Outboard HP needed for Tangaroa Catamaran

How much hp power or size of single outboard would be needed for a Tangaroa Mk 4, at 38' (stretched 2'-6") LOA, 22'-6" beam & 8000 lb max?

Would like to be able to make 4-4.5 knots in moderate to calm conditions.
Used mostly to motor out of bays or away from beach before raising sails, moving short distance when diving, coming in when wind dies down etc...
Not needing twin outboards for redundancy or increased maneuverability in confined marinas or harbors.

Would like to use the Yamaha T9.9 High Thrust if it would be powerful enough. It has 2.92:1 gear ratio for low speed thrust.

But I don't know how much thrust would be needed or how to figure the thrust of different outboards in the 9.9 - 20 hp range. The Tohatsu is one of lower geared ones, 20 hp & 2.15:1 compared to about 2.08:1 some of the others in the 9.9 - 20 hp range.
Yamaha has the T25 with low gearing but it weighs over 200 lbs, so is a bit much to pull the motor for storage part of the time when on a mooring line or maintance and that is probably a good bit more power than needed also.

How does one compare thrust between different outboards in the 9.9-20 hp range when there is a difference in gearing, prop diameter and pitch?

How can I determine how much power/thrust is needed for the Tangaroa?

Cheers, Allen

 

The original specifications said that a 9.9 was OK. I had one with a 35HP and it was overpowered. I think that 15 to 20 would be ideal.

 

I would use the 9.9 high thrust Yamaha. It is what I have used on most of my own cruising boats - 28ft Gypsy, 30ft Sagitta, 32ft Eclipse, 35ft Banshee and, most recently, 34ft Romany. Sometimes boat speed would drop to 2-2.5 knots when motoring into 30 knots of wind and sea, but usually we'd cruise at 5.5-6

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

The down side of the 15-20 hp outboards is that they weigh close to 50% more than the Yamaha T9.9 and have higher gearing directed towards lighter faster boats.

The T9.9 is supposed to be better for slow displacement boats.
If I understand it correctly it can push a heavy displacement sailboat better than a higher hp outboard (within reason) because of the lower gearing and large prop and has a pretty robust lower unit also.

I'm guessing the T9.9 in particular, might push a Tangaroa along about as well but maybe not quite as fast as a 15 hp with higher gearing. I don't think the other higher geared outboards would though. I think the Yamaha T9.9 is alone in it's class as far as thrust produced on a low speed displacement boat.

If the T9.9 can move your boats along at 2-2.5 knots in 30 knot wind and sea that's plenty potent. I'd be happy with that and 4 knots cruise in calm would good and anything faster would be bonus.

So it sounds like the T9.9 will fill the bill just fine.

 

With a cat that is light with an easy high speed potential.

I think a more powerful engine would be a far better choice.

Why limit the boat to 4K when the same effort will give 12 or 16KK?

Happily you should be able to simply borrow a couple of different sized OB and see what pleases you.

 

Getting more speed is less important than minimum engine weight. Only want an engine for moving short distance for diving, fishing or motoring out of bay with inexperienced people helping with sails so they have time to raise & set sails and occasionally motor in when wind has died down.

It will be primarily sailed and engine used minimally. I would prefer to not even have an engine, but I know having one can be safer at times and at very convenient at times. So practicality wins over being a purist.

Cheers, Allen

 

The difficulty will be in keeping the prop in the water and hooked up. Use the longest shaft you can get your hands on. You can't get the prop too deep. Even the 25" extra long shaft Yamaha would be best mounted on a sled. There are massive gains to be had by getting the prop of a slow boat deep under the waves and isolated from the boat's plunging motion. Otherwise, on a boat this size, you will be limited to canals and mill ponds. Outboards on displacement boats become a big headache on boats over about 23 feet. They just can't keep up with the seakeeping expectations of larger boats. Forget about the weight of the motor, a practical mount will be heavier than the motor. And it's a bargain compared to the several hundred pounds of extra ground tackle that you will need if you don't have a bullet-proof kicker.

See this thread as well. http://www.boatdesign.net/forums/multihulls/outboards-nacelles-51157.html

 

second that!
had a 15hp standfard 2-stroke with standard prop on the tangaroa - power was not the prob, it was keeping the prop in the water.
would be interesting to investigate a "floating" nacelle (has to be lift-able of course)

 

Will be using 25" shaft and a retractable sled of some sort to get prop as deep as possible and mounting engine closer to midship and not off stern.
One goal is to have the sled to give as much protection from splash water as possible when retracted and not in use.

Won't be able to protect the engine completely from from sea water, but want to work on the design to minimize the water that gets on the engine as much as feasible.

Once beams are set and deck is laid out will work on the sled and engine mount design.

The main corn about engine weight is being able to dismount it when boat is going to be moored for a while for theft protection. Not as concerned with an extra 90 lbs of engine weight as far as weight the boat has to carry, though working to weight as light as possible.

As Phil mentioned, the sled and mounting system will weigh more then the T9.9 engine it's self. But the sled & system doesn't need to be disconnected and carried to shore like the engine does on occasion.
Cheers, Allen

 

"Only want an engine for moving short distance for diving, fishing or motoring out of bay with inexperienced people helping with sails so they have time to raise & set sails and occasionally motor in when wind has died down."

This overlooks the fact that some cats actually are fast under sail.

Lets say you have a great wind and in an hour are 12 miles from home and the wind dies.

Do you really want to spend 3 or more hours putting back with with inexperienced people needing to be entertained?

An extra 40 lbs of motor might be a gift from the Gods for your mental health , and for the enjoyment of all.

I helped a fellow install a tiny engine on his Hoby Cat for just that reason.

He could come home at the speed he went.

 

Good point FF. I will definitely make the sled and mountings capable of handling the Yamaha T25 so that will be an option later if wanted or needed.

The T25 is abut 90lbs heavier than the T9.9, but if the T25 ends up being preferable the system will be built to accommodate it.

Part of the problem is knowing how much thrust the T9.9 actually produces and how much will be needed with the Tangaroa weight. From what I have gathered the T9.9 is has much better thrust in situation I will be using. So hopefully will be able to borrow one to actually test and see what kind of performance it will give in my specific situation.

There is a Tiki 46 builder that will be testing the performance difference between a Tohatsu 9.8 and Yamaha T9.9. He bought one of each and will test them to see what they each are capable of. That will be nice to get some real world performance data. The Tiki 46 is a substantially bigger and heavier boat than the Tangaroa, but will give some idea of potential on a Tangaroa.

Cheers, Allen