Powering a slippery cat with 20 hp. Fuel injected or carb.

Obviously marine engines are designed for continuous high RPM, there is not much loafing with boats, as the fact they don't have brakes, attests !

 

Of course, but...

Take two engines of the same design (or similar) and run one at 5000 and one at 5500.

In the simplest of views; the lower rpm engine is operating about 10% less. I had a 25hp merc and a 30hp Johnson. The Merc and Johnson had similar speeds, but the bigger engine ran at low rs and was quieter and I liked it much better overall. The Merc really ran hard and loud. While rpms were not a factor in recent decision making; if I sit next to a tiller; louder is less pleasant.

call me silly if need be

 

I wouldn't call you silly at all, but there is a band between say 3500-4500 RPM where outboards are quite happy to run without any worry that they are being over-operated. Full throttle is another matter, you certainly risk increasing engine wear doing that, not to mention use more fuel per mile. The noise factor might be as much a matter of how good the cowl insulation is.

 

I've been looking again at the Suzuki. Thinking that it may be worth building the fairing around the giant power tilt mount just to see if I can save some fuel. The remote 20 hp only comes with power tilt, but it's the only 20 that seems to burn significantly less fuel than the older carb 20's.

 

hi russell, i would love to see some pics of your cat if you have any. why are you fitting 1 engine. cats and twins usually go together. i am not critisizing, just curious.

 

Single engine because that's the way to save fuel, especially when the boat only needs 20 hp, plus the hulls are tiny, so nowhere to put a motor. The boat looks a lot like Grasshopper. Have you seen photos of that?

 

thanks . i will look up the grasshopper

 

had a look at the grasshopper video. .looks great . super efficient,

 

I'm leaning heavily towards buying the Suzuki 20, even though it has the power tilt bracket issue I mentioned above. I couldn't find a pre EFI Yamaha 20 and I'm ready for the motor soon. It's a fairly good deal for the remote Suzuki as it comes with the control box for about $3500 if you buy from the mail-order Suzuki place. My friend got one and it arrived up in 2 days. He had a cooling issue which he sorted out and it seems good. Anyone have anything to say before I pull the trigger?

 

i see parsons 20hp longshaft advertised on ebay.. neat looking motor 4 stroke . efi. manual tilt. comes with forward controls. i have no idea what the quality is like though.
i think they are yamaha copys.

 

I bought the Suzuki 20 with the power trim/tilt bracket (the only way to get the remote version). My plan is to completely remove the trim/tilt bracket, which must weigh at least 15 lbs and replace the necessary part with a custom composite bracket.
If it works, it will be the lightest 20 I could get.

 

The power trim/tilt and bracket weighs 14.8 lbs. That part is not going along for the ride. I'll post photos of the mount as it Progresses. This motor is pretty impressive. It's super complex, but it sure is quiet.


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Richard,
Sorry to hear of the loss of Skoota.
Sensible little boat with very clever accomodation.
Re injected 4 stroke outboards.
Our experience has shown the injected motors tend to be more economical at lower revs than carburetted motors. At higher revs there isn't alot in it.
We have a 15mt x 7000 kgs sailing cat with the older heavier 500cc ? 25hp 4 stroke 25" leg yammys on it that has circumnavigated.
No issues at all but thirsty in comparison to the 20 hp Tohatsu and Honda's. We havn't tried the new Yammy 25's ourselves but rumour has it, they are still thirstier than the 20's, which is logical.
We have about 10 x 12 to 14 mt sailing cats displacing 4800kgs/6000kgs [with 30,000/40,000 miles on them] with the 20 hp x 25" legs Hondas and Tohatsu's.
They have proven to be very economical to fit , maintain and operate with life spans of up to 10 yrs. They are also considerably lighter then diesels.
The boats generally use around 2 lph on one engine at around 5.5 kts. Running both engines at 7.5kts will use around 4 lph.
When bashing down from Princess Charlotte bay last year into 20//25 kts I was using 5/6lpm to achieve 5 kts. Pushing a rig thru the air makes a huge difference in consumption and performance.
Good sensible power cats/ tris/ outriggers that are kept simple, reasonably light, and with low windage can be powered with outboards and do big mileage, economically, reliabily and with good longevity.
My 12mt x 5.5mt x 3500kgs Pod Power cat is still going strong after 11 years on a pair of 60hp Yamaha 4 strokes.
The boat would be just as succesful with a pair of 30hp motors, just a bit slower.
Slowly firming up my new boat.
Either a single outrigger as done by Russell although at about 12mts or a Pod power cat with a single 30hp motor on one hull.
They are very nearly the same thing in reality although the outrigger will be faster.
Built in plywood, as 40 + years of experience has shown it to be wonderful material with great longevity, best weight for build/ dollar / time return there is.
Some builder experience/discipline is required though.
I'm also a big believer in water ballast in light boats for many reasons. It can be used to induce a slight bow down trim in smooth to slight conditions to induce slow speed surfing which increases speed/economy a surprising amount. Or to give the boat a bit of mass to ease the motion in lumpy offshore conditions.

 

What issues are there running with a single engine (the other tilted up) on those wide-spaced displacement cats Bob ? It seems an obvious way to extend range, but I suspect there is a snag in it somewhere.

 

Now that you've got your motor, you may want various props to optimize your rig for fuel efficiency under various conditions.

It is my understanding that motors can usually drive a little larger pitch at mid rpm than they want at wide open throttle.

Probably not all that much available for a 20 hp Zuke though.

Dan N.

(I'm a lurker!)