Yanmar Pleasure Boat Marine Engine Help

Propeller

The propeller controls the maximum rpm under load.
On slower vessels there is no substitute for propeller diameter. Fit the largest diameter prop that will fit in the space available. See the DIAGRAM for recommended clearances from the propeller to various parts of the hull. For faster power boats, too large a prop diameter may increase drag. Check with the designer or a reputable propeller supplier.
A 'MAXPROP' is an excellent cruising sailboat propeller with long life and low maintenance. If you have lost your handbook and need a new one you can download one here in .pdf format. 2 BLADE  3 BLADE AND 3 BLADE VP

CAUTION: Don't use a 'Maxprop' feathering propeller with a vessel that may exceed 20 knots under sail, it will auto-unfeather and the result is depressing, especially if the gearbox is engaged.

PROPELLER MATCHING

The graph shows Specific Fuel Consumption , Torque and kW / horsepower output for a YANMAR 4LH -STE engine. This type of graph is usually on every engine brochure.
Firstly, ignore the torque curve, it has no meaning in this application.
Secondly, ignore the max. output w/o marine gear and the max. output with HSW630A marine gear, they too are meaningless in this application. I must assume they are there because the marinised truck engine brochures have them? The two useful curves are the propeller power curve and the specific fuel consumption curve. To help explain how they work let's bring the prop and engine to life and give them a name. Prop is 'Prop' and the engine is 'Grumpy'. (It's always 'Grumpy' isn't it?)
You move the 'go faster' lever and tell Grumpy you want to cruise at 2600rpm. Prop tells Grumpy he needs 120hp to run at 2600rpm so Grumpy adjusts his fuel flow to suit.
You tell Grumpy to run at 2900rpm and Prop tells Grumpy he needs 160hp at this speed so Grumpy adjusts his fuel flow to suit. You tell Grumpy to run at 3300rpm and Prop tells Grumpy he needs 230 hp. Grumpy supplies everything he can and is happy because he and Prop are in harmony.
However, some nasty people give Grumpy a hard time by making him drive a Prop that is too big. Grumpy huffs and puffs and pours in a lot of fuel but cant reach 3300rpm. He takes up 'black' smoking and cracks up internally. Because he cant reach 3300rpm he can't pump enough water or suck enough air to keep cool. Eventually he has a hernia and the 'doc' makes some more money. Other people give Grumpy a Prop that is too small. He whips it around and makes lots of froth. Pretty, but useless. He takes up 'blue' smoking because he hasn't much else to do.
Some engine installers prop the vessel up to 100rpm above the max. at full power, to allow for a lightly loaded vessel during the initial sea trial. This is a good idea as most vessels are nowhere near their fully laden weight at this time. As the weight increases the boat will slow down and so will the rpm as the load on the engine is increased.
The two things that dictate how FAST your planning hull can go are weight and horsepower.
Weight is a little less critical in displacement vessels, e.g. cruising sailboats, and the thing that dictates maximum SPEED is the length of the waterline.
Jet units are affected very little by an increase in weight . Once the impeller is matched to the engine the load will very slowly decrease and the RPM will increase as the impeller wears. (Note, however, the boat will slow down)

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