Bottom Loading

[Willallison]

In a former thread ( http://boatdesign.net/forums/showthread.php?s=&threadid=539 ) Tom wrote about the need to keep weight/power and weight/area of planing area, under control.
Tom - if you're out there......
You refer to needing to keep the weight down to 240 km / sq.m
Do you simply use the waterplane area for this calculation, or is it the total (planing) surface area of the hull (measured up each side of the "v" etc)?

[tom28571]

Will,

I use a percentage of the at-rest bottom surface area used by some some naval architects. The number is 65% and is the actual surface area and not the waterplane area. I doubt that it matters if this percentage is excactly right as long as we are consistent. The area will obviously change with changes in trim angle so it is only exact at one speed and trim angle.

Assuming that we have a "normal" hull shape of normal aspect ratio, I think that power to weight ratio and weight per sq ft of bottom contact area are the main determinants of how easy it will be to get the boat onto and hold onto the planing mode. Neither of these factors can be far out of line if the boat is to plane at low speed. Lots of power per unit of weight will be no good is the weight is too large with respect to the area of the bottom in contact with the water.

Boats like this tend to stick their nose in the air above displacement speed until the dynamic lift per unit of bottom area begins to counteract the force of gravity per unit area and the boat starts to lift up. As the speed builds, the trim angle required to counteract gravity becomes less and wavemaking resistance becomes less and so on, and so on. As long as more power is applied, the trim angle will continue to decrease and the boat to lift until it starts to dance on the water like the offshore deep V's. Adding more power then makes the boat so unstable the it can not be controlled.

The relationship between weight per unit of bottom area, trim angle and speed must be understood to make any sense of these arguments. Dynamic lift and speed are both directly proportional to trim angle. That is why the trim angle get lower as speed increases.

The above type of boat simply can not be driven comfortably at what I call desirable cruising range of speeds and need to go too fast to get over the high trim angle "hump". I don't object to fast boats as such but, if they will not operate comfortably and efficiently down to the low teens, then I would not like one as a cruising boat. Fine for a week end or other special purposes but not a "cruising boat " for the lower classes that did not make a gazillion bucks off the stock market.

Back to the subject:

Any performance machine will almost always be faster if he weight is lower. That is just Newton in a different guise. What better way to state the weight of a planing boat than to give it in relation to the area of the bottom that must support that weight? Clearly that makes a lot of sense. Lower is better and higher will delay planing. Of course, very light is no good if there is no power to propel it, so these two factors must work in concert to make the boat a success.

One essential truth to get from this is that the trim angle is not an independent factor that the designer can choose. He/she must make other things work so that an efficient trim angle can be achieved. Most designers say that a trim angle of about 4 to 5 degrees gives the best balance between wavemaking and friction drag forces. With my own boat, I chose to go a slightly different route by accepting more friction drag (and therefore a lower top speed) in favor of lower wave making and consequent better low planing speed performance. Unlike other boats, the Bluejacket 24 (which I call the design of "Liz") comes up level and the trim angle slowly increases until it is a maximum of about 2 degrees at top speed of 20kts.

I know that this has been a very long explanation but I lack the time to make it shorter.

[Willallison]

Thanks Tom,
I'll go off and play with some numbers.
I'm intersted as to why the entire bottom surface area is used.
When pressure is exerted on the hull there are two forces at work - one is vertical and acts through the area of the water plane. The other is horizontal and would act through the longitudinal section of the immersed area. As it is only the vertical forces that support the hull, I would have thought it was logical do do the calculation based on this area.
That's just my (il)logical thought process - where does it go wrong?

[tom28571]

Will,

I wondered if anyone would ever gnaw on this bone. I may have been mistaken about the waterplane vs actual area in this calculation and suspect that I wrote it backwards. On my boat, with only 10 degrees deadrise, there is not much difference and I paid no attention to it.