Resistance factors, planing hull at low speed

Why is the planing cat curve starting at 18 knots ? Doesn't give any indication of the low speed end of things.

 

1)Correct.
This is from “Mechanics of Marine Vehicles”, By Clayton & Bishop.

2) This is from PNA. “The curves were based upon data from a variety of sources, and result in two pairs of empirical curves which define two ‘design lanes’”

3) These are from varying only the displacement and then again repeated varying the length with constant displacement (to validate). The effects of hull shape are, or rather lack of them, is shown in the first graph I posted.

 

Original graph is mean values of resistance for series 62 and 65 from Hubble.

 

Because we desgined that boat for 25kts...

 

Could you also post some rough sketch of hull form variations?
othervise it is not possible to see, what is compared to what.

 

please note, that in PDF file graph is for the same Froude Number = same S/L ratio

 

These boats (australian 18 ft skiffs) sail at 14 knots close hauled. Also his HSP boats are capable of similar performance.
Of course, there is no science and naval architecture involved...

 

To me, it does not matter how fast they sail; these are just small boats where design by trial and error works. And thus You are right - no science and NA involved. The graph they show in the book is just nonsense because every naval architect knows how to recalculate resistance from one size of boat to another. With this graph, their intuition was wrong

 

I have the final shape completed only - planing shape.

But the purpose of study was: what is better round bilge displacement or chine planing for this speed and load? It is a bit tricky issue, especially to convince the customer. Round bilge was studied with Molland series, so the parent hull is from there (actually it is NPL-series hull).

 

I see a lots of graphs here, but each one appears to be valid inside it's own fence.
The original question was about the behaviour of various hull forms in the low-speed range, so I don't think it's useful to show a curve for a planing cat for 18+ kts, compared to curves of displacement hulls at lower speeds.

There was a similar thread started by Easy Rider, where I had attached a paper by Van Oossanen there (he does know something about ship design) which shows a general resistance trend as a function of immersed transom area:
http://www.boatdesign.net/forums/boat-design/planing-hull-disp-speeds-31464-4.html#post405575
It was much debated afterwards due to the lack of info on the method used by Van Oossanen et al. in that research. However, it does show a 100% (or 2 times) higher resistance of a hull with immersed transom (like a planing hull), compared to a dry-transomed hull.

So now I'll put some more fuel in this fire by showing another paper, by Blount and McGrath. Several hard-chine and round-bilge hulls resistance data have been scaled to 500 t, and the variation of both LWL and wetted surface has reportedly been accounted for. Again, a significant difference (of the order of 100%, or 2 times) between the two hull types is shown in the low speed range (figures 1 and 2). Also, the influence of L/D^0.33 (or simply LD) ratio is shown in the figure 3, for the two hull types at Fn=0.6 (semi-displacement), with R/W data points again in favor of round bilge hulls.

I'd like to hear your opinions about this paper, it's conclusions and eventual flaws.

Cheers

 

Good paper; all based on well-known series though.

 

The figure n.8 (Design Guidance) is particularily interesting and valuable, imho. It shows where various hull features jump in as the Froude number is increased.

If you take into consideration both the Blount's paper seen before and Van Oossanen's (which I'm linking here again, for comodity: http://www.oossanen.nl/download/perryvanoossanen_-_motor_yacht_hull_form_design.pdf) you get a much larger picture about the factors influencing low-speed resistance, which goes beyond just considering LD or hard-chine or round-bilge.

The way I read these results in the displacement and semi-displacement speed range is:

1. the LD ratio is the single most important factor for Fn>0.5 and when LD is less than 7. For LD>7 the difference between hulls with different LD gets much smaller and less important.
2. For displacement speeds (Fn<0.4), LD has a very little influence (NPL data).
3. the transom immersion can double the resistance at displacement speeds (Van Oossanen).
4. the LCB (or LCG) location is of great importance in the displacement and semi-displacement range, and can double the resistance if placed too-far fwd or too-far aft of the optimum location.
5. the L/B factor is important for low LD's, and the resistance increse of a L/B=3 hull over an L/B=5 can be as much as by 40%-50% in the semi-disp. range. For LD>6 it becomes much less significant. (NPL data).

Comments, corrections?

Cheers

 

OK, I have seen that paper, but we do not know what the parameters of tested hulls are. Use of Telfer coefficient makes me think that they could have DLR also.

Another concern there is use of CFD for transom hulls and how they model transom effects.

 

Generally seems correct.

These conclusions and numbers are different from series to series.

Transom submersion might influence the resistance, but I would say less than double in range of displacement speeds.

CP would have more effect on resistance compared to LCB.

 

Un-scientific input...

My home-made 1/4" plywood rowboat with round chines and a submerged transom had an interesting characteristic. With some overpitched props on the small dc motors, she would only run about 4 MPH. If I moved my weight all the way up front and lifted the transome out of the water, the speed would go up and the motors would unload. Just an estimate but I would say at least a 2 MPH increase with the transom lifted out of the water. The bow has a fine entry so it didn't seem to mind plowing.

Shifting the weight forward never worked well with off the shelf trolling motors on this hull because they are very speed limited by the rpm and pitch of the props.

This hull is pictured in my profile with a bad attempt at a surface drive. I never got over 8 MPH (GPS) with the surface drive. The was a giant 'hole' behind the transom above 5 mph and I couldn't climb up on plane. I never got a chance to see if shifting weight forward would have provided the extra speed to plane her out. Termites took her over last summer but got a really light Al hull with a similiar shape to play with next year.