Not able to reach maximum RPM

[lambros]

I have a Yamaha 150 outboard engine. I have a 19" inox prop. The problem is that I cannot reach the full 5500 rpm (according to the manual). The loads are usually 250 lt of fuel and two people on board. As much I try to trim the engine I cannot reach more than 5000 rpm and 45 miles (not knots). My RIB is a deep V (27 deg).

The question is: Not being able to reach the full 5500 rpm, am I facing problems of engine wear??

Thanks a lot.

[Alan]

If you are not pulling Wide Open Throttle with your 19inch prop then, assuming the engine is in good order and nothing else is wrong, you probably need to drop down a prop size to a 17inch prop. This should enable you to reach the 5500 WOT setting.

You are probably straining and wearing your engine if you cannot make WOT the theory goes. In practice I'm not an engineer enough to understand exactly what the consequences are!

We used to only be able to turn a 17inch prop on our Johnson V6 150 but with the new Merc Optimax can swing a 19inch no probs.

HTH, Alan

[eksrae]

the first thing i would do is raise the motor one or two holes up. this will increase your rpm as well as speed. we run our rescue boat with the props basically level with the bottom of the boat. give it a whirl

[lambros]

Eksrae, thanks.
I will , by all means, give it a try. My engine is mounted at the highest hole so I will try to lift it one or two holes and see what happens.

I have a question though which might sound silly:
If you raise the motor doesn't this automatically mean that the boat's hull (while cruising) will be more in the water thus increasing drag?

Thanks a lot

Lambros

[batfalcon]

The depth of the hull while cruising is the result of boat's weight, load distibution and hull's overall shape. It has nothing to do with engine's installation height.
It would have if you used a powerful engine in a rather small boat. Then the engine would enforce it's position in the water against the boat's tendency.
Maybe you are confusing the trim's effect but it's a totaly different matter.
Normally the anticavitation plate of the engine must be level with boat's hull. The heigher you get (towards the surface), will result less drag so engine's rpm will go higher.

All that assuming that you have a 3 blade stainless prop, which requires to be fully submerged. If you switch to 5 blade then you must install the engine even higher, because 5 blade props are supposed to work somehow with 3 blades in the water while the rest 2 are out. The logic is more or less the same. Less moving mass in the water equals less drag and so on.

[eksrae]

just a note about the props. we use three blade stainless on our boat and the blade can be seen piercing the water. it depends on the design of the blades. we can still tow larger boats with no problems.