Not the right prop

[fstbttms]

Originally Posted by 01kiwijohn

Doubt it's a half inch shaft

OK- again, there is no such thing as a 1/2" prop shaft. Go into any West Marine or other chandlery and tell me how many 1/2" shaft anodes you can find.

[Guus]

Originally Posted by HappyMdRSailor

Welcome to CF Guus !

Propping advice is not my my strong suit... But certainly buying a prop with a lower pitch will be the cheapest and fastest solution... Hopefully selling the non appropriate props will make this a low cost labor investment solution...

p { margin-bottom: 0.1in; line-height: 120%; } Thanks for your advise!
Yes we have considered to buy a new prop. But a prop with a smaller pitch will change the clearance between the tip and the bottom. The diameter of the prop cannot be changed.
A longer shaft improves the clearance. We hope this will result in a better performance. To avoid swaying of the enlarged shaft the size of the shaft need to be bigger (19mm to 22mm).
Adjusting of the pitch of the existing prop is possible.

[Guus]

Originally Posted by fstbttms

OK- again, there is no such thing as a 1/2" prop shaft. Go into any West Marine or other chandlery and tell me how many 1/2" shaft anodes you can find.

you are right!
the size of the (existing) shaft is 3/4"/19 mm.
to avoid swaying of a longer shaft the size need to be at least 22 mm, which is maximum possible.

[fstbttms]

Originally Posted by Guus

you are right!
the size of the (existing) shaft is 3/4"/19 mm.

Don't go looking for a prop for a 19mm shaft, because that doesn't exist either. 3/4" or 20mm- yes. 19mm- no.

[a64pilot]

You know, just maybe this is where you fit either an Autoprop, or one of the featherers that you can adjust the pitch in the water to dial it in?
I know the water is cold up there, but people do ice dive so there are dry suits.

[Cheechako]

Originally Posted by Guus

p { margin-bottom: 0.1in; line-height: 120%; } Thanks for your advise!
Yes we have considered to buy a new prop. But a prop with a smaller pitch will change the clearance between the tip and the bottom. The diameter of the prop cannot be changed.
A longer shaft improves the clearance. We hope this will result in a better performance. To avoid swaying of the enlarged shaft the size of the shaft need to be bigger (19mm to 22mm).
Adjusting of the pitch of the existing prop is possible.

Why cant the diameter be changed? Go smaller.

[a64pilot]

I think he is saying diameter can't be bigger, he's suffering from a lack of power into high winds / waves, normal answer of course is go bigger diameter / flatter pitch, but he can't

[Guus]

Originally Posted by Guus

A few weeks ago I have sent the following question to Tartan Yachts. Until yet I got no answer..

" Our Tartan 28 Piper, hull no. 137, is still a reliable and fast sailer. The ship is sailing in the Netherlands. But motoring is sometimes a problem. Against strong wind, over 18 knots, and waves the ship doesn't perform well. The speed is far too low. Above 2500 rpm the engine starts smoking. A higher rpm will not result in a higher speed. The engine will and cannot deliver the necessary power. The Yanmar2GM20F is in a excellent condition but the gearbox ratio is 2,67: 1 as built. According the specification this should be 2:1. This results in a too low prop velocity. The installed prop is instead of a two blade, a three blade 12x14x1/2 which is not ok for a ratio 2:1. A second problem is the too small clearance between the tip of the prop and bottom of the ship. Instead of at least 10% of the diameter of the prop we measure less than 1 inch. The pitch of the prop seems not to be to appropriate for the gear ratio. Are you familiar with the problems as described? We are searching for a solution. Do you have suggestions how to solve this?"

Searching elsewhere I got the following advise:
1. change the pitch of the prop from 14 to around 12,5 which is maximum for the existing prop.
2. replace the 1/2'-shaft by a 4 inch longer shaft; diameter 1 inch.
3. bearings and seal must also be replaced.

What is your advise?

Guus

the existing shaft is 3/4" (19mm) . not 1/2". The maximum diameter of the proposed shaft is a bit less than 1": 22mm; due to existing diameter of the bearingholder.

[Guus]

Originally Posted by Derek Lumb

Google "propking.xls" which is a free, very comprehensive spreadsheet which will enable you to experiment with the effect of changing pitch, diameter,gearbox ratio etc and will tell you what size propshaft you need and the effect of using more exotic ss alloys if you are pushed for space.

1/2" shaft sounds too small to me for a 20HP engine.

We did all the possible calculations . The max hullspeed is approx 6.5 mph / 8,5 hp. Unfortunally the diameter of the prop is limited to 14" due to dimensions of the ship. The pitch of the prop can be adjusted from 14 to 12,5". This will result in less water displacement at a certain rpm. Reducing the pitch makes it possible to run the engine on a higher speed and higher powerdelivery.

[fstbttms]

Originally Posted by Guus

the existing shaft is 3/4" (19mm) . not 1/2". The maximum diameter of the proposed shaft is a bit less than 1": 22mm; due to existing diameter of the bearingholder.

Here are the standard shaft diameters in both in both SAE and metric. You will not find shafts in other diameters:

3/4", 7/8", 1", 1 1/8", 1 1/4", 1 3/8", 1 1/2", 1 3/4", 2" and up

20mm, 25mm, 30mm, 35mm, 40mm, 50mm

[Guus]

Originally Posted by phstol

Hi and welcome Guus.
I had a similar challenge with my 1966 Alberg 35. It had been repowered with a 30 hp diesel, replacing the original Atomic 4 petrol engine.... so, the prop shaft now is smaller dia than optimum for the 30 hp, the space for the prop is too small for the optimum prop dia.
I wanted to end up with a 3 blade to push the hull at hull speed against the strong currents around here, and to push it off a lee shore in weather. The 30 hp at specified 3,500 rpm is way overpowered for this hull and the smaller prop shaft. So I asked the shop to provide me a prop that would push me at hull speed, at 2,500 rpm (the lower end of cruising rpm for the 30 hp).

Fortunately I had a skilled prop shop near the marina. I was nervous given the cost but he said not only could he do it but he would guarantee it for hull speed at 2,500. He bought in a prop, and in his forge, he "tuned" it for his calculated dimensions for my set-up. Sure enough he did it, said there is nothing magic about it, it is "just" math and skill in the forge. So now I have very good fuel consumption, and plenty of reserve power for whatever , before I reach 3,500 rpm. Very pleased that I took this route rather than all the advice I got around the marina.
BTW, ref disposing of your old props .... take a close look at them for any sign of pink in the metal ... this indicates old brittle metal and you won't be doing anybody a favor by selling them.

Fair winds, Peter

I also have found a skilled engineer. He is convinced that reducing the pitch will help. The longer shaft will result in a better clearance.
Thanks for your help:

[Guus]

Originally Posted by captlloyd

I really do not believe the OP's shaft size is 1/2''. Maybe he is measuring the small end of the taper?

The shaft is 3/4" . No 1/2"!
I made a mistake.

[blakeney]

I have a 3gm yanmar in my 1964 Alberg 35 with a 3 blade prop on a 7/8" shaft.Didn't have enough push into the chop so I had the pitch increased by 2".Now I can steam 6kns at 2800 rpm.Much improvement. A 2" increase in pitch will decrease the rpm by 300.

Don

[Cheechako]

Originally Posted by blakeney

I have a 3gm yanmar in my 1964 Alberg 35 with a 3 blade prop on a 7/8" shaft.Didn't have enough push into the chop so I had the pitch increased by 2".Now I can steam 6kns at 2800 rpm.Much improvement. A 2" increase in pitch will decrease the rpm by 300.

Don

an INCREASE in pitch made it punch thru chop better? as well as lower HP at lower RPM?

[roverhi]

That's the opposite of what you'd expect. Increasing pitch or diameter gives you more speed at lower RPM but will limit the RPM at the upper end reducing HP/power. That is if the propellor wasn't undersized to begin with.

Just to be sure you know that you probably don't need a new prop just get your current prop tweaked by a prop shop. Can usually change the pitch by two inches and reduce the diameter without difficulty.

Originally Posted by blakeney

I have a 3gm yanmar in my 1964 Alberg 35 with a 3 blade prop on a 7/8" shaft.Didn't have enough push into the chop so I had the pitch increased by 2".Now I can steam 6kns at 2800 rpm.Much improvement. A 2" increase in pitch will decrease the rpm by 300.

Don