Catamaran Propulsion Thoughts?

I'm building a 45' x 25', 11,000lbs catamaran.

I'm looking at fitting it with outboards because I don't like the idea of all the "junk" associated with inboard engines hanging below the boat slowing her down. Also, the prop would be the lowest point (deepest point) when the dagger boards are up - not a good situation. The boat will be used for cruising.

Any general thoughts on propulsion here? Ideally, I'd like to have something retract like an outboard. Any reason I shouldn't use outboards?

What about 15HP outboards, 20HP outboards, or a small 9.9 on one side and a larger main outboard to use when I need speed? Thoughts on how to set this up?

 

Hi CB,

A pair of 20 hp engines would sound like about the right ballpark for such a boat.

Problems with outboards on a larger craft? Well, there's the prop diameter problem (few outboards can swing the large diameter / low rpm props that a big boat calls for). There's the ventilation problem (outboards can lift out of the water as the boat pitches, sucking air into the prop). And of course there's the alternator problem (good luck getting more than 20 amps out of one of these things).

Some will also mention the "gasoline on board problem"- a moot point, IMHO, since even if you pick diesel as your main fuel, there's a good chance you'll have propane on board for the stove and gasoline for the dinghy. Proper tankage and fuel lines to ABYC and Coast Guard specs pretty much nullifies the perceived hazard of gasoline.

Advantages- well, outboards are pretty hard to beat for capital cost, weight and convenience, and it's fairly easy to pull them right up out of the water when they're not in use.

 

Considering high windage of 45' sailing cat, I would definitely recommend more horsepower. I would take about 6-10HP per 1 ton of displacement, higher numbers for lighter boats.

 

Any reason I shouldn't use outboards?


The other problem with outboards in some countries is the use of ethanol in the fuel.

Storage life about 1 month !


Is you can avoid purchasing this rotten fuel outboards are fine.Mix the oil in JUST before the fuel is to be used.

If you are stuck where ethanol is mandated try AV Gas , expensive but usually alcohol free.

FF

 

I was going to use brand new Honda or Yamaha outboards, made to work with the new E10 gasoline. I figured I'd have to put in stabilizer and such, like I have done over the years with my Honda 4 stroke dinghy motor, which is very sensitive to ethanol and moisture problems.

8HP per ton of displacement would be about 80 HP. Is that right on a cat? I haven't seen many cats with 40HP outboards on them. Not sure if that's right? I see plenty of 45' cat with 20HP engines though (the Powerthrust style, with large props and slower rotation). Are we maybe talking about two different types of outboards?

I agree the gasoline on board problem can be a moot point because it's not actually inside the boat at all. It's a cat, so the tanks are out on the aftmost part of the deck with filtered fuel lines running to the outboards, outside the hull. No gasoline inside the boat. I think following the ABYC rules for the installation will work out well. Thanks for reminding me of those.

I figured I'd place these in wells through the bridgedeck, about 1/3 of the way forward, so they don't breach as often as aft mounted ones. I don't plan on getting any of my power budget from the outboards. I am considering it a bonus if I get anything!

Well, thanks for the discussion. The other prohibitive factor is that I can in no way, shape or form afford inboard diesels at this point.

 

I have a PDQ 36, it's over-weight, pushing 11,000 pounds. I have two Yamaha High Thrust 8's, while most other Gas PDQ's use Yamaha 9.9 High Thrust Engines.
Here are some catamaran facts of life: Gas outboards develop their best power at 5 to 6000 rpm. Most have 2.3 or 2.4:1 gear ratios, and 9 to 11" pitch props. That means the cat has to get up to 15 knots for the engines to turn near their peak power. Yamaha High Thrust (as opposed to their plain 9.9 hp portables) have 2.92:1 gear ratios and reach peak power around 11 knots. If a total of 20 hp is only going to move your cat at 7 knots, the Yamahas will move it faster than say the Honda 20 that is only turning 4600 rpm.

You can buy stump-pulling props for outboards from SOLAS.com and that might improve the equations above, but you would be spending more money.

PDQ's mound their engines 2/3 LOA back, and as a result they rarely airiate. Longer shafts help. Since your boat is 4' longer than mine, but wieghs the same, I think you can reasonably expect to power at 8 or (possibly) 9 knots, still too slow for a 20 hp outboard to reach its peak. So I think your best bet is still the High Thrust 9.9s. Maybe you could mount three of them!

Check out the Maine Cat 41: it uses two Yanmar 29hp saildrives, weighs 19,000# and powers at 10 knots. on two engines. If you can keep the weight down to half that, then half the power should do you fine.

 

Thanks for the real life answer, Sandy, even if I was a little slow on responding.

One follow up question: How is it that the Yamaha "High Thrust" is any better than the Honda "Power Thrust?" Aren't they the same thing (increased gear ratios, larger props)?

The way I'm reading your post, it says the two don't compare. Am I missing something?

I see the Maine Cat you're referring to. My issue is that I want this to be a sailing vessel. For once, I don't want a boat with all that crap below the waterline slowing me down in light air. She's to have a clean bottom, except rudders and daggerboards. That's why I'm going with outboards in the first place. To sail and use them as true auxiliary power.

 

Hi catbuilder! Everyone is suffering from the same blight: marketing. High Thrust and Power Thrust are just words, invented to fill in the gaps mere numbers reveal!.

I too wish that my loyalty to Honda Cars and motorcycles were justified in their outboard motors, but its just not the case. Surly, secretive, and brooding Yamaha builds a better outboard for catamarans. And then ignores their customers, foisting them off on the ex-used-car sales types.

Re-read my previous post. The Yamahas are properly geared to give their best power at about the start of a 36 to 40 something catamarans power required sweet spot. Because the Honda's are geared for a higher speed thru the water, they can't reach their top end rpm, are turning slower and have less thrust available to the boat. Its like pulling a heavy trailer up a hill in third gear instead of second. The only way to shift down it to switch motors.

The kicker is this: the Yamaha High Thrust series has a near 3:1 gear ratio. The Honda has less than 2.5 to 1. Think of the prop as a tire; the yamaha has more blade area filling that big circle, and the Honda is puny by comparison. Honda can't use a larger diameter prop or a higher numerical geat ratio without completely redisigning their lower units, so the answer (until they do that) is Yamaha is better than Honda IN THIS PARTICULAR CASE.

WARNING: Both manufacturers blew it when they introduced bigger motors they called High or Power Thrust. They went back to ski-boat gear ratios.

 

You're right Sandy, they all use some confusing words to describe their products. I used one of these Yamaha 9.9 engines with the true gear reduction in it to push a 37' Louisiane catamaran to 7.5 knots in calm water. I even cupped the stock prop a little to get a little more pitch.

If I remember correctly there was another 9,9 Yamaha engine as well that did not have this bigger gear ratio, but it was not so clearly designated...in other words there could be some confusion between the two.

 

Excellent advice, Sandy! Thank you very much. The devil is in the details (or was it gear ratios) and I had missed that they was such a difference between the Honda and Yamaha here.

I'm going to take a look at those Yamahas again with this new information.

 

Just a thought..

Ever consider a liftable sonic drive leg like you see on the Gemini cats? I love the fact you can pull it out of the water and not worry about galvanic corrosion or barnacle growth. Plus it is better for rougher waters to avoid flooding like you would see in an outboard application. The company is based out of the United Kingdom. A great company to deal with as well.

 

I had considered one but IIRC I wasn't able to find any of them that worked for this application.

Have you seen pricing or any availability on these?

 

Try This..

My best advice if you are curious about looking into the sonic drive leg is to contact the manufacturer directly and ask if they can help with a custom installation. I have used this company twice on two different projects and they took alot of time in helping me before I made any purchases. Their website is at:

http://www.sillette.co.uk/

 

Thank you. I understand who they are. However, the website makes no mention of the Sonic Drive Leg existing anymore. They seem to sell regular outdrives and saildrives only.