Resistance factors, planing hull at low speed

So.....if we tow a bullet-shaped object that is fully underwater, there is a point where the drag reduces when the rear end of it is no longer 'wet', with a tapered cavity appearing in the wake ?

 

If it is completely submerged that would only occur if it was cavitating.
If it is a standard vessel with a standard transom, there will be a loss of
hydrostatic pressure on the transom when it runs dry. That loss can be
considered as a component of resistance.

 

Ok, so I have obviously misinterpreted Ad Hoc's story about the hand moving through the water, as soon as the back of the hand "dries", the resistance increases due to the loss of hydrostatic pressure against the "transom". I'll try and pay more attention. I somehow assumed he meant the resistance decreased at that point.

 

Wasn't this thread about canoe vs. transom stern resistance at low speeds, when the transom is wet? Or did I misunderstand it right from the beginning?

 

I'll go back to the post which revived this thread:

This can happen without propellers. Have a look at Qualitative Investigation of Transom Stern Flow Ventilation by Maki, Troesch and Beck
http://www.iwwwfb.org/Abstracts/iwwwfb19/iwwwfb19_32.pdf
They investigated the flow past a transom at various speeds

 

I went back to the first post and the question asked was about the resistance of planing boats at low speeds, same as the title. The Original Poster asked about both the effects of chines and sterns. During the almost 20 months and over 250 posts since then the thread has wondered around a bit.

 

1. The simple equation due to Vanden-Broeck and Tuck is a 2D result. It is
really only applicable to wide transoms.
2. The rate at which the transom becomes unwetted has been investigated
by Doctors and his colleagues in several papers since the Make et al paper,
and a good thesis by Simon Robards I have mentioned before. The
coefficients in the regression equations that determine the rate of
unwetting seem different with every new paper.

 

My mistake. Maki, Troesch and Beck did not develop the simple criteria based on a Froude number using the submerged depth of the transom, nor did they advocate it as a predictive tool. The previous post was corrected.

 

It is possible, depending on hull shape, for a "submerged" transom to be dry at low speeds (Fn based on LWL considerably less than 0.3). This can be seen in various experiments including those reported in the Maki et al paper.

The Froude number based on submerged transom depth appears to be an approximate indicator of the speed required for a dry transom.

 

Correct, my fault. I have mistaken this discussion for this other old thread: http://www.boatdesign.net/forums/boat-design/planing-hull-disp-speeds-31464.html
Cheers

 

Doesn't surprise me if the papers are like those posted by DC. It is not consistent in its methodology.

The prism hull is a different length, the trims, which are fixed!!!, are different, the B/T's different from model to model. If their motivation was to investigate ventilation, in their approach, then a hull and hull shape is pretty much irrelevant, given their inconsistencies when comparing results.

 

Coming back to the original question of this posting. Low speed resistance of planing monohulls is more complex than saying it is dependent on mainly length displacement ratio (ok for catamarans), transom subergence etc. As Almeter has shown the low speed resistance in reality is mainly dependent on the Clement number which is a function of the Displacement, the LCG and the Beam of the hull.

The attached paper explains it all....

 

Tests/calculations for planing hulls at displacment speeds are important. In particularly hump speed. All of us in the industry have heard about planing boats that get stuck on the hump and cant make it onto the plane because their hump resistance is too high. That is clear enough reason to allways consider hump resistance in a planing hull design - especially heavily loaded ones.

 

Hull Speed and SOR ? I know this is an old thread, but maybe it will get me some info.
Please educate me to the two terms - Hull Speed and SOR.

 

That's because there is insufficient installed power to over come the prismatic hump or too much weight or both. Which is often owing to the designer not tank testing the hull form or only looking at top end speed where they do tank test.

I wouldn't call testing at hump speed slow! For all vessels, cats or mono's running at high Fn's, you need to know what the main hump resistance is, otherwise you'll end up as you've described.

Not doing this is just poor design practice.