Double Outboards on a Catamaran

[smj]

Originally Posted by Rumrace

I have a newish 40hp Yanmar. It’s kinda putting out 40hp around 2100 RPM from there to 3,000 rpm the motor doesn’t do much cause of hull speed. A Cat with its daggers up motoring I imagine has a pretty good hull speed. A pair of 29 Yanmar with Sea Drives would cost less than two Yamaha 25hp which reach their peak HP above 4,000 RPM. About $3,000 more. Fuel density would require bigger tanks for the gas option than the diesel killing your space saving theory.

Also the crappy alternator the Yanmar comes with is for a boat to charge a house bank of sorts.

You could blow the bundle put in two 5,000 psi hydrogen fuel cells and Yanmar pending little H2 motor. Toyota is finally producing hydrogen on board a Cat using salt water intake as generator. Japan settling up shore line hydrogen plants. Stored super cold in liquid then gases off specific pressure to system. US has a format goal for hydrogen cars with a range of 300 miles. Boats in salt water have an edge parked in a conductor in generating electricity. Now that Toyota Yamaha and Yanmar are producing hydrogen on board with excess energy consumption and pollution leave the conversation.

Can’t imagine a pair of diesels with saildrives would cost less than a pair of Yamaha 25’s. In fact I would imagine you could probably purchase the 2 Yamaha 25’s for about the same price as a single diesel with sail drive, or possibly for less.

[Rumrace]

I’ve been a BMW nut. Nissan Exhaust valves burn out Honda’s dropping body parts are spectator sports on one popular run. Honda screwed every marine dealer. In recent years to redeem themselves a fewLexus and Toyota Pick ups got an amazing Yamaha V8.
Same block recast
They have changed the stroke longer for torque coils and intake to keep balanced pressure. It’s down a few HP but up on torque. The Gas versions in Lexus were 6400 RPM 540hp. The Pick ups had various markings but Yamaha tuning folk snuck in. 500 Hp at 4400 rpm red line marine version and a handful built. You will see this motor showing up on performance boats with the ZF SeaRex.

The other i6 3.0L came out of Toyota’s version of a landrover. It’s cast iron so for experiment not production. The 30’ Toyota cruiser is getting a new Aluminum i6

Yanmar took a 3cylinder block and made a 130hp Hydrogen which they have attached to the ZF SeaDrive.

[Americat3014]

I have a 30 foot (9.1 m) non-performance catamaran with a 9.9 plus Suzuki high thrust outboard with a 25 inch shaft length and the answer to your question is that it is complicated..... I have increased the horsepower from 9.9 to 15 HP and changed the pitch of the prop from 5 inches to 9 inches. I could increase the horsepower from 15 HP to something close to 20 HP, but I don't think it will be beneficial. The problem with the smaller outboards is that they turn a smaller diameter prop. Once the engine rpm increases above about 3500 rpm, the prop cavitates and the speed of the boat does not increase. So, nothing is gained by adding more horsepower because of the necessity of keeping the rpm low. The increased pitch of the props did help increase the boat speed at the same rpm. The boat originally had a 115 HP outboard with a large prop and that motor did not cavitate, but the boat did sit lower in the stern with the heavier outboard so that complicates the comparison. I think I only ran that outboard above about 2500 rpm once and I don't remember the speed of the boat. The benefit to a larger engine is the larger diameter prop. Now, I would like to try the boat with the 5 inch longer (deeper) shaft because the small prop would be less likely to cavitate due to the higher pressure at the lower depth. The prop would also be less likely to slip when the boat hobbyhorse's in larger waves. This is a single motor mounted on the boat centerline. I would like to know from the people with the Seawind boats or others with the engines mounted midship if they are able to run their outboards at higher rpm without the prop slipping.
I do think 2 outboards running on the boat would increase the boat speed, but I would still be restricted to the lower rpm. The boat is more fun to sail with the 100 LBS outboard on the back than it was with the 320 LBS 115 HP monster.

[sailjumanji]

Originally Posted by Americat3014

I would like to know from the people with the Seawind boats or others with the engines mounted midship if they are able to run their outboards at higher rpm without the prop slipping.
I do think 2 outboards running on the boat would increase the boat speed, but I would still be restricted to the lower rpm. The boat is more fun to sail with the 100 LBS outboard on the back than it was with the 320 LBS 115 HP monster.

The Seawind 1000 - 33 ft cat - with twin 9.9 hp Yamaha high thrust could run at max speed in flat water without the props aerating. I don't remember RPM's exactly, but I think it was about 4500. That said, if motoring into closely-spaced waves or chop, the boat would hobby horse and the props would aerate. Motoring into the wind and chop was a bit of a nightmare. We'd starting moving weight to the back of the cockpit to keep the stern lower even when it did hobby horse.

The Seawind XL - which was a 36 ft boat - had less of a propensity to hobby horse. The Seawind 1160 Lite - 38 ft cat with twin Yamaha 25 high thrust - will rarely hobby horse. At WOT, the motors do 5000-5300 RPM and will push the boat about 8 knots in flat water and no wind. We usually sail, but when we motor, we run both motors at about 3500 RPM and make 6.5 knots. Fuel burn is 1.7 gal/hr total for the two motors. Running one motor at 3500 RPM used 1.0 gal/hr at about 5.5 knots. These motors are pushing a 12.25 inch diameter prop, 9 inch pitch.

Given all of that, I'd really be concerned about a 30 ft cat hobby horsing, and the motors coming shallow enough to aerate the prop. All of our Seawinds (1000 and 1160) had XL shaft lengths.

Finally, a friend just repowered an 1160 Lite with the new Yamaha 25 EFI motor - T25XWTC. I don't know specifics exactly, but he is reporting similar problem to what you are having, that the motors spin the prop very fast - around 6000 RPM. Doesn't bite. Yamaha puts about eight different props on that motor, but all (from memory) are 9-10 inch diameter, but larger pitch than 9 inch on my Yamahas. He's going thru the nightmare of trying to get the prop right now. Dealer says try the 12.25 x 9 inch he had on the carb'd Yamaha 25, but I don't think it's that simple. Unless gearing is the same.

[Chotu]

In. I have the exact same problems with the small diameter prop associated with smaller outboard‘s. I have twin 30 hp on a 50 foot boat. And they behave the same. They start slipping if you get the RPMs too high. These are the high thrust props and with low pitch. But they slip because they are too narrow in diameter.

[Bunji]

Originally Posted by fourlyons

It is possible that the boats you are comparing where there is little improvement by going to 25hp engines, are achieving hull speed with the 9.9 motors. I suspect that would be the case with your boat.

Spot on, understanding hull speed (sailing catamaran) under power (outboard) is the simple answer

[catsketcher]

Originally Posted by CMNZ

I have a question that does not seem to have been specifically answered in this topic and I have searched the forum and haven't been able to find an answer elsewhere either.

The two options for twin outboard sailing cats below 12m seem to be 9.9hp or 25hp and as far as I can see similair sized boats seem to use both equally. I understand that these two choices are because they are specifically specc'd with extended shafts and high thrust props (certainly in the Yamaha lineup) but what I cannot figure out is why they do not appear, from the posts I have read, to have an awful lot of difference in performance.

Am I wrong in this last observation and if so how wrong? Not really understanding how HP affects performance in an outboard I would have expected that a 25hp engine should provide significantly higher performance (2.5×?) than a 9.9HP.

Would appreciate some enlightenment.

Yep you are very wrong - there is a huge difference in power between the two, and a massive difference in the speed produced - if your cat is light enough not to have huge issues with hull speed. (Hull speed is not really an issue when sailing but my cat does produce some big waves when motoring)

My 38ft cat used to have a single 25 Yammie. I then wanted to go twins. mostly to get rid of my central pod which sometimes pounded offshore (even though I lifted it - I really dislike banging offshore). I didn't need any more power so I got a new (smaller capacity than old style derated 15) Yammie 9.9 for the other side. Two 9.9s were too little, so I kept the 25 and put it on the starboard side. Lopsided cat.

With just the 25 my cat can do 9 knots when very clean - and she is usually clean. I can see all my fuel money getting spent in making large waves but we slide along nicely. Coming back from Tassie we motored a lot across Bass Strait. We sat on 7.0 knots with the Yammie clicking along easily. Chewing about 3-4 litres an hour. There is a huge difference in power required to get from 7.0 to 9.0 knots and 7.0 knots is pretty easy on the motor.

Powering along with the 9.9 as well gets us to 10 knots flat out. Powering with JUST the 9.9 gets us up to 6 knots. But with absolutely no capacity for a good burst if needed and it feels really aneamic compared to the 25. The amount of power difference between the two is stark if any real thrust is required. I do a little bit of motoring with the two of them (usually just to give them a run) and I can notice a difference in windy conditions. With both running the boat has more glue to the water. Still they work well together and we can now do all those tricky things that twin motor cats can do.

I looked at replacing the 25 with 2 new 25s but I didn't. For one the new 25 has a typical 2.1:1 reduction, instead of the 2.4:1. That is not much of a difference but every little bit helps. I am happy keeping my old 25 for as long as I can and the 9.9 was a pretty good deal.

I think a Seawind would hit the wall quicker than my boat as it weighs about 2000kg more. Sailjumanji says his 1160 sits on 5.5 at about moderate throttle and as I said before, we motor at about 7 knots at a moderate throttle setting. It shows that even similar style and length boats get very different results from similar engines if they differ in weight.

I would not advise you to go twin 9.9s on a 38ft cruising cat like the one you are building. I don't think you would have the oomph required in critical situations. For me, top speed is pretty immaterial. It is "kick in the pants and let's get out of here" power that I like, like when I dragged anchor and needed to get the boat out of the moorings in a nasty squall. Extra power is really nice to have in those situations, and it allows you to be nice to the motor and run it at lower throttle settings when doing the most common motoring - clicking along in a calm before the wind breathes in and the Code 0 can go up.

I wouldn't save any money in fuel by running the 9.9 instead of the 25 (I almost always run only one motor when offshore in calms). This is because I would have to run it at full throttle to get to 6 knots (and I don't motor at 6 knots anyway - we go 7) whereas the 25 is just humming at easy throttle settings.

cheers

Phil

[CMNZ]

Thank you Phil, exactly the answer I was looking for.

[Tupaia]

What about one of each a 9.9 and a 25. You really only use both for mooring or anchoring. The bigger engine for motoring, the smaller on for motor sailing.


I have come across a number of larger cats with 1x40hp and 1x10hp



Downside is redundancy of spare parts.

[valhalla360]

Originally Posted by Tupaia

What about one of each a 9.9 and a 25. You really only use both for mooring or anchoring. The bigger engine for motoring, the smaller on for motor sailing.

I have come across a number of larger cats with 1x40hp and 1x10hp

Downside is redundancy of spare parts.

For most 30-35ft cats a pair of 9.9s (20hp) is plenty. In most moderate conditions a single 9.9 is plenty. Running both are useful when bucking a heavy current or strong headwinds (and dockside maneuvering)

Most cats in this size range are still subject to hull speed, so 50hp total (2 @ 25hp) is overkill.

While our cat had a single central engine, it came with a 40hp. Max throttle was a lot of noise, a big wake and about 8.0kt in calm conditions. We swapped out for a 25hp with a lot pitch work prop. Top speed dropped to 7.8kt in similar conditions (less noise but still a big wake). With the switch to work prop (ie: low pitch so it acts more like a low gear on a car), maneuvering was actually better.

[smj]

Originally Posted by catsketcher

Yep you are very wrong - there is a huge difference in power between the two, and a massive difference in the speed produced - if your cat is light enough not to have huge issues with hull speed. (Hull speed is not really an issue when sailing but my cat does produce some big waves when motoring)



My 38ft cat used to have a single 25 Yammie. I then wanted to go twins. mostly to get rid of my central pod which sometimes pounded offshore (even though I lifted it - I really dislike banging offshore). I didn't need any more power so I got a new (smaller capacity than old style derated 15) Yammie 9.9 for the other side. Two 9.9s were too little, so I kept the 25 and put it on the starboard side. Lopsided cat.



With just the 25 my cat can do 9 knots when very clean - and she is usually clean. I can see all my fuel money getting spent in making large waves but we slide along nicely. Coming back from Tassie we motored a lot across Bass Strait. We sat on 7.0 knots with the Yammie clicking along easily. Chewing about 3-4 litres an hour. There is a huge difference in power required to get from 7.0 to 9.0 knots and 7.0 knots is pretty easy on the motor.



Powering along with the 9.9 as well gets us to 10 knots flat out. Powering with JUST the 9.9 gets us up to 6 knots. But with absolutely no capacity for a good burst if needed and it feels really aneamic compared to the 25. The amount of power difference between the two is stark if any real thrust is required. I do a little bit of motoring with the two of them (usually just to give them a run) and I can notice a difference in windy conditions. With both running the boat has more glue to the water. Still they work well together and we can now do all those tricky things that twin motor cats can do.



I looked at replacing the 25 with 2 new 25s but I didn't. For one the new 25 has a typical 2.1:1 reduction, instead of the 2.4:1. That is not much of a difference but every little bit helps. I am happy keeping my old 25 for as long as I can and the 9.9 was a pretty good deal.



I think a Seawind would hit the wall quicker than my boat as it weighs about 2000kg more. Sailjumanji says his 1160 sits on 5.5 at about moderate throttle and as I said before, we motor at about 7 knots at a moderate throttle setting. It shows that even similar style and length boats get very different results from similar engines if they differ in weight.



I would not advise you to go twin 9.9s on a 38ft cruising cat like the one you are building. I don't think you would have the oomph required in critical situations. For me, top speed is pretty immaterial. It is "kick in the pants and let's get out of here" power that I like, like when I dragged anchor and needed to get the boat out of the moorings in a nasty squall. Extra power is really nice to have in those situations, and it allows you to be nice to the motor and run it at lower throttle settings when doing the most common motoring - clicking along in a calm before the wind breathes in and the Code 0 can go up.



I wouldn't save any money in fuel by running the 9.9 instead of the 25 (I almost always run only one motor when offshore in calms). This is because I would have to run it at full throttle to get to 6 knots (and I don't motor at 6 knots anyway - we go 7) whereas the 25 is just humming at easy throttle settings.



cheers



Phil

I think the most important part of your post is where you state “ if your cat is light enough not to have huge issues with hull speed”.
This is a rarity with most catamarans produced nowadays, sadly.

[Manateeman]

There are a series of videos by College Quaterdeck on repairing the fuel injection systems for Suzuki 20 hp fuel injection outboards.
It includes cleaning the injectors and the high pressure pump which some think cannot be repaired. We owned a Yamaha 25 fuel injection outboard and it ran well but I always worried about being unable to repair it. The videos on the Suzuki are very helpful and I wish I could find similar videos for Yamaha.
Generally, we detest outboards, and don’t trust them even for a dinghy motor.
We sailed for years without any engine. Our dinghy just had oars.
We’ve built catamarans and have actually done a few new boat sea trials on them. We really like Chris White designs but would never own a catamaran.
Just our personal choice of vessels. No stone throwing please.
The ability to repair your vessel at sea is what we see at the heart of cruising and we added the outboard repair information for those who wish to be more self sufficient. Again, just an opinion.
Happy trails to you.
Captain Mark and his sedentary, listless manatee crew.

[valhalla360]

Originally Posted by Manateeman

There are a series of videos by College Quaterdeck on repairing the fuel injection systems for Suzuki 20 hp fuel injection outboards.
It includes cleaning the injectors and the high pressure pump which some think cannot be repaired. We owned a Yamaha 25 fuel injection outboard and it ran well but I always worried about being unable to repair it. The videos on the Suzuki are very helpful and I wish I could find similar videos for Yamaha.
Generally, we detest outboards, and don’t trust them even for a dinghy motor.

Reminds me of when fuel injection came to cars.

Lots of fear based on the fact that carbs required regular maintenance and repair and how would they do that with this new fangled fuel injection...when the fact is fuel injection systems are far less likely to fail in the first place.

When was the last time, you had to rebuild the fuel injection system on your car? I never have and I don't know anyone who has. Not saying they have never failed but it's pretty darn rare.

Had a 25hp FI Outboard for several years and it was great. It simply ran. Other than fluid/filter changes...basically no maintenance and because it pulls completely clear of the water, no fouling or corrosion issues. No mucking about with clogged or dirty carbs. I would take it in a heartbeat over a diesel on a small catamaran.