Propeller dilemma: pitch v diameter

[Krusty]

I've been back over 2 years posts and found no answer to this one; but hope springs eternal! Fifteen years ago I installed a 14hp, 3000rpm engine and Gori 2-blade folding 14x10 and it met my needs at that time. Later, sailing short-handed, the boat really needed more 'Shove' in really hard weather, so 2 years ago I installed a 20hp, 3600rpm engine (same reduction) and kept the old propeller : much better in hard weather; more shove, sooner; and great astern! Old propeller now worn out: Gori agent says right prop is 15x10.
Since the only standard 14 Gori offer is 9.5 pitch, a retrograde step, I have three options:
A) take the 15x10 and sacrifice clearance; 2.0'' down to 1.5'', (10%), the bare minimum according to some experts, others say 15%.
or B) take a 14x12, (cut down from a 15x12) and keep the present clearance
or C) take a 14x10 (cut down from a 15x10) and continue as at present.
Any views on the pro's & con's out there? I would be grateful for any expert evaluations and/or experience.

 

[Talbot]

IMHO I would take the 15x10 and try it. If it didnt work due to clearance, I would consider making it 14.5x10 or even merely having the edges bent over.

 

[alahol2]

If the new engine would rev the old 14x10 to just short of 3600 then it would be the right prop. If you could get to 3600 before full throttle then you could increase pitch and/or diameter. I'd be inclined to try 15x10 then get it machined down if it causes problems. All IMHO

 

[bedouin]

When I replaced my prop a couple of years ago I went for one with only 10% clearance - and it hasn't caused any problems. If 15x10 is right, then 14x10 is definitely wrong - 14x12 would be better

 

[kengill]

Be very wary of increasing the pitch. We re-engined with a 2030(Yes I did wash my mouth out with soap!) from a 2003 . Original prop was 15 x 11 . The first prop they fitted was 17 x 13!!, after trimming to 16 and then to 15 shaving the area (which had gone up 30% as well) they wrung it back to 12 and finally to 11(They then had to throw it away) I now have a new folding prop 15 x 11 which works perfectly and get 7.2 knots at 3600 (and no black exhaust).

MHO would be to keep the pitch the same and have the slightly larger diameter. You can always trim it a bit if you don't make the revs but its more efficient keeping the same pitch as you started with and keep with the same pitch along the Blade - which you don't do if you then have to wring the prop.

Fair winds

Ken

 

[peteandthira]

Vitalman

I have a Legend 36 with an MD2030 which we want to re-prop with a 3-bladed, possibly folding one. The existing one is 2 bladed fixed which is worryingly unimpressive against a short sharp sea and strong headwind. What is your boat, and do you think a similar prop to yours will suit me?

With thanks

Pops

 

[peteandthira]

PS

Don't knock the old 2030. Allegedly they are Perkins engines re-named by Volvo and therefore VG.

 

[boatmike]

The clearance you have for a 15" is tight but sufficient. HOWEVER before spending all that money on a Gori I would seriously consider an Autoprop from Bruntons. The Autoprop will most certainly give you more thrust per HP than the Gori and you may find a 14" Autoprop quite big enough. At least speak to Bruntons and ask them before deciding. They are very helpful.

 

[charles_reed]

I've got a 14" Autoprop with a 2gm20 Yanmar on a very light and slippery hull.

Definitely overpropped but then the motor is down to about 80% compression, so I'll be changing to the 3YM20 which produces about 30% more power. I don't expect to have change the prop as it effectively covers the range of 9" to 14" in pitch.

Certainly fitting the Autoprop will be a certain way of avoiding the challenge, but at a cost - it's about x3 the price of a folding Gori (or was when I made the change in 1992)

 

[PCUK]

My 14 ton motor cruiser with a 30hp wing engine did 4 knots with a two blade folding prop of the correct size. I later changed it for a 3 blade Volvo folding propeller. Revs stayed the same but speed increased to 5.8 knots with real bite in astern. With more blades you can reduce the diameter to give more blade area.
If you would like to send me the followng details I will do a computer check on the correct size of prop' (diameter and pitch) required.
Boat W/L length.
Boat displacement in cruising trim
Engine HP
Engine max revs
Gearbox reduction ratio

 

[kengill]

Hi

Vitality is a Gib'sea Master 96 - About three foot shorter than you.

What you need to understand first is what RPM you acheive with the current prop - which you say is unimpressive- Is it Less than the 3600 Ideal or does it tend to over rev? If you under rev? do you have a black exhaust when running under full power, at what rpm does the exhaust turn black(If it does), if you over rev then by how much and what boatspeed do you achieve flat out both ways in calm flat water.

Cheers
Ken

 

[TheBoatman]

[ QUOTE ]
Vitality is a Gib'sea Master 96

[/ QUOTE ]

Great boat - I sail one that belongs to my uncle - safe - great acommodation and all round good vessel sails like a dream on all points BUT definately a product of the era of boat builders cutting costs by under-engining, ours has a 2GM20 which is about 10hp short of a good boat (should have had a 3GM30). On a flat sea she'll do 7 knts but in a short head sea she gets knocked back to 3-4 knts very easily.

You can play with prop sizes/pitches all you like but at the end of the day it's all to do with available HP and boat shape. You have to chose whether you want good pushing power ie a prop that will give you 5 knts what ever you do or do you want a top end speed of 7+ knts but die in choppy sea conditions. Put it another way if you only have 5hp available then prop size doesn't come into it, but if you have 50 hp prop size is important because you have "available" power to use!!!!

Liken it to a car, you have a set HP and one forward gear if that gear is 1st the engine revs easily and you go up and down hill with no probs, all be it slowly, however if you have 4th gear your slow to get up to speed, the engine struggles to rev and on the flat you eventually get some speed up - but as soon as you reach a hill the revs die the engine struggles and your speed drops off. Now if those hills are likened to the sea in a short choppy sea its like going up and down hills very quickly i.e you never get up any speed. The only way to improve the situation is to increase the amount of power you have not the gears you have(prop)?????
/forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[peteandthira]

Ken

Top rpm is 2300, which gives us about 7 kts in a smooth sea. That gets reduced to 3 or 4 in a short sharp Clyde chop against 25-30kts of headwind.

The other issue is noise - the prop spins under sail at anything over 3 knots and I like silence. Vovlo manual says leave in neutral and allow to spin. Is it damaging to put it in gear (saildrive)?

I'm torn with the decision to put a folding 3-blader on it (about £1000) or just a fixed 3 blader for about £250. Help!

We may need to tow a generator and also don't really want TWO propellers holding us back.

Any views on that Ken, or anybody? Tame engineer is currently pricing up both jobs, but I'm of the opinion that ANY 3 blader has to be better than 2.

Thanks

Pops

 

[Micky]

http://www.castlemarine.co.uk/aboutus.htm

This program, easy to use, will give you the exact answers you are looking for /forums/images/graemlins/smile.gif

 

[Krusty]

Many thanks for useful contributions: my decision is to take 15x10 ,the pitch that has been OK so far, and shave the diameter if lower clearance becomes a problem: seems obvious after consultation!
Great to have the Forum!

 

[LeonF]

Interesting to read your post. I have a 2010, and replaced the two bladed 12:10 with a three bladed 13:11.. Definitely overpitched I think. I was in a hurry and did not do the sums. Chatted to a fellow class member who has also changed but went for a 12:9 three blader as he believed he was overpitched at 12:10. He did all the sums and it was suggested he could have 11 degrees of pitch, but reckoned that would be ok at the start of the season with a clean hull and a lightly loaded boat. He is very happy with the results. I spoke to T.W. Norris, and they said that as a rough guide every degree of pitch equates 250 revs.