Still Underpowered

[Stemar]

I’m still underpowered with my Snapdragon 24 and MD1.

I’ve adjusted the throttle linkage so I’m getting something like full revs in neutral, changed all the filters and checked the valve clearances. I might be getting a little more umph, but nothing significant. A clean bum would certainly help a bit, but I don’t think that’s the whole story.

Amongst the papers that came with the boat, I’ve found a receipt for a new prop, described as 13 X 9. Castle Marine’s prop software suggests 11.5 X 9.4 as a suitable prop, using a waterline length of 22 ft, 2200kg as the boat’s weight and 2850 max engine rpm, with a forward gear ratio of 1.87. The weight and the length don’t seem to be critical to the calculations, and I’m reasonably sure of the rpm and gear ratio.

If this is the case, I’m way over-propped, which would explain everything, but the receipt is 14 years old…

Is there a way to find the right prop without buying one, trying and buying another, ad nauseam – and ad bankruptcy? Is the software likely to be accurate enough for me just to go out and buy a new prop? Anyone got a prop I could try (N London or around Portsmouth), or recommendations for expert help around Portsmouth Harbour?

Or am I barking up the wrong tree?


<hr width=100% size=1>

 

[dickh]

Check your Exhaust Elbow! See my previous post under "Prop Service"

If this is not the cause you can get your prop "re-pitched" to a smaller pitch for about £30.(I reduced the pitch by 1" and it raised the under load RPM by 400RPM)


<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif<P ID="edit"><FONT SIZE=-1>Edited by dickh on 26/08/2003 14:04 (server time).</FONT></P>

 

[tgalea]

Hi,

I have a Snap 24 too and feel the same about my situation. Would appreciate if you keep me updated on any developments.


Regards

Tyrone Galea
Malta


<hr width=100% size=1>

 

[tgalea]

Hi,

In my case as a said to the original author of this thread I have his same boat model and my concern is that from 50% RPM up to MAX i only get an increase of let's say 0.5 knots. Max speed at max throttle is 4.9kts on the log.

I did check my exhaust elbow and noted a slight collapse in one of the bends on theh exhaust pipe so i will be looking further into it.

Any ideas ?

Regards
Tyrone Galea


<hr width=100% size=1>

 

[dickh]

Tyrone,
My experience is that when not under load the engine will rev to full revs easily - I assume your max revs are 3000 RPM as the first poster said max revs is 2850 under load which sounds right. You should certainly get a substantial increase in speed from say 1500 to 2850 rpm.
On my boat at 1500 I get about 3½ knots in calm water and almost 6½ knots at 3100 RPM, with a max no load speed of 3500 - which I'm reasonable happy with.
If you can get full revs under load then you are not overpropped.
Is your prop clean? - a few barnacles makes a lot of difference.
Are you getting a black smokey exhaust? with black smuts/unburnt fuel? If so, you should check your exhaust elbow - you might have to remove it completely to inspect it on the inside, which is where both mine and my friends was blocked(impossible to see when on the manifold)
You mention a slightly collapsed bend - I assume the flexible exhaust hose, this can collapse internally and reduce the flow considerably and the only option is renewal.
And make sure you don't have blocked filters,
Have your injectors serviced if they haven't been serviced for a long time.
Keep us informed of progress.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[Johnjo]

Might be a silly idea, but is the power reaching the prop.
Possibly slippage in the gearbox !

<hr width=100% size=1>

 

[philwebb]

Start by drying out somewhere and giving the bottom and prop a good scrub then
see what your max speed is. You may get a surprise-I did.Barnacles and weed on
the prop and saildrive leg dropped my speed by about 1.5kt. You could have picked
up something on the prop-but this usually causes overheating as well.

<hr width=100% size=1>