so much to hydrodynamics of speed boats, and then lowering their top speeds. If it is true, with large appendages like shaft brackets or propeller tunnels, what are the chances of estimating top speed of boats?
In essence, how much margin(%) do you practically set when you estimate top speed/ required power using Savitsky method/(similar methods)?

(Meanwhile, I am also doing my search on Savitsky's method discussions in the forum, so, pardon me if I am asking redundant questions.)

 

Very complicated subject, some planing hulls carry weight better than others, strakes and chine flats can make a big difference too, there is such an element of "suck it and see" that no generic formula will be strictly accurate.

 

Hi,
wy not looking at the fastes modelboat in the monohull class. Monohull without steps and only very small stringer. So a very clean design. The drive are surface propeller forced by electric motor. The current records are hold by Ralf Moser /Germany. The fastes record was 127 mph . The boats are clean monohull V with outrigged propellerdrive shaft. For propwalk free operation they are counterrotating and are placed far outside of the boat centerline. see pic.
At such speed the boat climbe high on the V only touching the water with a small ski like centerline. The outrigged Propelles stabilised the boat from rolling left and right hand . This can also be done with outbord motors that lay on each sinde at 45 degree and cut away skeg. Stearing by a ruder in the middle like the modelboats show.
The light green monoboat with the Gurny flap at the trasom was driven by a singel propeller was the firstboat hit 100 mph in it´s class. The blach with the twin counter driven propellers was a very efficancy boat running with much less power than the light green hull but reach the same top speed.
Here you can read at the Intelwater forum about this modelboat class record designing boats.
Monos built strickly for SAW? https://www.intlwaters.com/threads/monos-built-strickly-for-saw.53230/ , at page 16 you will see a video of the last record run. Our record water is the munich Olympic rawing stadium where also run the fastest modelboat records over all outrigger style boat 300 km/h plus.

Happy Amps Christian

 

that is fast for a model boat, blink and you miss it.

 

If you don't study Savitsky papers, you won't get far. There is also a paper by Molland with all the parameters of the planing boat. An Excel spreadsheet regarding the design of planing hull was posted here a long time ago by Dingo Tweedie. Lots of in depth discussion there if you want to get your feet wet.

Here; Savitsky Power Prediction https://www.boatdesign.net/threads/savitsky-power-prediction.2187/page-6