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Old 21-02-2016, 05:47   #1
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Not the right prop

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
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Old 21-02-2016, 05:58   #2
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Re: Not the right prop

You may send an email message to www.west by north enterprises
they have been very good at custom building their own design of 3 bladed props to solve performance issues. I can say that my previous 8.5meter sailboat had headway issues in bigger sea states that were solved with their design and not outrageously priced.
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Old 21-02-2016, 06:01   #3
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Re: Not the right prop

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...
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Old 21-02-2016, 06:57   #4
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Re: Not the right prop

Greetings and welcome aboard the CF, Guus.
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Old 21-02-2016, 08:36   #5
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Re: Not the right prop

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.
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Old 21-02-2016, 09:01   #6
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Re: Not the right prop

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Originally Posted by Derek Lumb View Post
1/2" shaft sounds too small to me for a 20HP engine.

I had to laugh at that comment as it brought back memories from a previous life when I hopped up a early 60's Corvair. GM in their wisdom used a 1/2" x 26" long shaft to couple power from the engine to the tranny through the differential. That was when I learned the term "Torsional flexibility" and how it destroyed 3 differential/transmission units until I learned how to control it. No drag racing a Corvair! Rev the engine, drop the clutch and parts broke. Since then I tend to over-engineer things and am very leery of mechanical gizmos under load.
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Old 21-02-2016, 09:16   #7
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Re: Not the right prop

This may help in your prop calculations:

http://www.akcaymarin.com/Products/%...%20klavuzu.pdf

Standard prop shaft dimensions:

http://www.propellerpages.com/?c=tech_info&f=tapers
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Old 21-02-2016, 09:20   #8
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Re: Not the right prop

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
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Old 21-02-2016, 10:02   #9
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Re: Not the right prop

I had similiar problems with my Newport 28 and Yanmar YSM12. I googled prop analysis and several companies were listed. Wilmington prop in Long Beach CA responded right away and sent me a form to enter the boat and engine specs. The next day they advised a 12-7 prop and I found out I had a 12-12. Way wrong Now my engine gets the full 3000 rpm if I want and it runs perfect. 5.5 mph at 80% throttle
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Old 21-02-2016, 10:17   #10
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Re: Not the right prop

I really do not believe the OP's shaft size is 1/2''. Maybe he is measuring the small end of the taper?
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Old 21-02-2016, 10:34   #11
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Re: Not the right prop

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Originally Posted by captlloyd View Post
I really do not believe the OP's shaft size is 1/2''. Maybe he is measuring the small end of the taper?
Yeah... I cant believe it's 1/2" either.
The answer is get a 3 blade prop of correct diameter and pitch. Another comment though is the 2 GM engine is not much HP in the best of circumstances, Prop it for near max RPM.
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Old 21-02-2016, 12:49   #12
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Re: Not the right prop

I would think a 1/2" shaft would have to be a very special order..any factory install would most likely have been .750" to go down to half would require some special tooling and would be a foolish thing to do. On older 29 columbia with 25 hp diesel our reduction was where yours is on smaller diesels that would be a common ratio not 2:1. on smaller engines the 2:1 would work fine on light boat but not ob a heavy duty type hull like yours..the 29 col had 10"X8" and was not under propped. there are many good sailboat props out there I would rec. the Sailer prop in Canada..
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Old 21-02-2016, 13:33   #13
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Re: Not the right prop

If the engine is smoking and not reaching maximum RPM, you are probably swinging too large a diameter and/or too steep a pitch propellor. A 2.1 gear ratio transmission would be even worse for delivering maximum RPMs with that prop. Go to a local prop expert and they should be able to decrease diameter and or pitch to correct your current prop for maximum RPM.

As others have said, doubt that the shaft diameter is 1/2"/12mm. Have a 7/8" shaft on my 3 GM and the prop/engine guys I've had work on the boat think it's weird because of the small diameter.
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Old 21-02-2016, 15:22   #14
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Re: Not the right prop

Quote:
Originally Posted by captlloyd View Post
I really do not believe the OP's shaft size is 1/2''. Maybe he is measuring the small end of the taper?
It's not. There is no such thing as a 1/2" prop shaft.
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Old 21-02-2016, 18:36   #15
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Re: Not the right prop

Doubt it's a half inch shaft
Prop looks to big and too much pitch.
Properly sized, adn pitched, the boat should run , with a clean bottom, in flat water, at about 95% of it's high idle RPM
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