Advanced prop calculator on boatdiesel.com – hull factor

[Magnus W]

I posted the following on boatdiesel.com but I figure why not ask the same question here:

"I´m in the process of planning a stretch and repowering of one of my workboats. 8,5 m LWL and beam at chines of about 2,9 m.

I´m about to stretch her 3,1 m by adding a new stern section. From approximately where the cabin sits and aft the hull has the same shape (save for keel). The LxB ratio will increase from circa 2,9 to 4,0.

Aside from the added length of the hull the keel will be extended and made bigger in order to fit a 28" prop sitting approximately 2,0 meters aft of the current position.

With the advanced prop calculator I was able to determine a hull factor (6,3) for the boat in it´s current state.

Is it fair to assume that the new hull factor will be about the same as the old one? On the one hand the LxB ratio is more favorable, on the other the appendage drag will be higher due to the bigger keel, prop and rudder."

 

[JSL]

I have not checked out 'boatdiesel' yet but ---
what is the power, displacement, hull form.......Boat for speed, towing, etc.???? You may have a lower disp/length ratio which is good.
Not sure what they mean by hull factor.... type, speed, a coefficient (Cb , Cp, Cwp. ???)

 

[HJS]

Much more important than the length-width is the length-displacement ratio and the planing area in relation to the displacement. In addition, the overall center of gravity affects the result. If not all of these factors are included in the calculation model, there is a risk that the result will not be correct for a planing boat with V-bottom.
JS

 

[Magnus W]

I have not checked out 'boatdiesel' yet but ---
what is the power, displacement, hull form.......Boat for speed, towing, etc.???? You may have a lower disp/length ratio which is good.
Not sure what they mean by hull factor.... type, speed, a coefficient (Cb , Cp, Cwp. ???)

I'm not sure what hull factor is either but I believe it's some sort of index by which the end result is adjusted. Hull factor 1,o is slower, 10,0 is faster (for the hull type entered). Also, there's a way of determining your current factor by knowing your current performance but as I said I'm stretching the hull so the current factor doesn't apply.

I think the calculator is rather good, for instance all engines and transmissions are selectable so the calculator "knows" a bit more than the simpler ones.

Much more important than the length-width is the length-displacement ratio and the planing area in relation to the displacement. In addition, the overall center of gravity affects the result. If not all of these factors are included in the calculation model, there is a risk that the result will not be correct for a planing boat with V-bottom.
JS

The length/displacement ratio will in this case increase from 1,55 to 1,66 (if length in meters divided by weight in tons is the correct formula).

I can only speculate as far as the exact new cg position but it will be more aft than today, not only in meters relative the current cg but in percentage of lwl as well as in relation to hull buoyancy (what I mean by that is what we refer to as center of lift in aviation). I will have the ability to adjust the cg so if it's way off (whatever way off is) I can fix it.

Planing area, I have no idea and I'm not even sure what it means. But if you mean the wet area of the hull at planing speed I would suspect it to be about the same as today given that I'll be both heavier and faster which, I assume, to some degree will compensate for each other.