Prop Help getting desperate!!!!

Hi

I am new to this but am in dire need of some help.
I have a Birchwood 31, Charter boat,

I have just had to replace the gearbox, this then meant the RH prop I had was no good as the new box is LH, I also dropped from 2:1 to 1.5:1 gear ratio.
After speaking to various people I called Freedom Marine, they told me that because I am restricted to an 18" dia I would need to have a three bladed 18 x 18 made to suit. They did this at a cost of £350 to me, and took four and a half weeks and guess what it is not right, where as before i could get 14kts now I can only get 8kts tops and she has the biggest rooster tail of a wake going.

So I got back in touch and they then sent me a four bladed 18 x 21 with a taper insert to drop it down to fit the shaft,

Now it is vibrating like a ********* and will not go over 8kts even when fully throttled, by that I mean changing the revs from 2000 to 2600 makes no difference at all. She still has the rooster tail but now instead of being out the back of the boat it is about four foot from the back of the boat before it starts.

PLEASE CAN ANYBODY HELP

parameters

waterline length 31.5 ft
bhp 150
max rpm 2600
max dia prop 18"
gearbox 1.5:1
semi planing hull (design speed of hull around 25tks)

 

In order to recommend a Prop, one needs an idea as to total displacement [weight]

In order to obtain 14 kts with 150 HP and a 1.5 gear, A 3 bladed 18 x 13, 65% D.A.R.min. or a 4 bladed prop is needed. This assumes a total displacement of about 5500 kgs which is probably low. [perhaps you have more than 150 shaft HP ?]

If weight is not available, prop size can be reasonably determined if the prop size and pitch with the 2.0 gear is known.

ALSO

( make sure your transmission is NOT running in REVERSE gear to obtain Forward motion which would be necessary if the wrong hand prop is being used )

 

Is the engine properly aligned? Did the overall length of the shaft overhanging change?

 

I would say the Vibration could also be caused by the "taper insert" which is supposed to adjust for the larger shaft size in the latest prop. Probably not the best idea?

 

Thanks Guys

Hi Guys

Thanks for the above,


I forgot to say that the boat weighs 4 ton.
The engine HP is definitely 150 HP and she is not running in reverse.

The original gear box was a 2:1 Borg Warner with an 18 x 23 RH four bladed, but this was not exactly correct as it was not giving us the speeds we
should have had.

I agree with the insert causing the vibration but I am just a little stumped with all this.

 

Sorry forgot to say the overhang has not changed at all, and we have a aqua drive coupling running inbetween the box and the prop, so alignment is not an issue.

Ta

 

Is the 4 ton [4073 kg] maximum w. passengers, fuel etc?

 

Hi

Sorry for the delay, the boat is being put in the slings tomorrow morning to get the exact weight so I will let you know then.
Passengers (in the coding) are based on 75kg per person and we carry 10 plus two crew.
The fuel tank holds about 200 litres and is generally kept full.

 

Regarding the Propulsion Unit

1. Could you please measure the Revolution per minute of the Propeller Shaft at 2200 rpm of the Engine's speed.
2. Please verify at forward position of the Gearbox if the Engine Flywheel and the propeller turns to the same or to the opposite sense of rotation.