Cat with Outboards - Performance Under Power ?

[terhohalme]

What was the gear ratios of Honda 35 and Honda 40? And the dimensions of propellers? Any idea of the rpm's of engines at the top speed?

[bastonjock]

at what wave height does cavitation become a problem? will an outboard push a cat through choppy seas?

i sail mainly in the North sea,i get a lot of "washing machine seas"

[Talbot]

Originally Posted by bastonjock

at what wave height does cavitation become a problem? will an outboard push a cat through choppy seas?"

There is no one answer to fit all questions here.

The performance of the outboard in bad weather is dependent on a number of characteristics several of which are unique to the individual situation.

Position fitted. - Right aft provides the most space in the boat, but the worst performance in bad weather

Power - a 9.6 high thrust is a great motor up to the point where there just isnt enough horsepower to push through the water against the wind/sea.

Propellor type - the high thrust prop has a larger area of metal. thus pushes the water better at sailing boat speeds.

depth of propellor - (below the waterline). A fundamental point as the deeper it is, the less likely to cavitate, and the more solid the water it is pushing. This is also combined with position on the fore/aft line

engine/gearbox type - a motor designed to turn a bigger prop more slowly, works much better in the aereated water in bad seas.

Cavitation plate efficiency - the larger this is the better. I added something like the doel-fins to the plate and this significantly improved the thrust in bad weather.

Characteristics of the sea - how high and steep is the wave, and what is the periodicity. If you cant make way directly to windward, try doing 20 or 30 degrees to the wind/sea. It will change how the boat reacts to the wave, reduce the wind pressure against the boat, and make allow the engine to make progress.

Characteristics of the boat - a catalac has a very high wind profile, whereas a wharram has a much lower one. The Catalac will need much more power to punch against a high wind. Some boats are more prone to hobbyhorseing than others, this can pull the propellor high enough so that it cavitates or even comes out of the water.

My old 9m Catalac was originally fitted with a 30 hp yamaha. On one occassion I was in bad conditions such that she was hobbyhorseing so violently that I could not get her to climb over a wave without several attempts. I changed the motor to a 27hp ultra long diesel outboard with wings on the cavitation plate, and in the following 13 years never experienced cavitation.

[Pat Ross]

Originally Posted by Talbot

Power - a 9.6 high thrust is a great motor up to the point where there just isnt enough horsepower to push through the water against the wind/sea.

Propellor type - the high thrust prop has a larger area of metal. thus pushes the water better at sailing boat speeds.

Thank you for putting all that together. Tell me is Yamaha the only manufacturer of the High Thrust type engines you mention? I have checked around, they appear to be the only outboard manufacturers using that designation. Could other manufacturers such as Tahatsu or Nissan have a comparable engines but not be using the designation HT?

Thanks,

Pat

[Talbot]

Originally Posted by Pat Ross

is Yamaha the only manufacturer of the High Thrust type engines you mention?

Sorry but I dont have any information on the modern engines.

15 years ago, Yamaha did a high thrust prop for other engines - for example I got one for the 30 hp I had as the original engine on my boat. It didnt make a big difference in forward power, but was a dramatic difference when going astern.

[Pat Ross]

Here is an excellent read on outboards in a Seawind 1000

KatieKat FAQ

[sandy daugherty]

Thanks, Pat Ross, that reference filled in a few blanks for me too. As the author points out, engine manufacturers are singularly silent about one measure buyers would be very interested in, a thrust-to-speed curve. What information we are left with only gives us an approximation of what we are looking for.

By law, Brake horse power is measured at the propeller shaft, at an rpm range. That takes care of variations in efficiency of the drive gears and bearings, but it is always quoted at a range of rpm. Given the reduction ratio and prop pitch, we can make an educated guess about thrust to speed curves, but we can't narrow it down because we don't know slippage, prop efficiency at certain speeds or the volume of water displaced.

Even 10 hp motors can cavitate or snorkle [technical term is ventilate] at rest or at speed. Placement on the boat governs that. Cavitation plates are designed (and tested) to perform best, with least drag, under specified parameters. Non of the aftermarket fins, cowls or cowlfins are evaluated with those concerns.

What IS important to know is this: Are we getting the best thrust from our engines at the boat speeds we can attain? The answer is a universal no. Outboard engine equipped catamarans, with few exceptions, go faster than 8 knots under sail. While most 10 hp outboards are designed to reach their peek power at 16 mph, and have only reached about 70% at 8 knots, the Yamaha High Thrust have 2.91:1 gear ratios and low pitch propellers with three big blades and a large diameter, all contributing to moving more water at a lower speed. The Yamaha High Thrust Engines are alone in this regard and come closest to meeting our needs. They prove 85% of their available power at 8 knots. I now suspect that two of these engines give us about 520 pounds of thrust at best speed. By comparison, a Honda BF15 (my second choice) will provide less thrust at a higher fuel burn, more weight, and more cost.

Yamaha's term High Thrust has no particular meaning, just as Jeep's "Trail-Rated" bs is hype.

Evinrude has a multifuel engine that burns gas and aviation fuels. I don't know if it weighs more than their conventional outboard, and it doesn't appear to have a longshaft or low gear ratio option, but I would really like to see them do that, and tell us it will run good on fuel dock diesel. Until then, there's really only one choice.

[44'cruisingcat]

There is also the Honda "Powerthrust" 20 hp. It has the longest leg, 27" (I think), and a lower pitch, bigger diameter prop. Doesn't have as low a gear ratio as the Yamaha though. Weighs about the same as the Yam 9.9. An Oram 44C powered by them motors at up to 11 knots, cruises at 9, or 7 1/2 - 8 knots on one motor.

[Cadence]

Am I reading this right? "While most 10 hp outboards are designed to reach their peek power at 16 mph", on what?

[sandy daugherty]

On runabouts, dinghies, small fishing boats, crabbers, jonboats, etc. That represents 99% of the market for these engines. Is that what you were asking, Cadence?

Precisely, 44 Cruising. I seem to remember that those engines were repropped with SOLAS propellers, not widely available outside of OZ. And the prop ratio on the hondas are still 2.15:1? I thought that the 25" shaft was only available on the 15. MY point stands, the Honda is a distant second choice, for me, in the U.S.

I failed to mention that a big plus for me is the Yamaha's power tilt function, solving a major PITA . It makes it heavier than the Honda, but it still more economical tpo run.

[44'cruisingcat]

The Honda 20 has power tilt available.

The shaft length is 703mm = 27 3/4 inches.

In "Powerthrust" version they come fitted with a Honda 10 x 7 5/8 prop, with zero blade cup for better reverse thrust. As opposed to the standard 9 1/4 x 10 prop.

I'm pretty sure the Hondas have a bigger alternator too.

edit: It took a while to find, Honda must have the worlds WORST website - http://www.hondampe.com.au/repositor...fications.aspx

Look at the "x" type.

[Whimsical]

Originally Posted by sandy daugherty

What IS important to know is this: Are we getting the best thrust from our engines at the boat speeds we can attain? The answer is a universal no. Outboard engine equipped catamarans, with few exceptions, go faster than 8 knots under sail. While most 10 hp outboards are designed to reach their peek power at 16 mph, and have only reached about 70% at 8 knots, the Yamaha High Thrust have 2.91:1 gear ratios and low pitch propellers with three big blades and a large diameter, all contributing to moving more water at a lower speed. The Yamaha High Thrust Engines are alone in this regard and come closest to meeting our needs. They prove 85% of their available power at 8 knots. I now suspect that two of these engines give us about 520 pounds of thrust at best speed. By comparison, a Honda BF15 (my second choice) will provide less thrust at a higher fuel burn, more weight, and more cost.

This is the conclusion i came to and why i opted for diesels. Working out the pitch times Prop rpm gives some quite high speeds even allowing for slip. You end up with not a lot of diameter which is what gives thrust.
I would prefer to pitch for 8 knots and use the power for as big a diameter as possable for the times when **** hits the fan. Not so easy to do with ob's.
I'm not sure what rpm all the ob's get their power but for 20 hp i would think a 7.5" pitch would equate to about the 10" pitch i have with the diesel. The comparison then becomes 15" diameter for the diesel versus 10 for the ob.
Unless of course the ratios are significanly different but the Honda and my diesels are the same

Mike

[Cadence]

Originally Posted by 44'cruisingcat

The Honda 20 has power tilt available.

The shaft length is 703mm = 27 3/4 inches.

In "Powerthrust" version they come fitted with a Honda 10 x 7 5/8 prop, with zero blade cup for better reverse thrust. As opposed to the standard 9 1/4 x 10 prop.

I'm pretty sure the Hondas have a bigger alternator too.

edit: It took a while to find, Honda must have the worlds WORST website - Specifications | BF20 | Marine | Honda MPE

Look at the "x" type.

I believe it was the same in 15HP I had (Powertrust). I found it to give excellent preformance on a 31' Brown tri. The only complaint was it was so quiet I couldn't tell it was running without looking for the exhaust water telltale. I believe that's a complaint we can all live with.

[sandy daugherty]

Here's a partial solution to the mystery. Honda doesn't sell the same engines everywhere. Neither does Yamaha for that matter. The Honda BF15 x is very appealing (still my second choice for my particular interests) but the the numbers are revealing: 15 hp at 5500 rpm thru a 2.08:1 gear ratio to a 7.625" pitch equals 19 mph theoretical, discounting any slippage and special prop characteristics. Since Slippage is typically somewhere between 10 and 20% , we can SPECULATE that this engine is designed to develope best power at 15 to 17 mph. Passing speculation into the realm of wild ass guessing [as in feral donkey estimation] we could say that the Hondas are putting out 75 to 80% of their best power at 8 knots, and because they are higher displacement engines, they are burning more fuel than the Yamaha at that speed, developing the same or less power.

As far as I can tell, the BF15 is the only engine available with the X-tra long shaft in the US.

I've had the pleasure of owning seven different Yamaha Hight Thrust Engines, and one Honda. I am biased toward Honda cars, motorcycles, and generators, but when it comes to outboards the best fit is not a Honda

[sandy daugherty]

Gas outboards vs diesel? I don't know the exact numbers, but when PDQ offered a choice between outboard gasoline and inboard saildrive power on virtually identical catamarans, buyers chose gas something like five to one. If present owners were polled, I would guess that some of the Diesel owners would now prefer outboards after sailing the boats for a number of years.

I suggest that this is true for other cats of less than 10,000 # displacement, and does not apply to bigger, heavier boats, because there are at present no outboards with IDEAL thrust to speed ratios. I think a 30 hp longshaft diesel outboard that weighs less than 200 pounds would change that. Unfortunately, the market for such an engine would be very small.