Small Craft(3) - 8m yacht hull (Maxsurf Ver)

Maxsurf -> Auto CAD

MACHINARY SYSTEM

Power and Estimate speed

EHP - Effective Horsepower. This is very close to the amount of power required to tow the vessel.

IHP – Indicated Horsepower. – to drive vessel at a given speed. Ratio of EHP/IHP is usually 50 % (1:2).

BHP –break horsepower, max horsepower generated by the engine at a given rpm.

SHP – Shaft horsepower, transmitted along the propeller shaft to the propeller at a given RPM. Max SHP is the MAX power delivered to propeller, almost always at max attainable RPM.

*power lost in the gear box 3%

*power lost in the shaft bearing 1.5%

Power and Energy losses

35% lost in heat to the atmosphere

25% lost in heat and vibration to the water

2% lost at the propeller shaft

Torque – twisting force.

The lower the RPM and the higher the HP, the greater the torque

T = (5252 X HP) / RPM

SL ratio

SL – Speed length

Kts – Speed in knots

SHP – Shaft horse power at propeller

LB – Displacement in pounds

WL – Waterline length in feet

SL ratio =  = 1.34

Displacement boat speed formula

SL ratio =   = Kts/

Recommended RPM for Continuous Operation

Type of engine	% of max RPM
Light – duty petrol and diesel automotive conversions	70~ 80%
Light – duty or high out – put marine diesels	80 ~85%
Intermittent – duty marine diesels	88 ~ 92%
Continuous – duty marine diesesele	98 ~ 100%

The Torque Curve

The Max torque of most engines occurs below max RPM. For that reason, the most economical and efficient speed of operation of many engines is around 70 to 85% of top rated RPM. – choose cruising speed.

Measuring to find Displacement

Must be sure to include the weight of:

1. Full crew and passengers

2. All normal ship’s stores and gear

3. 2/3 of all fuel and water tank

4. 2/3 of all cargo

Displacement hulls

Sailboats, typically have displacement hull. Dome powerboats, some powerboats cruising trawlers.

*Design to ride in (not ride on) the water.

*Generally more fuel efficient than planing hulls.

* Can carry heavy stuff.

*It offers soft ride, gently push water aside.

* Susceptible to roll from the ‘beam sea’ / powerboats are more vulnerable than sailboats –they install keel(with the ballast) small riding sails. Or roll stabilizers

Displacement hull speed

The wave move at a 1.34 times the square root

1.34 X  = hull speed

Speed Length ratio is 1.5 to 2

•      DL ratio =DispT/(0.01 x WL)³ 

DispT – Displacement in long tons of

                  2,240 pounds

WL – water line in feet

•      Max. speed-length ratio vs DL ratio

SL Ratio = 8.26/(DL Ratio)^0.311  

Semi –Displacement Hulls

Lift up forward that achieve higher speed than pure displacement hulls. However, it is high fuel consumption.  

*Speed to length ratio in the 2.0 t0 3.5

Planing Hulls

Lighter than similar size of displacement or semi displacement

Tend to bounce around in choppy water.

Require substantial engine power to achieve planing speed. – Once planing, efficiency jumps – less power need/ total wetted surface drop – less friction 

Planing Boat

•      SL Ratio is over 2.5 or 3

•      Quarter beam buttock is 0 to 2 degree

•      Crouch’s Planing Speed Formula

           Kts = C / (LB/SHP)^0.5 

           Kts – Speed in knots

           C – Constant chosen for the type of vessel

           SHP – Horsepower at propeller shaft

Reference

Dave Gerr (1989) Propeeler Handbook.  International Marine

Bob Sweet (2007) Powerboat Handling Illustrated. International Marine

MACHINARY & SYSTEMS

MARINE DRAFTING - 13M BOAT SUPERSTRUTURE

SMALL CRAFT DESIGN (2) - 8m yacht hull

8meter Yacht hull drawing

Machinery & system (2) - SL ratio

DISPLACEMENT BOAT
example	-	finding SL ratio
WL	70	ft
DL	220000	Lb
Speed	11	Knt
Sl ratio =	Kts/(WL)^0.5
	1.314751
SHP=	Lb*(SLR)^3/10.665^3
	412.165	=	crusing speed		(85% of Max speed)
	484.9	=	MAX speed
	LB/HP
	533.7668
SEMIDISPLACEMENT BOAT
example	-			DispT – Displacement in long tons of
					2,240 pounds
Light vessel
WL	50	feet
DL	30000	Lb		Dis T	13.39285714
					13.4
	•DL ratio =DispT/(0.01 x WL)3
				DL ratio	107.2
	SL Ratio = 8.26/(DL Ratio)0.311
				Sl ratio	1.930142924
					1.93
	Pict
					LB/SHP= (10.665 / SL RATIO)3			168.7371
								168.7
					SHP=LB/168.7		177.8305
							178
PLANING BOAT
example					Kts = C / (LB/SHP)0.5
Twin outboard runaboat					25=150/(10890/SHP)^0.5
	WL	30	ft
	DL	10890	lb		SHP= 10,890 /(150/ 25) 2
Operate speed		25	knt
					302.5
				HP(70%)	302
				MAX	431.4285714
					431
				Twin	215.5
					215	each