Prop choice

[shibbs]

Evening all, dont you just hate it when you've nearly finished your post and you press the wrong button, lose it all and have to start again!!
Anyway,
I'm after some info if possible from you guys that know.
At present, i have a High 5 with 21P 48 16318 fitted. This shoots the boat up on the plane super quick but tops out at about 30 kts @ around 4000RPM.
My questions are:
a. What does 48 16318 mean?
b. Should the engine rev higher?
c. Would a 3 blade prop increase top end / engine revs?
and
d. How do you work out which props will fit? (Is it this simple? )
I have a Mercury Mercury Mirage Plus 19P 48 13700, would this be an option?
The boat is approx 2200kg, Mercruiser 5.7, Alpha 1 leg.
I've never played about with props or their different effects in the past so thanks for any help in advance...

[mlines]

I cannot answer your prop questions but I can give you some comparisons.

Our Bayliner 185 with a 3.0 litre engine, Alpha 1 and weighing around 1000kg would top out at around 4700 rpm and 38knots.

Our Regal 2250 with 5.7 350 MAG, Bravo 3 and weighing around 2200kg tops out at 5100 rpm and 46knots

[shibbs]

hmm, that's what i thought, RPM a little low and speed quite a bit slower. Obviously will be a slower hull to yours and an older engine and leg etc but thought i should be able to get a little more top end.
I had put it down to the High 5 and its great hole shot?
Thanks for the reply though.
TBH, not that it really bothers me anyway, i very rarely want to go more than about 25kts.. i was just curious
When i do, all i see is £££££ signs at the fuel burning

[Ianfs]

Stu

I think the numbers (48-123456) are part numbers.

Several other things here to think about.......

Your engine should have it's sweat spot, we like flat out, (also known annoyingly as WOT) at about 4,400 - 4,800. Martins 350Mag engine tops out at 5,000 rpms.

Consequently, if you're not getting there, your gearing may be wrong i.e. the Prop. But also consider that your engine may not be performing, or the hull is dirty!!

A High Five on a Cruiser? Well it's either a great combination or a disaster!! Simply put the High Five is a standard design for all types of hull. It's blade sizes are smaller than those of a 3 blade propeller!

In order to really check, put the 19p on and see if you can achieve your sweat spot, i.e. 4,400 - 4,800rpms.

If you don't achieve the correct results, this could possibly mean a carb rebuild, bad fuel, an engine tune or a hull clean, is reducing your speed. If it's the same, then propellers are the answer and we can take it from there!

[shibbs]

Thanks very much for the info Ian,
Certainly at this time of year I can rule out a dirty hull (for a while anyway ) and bad fuel, new fresh went in very recently. The hull is anti fouled though so this might knock a little off top end/revs..
I will certainly try the other prop when I next get out and report back.

I gave the engine a full service just before she went In so we'll see..

Thanks once again, I'll let you know.

[Ianfs]

I thought this might be the case.

Therefore I really do think that the 19p will give us some more data?

[shibbs]

Hi again, i finally got out today with the 19p Mirage Plus on in place of the High Five, the results were a little confusing..
with the High five 19p i could achieve around 30 kts at max, @ around 3750rpm
the Mirage Plus 19p gave an increase to about 35kts max but revs maxed out at 3000rpm.
It has made me wonder if the rev counter is reading correctly but then it sits at idle at approx 750rpm and sounds correct.
It's odd because it doesn't sound like it should run any quicker at full throttle either but surely it should reach more than 3000rpm??
It will cruise comfortably at 25kts at just over 2000rpm with the Mirage.
I'm not fussed at top speed at all but it seems odd, i am fully aware though that i do have a full tank of water and fuel too so top end will change with that but not revs??
Any thoughts would be appreciated..
(On a separate note, had a good run out this morning with a bit of fishing for my youngest - no fish to be seen anywhere though )

[shibbs]

I'm back down at the boat now and have been fiddling, I blipped the engine very briefly in neutral and it revs freely well past 4000rpm, went out for another spin as me and my youngest are staying onboard tonight and have noticed that I have a bit of a Mis fire, I can now only imagine this will have something to do with it struggling to rev under load??
Anyone had this before or any ideas on these symptoms?

[Ianfs]

1/ Carb rebuild
2/ Spark Plug
3/ Leads
4/ Distributor Cap
5/ Bad Earth
6/ Thunderbolt Ignition connectors
I didn't want to write this next one but a misfire on a raw water cooled engine can and I say with trepidation the word can and only possibly be water leaking into the cylinders from the manifolds.
It's more likely to be one of the other reasons!
Which Carburetor is fitted to your engine?

[shibbs]

1/ Carb rebuild
2/ Spark Plug
3/ Leads
4/ Distributor Cap
5/ Bad Earth
6/ Thunderbolt Ignition connectors
I didn't want to write this next one but a misfire on a raw water cooled engine can and I say with trepidation the word can and only possibly be water leaking into the cylinders from the manifolds.
It's more likely to be one of the other reasons!
Which Carburetor is fitted to your engine?

Please nooooo...
Ran back to its berth today with a steady noticeable mis fire again.
The plugs and leads are new so i'm fairly happy its not them.
Took distributor cap off yesterday and checked and all seems fairly new and in good condition in there.
Earths i'll have to look into, likewise the Thunderbolt ignition as yet i know nothing about them!
The carb that is fitted is pictured below, i do not know what type yet.
I have noticed though that the STBD riser is hotter than the Port, not hugely and still able to touch without discomfort with a bare hand, but still hotter, could this be another sign of something?
My next plan is to check the spark on each plug just to rule out, and then do a compression test additionally turning over with the fuel off to see if any water comes out of any of the cylinders.



One other question if anyone knows, should this elbow joint on the back of the distributor have anything connected to it?

Why would it have ever just of been the wrong pitch prop, that would have been far too easy!!

As always, thanks for any advice / suggestions anyone can offer..