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Old 23-01-2009, 09:40   #76
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Sandy,

You may well be right. I hadn't had a Yamaha.

I believe conventional wisdom on slippage is between 45 and 55% depending on the unknown # of variables, ie.pitch, dia, prizmatic coef. etc,etc,etc. For what it is worth. I guess if we could nail that down we would have it made.
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Old 23-01-2009, 09:47   #77
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I'm too lazy at the moment to drag out my prop book, but I vaguely recall that slippage is lower on slower turning props. If Practical Sailor had just found a load cell to mount all those small engines they tested last year! What a teapot tempest that would have stirred up!

And why do I have to go to Australia for all the really neat cat stuff?
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Old 23-01-2009, 09:48   #78
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Sandy and All,

Back again. I met a couple that had an outboard diesel on a 30' Endeveour Cat.

As I recall 25HP Yanmar? They didn't have to worry about fog. They could be heard for miles.
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Old 23-01-2009, 09:53   #79
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I think we sold that boat. I never got a chance to sail it while it was at Chesapeake Catamaran Center, and didn't know it made a lot of noise. But that seems like a good match.
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Old 23-01-2009, 10:02   #80
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Sandy,

You may well have sold it. I met the couple in Ft Myers, they were from NC. I understand they have since sold it.

It wasn't a good match. The only diesel OB I had seem but the noise was not worth whatever preformance it may or may not have had. A conversation in the cockpit was not an option.
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Old 23-01-2009, 13:47   #81
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Quote:
Originally Posted by sandy daugherty View Post
Here's a partial solution to the mystery. Honda doesn't sell the same engines everywhere. Neither does Yamaha for that matter. The Honda BF15 x is very appealing (still my second choice for my particular interests) but the the numbers are revealing: 15 hp at 5500 rpm thru a 2.08:1 gear ratio to a 7.625" pitch equals 19 mph theoretical, discounting any slippage and special prop characteristics. Since Slippage is typically somewhere between 10 and 20% , we can SPECULATE that this engine is designed to develope best power at 15 to 17 mph. Passing speculation into the realm of wild ass guessing [as in feral donkey estimation] we could say that the Hondas are putting out 75 to 80% of their best power at 8 knots, and because they are higher displacement engines, they are burning more fuel than the Yamaha at that speed, developing the same or less power.


For sustained cruising you wouldn't want to be operating at the max power 5500rpm though. More like 3500 - 4000 rpm, and you want the boat to be going at a good cruising speed at those rpm.

A bigger displacement engine running at 10 hp won't neccessarily use more fuel than a smaller displacement engine at 10 hp either.
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Old 23-01-2009, 16:42   #82
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Your correct in my opinion. Running a 6 71 or a 12 567 run cheaper at about 2/3 or less. So does a 3.5 Circa 50s Johnson OB.
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Old 25-01-2009, 14:09   #83
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I own one of the few Gemini 105M's equipped with twin 9.9's. I currently have 1996 Evinrude 9.9 4-stroke "High Thrust" engines with 4-blade props. I want to upgrade and am considering going up to twin 15's.
I don't want to over simplify, but when weighing the benefits of new 9.9's versus 15's, I can get 2 x Merc 15 Pro Kickers for the same price as 2 x Honda 9.9's.

Going in favour of the Honda is also the alternator output is better at lower RPM's, the Merc's 11A is closer to WOT.

Right now I can push our Gem at about 7.0kts 3/4 throttle with both engines, flat water, minimal wind, loaded for 3 weeks with wife and 2 kids. Can manage around 5kts WOT on one engine only.

I'm not looking for more speed with the 15's, just that extra "punch" against headwinds or counter currents.

Looking for opinions on the addage, "There no substitute for displacement".

Thanks,
Mark.
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Old 27-01-2009, 11:00   #84
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Mercury, Honda, and others met the engineering challenge presented by Yamaha by changing props and advertising. Period.

More efficient propellers have a higher aspect ratio, just as sailplanes have longer wings. They move throught the water slower, which means they turn at lower rpm. For outboard engines, that means a higher gear ratio. Only Yamaha has a near 3:1 ratio; the other have 2:1[ish] gear ratios. That kind of a change would require a major redesign of the lower unit, to house bigger gears and accomodate a larger diameter prop. Doing so would mean that their engines would overspeed on faster boats which represent the majority of their market.

I am not happy that the only place I can get what I need is from the Yamaha store; they have a monopoly and don't care very much about servicing our tiny little market segment. I would LOVE to see an American made engine that did what Yamaha does.

I sell mercury outboards. They don't cut it for cats.

A higher displacement engine uses more gas to produce the same power as a smaller displacement engine because the smaller displacement engine is running at a more optimum rpm. Think gas mileage differences between identical cars with smaller and larger displacements.

There are other, clearly superior advantages to a larger displacement; lower temps, less wear, lower compression, etc., but the discussion is fuel efficiency.

DelMitch: Do both of your engines mount on a single, raisable 'skid'? Can you use short shafts?
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Old 30-01-2009, 16:39   #85
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Sandy,
To answer your questions.
1. The engines are on 2 seperate lifting "buckets". The current set-up isn't ideal as the engines are connected, but I have given it some thought and plan to make it a bit more user friendly this spring once I add the new engines.

2. I am currently running 25" legs and wouldn't want to go any shorter.

After reading your post (plus a few other factors) I am leaning more towards sticking with 9.9's. The Yamaha's seem to be the way to go. As far as gear ratios, off the top of my head the Merc BF is 2.42:1 and the Yamaha is 2.92:1. Honda's are 2.33:1, but they do have the best alternator output at low RPM's.

I won't pretend to know a great deal about props (but I'm trying to learn), but is the 3-blade on the Yamaha just as good as the 4-blades all the other "high thrust" offer?
Would there be an advantage to putting 4 blades on a Yamaha?



We are leaving Lake Ontario this summer, down the ICW, winter in the Bahamas. My wife and I did a similar trip 9 years ago (a full-keel Ted Brewer design, no kids, seems like another life!!). Just want to make sure I won't have too many regrets with the 9.9's, as I mentioned I have been happy with the current 9.9's but have not had to deal with tidal currents.

Have you been tempted to go to twin 15's??

Thanks for the info,
Mark.
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Old 30-01-2009, 16:44   #86
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I messed up the request you had for contact. You can e-mail me at mdmaclean@oceaneering.com.

Mark.
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Old 02-02-2009, 12:39   #87
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Mark: I started my education about outboard and props on my first cruising cat, a PDQ 32. I had tried to get some information off the Yamaha web site, which was just so much sales pitch. When I called Yamaha, all I got was steamed. I'm still upset with them, but I've learned enough to keep giving them my money!

The differences in gear ratios between Yamaha and the others is about the same as the difference between first and second gear in a car. And having four blades is plainly irrelevant. The most efficient propeller has ONE blade, as in those used by indoor endurance gliders, and possibly one of the human powered contest vessels. Hartzell came out with a five bladed propeller for the Shorts Skyvan [?] because there wasn't enough room for longer, higher aspect ratio blades. Helicopters have 3, four or five blades because a longer blade goes transonic at higher airspeeds. Honda and Mercury have 4 bladed props because their cavitation plates are too close for a larger diameter, slower turning three bladed propeller.

Yamaha 8 and 9.9 high thrust engines have three blades because they were designed from the start to push best at slower speeds.

Remember that most outboards can spin up to high rpm even when they are dead in the water. Look at the stream of prop flow and you will see its white with air generated by the props cavitating. Pull the throttle back a bit, see the wash become less distinct, and feel the boat accelerate a little faster. Thats because the prop is slowing down and getting a bite on the water. When Gemini's were equipped with a single 2 stroke HP Tohatsu, they cavitated continuously!

My second catamaran, a 34' Simpson, had a single HP Honda. If I went from half throttle to full throttle, the engine would roar, cavitate, and the boat would slow down! It was obscenely over propped, so that the previous owner could run it at a low rpm, to save gas and reduce noise and vibration, because the boat only needed 20 hp to hit best speed. Unfortunately, you could never get the full 50 out of the engine when you needed it.

So Mark, the short answer is go with the 25" Yamaha 9.9s, and join me as I petition the gods of commerce to inspire Honda, or Evinrude, make a 30 hp 150 pound long shaft diesel with remote controls and power lift! One would be great for Cats under 40', and two would be the dream movers on cats from 38 to 44 feet!
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Old 02-02-2009, 16:12   #88
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Thanks Sandy, you have just greatly simplified my life!!!!

I have been debating my repower options for quite awhile, research kept coming back to the Yamaha's. After your convincing arguement all I have to concentrate on is getting the best price possible on a pair of T9.9's.

FYI ,I am currently stuck in a hotel in St. John's, Newfoundland. I didn't go offshore with my crew this week as I had meetings in town, but they had a rough night last night of 17.5m seas out on the Grand Banks! Not a record for us, the worst was 23m a few years back!

Looking forward to some calmer waters this spring with my Gemini and new T9.9's

Thanks again,
Mark.
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Old 03-02-2009, 11:23   #89
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Arrrrghhhh! 57' seas on the Grand Banks in February!!!! I'm going back to bed and roll up in a comforter!
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Old 23-03-2010, 14:26   #90
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I like the idea of outboards and see some lift vertical

I forget which boat I saw that on. It had jackplates, like the some bay
boats do for fishing in the skinny water.
Ive been looking at the Gemini but real crazy about that diesel they keep
using in it, but you have no other choice in their design.
Outboards are so easy to work on since you can take them right out.
That jackplate lift idea I saw at a show is nice too.
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