Shaft angle problem

[Stason4ik]

Hi,

I have a problem with performance of my boat.
As i think, the main mistake is the angle of the shaft.
As you can see on the picture the angle is too big.
The force vector is going down an rising the stern up.



As a result i get low speed 8-9.5 knows at full gas.
I have marinised diesel motor from citroen xantia TD 2.1

As a solution to the problem, i thought about some kind of quard ring that will be parellel to the boat and will correct the diriction of water flow.

Any ideas?

[daiquiri]

Yes, that shaft is pretty inclined (I have measured some 18° from the photo) but I've seen worse and the other things should be checked out first... What are your boat's data? Dimensions, displacement, nominal speed etc. Do you perhaps know the position of your longitudinal CoG?
A 2.1 TD by PSA gives some 105-110 HP, you need to evaluate if that is enough power for the target speed you want to obtain.

[Stason4ik]

Hi daiquiri,

Thanx for reply.
The boat's dimensions:
length 7 meters
width 2.4 meters
displacement is about 2.8782 tonn(as i know it underwater boat's volume * 1.025)

The engine's power is about 100 hp. Gear 2.91:1
Engine max output (before gear) 2700 RPM, after gear on shaft 2700/2.91=928 rpm

At 2000 rpm - boat's speed about 4.5 km/h
at 2700 rpm - about 7 knots

[CDK]

As you figured out yourself already, a substantial part of the power is wasted on lifting the stern, but there is not much you can do about that except reconstructing the drive line.

Because the available power is insufficient to reach planing speed, you'd better install a prop with more pitch, so you'll be able to cruise at 7 kts with approx. 2000 rpm, saving a lot of fuel.

[Gilbert]

As mentioned you should just increase the pitch of the prop. There is no need to worry about the shaft angle, it should not be a problem on this boat.
If the bow plows you might try to shift some weight aft if you can.

[daiquiri]

All has been said above. Your boat is underpowered for getting over the hump, up to the planing speeds. So either change the engine (watch out for weights!) or stick to the economy displacement speeds, though the hull of that boat was not intended for displacement cruising. If you can live with that, you can save a lots of fuel.

[Stason4ik]

Thanks for your posts.

I have question about pitch.
I don't know the pitch for my propeller.
I there any method to calculate the pitch of my propeller?
Let's say that i know the current pitch of the propeller. How can i know that it's not a maximum pitch for current power of the engine (i mean that the power of the engine is too low for higher pitch)?