Birchwood 25 - Volvo Penta 145 question

[ms1]

I am a Fairline Mirage 29 owner
I am still think 15 x 17 for the AQ145 set up ...
but happy to be proved wrong
LH part 854982
or
RH part 854997

when you borrow your friends prop these are the numbers to look for

-------------LH ------ RH
14 x 17 854977 854992
14 x 19 854978 854993
14 x 21 854979 854994
14 x 23 854980 854995
15 x 15 854981 854996
15 x 17 854982 854997
15 x 19 854983 854998
15 x 21 854984 854999
16 x 9 854985 —
16 x 13 854986 872000
16 x 15 854987 872001
16 x 17 854988 872002
16 x 19 854989 872003
16 x 21 854990 872004
16 x 23 854991 872005
These are the 'long hub' part numbers
also
15x17 is the largest 'short hub' Volvo propeller

if you take your waterline length as 25 feet then you 'hull' speed (or max displacement speed) is 6.7 knots
this means after that sort of speed you have to get over the 'hump' caused by the bow, for which you need a whole bunch of power. Trim tabs, if you have them, help by getting the stern up a bit to help you beat the hump.

If you want to talk speed and prop stuff then Mike Bellamy at Lancing Marine is likely to be able to give you some guidance.
Sadly propellers are not cheap which means experimenting is likely to be costly. Ebay often has propellers on offer which might be helpful.
I previously owned a B 25, a I remember the tunnel prop version had a traditional engine, gearbox, prop shaft and, whilst the hull above the waterline looked identical, the planing hull shape was fitted with outdrive transmission.
As previously mentioned by other subscribers the manufacturers could be quite optimistic about their speed claims, but I still like to think that you will succeed eventually.
Also as mentioned I suspect you will be either up or down .. .as soon as you throttle back a bit it may well fall off the plane. Throttle wide open trying to beat the hump , if unsuccessful, will give you bigtime fuel consumption of course.
If you decide what will be successful and then get a cheap mashed prop off eBay, then Steel Developments in London will fix/ rebuild it for you.

 

[ms1]

Just a thought
You mentioned a cruddy prop
In the same way that a clean hull impacts speed ..... a clean prop is important as well.
If its caked in limescale you might soak it in some distilled vinegar (the clear stuff) which will clean it up well ... if its salt water crud then I guess only friction will get it off (wooden scraper maybe)

 

[apollo]

It does sound like I need to speak to someone to work out the correct prop sizing. Does anyone know of a website that can work this out by any chance?

Try Here
https://www.vicprop.com/planing_size.php

REMEMBER your waterline length is not actually 25feet but less i would put in 22-23

 

[plumpton]

Thanks for the link.

The prop im collecting on Saturday is a 15x15 I believe. Its from a twin engined Fairline 29 so not a perfect match...

 

[plumpton]

Popped my prop off and went at the hub with some sand paper. My current prop is a 14x15.

Popped the loan prop which is a 15x15 on and had a quick burn up. Only had a few seconds to try so not a real run but bow was a lot higher to the point where i could not see over it so there is a definate change. Noted engine revs were at 2700 instead of 2800rpm but as I said, I only had a few seconds to try. I'll pop downstream in the coming weeks to see what happens if the boat is left at full speed for longer.

I had a look on the Keyparts website and they dont seem to sell a prop of my size so its still possible that I do have the wrong sized prop fitted.

Got more important problems to deal with now. Someone on my boat managed to break my sea toilet so thats my next job... All part of the fun of ownership

Here is a photo of my prop