Keel modifications

Greetings All, I wish to swing a bigger propeller on my boat. To achieve this I need to move the stern tube down about 2", build up the skeg 2" and refit the keel band which runs from skeg to the rudder base mounting plate. Engine will be realigned once the above has been completed. Can anybody point out any pitfalls or recommend any books to help with this project?
Many thanks Rammy

 

To start with, calculate the angle of the shaft. It should not be more than 15 degrees from the waterline when running. You need to add the angle of attitude at speed to the angle of the shaft from the static waterline.

 

Thanks gonzo I"ll make that my first task then, a lot easier to do now with the boat still in the water I think!

 

Greetings, The angle of the shaft is 5 degrees. I have contacted a couple of prop people with mixed results. Anybody offer any advice as to optimum size please? Here are the details.
L.O.A. 27ft.
L.O.W. 24ft
BEAM. 9ft 2"
WEIGHT. 11,200LBS about 5 English tons.
TOTAL B.H.P. 120
MAX R.P.M. 2800
GEARBOX RATIO. 1.21

Current speed is 8knts WOT. I had a new prop made and was promised 12knts. This never happened. Current prop is 4 bladed 14x12.5. Any thoughts greatly appreciated as I seem to be wasting my life (and cash) on this problem at the moment.

 

Um, I could be wrong here but Hull speed on this boat is about 6-8 knts I dont think your going to ever get 12 knts.

source http://www.sailingusa.info/cal__hull_speed.htm

K9

 

Hi Kay, My boat is an Aquabell, which is semi-displacement. I know of 2 others that run at 20+knts. These are stern-driven, which I belive mine was at some stage. The boat is 1980, The engine is a Perkins 6354, somewhat later I believe.

 

Well, like I said "I could be wrong".

Ill get you some info on prop sizing.

K9

 

As promised:

http://www.propline.com/Propeller-General-Information/Propeller_Sizing.htm

http://www.acepropeller.com/prop-spec.asp

http://www.tacomapropeller.com/8467/Inboard-Propeller-Size-Help.html

I hope this helps

K9

 

Much appreciated K9, I will run the figures through them. Rammy.

 

It is a very heavy boat for a 14" prop so fitting a bigger prop will improve efficiency.

The performance figures you have provided do not quite add up. You should be able to get better than 8kts even with that dinky prop if the prop is actually spinning at 2800/1.2 rpm. Are you certain you are getting full engine rpm? I estimate it will be doing around 1900rpm rather than 2300rpm if it is the size you say.

The current shaft angle is not particularly severe. Even a larger prop will operate at high slip so the moderate shaft angle is not a great concern.

Rick W

 

Another thought.

Does the boat sit on the waterline shown is it is trimmed bow down with the Perkins rather than the stern drive. If it is trimmed badly it might not be getting any lift so it will just dig a big hole. This should be obvious.

Is it something you have observed?

If this is the case then improving the prop will produce a bigger hole without much increase in speed.

Rick W

 

Hi Rick, The shaft rpm are 2314 according to Castle marines calculation. There was no serial number on the gearbox (borg-warner velvet drive, on a vee-drive) I calculated the ratio with an optical tachometer on the shaft and my new vetus dashboard tachometer. The nearest ratio in the borg-warner book was 1.21 . I have tried different prop diameters on Castles site and they all say that the ratio is not ideal,although they do not tell me the ideal ratio. It looks as though I may have to alter or replace gearbox. What do you think? My main concern at the moment is deciding on the approx diameter of ideal prop 16" 17" 18" 20"? so I can work out new stern tube position. Thanks for your interest. Rammy
P.S. my taco shows 2800 rpm

 

Sorry, just noticed your other post, She sits nicely on the water, engine is mounted aft of midship. Engine (back to front on vee drive) to allow a bigger wheelhouse I imagine.

 

Having the boat in trim means she should be really starting to lift the bow at 8kts.

It should be possible to check the gearbox ratio by turning the motor and comparing rotation with the prop. I expect your motor tacho will be OK. Any increase in diameter will help. The bigger the diameter the better up to the point where the shaft angle is excessive. Aim for less than 15 degrees.

A large step up in diameter can be offset by selecting a smaller pitch or altering the gearbox ratio. If you can fit a 20" prop then the pitch will be about 10". A P/D ratio of 0.5 is less common than say 0.8 to 1.2. Hence, if the ratio is 1.2, it will be low for swinging a more suitably sized prop.

How do you measure speed? It would be good to be certain about the current operating point for your 14 x 12.5 prop as 2333rpm and 8kts. The trim of the hull and weight are other main variables. I would expect better than 8kts under the conditions you have given. Have you any idea of the trim when at 8kts.

Rick W

 

Looking at my charts for boats of this type, engine etc etc, if your RPM is 2300-ish and the power is 120-ish...then you're looking at around 16" diamater. But.....!!!