What size propellers do i need to lift the rpms on my boat? its a 19ft sealine fitted with volvopenta 250,4cyl

Question

The last owner fitted it with a duo prop outdrive wich was fine on the river thames at 6mph,i use it at sea and it seems a bit lazy taking off,it will only rev to 4800 (it should do 5500) on checking the prop sizes i have found that they are actualy designed for diesel engines being A0 series and 7 blade,will changing to B series 6 blade (petrol engine props) improve the performance or will i be wasting my money?

Answer 1

if you are going to the trouble of changing the prop then find what the original spec was and put that back on.

you will find that the manufacturers put on the biggest that that engine was capable of without overloading it (they can only push so much water)

if you really want to go faster you have to slip a bigger engine in or just get a quicker boat.

I know a guy that put extensions (welded extra bits on) to his prop, he wrecked it <he he> 1) it was out of balance 2) would not rev bcoz too much to push

Answer 2

If you are in England, you could contact their factory in Kidderminster, Worcs.

It's on the Worcester Road.  I'm sure they have a Customer relations person who could find the info straight from their technical department.

Answer 3

I dont think you will be wasting your money, I had a large boat Semi displacement, and it was designed to take two 63544 Perkins deisel engines, it would never come up to correct RPM and had to have the props reduced in diameter by 1/2 inch

A displacement hull, has a top speed of 1.4 times the square route of the length of the hull (in feet)  it will make no difference to the speed by increasing engine capacity, ie if it was 25 feet long it would have a speed = 7 knots. this does not apply to planning hulls

Answer 4

I would have thought reaching 4800 out of a possible 5500 whilst under power was pretty fair.