[MHml] outboard gearing

Tue Feb 13 02:45:24 EST 2007

Bill,

I thought this must help. Real world experience on a set up close to  
what you intend on Cats Away (Shuttleworth Tek 35) with the engines  
mounted at the transom.

For the designer comments go to: http://www.john-shuttleworth.com/ 
New.html and scroll down to:

  John Shuttleworth has just returned from the Caribbean.

Below is what the owner had to say.

> Hi;
> I sold my boat around 5 months ago, so you can also ask the new owner.
> I looked into the difference of high thrust and regular outboards  
> around 3
> years ago so the exact numbers I can not give you.
>
> The high thrust basically is for slower boat speed compared to  the  
> regular
> outboards, which are basically designed for planning boats. So for my
> catamaran (which weighs around 9 to 10,000) the Yamaha T50 is a  
> good match
> becuase it is designed for a boat speed of generally 15 or so mph.
> They do this by using a large diameter prop (13 inches) and little  
> pitch in
> the blades.
>
> At around 80% throttle we would do 14 knots in small chop and some  
> wind
> (chop 2 feet or less, wind 10 knots or less),  As the chop got  
> worse we
> would throttle down (to much spray, and then motion gets a little  
> to much,
> better to be sailing).   top speed was close to 16 , and down wind  
> we could
> do 17knots.
> At slow speeds it has a lot of power,  Thrust for manuvering was  
> excellent
> and having the two engines gives you a lot of control( with dagger  
> board
> down especially). My outboards were place behind the rudders, but  
> if they
> had been in front , I imagine one could "crab" side weighs like  
> motor boats.
>
> I would definantly go with high thrust for sailboats.
> Hope this helps .
>
> tony amador

Regards,
Andre

On Feb 12, 2007, at 12:50 AM, Bill Gibbs wrote:

> After some hours spent reading outboard specs and performance  
> reviews, I am amazed that all manufacturers only post boat reviews  
> where the top speed is 50+ knots.  Hundreds of different boats.   
> Even if they have to put 2 or 3 motors on the transom.  Even  
> pontoon boats doing 50+ knots.  Ok, I exagerate slightly, there  
> might have been a few topped out at 48 knots.  Are there no  
> outboards on slow boats?
>
> You look at a motor whose rated horsepower occurs at 5200 rpm,  
> spinning a small high pitch prop, and you've got to wonder if all  
> I'm going to do is rip holes in the water trying to get to 20  
> knots.  I haven't found any power curve data for these motors, but  
> it seems most peak at high rpm.  Yamaha's high thrust models  
> (exactly paralleled by Mercury's Bigfoot models, odd?) advertise  
> lower gearing and bigger props, but max at 60hp.
> Larry is correct in pointing out only Suzuki (of the brands I've  
> looked at) has lower gearing and bigger props in higher  
> horsepower.  They are also larger displacement engines.  More  
> torque at lower rpm?
>
> Simply going to a smaller pitch can keep the prop matched to your  
> desired boat speed, but may only use a fraction of the power  
> available, with over-reving a danger.  And no guarantee you'll  
> reach your target speed.  So which is better, a 175 hp high  
> displacement motor geared 2.5:1 and a 17 pitch 16" dia prop  
> matching 20 knots BS at 15% slip & 4,000 rpm?  Will it push AB to  
> 20 knots?  Maybe not enough hp?
> Or a 250 hp geared 1.85 spinning a 14.5" diameter prop that needs a  
> custom low 14 pitch for 15% slip at 4k rpm & 20 knots?  But maybe  
> your not using all the hp at that low a pitch?  Maybe we're just  
> spinning?
>
> I say 4k in both cases because the case histories show that optimal  
> range occurs at 3k-3.5k rpm.  By 6k rpm fuel consumption is up  
> 2-4x, ok for boats going a lot faster, biut not for a knot gain.
>
> My head hurts from the variables.  No 10k pound cats in the test  
> studies for any brand. :-(
>
> I've asked Malcolm for an opinion.  Maybe he'll have one to share.
>
> Bill
> Afterburner
>
>
> From: multihulls-bounces at steamradio.com [mailto:multihulls- 
> bounces at steamradio.com] On Behalf Of Larry H
> Sent: Saturday, February 10, 2007 12:39 AM
> To: multihulls at steamradio.com
> Subject: Re: [MHml] outboard gearing
>
> Bill,
>
> From 20 years in the Evinrude outboard business:
>
> The short answer is...no,  you cannot change the gear ratio of a  
> outboard.
>
> The lower unit casting is made to fit certain pinion and drive  
> gears and no others.
>
> Any change in overall gear ratio would have to be done between the  
> power head and the lower unit.  Suzuki uses a gear set at the  
> bottom of some powerheads (90hp)(between the powerhead crankshaft  
> and the driveshaft) to change the actual lower unit drive ratio  
> from 2:1  to a final ratio of 2.5:1(approx)  See the Suzuki web  
> site for more info.  This allows the Suzuki 90 to turn a larger  
> diameter prop with a higher pitch than say a Honda or Yamaha which  
> would have only the 2:1 (approx) ratio.
>
> A Suzuki might be more suited to pushing your sled.
>
> Interesting project!!
>
> Larry H
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