Sterndrive on semi-displacement hull ?

Sterndrives are used successfully in many displacement boats. For example, houseboats.
http://www.usedboatlist.com/used-1970-nauta-line-for-sale/

 

It is always going to be attractive to be able to nudge across sand-banks by tilting up the drive mechanism.

 

on modern semi-displacement hulls sterndrives also has the advantage of adjusting the trim in negative or positive to change the longitudinal lift of the boat at the varying loads or marine conditions

 

I'd like to know the hydrodynamic characteristics in terms of planing or semi-displacement for the hull of the following, similar boat as well >
Jeanneau Merry Fisher 725

As I don't want to bother you much more, but would you please point me where I could find out more on your determination method, so I can deduce myself ??

MANY THANKS,
V.

 

V

It is basic hydrodynamics. However there is a significant weight of information behind all this "simplistic" explanation, which may confuse, when probed further. So I'll briefly summarise.

See the graph below:



This is basic overall resistance versus speed. But we use a non-dimensional number called a Froude number, or Fn.

You can calculate it by Fn = Speed (m/s) / sqRt [g.L (m)]

So for that 6.83m hull LOA and assuming a length, waterline length that is of 6.0m.

WE can find the speed in m/s => 29 x 0.5148 = 14.93m/s
g = gravity = 9.81m/s^2 and is a constant
L = 6.0m

So we have Fn = 14.93/(9.81 x 6.0 ) ^0.5 = 1.94 or roughly 1.9

If you look on the graph a value of 1.9 is off the chart..it is past the main hump which occurs around 0.5. After 0.5 you can see the curve reducing to 0.6. Once you go past 0.6 the resistance changes but, past 0.6 the vessel, if a hull shape as you have shown it will be technically planing.

The same calculation for 19 knots = Fn 1.3, which is still off the chart..again therefore again suggest it is planing.

Semi-displacement boats, are in the 0.35 to 0.6/0.8 Fn range...although many question the values and try to be "exact" but there is no real official exact definition. Except that the vessels are not fully planning nor are they pure displacement, as their hull shape suggests otherwise too.

Displacement hulls are below 0.45 on the graph.

A typical displacement hull will never get past Fn 0.5. The waterlines around the aft section do not promote that which is needed to plan. A wide flat transom. If you look at a tanker, as an example, the hull shape is about flow of water into the prop. This is where non-technical people get confused and call this "hull-speed". There is no such thing, it does not exist in the vocabulary or nomenclature of hydrodynamics. Only in laypersons minds - they use this term to describe something that they don't understand, hence the conundrum!. Simply because it requires a degree of understanding of hydrodynamics that is beyond their current knowledge or level of expertise. It only has some reference to what is called the prismatic hump, the hump you see on the graph above.

The speed length rtaio (SLR) or Taylor Quotient (Tq) is basically the same looking graph as the Fn graph above, but the cut-off is a value of 2.0. Beyond a value of 2 one is considered planing.

As you can see the Fn is metric system, and the SLR is simply speed (knots) divided by length (feet) is old imperial system of units hence different values but basically the same shape graph.

 

Typical traditional semi-displacement boat.
You can see that the self-generated wave is longer than the hull, but the boat is not planing.

http://nautipedia.it/images/thumb/b/b7/MOTOMAR31.JPG/1280px-MOTOMAR31.JPG


http://nautipedia.it/index.php/MOTOMAR#mediaviewer/File:MOTOMAR31.JPG

 

The point seems to be the OP was concerned the boat was a semi-planing type and he probably hadn't noticed many such combinations, that type of hull with a sterndrive, however it is obviously a full planing type, which settles that.

 

cats are sleds and IMHO never actually plane