Lend me a Prop?

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Slowboat35

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For troubleshooting purposes I would very much like to try a 3 blade prop of 17 or 18" dia - 1 ¼ in bore, 1:12 taper pitched for a 35hp engine or thereabouts. Dia is important, pitch much less so. Anything from a Yanmar 3GM/YM up tp 35BHP or similar would suit - I need to discover whether the vibrations I'm getting are prop tip clearance related (I am running a feathering prop at present) or if it is something else.

I only need it as a substitute to run very briefly -only for a few minutes over a tide or two, that's all...

Levington based. Of course I'll indemnify againt damage/put down deposit if required.

Any help much appreciated.
 
Think you will find 18" is too big. Would normally be a 16" or at most 17". Probably a 17*10 3 blade based on having a 2.4:1 reduction..

Have you actually measured the tip clearance with your current prop? and what diameter is it?

Would be nice to do this comparison, but would only make sense if you used the optimum fixed prop for your boat - and that may actually be constrained by the clearance in the aperture. In other words a 17" may be best but tip clearance limits to 16" so you have to go with the smaller diameter and steeper pitch.

Have you actually talked to Darglow? They know more about propellers than just about anybody in the country.
 
I replaced a 17" fixed with a 18" feathering from Darglow. I now have vibrations from 1800 rpm upwards. I thought it might have been tip clearance, but I am becoming more convinced that it is shaft whip. I am now going to modify the P bracket to get support closer to the prop.
 
I’ve just got back from the boat with my fixed prop. It’s 15” diameter, one and a quarter inch shaft. Don’t know the pitch or taper I’m afraid, but boss is 80mm in length and the narrow end of shaft hole measured 15nm. It’s off a 3GM30 engine. Any use to you?
 
fredrussell said:
I’ve just got back from the boat with my fixed prop. It’s 15” diameter, one and a quarter inch shaft. Don’t know the pitch or taper I’m afraid, but boss is 80mm in length and the narrow end of shaft hole measured 15nm. It’s off a 3GM30 engine. Any use to you?
Click to expand...
It's a pretty big prop if the hole is 15 nautical miles across ???
 
pyrojames said:
I replaced a 17" fixed with a 18" feathering from Darglow. I now have vibrations from 1800 rpm upwards. I thought it might have been tip clearance, but I am becoming more convinced that it is shaft whip. I am now going to modify the P bracket to get support closer to the prop.
Click to expand...
The overhang aft of the P bracket should not exceed 60mm on a 1" or 1 1/4". proportionately more for larger diameters. If it is greater than this, simplest solution is to shorten the shaft assuming that tip clearance is still OK with the prop moved forward.
 
Hi Pyrojames,

+1 for Tranona's advice on overhang aft of P bracket. If that's not the problem, the other feature that can make an insyallation prone to shaft whip is excessive lenth from the aft gearbox bearing to the P bracket bearing (sorry, I forget how long is excessive). A simpler cure for this than modifying the P bracket is to install an additional cutless bearing in the forward end of the stern tube. If hat's not practical, yu can install a ball race just aft of the shaft couping. In both cases, it's obviously important to get the alignment right.

Peter.
 
Shaft is same length as it was with previous engine
Too little overhang for shaft whip I'd have thought.
My concern is its most likely to be tip clearance at the top.
Opinions?
IMG_3236.JPGIMG_3238.jpgIMG_3242.jpg
 
Tip clearance may well be an issue . Ideally 15% diameter or approx 2.5" for a 16/17" prop. Your aperture (and the blunt forward face of the rudder) is very similar to my old boat. As I think I said on your other thread when I replaced the 14" fixed with a 15" JF which is physically identical to your later Featherstream it was definitely noisier. Later when I sold the boat, for various reasons I removed the JF and replaced the old fixed prop which was indeed quieter.

No problem with the gap between the cutless housing and the prop - quiet normal. Only observation is that the cutter is far too close to the cutless and could restrict waterflow into what is a water lubricated bearing. Gap should be minimum 15mm.

Hope this helps.
 
fredrussell said:
I’ve just got back from the boat with my fixed prop. It’s 15” diameter, one and a quarter inch shaft. Don’t know the pitch or taper I’m afraid, but boss is 80mm in length and the narrow end of shaft hole measured 15nm. It’s off a 3GM30 engine. Any use to you?
Click to expand...

I reckon that’s not quite what I’m after Fred but cheers for going down to the boat and getting it anyway.

Yours,
Slowboat35
 
Apologies Fred, I didn't intend to be rude.
It wasn't at all clear that you'd made a special trip to retrieve it.
Sorry.
 
Slowboat35 said:
Apologies Fred, I didn't intend to be rude.
It wasn't at all clear that you'd made a special trip to retrieve it.
Sorry.
Click to expand...

No worries, my post was a bit ‘precious’ to be honest. Post in haste, repent at leisure eh! No hard feelings and good luck sorting your prop issue out.
 
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