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Old 21-11-2015, 12:12   #1
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Prop upgrade.

I'm currently sailing a Gemini Legacy 35. Under sail on a broad or beam reach it is pretty easy to hit 8-12 knots but under power at 3000 rpm only about 5.5 knots if no wind. The engines don't seem to be in any strain even motoring into stiff winds. I was considering replacing the two blade props for the folding variety until looking at prices. I suppose the real question is can I go to three blade props on the Yanmar 15 hp engines. If so what diameter?
Thoughts appreciated
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Old 21-11-2015, 14:52   #2
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Re: Prop upgrade.

Hello rbbaker,
matching a prop to a motor is not a straight forward proposition. If i can suggest contacting a prop maker directly as they will have a series of questions prior to making a suggestion. We have done this previously when upgrading engines. For instance shaft drive or sail drive, distance from rudder, distance from top of blade to bottom of boat, details of current prop, engine specifications.
I would suggest that 15 HP motors are also part of the problem. While they are reliable engines they are trying to push a lot of boat. Have you spoken to other Gemini owners to ascertain their engine performance or the actual current builders to see the designed specs and recommendations.


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Old 21-11-2015, 15:10   #3
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Re: Prop upgrade.

I'm surprised they have 2 blade now. But I would also be surprised if going to the proper 3 blade will increase your top speed. More power or more speed in rough water maybe though. Maybe your 2 blade is not pitched properly for your boat? Well... I guess not if you can get 3000 rpm.
Going from a 2 blade to 3 blade in a mono didn't increase my flat water top speed... just gave more power in reverse or sloppy water.
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Old 21-11-2015, 16:32   #4
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Re: Prop upgrade.

You can but it will cost you plenty in sailing performance. Perhaps in excess of one knot.

Stay with folding props but get better ones.

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Old 22-11-2015, 00:56   #5
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Re: Prop upgrade.

Hi rbbaker,

folding props and especially the 2 blades have and high prop slip because the blades surface is not large enough to overcome hull resistance.
Installing appropriate fixed props would be good when motoring but bad when sailing.
Feathering props with high DAR (Disc Area Ratio) would give benefits when motoring, when sailing (feathering) and in maneuverability.
I took the liberty to make a calculation for you and with 2 EWOL Hi-Speed props 15" and your Gemini Legacy we would estimate a speed of over 7 knots.

We have considerable experience in increasing boat speed where the prop is at the origin of the problem, especially catamarans suffer this problem. Get in touch with us and we can discuss the matter in details.
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Old 22-11-2015, 04:04   #6
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Re: Prop upgrade.

The advice about consulting both with other Gemini owners, as well as prop specialists is wise. Especially as determining how your engines are performing under load isn't nearly as easy as it is in a car for example.

Also, given your concerns about $, a valid one. The fancier you go, prop wise, typically the more expensive they run. Though finding pre-loved ones, especially those which have been or can be (factory) reconditioned is a good option.
And those which can be re-pitched in the water, on the shaft, by you or a diver, is a Great feature to have. Which you can do with some, like Max Props, for example. So that you can tune them to your boat & engine combo.

As to 3-bladed fixed props, yeah, they'll cost you a Lot, in terms of drag under sail, as already stated.
It's worth your doing a little research into what's available prop wise; in terms of prop efficency (under sail & when motoring), what you can find new & used per/$, & their evaluations in places like Practical Sailor.
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Old 22-11-2015, 06:37   #7
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Re: Prop upgrade.

Quote:
Originally Posted by UNCIVILIZED View Post
The advice about consulting both with other Gemini owners, as well as prop specialists is wise. Especially as determining how your engines are performing under load isn't nearly as easy as it is in a car for example.
Excellent advice.

Quote:
Also, given your concerns about $, a valid one. The fancier you go, prop wise, typically the more expensive they run.
More good advice.

Quote:
And those which can be re-pitched in the water, on the shaft, by you or a diver, is a Great feature to have. Which you can do with some, like Max Props, for example. So that you can tune them to your boat & engine combo.
End of good advice, IMHO. And this contradicts the good points made above. If you've consulted with other owners and prop makers, you'll arrive at the correct diameter and pitch and won't have any need for variable pitch props, which are fancier and more expensive.

Dave
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Old 22-11-2015, 07:56   #8
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Re: Prop upgrade.

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Originally Posted by 2Hulls View Post
End of good advice, IMHO. And this contradicts the good points made above. If you've consulted with other owners and prop makers, you'll arrive at the correct diameter and pitch and won't have any need for variable pitch props, which are fancier and more expensive.

Dave
The choice of a prop isn't quite as simple as it seems at first glance:

Firstly, no two engines, even ones of the same; make, model, year, & production run (even with only 1 digit of difference, seriel # wise) are the same. They can, & do, at times have more than enough difference in terms of HP & Torque output at various RPM's so as to make getting them perfectly matched with a propeller, a thing requiring a bit of fine tuning.

For example, you can pull two Chevy's off of the lot or production line, which meet the criteria state above. And they can have several percent of HP difference (sometimes in the double digit range, percentage wise). And that's in engines which have built in computer control modules. As well as several other features to optimize their engine's output.

As opposed to, say, a Marine Diesel, having mechanical fuel injection. And also with no/to minimal built in systems for optimizing the various aspects of timing, for example. Or to "re-tune" the engine (many times per second) for things based on; the sensed octane of the fuel, the fuel's temp., ambient air temp., measured air density, engine temp., engine load, etc., etc.
Not all diesels are so low tech as to not have some of these features, but... there are plenty which have few to none of them. At least in (relatively) low priced yacht engines.

The reasons that one (Chevy) Mustang may be 3-5mph faster than another, or be 0.5 sec quicker in the quarter mile. Both of which constitute a Lot of HP & Torque, BTW. Are due to many of the above stated factors (& more).

For example, one could have more aerodynamic drag, or in a boat's case, hydrodynamic, than the other. A common thing. For which I could list 2 dozen+ factors.
And too, said vessel could have more significantly more or less aerodynamic drag than it's average sisterships; on which the prop selection model is based.

Bottom line. These types of things all add up to make 2 different vessels of the same type, differ when it comes to what kind of loads they put on their engines while powering through the water at different speeds. So then, being able to fine tune one's prop is a grand perk. Adding to efficiency, plus, in the long run, engine life as well.

Then, for instance, a common external influencing factor is this. In some locales the seas will vary greatly in; frequency, size, & shape, etc. as compared to other locations. Which again, would add up to wanting a differntly pitched prop than that provided by a generic recommendation. And such factors might be seasonal, or based on the environmentals of a specific voyage.
Yes, one can have a fixed bladed prop re-pitched, but, there's a reason that whole classes & designs of vessels have controllable, variable pitch propellers.

Meaning that, such vessels even able to alter their propeller's pitch while underway. In order to accomodate some of the above mentioned factors, & thus optimize their engine's transfer of power into the water. IE: Get the most possible speed, for the least amount of fuel burned.
Such as, for instance, when making a long, primarily uphill voyage, most of it under power, or motor sailed. Like the infamous Baja Bash, or when going from Hawaii back to the lower mainland.
Me, I'd love to have such a propeller, especially for such trips... if my Visa card gave it the nod.

Thus my comments about it being nice to have the option to tune one's prop to the boat.

My apoligies for the dissertation. I'm simply attempting to clarify the why behind my statement(s).
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Old 22-11-2015, 08:35   #9
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Re: Prop upgrade.

Mr. Uncivilized - I don't disagree with any of your reasons for perhaps choosing a multi-pitch prop, especially on a mono. But I do suggest you change your screen name to "Splitting Hairs."

Dave
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Old 22-11-2015, 09:44   #10
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Re: Prop upgrade.

Yes, I do at times get a bit ornery. But I also prefer that folks make educated decisions when possible. Call me a Curmudgeon if you like
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Old 22-11-2015, 09:48   #11
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Re: Prop upgrade.

The advice on a repitchable prop is sound. Assuming you have a Yanmanr 2GM20, it's rated for continuous hp at 3400 rpm and max at 3600 rpm. If you're turning 3000, you're not getting the hp the engine can deliver. That doesn't mean you have to run at 3400 or 3600 (intermittent), but that you have the option. In addition to the additional hp, your engines might run smoother and conceivably get better fuel consumption.

At a minimum you'll be able to tune the props to your boat. That's the hard part of finding the right prop.
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